| 1. |
- Alberto, H. V., et al.
(författare)
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Slow-muon study of quaternary solar-cell materials : Single layers and p-n junctions
- 2018
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Ingår i: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 2:2
-
Tidskriftsartikel (refereegranskat)abstract
- Thin films and p-n junctions for solar cells based on the absorber materials Cu(In, Ga) Se-2 and Cu2ZnSnS4 were investigated as a function of depth using implanted low energy muons. The most significant result is a clear decrease of the formation probability of the Mu(+) state at the heterojunction interface as well as at the surface of the Cu(In, Ga)Se-2 film. This reduction is attributed to a reduced bonding reaction of the muon in the absorber defect layer at its surface. In addition, the activation energies for the conversion from a muon in an atomiclike configuration to a anion-bound position are determined from temperature-dependence measurements. It is concluded that the muon probe provides a measurement of the effective surface defect layer width, both at the heterojunctions and at the films. The CIGS surface defect layer is crucial for solar-cell electrical performance and additional information can be used for further optimizations of the surface.
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| 2. |
- Amirabbasi, Mohammad, et al.
(författare)
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Lattice distortions and magnetic interactions in single-layer VOCl
- 2023
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Ingår i: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 7:7
-
Tidskriftsartikel (refereegranskat)abstract
- Atomically thin layers exfoliated from magnetic van der Waals layered materials are currently of high interest in solid state physics. VOCl is a quasi-two-dimensional layered antiferromagnet which was recently synthesized in monolayer form. Previous theoretical studies have assumed the high-temperature orthorhombic lattice symmetry also in the low-temperature range, where the bulk system is known to be monoclinic due to a strong magnetoelastic coupling. We demonstrate from ab initio calculations that this monoclinic distortion is prevalent also in monolayers, which is in line with recent experimental indications of monoclinic symmetry. Our calculations also show that competing ferromagnetic and antiferromagnetic interactions cause a frustrated twofold magnetic superstructure where higher-order magnetic interactions play a key role.
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| 3. |
- Andersson, Mikael, 1988, et al.
(författare)
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Interplay of NH4+ and BH4- reorientational dynamics in NH4BH4
- 2020
-
Ingår i: Physical Review Materials. - 2475-9953. ; 4:8
-
Tidskriftsartikel (refereegranskat)abstract
- The reorientational dynamics of ammonium borohydride (NH4BH4) was studied using quasielastic neutron scattering in the temperature interval from 10 to 240 K, which covers both the dynamically ordered and disordered polymorphs of NH4BH4. In the low-temperature (50 K) ordered polymorph of NH4BH4, analysis of the quasielastic neutron scattering data reveals that no reorientational dynamics is present within the probed timescale region of 0.1 to 100 ps. In the high-temperature (50 K) disordered polymorph, the analysis establishes the onset of NH4+ and BH4- dynamics at around 50 and 125 K, respectively. The relaxation time at 150 K for NH4+ is approximately 1 ps, while around 100 ps for BH4- . The NH4+ dynamics at temperatures below 125 K is associated with preferential tetrahedral tumbling motions, where each of the hydrogen atoms in the NH4+ tetrahedron can visit any of the four hydrogen sites, however, reorientations around a specific axis are more frequently occurring (C-2 or C3). At higher temperatures, the analysis does not exclude a possible evolution of the NH4+ dynamics from tetrahedral tumbling to either cubic tumbling, where the hydrogen atoms can visit any of the eight positions corresponding to the corners of a cube, or isotropic rotational diffusion, where the hydrogen atoms can visit any location on the surface of a sphere. The BH4- dynamics can be described as cubic tumbling. The difference in reorientational dynamics between the two ions is related to the difference of the local environment where the dynamically much slower BH4- anion imposes a noncubic environment on the NH4+ cation.
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| 4. |
- Anzellini, S., et al.
(författare)
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Phase diagram of calcium at high pressure and high temperature
- 2018
-
Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 2:8
-
Tidskriftsartikel (refereegranskat)abstract
- Resistively heated diamond-anvil cells have been used together with synchrotron x-ray diffraction to investigate the phase diagram of calcium up to 50 GPa and 800 K. The phase boundaries between the Ca-I (fcc), Ca-II (bcc), and Ca-III (simple cubic, sc) phases have been determined at these pressure-temperature conditions, and the ambient temperature equation of state has been generated. The equation of state parameters at ambient temperature have been determined from the experimental compression curve of the observed phases by using third-order Birch-Murnaghan and Vinet equations. A thermal equation of state was also determined for Ca-I and Ca-II by combining the room-temperature Birch-Murnaghan equation of state with a Berman-type thermal expansion model.
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| 5. |
- Arapan, S., et al.
(författare)
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Influence of antiphase boundary of the MnAl tau-phase on the energy product
- 2019
-
Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 3:6
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we use a multiscale approach to describe a realistic model of a permanent magnet based on MnAl tau-phase and elucidate how the antiphase boundary defects present in this material affect the energy product. We show how the extrinsic properties of a microstructure depend on the intrinsic properties of a structure with defects by performing micromagnetic simulations. For an accurate estimation of the energy product of a realistic permanent magnet based on the MnAl tau-phase with antiphase boundaries, we quantify exchange interaction strength across the antiphase boundary defect with a simple approach derived from first-principles calculations. These two types of calculations, performed at different scales, are linked via atomistic spin-dynamics simulations.
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| 6. |
- Arpaia, Riccardo, 1985, et al.
(författare)
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Engineering underdoped CuO2 nanoribbons in nm-thick a -axis YBa2Cu3 O7-δ films
- 2024
-
Ingår i: Physical Review Materials. - 2475-9953. ; 8:4
-
Tidskriftsartikel (refereegranskat)abstract
- In underdoped cuprate high-Tc superconductors, various local orders and symmetry-breaking states, in addition to superconductivity, reside in the CuO2 planes. The confinement of the CuO2 planes can therefore play a fundamental role in modifying the hierarchy between the various orders and their intertwining with superconductivity. Here we present the growth of a-axis oriented YBa2Cu3O7-δ films, spanning the whole underdoped side of the phase diagram. In these samples, the CuO2 planes are confined by the film thickness, effectively forming unit-cell-thick nanoribbons. The unidirectional confinement at the nanoscale enhances the in-plane anisotropy of the films. By x-ray diffraction and resistance vs temperature measurements, we have discovered the suppression of the orthorhombic-to-tetragonal transition at low dopings, and a very high anisotropy of the normal state resistance in the b-c plane, the latter being connected to a weak coupling between adjacent CuO2 nanoribbons. These findings show that the samples we have grown represent a novel system, different from the bulk, where future experiments can possibly shed light on the rich and mysterious physics occurring within the CuO2 planes.
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| 7. |
- Arpaia, Riccardo, 1985, et al.
(författare)
-
Engineering underdoped CuO2 nanoribbons in nm-thick a -axis YBa2Cu3 O7-δ films
- 2024
-
Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 8:4
-
Tidskriftsartikel (refereegranskat)abstract
- In underdoped cuprate high-Tc superconductors, various local orders and symmetry-breaking states, in addition to superconductivity, reside in the CuO2 planes. The confinement of the CuO2 planes can therefore play a fundamental role in modifying the hierarchy between the various orders and their intertwining with superconductivity. Here we present the growth of a-axis oriented YBa2Cu3O7-δ films, spanning the whole underdoped side of the phase diagram. In these samples, the CuO2 planes are confined by the film thickness, effectively forming unit-cell-thick nanoribbons. The unidirectional confinement at the nanoscale enhances the in-plane anisotropy of the films. By x-ray diffraction and resistance vs temperature measurements, we have discovered the suppression of the orthorhombic-to-tetragonal transition at low dopings, and a very high anisotropy of the normal state resistance in the b-c plane, the latter being connected to a weak coupling between adjacent CuO2 nanoribbons. These findings show that the samples we have grown represent a novel system, different from the bulk, where future experiments can possibly shed light on the rich and mysterious physics occurring within the CuO2 planes.
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| 8. |
- Arpaia, Riccardo, 1985, et al.
(författare)
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Probing the phase diagram of cuprates with YBa2Cu3O7−δ thin films and nanowires
- 2018
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Ingår i: Physical Review Materials. - 2475-9953. ; 2:2
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Tidskriftsartikel (refereegranskat)abstract
- We have grown and characterized 30-nm-thick YBa2Cu3O7−δ (YBCO) films, deposited by pulsed laser deposition on both MgO (110) and SrTiO3 (001) substrates, which induce opposite strain to the superconducting layer. By carefully tuning the in situ post-annealing oxygen pressure, we achieved, in a reproducible way, films at different oxygen doping, spanning from the slightly overdoped down to the strongly underdoped region of the phase diagram. The transport properties of the films, investigated through resistance versus temperature measurements, are in perfect qualitative agreement with single crystals. Starting from these films, we have also successfully fabricated nanowires with widths down to 65 nm, at different oxygen doping. The nanostructures exhibit characteristic temperatures (as the critical temperature Tc and the pseudogap temperature T∗) similar to those of the as-grown films and carry critical current densities Jc close to the critical depairing value, limited by vortex entry. This implies that the superconducting and the normal state properties of underdoped YBCO are preserved in our films, and they can be studied as a function of the dimensionality of the system, down to the nanoscale.
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| 9. |
- Arpaia, Riccardo, 1985, et al.
(författare)
-
Untwinned YBa2Cu3O7-delta thin films on MgO substrates: A platform to study strain effects on the local orders in cuprates
- 2019
-
Ingår i: Physical Review Materials. - 2475-9953. ; 3:11
-
Tidskriftsartikel (refereegranskat)abstract
- We have grown untwinned YBa2Cu3O7-delta (YBCO) films on (110) MgO substrates that were preannealed at high temperature in oxygen atmosphere. The annealing results in surface reconstruction with shallow facets, which induce the suppression of the YBCO twinning domains, and the preferential alignment of the CuO chains along one of the in-plane directions of the substrate. Because of the large mismatch between the in-plane lattice parameters of film and substrate, the strain induced by the MgO into the YBCO layer is strong and very peculiar. The YBCO film is compressed, with respect to the bulk, and presents a unidirectional buckling of the atomic planes, along the chains' direction, due to a deformation of the copper-oxygen octahedra. The YBCO films, which can be grown with thicknesses down to few unit cells and oxygen doping levels spanning most of the superconducting dome, are patterned into nanowires with dimensions down to 50 nm. The anisotropies due to the untwinning state are preserved in these structures; moreover, additional anisotropies appear in ultrathin structures where strain effects become more pronounced. Such untwinned and compressively strained films can therefore be used as a platform to study the interplay between strain and the various local orders in the normal state of YBCO.
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| 10. |
- Baba, Elbruz Murat, et al.
(författare)
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Light-induced breathing in photochromic yttrium oxyhydrides
- 2020
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Ingår i: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 4:2
-
Tidskriftsartikel (refereegranskat)abstract
- When exposed to air, metallic yttrium dihydride YH2 films turn into insulating and transparent yttrium oxyhydride (YHO). The incorporation of oxygen causes the lattice expansion of YH2 and the emergence of photochromic properties, i.e., YHO darkens reversibly when illuminated with light of adequate energy and intensity. However, the adequate bleaching of the photodarkened samples once the illumination has stopped is much faster in air than in inert atmosphere. According to this experimental evidence, the photochromic mechanism has to be related to an oxygen diffusion and exchange process. Since this process is accompanied by a lattice expansion/contraction, it can be said that YHO "breathes" when subjected to illumination/darkness cycling. Another interesting side effect of the breathing is the unexpected enhancement of the hydrophobicity of the YHO samples under illumination. A theoretical model able to explain the breathing in YHO is presented, together with the discussion of other alternative explanations.
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| 11. |
- Benshalom, Nimrod, et al.
(författare)
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Dielectric response of rock-salt crystals at finite temperatures from first principles
- 2022
-
Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 6:3
-
Tidskriftsartikel (refereegranskat)abstract
- We combine ab initio simulations and Raman scattering measurements to demonstrate explicit anharmonic effects in the temperature-dependent dielectric response of a NaCl single crystal. We measure the temperature evolution of its Raman spectrum and compare it to both a quasiharmonic and anharmonic model. Results demonstrate the necessity of including anharmonic lattice dynamics to explain the dielectric response of NaCl, as it is manifested in Raman scattering. Our model fully captures the linear dielectric response of a crystal at finite temperatures and may therefore be used to calculate the temperature dependence of other material properties governed by it.
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| 12. |
- Bergqvist, Lars, 1976-, et al.
(författare)
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Realistic finite temperature simulations of magnetic systems using quantum statistics
- 2018
-
Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 2:1
-
Tidskriftsartikel (refereegranskat)abstract
- We have performed realistic atomistic simulations at finite temperatures using Monte Carlo and atomistic spin dynamics simulations incorporating quantum (Bose-Einstein) statistics. The description is much improved at low temperatures compared to classical (Boltzmann) statistics normally used in these kind of simulations, while at higher temperatures the classical statistics are recovered. This corrected low-temperature description is reflected in both magnetization and the magnetic specific heat, the latter allowing for improved modeling of the magnetic contribution to free energies. A central property in the method is the magnon density of states at finite temperatures, and we have compared several different implementations for obtaining it. The method has no restrictions regarding chemical and magnetic order of the considered materials. This is demonstrated by applying the method to elemental ferromagnetic systems, including Fe and Ni, as well as Fe-Co random alloys and the ferrimagnetic system GdFe3.
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| 13. |
- Bernardes, Yuri, et al.
(författare)
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Direct observation of large-area strain propagation on free-standing nanomembranes
- 2023
-
Ingår i: Physical Review Materials. - 2475-9953. ; 7:2
-
Tidskriftsartikel (refereegranskat)abstract
- Investigations on epitaxial nanostructures with size of tens of nanometers have been a challenging issue for techniques that present high strain sensitivity but restricted spatial resolution. This is the case of recently developed x-ray nanoprobe techniques. Despite its inherent nondestructive character, submicron x-ray spots have only been successfully applied to the study of individual nanostructures which are either strain free or present extremely mild spatial lattice parameter gradients. Such limitation, with an uttermost barrier given by the diffraction limit, leads to voxel or pixel sizes between 5 and 10 nm obtained in coherent diffraction imaging or ptychographic reconstructions of real-space objects. Whenever the strain field of a nanostructure is successfully reconstructed from reciprocal space measurements, it cannot vary considerably in short distances since this would induce diffraction peak broadening and cause abrupt phase variations, leading to convergence issues on reconstruction algorithms. Here we show how epitaxial systems with large lattice mismatch and appreciable interfacial strain can be identified and directly analyzed throughout their strain field propagation in nanometer-thin crystalline membrane platforms, using the InGaAs/GaAs Stranski-Krastanov system as a model. The strain-induced footprint becomes observable along a few microns if the membrane thickness is comparable to the nanostructure size. It is possible to retrieve both interfacial strain and nanostructure size by probing individual objects.
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| 14. |
- Bischoff, Thomas, et al.
(författare)
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Nonempirical hybrid functionals for band gaps of inorganic metal-halide perovskites
- 2019
-
Ingår i: Physical Review Materials. - 2475-9953. ; 3:12
-
Tidskriftsartikel (refereegranskat)abstract
- Nonempirical hybrid functionals are investigated for band-gap predictions of inorganic metal-halide perovskites belonging to the class CsBX3, with B = Ge, Sn, Pb and X = Cl, Br, I. We consider both global and range-separated hybrid functionals and determine the parameters through two different schemes. The first scheme is based on the static screening response of the material and thus yields dielectric-dependent hybrid functionals. The second scheme defines the hybrid functionals through the enforcement of Koopmans' condition for localized defect states. We also carry out quasiparticle self-consistent GW calculations with vertex corrections to establish state-of-the-art references. For the investigated class of materials, dielectric-dependent functionals and those fulfilling Koopmans' condition yield band gaps of comparable accuracy (similar to 0.2 eV), but the former only require calculations for the primitive unit cell and are less subject to the specifics of the material.
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| 15. |
- Blixt, Kevin H., et al.
(författare)
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Phase field crystal modeling of grain boundary structures in diamond cubic systems
- 2024
-
Ingår i: Physical Review Materials. - 2475-9953. ; 8:3
-
Tidskriftsartikel (refereegranskat)abstract
- Phase field crystal (PFC) modeling has proved to be a versatile numerical tool in the analysis of crystalline microstructures. Most often, however, the focus is put on bulk crystal behavior, while crystal defects such as grain boundaries (GBs) are less explored. This is, in particular, the case for crystal structures beyond fcc and bcc. In this work, the possibilities and challenges in adopting PFC to diamond cubic (DC) crystal structures is investigated. Three different PFC models are considered for this purpose. One of them was published previously, and two are modifications proposed in the present work. The models are compared in terms of both DC phase stabilization and their ability to provide relevant GB structures. The models employ combinations of two- and three-point correlations, and the addition of a three-point correlation is found to be required for stabilization of the expected DC GB structures. It is concluded that although each of the models has limitations in terms of the GB structures which can be stabilized and performance in terms of phase stability, key PFC components for successful modeling of DC structures can be identified.
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| 16. |
- Bodlos, R., et al.
(författare)
-
Ab initio investigation of the atomic volume, thermal expansion, and formation energy of WTi solid solutions
- 2021
-
Ingår i: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 5:4
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Tidskriftsartikel (refereegranskat)abstract
- WTi is used as an adhesive layer in integrated circuit devices. The temperature dependent mechanical properties of WTi are still largely unexplored. In this paper we investigate WTi solid solutions with density functional theory calculations to determine the temperature and concentration dependent behavior of volume and coefficient of thermal expansion. The coefficient of thermal expansion is analyzed in terms of the bulk modulus, heat capacity, and Gruneisen parameter. Furthermore, we gain insight into the bonding of the system via investigation of the electronic structure, phonon density of states, and analysis of the formation energy. Low Ti concentrations lead to strong W-Ti bonding, as manifested in additional high frequency peaks in the phonon density of states. As a consequence, deviations from Vegard's law are found at low Ti concentrations, with a minimum of the lattice constant at about 15 at.% Ti. The CTE as a function of Ti concentration shows a negative trend at low temperatures and Ti concentrations, which is related to a strong decrease of heat capacity. Finally we show that the Debye-Gruneisen model yields results for WTi comparable to the quasiharmonic approach at a fraction of the computational cost.
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| 17. |
- Borisov, Vladislav, et al.
(författare)
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Tuning skyrmions in B20 compounds by 4d and 5d doping
- 2022
-
Ingår i: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 6:8
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Tidskriftsartikel (refereegranskat)abstract
- Skyrmion stabilization in novel magnetic systems with the B20 crystal structure is reported here, primarily based on theoretical results. The focus is on the effect of alloying on the 3d sublattice of the B20 structure by substitution of heavier 4d and 5d elements, with the ambition to tune the spin-orbit coupling and its influence on magnetic interactions. State-of-the-art methods based on density functional theory are used to calculate both isotropic and anisotropic exchange interactions. Significant enhancement of the Dzyaloshinskii-Moriya interaction is reported for 5d-doped FeSi and CoSi, accompanied by a large modification of the spin stiffness and spiralization. Micromagnetic simulations coupled to atomistic spin-dynamics and ab initio magnetic interactions reveal the spin-spiral nature of the magnetic ground state and field-induced skyrmions for all these systems. Especially small skyrmions similar to 50 nm are predicted for Co0.75Os0.25Si, compared to similar to 148 nm for Fe0.75Co0.25Si. Convex-hull analysis suggests that all B20 compounds considered here are structurally stable at elevated temperatures and should be possible to synthesize. This prediction is confirmed experimentally by synthesis and structural analysis of the Ru-doped CoSi systems discussed here, both in powder and in single-crystal forms.
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| 18. |
- Brem, Samuel, 1991, et al.
(författare)
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Electrically pumped graphene-based Landau-level laser
- 2018
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Ingår i: Physical Review Materials. - 2475-9953. ; 2:3
-
Tidskriftsartikel (refereegranskat)abstract
- Graphene exhibits a nonequidistant Landau quantization with tunable Landau-level (LL) transitions in the technologically desired terahertz spectral range. Here, we present a strategy for an electrically driven terahertz laser based on Landau-quantized graphene as the gain medium. Performing microscopic modeling of the coupled electron, phonon, and photon dynamics in such a laser, we reveal that an inter-LL population inversion can be achieved resulting in the emission of coherent terahertz radiation. The presented paper provides a concrete recipe for the experimental realization of tunable graphene-based terahertz laser systems.
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| 19. |
- Carvalho, P. C., et al.
(författare)
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Complex magnetic textures in Ni/Ir-n/Pt(111) ultrathin films
- 2021
-
Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 5:12
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Tidskriftsartikel (refereegranskat)abstract
- A combined approach using first-principles calculations and spin dynamics simulations is applied to study Ni/Ir-n/Pt(111) (n = 0, 1, 2) films. The lowest-energy states are predicted to be spin spirals but with a minute (of the order of a few mu eV/atom) energy difference with skyrmionic states. The spontaneous low-temperature skyrmions, with similar to 15 nm to similar to 35 nm size, arise from a large Dzyaloshinskii-Moriya (DM) and Heisenberg exchange interactions ratio and, in particular, from a large in-plane DM vector component for nearest neighbors. The skyrmions become larger and more dispersed with the enhancement of the Ir buffer thickness. Also, with increasing n, the skyrmions' stability decreases when an external magnetic field is applied or the temperature is raised. For n = 0 and n = 1, we find that metastable skyrmioniums can occur, which are characterized by a slightly lower stability with respect to the external fields and larger critical currents, compared to skyrmions.
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| 20. |
- Casillas Trujillo, Luis, et al.
(författare)
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Identification of materials with strong magnetostructural coupling using computational high-throughput screening
- 2021
-
Ingår i: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 5:3
-
Tidskriftsartikel (refereegranskat)abstract
- Important phenomena such as magnetostriction, magnetocaloric, and magnetoelectric effects arise from, or could be enhanced by, the coupling of magnetic and structural degrees of freedom. The coupling of spin and lattice also influence transport and structural properties in magnetic materials, in particular around phase transitions. In this paper we propose a method for screening materials for a strong magnetostructural coupling by assessing the effect of the local magnetic configuration on the atomic forces using density functional theory. We have employed the disordered local moment approach in a supercell formulation to probe different magnetic local configurations and their forces and performed a high-throughput search on binary and ternary compounds available in the Crystallography Open Database. We identify a list of materials with a strong spin-lattice coupling out of which several are already known to display magnetolattice coupling phenomena such as Fe3O4 and CrN. Others, such as Mn2CrO4 and CaFe7O11, have been less studied and have yet to reveal their potentials in experiments and applications.
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| 21. |
- Casillas Trujillo, Luis, et al.
(författare)
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Interstitial carbon in bcc HfNbTiVZr high-entropy alloy from first principles
- 2020
-
Ingår i: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 4:12
-
Tidskriftsartikel (refereegranskat)abstract
- The remarkable mechanical properties of high-entropy alloys can be further improved by interstitial alloying. In this work we employ density functional theory calculations to study the solution energies of dilute carbon interstitial atoms in tetrahedral and octahedral sites in bcc HfNbTiVZr. Our results indicate that carbon interstitials in tetrahedral sites are unstable, and the preferred octahedral sites present a large spread in the energy of solution. The inclusion of carbon interstitials induces large structural relaxations with long-range effects. The effect of local chemical environment on the energy of solution is investigated by performing a local cluster expansion including studies of its correlation with the carbon atomic Voronoi volume. However, the spread in solution energetics cannot be explained with a local environment analysis only pointing towards a complex, long-range influence of interstitial carbon in this alloy.
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| 22. |
- Champagne, A., et al.
(författare)
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First-order Raman scattering of rare-earth containing i-MAX single crystals (Mo2/3RE1/3)(2)AlC (RE = Nd, Gd, Dy, Ho, Er)
- 2019
-
Ingår i: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 3:5
-
Tidskriftsartikel (refereegranskat)abstract
- Herein, we report on the growth of single crystals of various (Mo2/3RE1/3)(2)AlC (RE = Nd, Gd, Dy, Ho, Er) i-MAX phases and their Raman characterization. Using first principles, the wave numbers of the various phonon modes and their relative atomic displacements are calculated and compared to experimental results. Twelve high-intensity Raman peaks are identified as the fingerprint of this new family of rare-earth containing i-MAX phases, thus being a useful tool to investigate their corresponding composition and structural properties. Indeed, while a redshift is observed in the low-wave-number range due to an increase of the rare-earth atomic mass when moving from left to right on the lanthanide row, a blueshift is observed for most of the high-wave-number modes due to a strengthening of the bonds. A complete classification of bond stiffnesses is achieved based on the direct dependence of a phonon mode wave number with respect to the bond stiffness. Finally, STEM images are used to confirm the crystal structure.
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| 23. |
- Chen, Zhesheng, et al.
(författare)
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Time-resolved photoemission spectroscopy of electronic cooling and localization in CH3 NH3 PbI3 crystals
- 2017
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Ingår i: Physical Review Materials. - 2475-9953. ; 1:4
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Tidskriftsartikel (refereegranskat)abstract
- We measure the surface of CH3NH3PbI3 single crystals by making use of two-photon photoemission spectroscopy. Our method monitors the electronic distribution of photoexcited electrons, explicitly discriminating the initial thermalization from slower dynamical processes. The reported results disclose the fast-dissipation channels of hot carriers (0.25 ps), set an upper bound to the surface-induced recombination velocity (<4000 cm/s), and reveal the dramatic effect of shallow traps on the electrons dynamics. The picosecond localization of excited electrons in degraded CH3NH3PbI3 samples is consistent with the progressive reduction of photoconversion efficiency in operating devices. Minimizing the density of shallow traps and solving the aging problem may boost the macroscopic efficiency of solar cells to the theoretical limit.
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| 24. |
- Ciuciulkaite, Agne, MSc, 1991-, et al.
(författare)
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Magnetic and all-optical switching properties of amorphous TbxCo100-x alloys
- 2020
-
Ingår i: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 4:10
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Tidskriftsartikel (refereegranskat)abstract
- Amorphous TbxCo100-x magnetic alloys exhibit a list of intriguing properties, such as perpendicular magnetic anisotropy, high magneto-optical activity, and magnetization switching using ultrashort optical pulses. Varying the Tb:Co ratio in these alloys allows for tuning properties such as the saturation magnetic moment, coercive field, and the performance of light-induced magnetization switching. In this paper, we investigate the magnetic, optical, and magneto-optical properties of various TbxCo100-x thin-film alloy compositions. We report on the effect the choice of different seeding layers has on the structural and magnetic properties of TbxCo100-x layers. We also demonstrate that for a range of alloys, deposited on fused silica substrates, with Tb content of 24-30 at. %, helicity-dependent all-optical switching of magnetization can be achieved, albeit in a multishot framework. We explain this property to arise from the helicity-dependent laser-induced magnetization on the Co sublattice due to the inverse Faraday effect. Our paper provides an insight into material aspects for future potential hybrid magnetoplasmonic TbCo-based architectures.
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| 25. |
- Colvin, Jovana, et al.
(författare)
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Local defect-enhanced anodic oxidation of reformed GaN nanowires
- 2020
-
Ingår i: Physical Review Materials. - 2475-9953. ; 4:7
-
Tidskriftsartikel (refereegranskat)abstract
- Understanding formation and distribution of defects in GaN substrates and device layers is needed to improve device performance in rf and power electronics. Here we utilize conductive atomic force microscopy (c-AFM) for studying defect-related leakage paths in an unintentionally doped GaN film formed by nanowire reformation. A nanoscopic Schottky contact is formed between the c-AFM probe and the GaN surface, which, under reverse-bias conditions, reveals local leakage currents at the positions of the nanowires. Cathodoluminescence shows these areas to be dominated by yellow-band luminescence, in contrast to the surrounding GaN matrix, which mainly shows near-band-gap luminescence. These results are attributed to a high density of native and residual defects, confined to the nanowires. In addition, we use anodic oxidation to map defect-related conductive paths through locally induced growth of gallium oxide. The oxide yield, which is known to depend on the local electric field strength between the AFM tip and the sample, correlates well with the level of reverse-bias leakage current. Local irregularities in oxide height reveal extended oxidation attributed to defect-related deep-level states. Thisis confirmed by controlled dissolution of the oxide in NaOH, showing that a deeper oxide film is grown over areas where defect-related conductive paths are formed. Finally, we demonstrate how this approach can be used as a quick and easy diagnostic tool for evaluating the influence of specific growth conditions and process steps on defect-induced leakage current levels and defect distribution in GaN structures, demonstrating its potential for accelerated test of leakage degradation at critical positions in GaN-based devices.
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| 26. |
- Colvin, Jovana, et al.
(författare)
-
Surface and dislocation investigation of planar GaN formed by crystal reformation of nanowire arrays
- 2019
-
Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 3:9
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper we present a process of forming monolithic GaN surface from an ordered nanowire array by means of material redistribution. This process, referred to as reformation, is performed in a conventional MOVPE crystal growth system with the gallium supply turned off and allows a crystal nanostructure to change shape according to differences in surface energies between its facets. Using reformation, coalescence may proceed closer to thermodynamic equilibrium, which is required for fabrication of high-quality substrate material. Scanning probe techniques are utilized, complemented by cathodoluminescence and electron microscopy, to investigate structural and electrical properties of the surface after reformation, as well as to assess densities, location, and formation of different types of defects in the GaN film. Spatial variations in material properties such as intrinsic majority-carrier types can be attributed to the radical changes in growth conditions required for sequential transition between nanowire growth, selective shell growth, and reformation. These properties enable us to assess the impact of the process on densities, locations, and formation of different types of dislocations in the GaN film. We find a fraction of the nanowires to comprise of a single electrically neutral edge dislocation, propagating from the GaN buffer, while electrically active dislocations are found at coalesced interfaces between nanowires. By decreasing the mask aperture size and changing the nucleation conditions the prevalence of nanowires comprising edge dislocation was significantly reduced from 6% to 3%, while the density of interface dislocations was reduced from 6×108 to 4×107cm-2. Using a sequential reformation process was found to create inversion domains with low surface potential N-polar regions in an otherwise Ga-polar GaN film. The inversion domains were associated with pinned dislocation pairs, and were further confirmed by selective wet etching in NaOH. This lateral polarity inversion was thoroughly eliminated in samples formed by a continuous reformation process. These results reveal a path and challenges for growing GaN substrates of superior crystal quality through nanowire reformation.
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| 27. |
- Corona, Patrick T., et al.
(författare)
-
Fingerprinting soft material nanostructure response to complex flow histories
- 2022
-
Ingår i: Physical Review Materials. - 2475-9953. ; 6:4
-
Tidskriftsartikel (refereegranskat)abstract
- Understanding the complex connection between flow-processing history, fluid structure, and fluid properties represents a grand challenge for the engineering and fundamental study of nanostructured soft materials. To address this challenge, we report measurements using scanning small-angle x-ray scattering in a fluidic four-roll mill (FFoRM), which enables rapid nanostructural characterization of complex fluids under an unprecedentedly diverse range of flow histories. Combining this technique with analysis of the Lagrangian deformation history of fluid elements, we demonstrate rapid mapping of orientational ordering of fluids' nanostructure along diverse deformation trajectories that emulate those encountered in flow processing. Using demonstrative experiments on model rodlike nanoparticle dispersions, we show that differences in rod flexibility and rod-rod interactions play a significant role in determining the fluid's anisotropic structural response to similar flow histories. An analysis of the coupling between measured particle alignment and interparticle correlations reveals these differences to arise from the nature and strength of interparticle interactions in flow. These measurement and analysis techniques produce large datasets that hold promise toward advancing process-structure-property models and inverse design processes of flows that are tailored to produce targeted nanostructures.
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| 28. |
- De Knoop, Ludvig, 1972, et al.
(författare)
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Electric-field-controlled reversible order-disorder switching of a metal tip surface
- 2018
-
Ingår i: Physical Review Materials. - 2475-9953. ; 2:8
-
Tidskriftsartikel (refereegranskat)abstract
- While it is well established that elevated temperatures can induce surface roughening of metal surfaces, the effect of a high electric field on the atomic structure at ambient temperature has not been investigated in detail. Here we show with atomic resolution using in situ transmission electron microscopy how intense electric fields induce reversible switching between perfect crystalline and disordered phases of gold surfaces at room temperature. Ab initio molecular dynamics simulations reveal that the mechanism behind the structural change can be attributed to a vanishing energy cost in forming surface defects in high electric fields. Our results demonstrate how surface processes can be directly controlled at the atomic scale by an externally applied electric field, which promotes an effective decoupling of the topmost surface layers from the underlying bulk. This opens up opportunities for development of active nanodevices in, e.g., nanophotonics and field-effect transistor technology as well as fundamental research in materials characterization and of yet unexplored dynamically controlled low-dimensional phases of matter.
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| 29. |
- Dwedari, Magdulin, et al.
(författare)
-
Disorder-induced broadening of excitonic resonances in transition metal dichalcogenides
- 2019
-
Ingår i: Physical Review Materials. - 2475-9953. ; 3:7
-
Tidskriftsartikel (refereegranskat)abstract
- The presence of impurities and disorder has an important impact on the optical response of monolayer transition metal dichalcogenides (TMDs). Here, we investigate elastic exciton-impurity scattering and its influence on the linewidth of excitonic resonances in different TMD materials. We derive an analytic expression for the linewidth broadening within the density matrix formalism. We find that the exciton linewidth increases for states up to the 3s exciton due to the scattering with impurities. For higher states, the impurity contribution decreases, reflecting the reduced scattering cross section. Furthermore, we reveal that the scattering efficiency is the largest for transitions between s and p exciton states. Finally, different TMDs show generally a similar behavior. The quantitatively smaller broadening in tungsten-based TMDs can be ascribed to their smaller effective masses resulting in a less efficient scattering.
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| 30. |
- Ehteshami, Hossein, et al.
(författare)
-
High-temperature thermophysical properties of gamma- and delta-Mn from first principles
- 2018
-
Ingår i: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 2:3
-
Tidskriftsartikel (refereegranskat)abstract
- Thermophysical properties of gamma-and delta-Mn phases have been investigated using first-principles calculations in their thermodynamically stable temperature range. An adiabatic approximation is used for partitioning of the Helmholtz free energy into electronic, magnetic, and vibrational contributions from the corresponding temperature induced excitations, where the fastest degree of freedom has been included in the slower ones. Namely, electronic excitations (on a one-electron level) have been included directly in the first-principles calculations at the corresponding temperatures. Magnetic excitations in the paramagnetic state then have been taken into consideration in the two opposite limits: localized, considering only transverse spin fluctuations (TSF), and itinerant, allowing for the full coupling of transverse and longitudinal spin fluctuations (LSF). Magnetic contribution to the free energy has been included in the calculations of the vibrational one, which has been obtained within the Debye-Gruneisen model. The calculated thermophysical properties such as lattice constance, thermal lattice expansion, and heat capacity are in good agreement with available experimental data, especially in the case when the itinerant magnetic model is chosen. We also present our results for elastic properties at high temperatures.
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| 31. |
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| 32. |
- Ehteshami, Hossein, et al.
(författare)
-
Thermodynamic properties of paramagnetic α- and β−Mn from first principles: The effect of transverse spin fluctuations
- 2017
-
Ingår i: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 1:073803
-
Tidskriftsartikel (refereegranskat)abstract
- First-principles-based thermodynamic modeling of cubic alpha and beta phases of Mn represent a challenge due to their structural complexity and the necessity of simultaneous treatment of several types of disorder (electronic, magnetic, and vibrational) that have very different characteristic time scales. Here we employ mean-field theoretical models to describe the different types of disorder and then we connect each layer of theory to the others using the adiabatic principle of separating faster and slower degrees of freedom. The slowest (vibrational) degrees of freedom are treated using the Moruzzi, Janak, and Schwarz formalism [Phys. Rev. B 37, 790 (1988)] of the Debye-Gruneisen model parametrized based on the first-principles calculated equation of state which includes the free-energy contributions due to the fast (electronic and magnetic) degrees of freedom via the Fermi-Dirac distribution function and a mean-field theory of transverse spin fluctuations. The magnetic contribution due to transverse spin fluctuations has been computed self-consistently within the disordered local moment picture of the paramagnetic state. The obtained results for thermodynamic properties such as lattice parameter, linear thermal expansion coefficient, and heat capacity of both phases show a good agreement with available experimental data. We also tested the assumption about the nature (localized versus delocalized) of magnetic moment on site IV in alpha-Mn and site I in beta-Mn on the thermodynamic properties of these two phases. Similar to the findings of experimental studies, we conclude that magnetic moment on site IV in alpha-Mn is not of a localized character. However, a similar analysis suggests that the magnetic moment of site I in beta-Mn should be treated as localized.
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| 33. |
- Ekström Kelvinius, Filip, et al.
(författare)
-
Graph-based machine learning beyond stable materials and relaxed crystal structures
- 2022
-
Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 6:3
-
Tidskriftsartikel (refereegranskat)abstract
- There has been a recent surge of interest in using machine learning to approximate density functional theory in materials science. However, many of the most performant models are evaluated on large databases of computed properties of, primarily, materials with precise atomic coordinates available, and which have been experimentally synthesized, i.e., which are thermodynamically stable or metastable. These aspects provide challenges when applying such models on theoretical candidate materials, for example for materials discovery, where the coordinates are not known. To extend the scope of this methodology, we investigate the performance of the crystal graph convolutional neural network on a data set of theoretical structures in three related ternary phase diagrams (Ti,Zr,Hf)-Zn-N, which thus include many highly unstable structures. We then investigate the impact on the performance of using atomic positions that are only partially relaxed into local energy minima We also explore options for improving the performance in these scenarios by transfer learning, either from models trained on a large database of mostly stable systems, or a different but related phase diagram. Models pretrained on stable materials do not significantly improve performance, but models trained on similar data transfer very well. We demonstrate how our findings can be utilized to generate phase diagrams with a major reduction in computational effort.
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| 34. |
- Ektarawong, Annop, et al.
(författare)
-
Effect of temperature and configurational disorder on the electronic band gap of boron carbide from first principles
- 2018
-
Ingår i: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 2:10
-
Tidskriftsartikel (refereegranskat)abstract
- The overestimation, rather than the usual underestimation, of the electronic band gap at 0 K of boron carbide with the ideally stoichiometric composition of B4C, represented by B11CP (CBC), in density functional theory calculations is one of the outstanding controversial issues in the field of icosahedral boron-rich solids. Using a first-principles approach, we explore the effect of temperature and configurational disorder on the electronic band gap of B4C. Ab initio molecular dynamics simulations are performed to account for the effects of vibrational disorder. The results reveal that the volumetric thermal expansion as well as the thermally induced configurational disorder of icosahedral C-P atoms residing in the B11CP icosahedra have a minimal impact on the band gap of B4C, while a major decrease of the band gap is caused by explicit atomic displacements, induced by lattice vibrations. At 298 K, the band gap of B4C is overestimated, as compared to the experimental value, by approximately 31%. However, configurational disorder induced by introducing a small fraction of B-12 (CBC) and B-12 (B-4) into a matrix of B11CP (CBC) to make the composition of boron carbide approximately B4.3C, claimed to be the carbon-rich limit of the material in experiment, leads to a smaller band gap due to the appearance of midgap states. These results can explain at least a part of the previous discrepancies between theory and experiments for the band gap of boron carbide.
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| 35. |
- Ektarawong, Annop, et al.
(författare)
-
Phase stability of two-dimensional monolayer As1-xPx solid solutions revealed by a first-principles cluster expansion
- 2019
-
Ingår i: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 3:5
-
Tidskriftsartikel (refereegranskat)abstract
- The phase stability of two-dimensional monolayer As1-xPx solid solutions, exhibiting the puckered (alpha phase) and buckled (beta phase) structures are investigated using a first-principles cluster-expansion method. Canonical Monte Carlo simulations, together with harmonic approximation, are performed to capture the influences of thermally induced configurational disorder and lattice vibrations on the phase stability of monolayer As1-xPx. We first demonstrate that, as the temperature approaches 0 K, the monolayer As1-xPx displays a tendency toward phase separation into its constituent elemental phases, and thus no stable ordered structures of As1-xPx, both alpha and beta phases, are predicted to be thermodynamically stable. We further reveal with the inclusion of the lattice vibrational contributions that beta-As1-xPx is thermodynamically favored over alpha-As1-xPx, across the entire composition range even at 0 K and increasingly so at higher temperature, and a continuous series of disordered solid solution of beta-As1-xPx, where 0 amp;lt;= x amp;lt;= 1, is predicted at the temperature above 550 K. These findings not only indicate that the ordered structures of monolayer alpha-As1-xPx, and beta-As1-xPx, frequently studied in the literature, may not exist in nature, but also presumably suggest that monolayer alpha-As1-xPx is metastable with respect to monolayer beta-As1-xPx.
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| 36. |
- Eliasen, K. L., et al.
(författare)
-
High-frequency dynamics and test of the shoving model for the glass-forming ionic liquid Pyr14-TFSI
- 2021
-
Ingår i: Physical Review Materials. - 2475-9953. ; 5:6
-
Tidskriftsartikel (refereegranskat)abstract
- In studies of glass-forming liquids, one of the important questions is to understand to which degree chemically different classes of liquids have the same type of dynamics. In this context, room-temperature ionic liquids are interesting because they exhibit both van der Waals and Coulomb interactions. In this work we study the α relaxation and faster relaxation dynamics in the room-temperature ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14-TFSI). The paper presents quasielastic neutron and shear mechanical spectroscopy data measured over seven decades in frequency (10-3-104 Hz). The use of these two methods in combination reveal the α relaxation and four separate, faster modes. Two of these faster modes, based on the partial deuterations, can be assigned to the methyl group and the methyl end of the butyl chain of the cation. The neutron data are also used to determine the mean-square displacement (MSD) on the nanosecond timescale. It is shown that the temperature dependence of the MSD can account for the super-Arrhenius behavior of the α relaxation as predicted by the shoving model [Dyre, Rev. Mod. Phys. 78, 953 (2006)RMPHAT0034-686110.1103/RevModPhys.78.953], similarly to what is seen in simpler glass-forming liquids.
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| 37. |
- Erkensten, Daniel, 1995, et al.
(författare)
-
Microscopic origin of anomalous interlayer exciton transport in van der Waals heterostructures
- 2022
-
Ingår i: Physical Review Materials. - 2475-9953. ; 6:9
-
Tidskriftsartikel (refereegranskat)abstract
- Van der Waals heterostructures constitute a platform for investigating intriguing many-body quantum phenomena. In particular, transition-metal dichalcogenide (TMD) heterobilayers host long-lived interlayer excitons which exhibit permanent out-of-plane dipole moments. Here, we develop a microscopic theory for interlayer exciton-exciton interactions including both the dipolar nature of interlayer excitons as well as their fermionic substructure, which gives rise to an attractive fermionic exchange. We find that these interactions contribute to a drift force resulting in highly nonlinear exciton propagation at elevated densities in the MoSe2 - WSe2 heterostructure. We show that the propagation can be tuned by changing the number of hBN spacers between the TMD layers or by adjusting the dielectric environment. In particular, although counterintuitive, we reveal that interlayer excitons in freestanding samples propagate slower than excitons in hBN-encapsulated TMDs-due to an enhancement of the net Coulomb drift with stronger environmental screening. Overall, our work contributes to a better microscopic understanding of the interlayer exciton transport in technologically promising atomically thin semiconductors.
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| 38. |
- Fallqvist, Amie, et al.
(författare)
-
Resolving the debated atomic structure of the metastable cubic SiNx tissue phase in nanocomposites with TiN
- 2018
-
Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 2:9
-
Tidskriftsartikel (refereegranskat)abstract
- The TiN/SiNx nanocomposite and nanolaminate systems are the archetype for super if not ultrahard materials. Yet, the nature of the SiNx tissue phase is debated. Here, we show by atomically resolved electron microscopy methods that SiNx is epitaxially stabilized in a NaCl structure on the adjacent TiN(001) surfaces. Additionally, electron energy loss spectroscopy, supported by first-principles density functional theory calculations infer that SiNx hosts Si vacancies.
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| 39. |
- Feierabend, Maja, 1990, et al.
(författare)
-
Molecule signatures in photoluminescence spectra of transition metal dichalcogenides
- 2018
-
Ingår i: Physical Review Materials. - 2475-9953. ; 2:1
-
Tidskriftsartikel (refereegranskat)abstract
- Monolayer transition metal dichalcogenides (TMDs) show an optimal surface-to-volume ratio and are thus promising candidates for novel molecule sensor devices. It was recently predicted that a certain class of molecules exhibiting a large dipole moment can be detected through the activation of optically inaccessible (dark) excitonic states in absorption spectra of tungsten-based TMDs. In this paper, we investigate the molecule signatures in photoluminescence spectra in dependence of a number of different experimentally accessible quantities, such as excitation density, temperature, as well as molecular characteristics including the dipole moment and its orientation, molecule-TMD distance, molecular coverage, and distribution. We show that under certain optimal conditions even room-temperature detection of molecules can be achieved.
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| 40. |
- Ferreira, Beatriz, 1996, et al.
(författare)
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Microscopic modeling of exciton-polariton diffusion coefficients in atomically thin semiconductors
- 2022
-
Ingår i: Physical Review Materials. - 2475-9953. ; 6:3
-
Tidskriftsartikel (refereegranskat)abstract
- In the strong light-matter coupling regime realized, e.g., by integrating semiconductors into optical microcavities, polaritons as new hybrid light-matter quasiparticles are formed. The corresponding change in the dispersion relation has a large impact on optics, dynamics, and transport behavior of semiconductors. In this paper, we investigate the strong-coupling regime in hBN-encapsulated MoSe2 monolayers focusing on exciton-polariton diffusion. Applying a microscopic approach based on the exciton density matrix formalism combined with the Hopfield approach, we predict a drastic increase of the diffusion coefficients by two to three orders of magnitude in the strong-coupling regime. We explain this behavior by the much larger polariton group velocity and suppressed polariton-phonon scattering channels with respect to the case of bare excitons. Our study contributes to a better microscopic understanding of polariton diffusion in atomically thin semiconductors.
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| 41. |
- Filippov, Stanislav, et al.
(författare)
-
Temperature-induced phase transition and Li self-diffusion in Li2C2: A first-principles study
- 2019
-
Ingår i: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 3:2
-
Tidskriftsartikel (refereegranskat)abstract
- Lithium carbide, Li2C2, is a fascinating material that combines strong covalent and weak ionic bonding resulting in a wide range of unusual properties. The mechanism of its phase transition from the ground-state orthorhombic (Immm) to the high-temperature cubic (Fm (3) over barm) crystal structure is not well understood and here we elucidate it with help of first-principles calculations. We show that stabilization of the cubic phase is a result of a temperature-induced disorientation of the C-C dumbbells and their further thermal rotations. Due to these rotations rather large deviatoric stress, which is associated with the dumbbell alignment along one of the crystallographic axes, averages out making the cubic structure mechanically stable. At high temperature we observe a type-II superionic transition to a state of high Li self-diffusion involving collective ionic motion mediated by the formation of Frenkel pairs.
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| 42. |
- Forslund, Axel, et al.
(författare)
-
Structural vacancies in (Ti,Al)N : An ab initio study
- 2022
-
Ingår i: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 6:1
-
Tidskriftsartikel (refereegranskat)abstract
- The zero K formation energies of metal and nitrogen vacancies in several (Ti,Al)N alloys and at the (001) (Ti,Al)N/AlN interface are obtained in ab initio supercell calculations. The dependence of the formation energies of metal vacancies on their local environment and type are analyzed and explained in terms of effective cluster interactions for unrelaxed calculations. The common trend for all investigated types of vacancies is that their formation energy increases with the number of Al nearest neighbors if local lattice relaxations are not allowed. However, local lattice relaxations produce a dramatic effect especially in the case of metal vacancies leading to a complicated nonlinear dependence on the local environment indicating the existence of strong multisite strain-induced interactions.
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| 43. |
- Fransson, Erik, 1990, et al.
(författare)
-
First-principles modeling of complexions at the phase boundaries in Ti-doped WC-Co cemented carbides at finite temperatures
- 2021
-
Ingår i: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 5:9
-
Tidskriftsartikel (refereegranskat)abstract
- WC-Co cemented carbides have a unique combination of high hardness and good toughness, making them ideal as tool materials in applications such as metal machining or rock drilling. Dopants are commonly added to retard grain growth and thereby creating a harder material. Thin films with cubic structure have been observed experimentally at phase boundaries between hexagonal WC and fcc Co-rich binder when doping with, e.g., Ti, V, or Cr. These films are generally considered to play a crucial role in the grain growth inhibition effect. Therefore, the thermodynamics of these thin cubic films is important to understand. Here, we construct, using ab initio calculations and modeling, an interfacial phase diagram for thin cubic films in Ti-doped WC-Co. We consider C <-> vacancy and W <-> Ti substitutions by constructing alloy cluster expansions and use Monte Carlo simulations to calculate the configurational free energy. Furthermore, force-constant fitting is used to extract the harmonic free energy for the ground-state structures. Additionally, we use information from thermodynamic databases to couple our atomic-scale calculations to overall compositions of typical WC-Comaterials. We predict that Ti segregates to WC/Co phase boundaries to form thin cubic films of two metallic layer thickness, both at solid-state and liquid-phase sintering temperatures. Furthermore, we predict that these films are stable also for low doping concentrations when no Ti-containing carbide phase precipitates in the material. We show that Ti essentially only segregates to the inner layer of the thin cubic film leaving an almost pure W layer towards Co, an ordering which has been observed in recent experimental high-resolution transmission electron microscopy studies.
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| 44. |
- Fransson, Jonas, 1970-, et al.
(författare)
-
Microscopic theory for coupled atomistic magnetization and lattice dynamics
- 2017
-
Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 1:7
-
Tidskriftsartikel (refereegranskat)abstract
- A coupled atomistic spin and lattice dynamics approach is developed which merges the dynamics of these two degrees of freedom into a single set of coupled equations of motion. The underlying microscopic model comprises local exchange interactions between the electron spin and magnetic moment and the local couplings between the electronic charge and lattice displacements. An effective action for the spin and lattice variables is constructed in which the interactions among the spin and lattice components are determined by the underlying electronic structure. In this way, expressions are obtained for the electronically mediated couplings between the spin and lattice degrees of freedom, besides the well known interatomic force constants and spin-spin interactions. These former susceptibilities provide an atomistic ab initio description for the coupled spin and lattice dynamics. It is important to notice that this theory is strictly bilinear in the spin and lattice variables and provides a minimal model for the coupled dynamics of these subsystems and that the two subsystems are treated on the same footing. Questions concerning time-reversal and inversion symmetry are rigorously addressed and it is shown how these aspects are absorbed in the tensor structure of the interaction fields. By means of these results regarding the spin-lattice coupling, simple explanations of ionic dimerization in double-antiferromagnetic materials, as well as charge density waves induced by a nonuniform spin structure, are given. In the final parts, coupled equations of motion for the combined spin and lattice dynamics are constructed, which subsequently can be reduced to a form which is analogous to the Landau-Lifshitz-Gilbert equations for spin dynamics and a damped driven mechanical oscillator for the ionic motion. It is important to notice, however, that these equations comprise contributions that couple these descriptions into one unified formulation. Finally, Kubo-like expressions for the discussed exchanges in terms of integrals over the electronic structure and, moreover, analogous expressions for the damping within and between the subsystems are provided. The proposed formalism and types of couplings enable a step forward in the microscopic first principles modeling of coupled spin and lattice quantities in a consistent format.
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| 45. |
- Frisk, Andreas, et al.
(författare)
-
Magnetic and structural characterization of CoFeZr thin films grown by combinatorial sputtering
- 2019
-
Ingår i: Physical Review Materials. - 2475-9953. ; 3:7
-
Tidskriftsartikel (refereegranskat)abstract
- We report on a detailed investigation of structural and magnetic properties of thin CoFeZr films, produced by combinatorial cosputtering, with compositions in the ranges 30-85 at.% Co, 12-63 at.% Fe, and 4-8 at.% Zr. Extended x-ray absorption fine structure and x-ray diffraction measurements reveal that alloys with a Zr content below 5 at.% are polycrystalline with a bcc structure, while an amorphous morphology is stabilized at Zr contents above 6 at.%. All samples display a growth-induced in-plane uniaxial anisotropy, which is closely related to the Zr concentration gradients across the wafers. A model for the angular dependence of the reduced remanence, including a Gaussian distribution of easy/hard anisotropy axes, is presented and successfully used to fit the data for all samples. The magnetic moments of the polycrystalline films approximately follow the Slater-Pauling curve, and the magnetic moments of the amorphous films follow a similar trend, but with about 20 % lower values. X-ray magnetic circular dichroism measurements show, for the amorphous films, that the Co moments are virtually constant at 1.7(2)μB/atom.
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| 46. |
- Gangaprasad Rao, Smita, et al.
(författare)
-
Phase formation in CrFeCoNi nitride thin films
- 2023
-
Ingår i: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 7:4
-
Tidskriftsartikel (refereegranskat)abstract
- As a single-phase alloy, CrFeCoNi is a face centered cubic (fcc) material related to the archetypical highentropy Cantor alloy CrFeCoNiMn. For thin films, CrFeCoNi of approximately equimolar composition tends to assume an fcc structure when grown at room temperature by magnetron sputtering. However, the single-phase solid solution state is typically not achieved for thin films grown at higher temperatures. The same holds true for Cantor alloy-based ceramics (nitrides and oxides), where phase formation is extremely sensitive to process parameters such as the amount of reactive gas. This study combines theoretical and experimental methods to understand the phase formation in nitrogen-containing CrFeCoNi thin films. Density functional theory calculations considering three competing phases (CrN, Fe-Ni and Co) show that the free energy of mixing, Delta G of (CrFeCoNi)(1-x)N-x solid solutions has a maximum at x = 0.20-0.25, and AG becomes lower when x < 0.20 and x > 0.25. Thin films of (CrFeCoNi)1-xNx (0.14 >= x <= 0.41) grown by magnetron sputtering show stabilization of the metallic fcc when x <= 0.22 and the stabilization of the NaCl B1 structure when x > 0.33, consistent with the theoretical prediction. In contrast, films with intermediate amounts of nitrogen (x = 0.22) grown at higher temperatures show segregation into multiple phases of CrN, Fe-Ni-rich and Co. These results offer an explanation for the requirement of kinetically limited growth conditions at low temperature for obtaining single-phase CrFeCoNi Cantor-like nitrogen-containing thin films and are of importance for understanding the phase-formation mechanisms in multicomponent ceramics. The results from the study further aid in making correlations between the observed mechanical properties and the crystal structure of the films.
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| 47. |
- Ganguly, Saswati, et al.
(författare)
-
Crystal to crystal transformation in soft ionic microgels : Kinetics and the role of local mechanical susceptibilities
- 2023
-
Ingår i: Physical Review Materials. - 2475-9953. ; 7:8
-
Tidskriftsartikel (refereegranskat)abstract
- We report confocal microscopy experiments of electric-field-driven crystal-crystal transformations in microgel colloids with varying particle softness. The transformation kinetics are well described by phenomenological nucleation and growth theory. Using a spatial projection formalism, we determine local susceptibilities to affine shear deformation and nonaffine defect formation in the parent crystal. Curiously, the softer particles have lower shear strain susceptibility, while nonaffine susceptibility and phenomenological growth coefficients depend nonmonotonically on particle softness.
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| 48. |
- Garbrecht, Magnus, et al.
(författare)
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Void-mediated coherency-strain relaxation and impediment of cubic-to-hexagonal transformation in epitaxial metastable metal/semiconductor TiN/Al0.72Sc0.28N multilayers
- 2017
-
Ingår i: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 1:3
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Tidskriftsartikel (refereegranskat)abstract
- Bulk metastable phases can be stabilized during thin-film growth by employing substrates with similar crystal structure and lattice parameter, albeit over a thickness range limited by coherency-strain relaxation. Expanding that strategy, growth of superlattices comprising one stable and another metastable compound with similar crystal structure and lattice parameters are known to yield epitaxial stabilization over a few nanometers of thickness. In this work, the high-pressure rocksalt (B1) phase of Al0.72Sc0.28N was stabilized epitaxially in a multilayer with TiN with thicknesses of up to 26 nm. In order to investigate the microstructural changes leading to the phase transformation of the metastable B1 phase to its wurtzite allomorph, we demonstrate a design based on a multilayer architecture with systematically varying thicknesses of the metastable compound within a constant-thickness lattice of stable metallic TiN with the cubic rocksalt structure. The multilayer films show an increasing hardness and elastic modulus for decreasing period thickness, in correspondence with both coherency-strain and Koehler hardening. The phase transition is accompanied by an increase of lattice strain with increasing multilayer periods, and resulting ultimately in coherency-strain relaxation upon phase transformation. Further, we show that the phase transformation is mediated by voids decorating the {130} planes that separate regions of different growth rates and act as additional growth fronts for wurtzite growth during the phase transformation. The TiN/(Al, Sc) N interfaces themselves remain atomically sharp and smooth until the interface structure roughens along with the epitaxial rocksalt to wurtzite transition of (Al, Sc) N. These results show the strong influence of the voids on controlling the target thickness of epitaxially stabilized thin-film growth to the range relevant for applications, such as coatings, plasmonic materials, and electronic device technology, where the mechanical integrity of the material is critical.
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| 49. |
- Geilhufe, R. Matthias, et al.
(författare)
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Towards novel organic high-T-c superconductors : Data mining using density of states similarity search
- 2018
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Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 2:2
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Tidskriftsartikel (refereegranskat)abstract
- Identifying novel functional materials with desired key properties is an important part of bridging the gap between fundamental research and technological advancement. In this context, high-throughput calculations combinedwith data-mining techniques highly accelerated this process in different areas of research during the past years. The strength of a data-driven approach for materials prediction lies in narrowing down the search space of thousands of materials to a subset of prospective candidates. Recently, the open-access organic materials database OMDBwas released providing electronic structure data for thousands of previously synthesized three-dimensional organic crystals. Based on the OMDB, we report about the implementation of a novel density of states similarity search tool which is capable of retrieving materials with similar density of states to a reference material. The tool is based on the approximate nearest neighbor algorithm as implemented in the ANNOY library and can be applied via the OMDB web interface. The approach presented here is wide ranging and can be applied to various problems where the density of states is responsible for certain key properties of a material. As the first application, we report about materials exhibiting electronic structure similarities to the aromatic hydrocarbon p-terphenyl which was recently discussed as a potential organic high-temperature superconductor exhibiting a transition temperature in the order of 120 K under strong potassium doping. Although the mechanism driving the remarkable transition temperature remains under debate, we argue that the density of states, reflecting the electronic structure of a material, might serve as a crucial ingredient for the observed high T-c. To provide candidates which might exhibit comparable properties, we present 15 purely organic materials with similar features to p-terphenyl within the electronic structure, which also tend to have structural similarities with p-terphenyl such as space group symmetries, chemical composition, and molecular structure. The experimental verification of these candidates might lead to a better understanding of the underlying mechanism in case similar superconducting properties are revealed.
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| 50. |
- Ghorbani, Elaheh, et al.
(författare)
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Energy level alignment of Cu(In,Ga)(S,Se)₂ absorber compounds with In₂S₃, NaIn₅S₈, and CuIn₅S₈ Cd-free buffer materials
- 2019
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Ingår i: Physical Review Materials. - 2475-9953. ; 3:7
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Tidskriftsartikel (refereegranskat)abstract
- Motivated by environmental reasons, In2S3 is a promising candidate for a Cd-free buffer layer in Cu(In, Ga)(S, Se)(2) (CIGSSe)-based thin-film solar cells. For an impactful optimization of the In-2 S-3 alternative buffer layer, however, a comprehensive knowledge of its electronic properties across the absorber-buffer interface is of foremost importance. In this respect, finding a favorable band offset between the absorber and the buffer layers can effectively reduce the carrier recombination at the interface and improve open-circuit voltage and fill factor, leading to higher conversion efficiencies. In this study, we investigate the band alignment between the most common CIGSSe-based absorber compounds and In2S3. Furthermore, we consider two chemically modified indium sulfide layers, NaIn(5)S(8 )and CuIn5S8, and we discuss how the formation of these secondary phases influences band discontinuity across the interface. Our analysis is based on density functional theory calculations using hybrid functionals. The results suggest that Ga-based absorbers form a destructive clifflike conduction-band offset (CBO) with both pure and chemically modified buffer systems. For In-based absorbers, however, if the absorber layer is Cu-poor at the surface, a modest favorable spikelike CBO arises with NaIn5S8 and CuIn5S8.
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