| 61. |
- Burgos-Parra, E., et al.
(författare)
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Investigation of magnetic droplet solitons using x-ray holography with extended references
- 2018
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- A dissipative magnetic soliton, or magnetic droplet, is a structure that has been predicted to exist within a thin magnetic layer when non-linearity is balanced by dispersion, and a driving force counteracts the inherent damping of the spin precession. Such a soliton can be formed beneath a nano-contact (NC) that delivers a large spin-polarized current density into a magnetic layer with perpendicular magnetic anisotropy. Although the existence of droplets has been confirmed from electrical measurements and by micromagnetic simulations, only a few attempts have been made to directly observe the magnetic landscape that sustains these structures, and then only for a restricted set of experimental parameter values. In this work we use and x-ray holography technique HERALDO, to image the magnetic structure of the [ Co/ Ni] x4 multilayer within a NC orthogonal pseudo spin-valve, for different range of magnetic fields and injected electric currents. The magnetic configuration imaged at -33 mA and 0.3 T for devices with 90 nm NC diameter reveals a structure that is within the range of current where the droplet soliton exist based on our electrical measurements and have it is consistent with the expected size of the droplet (similar to 100 nm diameter) and its spatial position within the sample. We also report the magnetisation configurations observed at lower DC currents in the presence of fields (0-50 mT), where it is expected to observe regimes of the unstable droplet formation.
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| 62. |
- Catena, Riccardo, 1978, et al.
(författare)
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Crystal responses to general dark matter-electron interactions
- 2021
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Ingår i: Physical Review Research. - 2643-1564. ; 3:3
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Tidskriftsartikel (refereegranskat)abstract
- We develop a formalism to describe the scattering of dark matter (DM) particles by electrons bound in crystals for a general form of the underlying DM-electron interaction. Such a description is relevant for direct-detection experiments of DM particles lighter than a nucleon, which might be observed in operating DM experiments via electron excitations in semiconductor crystal detectors. Our formalism is based on an effective theory approach to general nonrelativistic DM-electron interactions, including the anapole, and magnetic and electric dipole couplings, combined with crystal response functions defined in terms of electron wave function overlap integrals. Our main finding is that, for the usual simplification of the velocity integral, the rate of DM-induced electronic transitions in a semiconductor material depends on at most five independent crystal response functions four of which are distinct from the usual scalar response. We identify these crystal responses and evaluate them using density functional theory for crystalline silicon and germanium, which are used in operating DMdirect-detection experiments. Our calculations allow us to set 90% confidence level limits on the strength of DM-electron interactions from data reported by the SENSEI and EDELWEISS experiments. The crystal response functions discovered in this paper encode properties of crystalline solids that do not interact with conventional experimental probes, suggesting the use of the DM wind as a probe to reveal new kinds of hidden order in materials.
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| 63. |
- Chico, Jonathan, et al.
(författare)
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First-principles studies of the Gilbert damping and exchange interactions for half-metallic Heuslers alloys
- 2016
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 93:21
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Tidskriftsartikel (refereegranskat)abstract
- Heusler alloys have been intensively studied due to the wide variety of properties that they exhibit. One of these properties is of particular interest for technological applications, i.e., the fact that some Heusler alloys are half-metallic. In the following, a systematic study of the magnetic properties of three different Heusler families Co(2)MnZ, Co(2)FeZ, and Mn(2)VZ with Z = (Al, Si, Ga, Ge) is performed. A key aspect is the determination of the Gilbert damping from first-principles calculations, with special focus on the role played by different approximations, the effect that substitutional disorder and temperature effects. Heisenberg exchange interactions and critical temperature for the alloys are also calculated as well as magnon dispersion relations for representative systems, the ferromagnetic Co2FeSi and the ferrimagnetic Mn2VAl. Correlation effects beyond standard density-functional theory are treated using both the local spin density approximation including the Hubbard U and the local spin density approximation plus dynamical mean field theory approximation, which allows one to determine if dynamical self-energy corrections can remedy some of the inconsistencies which were previously reported for these alloys.
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| 64. |
- Dissanayake, M. A. K. L., et al.
(författare)
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Highly efficient, PbS:Hg quantum dot-sensitized, plasmonic solar cells with TiO2 triple-layer photoanode
- 2019
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Ingår i: Journal of Solid State Electrochemistry. - : Springer Science and Business Media LLC. - 1433-0768 .- 1432-8488. ; 23:6, s. 1787-1794
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Tidskriftsartikel (refereegranskat)abstract
- Highly efficient, PbS:Hg quantum dot-sensitized, plasmonic solar cells with TiO2 triple-layer photoanode were fabricated by successive ionic layer adsorption and reaction (SILAR) method. These nanostructured photoanodes were characterized by optical and morphological techniques and the solar cells were characterized by optical and electrical techniques. The light absorption by the photoanode was enhanced by effective light scattering process using a triple-layer TiO2 nanostructure, fabricated with a TiO2 nanofiber layer sandwiched between two TiO2 nanoparticle layers. The best plasmon-enhanced quantum dot-sensitized solar cell showed an efficiency of 5.41% with short circuit current density of 18.02mAcm(-2) and open-circuit voltage of 679.83mV. The overall efficiency and photocurrent density of the Q-dot-sensitized solar cell are enhanced by 15.84% and 38.83% respectively due to the plasmonic effect. The enhanced efficiency appears to be due to the improved short circuit current density by increased light absorption by the triple-layered photoanode nanostructure as well as by the localized surface plasmon resonance (LSPR) effect of the plasmonic gold nanoparticles. This is the first report on plasmon-enhanced, triple-layered TiO2 photoanode sensitized with PbS:Hg Q-dots.
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| 65. |
- Edlund, Erik, 1986, et al.
(författare)
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Chiral Surfaces Self-Assembling in One-Component Systems with Isotropic Interactions
- 2012
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 108:16, s. 165502-
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Tidskriftsartikel (refereegranskat)abstract
- We show that chiral symmetry can be broken spontaneously in one-component systems with isotropic interactions, i.e., many-particle systems having maximal a priori symmetry. This is achieved by designing isotropic potentials that lead to self-assembly of chiral surfaces. We demonstrate the principle on a simple chiral lattice and on a more complex lattice with chiral supercells. In addition, we show that the complex lattice has interesting melting behavior with multiple morphologically distinct phases that we argue can be qualitatively predicted from the design of the interaction.
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| 66. |
- Erhart, Paul, 1978, et al.
(författare)
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Low-Temperature Criticality of Martensitic Transformations of Cu Nanoprecipitates in alpha-Fe
- 2013
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 111:2, s. 025701-
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Tidskriftsartikel (refereegranskat)abstract
- Nanoprecipitates form during nucleation of multiphase equilibria in phase segregating multicomponent systems. In spite of their ubiquity, their size-dependent physical chemistry, in particular, at the boundary between phases with incompatible topologies, is still rather arcane. Here, we use extensive atomistic simulations to map out the size-temperature phase diagram of Cu nanoprecipitates in alpha-Fe. The growing precipitates undergo martensitic transformations from the body-centered cubic (bcc) phase to multiply twinned 9R structures. At high temperatures, the transitions exhibit strong first-order character and prominent hysteresis. Upon cooling, the discontinuities become less pronounced and the transitions occur at ever smaller cluster sizes. Below 300 K, the hysteresis vanishes while the transition remains discontinuous with a finite but diminishing latent heat. This unusual size-temperature phase diagram results from the entropy generated by the soft modes of the bcc-Cu phase, which are stabilized through confinement by the alpha-Fe lattice.
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| 67. |
- Eriksson, Martin, 1989
(författare)
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There's Plenty of Room in Higher Dimensions - Nonlinear Dynamics of Nanoelectromechanical Systems
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nanoelectromechanical systems (NEMS) couple the dynamics of electrons to vibrating nanostructures such as suspended beams or membranes. These resonators can be used in for instance nanoelectronics and sensor applications. NEMS are also of fundamental interest since electrons exhibit strong quantum effects when confined in nanoobjects. Furthermore, NEMS such as graphene resonators are strongly nonlinear, which opens the door for complex dynamical response. The operation of nanoresonators often rely on actuation of mechanical vibrations driven by an electric ac-field. The first part of this thesis theoretically investigates high-frequency nonresonant actuation relying on electromechanical back action (Papers I-II). The nonresonant phenomenon can be utilized to study nonlinear dissipation and to selectively actuate different vibrational modes, also asymmetric ones, even though the driving field is homogeneous (Paper III). Another nonresonant actuation mechanism converts heat into mechanical energy and relies on electron-electron interaction in a movable quantum dot (Paper IV).Furthermore, parametric actuation of a nanoresonator can be used to generate a supercurrent through a superconducting weak link even though the superconducting phase difference across the link is zero (Paper V). The excitation leads to a spontaneous symmetry breaking, which allows for a new possibility to switch between the two current directions.Actuation of mechanical vibrations is also used to study nonlinear dynamics and mode coupling in nanoresonators. The strength of nonlinearities and vibrational frequencies can be tuned by electrostatic means (Paper VI). This tunability and the low dissipation in nanoresonators make it possible to selectively address individual or combinations of modes. Coupled modes allow for much richer nonlinear dynamics, such as internal resonances (Paper VII), due to the increased dimensionality of the relevant phase space. Furthermore, exotic dynamical regions may be hidden and not observed in standard experiments. However, bifurcation theory can help to construct maps which reveal the hidden regions. A lot more is therefore to be expected from coupled mode dynamics, since there’s plenty of room in higher dimensions.
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| 68. |
- Fan, Qunping, 1989, et al.
(författare)
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Over 14% efficiency all-polymer solar cells enabled by a low bandgap polymer acceptor with low energy loss and efficient charge separation
- 2020
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Ingår i: Energy & Environmental Science. - : Royal Society of Chemistry. - 1754-5692 .- 1754-5706. ; 13:12, s. 5017-5027
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Tidskriftsartikel (refereegranskat)abstract
- Obtaining both high open-circuit voltage (V-oc) and short-circuit current density (J(sc)) has been a major challenge for efficient all-polymer solar cells (all-PSCs). Herein, we developed a polymer acceptor PF5-Y5 with excellent optical absorption capability (onset extending to similar to 880 nm and maximum absorption coefficient exceeding 105 cm(-1) in a film), high electron mobility (3.18 x 10(3) cm(2) V-1 s(-1)) and high LUMO level (-3.84 eV) to address such a challenge. As a result, the PBDB-T:PF5-Y5-based all-PSCs achieved a high power conversion efficiency of up to 14.45% with both a high Voc (0.946 V) and a high Jsc (20.65 mA cm(-2)), due to the high and broad absorption coverage, small energy loss (0.57 eV) and efficient charge separation and transport in the device, which are among the best values in the all-PSC field. In addition, the all-PSC shows a similar to 15% improvement in PCE compared to its counterpart small molecule acceptor (Y5)-based device. Our results suggest that PF5-Y5 is a very promising polymer acceptor candidate for applications in efficient all-PSCs.
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| 69. |
- Fransson, Jonas, 1970-, et al.
(författare)
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Microscopic theory for coupled atomistic magnetization and lattice dynamics
- 2017
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Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 1:7
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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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| 70. |
- Geilhufe, Matthias
(författare)
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Quantum Buckling in Metal-Organic Framework Materials
- 2021
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 21:24, s. 10341-10345
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Tidskriftsartikel (refereegranskat)abstract
- Metal-organic frameworks are porous materials composed of metal ions or clusters coordinated by organic molecules. As a response to applied uniaxial pressure, molecules with a straight shape in the framework start to buckle. At sufficiently low temperatures, this buckling has a quantum nature described by a superposition of degenerate buckling states. Buckling states of adjacent molecules couple in a transverse field Ising type behavior. Based on the example of the metal organic framework topology MOF-5, we derived the phase diagram under applied strain, showing a normal phase, a parabuckling phase, and a ferrobuckling phase. At zero temperature, quantum phase transitions between the three phases can be induced by strain. This novel type of order opens a new path toward strain induced quantum phases.
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