| 1. |
- Bossini, D., et al.
(författare)
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Magnetoelectrics and multiferroics : theory, synthesis, characterisation, preliminary results and perspectives for all-optical manipulations
- 2023
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Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 56:27
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Forskningsöversikt (refereegranskat)abstract
- Solid state compounds exhibiting multiple and coupled macroscopic orders, named multiferroics, represent a challenge for both theoretical and experimental modern condensed-matter physics. Spins and the electric polarisation in conventional magnetic and ferroelectric materials can be manipulated on their fundamental timescales, by means of femtosecond laser pulses. In view of the resounding success and popularity of the all-optical approach, it is only natural to wonder about the application of this scheme to study the intrinsic coupling between spins and charges in multiferroics. Deeply fundamental questions arise: can ultrashort laser pulses deterministically activate, enhance or suppress the magnetoelectric coupling on the femtosecond timescale? Can these processes be triggered in a fully coherent fashion, thus being unrestrained by any thermal load? Which mechanism of spin-charge coupling is most favourable to overcome these overarching and daunting challenges? This problem is interdisciplinary in nature, requiring contributions from materials science and condensed matter physics from both theoretical and experimental perspectives. High-quality materials suitable for optical investigations have to be identified, synthetized and characterised. General and valid models offer then a guide to the plethora of possible light-induced processes, resulting in the desired ultrafast multiferroic manipulations. Finally, healthy experimental schemes, able to unambiguously track the ultrafast dynamics of either the ferroelectric or the magnetic order parameter have to be developed and implemented. Our motivation to write this review is to lay a broad and multidisciplinary foundation, which may be employed as a starting point for non-equilibrium approaches to the manipulation of the multiferroicity on the femtosecond timescale. This was also one of the main goals of the COST Action MAGNETOFON, whose network constitutes the core of the authors of this review. The present work thus represents a part of the scientific legacy of MAGNETOFON itself.
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| 2. |
- Balatsky, Alexander V., et al.
(författare)
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Resource Demand Growth and Sustainability Due to Increased World Consumption
- 2015
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Ingår i: Sustainability. - : MDPI AG. - 2071-1050. ; 7:3, s. 3430-3440
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Tidskriftsartikel (refereegranskat)abstract
- The paper aims at continuing the discussion on sustainability and attempts to forecast the impossibility of the expanding consumption worldwide due to the planet's limited resources. As the population of China, India and other developing countries continue to increase, they would also require more natural and financial resources to sustain their growth. We coarsely estimate the volumes of these resources (energy, food, freshwater) and the gross domestic product (GDP) that would need to be achieved to bring the population of India and China to the current levels of consumption in the United States. We also provide estimations for potentially needed immediate growth of the world resource consumption to meet this equality requirement. Given the tight historical correlation between GDP and energy consumption, the needed increase of GDP per capita in the developing world to the levels of the U.S. would deplete explored fossil fuel reserves in less than two decades. These estimates predict that the world economy would need to find a development model where growth would be achieved without heavy dependence on fossil fuels.
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| 3. |
- Khaetskii, Alexander, et al.
(författare)
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Thermal magnetic fluctuations of a ferroelectric quantum critical point
- 2021
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 33:4
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Tidskriftsartikel (refereegranskat)abstract
- Entanglement of two different quantum orders is of an interest of the modern condensed matter physics. One of the examples is the dynamical multiferroicity, where fluctuations of electric dipoles lead to magnetization. We investigate this effect at finite temperature and demonstrate an elevated magnetic response of a ferroelectric near the ferroelectric quantum critical point (FE QCP). We calculate the magnetic susceptibility of a bulk sample on the paraelectric side of the FE QCP at finite temperature and find enhanced magnetic susceptibility near the FE QCP. We propose quantum paraelectric strontium titanate as a candidate material to search for dynamic multiferroicity. We estimate the magnitude of the magnetic susceptibility for this material and find that it is detectable experimentally.
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| 4. |
- Tyner, Alexander C., 1994-, et al.
(författare)
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Electron–Phonon Coupling in Copper-Substituted Lead Phosphate Apatite
- 2024
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Ingår i: Journal of Low Temperature Physics. - 0022-2291 .- 1573-7357.
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Tidskriftsartikel (refereegranskat)abstract
- Recent reports of room-temperature, ambient pressure superconductivity in copper-substituted lead phosphate apatite, commonly referred to as LK99, have prompted numerous theoretical and experimental studies into its properties. As the electron–phonon interaction is a common mechanism for superconductivity, the electron–phonon coupling strength is an important quantity to compute for LK99. In this work, we compare the electron–phonon coupling strength among the proposed compositions of LK99. The results of our study are in alignment with the conclusion that LK99 is a candidate for low-temperature, not room-temperature, superconductivity if electron–phonon interaction is to serve as the mechanism.
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| 5. |
- Zakharchenko, Konstantin V., et al.
(författare)
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Controlled healing of graphene nanopores
- 2014
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Ingår i: Carbon. - : Elsevier BV. - 0008-6223 .- 1873-3891. ; 80, s. 12-18
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Tidskriftsartikel (refereegranskat)abstract
- Nanopores - nanometer-size channels hold significant promise for numerous applications: DNA sequencing, sensing, biosensing and molecular detectors, and catalysis and water desalination. However, these applications require accurate control over the size of the nanopores. Since graphene is often mentioned as a promising material to host nanopores, we have performed realistic computer simulation studies of regrowth and healing of graphene nanopores of different sizes ranging from 30 to 5 angstrom. Our simulations clearly point to at least two distinct healing mechanisms for graphene sheets: edge attachment (where carbons are attached to the edges of the graphene sheet/pore) and direct insertion (where individual atoms insert directly into a sheet of graphene, even in the absence of the edges). The insertion mechanism is a surprising prediction that points to the growth process that would be operational in pristine graphene. We have uncovered an unusual dependence in the speed of nanopore regrowth and the structure of "healed" areas as a function of its size in a wide range of temperatures. Our findings point to significantly more complicated pathways for graphene annealing. They also provide an important enabling step in the development of graphene-based devices for numerous applications in nanotechnology. Our results suggest the possibility to control the final size of healed nanopore by terminating the annealing at a prescribed time dependent on the temperature.
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| 6. |
- Abergel, David S. L., et al.
(författare)
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The role of spin-orbit coupling in topologically protected interface states in Dirac materials
- 2014
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 16, s. 065012-
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Tidskriftsartikel (refereegranskat)abstract
- We highlight the fact that two-dimensional (2D) materials with Dirac-like low energy band structures and spin-orbit coupling (SOC) will produce linearly dispersing topologically protected Jackiw-Rebbi modes at interfaces where the Dirac mass changes sign. These modes may support persistent spin or valley currents parallel to the interface, and the exact arrangement of such topologically protected currents depends crucially on the details of the SOC in the material. As examples, we discuss buckled 2D hexagonal lattices such as silicene or germanene, and transition metal dichalcogenides such as MoS2.
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| 7. |
- Ahmed, Towfiq, et al.
(författare)
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Correlation dynamics and enhanced signals for the identification of serial biomolecules and DNA bases
- 2014
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 25:12, s. 125705-
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Tidskriftsartikel (refereegranskat)abstract
- Nanopore-based sequencing has demonstrated a significant potential for the development of fast, accurate, and cost-efficient fingerprinting techniques for next generation molecular detection and sequencing. We propose a specific multilayered graphene-based nanopore device architecture for the recognition of single biomolecules. Molecular detection and analysis can be accomplished through the detection of transverse currents as the molecule or DNA base translocates through the nanopore. To increase the overall signal-to-noise ratio and the accuracy, we implement a new 'multi-point cross-correlation' technique for identification of DNA bases or other molecules on the single molecular level. We demonstrate that the cross-correlations between each nanopore will greatly enhance the transverse current signal for each molecule. We implement first-principles transport calculations for DNA bases surveyed across a multilayered graphene nanopore system to illustrate the advantages of the proposed geometry. A time-series analysis of the cross-correlation functions illustrates the potential of this method for enhancing the signal-to-noise ratio. This work constitutes a significant step forward in facilitating fingerprinting of single biomolecules using solid state technology.
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| 8. |
- Ahmed, T., et al.
(författare)
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G W quasiparticle calculations with spin-orbit coupling for the light actinides
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 89:3, s. 035104-
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Tidskriftsartikel (refereegranskat)abstract
- We report on the importance of GW self-energy corrections for the electronic structure of light actinides in the weak-to-intermediate coupling regime. Our study is based on calculations of the band structure and total density of states of Np, U, and Pu using a one-shot GW approximation that includes spin-orbit coupling within a full potential LAPW framework. We also present RPA screened effective Coulomb interactions for the f-electron orbitals for different lattice constants, and show that there is an increased contribution from electron-electron correlation in these systems for expanded lattices. We find a significant amount of electronic correlation in these highly localized electronic systems.
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| 9. |
- Ahmed, Towfiq, et al.
(författare)
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Next-Generation Epigenetic Detection Technique : Identifying Methylated Cytosine Using Graphene Nanopore
- 2014
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 5:15, s. 2601-2607
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Tidskriftsartikel (refereegranskat)abstract
- DNA methylation plays a pivotal role in the genetic evolution of both embryonic and adult cells. For adult somatic cells, the location and dynamics of methylation have been very precisely pinned down with the 5-cytosine markers on cytosine-phosphate-guanine (CpG) units. Unusual methylation on CpG islands is identified as one of the prime causes for silencing the tumor suppressant genes. Early detection of methylation changes can diagnose the potentially harmful oncogenic evolution of cells and provide promising guideline for cancer prevention. With this motivation, we propose a cytosine methylation detection technique. Our hypothesis is that electronic signatures of DNA acquired as a molecule translocates through a nanopore would be significantly different for methylated and nonmethylated bases. This difference in electronic fingerprints would allow for reliable real-time differentiation of methylated DNA. We calculate transport currents through a punctured graphene membrane while the cytosine and methylated cytosine translocate through the nanopore. We also calculate the transport properties for uracil and cyanocytosine for comparison. Our calculations of transmission, current, and tunneling conductance show distinct signatures in their spectrum for each molecular type. Thus, in this work, we provide a theoretical analysis that points to a viability of our hypothesis.
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| 10. |
- Altfeder, Igor, et al.
(författare)
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Scanning Tunneling Microscopy Observation of Phonon Condensate
- 2017
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Ingår i: Scientific Reports. - : Nature Publishing Group. - 2045-2322. ; 7
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Tidskriftsartikel (refereegranskat)abstract
- Using quantum tunneling of electrons into vibrating surface atoms, phonon oscillations can be observed on the atomic scale. Phonon interference patterns with unusually large signal amplitudes have been revealed by scanning tunneling microscopy in intercalated van der Waals heterostructures. Our results show that the effective radius of these phonon quasi-bound states, the real-space distribution of phonon standing wave amplitudes, the scattering phase shifts, and the nonlinear intermode coupling strongly depend on the presence of defect-induced scattering resonance. The observed coherence of these quasi-bound states most likely arises from phase-and frequency-synchronized dynamics of all phonon modes, and indicates the formation of many-body condensate of optical phonons around resonant defects. We found that increasing the strength of the scattering resonance causes the increase of the condensate droplet radius without affecting the condensate fraction inside it. The condensate can be observed at room temperature.
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| 11. |
- Balatsky, Alexander V., et al.
(författare)
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Dark sound : Collective modes of the axionic dark matter condensate
- 2022
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Ingår i: Physical Review D. - : American Physical Society (APS). - 2470-0010 .- 2470-0029. ; 105:2
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Tidskriftsartikel (refereegranskat)abstract
- We discuss the axion dark matter (DM) condensate and the consequences the interactions of dark matter would have on the spectrum of collective modes. We find that DM self-interactions change the spectrum of excitations from a quadratic to a linearlike dispersion with velocity upsilon(s) which is set by the interactions, but dominated by gravity. For typical DM densities and interactions we find upsilon(s) similar to 10(-12)c. This soundlike mode corresponds to DM density oscillations just like in any other Bose liquid, hence we call it dark sound (DS). The DS mode is well defined and describes stable density oscillations at intermediate length scales k >= k(min) similar to 10(4) lyr(-1). In the extreme long-wavelength limit gravity dominates and leads to Jeans instability of the sound mode at the scale of clump formation k <= k(mi)(n). We also discuss the possible observable consequences of the DS, including quantized DS modes inside clumps, their characteristic energy, and noise features that might facilitate the observation of DM.
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| 12. |
- Balatsky, Alexander V., et al.
(författare)
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Dynamic Quantum Matter
- 2020
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Ingår i: Annalen der Physik. - : John Wiley & Sons. - 0003-3804 .- 1521-3889. ; 532:2
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 13. |
- Balatsky, Alexander V., et al.
(författare)
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Functional Dirac Materials : Status and Perspectives
- 2018
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Ingår i: Physica Status Solidi. Rapid Research Letters. - : Wiley-VCH Verlagsgesellschaft. - 1862-6254 .- 1862-6270. ; 12:11
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 14. |
- Balatsky, Alexander V., et al.
(författare)
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Quantum Pairing Time Orders
- 2020
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Ingår i: Annalen der Physik. - : Wiley-VCH Verlagsgesellschaft. - 0003-3804 .- 1521-3889. ; 532:2
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Tidskriftsartikel (refereegranskat)abstract
- The concept of the time-independent correlators for the even- and odd-frequency pairing states that can be defined for both bosonic and fermionic quasiparticles is proposed. These correlators explicitly capture the existence of two distinct classes of pairing states and provide a direct probe of the hidden Berezinskii order. This concept is illustrated in the cases of pairings for Majorana fermions and quasiparticles in Dirac semimetals. It is shown that the time-independent correlator is able to effectively capture the energy scale relevant for pairing.
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| 15. |
- Banerjee, Saikat, et al.
(författare)
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Granular superconductor in a honeycomb lattice as a realization of bosonic Dirac material
- 2016
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Ingår i: PHYSICAL REVIEW B. - : American Physical Society. - 2469-9950. ; 93:13
-
Tidskriftsartikel (refereegranskat)abstract
- We examine the low-energy effective theory of phase oscillations in a two-dimensional granular superconducting sheet where the grains are arranged in a honeycomb lattice structure. Using the example of graphene, we present evidence for the engineered Dirac nodes in the bosonic excitations: the spectra of the collective bosonic modes cross at the K and K' points in the Brillouin zone and form Dirac nodes. We show how two different types of collective phase oscillations are obtained and that they are analogous to the Leggett and the Bogoliubov-Anderson-Gorkov modes in a two-band superconductor. We show that the Dirac node is preserved in the presence of an intergrain interaction, despite induced changes of the qualitative features of the two collective modes. Finally, breaking the sublattice symmetry by choosing different on-site potentials for the two sublattices leads to a gap opening near the Dirac node, in analogy with fermionic Dirac materials. The Dirac node dispersion of bosonic excitations is thus expanding the discussion of the conventional Dirac cone excitations to the case of bosons. We call this case as a representative of bosonic Dirac materials (BDM), similar to the case of Fermionic Dirac materials extensively discussed in the literature.
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| 16. |
- Banerjee, Saikat, et al.
(författare)
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Interacting Dirac materials
- 2020
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 32:40
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the extent to which the class of Dirac materials in two-dimensions provides general statements about the behavior of both fermionic and bosonic Dirac quasiparticles in the interacting regime. For both quasiparticle types, we find common features for the interaction induced renormalization of the conical Dirac spectrum. We perform the perturbative renormalization analysis and compute the self-energy for both quasiparticle types with different interactions and collate previous results from the literature whenever necessary. Guided by the systematic presentation of our results in table1, we conclude that long-range interactions generically lead to an increase of the slope of the single-particle Dirac cone, whereas short-range interactions lead to a decrease. The quasiparticle statistics does not qualitatively impact the self-energy correction for long-range repulsion but does affect the behavior of short-range coupled systems, giving rise to different thermal power-law contributions. The possibility of a universal description of the Dirac materials based on these features is also mentioned.
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| 17. |
- Banerjee, Saikat, 1989-
(författare)
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Interacting Dirac Matter
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The discovery of graphene in 2004 has led to a surge of activities focused on the theoretical and experimental studies of materials hosting linearly dispersive quasiparticles during the last decade. Rapid expansion in the list of materials having similar properties to graphene has led to the emergence of a new class of materials known as the Dirac materials. The low energy quasiparticles in this class of materials are described by a Dirac-like equation in contrast to the Schrödinger equation which governs the low energy dynamics in any conventional materials such as metals. The Dirac fermions, as we call these low-energy quasiparticles, in a wide range of materials ranging from the d-wave superconductors, graphene to the surface states of topological insulators share the common property. The particles move around as if they have lost their mass. This feature results in a completely new set of physical effects consisting of various transport and thermodynamic quantities, that are absent in conventional metals.This thesis is devoted to studying the properties of bosonic analogs of the commonly known Dirac materials where the quasiparticle are fermionic. In chapter one, we discuss the microscopic origin of the Dirac equation in several fermionic and bosonic systems. We observe identical features of the Dirac materials with quasiparticles of either statistics when the interparticle interaction is absent. Dirac materials with both types of quasiparticles possess the nodal excitations that are described by an effective Dirac-like equation. The possible physical effects due to the linear dispersions in fermionic and bosonic Dirac materials are also outlined.In chapter two, we propose a system of superconducting grains arranged in honeycomb lattice as a realization for Bosonic Dirac Materials (BDM). The underlying microscopic dynamics, which give rise to the emergence of Dirac structure in the spectrum of the collective phase oscillations, is discussed in detail. Similarities and differences of BDM systems to the conventional Dirac materials with fermionic quasiparticles are also mentioned. Chapter three is dedicated to the detailed analysis of the interaction effects on the stability and renormalization of the conical Dirac band structure. We find that the type of interaction dictates the possible fate of renormalized Dirac cone in both fermionic and bosonic Dirac materials. We study interaction effects in four different individual systems : (a) Dirac fermions in graphene interacting via Coulomb interactions, (b) Dirac fermions subjected to an onsite Hubbard repulsion, (c) Coulomb repulsion in charged Cooper pairs in honeycomb lattice and (d) Dirac magnons interacting via Heisenberg exchange interaction. The possibility of interaction induced gap opening at the Dirac nodal point described is also discussed in these cases.Chapter four mainly concerns the study of a related topic of the synthetic gauge fields. We discuss the possibility of Landau quantization in neutral particles. Possible experimental evidence in toroidal cold atomic traps is also mentioned. A connection to Landau levels in case of magnons is also described. We finally conclude our thesis in chapter five and discuss the possible future directions that can be taken as an extension for our works in interacting Dirac materials.
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| 18. |
- Banerjee, Saikat, et al.
(författare)
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Landau-like states in neutral particles
- 2016
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Ingår i: PHYSICAL REVIEW B. - 2469-9950. ; 93:23
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Tidskriftsartikel (refereegranskat)abstract
- We show the emergence of a new type of dispersion relation for neutral atoms with an interesting similarity to the spectrum of two-dimensional electrons in an applied perpendicular constant magnetic field. These neutral atoms can be confined in toroidal optical traps and give quasi-Landau spectra. In strong contrast to the equidistant infinitely degenerate Landau levels for charged particles, the spectral gap for such two-dimensional neutral particles increases in particular electric-field configurations. The idea in the paper is motivated by the development in cold atom experiments and builds on the seminal paper of Aharonov and Casher.
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| 19. |
- Björnson, Kristofer, et al.
(författare)
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Spin-polarized edge currents and Majorana fermions in one- and two-dimensional topological superconductors
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 92:21
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the persistent currents, spin-polarized local density of states, and spectral functions of topological superconductors constructed by placing ferromagnetic impurities on top of an s-wave superconductor with Rashba spin-orbit interaction. We solve self-consistently for the superconducting order parameter and investigate both two-dimensional blocks and one-dimensional wires of ferromagnetic impurities, with the magnetic moments pointing both perpendicular and parallel to the surface. We find that the topologically protected edge states of ferromagnetic blocks give rise to spin-polarized edge currents, but that the total persistent current flows in opposite direction to what is expected from the dispersion relation of the edge states. We also show that the Majorana fermions at the end points of one-dimensional wires are spin polarized, which can be directly related to the spin polarization of the edge currents of two-dimensional blocks. Connections are also made to the physics of the Yu-Shiba-Rusinov states for zero-dimensional impurities.
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| 20. |
- Björnson, Kristofer, et al.
(författare)
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Superconducting order parameter π-phase shift in magnetic impurity wires
- 2017
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 95:10
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Tidskriftsartikel (refereegranskat)abstract
- It has previously been found that a magnetic impurity in a conventional s-wave superconductor can give rise to a local π-phase shift of the superconducting order parameter. By studying a finite wire of ferromagnetic impurities, we are able to trace the origin of the π-phase shift to a resonance condition for the Bogoliubov-de Gennes quasiparticle states. When non-resonating states localized at the impurity sites are pulled into the condensate for increasing magnetic strength, the superconducting order parameter is reduced in discrete steps, eventually resulting in a π-phase shift. We also show that for a finite spin-orbit coupling, the π-phase shift is preserved and occurs in a large portion of the topologically non-trivial phase.
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| 21. |
- Black-Schaffer, Annica M., et al.
(författare)
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Odd-frequency superconducting pairing in multiband superconductors
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 88:10, s. 104514-
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Tidskriftsartikel (refereegranskat)abstract
- We point out that essentially all multiband superconductors have an odd-frequency pairing component, as follows from a general symmetry analysis of even- and odd-frequency pairing states. We show that odd-frequency superconducting pairing requires only a finite band hybridization, or scattering, and nonidentical intraband order parameters, of which only one band needs to be superconducting. Under these conditions odd-frequency odd-interband pairing is always present. From a symmetry analysis we establish a complete reciprocity between parity in band index and frequency.
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| 22. |
- Black-Schaffer, Annica M., et al.
(författare)
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Odd-frequency superconducting pairing in topological insulators
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:14, s. 144506-
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Tidskriftsartikel (refereegranskat)abstract
- We discuss the appearance of odd-frequency spin-triplet s-wave superconductivity, first proposed by Berezinskii [J. Exp. Theor. Phys. 20, 287 (1974)], on the surface of a topological insulator proximity coupled to a conventional spin- singlet s-wave superconductor. Using both analytical and numerical methods, we show that this disorder robust odd-frequency state is present whenever there is an in-surface gradient in the proximity induced gap, including superconductor-normal state junctions. The time-independent order parameter for the odd-frequency superconductor is proportional to the in-surface gap gradient. The induced odd-frequency component does not produce any low-energy states.
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| 23. |
- Black-Schaffer, Annica M., et al.
(författare)
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Proximity-induced unconventional superconductivity in topological insulators
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 87:22, s. 220506-
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Tidskriftsartikel (refereegranskat)abstract
- We study and classify the proximity-induced superconducting pairing in a topological insulator (TI)-superconductor (SC) hybrid structure for SCs with different symmetries. The Dirac surface state gives a coupling between spin-singlet and spin-triplet pairing amplitudes as well as pairing that is odd in frequency for p-wave SCs. We also find that all SCs induce pairing that is odd in both frequency and orbital (band) index, with oddness in frequency and orbital index being completely interchangeable. The different induced pairing amplitudes significantly modify the density of states in the TI surface layer.
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| 24. |
- Black-Schaffer, Annica M., et al.
(författare)
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Strong potential impurities on the surface of a topological insulator
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 85:12, s. 121103-
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Tidskriftsartikel (refereegranskat)abstract
- Topological insulators (TIs) are said to be stable against nonmagnetic impurity scattering due to suppressed backscattering in the Dirac surface states. We solve a lattice model of a three-dimensional TI in the presence of strong potential impurities and find that both the Dirac point and low-energy states are significantly modified: Low-energy impurity resonances are formed that produce a peak in the density of states near the Dirac point, which is destroyed and split into two nodes that move off center. The impurity-induced states penetrate up to ten layers into the bulk of the TI. These findings demonstrate the importance of bulk states for the stability of TIs and how they can destroy the topological protection of the surface.
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| 25. |
- Black-Schaffer, Annica M., et al.
(författare)
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Subsurface impurities and vacancies in a three-dimensional topological insulator
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:11, s. 115433-
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Tidskriftsartikel (refereegranskat)abstract
- Using a three-dimensional microscopic lattice model of a strong topological insulator (TI) we study potential impurities and vacancies in surface, subsurface, and bulk positions. For all impurity locations we find impurity-induced resonance states with energy proportional to the inverse of the impurity strength, although the impurity strength needed for a low-energy resonance state increases with the depth of the impurity. For strong impurities and vacancies as deep as 15 layers into the material, resonance peaks will appear at and around the Dirac point in the surface energy spectrum, splitting the original Dirac point into two nodes located off-center. Furthermore, we study vacancy clusters buried deep inside the bulk and find zero-energy resonance states for both single and multiple-site vacancies. Only fully symmetric multiple-site vacancy clusters show resonance states expelled from the bulk
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| 26. |
- Borysov, Stanislav S., et al.
(författare)
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Cross-Correlation Asymmetries and Causal Relationships between Stock and Market Risk
- 2014
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Ingår i: PLOS ONE. - : Public Library of Science (PLoS). - 1932-6203. ; 9:8, s. e105874-
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Tidskriftsartikel (refereegranskat)abstract
- We study historical correlations and lead-lag relationships between individual stock risk (volatility of daily stock returns) and market risk (volatility of daily returns of a market-representative portfolio) in the US stock market. We consider the cross-correlation functions averaged over all stocks, using 71 stock prices from the Standard & Poor's 500 index for 1994-2013. We focus on the behavior of the cross-correlations at the times of financial crises with significant jumps of market volatility. The observed historical dynamics showed that the dependence between the risks was almost linear during the US stock market downturn of 2002 and after the US housing bubble in 2007, remaining at that level until 2013. Moreover, the averaged cross-correlation function often had an asymmetric shape with respect to zero lag in the periods of high correlation. We develop the analysis by the application of the linear response formalism to study underlying causal relations. The calculated response functions suggest the presence of characteristic regimes near financial crashes, when the volatility of an individual stock follows the market volatility and vice versa.
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| 27. |
- Borysov, Stanislav S., et al.
(författare)
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Online search tool for graphical patterns in electronic band structures
- 2018
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Ingår i: npj Computational Materials. - : Springer Science and Business Media LLC. - 2057-3960. ; 4
-
Tidskriftsartikel (refereegranskat)abstract
- Many functional materials can be characterized by a specific pattern in their electronic band structure, for example, Dirac materials, characterized by a linear crossing of bands; topological insulators, characterized by a Mexican hat pattern or an effectively free electron gas, characterized by a parabolic dispersion. To find material realizations of these features, manual inspection of electronic band structures represents a relatively easy task for a small number of materials. However, the growing amount of data contained within modern electronic band structure databases makes this approach impracticable. To address this problem, we present an automatic graphical pattern search tool implemented for the electronic band structures contained within the Organic Materials Database. The tool is capable of finding user-specified graphical patterns in the collection of thousands of band structures from high-throughput calculations in the online regime. Using this tool, it only takes a few seconds to find an arbitrary graphical pattern within the ten electronic bands near the Fermi level for 26,739 organic crystals. The source code of the developed tool is freely available and can be adapted to any other electronic band structure database.
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| 28. |
- Borysov, Stanislav S., et al.
(författare)
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Organic materials database : An open-access online database for data mining
- 2017
-
Ingår i: PLOS ONE. - : PUBLIC LIBRARY SCIENCE. - 1932-6203. ; 12:2
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Tidskriftsartikel (refereegranskat)abstract
- We present an organic materials database (OMDB) hosting thousands of Kohn-Sham electronic band structures, which is freely accessible online at http://omdb.diracmaterials.org. The OMDB focus lies on electronic structure, density of states and other properties for purely organic and organometallic compounds that are known to date. The electronic band structures are calculated using density functional theory for the crystal structures contained in the Crystallography Open Database. The OMDB web interface allows users to retrieve materials with specified target properties using non-trivial queries about their electronic structure. We illustrate the use of the OMDB and how it can become an organic part of search and prediction of novel functional materials via data mining techniques. As a specific example, we provide data mining results for metals and semiconductors, which are known to be rare in the class of organic materials.
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| 29. |
- Borysov, Stanislav S., et al.
(författare)
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Reconstruction of tip-surface interactions with multimodal intermodulation atomic force microscopy
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 88:11, s. 115405-
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Tidskriftsartikel (refereegranskat)abstract
- We propose a theoretical framework for reconstructing tip-surface interactions using the intermodulation technique when more than one eigenmode is required to describe the cantilever motion. Two particular cases of bimodal motion are studied numerically: one bending and one torsional mode, and two bending modes. We demonstrate the possibility of accurate reconstruction of a two-dimensional conservative force field for the former case, while dissipative forces are studied for the latter.
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| 30. |
- Borysov, Stanislav, et al.
(författare)
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U.S. stock market interaction network as learned by the Boltzmann machine
- 2015
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Ingår i: European Physical Journal B. - : Springer Berlin/Heidelberg. - 1434-6028 .- 1434-6036. ; 88:12, s. 1-14
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Tidskriftsartikel (refereegranskat)abstract
- We study historical dynamics of joint equilibrium distribution of stock returns in the U.S. stock market using the Boltzmann distribution model being parametrized by external fields and pairwise couplings. Within Boltzmann learning framework for statistical inference, we analyze historical behavior of the parameters inferred using exact and approximate learning algorithms. Since the model and inference methods require use of binary variables, effect of this mapping of continuous returns to the discrete domain is studied. The presented results show that binarization preserves the correlation structure of the market. Properties of distributions of external fields and couplings as well as the market interaction network and industry sector clustering structure are studied for different historical dates and moving window sizes. We demonstrate that the observed positive heavy tail in distribution of couplings is related to the sparse clustering structure of the market. We also show that discrepancies between the model’s parameters might be used as a precursor of financial instabilities.
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| 31. |
- Boyko, D., et al.
(författare)
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Evolution of magnetic Dirac bosons in a honeycomb lattice
- 2018
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 97:1
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Tidskriftsartikel (refereegranskat)abstract
- We examine the presence and evolution of magnetic Dirac nodes in the Heisenberg honeycomb lattice. Using linear spin theory, we evaluate the collinear phase diagram as well as the change in the spin dynamics with various exchange interactions. We show that the ferromagnetic structure produces bosonic Dirac and Weyl points due to the competition between the interactions. Furthermore, it is shown that the criteria for magnetic Dirac nodes are coupled to the magnetic structure and not the overall crystal symmetry, where the breaking of inversion symmetry greatly affects the antiferromagnetic configurations. The tunability of the nodal points through variation of the exchange parameters leads to the possibility of controlling Dirac symmetries through an external manipulation of the orbital interactions.
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| 32. |
- Commeau, Benjamin, et al.
(författare)
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Structural and electronic properties of alpha-(BEDT-TTF)(2)I-3, ss-(BEDT-TTF)(2)I-3, and kappa-(BEDT-TTF)(2)X-3 (X = I, F, Br, Cl) organic charge transfer salts
- 2017
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 96:12
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Tidskriftsartikel (refereegranskat)abstract
- (BEDT-TFF)(2)I-3 charge transfer salts are reported to show superconductivity and pressure-induced quasi-twodimensional Dirac cones at the Fermi level. By performing state of the art ab initio calculations in the framework of density functional theory, we investigate the structural and electronic properties of the three structural phases alpha, beta, and kappa(.) We furthermore report about the irreducible representations of the corresponding electronic band structures, symmetry of their crystal structure, and the origin of band crossings. Additionally, we discuss the chemically induced strain in kappa-(BEDT-TTF)(2)I-3 achieved by replacing the iodine layer with other halogens: fluorine, bromine, and chlorine. In the case of kappa-(BEDT-TTF)(2)F-3, we identify topologically protected crossings within the band structure. These crossings are forced to occur due to the nonsymmorphic nature of the crystal. The calculated electronic structures presented here are added to the organic materials database (OMDB).
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| 33. |
- Creange, Nicole, et al.
(författare)
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Computational Investigation of the Electronic and Optical Properties of Planar Ga-Doped Graphene
- 2015
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Ingår i: Advances in Condensed Matter Physics. - : Hindawi Limited. - 1687-8108 .- 1687-8124.
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Tidskriftsartikel (refereegranskat)abstract
- We simulate the optical and electrical responses in gallium-doped graphene. Using density functional theory with a local density approximation, we simulate the electronic band structure and show the effects of impurity doping (0-3.91%) in graphene on the electron density, refractive index, optical conductivity, and extinction coefficient for each doping percentage. Here, gallium atoms are placed randomly (using a 5-point average) throughout a 128-atom sheet of graphene. These calculations demonstrate the effects of hole doping due to direct atomic substitution, where it is found that a disruption in the electronic structure and electron density for small doping levels is due to impurity scattering of the electrons. However, the system continues to produce metallic or semimetallic behavior with increasing doping levels. These calculations are compared to a purely theoretical 100% Ga sheet for comparison of conductivity. Furthermore, we examine the change in the electronic band structure, where the introduction of gallium electronic bands produces a shift in the electron bands and dissolves the characteristic Dirac cone within graphene, which leads to better electron mobility.
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| 34. |
- Crook, Charles B., et al.
(författare)
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Spatial dependence of the super-exchange interactions for transition-metal trimers in graphene
- 2018
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 123:1
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Tidskriftsartikel (refereegranskat)abstract
- This study examines the magnetic interactions between spatially variable manganese and chromium trimers substituted into a graphene superlattice. Using density functional theory, we calculate the electronic band structure and magnetic populations for the determination of the electronic and magnetic properties of the system. To explore the super-exchange coupling between the transition-metal atoms, we establish the magnetic ground states through a comparison of multiple magnetic and spatial configurations. Through an analysis of the electronic and magnetic properties, we conclude that the presence of transition-metal atoms can induce a distinct magnetic moment in the surrounding carbon atoms as well as produce a Ruderman-Kittel-Kasuya-Yosidalike super-exchange coupling. It is hoped that these simulations can lead to the realization of spintronic applications in graphene through electronic control of the magnetic clusters.
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| 35. |
- Dakovski, Georgi L., et al.
(författare)
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Anomalous femtosecond quasiparticle dynamics of hidden order state in URu(2)Si(2)
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:16, s. 161103-
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Tidskriftsartikel (refereegranskat)abstract
- At T(0) = 17.5 K an exotic phase emerges from a heavy fermion state in URu(2)Si(2). The nature of this hidden order (HO) phase has so far evaded explanation. Formation of an unknown quasiparticle (QP) structure is believed to be responsible for the massive removal of entropy at the HO transition, however, experiments and ab initio calculations have been unable to reveal the essential character of the QP. Here we use femtosecond pump-probe time-and angle-resolved photoemission spectroscopy (tr-ARPES) to elucidate the ultrafast dynamics of the QP. We show how the Fermi surface is renormalized by shifting states away from the Fermi level at specific locations, characterized by vector q(< 110 >) = 0.56 +/- 0.08 angstrom(-1). Measurements of the temperature-time response reveal that, upon entering the HO, the QP lifetime in those locations increases from 42 fs to few hundred fs. The formation of the long-lived QPs is identified here as a principal actor of the HO.
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| 36. |
- Das, Tanmoy, et al.
(författare)
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Engineering three-dimensional topological insulators in Rashba-type spin-orbit coupled heterostructures
- 2013
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 4, s. 1972-
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Tidskriftsartikel (refereegranskat)abstract
- Topological insulators represent a new class of quantum phase defined by invariant symmetries and spin-orbit coupling that guarantees metallic Dirac excitations at its surface. The discoveries of these states have sparked the hope of realizing non-trivial excitations and novel effects such as a magnetoelectric effect and topological Majorana excitations. Here we develop a theoretical formalism to show that a three-dimensional topological insulator can be designed artificially via stacking bilayers of two-dimensional Fermi gases with opposite Rashba-type spin-orbit coupling on adjacent layers, and with interlayer quantum tunneling. We demonstrate that in the stack of bilayers grown along a (001)-direction, a non-trivial topological phase transition occurs above a critical number of Rashba bilayers. In the topological phase, we find the formation of a single spin-polarized Dirac cone at the G-point. This approach offers an accessible way to design artificial topological insulators in a set up that takes full advantage of the atomic layer deposition approach. This design principle is tunable and also allows us to bypass limitations imposed by bulk crystal geometry.
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| 37. |
- Das, Tanmoy, et al.
(författare)
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Origin of pressure induced second superconducting dome in AyFe(2-x)Se(2) [A = K, (TI, Rb)]
- 2013
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 15, s. 093045-
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Tidskriftsartikel (refereegranskat)abstract
- Recent observation of a pressure induced second superconducting phase in A(y)Fe(2-x)Se(2) [A = K, (TI, Rb)] calls for the models of superconductivity that are rich enough to allow for multiple superconducting phases. We propose the model where pressure induces renormalization of band parameters in such a way that it leads to changes in Fermi surface topology even for a fixed electron number. We develop a low-energy effective model, derived from first-principles band-structure calculation at finite pressure, to suggest the phase assignment where a low pressure superconducting state with no hole pocket at the 0 point is a nodeless d-wave state. It evolves into a s(+/-) state at higher pressure when the Fermi surface topology changes and the hole pocket appears. We analyze the pairing interactions using a five band tight binding fitted band structure and find that a strong pairing strength is dependent on pressure. We also evaluate the energy and momentum dependence of neutron spin resonances in each of the phases as verifiable predictions of our proposal.
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| 38. |
- Dunnett, Kirsty, et al.
(författare)
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Dynamic Multiferroicity of a Ferroelectric Quantum Critical Point
- 2019
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Ingår i: Physical Review Letters. - : AMER PHYSICAL SOC. - 0031-9007 .- 1079-7114. ; 122:5
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Tidskriftsartikel (refereegranskat)abstract
- Quantum matter hosts a large variety of phases, some coexisting, some competing; when two or more orders occur together, they are often entangled and cannot be separated. Dynamical multiferroicity, where fluctuations of electric dipoles lead to magnetization, is an example where the two orders are impossible to disentangle. Here we demonstrate an elevated magnetic response of a ferroelectric near the ferroelectric quantum critical point (FE QCP), since magnetic fluctuations are entangled with ferroelectric fluctuations. We thus suggest that any ferroelectric quantum critical point is an inherent multiferroic quantum critical point. We calculate the magnetic susceptibility near the FE QCP and find a region with enhanced magnetic signatures near the FE QCP and controlled by the tuning parameter of the ferroelectric phase. The effect is small but observable-we propose quantum paraelectric strontium titanate as a candidate material where the magnitude of the induced magnetic moments can be similar to 5 x 10(-7) mu(B) per unit cell near the FE QCP.
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| 39. |
- Dunnett, Kirsty, et al.
(författare)
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Strain and ferroelectric soft-mode induced superconductivity in strontium titanate
- 2018
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 97:14
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the effects of strain on superconductivity with particular reference to SrTiO3. Assuming that a ferroelectric mode that softens under tensile strain is responsible for the coupling, an increase in the critical temperature and range of carrier densities for superconductivity is predicted, while the peak of the superconducting dome shifts towards lower carrier densities. Using a Ginzburg-Landau approach in 2D, we find a linear dependence of the critical temperature on strain: if the couplings between the order parameter and strains in different directions differ while their sum is fixed, different behaviors under uniaxial and biaxial strain can be understood.
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| 40. |
- Edge, Jonathan M., et al.
(författare)
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Quantum Critical Origin of the Superconducting Dome in SrTiO3
- 2015
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 115:24
-
Tidskriftsartikel (refereegranskat)abstract
- We expand the well-known notion that quantum criticality can induce superconductivity by proposing a concrete mechanism for superconductivity due to quantum ferroelectric fluctuations. To this end, we investigate the origin of superconductivity in doped SrTiO3 using a combination of density functional and strong coupling theories within the framework of quantum criticality. Our density functional calculations of the ferroelectric soft mode frequency as a function of doping reveal a crossover related to quantum paraelectricity at a doping level coincident with the experimentally observed top of the superconducting dome. Thus, we suggest a model in which the soft mode fluctuations provide the pairing interaction for superconductivity carriers. Within our model, the low doping limit of the superconducting dome is explained by the emergence of the Fermi surface, and the high doping limit by departure from the quantum critical regime. We predict that the highest critical temperature will increase and shift to lower carrier doping with increasing O-18 isotope substitution, a scenario that is experimentally verifiable. Our model is applicable to other quantum paraelectrics, such as KTaO3.
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| 41. |
- Edge, Jonathan M., et al.
(författare)
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Upper Critical Field as a Probe for Multiband Superconductivity in Bulk and Interfacial STO
- 2015
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Ingår i: Journal of Superconductivity and Novel Magnetism. - : Springer Science and Business Media LLC. - 1557-1939 .- 1557-1947. ; 28:8, s. 2373-2384
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the temperature dependence of the upper critical field H-c2 as a tool to probe the possible presence of multiband superconductivity at the interface between LaAlO3 and SrTiO3 (LAO/STO). The behaviour of H-c2 can clearly indicate two-band superconductivity through its nontrivial temperature dependence. For the disorder scattering dominated two-dimensional LAO/STO interface, we find a characteristic non-monotonic curvature of the H-c2(T). We also analyse the H-c2 for multiband bulk STO and find similar behaviour.
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| 42. |
- Fang, Kun, et al.
(författare)
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Pairing modulations and phase separation instabilities in Bi2Sr2CaCu2O8+delta
- 2014
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Ingår i: Physics Letters A. - : Elsevier BV. - 0375-9601 .- 1873-2429. ; 378:3, s. 243-248
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Tidskriftsartikel (refereegranskat)abstract
- There is growing evidence that the unconventional spatial inhomogeneities in the doped high-T-c superconductors are accompanied by the pairing of electrons, subsequent phase transitions and condensation into coherent states. We show that such pairing states can be obtained from phase separation instabilities near level crossings. Conditions for coherent pairing instabilities are examined using exact diagonalization of Hubbard-like pyramid structures under variation of coupling and interaction strengths. We also evaluate the behavior of the energy charge gap in the vicinity of level crossings using a parametrization of coupling to the apical site to represent out-of-plane effects. These results provide a simple microscopic explanation of (correlation induced) supermodulation of the coherent pairing gap observed in scanning tunneling microscopy measurements at atomic scale in Bi2Sr2CaCu2O8+delta.
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| 43. |
- Fang, Kun, et al.
(författare)
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Possible phase separation in square and honeycomb Hubbard model : A variational cluster study
- 2015
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Ingår i: Physics Letters A. - : Elsevier BV. - 0375-9601 .- 1873-2429. ; 379:37, s. 2230-2238
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Tidskriftsartikel (refereegranskat)abstract
- The VCA ground state of the 2D Hubbard model is examined for possible phase separation under hole doping manifested by spatial inhomogeneities of coexisting different electron densities at equilibrium. Phase separation is accompanied by spectral weight loss and first Brillouin zone boundary deformation. Such an instability is observed in square structures and it is absent in honeycomb lattices. To our knowledge, no previous publications have revealed relationship between a Fermi surface instability and phase separation. Our VCA calculations provide strong support for this spontaneous instability, driven by electron correlations in specific lattice geometries, proposed in our earlier publications using exact quantum cluster calculations.
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| 44. |
- Fernandes, R. M., et al.
(författare)
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Two-band superconductivity in doped SrTiO3 films and interfaces
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 87:1, s. 014510-
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the possibility of multiband superconductivity in SrTiO3 films and interfaces using a two-dimensional two-band model. In the undoped compound, one of the bands is occupied whereas the other is empty. As the chemical potential shifts due to doping by negative charge carriers or application of an electric field, the second band becomes occupied and gives rise to a strong enhancement of the transition temperature and a sharp feature in the gap functions, which is manifested in the local density of states spectrum. By comparing our results with tunneling experiments in Nb-doped SrTiO3, we find that intraband pairing dominates over interband pairing, unlike other known multiband superconductors. Given the similarities with the value of the transition temperature and with the band structure of LaAlO3/SrTiO3 heterostructures, we speculate that the superconductivity observed in SrTiO3 interfaces may be similar in nature to that of bulk SrTiO3, involving multiple bands with distinct electronic occupations.
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| 45. |
- Fransson, Jonas, et al.
(författare)
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Inelastic electron tunneling spectroscopy at local defects in graphene
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 87:24, s. 245404-
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Tidskriftsartikel (refereegranskat)abstract
- We address local inelastic scattering from the vibrational impurity adsorbed onto graphene and the evolution of the local density of electron states near the impurity from a weak to strong coupling regime. For weak coupling the local electronic structure is distorted by inelastic scattering developing peaks or dips and steps. These features should be detectable in the inelastic electron tunneling spectroscopy d(2)I/dV(2) using local probing techniques. Inelastic Friedel oscillations distort the spectral density at energies close to the inelastic mode. In the strong coupling limit, a local negative U center forms in the atoms surrounding the impurity site. For those atoms, the Dirac cone structure is fully destroyed, that is, the linear energy dispersion as well as the V-shaped local density of electron states is completely destroyed. We further consider the effects of the negative U formation and its evolution from weak to strong coupling. The negative U site effectively acts as a local impurity such that sharp resonances appear in the local electronic structure. The main resonances are caused by elastic scattering off the impurity site, and the features are dressed by the presence of vibrationally activated side resonances. Going from weak to strong coupling, changes the local electronic structure from being Dirac-cone-like including midgap states, to a fully destroyed Dirac cone with only the impurity resonances remaining.
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| 46. |
- Fransson, Jonas, et al.
(författare)
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Magnon Dirac materials
- 2016
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 94:7
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate how a Dirac-like magnon spectrum is generated for localized magnetic moments forming a two-dimensional honeycomb lattice. The Dirac crossing point is proven to be robust against magnon-magnon interactions, as these only shift the spectrum. Local defects induce impurity resonances near the Dirac point, as well as magnon Friedel oscillations. The energy of the Dirac point is controlled by the exchange coupling, and thus a two-dimensional array of magnetic dots is an experimentally feasible realization of Dirac magnons with tunable dispersion.
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| 47. |
- Gao, Jun, et al.
(författare)
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Coexistence of extended and localized states in finite-sized mosaic Wannier-Stark lattices
- 2023
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 108:14
-
Tidskriftsartikel (refereegranskat)abstract
- Quantum transport and localization are fundamental concepts in condensed matter physics. It is commonly believed that in one-dimensional systems, the existence of mobility edges is highly dependent on disorder. Recently, there has been a debate over the existence of an exact mobility edge in a modulated mosaic model without quenched disorder, the so-called mosaic Wannier-Stark lattice. Here, we experimentally implement such disorder-free mosaic photonic lattices using a silicon photonics platform. By creating a synthetic electric field, we could observe energy-dependent coexistence of both extended and localized states in a finite number of waveguides. The Wannier-Stark ladder emerges when the resulting potential is strong enough, and can be directly probed by exciting different spatial modes of the lattice. Our studies provide the experimental proof of coexisting sets of strongly localized and conducting (though weakly localized) states in finite-sized mosaic Wannier-Stark lattices, which hold the potential to encode high-dimensional quantum resources with compact and robust structures.
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| 48. |
- Geilhufe, Matthias, et al.
(författare)
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Data Mining for Three-Dimensional Organic Dirac Materials : Focus on Space Group
- 2017
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Ingår i: Scientific Reports. - : Nature Publishing Group. - 2045-2322. ; 7:1
-
Tidskriftsartikel (refereegranskat)abstract
- We combined the group theory and data mining approach within the Organic Materials Database that leads to the prediction of stable Dirac-point nodes within the electronic band structure of three-dimensional organic crystals. We find a particular space group P212121 (#19) that is conducive to the Dirac nodes formation. We prove that nodes are a consequence of the orthorhombic crystal structure. Within the electronic band structure, two different kinds of nodes can be distinguished: 8-fold degenerate Dirac nodes protected by the crystalline symmetry and 4-fold degenerate Dirac nodes protected by band topology. Mining the Organic Materials Database, we present band structure calculations and symmetry analysis for 6 previously synthesized organic materials. In all these materials, the Dirac nodes are well separated within the energy and located near the Fermi surface, which opens up a possibility for their direct experimental observation.
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| 49. |
- Geilhufe, Matthias, et al.
(författare)
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Dynamically induced magnetism in KTaO3
- 2021
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Ingår i: Physical Review Research. - : American Physical Society (APS). - 2643-1564. ; 3:2
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Tidskriftsartikel (refereegranskat)abstract
- Dynamical multiferroicity features entangled dynamic orders: fluctuating electric dipoles induce magnetization. Hence, the material with paraelectric fluctuations can develop magnetic signatures if dynamically driven. We identify the paraelectric KTaO3 (KTO) as a prime candidate for the observation of the dynamical multiferroicity. We show that when a KTO sample is exposed to a circularly polarized laser pulse, the dynamically induced ionic magnetic moments are of the order of 5% of the nuclear magneton per unit cell. We determine the phonon spectrum using ab initio methods, and we identify T-1u as relevant phonon modes that couple to the external field and induce magnetic polarization. We also predict a corresponding electron effect for the dynamically induced magnetic moment, which is enhanced by several orders of magnitude due to the significant mass difference between electron and ionic nucleus.
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| 50. |
- Geilhufe, Matthias, et al.
(författare)
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Symmetry analysis of odd- and even-frequency superconducting gap symmetries for time-reversal symmetric interactions
- 2018
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 97:2
-
Tidskriftsartikel (refereegranskat)abstract
- Odd-frequency superconductivity describes a class of superconducting states where the superconducting gap is an odd function in relative time and Matsubara frequency. We present a group theoretical analysis based on the linearized gap equation in terms of Shubnikov groups of the second kind. By discussing systems with spin-orbit coupling and an interaction kernel which is symmetric under the reversal of relative time, we show that both even-and odd-frequency gaps are allowed to occur. Specific examples are discussed for the square lattice, the octahedral lattice, and the tetragonal lattice. For irreducible representations that are even under the reversal of relative time the common combinations of s- and d-wave spin singlet and p-wave spin triplet gaps are revealed, irreducible representations that are odd under reversal of relative time give rise to s- and d-wave spin triplet and p-wave spin singlet gaps. Furthermore, we discuss the construction of a generalized Ginzburg-Landau theory in terms of the associated irreducible representations. The result complements the established classification of superconducting states of matter.
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