| 1. |
- Bagrov, Andrey A., et al.
(författare)
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Detecting quantum critical points in the t-t ' Fermi-Hubbard model via complex network theory
- 2020
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Ingår i: Scientific Reports. - : NATURE RESEARCH. - 2045-2322. ; 10
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Tidskriftsartikel (refereegranskat)abstract
- A considerable success in phenomenological description of high-Tc superconductors has been achieved within the paradigm of Quantum Critical Point (QCP)-a parental state of a variety of exotic phases that is characterized by dense entanglement and absence of well-defined quasiparticles. However, the microscopic origin of the critical regime in real materials remains an open question. On the other hand, there is a popular view that a single-band t-t ' Hubbard model is the minimal model to catch the main relevant physics of superconducting compounds. Here, we suggest that emergence of the QCP is tightly connected with entanglement in real space and identify its location on the phase diagram of the hole-doped t-t ' Hubbard model. To detect the QCP we study a weighted graph of inter-site quantum mutual information within a four-by-four plaquette that is solved by exact diagonalization. We demonstrate that some quantitative characteristics of such a graph, viewed as a complex network, exhibit peculiar behavior around a certain submanifold in the parametric space of the model. This method allows us to overcome difficulties caused by finite size effects and to identify precursors of the transition point even on a small lattice, where long-range asymptotics of correlation functions cannot be accessed.
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| 2. |
- Alexander, Gerianne, et al.
(författare)
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The sounds of science-a symphony for many instruments and voices
- 2020
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Ingår i: Physica Scripta. - : IOP Publishing. - 0031-8949 .- 1402-4896. ; 95:6
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Tidskriftsartikel (refereegranskat)abstract
- Sounds of Science is the first movement of a symphony for many (scientific) instruments and voices, united in celebration of the frontiers of science and intended for a general audience. John Goodenough, the maestro who transformed energy usage and technology through the invention of the lithium-ion battery, opens the programme, reflecting on the ultimate limits of battery technology. This applied theme continues through the subsequent pieces on energy-related topics-the sodium-ion battery and artificial fuels, by Martin Mansson-and the ultimate challenge for 3D printing, the eventual production of life, by Anthony Atala. A passage by Gerianne Alexander follows, contemplating a related issue: How might an artificially produced human being behave? Next comes a consideration of consciousness and free will by Roland Allen and Suzy Lidstrom. Further voices and new instruments enter as Warwick Bowen, Nicolas Mauranyapin and Lars Madsen discuss whether dynamical processes of single molecules might be observed in their native state. The exploitation of chaos in science and technology, applications of Bose-Einstein condensates and the significance of entropy follow in pieces by Linda Reichl, Ernst Rasel and Roland Allen, respectively. Mikhail Katsnelson and Eugene Koonin then discuss the potential generalisation of thermodynamic concepts in the context of biological evolution. Entering with the music of the cosmos, Philip Yasskin discusses whether we might be able to observe torsion in the geometry of the Universe. The crescendo comes with the crisis of singularities, their nature and whether they can be resolved through quantum effects, in the composition of Alan Coley. The climax is Mario Krenn, Art Melvin and Anton Zeilinger's consideration of how computer code can be autonomously surprising and creative. In a harmonious counterpoint, his 'Guidelines for considering AIs as coauthors', Roman Yampolskiy concludes that code is not yet able to take responsibility for coauthoring a paper. An interlude summarises a speech by Zdenek Papousek. In a subsequent movement, new themes emerge as we seek to comprehend how far we have travelled along the path to understanding, and speculate on where new physics might arise. Who would have imagined, 100 years ago, a global society permeated by smartphones and scientific instruments so sophisticated that genes can be modified and gravitational waves detected?
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| 3. |
- Boukhvalov, D. W., et al.
(författare)
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Electronic structure of a Mn-12 molecular magnet : Theory and experiment
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:1, s. 014419-
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Tidskriftsartikel (refereegranskat)abstract
- We used site-selective and element-specific resonant inelastic x-ray scattering (RIXS) to study the electronic structure and the electron interaction effects in the molecular magnet [Mn12O12(CH3COO)(16)(H2O)(4)]center dot 2CH(3)COOH center dot 4H(2)O, and compared the experimental data with the results of local spin density approximation +U electron structure calculations which include the on-site Coulomb interactions. We found a good agreement between theory and experiment for the Coulomb repulsion parameter U = 4 eV. In particular, the p-d band separation of 1.8 eV has been found from the RIXS spectra, which is in accordance with the calculations. Similarly, the positions of the peaks in the XPS spectra agree with the calculated densities of p and d states. Using the results of the electronic structure calculations, we determined the intramolecular exchange parameters, and used them for diagonalization of the Mn-12 spin Hamiltonian. The calculated exchanges gave the correct ground state with the total spin S = 10.
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| 4. |
- Danilov, Michael, et al.
(författare)
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Degenerate plaquette physics as key ingredient of high-temperature superconductivity in cuprates
- 2022
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Ingår i: npj Quantum Materials. - : Springer Science and Business Media LLC. - 2397-4648. ; 7:1
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Tidskriftsartikel (refereegranskat)abstract
- We study the physics of high-temperature cuprate superconductors starting from the highly degenerate four-site plaquette of the t− t′− U Hubbard model as a reference system. The degeneracy causes strong fluctuations when a lattice of plaquettes is constructed. We show that there is a large binding energy between holes when a set of four plaquettes is considered. The next-nearest-neighbour hopping t′ plays a crucial role in the formation of these strongly bound electronic bipolarons whose coherence at lower temperature could be the explanation for superconductivity. A complementary approach is cluster dual fermion starting from a single degenerate plaquette, which contains the relevant short-ranged fluctuations from the beginning. It gives d-wave superconductivity as the leading instability under a reasonably broad range of parameters. The origin of the pseudogap is also discussed in terms of the coupling of degenerate plaquettes. Thus, some of the essential elements of cuprate superconductivity appear from the local plaquette physics.
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| 5. |
- Rudenko, A.N., et al.
(författare)
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Strong electron-phonon coupling and phonon-induced superconductivity in tetragonal C3N4 with hole doping
- 2024
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 109:1
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Tidskriftsartikel (refereegranskat)abstract
- C3N4 is a recently discovered phase of carbon nitrides with the tetragonal crystal structure [D. Laniel et al., Adv. Mater. (2023), doi:10.1002/adma.202308030] that is stable at ambient conditions. C3N4 is a semiconductor exhibiting flat-band anomalies in the valence band, suggesting the emergence of many-body instabilities upon hole doping. Here, using state-of-the-art first-principles calculations we show that hole-doped C3N4 reveals strong electron-phonon coupling, leading to the formation of a gapped superconducting state. The phase transition temperatures turn out to be strongly dependent on the hole concentration. We propose that holes could be injected into C3N4 via boron doping which induces, according to our results, a rigid shift of the Fermi energy without significant modification of the electronic structure. Based on the electron-phonon coupling and Coulomb pseudopotential calculated from first principles, we conclude that the boron concentration of 6 atoms per nm3 would be required to reach the critical temperature of ∼36 K at ambient pressure.
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| 6. |
- Szilva, Attila, et al.
(författare)
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Quantitative theory of magnetic interactions in solids
- 2023
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Ingår i: Reviews of Modern Physics. - : American Physical Society. - 0034-6861 .- 1539-0756. ; 95:3
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Forskningsöversikt (refereegranskat)abstract
- This review addresses the method of explicit calculations of interatomic exchange interactions of magnetic materials. This involves exchange mechanisms normally referred to as a Heisenberg exchange, a Dzyaloshinskii-Moriya interaction, and an anisotropic symmetric exchange. The connection between microscopic theories of the electronic structure, such as density functional theory and dynamical mean-field theory, and interatomic exchange is examined. The different aspects of extracting information for an effective spin Hamiltonian that involves thousands of atoms, from electronic structure calculations considering significantly fewer atoms (1-50), is highlighted. Examples of exchange interactions of a large group of materials is presented, which involves heavy elements of the 3d period, alloys between transition metals, Heusler compounds, multilayer systems as well as overlayers and adatoms on a substrate, transition metal oxides, 4f elements, magnetic materials in two dimensions, and molecular magnets. Where possible, a comparison to experimental data is made that becomes focused on the magnon dispersion. The influence of relativity is reviewed in a few cases, as is the importance of dynamical correlations. Development to theories that handle out-of-equilibrium conditions is also described here. The review ends with a description of extensions of the theories behind explicit calculations of interatomic exchange to nonmagnetic situations, such as those that describe chemical (charge) order and superconductivity.
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| 7. |
- Wu, Han-Chun, et al.
(författare)
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Large positive in-plane magnetoresistance induced by localized states at nanodomain boundaries in graphene
- 2017
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- Graphene supports long spin lifetimes and long diffusion lengths at room temperature, making it highly promising for spintronics. However, making graphene magnetic remains a principal challenge despite the many proposed solutions. Among these, graphene with zig-zag edges and ripples are the most promising candidates, as zig-zag edges are predicted to host spin-polarized electronic states, and spin-orbit coupling can be induced by ripples. Here we investigate the magnetoresistance of graphene grown on technologically relevant SiC/Si(001) wafers, where inherent nanodomain boundaries sandwich zig-zag structures between adjacent ripples of large curvature. Localized states at the nanodomain boundaries result in an unprecedented positive in-plane magnetoresistance with a strong temperature dependence. Our work may offer a tantalizing way to add the spin degree of freedom to graphene.
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| 8. |
- Yudin, Dmitry, et al.
(författare)
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Fermi Condensation Near van Hove Singularities Within the Hubbard Model on the Triangular Lattice
- 2014
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 112:7, s. 070403-
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Tidskriftsartikel (refereegranskat)abstract
- The proximity of the Fermi surface to van Hove singularities drastically enhances interaction effects and leads to essentially new physics. In this work we address the formation of flat bands ("Fermi condensation") within the Hubbard model on the triangular lattice and provide a detailed analysis from an analytical and numerical perspective. To describe the effect we consider both weak-coupling and strong-coupling approaches, namely the renormalization group and dual fermion methods. It is shown that the band flattening is driven by correlations and is well pronounced even at sufficiently high temperatures, of the order of 0.1-0.2 of the hopping parameter. The effect can therefore be probed in experiments with ultracold fermions in optical lattices.
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| 9. |
- Baglai, Mikhail, et al.
(författare)
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Giant anisotropy of Gilbert damping in a Rashba honeycomb antiferromagnet
- 2020
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 101:10
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Tidskriftsartikel (refereegranskat)abstract
- Giant Gilbert damping anisotropy is identified as a signature of strong Rashba spin-orbit coupling in a two-dimensional antiferromagnet on a honeycomb lattice. The phenomenon originates in spin-orbit-induced splitting of conduction electron subbands that strongly suppresses certain spin-flip processes. As a result, the spin-orbit interaction is shown to support an undamped nonequilibrium dynamical mode that corresponds to an ultrafast in-plane Neel vector precession and a constant perpendicular-to-the-plane magnetization. The phenomenon is illustrated on the basis of a two-dimensional s-d-like model. Spin-orbit torques and conductivity are also computed microscopically for this model. Unlike Gilbert damping, these quantities are shown to reveal only a weak anisotropy that is limited to the semiconductor regime corresponding to the Fermi energy staying in the close vicinity of the antiferromagnetic gap.
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| 10. |
- Bagrov, Andrey A., et al.
(författare)
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Multiscale structural complexity of natural patterns
- 2020
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : NATL ACAD SCIENCES. - 0027-8424 .- 1091-6490. ; 117:48, s. 30241-30251
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Tidskriftsartikel (refereegranskat)abstract
- Complexity of patterns is key information for human brain to differ objects of about the same size and shape. Like other innate human senses, the complexity perception cannot be easily quantified. We propose a transparent and universal machine method for estimating structural (effective) complexity of two-dimensional and three-dimensional patterns that can be straightforwardly generalized onto other classes of objects. It is based on multistep renormalization of the pattern of interest and computing the overlap between neighboring renormalized layers. This way, we can define a single number characterizing the structural complexity of an object. We apply this definition to quantify complexity of various magnetic patterns and demonstrate that not only does it reflect the intuitive feeling of what is "complex" and what is "simple" but also, can be used to accurately detect different phase transitions and gain information about dynamics of nonequilibrium systems. When employed for that, the proposed scheme is much simpler and numerically cheaper than the standard methods based on computing correlation functions or using machine learning techniques.
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