Recent demonstrations of magnetic ordering and spin transport in two-dimensional heterostructures have opened research venues in these material systems. In order to control and enhance the tunneling magnetoresistance phenomena in 2D magnetic heterostructures beyond phenomenological descriptions, quantitative estimates of tunneling rates in terms of the atomic details are required. Here we combine first principles and quantum ballistic transport calculations to shed important insights from an atomistic viewpoint on the underlying mechanisms governing spin transport in graphene/${\mathrm{CrI}}_3$ junctions. Descriptions of the electronic structure reveal that tunneling is the dominant transport mechanism in these heterostructures and helps differentiate intermediate metamagnetic states present in the switching process. We find that quantum confinement and layer-layer interactions are key to describing transport in these two-dimensional systems. Ballistic transport calculations further support these findings and yield magnetoresistance values in remarkable agreement with experiments. The short width of these barriers limits analysis solely based on the bulk complex band structure often employed in the description of magnetic tunnel junctions. Our work devises mechanisms to attain larger tunneling magnetoresistances, proving valuable to the advancement of spin valves in layered heterostructures.

Searching for the $p+ip$ superconducting (SC) state has become a fascinating subject in condensed matter physics recently, as a dream application awaiting in topological quantum computation. Unfortunately, so far there is no universal principle for realizing $p+ip$ in generic solid-state systems. Here we report a theoretical discovery of a $p+ip$ SC ground state (coexisting with ferromagnetic order) in the honeycomb lattice Hubbard model in the extremely strong-coupling limit (e.g., infinite $U)$ at low doping $\left(\delta <0.2\right)$, by using both the state-of-art Grassmann tensor product state approach and a continuum quantum field theory approach. Our discovery suggests a mechanism for the $p+ip$ SC state in generic strongly correlated systems based on spin-charge separation and the charge current–current coupling scenario, which opens a door towards experimental realization. The $p+ip$ SC state has an instability towards a potential non-Fermi liquid with a large but finite $U$. Nevertheless, by applying an in-plane Zeeman field, such a $p+ip$ SC state can be stabilized with finite $U$ in a very wide range of doping. Relevant realistic materials are also proposed.

Based on first-principles simulations and calculations, we explore the evolutions of crystal structure, electronic structure, and transport properties of quasi-two-dimensional layered $\mathrm{Pd}{\mathrm{S}}_2$ under compression by uniaxial stress and hydrostatic pressure. An interesting ferroelastic phase transition with lattice reorientation is revealed under uniaxial compressive stress, which originates from the bond reconstructions of the unusual $\mathrm{Pd}{\mathrm{S}}_4$ square-planar coordination. By contrast, the layered structure transforms into a three-dimensional cubic pyrite-type structure under hydrostatic pressure. In contrary to the experimentally proposed coexistence of layered $\mathrm{Pd}{\mathrm{S}}_2$-type structure with cubic pyrite-type structure at intermediate pressure range, we predict that the compression-induced intermediate phase will show the same structure symmetry as the ambient phase, except for sharply shrinking interlayer distances. The coordination of the Pd ions not only plays crucial roles in the structural transition, but also leads to electronic structure and transport property variations, which changes from square planar to distorted octahedron in the intermediate phase, resulting in bandwidth broadening and orbital-selective metallization. In addition, the superconductivity in the cubic pyrite-type structure comes from the strong electron-phonon coupling in the presence of topological nodal-line states. The strong interplay between structural transition, metallization, and superconductivity in $\mathrm{Pd}{\mathrm{S}}_2$ provides a good platform to study the fundamental physics of the interactions between crystal structure and transport behavior, and the competition or cooperation between diverse phases.

We study from first principles the structural and electronic properties of head-to-head (HH) and tail-to-tail (TT) ${180}^{\circ }$ domain walls in isolated free-standing ${\mathrm{PbTiO}}_3$ slabs. For sufficiently thick domains $(n=16$ unit cells of ${\mathrm{PbTiO}}_3)$, a transfer of charge from the free surfaces to the domain walls to form localized electron (in the HH) and hole (in the TT) gases in order to screen the bound polarization charges is observed. The electrostatic driving force behind this electronic reconstruction is clearly visible from the perfect match between the smoothed free charge densities and the bound charge distribution, computed from a finite difference of the polarization profile obtained after the relaxation of the lattice degrees of freedom. The domain wall widths, of around six unit cells, are larger than in the conventional ${180}^{\circ }$ neutral configurations. Since no oxygen vacancies, defects, or dopant atoms are introduced in our simulations, all the previous physical quantities are the intrinsic limits of the system. Our results support the existence of an extra source of charge at the domain walls to explain the enhancement of the conductivity observed in some domains walls of prototypical, insulating in bulk, perovskite oxides.

Some facets of the way sound waves travel through glasses are still unclear. Recent works have shown that in the low-temperature harmonic limit a crucial role in controlling sound damping is played by local elastic heterogeneity. Sound waves propagation has been demonstrated to be strongly affected by inhomogeneous mechanical features of the materials, which add to the anharmonic couplings at finite temperatures. We describe the interplay between these two effects by molecular dynamics simulation of a model glass. In particular, we focus on the transverse components of the vibrational excitations in terms of dynamic structure factors, and characterize the temperature dependence of sound attenuation rates in an extended frequency range. We provide a complete picture of all phenomena, in terms encompassing both theory and experiments.

The Ising spin chain with longitudinal and transverse magnetic fields is often used in studies of quantum chaos, displaying both chaotic and integrable regions in its parameter space. However, even at a strongly chaotic point this model does not exhibit Lyapunov growth of the commutator squared of spin operators, as this observable saturates before exponential growth can manifest itself (even in situations where a spatial suppression factor makes the initial commutator small). We extend this model from the spin 1/2 Ising model to higher spins, demonstrate numerically that a window of exponential growth opens up for sufficiently large spin, and extract a quantity which corresponds to a notion of a Lyapunov exponent. In the classical infinite-spin limit, we identify and compute the appropriate classical analog of the commutator squared, and show that the corresponding exponent agrees with the infinite-spin limit extracted from the quantum spin chain.

Motivated by the quest for experimentally accessible dynamical probes of Floquet topological insulators, we formulate the linear response theory of a periodically driven system. We illustrate the applications of this formalism by giving general expressions for optical conductivity of Floquet systems, including its homodyne and heterodyne components and beyond. We obtain the Floquet optical conductivity of specific driven models, including two-dimensional Dirac material such as the surface of a topological insulator, graphene, and the Haldane model irradiated with circularly or linearly polarized laser, as well as semiconductor quantum well driven by an ac potential. We obtain approximate analytical expressions and perform numerically exact calculations of the Floquet optical conductivity in different scenarios of the occupation of the Floquet bands, in particular the diagonal Floquet distribution and the distribution obtained after a quench. We comment on experimental signatures and detection of Floquet topological phases using optical probes.

The ferrimagnetic insulator yttrium iron garnet (YIG) is one of the most important materials in the active fields of insulator-based spintronics and spin caloritronics. Nevertheless, and despite the fact that this material has been studied for over six decades, the thermal properties of magnons in YIG have not been sufficiently characterized, mainly because at not very low temperatures they are overwhelmed by the contribution of phonons. Here, we report measurements of the thermal conductivity in YIG under magnetic fields up to 31.4 T to increase the magnon energy gap, to suppress the magnon contribution, and to isolate that of the phonons relative to their behavior at zero field. We observe that at a temperature of 20 K, even with a field as large as 31.4 T, the magnon contribution is not completely suppressed. The magnon thermal conductivity, measured by subtracting the value of the total thermal conductivity at 31.4 T from the value at zero field, has a peak at 16 K, with an amplitude that is over five times larger than the one obtained by measuring under a field of only 7 T, as previously reported.

The classical nearest-neighbor Kitaev-Heisenberg model on the triangular lattice is known to host ${\mathbb{Z}}_2$ spin vortices, forming a crystalline superstructure in the ground state. The ${\mathbb{Z}}_2$ vortices in this system can be understood as distortions of the local ${120}^{\circ }$ Néel parent order of the Heisenberg-only Hamiltonian. Here, we explore possibilities of stabilizing further types of ${\mathbb{Z}}_2$ vortex phases in Kitaev-Heisenberg models, including those which rely on more complicated types of noncollinear parent orders such as tetrahedral states. We perform extensive scans through large classes of Kitaev-Heisenberg models on different lattices employing a two-step methodology which first involves a mean-field analysis followed by a stochastic iterative minimization approach. When allowing for longer-range Kitaev couplings, we identify several ${\mathbb{Z}}_2$ vortex phases such as a state based on the ${120}^{\circ }$ Néel order on the triangular lattice which shows a coexistence of different ${\mathbb{Z}}_2$ vortex types. Furthermore, perturbing the tetrahedral order on the triangular lattice with a suitable combination of first- and second-neighbor Kitaev interactions, we find that a kagomelike superstructure of ${\mathbb{Z}}_2$ vortices may be stabilized, where vortices feature a counter-rotating winding of spins on different sublattices. This last phase may also be extended to honeycomb lattices where it is related to cubic types of parent orders. In total, this analysis shows that ${\mathbb{Z}}_2$ vortex phases appear in much wider contexts than the ${120}^{\circ }$ Néel-ordered systems previously studied.

We calculate the Andreev spectroscopy between a ferromagnetic lead and a Bi/Ni bilayer system. The bilayer system is described by the Anderson-Brinkman-Morel (ABM) state and mixing ABM and $S$-wave states. In both the ABM state and ABM state is mixed with small $S$-wave state, the Andreev conductance is consistent with that obtained in the point-contact experiment [Zhao et al., arXiv:1810.10403]. Moreover, the conductance peak near zero energy is induced by the surface state of the ABM state. Our work may provide helpful clarification for understanding recent experiments.

The appearance and origin of resonance phenomena are studied in an annular system of underdamped Josephson junctions. If no fluxon is trapped in the system, the dynamics is governed by the motion of fluxon-antifluxon pairs. If, on the other hand, trapped fluxons are present, in addition to their motion, the system can also exhibit the simultaneous motion of trapped fluxons and fluxon-antifluxon pairs. Locking between the rotating excitations (fluxons and antifluxons) and the Josephson frequency leads to the appearance of zero-field steps in the current-voltage characteristics, whose number is determined by the number of junctions and the total number of excitations present in the system. The obtained results clearly show that the branching of zero-field steps due to resonance between the rotating excitations and plasma oscillations in their tails appears only at the lower steps and completely disappears at the higher steps. A comparative analysis between systems without and those with trapped fluxons shows a correlation between their current-voltage characteristics. From a high-resolution analysis some special features of zero-field steps emerge such as an additional branch due to resonance between the pulsating fluxon and the Josephson frequency. Examination of systems with the same number but different types of excitations further reveals that their dynamics is determined not only by the number, but also by the type of excitations, i.e., systems with the same number but different types of excitations have different current-voltage characteristics.

The vanadium doping effects on the superconductivity and magnetism of iron pnictides are investigated in $\mathrm{Ba}{\left({\mathrm{Fe}}_{0.92-x}{\mathrm{Co}}_{0.08}{\mathrm{V}}_x\right)}_2{\mathrm{As}}_2$ by transport, susceptibility, and neutron scattering measurements. The doping of magnetic impurity V causes a fast suppression of superconductivity with $T_C$ reduced at a rate of approximately 7.4 K/1% V. On the other hand, for $x\ge 0.02$, long-range $C$-type antiferromagnetic order is recovered upon V doping, in contrast to the $x=0$ sample with no detectable magnetic order. The value of ordered magnetic moments of $\mathrm{Ba}{\left({\mathrm{Fe}}_{0.92-x}{\mathrm{Co}}_{0.08}{\mathrm{V}}_x\right)}_2{\mathrm{As}}_2$ follows a domelike evolution versus doping concentration $x$. A possible Griffiths-type antiferromagnetic region of multiple coexisting phases in the phase diagram of $\mathrm{Ba}{\left({\mathrm{Fe}}_{0.92-x}{\mathrm{Co}}_{0.08}{\mathrm{V}}_x\right)}_2{\mathrm{As}}_2$ is identified, in accordance with previous theoretical predictions based on the cooperative behavior of magnetic impurities and conduction electrons mediating the Ruderman-Kittel-Kasuya-Yosida interactions between them.

Doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is proposed to be a nematic superconductor originating from unusual interorbital pairing. We calculate all induced superconducting pair correlations in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and discover that intraorbital odd-frequency pairing clearly dominates over a significant range of frequencies. Moreover, we explore the contributions of even- and odd-frequency pairing to the Meissner effect, including separating intra- and interband processes in the response function. Contrary to expectations, and due to interband contributions, we find a diamagnetic Meissner effect from the odd-frequency pairing that stabilizes the superconducting order.

The electron-ion relaxation dynamics in warm dense iron were investigated by time-resolved x-ray absorption near-edge spectroscopy (XANES). A novel analysis, combining ab initio density functional theory (DFT) and two temperature model (TTM) simulations, was developed to calculate the x-ray absorption spectra as a function of delay time. Here we present experimental evidence of changes at the XAS L3 edge of iron that are consistent with the reduction of the electron-phonon coupling under warm dense matter conditions. The experimental results are in agreement with the model that takes both the electron ($T_e$) and the ion temperature ($T_i$) dependence of the thermophysical properties into consideration, while models where either constant electron-phonon coupling factor (G) or only $T_e$-dependent G are used do not agree with the observed relaxation dynamics of iron.

Domain walls in magnetic thin films display a complex dynamical response when subject to an external drive. It is claimed that different dynamic regimes are correlated with the domain-wall roughness, i.e., with the fluctuations of domain-wall position due to the inherent disorder in the system. Therefore, key to understanding the dynamics of domain walls is to have a statistically meaningful measure of the domain-wall roughness. Here we present a thorough study of the roughness parameters, i.e., roughness exponent and roughness amplitude, for domain walls in a ferrimagnetic GdFeCo thin film in the creep regime. Histograms of roughness parameters are constructed with more than 40 independent realizations under the same experimental conditions, and the average values and standard deviations are compared in different conditions. We found that the most prominent feature of the obtained distributions is their large standard deviations, which is a signature of large fluctuations. We show that even if the roughness parameters for a particular domain wall are well known, these parameters are not necessarily representative of the underlying physics of the system. In the low field limit, within the creep regime of domain-wall motion, we found the average roughness exponent and roughness amplitude to be around 0.75 and 0.45 $\mu {\mathrm{m}}^2$, respectively. When an in-plane magnetic field is applied we observed that, even though the distributions are wide, changes in the mean values of roughness parameters can be identified; the roughness exponent decreasing to values around 0.72 while the roughness amplitude increases to 0.65 $\mu {\mathrm{m}}^2$. Our results call for a careful consideration of statistical averaging over different domains walls when reporting roughness exponents.

Resistivity minima are commonly seen in itinerant magnets and they are often attributed to the Kondo effect. However, recent experiments are revealing an increasing number of materials showing resistivity minima in the absence of indications of Kondo singlet formation. In a previous work [Z. Wang, K. Barros, G.-W. Chern, D. L. Maslov, and C. D. Batista, Phys. Rev. Lett. 117, 206601 (2016)], we demonstrated that the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction can produce a classical spin liquid state at finite temperature, whose resistivity increases with decreasing temperature. The classical spin liquid exists over a relatively large temperature window because of the frustrated nature of the RKKY interaction produced by a 2D electron gas. In this work, we investigate the robustness of the RKKY-induced resistivity upturn against site dilution, which provides an alternative, and more robust, way of stabilizing the classical spin liquid state down to $T=0$. By using series expansions and stochastic Landau-Lifshitz dynamics simulation, we show that site dilution competes with thermal fluctuations and further stabilizes the resistivity upturn, which is accompanied by a negative magnetoresistivity due to suppression of the electron-spin scattering.

Recent experiments on a single crystal of ${\mathrm{U}}_2{\mathrm{Ni}}_2\mathrm{Sn}$ established a strong uniaxial magnetic anisotropy and an unusual magnetic structure that is antiferromagnetic both inside the U layers and between the layers. We apply fully relativistic first-principles calculations to investigate various magnetic configurations of the system. The calculations confirmed both the character of the magnetic anisotropy and the type of the ground-state magnetic structure. The study of the electron density of states shows that the lowest energy of the ground-state structure is related to the doubling of the magnetic unit cell and corresponding reduction of the Brillouin zone, leading to the formation of the energy gaps in the electronic structure. The mapping of the energies of the magnetic structures on the Heisenberg Hamiltonian of interacting atomic moments leads to a surprising result that only one of the interatomic exchange interactions is antiferromagnetic. We explain how this one antiferromagnetic interaction competing with several ferromagnetic interactions leads to the simultaneous presence of two types of antiferromagnetic behavior. We provide a simple mean-field estimate of the Néel temperature and conclude that short-range magnetic order, neglected on the mean-field level, may play an important role at temperatures around the critical temperature. We demonstrate the existence of self-consistent noncollinear magnetic states that limits the accuracy of treating ${\mathrm{U}}_2{\mathrm{Ni}}_2\mathrm{Sn}$ as a pure Ising magnet. We apply $\mathrm{GGA}+U$ method to examine the influence of the Hubbard parameter $U$ on the magnetic moments and energies of the magnetic configurations. An estimation of the influence of the contraction of the lattice on the magnetic properties is reported.

T-carbon had been proposed as a new carbon allotrope in 2011, which was successfully synthesized in recent experiments. Because of its fluffy structure, several kinds of atoms can be intercalated into T-carbon, making it a potential versatile candidate in various applications such as hydrogen storage, solar cells, lithium ion batteries, thermoelectrics, photocatalyst, etc. Here we show that superconductivity can appear in Na-doped T-carbon with a superconducting transition temperature $T_c$ of 11 K at ambient pressure, and $T_c$ can be enhanced to 19 K under a pressure of 14 GPa. The calculations on temperature dependence of specific heat and electrical and thermal conductivities show that the normal state of the Na-doped T-carbon superconductor is a non-Fermi liquid at temperature below 50 K, where the Wiedemann-Franz law is remarkably violated. The prediction of superconductivity in Na-doped T-carbon would spur great interest both experimentally and theoretically to explore novel carbon-based superconductors.

In this paper, we perform a systematic study on the electronic, magnetic, and transport properties of the hexagonal graphene quantum dots (GQDs) with armchair edges in the presence of a charged impurity using two different configurations: (1) a central Coulomb potential and (2) a positively charged carbon vacancy. The tight-binding and the half-filled extended Hubbard models are numerically solved and compared with each other in order to reveal the effect of electron interactions and system sizes. Numerical results point out that off-site Coulomb repulsion leads to an increase in the critical coupling constant to ${\beta }_c=0.6$ for a central Coulomb potential. This critical value of $\beta$ is found to be independent of the GQD size, reflecting its universality even in the presence of electron-electron interactions. In addition, a sudden downshift in the transmission peaks shows a clear signature of the transition from subcritical $\beta <{\beta }_c$ to the supercritical $\beta >{\beta }_c$ regime. On the other hand, for a positively charged vacancy, collapse of the lowest bound state occurs at ${\beta }_c=0.7$ for the interacting case. Interestingly, the local magnetic moment, induced by a bare carbon vacancy, is totally quenched when the vacancy is subcritically charged, whereas the valley splittings in electron and hole channels continue to exist in both regimes.