Veselago lens focusing in graphene $p\text{−}n$ junction is promising for realizations of new generation electron optics devices. However, the effect of the strain-induced Aharonov-Bohm interference in a $p\text{−}n$ junction has not been discussed before. We provide an experimentally feasible setup based on the Veselago lens in which the presence of strain can result in both the valley-dependent Lorentz force and Aharonov-Bohm interference. In particular, by employing the Green's function and tight-binding methods, we study the strain induced by dislocations and line defects in a $p\text{−}n$ junction and show how the resulting Aharonov-Bohm phase and interference can be detected. Furthermore, for a different strain configuration, e.g., corresponding to corrugated graphene, we find strong signatures of valley splitting induced by the fictitious magnetic field. Our proposal can be useful for mapping elastic deformations and defects, and for studying valley-dependent effects in graphene.

A formulation of the response of a system to strain and electric field perturbations in the pseudopotential-based density functional perturbation theory has been proposed by Hamann et al. [D. R. Hamann, X. Wu, K. M. Rabe, and D. Vanderbilt, Phys. Rev. B 71, 035117 (2005)], using an elegant formalism based on the expression of density functional theory (DFT) total energy in reduced coordinates, the key quantity being the metric tensor. We extend this formulation to the projector augmented-wave approach. In this context, we express the full elastic tensor including the clamped-atom tensor, the atomic-relaxation contributions, and the response to electric field change (piezoelectric tensor and effective charges). With this, we are able to compute the elastic tensor for all materials within a fully analytical formulation. The comparison with finite difference calculations on simple systems shows excellent agreement. This formalism has been implemented in the plane-wave-based DFT ABINIT software package. We apply it to the computation of elastic properties of low-symmetry systems of geophysical interest.

Structured metallic and magnetic materials have attracted increasing attention due to their potential for label-free molecular detection, based on the control of the phase of light. Traditionally, tuning magneto-optical signals via magnetoplasmon leads to an excessively low light energy at the critical position in the spectrum. Thus, improving the reflectivity of magnetoplasmon-based photonic devices is a promising method to enhance their environmental adaptability. Here, we introduce a strategy for decoupling magneto-optical signal control and modulation of the reflection spectral line shape in ferromagnetic metallic gratings. This strategy is based on the sensibility of metallic gratings to the incident polarization state and the conclusion that the spin-orbit coupling induced orthogonal plasmon mode can be treated as from a pseudolight source. The decoupling and other phenomena of the oft-repeated magneto-optical Kerr rotation reversals and sensitivity to incident angle are validated using a cobalt grating.

Topological spin textures, such as magnetic skyrmions, have been extensively discussed in magnetic materials with broken inversion symmetry. Most of these textures are stable at low temperatures and form long-range orders. However, the topological nature of these textures, characterized by the index called topological charge, is actually persistent at elevated temperatures. In this work, the topological charge of a two-dimensional frustrated spin model with the Dzyaloshinskii-Moriya (DM) interaction was calculated by the Monte Carlo simulations. At finite temperatures, nontrivial topology driven by thermal fluctuation was discovered in a spin-disordered phase. This thermally driven topology is the dominant topology at low magnetic fields. The magnitude of topological charge is quadratic in the DM interaction and linear in the external magnetic field or uniaxial magnetic anisotropy. We also propose a real frustrated system, the Mn-Bi monolayer film, with large DM interaction to enable such thermally driven topology. A full phase diagram of this system has been explored.

Microscopic origin of the ferromagnetic (FM) exchange coupling in two Cr trihalides, ${\mathrm{CrCl}}_3$ and ${\mathrm{CrI}}_3$, their common aspects and differences, are investigated on the basis of density functional theory combined with realistic modeling approach for the analysis of interatomic exchange interactions. For these purposes, we perform a comparative study based on the pseudopotential and linear muffin-tin orbital methods by treating the effects of electron exchange and correlation in generalized gradient approximation (GGA) and local spin density approximation (LSDA), respectively. The results of ordinary band structure calculations are used in order to construct the minimal tight-binding type models describing the behavior of the magnetic Cr $3d$ and ligand $p$ bands in the basis of localized Wannier functions, and evaluate the effective exchange coupling ($J_{\mathrm{eff}}$) between two Cr sublattices employing four different technique: (i) brute force total energy calculations; (ii) the second-order Green's function perturbation theory for infinitesimal spin rotations of the LSDA (GGA) potential at the Cr sites; (iii) enforcement of the magnetic force theorem in order to treat both Cr and ligand spins on a localized footing; and (iv) constrained total-energy calculations with an external field, treated in the framework of self-consistent linear response theory. We argue that the ligand states play crucial role in the ferromagnetism of Cr trihalides, though their contribution to $J_{\mathrm{eff}}$ strongly depends on additional assumptions, which are traced back to the fundamentals of adiabatic spin dynamics. Particularly, by neglecting the ligand spins in the Green's function method, $J_{\mathrm{eff}}$ can easily become antiferromagnetic, while by treating them as localized, one can severely overestimate the FM coupling. The best considered approach is based on the constraint method, where the ligand states are allowed to relax in response to each instantaneous reorientation of the Cr spins, controlled by a constraining field. Furthermore, the differences of the electronic structure of Cr trihalides in GGA and LSDA, and their impact on the exchange coupling are discussed in details, as well as the possible roles played by the on-site Coulomb repulsion $U$.

Motivated by the recent discovery of a macroscopically degenerate exactly solvable point of the spin-$1/2XXZ$ model for $J_z/J=-1/2$ on the kagome lattice [H. J. Changlani et al. Phys. Rev. Lett. 120, 117202 (2018)]—a result that holds for arbitrary magnetization—we develop an exact mapping between its exact ”quantum three-coloring” wave functions and the characteristic localized and topological magnons. This map, involving ”resonating two-color loops,” is developed to represent exact many-body ground state wave functions for special high magnetizations. Using this map we show that these exact ground state solutions are valid for any $J_z/J\ge -1/2$. This demonstrates the equivalence of the ground-state wave function of the Ising, Heisenberg, and $XY$ regimes all the way to the $J_z/J=-1/2$ point for these high magnetization sectors. In the hardcore bosonic language, this means that a certain class of exact many-body solutions, previously argued to hold for purely repulsive interactions $\left(J_z\ge 0\right)$, actually hold for attractive interactions as well, up to a critical interaction strength. For the case of zero magnetization, where the ground state is not exactly known, we perform density matrix renormalization group calculations. Based on the calculation of the ground state energy and measurement of order parameters, we provide evidence for a lack of any qualitative change in the ground state on finite clusters in the Ising $\left(J_z\gg J\right)$, Heisenberg $\left(J_z=J\right)$, and $XY$ $\left(J_z=0\right)$ regimes, continuing adiabatically to the vicinity of the macroscopically degenerate $J_z/J=-1/2$ point. These findings offer a framework for recent results in the literature and also suggest that the $J_z/J=-1/2$ point is an unconventional quantum critical point whose vicinity may contain the key to resolving the spin-$1/2$ kagome problem.

We present experimental observation of a phenomenon that we interpret as a NMR-like effect on an anisotropic magnetic moment of the surface Andreev bound states in topological superfluid ${}^3\mathrm{He}\text{−}\mathrm{B}$ at zero temperature limit. We show that an anisotropic magnetic moment formed near the horizontal surface of a mechanical resonator due to symmetry violation of the superfluid ${}^3\mathrm{He}\text{−}\mathrm{B}$ order parameter by the resonator's surface may lead to anomalous damping of the resonator motion in magnetic field. In difference to the classical NMR technique, here NMR was excited using own harmonic motion of the mechanical resonator and nuclear magnetic resonance was detected as a maximum in damping when the resonator's angular frequency satisfied the Larmor resonance condition.

We analyze a prototypical particle-in-a-box model for a hole spin qubit. This quantum dot is subjected to static magnetic and electric fields, and to a radio-frequency electric field that drives Rabi oscillations owing to spin-orbit coupling. We derive the equations for the Rabi frequency in a regime where the Rabi oscillations mostly result from the coupling between the qubit states and a single nearby excited state. This regime has been shown to prevail in, e.g., hole spin qubits in thin silicon-on-insulator nanowires. The equations for the Rabi frequency highlight the parameters that control the Rabi oscillations. We show, in particular, that [110]-oriented dots on (001) substrates perform much better than [001]-oriented dots because they take best advantage of the anisotropy of the valence band of the host material. We also conclude that silicon provides the best opportunities for fast Rabi oscillations in this regime despite small spin-orbit coupling.

We have studied cleaning procedures of Nb(110) by verifying the surface quality with low-energy electron diffraction, Auger electron spectroscopy, and scanning tunneling microscopy and spectroscopy. Our results show that the formation of a surface-near impurity depletion zone is inhibited by the very high diffusivity of oxygen in the Nb host crystal which kicks in at annealing temperatures as low as a few hundred degree Celsius. Oxygen can be removed from the surface by heating the crystal up to $T=2400{}^{\circ }\mathrm{C}$. Tunneling spectra measured on the clean Nb(110) surface exhibit a sharp conductance peak in the occupied states at an energy of about $-450$ meV. Density functional theory calculations show that this peak is caused by a $d_{z^2}$ surface resonance band at the $\overline{\mathrm{\Gamma }}$ point of the Brillouin zone which provides a large density of states above the sample surface. The clean Nb(110) surface is superconducting with a gap width and a critical magnetic field strength in good agreement to the bulk value. In an external magnetic field we observe the Abrikosov lattice of flux quanta (vortices). Spatially resolved spectra show a zero-bias anomaly in the vortex core.

We report the observation of counterflowing edge current in InAs quantum wells which leads to the breakdown of quantum Hall (QH) effects at high magnetic fields. Counterflowing edge channels arise from the Fermi-level pinning of InAs and the resultant sharp edge potential with downward bending. By measuring the counterflow conductance for varying edge lengths, we determine the effective number $\langle N_{\text{C}}\rangle$ of counterflowing modes and their equilibration length ${\lambda }_{\text{eq}}$ at a bulk integer filling factor $\nu =1–4$. ${\lambda }_{\text{eq}}$ increased exponentially with magnetic field $B$, reaching $200\mu \mathrm{m}$ for $\nu =4$ at $B\ge 7.6$ T. Our data reveal important roles of the innermost incompressible strip with even filling in determining $\langle N_{\text{C}}\rangle$ and ${\lambda }_{\text{eq}}$ and the impact of the contact configuration on the QH effect breakdown. Our results show that counterflowing edge channels manifest as transport anomalies only at high fields and in short edges. This in turn suggests that, even in the integer QH regime, the actual microscopic structure of the edge states can differ from that anticipated from macroscopic transport measurements, which is relevant to various systems including atomic-layer materials.

An infinite edge of a quantum Hall system prohibits indirect exchange coupling between two spins, whereas a quantum spin-Hall edge prohibits out-of-plane coupling. In this study, we analyze an unexpected breakdown of this behavior in a finite system, where the two spins can interact also via a longer path that traverses the whole perimeter of the system. We explain this using an analytical model as well as tight-binding models in real space. Based on this finding, we show how using a lead far away from the spins can switch the coupling on and off among them nonlocally.

The spectrum of the strongly correlated systems usually shows resonant peaks at finite energy, with examples in the 115 Ce family which are reproduced by the dynamical mean-field theory. A similar structure has been seen recently in the orbitally selective Mott phase of the two-band model, known as the doublon-holon bound state, with implications for the fate of such phase in the zero Hund's coupling limit. We show that these features can be captured with the slave-particle methods once their Hilbert space is taken into account. We use slave-spin calculations, justifiable in the limit of large dimensions, to explicitly demonstrate this and compare the results with dynamical mean-field theory.

We perform extensive first-principles calculations for heterostructures composed of monolayer graphene and hexagonal boron nitride (hBN). Employing a symmetry-derived minimal tight-binding model, we extract orbital and spin-orbit coupling (SOC) parameters for graphene on hBN, as well as for hBN encapsulated graphene. Our calculations show that the parameters depend on the specific stacking configuration of graphene on hBN. We also perform an interlayer distance study for the different graphene/hBN stacks to find the corresponding lowest energy distances. For very large interlayer distances, one can recover the pristine graphene properties, as we find from the dependence of the parameters on the interlayer distance. Furthermore, we find that orbital and SOC parameters, especially the Rashba one, depend strongly on an applied transverse electric field, giving a rich playground for spin physics. Armed with the model parameters, we employ the Dyakonov-Perel formalism to calculate the spin relaxation in graphene/hBN heterostructures. We find spin lifetimes in the nanosecond range, in agreement with recent measurements. The spin relaxation anisotropy, being the ratio of out-of-plane to in-plane spin lifetimes, is found to be giant close to the charge neutrality point, decreasing with increasing doping, and being highly tunable by an external transverse electric field. This is in contrast to bilayer graphene in which an external field saturates the spin relaxation anisotropy.

Coupled electron-hole states are realized in a system consisting of a combination of an electrostatic potential barrier and ring-shaped potential well, which resembles a circular dipole. A perpendicular magnetic field induces confined states inside the Landau gaps which are mainly located at the barrier or ring. Hybridizations between the barrier and ring states are seen as anticrossings in the energy spectrum. As a consequence, the energy levels show an oscillating dependence on the electrostatic potential strength in combination with an oscillating migration of the wave functions between the barrier and ring. At the anticrossing points the quantum state consists of a mixture of electron and hole. The present system mimics closely the behavior of a relativistic dipole on gapped graphene.

We study the Josephson effect in the multiterminal junction of topological superconductors. We use the symmetry-constrained scattering matrix approach to derive band dispersions of emergent subgap Andreev bound states in a multidimensional parameter space of superconducting phase differences. We find distinct topologically protected band crossings that serve as monopoles of finite Berry curvature. Particularly, in a four-terminal junction the admixture of $2\pi$ and $4\pi$ periodic levels leads to the appearance of finite-energy Majorana-Weyl nodes. This topological regime in the junction can be characterized by a quantized nonlocal conductance that measures the Chern number of the corresponding bands. In addition, we calculate current-phase relations, variance, and cross correlations of topological supercurrents in multiterminal contacts and discuss the universality of these transport characteristics. At the technical level these results are obtained by integrating over the group of a circular ensemble that describes the scattering matrix of the junction. We briefly discuss our results in the context of observed fluctuations of the gate dependence of the critical current in topological planar Josephson junctions and comment on the possibility of parity measurements from the switching current distributions in multiterminal Majorana junctions.

We use a classical analytical and separable elastic energy landscape describing SnO monolayers to estimate the softening of elastic moduli through a mechanical instability occurring at finite temperature in this material. Although not strictly applicable to this material due to its low-energy barrier $J$ that leads to a quantum paraelastic phase, the present exercise is relevant as it establishes a conceptual procedure to estimate such moduli straight from a two-dimensional elastic energy landscape. As additional support for the existence of a quantum paraelastic phase, we carry a qualitative WKB analysis to estimate escape times from an individual well on the landscape; escape times increase exponentially with the height of the barrier $J$. We also provide arguments against an additional transformation onto a planar lattice due to its high-energy cost. These results continue to establish a case for the usefulness of soft matter concepts in two-dimensional materials, and of the potential lurking of quantum effects into soft matter.

The magnetic, transport, and thermodynamic properties of $\mathrm{Nd}{\mathrm{Ir}}_3$ are reported. Its $\mathrm{PuN}{\mathrm{i}}_3$-type crystal structure (space group $R-3m$), with lattice parameters $a=5.3262\left(1\right)\AA$ and $c=26.2218\left(3\right)\AA$, was confirmed by powder x-ray diffraction. Our measurements indicate that $\mathrm{Nd}{\mathrm{Ir}}_3$ exhibits a previously unreported paramagnetic to ferromagnetic phase transition below $T_C=10.6\mathrm{K}$ (determined by using the Arrott plot). The magnetic susceptibility obeys the Curie-Weiss law with an effective magnetic moment of ${\mu }_{\mathrm{eff}}=3.63\left(2\right) {\mu }_B/\mathrm{Nd}$ and a paramagnetic Curie temperature ${\theta }_P=5.4\left(6\right)\mathrm{K}$. The heat-capacity anomaly at $T_C$ confirms a bulk nature of the transition, though $\mathrm{\Delta }{C}_p=11.7\mathrm{J}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{K}}^{-1}$ is lower than expected for $J=9/2$ and instead close to the $J=1/2$ system. This suggests that Nd ions are subject to the crystalline electrical field that removes spin degeneracy and leaves the Nd ions in a doublet ground state. Resistivity and heat-capacity measurements reveal an anomaly at around 70 K. The origin of this anomaly is unknown, but the lack of thermal hysteresis suggests that it is a second-order phase transition and may be related to electronic instabilities. The calculated electronic structure is reported.

We here propose a feasible method to detect hidden orders in high-$T_c$ superconductors. When we consider a $d$-wave superconductor being in proximity to a two-dimensional Weyl model, a topological superconductor with gapless edge states may be realized. The system can become topologically trivial when an additional $d$-density-wave order is also included. The edge states become gapped and may be detected in experiments. For this, the $d$-density-wave order can be detected experimentally, and different scenarios for the pseudogap in high-$T_c$ superconductors may be distinguished. This method may be used to detect various hidden orders through putting high-$T_c$ superconductors being in proximity to designated topologically nontrivial materials.

Systematic pressure- and temperature-dependent infrared studies on the quasi-two-dimensional organic quantum spin liquid ${\beta }^{\prime }-{\mathrm{EtMe}}_3\mathrm{Sb}{\left[\mathrm{Pd}{\left(dmit\right)}_2\right]}_2$ disclose the electronic and lattice evolution across the Mott insulator-metal transition. Increasing hydrostatic pressure continuously suppresses the insulating ground state; for $p>0.6$ GPa, a Drude-like component develops indicating the appearance of coherent quasiparticles at the Fermi level. In the vicinity of the Mott transition, not only the electronic state changes rapidly, but also the vibration modes exhibit pronounced changes in frequency and Fano constant, underlining the strong coupling between lattice and electrons. The anisotropy of the in-plane optical response becomes inverted above 0.6 GPa. The findings are discussed in detail and summarized in a phase diagram comprising different experimental approaches.