The results of extensive Monte Carlo simulations of classical spins on the two-dimensional kagome lattice with only dipolar interactions are presented. In addition to revealing the sixfold-degenerate ground state, the nature of the finite-temperature phase transition to long-range magnetic order is discussed. Low-temperature states consisting of mixtures of degenerate ground-state configurations separated by domain walls can be explained as a result of competing exchange-like and shape-anisotropy-like terms in the dipolar coupling. Fluctuations between pairs of degenerate spin configurations are found to persist well into the ordered state as the temperature is lowered until locking in to a low-energy state.

We derive the $\mathrm{Bogoliubov}+U$ formalism to study the thermodynamical properties of the Bose-Hubbard model. The framework can be viewed as the zero-frequency limit of bosonic dynamical mean-field theory (B-DMFT), but equally well as an extension of the mean-field decoupling approximation in which pair creation and annihilation of depleted particles is taken into account. The self-energy on the impurity site is treated variationally, minimizing the grand potential. The theory containing just three parameters that are determined self-consistently reproduces the $T=0$ phase diagrams of the three-dimensional and two-dimensional Bose-Hubbard model with an accuracy of $1\%$ or better. The superfluid to normal transition at finite temperature is also reproduced well and only slightly less accurately than in B-DMFT.

Based on the density-matrix renormalization group and the infinite time-evolving block decimation methods we study the interacting spin-orbit-coupled 1D Fermi gas in a transverse magnetic field. We find that the system with an attractive interaction can have a polarized insulator phase, a superconducting (SC) phase, a Luther-Emery (LE) phase, and a band insulator phase as we vary the chemical potential and the strength of the magnetic field. Spin-orbit coupling (SOC) enhances the triplet pairing order at zero momentum in both the SC and the LE phase, which leads to an algebraically decaying correlation with the same exponent as that of the singlet pairing one. In contrast to the Fulde-Ferrell-Larkin-Ovchinnikov phase found in the spin imbalanced system without SOC, pairings at finite momentum in these two phases have larger exponents hence do not dictate the long-range behavior. We also test for the presence of Majorana fermions in this system. Unlike results from the mean-field study, we do not find positive evidence of Majorana fermions.

The surface Fermi arc states in ${\mathbb{Z}}_2$ Weyl semimetals $A_3\mathrm{Bi} (A=\mathrm{Na},\mathrm{K},\mathrm{Rb})$ are studied by employing a continuum low-energy effective model. It is shown that the surface Fermi arc states can be classified with respect to the ud-parity symmetry. Because of the symmetry, the arcs come in mirror symmetric pairs. The effects of symmetry breaking terms on the structure of the Fermi arcs are also studied. Among other results, we find at least two qualitatively different types of the surface Fermi arcs. The arcs of the first type link disconnected sheets of the bulk Fermi surface, while arcs of the second type link different points of the same bulk Fermi surface sheet.

When an exciton in semiconductor is scattered and its energy is decreased far below the resonance energy of the bare exciton state, it has been considered that an exciton-polariton is created immediately by the scattering process because there is no exciton level at that energy. However, according to the recent time-resolved measurements of P emission originating from inelastic exciton-exciton scattering, it looks rather natural to consider that the exciton-polariton is created in a finite time scale which is restricted by a coherence volume of the exciton after the scattering. In this interpretation, the exciton remains in this time scale far below its resonance energy as a transient state in a series of processes. We propose an expression of the P-emission lifetime depending on the coherence volume of the scattered excitons through the conversion process from them to the polaritons. The coherence volume of the scattered excitons appears in the calculation of the inelastic scattering process on the assumption of a finite coherence volume of the bottleneck excitons. Time-resolved optical-gain measurements could be a way for investigating the validity of our interpretation.

We demonstrate that the charge-spin conversion efficiency of topological insulators (TI) can be experimentally determined by injecting spin-polarized tunneling electrons into a TI. Through a comparative study between bismuth selenide and bismuth antimony telluride, we verified the topological-surface-state origin of the observed giant spin signals. By injecting energetic electrons into bismuth selenide, we further studied the energy dependence of the effective spin polarization at the TI surface. The experimentally verified large spin polarization, as well as our calculations, provides new insights into optimizing TI materials for near room-temperature spintronic applications.

We report on the magnetic circular dichroism of nonresonant x-ray Raman scattering. The circular dichroism has been observed at the $L_{\mathrm{II},\mathrm{III}}$ edges on a pure iron sample. The observed magnetic effect is $\sim 2\%$. The dichroism spectra exhibit a significant dependence on the direction of the magnetization vector. We have constructed an effective theory to understand these behaviors. The dichroism arises from the orbital scattering and the spin scattering having different angular dependences. The calculation reproduces well the observed spectra.

We study one-dimensional Su-Schrieffer-Heeger (SSH) models with quantum phonons using a continuous-time quantum Monte Carlo method. Within statistical errors, we obtain identical results for the SSH model with acoustic phonons, and a related model with a coupling to an optical bond phonon mode. Based on this agreement, we first study the Peierls metal-insulator transition of the spinless SSH model, and relate it to the Kosterlitz-Thouless transition of a spinless Luttinger liquid. In the Peierls phase, the spectral functions reveal the single-particle and charge gap, and a central peak related to long-range order. For the spinful SSH model, which has a dimerized ground state for any nonzero coupling, we reveal a symmetry-related degeneracy of spin and charge excitations, and the expected spin and charge gaps as well as a central peak. Finally, we study the SSH-$UV$ model with electron-phonon and electron-electron interaction. We observe a Mott phase with critical spin and bond correlations at weak electron-phonon coupling, and a Peierls phase with gapped spin excitations at strong coupling. We relate our findings to the extended Hubbard model, and discuss the physical origin of the agreement between optical and acoustic phonons.

We have investigated the surface termination of ${\mathrm{LaAlO}}_3\left(001\right)$ at elevated temperatures by low-energy electron microscopy. The terminating layer can be LaO or ${\mathrm{AlO}}_2$. The LaO surface shows a $\sqrt{5}\times \sqrt{5}\mathrm{R}26$ reconstruction which can be used as a signature for the LaO termination, while the ${\mathrm{AlO}}_2$ termination is unreconstructed. We find that heating of as-delivered substrates in vacuum or heating substrates which were previously annealed in air or oxygen can lead to a reconstructed surface as observed in diffraction. However, the real-space image shows that the reconstructed areas only cover about one-third of the surface and that the termination is actually an ordered mixture of the reconstructed LaO and the unreconstructed ${\mathrm{AlO}}_2$ terminations. This conclusion is supported by data from atomic force microscopy. We also demonstrate how the disordered mixture of both terminations changes to large LaO islands in the middle of the ${\mathrm{AlO}}_2$ terraces upon heating.

We develop a theory for charge and spin currents between two canted magnetic leads flowing through a quantum dot with an arbitrary local interaction. For a noncollinear magnetic configuration, we calculate equilibrium and nonequilibrium currents biased by voltage or temperature difference or pumped by magnetic dynamics. We are able to explicitly separate the equilibrium and nonequilibrium contributions to the spin current, both of which can be written in terms of the full retarded Green's function on the dot. Taking the specific example of a single-level quantum dot with a large on-site Coulomb interaction, we calculate the total spin current near the Kondo regime, which we find to be generally enhanced in magnitude as compared to the noninteracting case.

Surface states in $\mathrm{Tl}/\mathrm{Si}\left(111\right)\text{−}\left(1\times 1\right)$ and $\beta \text{−}\mathrm{Bi}/\mathrm{Si}\left(111\right)\text{−}\left(\sqrt{3}\times \sqrt{3}\right)$ show non-Rashba-type spin splitting. We study spin-transport properties in these surface states. First, we construct tight-binding Hamiltonians for Tl/Si and Bi/Si surfaces, which respect crystallographic symmetries. As a result, we find specific terms in the Tl/Si surface Hamiltonian responsible for non-Rashba spin splitting. Using this model, we calculate current-induced spin polarization in the Tl/Si Hamiltonian in order to see the effect of non-Rashba spin-orbit interaction. We found that the induced spin polarization is in plane and perpendicular to the current, which is consequently the same with Rashba systems. We find that it follows from crystallographic symmetries. Furthermore, we numerically find bound states at the junction between two surface regions which have different signs of the spin-orbit interaction parameters in the Bi/Si system and in the Tl/Si system. We explain these numerical results with the results of our analytical calculations.

We investigate the nonequilibrium population of a vibrational mode in the steady state of a biased molecular junction, using a rate equation approach. We focus on the limit of weak electronic-vibrational coupling and show that, in the resonant transport regime and for sufficiently high bias voltages, the level of vibrational excitation increases with decreasing coupling strength, assuming a finite and nonzero value. An analytic behavior with respect to the electronic-vibrational coupling strength is only observed if the influence of environmental degrees of freedom is explicitly taken into account. We consider the influence of three different types of broadening: hybridization with the electrodes, thermal fluctuations, and the coupling to a thermal heat bath. Our results apply to vibrationally coupled electron transport through molecular junctions but also to quantum dots coupled to a microwave cavity, where the photon number can be expected to exhibit a similar behavior.

Entropy barriers and aging states appear in martensitic structural-transition models, slowly re-equilibrating after temperature quenches, under Monte Carlo dynamics. Concepts from protein folding and aging harmonic oscillators turn out to be useful in understanding these nonequilibrium evolutions. We show how the athermal, nonactivated delay time for seeded parent-phase austenite to convert to product-phase martensite arises from an identified entropy barrier in Fourier space. In an aging state of low Monte Carlo acceptances, the strain structure factor makes constant-energy searches for rare pathways to enter a Brillouin zone “golf hole” enclosing negative-energy states, and to suddenly release entropically trapped stresses. In this context, a stress-dependent effective temperature can be defined, that re-equilibrates to the quenched bath temperature.

Polarization reversal in ferroelectrics by the tip of a scanning probe microscope (SPM) has been intensively studied over the last two decades. In addition to classical domain formation and growth, a number of abnormal switching phenomena have been reported. In particular, it was experimentally and theoretically shown that slow dynamics of surface screening can control the kinetics of the ferroelectric switching, and result in backswitching and relaxation phenomena. Here we experimentally demonstrate the practical possibility of the history dependent polarization reversal by the grounded SPM tip. This phenomenon was attributed to the induction of the slowly dissipating charges into the surface, which in the presence of the grounded tip induce polarization reversal. Analytical and numerical electrostatic calculations allow additional insight into the mechanisms of the observed phenomena.

A novel method for the calculation of the energy dispersion relation and density of states (DOS) in one (1D) and two (2D) dimensions is introduced and applied to linear lattices (1D) and square and hexagonal lattices (2D). The (van Hove) singularities and (Dirac) zeros of the DOS are discussed. Results for the 2D hexagonal lattice (graphenelike materials) are compared with experimental data in microwave photonic crystals.

We investigate the local density of states, spectral function, and superconducting pair amplitudes for signatures of Majorana fermions in vortex cores in ferromagnetic and spin-orbit coupled semiconductor/superconductor heterostructures. We show that the Majorana fermion quasiparticle momentum distribution is always symmetrically distributed at a finite radius around a high-symmetry point, thereby providing a necessary condition for a low-energy state to be a Majorana fermion. In real-space profiles of the local density of states through the vortex core the Majorana fermion, together with other finite-energy vortex states, forms a characteristic $\mathsf{X}$-shape structure only present at nontrivial topology. Moreover, we find that the Mexican hat band structure property of the topologically nontrivial phase translates into multiple high-intensity band edges and also vortex core states located above the superconducting gap in the local density of states. Finally, we find no strong correlation between odd-frequency pairing and the appearance of Majorana fermions, but odd-frequency pairing exists as soon as ferromagnetism is present. In fact, we find that the only vortex superconducting pair amplitude directly related to any phase transition is the appearance of certain spin-triplet $p$-wave pairing components in the vortex core at a pre-topological vortex core widening transition.

Thermodynamic equilibrium in multicomponent solids subject to mechanical stresses is a complex nonlinear problem whose exact solution requires extensive computations. A few decades ago, Larché and Cahn proposed a linearized solution of the mechanochemical equilibrium problem by introducing the concept of open system elastic coefficients [Acta Metall. 21, 1051 (1973)]. Using the Ni-Al solid solution as a model system, we demonstrate that open system elastic coefficients can be readily computed by semigrand canonical Monte Carlo simulations in conjunction with the shape fluctuation approach. Such coefficients can be derived from a single simulation run, together with other thermodynamic properties needed for prediction of compositional fields in solid solutions containing defects. The proposed calculation approach enables streamlined solutions of mechanochemical equilibrium problems in complex alloys. Second order corrections to the linear theory are extended to multicomponent systems.

Solid ${\mathrm{CS}}_2$ is superficially similar to ${\mathrm{CO}}_2$, with the same $Cmca$ molecular crystal structure at low pressures, which has suggested similar phases also at high pressures. We carried out an extensive first-principles evolutionary search in order to identify the zero-temperature lowest-enthalpy structures of ${\mathrm{CS}}_2$ for increasing pressure up to 200 GPa. Surprisingly, the molecular $Cmca$ phase does not evolve into $\beta$-cristobalite as in ${\mathrm{CO}}_2$ but transforms instead into phases HP2 and HP1, both recently described in high-pressure ${\mathrm{SiS}}_2$. HP1 in particular, with a wide stability range, is a layered $P{2}_1/c$ structure characterized by pairs of edge-sharing tetrahedra and is theoretically more robust than all other ${\mathrm{CS}}_2$ phases discussed so far. Its predicted Raman spectrum and pair correlation function agree with experiment better than those of $\beta$-cristobalite, and further differences are predicted between their respective IR spectra. The band gap of HP1-${\mathrm{CS}}_2$ is calculated to close under pressure, yielding an insulator-metal transition near 50 GPa, in agreement with experimental observations. However, the metallic density of states remains modest above this pressure, suggesting a different origin for the reported superconductivity.

The high thermal conductivity of SiC prevents the improvement of its thermoelectric figure of merit, although excellent power factor has been achieved. Here we propose a different type of SiC superlattice, i.e., antiphase superlattice (APSL) nanowires (NWs), composed of only SiC components but with different stacking sequences. Our molecular dynamics simulations show that the thermal conductivity of period modulated APSL NWs can be significantly reduced by up to a factor of two at room temperature compared to the one of pristine NWs. The phonon density of states reveals that new vibrational modes emerge on the interfaces due to the formation of Si-Si and C-C bonds. We identify the increased phonon interfacial scattering as the predominant factor that hinders the thermal transport along the wires with period $L_p>6$ nm. Phonon coherent transport is also observed in the structures with period $L_p<6$ nm, which leads to a minimum thermal conductivity at the period of 6 nm. These results provide clear guidelines to design structures with minimal thermal conductivity and possibly promote SiC as a competitive thermoelectric material.