We study the behavior of a topological Josephson junction in which two topological superconductors are coupled through a quantum dot. We focus on the case with the bulk superconducting gap being the largest energy scale. Two parameter regimes are investigated: a weak tunneling between the dot and the superconductors, with the dot near its charge degeneracy point, and a strong-tunneling regime in which the transmission between the dot and the superconductors is nearly perfect. We show that in the former situation, the Andreev spectrum for each sector with fixed fermion parity consists of only two levels, which gives rise to the nontrivial current-phase relation. Moreover, we study the Rabi oscillation between the two levels and indicate that the corresponding frequency is a $4\pi$-periodic function of the phase difference between the two superconductors, which is immune to the quasiparticle poisoning. In the latter case, we find that the Coulomb charging energy enhances the effect of backscattering at the interfaces between the dot and the superconductors. Both the temperature and the gate-voltage dependence of the critical Josephson current are examined.

We address the question of why strongly correlated $d$-wave superconductors, such as the cuprates, prove to be surprisingly robust against the introduction of nonmagnetic impurities. We show that, very generally, both the pair-breaking and the normal state transport scattering rates are significantly suppressed by strong correlations effects arising in the proximity to a Mott insulating state. We also show that the correlation-renormalized scattering amplitude is generically enhanced in the forward direction, an effect which was previously often ascribed to the specific scattering by charged impurities outside the copper-oxide planes.

We report on highly polarization-selective three-photon absorption (3PA) in a Bridgman-grown single crystal of ${\mathrm{CsPbBr}}_3$ oriented along the (112) direction, which is an inorganic counterpart to emerging organic-inorganic hybrid halide perovskites for solar-cell and optoelectronic applications. The crystal exhibits strong photoluminescence (PL) at room temperature as a direct consequence of 3PA of fundamental radiation. Interestingly, 3PA disappears when the input polarization is parallel to the $\left(-110\right)$ direction. This 3PA effect is strongest when orthogonal to $\left(-110\right)$ and the corresponding 3PA coefficient was measured to be $\gamma =0.14\pm 0.03{\mathrm{cm}}^3/{\mathrm{GW}}^2$ under picosecond-pulse excitation at the fundamental wavelength of $\lambda =1200$ nm. The laser-induced damage threshold was also determined to be about 20 GW/${\mathrm{cm}}^2$ at the same wavelength. Based on relative PL intensities upon $\lambda$ tuning over the entire 3PA range ($1100\text{–}1700$ nm), we determined the nonlinear optical dispersion of the 3PA coefficient for ${\mathrm{CsPbBr}}_3$, which is consistent with a theoretical prediction. Experimentally observed significant polarization dependence of $\gamma$ was explained by relevant selection rules. The perovskite is potentially important for nonlinear optical applications owing to its highly efficient 3PA-induced PL response with a sharp on/off ratio by active polarization control.

In metallic condensed matter systems two different types of Weyl fermions can in principle emerge, with either a vanishing (type-I) or with a finite (type-II) density of states at the Weyl node energy. So far only ${\mathrm{WTe}}_2$ and ${\mathrm{MoTe}}_2$ were predicted to be type-II Weyl semimetals. Here we identify ${\mathrm{TaIrTe}}_4$ as a third member of this family of topological semimetals. ${\mathrm{TaIrTe}}_4$ has the attractive feature that it hosts only four well-separated Weyl points, the minimum imposed by symmetry. Moreover, the resulting topological surface states—Fermi arcs connecting Weyl nodes of opposite chirality—extend to about 1/3 of the surface Brillouin zone. This large momentum-space separation is very favorable for detecting the Fermi arcs spectroscopically and in transport experiments.

We report a systematic study of temperature- and field-dependent charge $\left(\mathbf{\rho }\right)$ and entropy $\left(\mathbf{S}\right)$ transport in the heavy-fermion superconductor ${\mathrm{CeIrIn}}_5$. Its large positive thermopower $S_{xx}$ is typical of Ce-based Kondo lattice systems, and strong electronic correlations play an important role in enhancing the Nernst signal $S_{xy}$. By separating the off-diagonal Peltier coefficient ${\alpha }_{xy}$ from $S_{xy}$, we find that ${\alpha }_{xy}$ becomes positive and greatly enhanced at temperatures well above the bulk $T_c$. Compared with the nonmagnetic analog ${\mathrm{LaIrIn}}_5$, these results suggest vortexlike excitations in a precursor state to unconventional superconductivity in ${\mathrm{CeIrIn}}_5$. This study sheds light on the similarity of heavy-fermion and cuprate superconductors and on the possibility of states not characterized by the amplitude of an order parameter.

We present a polarization-resolved and temperature-dependent Raman scattering study of $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($A=\mathrm{Ca}$, Eu). In the spin-density-wave phase, spectral weight redistribution is observed in the fully symmetric and nonsymmetric scattering channels at different energies. An anisotropic Raman response is observed in the fully symmetric channel in spontaneously detwinned ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ samples. We calculate the orbital-resolved electronic structures using a combination of density functional theory and dynamical mean field theory. We identify the electronic transitions corresponding to these two spectral features and find that the anisotropic Raman response originates from the lifted degeneracy of the $d_{xz/yz}$ orbitals in the broken-symmetry phase.

By measuring magnetoresistance and the Hall effect in a classically moderate perpendicular magnetic field in a Si-MOSFET-type macroscopic antidot array, we found a nonlinear with field, temperature- and density-dependent Hall resistivity. We argue that this nonlinearity originates from low mobility shells of the antidots with a strong temperature dependence of the resistivity and suggest a qualitative explanation of the phenomenon.

A numerical method, suitable for the simulation of the time evolution of quantum spin models of arbitrary lattice dimension, is presented. The method combines sampling of the Wigner function with evolution equations obtained from the Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY) hierarchy. Quantum correlations are treated through both the Wigner function sampling and the BBGKY evolution, bringing about highly accurate estimates of correlation functions. The method is particularly suitable for long-range interacting systems, and we demonstrate its power by comparing with exact results as well as other numerical methods. As an application we compute spin squeezing in a frustrated two-dimensional lattice with antiferromagnetic power-law interactions and a transverse field, making predictions pertinent to future ion trap experiments.

We extend the generalized Langevin equation (GLE) method [L. Stella, C. D. Lorenz, and L. Kantorovich, Phys. Rev. B 89, 134303 (2014)] to model a central classical region connected to two realistic thermal baths at two different temperatures. In such nonequilibrium conditions a heat flow is established, via the central system, in between the two baths. The GLE-2B (GLE two baths) scheme permits us to have a realistic description of both the dissipative central system and its surrounding baths. Following the original GLE approach, the extended Langevin dynamics scheme is modified to take into account two sets of auxiliary degrees of freedom corresponding to the mapping of the vibrational properties of each bath. These auxiliary variables are then used to solve the non-Markovian dissipative dynamics of the central region. The resulting algorithm is used to study a model of a short Al nanowire connected to two baths. The results of the simulations using the GLE-2B approach are compared to the results of other simulations that were carried out using standard thermostatting approaches (based on Markovian Langevin and Nosé-Hoover thermostats). We concentrate on the steady-state regime and study the establishment of a local temperature profile within the system. The conditions for obtaining a flat profile or a temperature gradient are examined in detail, in agreement with earlier studies. The results show that the GLE-2B approach is able to treat, within a single scheme, two widely different thermal transport regimes, i.e., ballistic systems, with no temperature gradient, and diffusive systems with a temperature gradient.

Guided elastic waves in plates, or Lamb waves, generally undergo reflection and mode conversion upon encountering a free edge. In the case where a backward-propagating Lamb wave is mode-converted to a forward-propagating wave or vice versa, the mode-converted wave is reflected on the same side of the surface normal as the incident wave. In this paper, we study such negative reflection and show that this effect can be achieved over a broad angular range at a simple plate edge. We demonstrate, through both numerical and experimental approaches, that a plate edge can act as a lens and focus a mode-converted Lamb wave field. Furthermore, we show that as the wave vectors of the incident and mode-converted Lamb waves approach each other, the mode-converted field nearly retraces the incident field. We propose that broad-angle negative reflection may find application in the nondestructive testing of structures supporting guided waves and in the development of new acoustic devices including resonators, lenses, and filters.

An electric current in the presence of spin-orbit coupling can generate a spin accumulation that exerts torques on a nearby magnetization. We demonstrate that, even in the absence of materials with strong bulk spin-orbit coupling, a torque can arise solely due to interfacial spin-orbit coupling, namely, Rashba-Eldestein effects at metal/insulator interfaces. In magnetically soft NiFe sandwiched between a weak spin-orbit metal (Ti) and insulator (${\mathrm{Al}}_2{\mathrm{O}}_3$), this torque appears as an effective field, which is significantly larger than the Oersted field and qualitatively modified by inserting an additional layer between NiFe and ${\mathrm{Al}}_2{\mathrm{O}}_3$. Our findings point to unconventional routes for tuning spin-orbit torques by engineering interfacial electric dipoles.

Due to the charge transfer effect at the contact of transition metal (TM) and $\mathrm{Mo}{\mathrm{S}}_2$, the use of ferromagnets in $\mathrm{Mo}{\mathrm{S}}_2$ based spin transistor is not suitable. On the other hand, $\beta \text{−}\mathrm{Ni}{\left(\mathrm{OH}\right)}_2$ is known to be a layered type material with antiparallel Ni spins in alternate layers. Here, an ultrathin layer of antiferromagnetic $\beta \text{−}\mathrm{Ni}{\left(\mathrm{OH}\right)}_2$ is grown on the $\mathrm{Mo}{\mathrm{S}}_2$ surface to achieve complete ferromagnetism with giant coercivity (2925 Oe). The origin of this ferromagnetic ordering is the reduction of Ni spin moments in $\mathrm{Ni}{\left(\mathrm{OH}\right)}_2$ layer adjacent to $\mathrm{Mo}{\mathrm{S}}_2$ surface due to charge transfer from S to Ni. The use of antiferromagnetic layered type material to achieve ferromagnetic ordering with giant coercivity is a new concept to realize perfect two-dimensional (2D) ferromagnets which have major advantages due to the huge change in coercivity with thickness.

Mn-Ga alloys close to the $\mathrm{M}{\mathrm{n}}_3\mathrm{Ga}$ stoichiometry can be synthesized in three different crystal modifications: hexagonal, tetragonal, and face-centered cubic, both in bulk and in thin-film forms. The magnetic ordering of these modifications is varying from noncollinear antiferromagnetic in the hexagonal case to ferrimagnetic order in the tetragonal one, whereas it is still unknown for the atomically disordered fcc structure. Here we study the onset of magnetic order at finite temperatures in these systems on a first-principles basis calculating the interatomic magnetic exchange interactions in the high-temperature paramagnetic regime. We employ the disordered local moment formalism and the magnetic force theorem within the framework of the local spin-density approximation and Monte Carlo simulations taking also the effects of atomic disorder in fcc alloys into account. In particular we find the origin of the stabilization of the noncollinear $3k$ structure in competition between antiferromagnetic inter- and in-plane couplings of frustrated kagome planes in hexagonal $\mathrm{M}{\mathrm{n}}_3\mathrm{Ga}$ and predict the antiferromagnetic-1 collinear order due to frustration in fcc alloys. Special attention is paid to the effects of the off-stoichiometry and the consequences of atomic disorder. We calculate the site-preference energy of Ga antisite atoms in the tetragonal structures in the range of the compositions from $\mathrm{M}{\mathrm{n}}_3\mathrm{Ga}$ to $\mathrm{M}{\mathrm{n}}_2\mathrm{Ga}$ and slightly beyond and confirm the earlier explanation of the effect of magnetization increase due to Ga preferentially occupying one of the Mn sites.

In a recent paper [Phys. Rev. B 90, 125150 (2014)], we showed that the random phase approximation with exchange (RPAx) gives accurate total energies for a diverse set of systems including the high and low density regime of the homogeneous electron gas, the ${\mathrm{N}}_2$ molecule, and the ${\mathrm{H}}_2$ molecule at dissociation. In this paper, we present results for the van der Waals bonded ${\mathrm{Ar}}_2$ and ${\mathrm{Kr}}_2$ dimers and demonstrate that the RPAx gives superior dispersion forces as compared to the RPA. We then show that this improved description is crucial for the bond formation of the ${\mathrm{Mg}}_2$ molecule. In addition, the RPAx performs better for the ${\mathrm{Be}}_2$ dissociation curve at large nuclear separation but, similar to the RPA, fails around equilibrium due to the build up of a large repulsion hump. For the strongly correlated LiH molecule at dissociation we have also calculated the RPAx potential and find that the correlation peak at the bond midpoint is overestimated as compared to the RPA and the exact result. The step feature is missing and hence the delocalization error is comparable to the RPA. This is further illustrated by a smooth energy versus fractional charge curve and a poor description of the LiH dipole moment at dissociation.

We theoretically discuss a momentum-space analog of the quantum Hall effect, which could be observed in topologically nontrivial lattice models subject to an external harmonic trapping potential. In our proposal, the Niu-Thouless-Wu formulation of the quantum Hall effect on a torus is realized in the toroidally shaped Brillouin zone. In this analogy, the position of the trap center in real space controls the magnetic fluxes that are inserted through the holes of the torus in momentum space. We illustrate the momentum-space quantum Hall effect with the noninteracting trapped Harper-Hofstadter model, for which we numerically demonstrate how this effect manifests itself in experimental observables. Extension to the interacting trapped Harper-Hofstadter model is also briefly considered. We finally discuss possible experimental platforms where our proposal for the momentum-space quantum Hall effect could be realized.

The surface structures of $\mathrm{SrTi}{\mathrm{O}}_3$ (100) single crystals were examined as a function of annealing time and temperature in either oxygen atmosphere or ultrahigh vacuum (UHV) using noncontact atomic force microscopy (NC-AFM), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). Samples were subsequently analyzed for the effect the modulation of their charge distribution had on their surface potential. It was found that the evolution of the $\mathrm{SrTi}{\mathrm{O}}_3$ surface roughness, stoichiometry, and reconstruction depends on the preparation scheme. LEED revealed phase transitions from a $\left(1\times 1\right)$ termination to an intermediate $c(4\times 2)$ reconstruction to ultimately a $(\surd 13\times \surd 13)-R33.7^{\circ }$ surface phase when the surface was annealed in an oxygen flux, while the reverse transition from $(\surd 13\times \surd 13)-R33.7^{\circ }$ to $c(4\times 2)$ was observed when samples were annealed in UHV. When the surface reverted to $c(4\times 2)$, AES data indicated decreases in both the surface Ti and O concentrations. These findings were corroborated by NC-AFM imaging, where initially $\mathrm{Ti}{\mathrm{O}}_2$-terminated crystals developed half-unit cell high steps following UHV annealing, which is typically attributed to a mix of SrO and $\mathrm{Ti}{\mathrm{O}}_2$ terminations. Surface roughness evolved nonmonotonically with UHV annealing temperature, which is explained by electrostatic modulations of the surface potential caused by increasing oxygen depletion. This was further corroborated by experiments in which the apparent roughness tracked in NC-AFM could be correlated with changes in the surface charge distribution that were controlled by applying a bias voltage to the sample. Based on these findings, it is suggested that careful selection of preparation procedures combined with application of an electric field may be used to tune the properties of thin films grown on $\mathrm{SrTi}{\mathrm{O}}_3$.

We present a theory of quantum tunneling between two-dimensional (2D) layers with Rashba and Dresselhaus spin-orbit interaction (SOI) in the layers. Accounting for SOI in the layers leads to a complex pattern in the tunneling characteristic with typical features corresponding to SOI energy. The resonant features strongly depend on the SOI parameters; for clear experimental observation the SOI characteristic energy should exceed the resonant broadening related to the particles' quantum lifetime in the layers. It appears that the experiments on hole tunneling are favorable to meet this criterion. We also consider a promising candidate for observing the effect, that is, $p$-doped SiGe strained heterostructures. As supported by our calculations, small adjustments of the parameters for experimentally studied AlGaAs/GaAs $p$-type quantum walls or designing a 2D-2D tunneling experiment for recently fabricated SiGe structures are very likely to reveal the SOI features in the 2D-2D tunneling.

The virial theorem for a system of interacting electrons in a crystal, which is described within the framework of the tight-binding model, is derived. We show that, in the particular case of interacting massless electrons in graphene and other Dirac materials, the conventional virial theorem is violated. Starting from the tight-binding model, we derive the generalized virial theorem for Dirac electron systems, which contains an additional term associated with a momentum cutoff at the bottom of the energy band. Additionally, we derive the generalized virial theorem within the Dirac model using the minimization of the variational energy. The obtained theorem is illustrated by many-body calculations of the ground-state energy of an electron gas in graphene carried out in Hartree-Fock and self-consistent random-phase approximations. Experimental verification of the theorem in the case of graphene is discussed.