We use angle-resolved photoemission spectroscopy to extract the band dispersion and the Fermi surface of ${\mathrm{BaCu}}_2{\mathrm{As}}_2$ and $\alpha -{\mathrm{BaCu}}_2{\mathrm{Sb}}_2$. While the Cu $3d$ bands in both materials are located around 3.5 eV below the Fermi level, the low-energy photoemission intensity mainly comes from As $4p$ states, suggesting a completely filled Cu $3d$ shell. The splitting of the As $3d$ core levels and the lack of pronounced three-dimensionality in the measured band structure of ${\mathrm{BaCu}}_2{\mathrm{As}}_2$ indicate a surface state likely induced by the cleavage of this material in the collapsed tetragonal phase, which is consistent with our observation of a ${\mathrm{Cu}}^{+1}$ oxidation state. However, the observation of Cu states at similar energy in $\alpha -{\mathrm{BaCu}}_2{\mathrm{Sb}}_2$ without the pnictide-pnictide interlayer bonding characteristic of the collapsed tetragonal phase suggests that the short interlayer distance in ${\mathrm{BaCu}}_2{\mathrm{As}}_2$ follows from the stability of the ${\mathrm{Cu}}^{+1}$ rather than the other way around. Our results confirm the prediction that ${\mathrm{BaCu}}_2{\mathrm{As}}_2$ is an $sp$ metal with weak electronic correlations.

We make extensive simulations over a spin chain model that combines the frustrated $J_1-J_2$ spin chain and the long-range nonfrustrated ${(-1)}^{(r-1)}{r}^{-\alpha }$ decay interactions through the variational matrix product state method for both finite and infinite lengths. We study both the ground-state entanglement and phase diagram. We find that it is most entangled in the rotation invariant long-range ordered antiferromagnetic phase, where the entanglement scales approximately logarithmically. We determine the development of the Majudar-Ghosh point to a disorder line from entanglement. And we approximately determine the transition from the dimerized and incommensurate phase of the $J_1-J_2$ model to a decoupled phase by studying spin correlation and the dimerization order parameter. Some implications for entanglement in systems with long-range interactions are stated.

The search for inversion-asymmetric topological insulators (IATIs) persists as an effect for realizing new topological phenomena. However, so far only a few IATIs have been discovered and there is no IATI exhibiting a large band gap exceeding 0.6 eV. Using first-principles calculations, we predict a series of new IATIs in saturated Group III-Bi bilayers. We show that all these IATIs preserve extraordinary large bulk band gaps, which are well above room temperature, allowing for viable applications in room-temperature spintronic devices. More importantly, most of these systems display large bulk band gaps that far exceed 0.6 eV and, part of them even are up to ∼1 eV, which are larger than any IATIs ever reported. The nontrivial topological situation in these systems is confirmed by the identified band inversion of the band structures, $Z_2$ topological invariants, and an explicit demonstration of the topological edge states. Owning to their asymmetric structures, remarkable Rashba spin splitting is produced in both the valence and conduction bands of these systems. These predictions strongly revive these new systems as excellent candidates for IATI-based novel applications.

The response of two-dimensional electron gas to a temperature gradient in perpendicular magnetic field under steady-state microwave irradiation is studied theoretically. The electric currents induced by the temperature gradient and the thermopower coefficients are calculated taking into account both diffusive and phonon-drag mechanisms. The modification of thermopower by microwaves takes place because of Landau quantization of the electron energy spectrum and is governed by the microscopic mechanisms which are similar to those responsible for microwave-induced oscillations of electrical resistivity. The magnetic-field dependence of microwave-induced corrections to phonon-drag thermopower is determined by mixing of phonon resonance frequencies with radiation frequency, which leads to interference oscillations. The transverse thermopower is modified by microwave irradiation much stronger than the longitudinal one. Apart from showing prominent microwave-induced oscillations as a function of magnetic field, the transverse thermopower appears to be highly sensitive to the direction of linear polarization of microwave radiation.

We show theoretically that the strong coupling of circularly polarized photons to an exciton in ringlike semiconductor nanostructures results in physical nonequivalence of clockwise and counterclockwise exciton rotations in the ring. As a consequence, the stationary energy splitting of exciton states corresponding to these mutually opposite rotations appears. This excitonic Aharonov-Bohm effect depends on the intensity and frequency of the circularly polarized field and can be detected in state-of-the-art optical experiments.

We consider a one-dimensional, time-reversal-invariant system with attractive interactions and spin-orbit coupling. Such a system is gapless due to the strong quantum fluctuations of the superconducting order parameter. However, we show that a sharply defined topological phase with protected, exponentially localized edge states exists. If one of the spin components is conserved, the protection of the edge modes can be understood as a consequence of the presence of a spin gap. In the more general case, the localization of the edge states arises from a gap to single-particle excitations in the bulk. We consider specific microscopic models and demonstrate both analytically and numerically (using density matrix renormalization group calculations) that they can support the topologically nontrivial phase.

In a traditional classical force-field model, the atomic point charge that generates the electrostatic potential, and the Born charge induced by atomic movement, are represented by the same charge parameter. But their actual values can be very different, and correct values for both of them are needed in order to yield the correct atomic structure (electrostatic charge) and phonon spectrum (Born charge). This is particularly true for nanostructure calculations. Here, we introduce a double-charge model (DCM) to reconcile the difference between the electrostatic charge and Born charge. The DCM allows us to reproduce the accurate ab initio phonon spectrum not only in bulk systems, but also for nanostructures (slabs and nanowires). This enables the use of classical force fields to study phonon spectra of large nanostructures, which are important for many phenomena from carrier dynamics to thermo conductivities.

The effects of voltage bias on magnetic hysteresis in single Ni particles 2 to 3 nm in diameter are measured between temperatures of 60 mK and 4.2 K by using sequential electron tunneling through the particle. While some Ni particles do not display magnetic hysteresis in tunneling current versus magnetic field, in the Ni particles that display hysteresis, the effect of bias voltage on magnetic switching field is nonlinear. The magnetic switching field changes weakly in the voltage interval $\sim 1$ mV above the tunneling onset voltage, and rapidly decreases versus voltage above that interval. A voltage-driven mechanism explaining this nonlinear suppression of magnetic hysteresis is presented, where the key effect is a magnetization blockade due to the addition of spin-orbit anisotropy ${\epsilon }_{\text{so}}$ to the particle by a single electron. A necessary condition for the particle to exhibit magnetization blockade is that ${\epsilon }_{\text{so}}$ increases when the magnetization is slightly displaced from the easy axis. In that case, an electron will be energetically unable to access the particle if the magnetization is sufficiently displaced from the easy axis, which leads to a voltage interval where magnetic hysteresis is possible that is comparable to ${\epsilon }_{\text{so}}/e$, where $e$ is the electronic charge. If ${\epsilon }_{\text{so}}$ decreases vs magnetization displacement from the easy axis, there is no magnetization blockade and no hysteresis.

We find that a new type of nonreciprocal modes exists at an interface between two parity-time- ($\mathcal{PT}$-) symmetric magnetic domains (MDs) near the frequency of zero effective permeability. This mode is nonpropagating and purely magnetic when the two MDs are semi-infinite, while it becomes propagating in the finite case. In particular, two pronounced nonreciprocal responses could be observed via the excitation of this mode: one-way optical tunneling for oblique incidence and unidirectional beam shift at normal incidence. When the two MDs system becomes finite in size, it is found that perfect-transmission mode could be achieved if $\mathcal{PT}$ symmetry is maintained. The unique properties of such an unusual mode are investigated by analytical modal calculation as well as numerical simulations. The results suggest a different approach to the design of compact optical isolator.

We report the observation of cooperative paramagnetic fluctuations of ${\mathrm{Ru}}^{4+}$ spins that coexist with the non-Fermi-liquid state in ${\mathrm{CaRuO}}_3$ perovskite below $T\simeq 21$ K. Detailed electrical, magnetic, and neutron scattering measurements reveal that the ${\mathrm{Ru}}^{4+}$ ions reside in magnetic-field-independent random domains with dynamic properties that are reminiscent of the cooperative paramagnetic fluctuations. The linear ($E/T$) scaling of the dynamic susceptibilities and divergence of the mean relaxation time as $T\to 0$ K suggest a quasilocal critical nature of the spin fluctuations. We argue that the non-Fermi-liquid behavior arises due to the quantum critical nature of the cooperative paramagnetic fluctuations in ${\mathrm{CaRuO}}_3$.

The role of the multiorbital effects on the emergence of frustrated electronic orders on the triangular lattice at half filling is investigated through an extended spinless fermion Hubbard model. By using two complementary approaches, unrestricted Hartree-Fock and exact diagonalizations, we unravel a very rich phase diagram controlled by the strength of both local and off-site Coulomb interactions and by the interorbital hopping anisotropy ratio $t^{\prime }/t$. Three robust unconventional electronic phases, a pinball liquid, an inverse pinball liquid, and a large-unit-cell $\sqrt{12}\times \sqrt{12}$ droplet phase, are found to be generic in the triangular geometry, being controlled by the band structure parameters. The latter are also stabilized in the isotropic limit of our microscopic model, which recovers the standard SU(2) spinful extended single-band Hubbard model.

We report temperature-dependent transport and x-ray diffraction measurements of the influence of Ti hole doping on the charge density wave (CDW) in $1T\text{−}{\mathrm{Ta}}_{1-x}{\mathrm{Ti}}_x{\mathrm{S}}_2$. Confirming past studies, we find that even trace impurities eliminate the low-temperature commensurate (C) phase in this system. Surprisingly, the magnitude of the in-plane component of the CDW wave vector in the nearly commensurate (NC) phase does not change significantly with Ti concentration, as might be expected from a changing Fermi surface volume. Instead, the angle of the CDW in the basal plane rotates, from $11.9{}^{\circ }$ at $x=0$ to $16.4{}^{\circ }$ at $x=0.12$. Ti substitution also leads to an extended region of coexistence between incommensurate (IC) and NC phases, indicating heterogeneous nucleation near the transition. Finally, we explain a resistive anomaly originally observed by Di Salvo [F. J. Di Salvo et al., Phys. Rev. B 12, 2220 (1975)] as arising from pinning of the CDW on the crystal lattice. Our study highlights the importance of commensuration effects in the NC phase, particularly at $x\sim 0.08$.

Charge carrier concentrations in acceptor-doped proton-conducting perovskites are to a large extent determined by the hydration and oxidation of oxygen vacancies, which introduce protons and holes, respectively. First-principles modeling of these reactions involves calculation of formation energies of charged defects, which requires an accurate description of the band gap and the position of the band edges. Since density-functional theory (DFT) with local and semilocal exchange-correlation functionals (LDA and GGA) systematically fails to predict these quantities this can have serious implications on the modeling of defect reactions. In this study we investigate how the description of band gap and band-edge positions affects the hydration and oxidation in acceptor-doped ${\text{BaZrO}}_3$. First-principles calculations are performed in combination with thermodynamic modeling in order to obtain equilibrium charge carrier concentrations at different temperatures and partial pressures. Three different methods have been considered: DFT with both semilocal (PBE) and hybrid (PBE0) exchange-correlation functionals, and many-body perturbation theory within the $G_0{W}_0$ approximation. All three methods yield similar results for the hydration reaction, which are consistent with experimental findings. For the oxidation reaction, on the other hand, there is a qualitative difference. PBE predicts the reaction to be exothermic, while the two others predict an endothermic behavior. Results from thermodynamic modeling are compared with available experimental data, such as enthalpies, concentrations, and conductivities, and only the results obtained with PBE0 and $G_0{W}_0$, with an endothermic oxidation behavior, give a satisfactory agreement with experiments.

We report on the topological Hall effect (THE) in centrosymmetric EuO thin films. This THE signal persists down to the lowest temperature in the metallic region below 50 K for the films thinner than 200 nm. The signal rapidly disappears by tilting the applied magnetic field from surface normal, suggestive of noncoplanar spin configuration such as two-dimensional skyrmions. This observation possibly substantiates the theoretical proposal of magnetic skyrmions in 2D Heisenberg ferromagnets in marked contrast to better established $B20\text{−}\mathrm{type}$ chiral helimagnets.

We have studied the body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close-packed (hcp) phases of Fe alloyed with 25 at.% of Ni at Earth's core conditions using an ab initio local density approximation + dynamical mean-field theory approach. The alloys have been modeled by ordered crystal structures based on the bcc, fcc, and hcp unit cells with the minimum possible cell size allowing for the proper composition. Our calculations demonstrate that the strength of electronic correlations on the Fe $3d$ shell is highly sensitive to the phase and local environment. In the bcc phase, the $3d$ electrons at the Fe site with Fe only nearest neighbors remain rather strongly correlated, even at extreme pressure-temperature conditions, with the local and uniform magnetic susceptibility exhibiting a Curie-Weiss-like temperature evolution and the quasiparticle lifetime Γ featuring a non-Fermi-liquid temperature dependence. In contrast, for the corresponding Fe site in the hcp phase, we predict a weakly correlated Fermi-liquid state with a temperature-independent local susceptibility and a quadratic temperature dependence of Γ. The iron sites with nickel atoms in the local environment exhibit behavior in the range between those two extreme cases, with the strength of correlations gradually increasing along the hcp-fcc-bcc sequence. Further, the intersite magnetic interactions in the bcc and hcp phases are also strongly affected by the presence of Ni nearest neighbors. The sensitivity to the local environment is related to modifications of the Fe partial density of states due to mixing with Ni $3d$ states.

We investigate the time evolution of optical conductivity in the half-filled one-dimensional extended Hubbard model driven by a transient laser pulse, by using the time-dependent Lanczos method. Photoinduced in-gap excitations exhibit a qualitatively different structure in the spin-density wave (SDW) in comparison to the charge-density-wave (CDW) phase. In the SDW, the origin of a low-energy in-gap excitation is attributed to the even-odd parity of the photoexcited states, while in the CDW an in-gap state is due to confined photogenerated carriers. The signature of the in-gap excitations can be identified as a characteristic oscillation in the time evolution of physical quantities.