We study the response to an applied flux of an interacting system of Dirac-Weyl fermions confined in a one-dimensional (1D) ring. Combining analytical calculations with density-matrix renormalization group results, we show that tuning of the interactions leads to a unique many-body system that displays either a suppression or an enhancement of the Drude weight—the zero-frequency peak in the ac conductivity—with respect to the noninteracting value. An asymmetry in the interaction strength between same- and different-pseudospin Dirac-Weyl fermions leads to Drude weight enhancement. Vice versa, symmetric interactions lead to Drude weight suppression. Our predictions can be tested in mixtures of ultracold fermions in 1D ring traps.

A spontaneous photocurrent, termed a shift current, can flow in noncentrosymmetric bulk crystals due to the topological nature of the constituting electronic bands. A shift current with a less dissipative character may have remarkable advantages over the conventional drift photocurrent driven by a built-in potential or external electric field. We revisit the generation and transport of the shift current in a prototypical ferroelectric semiconductor SbSI near its band-gap energy. It is revealed that a switchable shift current is steadily generated by photoexcitation down to low temperatures, appears over a distance of millimeter range in a highly insulating bulk without noticeable decay, and largely exceeds the polarization charge in the sample, reflecting its Berry phase origin.

A simple model for the transmission of pairs of electrons through a weak electric link in the form of a nanowire made of a material with strong electron spin-orbit interaction (SOI) is presented, with emphasis on the effects of Coulomb interactions and the Pauli exclusion principle. The constraints due to the Pauli principle are shown to “quench” the coherent SOI-induced precession of the spins when the spatial wave packets of the two electrons overlap significantly. The quenching, which results from the projection of the pair's spin states onto spin-up and spin-down states on the link, breaks up the coherent propagation in the link into a sequence of coherent hops that add incoherently. Applying the model to the transmission of Cooper pairs between two superconductors, we find that in spite of Pauli quenching, the Josephson current oscillates with the strength of the SOI, but may even change its sign (compared to the limit of the Coulomb blockade, when the quenching is absent). Conditions for an experimental detection of these features are discussed.

We study the Hubbard model on the AB-stacked bilayer honeycomb lattice with a repulsive on-site interaction $U$ in second-order perturbation theory and in self-consistent random phase approximation. We determine the changes in the antiferromagnetic magnetic ordering tendencies due to the real and imaginary parts of the self-energy at the band crossing points. In particular, we present an estimate for the threshold value $U^{*}$ below which the magnetic order is endangered by the splitting of the quadratic band touching points into four Dirac points by an interaction-induced interlayer skew hopping. For most of the parameter space, however, the quasiparticle degradation by the frequency-dependence of the sublattice-diagonal self-energies and the Dirac-cone steepening are more essential for suppressing the AF ordering tendencies considerably. Our results might help to understand the energy scales obtained in renormalization group treatments of the same model and shed light on recent quantum Monte Carlo investigations about the fate of the magnetic ordering down to lower $U$.

We consider the interacting, spin-conserving, extended Kane-Mele-Hubbard model, and we rigorously establish the exact quantization of the edge spin conductance and the validity of the helical Luttinger liquid relations for Drude weights and susceptibilities. Our analysis takes fully into account lattice effects, typically neglected in the helical Luttinger model approximation, which play an essential role for universality. The analysis is based on exact renormalization-group methods and on a combination of lattice and emergent Ward identities, which enable the emergent chiral anomaly to be related with the finite renormalizations due to lattice corrections.

Ferroelectrics have attracted a great deal of attention, but diatomic ferroelectrics are less common. Here we establish a class of diatomic ferroelectrics in the distorted NiAs-type structure based on state-of-the-art first-principles calculations. These compounds, with giant Rashba effect, possess rich phases, ranging from ferroelectric semiconductors to ferroelectric semimetals. Topological surface states and type-II bulk Dirac fermions are found in the undistorted phases. This class of multifunctional materials has potential applications in spin-orbitronics.

In the spectral density of Cr(001) surfaces, a sharp resonance close to the Fermi level is observed in both experiment and theory. For the physical origin of this peak, two mechanisms were proposed: a single-particle $d_{z^2}$ surface state renormalized by electron-phonon coupling and an orbital Kondo effect due to the degenerate $d_{xz}$/$d_{yz}$ states. Despite several experimental and theoretical investigations, the origin is still under debate. In this work, we address this problem by two different approaches of the dynamical mean-field theory: first, by the spin-polarized $T$-matrix fluctuation exchange approximation suitable for weakly and moderately correlated systems; second, by the noncrossing approximation derived in the limit of weak hybridization (i.e., for strongly correlated systems) capturing Kondo-type processes. By using recent continuous-time quantum Monte Carlo calculations as a benchmark, we find that the high-energy features, everything except the resonance, of the spectrum are captured within the spin-polarized $T$-matrix fluctuation exchange approximation. More precisely, the particle-particle processes provide the main contribution. For the noncrossing approximation, it appears that spin-polarized calculations suffer from spurious behavior at the Fermi level. Then, we turned to non-spin-polarized calculations to avoid this unphysical behavior. By employing two plausible starting hybridization functions, it is observed that the characteristics of the resonance are crucially dependent on the starting point. It appears that only one of these starting hybridizations could result in an orbital Kondo resonance in the presence of a strong magnetic field like in the Cr(001) surface. It is for a future investigation to first resolve the unphysical behavior within the spin-polarized noncrossing approximation and then check for an orbital Kondo resonance.

In this paper, we present the magnetotransport and magnetization properties of LaSbTe single crystals. Magnetic-field-induced turn-on behavior and low-temperature resistivity plateau have been observed. By adopting both a metal-semiconductor crossover and Kohler scaling analysis, we discuss the possible origin of the temperature and magnetic field dependence of resistivity. At 5 K and 9 T, a large, nonsaturating transverse magnetoresistance (MR) ∼$5\times {10}^3$% is been obtained. The MR shows considerable anisotropy when the magnetic field is applied along different crystallographic directions. The nonlinear field dependence of the Hall resistivity confirms the presence of two types of charge carriers. From the semiclassical two-band fitting of Hall conductivity and longitudinal conductivity, very high carrier mobilities and almost equal electron and hole densities are deduced, which result in large MR. The Fermi surface properties are analyzed from de Haas–van Alphen oscillation. From the magnetization measurement, the signature of the nontrivial surface state is detected, which confirms that LaSbTe is a topological insulator, consistent with the earlier first-principles calculations.

Pump-probe experiments have turned out as a powerful tool in order to study the dynamics of competing orders in a large variety of materials. The corresponding analysis of the data often relies on standard linear-response theory generalized to nonequilibrium situations. Here we examine the validity of such an approach for the charge and pairing response of systems with charge-density wave and (or) superconducting (SC) order. Our investigations are based on the attractive Hubbard model which we study within the time-dependent Hartree-Fock approximation. In particular, we calculate the quench and pump-probe dynamics for SC and charge order parameters in order to analyze the frequency spectra and the coupling of the probe field to the specific excitations. Our calculations reveal that the “linear-response assumption” is justified for small to moderate nonequilibrium situations (i.e., pump pulses) in the case of a purely charge-ordered ground state. However, the pump-probe dynamics on top of a superconducting ground state is determined by phase and amplitude modes which get coupled far from the equilibrium state indicating the failure of the linear-response assumption.

We propose an effective method to accurately determine the defect formation energy $E_{\mathrm{f}}$ and charge transition level $\varepsilon$ of the point defects using exclusively cohesive energy $E_{\mathrm{coh}}$ and the fundamental band gap $E_{\mathrm{g}}$ of pristine host materials. We find that $E_{\mathrm{f}}$ of the point defects can be effectively separated into geometric and electronic contributions with a functional form: $E_{\mathrm{f}}=\chi E_{\mathrm{coh}}+\lambda E_{\mathrm{g}}$, where $\chi$ and $\lambda$ are dictated by the geometric and electronic factors of the point defects ($\chi$ and $\lambda$ are defect dependent). Such a linear combination of $E_{\mathrm{coh}}$ and $E_{\mathrm{g}}$ reproduces $E_{\mathrm{f}}$ with an accuracy better than 5% for electronic structure methods ranging from hybrid density-functional theory (DFT) to many-body random-phase approximation (RPA) and experiments. Accordingly, $\varepsilon$ is also determined by $E_{\mathrm{coh}}/E_{\mathrm{g}}$ and the defect geometric/electronic factors. The identified correlation is rather general for monovacancies and interstitials, which holds in a wide variety of semiconductors covering Si, Ge, phosphorenes, ZnO, GaAs, and InP, and enables one to obtain reliable values of $E_{\mathrm{f}}$ and $\varepsilon$ of the point defects for RPA and experiments based on semilocal DFT calculations.

We study spin generation in lateral spin-valve structures consisting of ferromagnetic ${\mathrm{Fe}}_3\mathrm{Si}$ contacts on $n$-type GaAs transport channels. Different film-growth conditions are found to enable the comparison of ferromagnetic contacts with similar spin polarizations but different electrical characteristics. Strongly rectifying contacts are found to hinder the observation of spin-injection signals due to a strong bias dependence of the spin-generation efficiency. For contacts fabricated under optimized growth conditions, efficient spin generation enables the operation of nonlocal and local spin valves as well as the detection of spin signals in the three-terminal configuration. In all configurations, the observed signals exhibit a consistent bias-dependent spin generation and detection behavior which is explained by spin accumulation in the GaAs conduction band without the involvement of localized states.

Tamm plasmons are confined optical states at the interface of a metal and a dielectric Bragg mirror. Unlike conventional surface plasmons, Tamm plasmons may be directly excited by an external light source in both TE and TM polarizations. Here we consider the one-dimensional propagation of Tamm plasmons under long and narrow metallic stripes deposited on top of a semiconductor Bragg mirror. The spatial confinement of the field imposed by the stripe and its impact on the structure and energy of Tamm modes are investigated. We show that the Tamm modes are coupled to surface plasmons arising at the stripe edges. These plasmons form an interference pattern close to the bottom surface of the stripe that involves modification of both the energy and loss rate for the Tamm mode. This phenomenon is pronounced only in the case of TE polarization of the Tamm mode. These findings pave the way to application of laterally confined Tamm plasmons in optical integrated circuits as well as to engineering potential traps for both Tamm modes and hybrid modes of Tamm plasmons and exciton polaritons with meV depth.

The search for long electron phase-coherence length, which is the length that an electron can keep its quantum wavelike properties, has attracted considerable interest in the last several decades. Here, we report the long phase-coherence length of ∼3.7 μm in La-doped CdO thin films at 2 K. Systematical investigations of the La doping and the temperature dependences of the electron mobility and the electron phase-coherence length reveal contrasting scattering mechanisms for these two physical properties. Furthermore, these results show that the oxygen vacancies could be the dominant scatters in CdO thin films that break the electron phase coherence, which would shed light on further investigation of phase-coherence properties in oxide materials.

We discuss the quasiparticle entropy and heat capacity of a dirty superconductor/normal metal/superconductor junction. In the case of short junctions, the inverse proximity effect extending in the superconducting banks plays a crucial role in determining the thermodynamic quantities. In this case, commonly used approximations can violate thermodynamic relations between supercurrent and quasiparticle entropy. We provide analytical and numerical results as a function of different geometrical parameters. Quantitative estimates for the heat capacity can be relevant for the design of caloritronic devices or radiation sensor applications.

Two-dimensional electron gases (2DEG) at oxide interfaces, such as $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ (001), have aroused significant interest due to their high carrier density $\left(\sim {10}^{14}\mathrm{c}{\mathrm{m}}^{-2}\right)$ and strong lateral confinement (∼1 nm). However, these 2DEGs are normally hosted by the weakly dispersive and degenerate $d$ bands (e.g., $\mathrm{Ti}\text{−}3d$ bands), which are strongly coupled to the lattice, causing mobility of such 2DEGs to be relatively low at room temperature $\left(\sim 1\mathrm{c}{\mathrm{m}}^2/\mathrm{Vs}\right)$. Here, we propose using oxide host materials with the conduction bands formed from $s$ electrons to increase carrier mobility and soften its temperature dependence. Using first-principles density functional theory calculations, we investigate $\mathrm{LaSc}{\mathrm{O}}_3/\mathrm{BaSn}{\mathrm{O}}_3$ (001) heterostructure and as a model system, where the conduction band hosts the $s$-like carriers. We find that the polar discontinuity at this interface leads to electronic reconstruction resulting in the formation of the 2DEG at this interface. The conduction electrons reside in the highly dispersive $\mathrm{Sn}\text{−}5s$ bands, which have a large band width and a low effective mass. The predicted 2DEG is expected to be highly mobile even at room temperature due to the reduced electron-phonon scattering via the inter-band scattering channel. A qualitatively similar behavior is predicted for a doped $\mathrm{BaSn}{\mathrm{O}}_3$, where a monolayer of BaO is replaced with LaO. We anticipate that the quantum phenomena associated with these 2DEGs to be more pronounced owing to the high mobility of the carriers.

This paper reveals how the electronic structure, magnetic structure, and topological phase of two-dimensional (2D), single-layer structures of bismuth are modified by point defects. We first showed that a free-standing, single-layer, hexagonal structure of bismuth, named h-bismuthene, exhibits nontrivial band topology. We then investigated interactions between single foreign adatoms and bismuthene structures, which comprise stability, bonding, electronic structure, and magnetic structures. Localized states in diverse locations of the band gap and resonant states in band continua of bismuthene are induced upon the adsorption of different adatoms, which modify electronic and magnetic properties. Specific adatoms result in reconstruction around the adsorption site. Single vacancies and divacancies can form readily in bismuthene structures and remain stable at high temperatures. Through rebondings, Stone-Whales-type defects are constructed by divacancies, which transform into a large hole at high temperature. Like adsorbed adatoms, vacancies induce also localized gap states, which can be eliminated through rebondings in divacancies. We also showed that not only the optical and magnetic properties, but also the topological features of pristine h-bismuthene can be modified by point defects. The modification of the topological features depends on the energies of localized states and also on the strength of coupling between point defects.

We present temperature-dependent inelastic-neutron-scattering measurements, accompanied by ab initio calculations of the phonon spectra and elastic properties as a function of pressure to quantitatively explain an unusual combination of negative thermal expansion and negative linear compressibility behavior of ${\mathrm{ZnAu}}_2{\left(\mathrm{CN}\right)}_4$. The mechanism of the negative thermal expansion is identified in terms of specific anharmonic phonon modes that involve bending of the -Zn-NC-Au-CN-Zn- linkage. The soft phonon at the $L$ point at the Brillouin zone boundary quantitatively relates to the high-pressure phase transition at about 2 GPa. The ambient pressure structure is also found to be close to an elastic instability that leads to a weakly first-order transition.

The topology and creation energy of domain walls in different magnetic chains (called Single-Chain Magnets or SCMs) are discussed. As these domain walls, that can be seen as “defects”, are known to control both static and dynamic properties of these one-dimensional systems, their study and understanding are necessary first steps before a deeper discussion of the SCM properties at finite temperature. The starting point of the paper is the simple regular ferromagnetic chain for which the characteristics of the domain walls are well known. Then two cases will be discussed (i) the “mixed chains” in which isotropic and anisotropic classical spins alternate, and (ii) the so-called “canted chains” where two different easy axis directions are present. In particular, we show that “strictly narrow” domain walls no longer exist in these more complex cases, while a cascade of phase transitions is found for canted chains as the canting angle approaches ${45}^{\circ }$. The consequence for thermodynamic properties is briefly discussed in the last part of the paper.

We demonstrate the potential for manipulating the ultrafast dynamics of thermomagnetic flux avalanches (TMA) in superconducting films with soft magnetic stripes deposited on the film. By tuning the in-plane magnetization of the stripes, we induce lines of strong magnetic potentials for Abrikosov vortices, resulting in guided slow motion of vortices along the stripe edges and preferential bursts of TMA along the stripes. Furthermore, we show that transversely polarized stripes can reduce the TMA size by diverting magnetic flux away from the major trunk of the TMA into interstripe gaps. Our data indicate that TMAs are launched from locations with enhanced vortex entry barrier, where flux accumulation followed by accelerated vortex discharge significantly reduces the threshold of the applied field ramping speed required for the creation of TMAs. Finally, vortex-antivortex annihilation at the moving front of an expanding TMA can account for the enhanced TMA activity in the receding branches of the sample's magnetization cycle and the preferred propagation of TMAs into maximum trapped flux regions.