We use a second-order rotational invariant Green's-function method (RGM) and the high-temperature expansion (HTE) to calculate the thermodynamic properties of the kagome-lattice spin-$S$ Heisenberg antiferromagnet with nearest-neighbor exchange $J$. While the HTE yields accurate results down to temperatures of about $T/S(S+1)\sim J$, the RGM provides data for arbitrary $T\ge 0$. For the ground state we use the RGM data to analyze the $S$ dependence of the excitation spectrum, the excitation velocity, the uniform susceptibility, the spin-spin correlation functions, the correlation length, and the structure factor. We found that the so-called $\sqrt{3}\times \sqrt{3}$ ordering is more pronounced than the $q=0$ ordering for all values of $S$. In the extreme quantum case $S=1/2$, the zero-temperature correlation length is only of the order of the nearest-neighbor separation. Then we study the temperature dependence of several physical quantities for spin quantum numbers $S=1/2,1,\cdots ,7/2$. As $S$ increases, the typical maximum in the specific heat and that in the uniform susceptibility are shifted toward lower values of $T/S(S+1)$, and the height of the maximum is growing. The structure factor $\mathcal{S}\left(\mathbf{q}\right)$ exhibits two maxima at magnetic wave vectors $\mathbf{q}={\mathbf{Q}}_i,i=0,1,$ corresponding to the $q=0$ and $\sqrt{3}\times \sqrt{3}$ state. We find that the $\sqrt{3}\times \sqrt{3}$ short-range order is more pronounced than the $q=0$ short-range order for all temperatures $T\ge 0$. For the spin-spin correlation functions, the correlation lengths, and the structure factors, we find a finite low-temperature region $0\le T<T^{*}\approx a/S(S+1), a\approx 0.2$, where these quantities are almost independent of $T$.

Superconducting gap structure was probed in type-II Dirac semimetal ${\mathrm{PdTe}}_2$ by measuring the London penetration depth using the tunnel diode resonator technique. At low temperatures, the data for two samples are well described by a weak-coupling exponential fit yielding $\lambda (T=0)=230$ nm as the only fit parameter at a fixed $\mathrm{\Delta }\left(0\right)/T_c\approx 1.76$, and the calculated superfluid density is consistent with a fully gapped superconducting state characterized by a single gap scale. Electrical resistivity measurements for in-plane and inter-plane current directions find very low and nearly temperature-independent normal-state anisotropy. The temperature dependence of resistivity is typical for conventional phonon scattering in metals. We compare these experimental results with expectations from a detailed theoretical symmetry analysis and reduce the number of possible superconducting pairing states in ${\mathrm{PdTe}}_2$ to only three nodeless candidates: a regular, topologically trivial $s$-wave pairing, and two distinct odd-parity triplet states that both can be topologically nontrivial depending on the microscopic interactions driving the superconducting instability.

We study $\mathrm{Nb}-{\mathrm{AlO}}_x$-Nb Josephson junctions (JJs) with a phase-discontinuity $\kappa$ created by a pair of current injectors attached to one of the Nb electrodes. For $\kappa \approx \pi$ the Josephson potential energy $U$ as a function of the average phase $\psi$ across the JJ has the form of a $2\pi$-periodic double-well potential. Thus, the device behaves as a $\phi$ JJ with degenerate ground state phases $\psi =\pm \phi$ (the value of $\phi$ depends on the system parameters). Experimentally, the existence of two wells of the potential is confirmed by the observation of two different critical currents $I_{\mathrm{c}\pm }$, corresponding to the escape from different wells. We investigate the escape of the Josephson phase from both wells by collecting statistics of the switching currents. The histogram of switching current exhibits two peaks corresponding to $I_{\mathrm{c}\pm }$. The dependence of the width ${\sigma }_{+}$ and ${\sigma }_{-}$ of each peak on the bath temperature $T$ indicates the transition from thermal activation to macroscopic quantum tunneling (MQT) at $T^{★}\approx 260\mathrm{mK}$ as $T$ decreases. We argue that the observed saturation value of ${\sigma }_{+}$ and ${\sigma }_{-}$ below $T^{★}$ is indeed related to quantum tunneling rather than to parasitic noise in the system, as the histogram width can be reduced by tuning the value of $\kappa$ away from $\pi$. The comparison of the experimental escape rate $\mathrm{\Gamma }$ with theoretical predictions further confirms MQT of the phase $\psi$ from both wells.

When a 2D superconductor is subjected to a strong in-plane magnetic field, Zeeman polarization of the Fermi surface can give rise to inhomogeneous FFLO order with a spatially modulated gap. Further increase of the magnetic field eventually drives the system into a normal metal state. Here, we perform a renormalization group analysis of this quantum phase transition, starting from an appropriate low-energy theory recently introduced in Phys. Rev. B 93, 085112 (2016). We compute one-loop flow equations within the controlled dimensional regularization scheme with fixed dimension of Fermi surface, expanding in $\epsilon =5/2-d$. We find a new stable non-Fermi-liquid fixed point and discuss its critical properties. One of the most interesting aspects of the FFLO non-Fermi-liquid scenario is that the quantum critical point is potentially naked, with the scaling regime observable down to arbitrary low temperatures. In order to study this possibility, we perform a general analysis of competing instabilities, which suggests that only charge density wave order is enhanced in the vicinity of the quantum critical point.

The square lattices formed by main-group elements such as Bi, Sb, Sn, and Si in layered materials have attracted a lot of interest, since they can create rich topological phases. In this paper, we report the slightly distorted square lattice of Sn in a noncentrosymmetric compound ${\mathrm{YSn}}_2$ generates multiple topologically nontrivial bands, one of which likely hosts a nodal line and tunable Weyl semimetal state induced by the Rashba spin-orbit coupling and proper external magnetic field. The quasiparticles described as relativistic fermions from these bands are manifested by nearly zero mass and nontrivial Berry phases probed in de Haas-van Alphen (dHvA) oscillations. The dHvA study also reveals ${\mathrm{YSn}}_2$ has a complicated Fermi surface, consisting of several three-dimensional (3D) and one 2D pocket. Our first-principles calculations show the pointlike 3D pocket at Y point on the Brillouin zone boundary hosts the possible Weyl state. Our findings establish ${\mathrm{YSn}}_2$ as a new interesting platform for observing novel topological phases and studying their underlying physics.

We investigate in-gap states emerging when a single $3d$ transition metal impurity is embedded in topological insulators (${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Se}}_3$). We use a combined approach relying on first-principles calculations and an Anderson impurity model. By computing the local density of states of Cr, Mn, Fe, and Co embedded not only in surfaces of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ but also in their bulk phases, we demonstrate that in-gap states originate from the hybridization of the electronic states of the impurity with bulk bands and not with the topological surface states as is usually assumed. This finding is analyzed using a simplified Anderson impurity model. These observations are in contradiction with the prevailing models used to investigate the magnetic doping of topological insulators [R. R. Biswas et al., Phys. Rev. B 81, 233405 (2010)], which attribute the origin of the in-gap states to the hybridization with the topological surface states.

We report an experimental and theoretical study of the electronic band structure and Fermi surfaces (FSs) of ${\mathrm{V}}_2\mathrm{AlC}$ single crystals using angle-resolved photoemission spectroscopy and density functional theory (DFT) calculations. We provide evidence of the existence of equivalent, complex hole FSs with a nearly tubular form along the $c$ axis, and of an intricate electron FS exhibiting tubes parallel to $c$ connected to one another by smaller tubes with axes parallel to the $ab$ plane. The electron FS thus exhibits a small delocalization along the $c$ axis. The local orbital character of each observed band is experimentally and theoretically assessed. DFT calculations show an excellent agreement with experiment. We also report the observation of an unstable surface state.

The nonlocal response of chiral electron fluid in a semi-infinite Weyl semimetal slab with the electric current source and drain attached to its surface is studied by using the consistent hydrodynamic framework. It is found that the Chern-Simons terms lead to a spatial asymmetry of the electron flow and the electric field. Most remarkably, the corresponding topological terms could result in a negative nonlocal resistance. In addition, they give rise to the anomalous Hall current, which is sensitive to the spatial distribution of the electric field in the plane of the contacts and depends on the orientation of the chiral shift.

We show that the appropriate on-off switching of Josephson coupling between exciton-polaritons in coupled semiconductor microcavities can reveal the full capacity for generating entanglement with a recently proposed method which essentially enhances the nonlinearity of the system. The improvement achieved with this simple modulation of the coupling is substantial over the case where it is kept constant. We also show that even better results can be obtained with the additional on-off switching of the enhanced nonlinearity. The suggested procedure is expected to find also application in other research areas, where nonlinear interacting bosons are encountered.

Graphene has been widely used to construct low-resistance van der Waals (vdW) contacts to other two-dimensional (2D) materials. However, a rise of graphene's electron temperature under a current flow has not been seriously considered in many applications. Owing to its small electronic heat capacity and electron-phonon coupling, graphene's electron temperature can be increased easily by the application of current. The heat generated within the graphene is transferred to the contacted 2D materials through the vdW interface and potentially influences their properties. Here, we compare the superconducting critical currents of an ${\mathrm{NbSe}}_2$ flake for two different methods of current application: with a Au/Ti electrode fabricated by thermal evaporation and with a graphene electrode contacted to the ${\mathrm{NbSe}}_2$ flake through a vdW interface. The influence of the heat transfer from the graphene to ${\mathrm{NbSe}}_2$ is detected through the change of the superconductivity of ${\mathrm{NbSe}}_2$. We found that the critical current of ${\mathrm{NbSe}}_2$ is significantly reduced when the current is applied with the graphene electrode compared to that from the conventional Au/Ti electrode. Further, since the electron heating in graphene exhibits ambipolar back-gate modulation, we demonstrate the electric field modulation of the critical current in ${\mathrm{NbSe}}_2$ when the current is applied with graphene electrode. These results are attributed to the significant heat transfer from the graphene electrode to ${\mathrm{NbSe}}_2$ through the vdW interface.

We study the optical tunability of the charge carrier type in InAs/GaSb double quantum wells with its type-II broken band alignment and inverted band structure. Under constant optical excitation, the majority charge carrier type switches from electron to hole. Within the majority charge carrier type transition, the coexisting minority charge carrier contribution indicates electron-hole hybridization with a nontrivial topological insulating phase. The optical tuning is attributed to the negative photoconductivity of antimonide materials in combination with a persistent charge carrier buildup of photogenerated charges at the surface and substrate side of the device, respectively. Our study of the tuning of an InAs/GaSb double quantum well heterostructure reveals that an electro-optical switching is possible and paves the way to an optical control of the phase diagram of InAs/GaSb topological insulators.

Spin-resolved band structures of $L$-gap surface states on Ag(111) and Cu(111) are investigated by spin- and angle-resolved photoelectron spectroscopy (SARPES) with a vacuum-ultraviolet laser. The observed spin textures of the Ag(111) and Cu(111) surface states agree with that expected by the conventional Rashba effect. The Rashba parameter of the Ag(111) surface state is estimated quantitatively and is $80\%$ of that of Cu(111). The surface-state wave function is found to be predominantly of even mirror symmetry with negligible odd contribution by SARPES using a linearly polarized light. The results are consistent with our theoretical calculations for the orbital-resolved surface state.

Motivated by the rapid experimental progress on the spin-orbit-coupled Mott insulators, we propose and study a generic spin model that describes the interaction between the non-Kramers doublets on a triangular lattice and is relevant for triangular lattice rare-earth magnets. We predict that the system supports both pure quadrupolar orders and intertwined multipolar orders in the phase diagram. Besides the multipolar orders, we explore the magnetic excitations to reveal the dynamic properties of the systems. Due to the peculiar properties of the non-Kramers doublets and the selective coupling to the magnetic field, we further study the magnetization process of the system in the magnetic field. We point out the selective measurements of the static and dynamic properties of the intertwined multipolarness in the neutron scattering, NMR, and $\mu \mathrm{SR}$ probes and predict the experimental consequences. The relevance to the existing materials such as ${\mathrm{TmMgGaO}}_4$, Pr-based, and Tb-based magnets, and many ternary chalcogenides is discussed. Our results not only illustrate the rich physics and the promising direction in the interplay between strong spin-orbit-entangled multipole moments and the geometrical frustration, but also provide a general idea to use noncommutative observables to reveal the dynamics of the hidden orders.

Exciton condensation, a Bose-Einstein-like condensation of excitons, was recently reported in an electronic double layer (EDL) of graphene. We show that a universal quantum signature for exciton condensation can be used to both identity and quantify exciton condensation in molecular systems from direct calculations of the two-electron reduced density matrix. Computed large eigenvalues in the particle-hole reduced-density matrices of pentacene and hexacene EDLs reveal the beginnings of condensation, suggesting the potential for exciton condensation in smaller scale molecular EDLs.

ABA-stacked trilayer graphene (TLG), the simplest system consisting of both monolayerlike and bilayerlike Dirac fermions, is expected to exhibit many interesting broken-symmetry quantum Hall states and interaction-induced phenomena. However, difficulties in microscopically identifying the stacking order of the TLG and limited spectroscopic resolution have stymied experimental probes of these interesting states and phenomena in scanning tunneling microscopy (STM) studies. Here we studied the detailed features of the electronic structure in the ABA TLG by using high-resolution STM measurements. Landau-level (LL) crossings of the monolayerlike and bilayerlike Dirac fermions and effective-mass renormalization of the bilayerlike Dirac fermions were observed, indicating strong electron-electron interactions in the ABA TLG. Most unexpectedly, we observed unconventional splittings of the lowest LLs for both the monolayerlike and the bilayerlike Dirac fermions in high perpendicular magnetic fields. These splittings of the LLs, which are beyond the description of the tight-binding model, reveal unexplored broken-symmetry quantum Hall states in the ABA TLG induced by many-body effects.

We study the quantum corrections to the conductivity of the two-dimensional disordered interacting electron system in the diffusive regime due to inelastic scattering off rare magnetic impurities. We focus on the case of very different $g$ factors for electrons and magnetic impurities. Within the Born approximation for the inelastic scattering off magnetic impurities we find additional temperature-dependent corrections to the conductivity of the Altshuler-Aronov type. Our results demonstrate that the low-temperature transport in interacting disordered electron systems with rare magnetic impurities is more interesting than it was commonly believed on the basis of treatment of magnetic impurity spins as classical ones.

We propose a one-dimensional nonlinear system of coupled anharmonic oscillators that dynamically undergoes a topological transition switching from the disordered and topologically trivial phase to the nontrivial one due to the spontaneous symmetry breaking. The topological transition is accompanied by the formation of the topological interface state in the spectrum of linearized excitations of the equilibrium phase. Our findings thus highlight the potential of the nonlinear systems for hosting the topological phases and uncover a fundamental link between the spontaneous-symmetry-breaking mechanism and topological interface states.