The photonic spin Hall effect (SHE), featured by a spin-dependent transverse shift of an impinging optical beam driven by its polarization handedness, has many applications including precise metrology and spin-based nanophotonic devices. It is highly desirable to control and enhance the photonic SHE. However, such a goal remains elusive, due to the weak spin-orbit interaction of light, especially for systems with optical loss. Here we reveal a flexible way to modulate the photonic SHE via exceptional points, by exploiting the transverse shift in a parity-time (PT) symmetric system with balanced gain and loss. The underlying physics is associated with the near-zero value and abrupt phase jump of the reflection coefficients at exceptional points. We find that the transverse shift is zero at exceptional points, but it is largely enhanced in their vicinity. Moreover, the transverse shift switches its sign across the exceptional point, resulting from spontaneous PT-symmetry breaking. Due to the sensitivity of transverse shift at exceptional points, our work also indicates that the photonic SHE can enable a precise way to probe the location of exceptional point in photonic systems.

In spite of decades of work it has remained unclear whether or not superradiant quantum phases, referred to here as photon condensates, can occur in equilibrium. In this Rapid Communication, we first show that when a nonrelativistic quantum many-body system is coupled to a cavity field, gauge invariance forbids photon condensation. We then present a microscopic theory of the cavity quantum electrodynamics of an extended Falicov-Kimball model, showing that, in agreement with the general theorem, its insulating ferroelectric and exciton condensate phases are not altered by the cavity and do not support photon condensation.

We study the growth of entanglement in quantum systems with a conserved quantity exhibiting diffusive transport, focusing on how initial inhomogeneities are imprinted on the entropy. We propose a simple effective model, which generalizes the minimal cut picture of Jonay, Huse, and Nahum [arXiv:1803.00089] in such a way that the “line tension” of the cut depends on the local entropy density. In the case of noisy dynamics, this is described by the Kardar-Parisi-Zhang (KPZ) equation coupled to a diffusing field. We investigate the resulting dynamics and find that initial inhomogeneities of the conserved charge give rise to features in the entanglement profile, whose width and height both grow in time as $\propto \sqrt{t}$. In particular, for a domain wall quench, diffusion restricts entanglement growth to be $S_{\text{vN}}\lesssim \sqrt{t}$. We find that for charge density wave initial states, these features in the entanglement profile are present even after the charge density has equilibrated. Our conclusions are supported by numerical results on random circuits and deterministic spin chains.

Mean field approach, although a generally reliable tool that captures major short-range correlations, often fails in symmetric low dimensional strongly correlated electronic systems like those described by the Hubbard model. In these situations a symmetry is almost broken. The problem is linked to the restoration of the symmetry due to strong fluctuations (both quantum and thermal) on all scales. The restoration of symmetry in statistical models of scalar order parameter fields was treated recently successfully on the Gaussian approximation level by symmetrization of the correlators. Here the idea is extended to fermionic systems in which the order parameter is composite. Furthermore, the precision of the correlators can be improved perturbatively. Such a scheme (based on covariant Gaussian approximation) is demonstrated on the one dimensional (1D) and 2D one band Hubbard models by comparison of the correlator with exact diagonalization and MC simulations, respectively.

We explore the effect of inhomogeneity on electronic properties of the two-dimensional Hubbard model on a square lattice using dynamical mean-field theory (DMFT). The inhomogeneity is introduced via modulated lattice hopping such that in the extreme inhomogeneous limit the resulting geometry is a Lieb lattice, which exhibits a flat-band dispersion. The crossover can be observed in the uniform sublattice magnetization which is zero in the homogeneous case and increases with the inhomogeneity. Studying the spatially resolved frequency-dependent local self-energy, we find a crossover from Fermi-liquid to non-Fermi-liquid behavior happening at a moderate value of the inhomogeneity. This emergence of a non-Fermi liquid is concomitant of a quasiflat band. For finite doping the system with small inhomogeneity displays $d$-wave superconductivity coexisting with incommensurate spin-density order, inferred from the presence of oscillatory DMFT solutions. The $d$-wave superconductivity gets suppressed for moderate to large inhomogeneity for any finite doping while the incommensurate spin-density order still exists.

Thermally conductive molecular crystals are of fundamental interest because they are unlike typical complex crystals, which conduct heat poorly owing to their large phonon scattering phase space. While molecular crystals with high thermal conductivity in the range of tens of ${\mathrm{Wm}}^{-1}{\mathrm{K}}^{-1}$ have been known experimentally for decades, their intrinsic upper limits for thermal conductivity are unclear. Ab initio methods that have been successfully applied to simple crystals have proved difficult to adapt to molecular crystals due to quantum nuclear motion and their complex primitive cells. Here, we report the thermal transport properties of crystalline polythiophene with 28 atoms per primitive cell using an ab initio approach that rigorously includes finite-temperature anharmonicity and quantum nuclear effects. The calculated room temperature thermal conductivity is $198{\mathrm{Wm}}^{-1}{\mathrm{K}}^{-1}$ along the chain axis, a high value that arises from exceptional phonon focusing along the chain for both acoustic and optical branches for nearly all wave vectors and despite short lifetimes in the picosecond range. Our finding, along with other recent ab initio studies of polyethylene, suggests that the intrinsic upper bounds for the chain axis thermal conductivity of polymer crystals may exceed $100{\mathrm{Wm}}^{-1}{\mathrm{K}}^{-1}$.

Quantum phase transitions are studied in the nonchiral $p$-clock chain, and a new explicitly U(1)-symmetric clock model, by monitoring the ground-state fidelity susceptibility. For $p\ge 5$, the self-dual ${\mathbb{Z}}_p$-symmetric chain displays a double-hump structure in the fidelity susceptibility with both peak positions and heights scaling logarithmically to their corresponding thermodynamic values. This scaling is precisely as expected for two Beresinskii-Kosterlitz-Thouless (BKT) transitions located symmetrically about the self-dual point, and so confirms numerically the theoretical scenario that sets $p=5$ as the lowest $p$ supporting BKT transitions in ${\mathbb{Z}}_p$-symmetric clock models. For our U(1)-symmetric, non-self-dual minimal modification of the $p$-clock model we find that the phase diagram depends strongly on the parity of $p$ and only one BKT transition survives for $p\ge 5$. Using asymptotic calculus we map the self-dual clock model exactly, in the large $p$ limit, to the quantum $O\left(2\right)$ rotor chain. Finally, using bond-algebraic dualities we estimate the critical BKT transition temperatures of the classical planar $p$-clock models defined on square lattices, in the limit of extreme spatial anisotropy. Our values agree remarkably well with those determined via classical Monte Carlo for isotropic lattices. This work highlights the power of the fidelity susceptibility as a tool for diagnosing the BKT transitions even when only discrete symmetries are present.

We predict magnetization plateaus ground states for $S=1/2$ Heisenberg antiferromagnets on pyrochlore lattices by formulating arguments based on gauge and spin-parity transformations. We derive a twist operator appropriate to the pyrochlore lattice, and show that it is equivalent to a large gauge transformation. Invariance under this large gauge transformation indicates the sensitivity of the ground state to changes in boundary conditions. This leads to the formulation of an Oshikawa-Yamanaka-Affleck–like criterion at finite external magnetic field, enabling the prediction of plateaus in the magnetization versus field diagram. We also develop an analysis based on the spin-parity operator, leading to a condition from which identical predictions are obtained of magnetization plateaus ground states. Both analyses are based on the non-local nature of the transformations, and rely only on the symmetries of the Hamiltonian. This suggests that the plateaus ground states can possess properties arising from non-local entanglement between the spins.

We present inelastic neutron-scattering data on the quantum paramagnet ${\mathrm{AgVOAsO}}_4$ that establish this system as a $S=1/2$ alternating spin chain compound and provide a direct measurement of the spin gap $\mathrm{\Delta }=1.2\mathrm{meV}$. We also present experimental evidence for two different types of field-induced magnetic order between ${\mu }_0{H}_{c1}=8.4\mathrm{T}$ and ${\mu }_0{H}_{c2}=48.9\mathrm{T}$, which may be related to Bose-Einstein condensation (BEC) of triplons. Thermodynamic measurements in magnetic fields up to 60 T and temperatures down to 0.1 K reveal a $H-T$ phase diagram consisting of a dome encapsulating two ordered phases with maximum ordering temperatures of 3.8 and 5.3 K, respectively. This complex phase diagram is not expected for a single-$\vec{Q}$ BEC system and therefore establishes ${\mathrm{AgVOAsO}}_4$ as a promising multi-$\vec{Q}$ BEC candidate capable of hosting exotic vortex phases.

Proximity induced superconducting states in the surface of magnetically doped topological insulators can host chiral Majorana modes. We consider a Josephson junction in that system with changing the chemical potential, which drives a topological phase transition in the induced superconducting states as well as a metal-insulator transition in the surface states. The local density of states and the Josephson current are analytically calculated by McMillan's Green's function method in terms of the Andreev reflection coefficient. We show that although the magnitude of the Josephson current is greatly enhanced when the surface state changes from insulating to metallic, its temperature dependence drastically changes at the topological phase transition point, reflecting the appearance of the chiral Majorana modes.

We present a first-principles study of the electronic band structure and low-energy dielectric response properties of a representative group of graphite intercalated compounds—${\mathrm{LiC}}_6,{\mathrm{SrC}}_6$, and ${\mathrm{BaC}}_6$. Our results obtained from time-dependent density functional theory calculations reveal the presence of different plasmons in these compounds at energies below 12 eV. The presence of these collective electronic excitations is discussed in terms of intra- and interband transitions. In addition to the bulk plasmon, we find the $\pi$- and intraband plasmons. However, their properties vary greatly from one material to another. In ${\mathrm{LiC}}_6$ and ${\mathrm{BaC}}_6$, the $\pi$ plasmon is a two-dimensional excitation, whereas in ${\mathrm{SrC}}_6$ it has a three-dimensional character. As for the intraband plasmon, it is observed in all momentum-transfer symmetry directions, with rather strong anisotropy in the energy position between momentum transfers in the basal carbon plane and perpendicular to it. Also, we discuss the appearance in ${\mathrm{SrC}}_6$ and ${\mathrm{BaC}}_6$ of a low-energy collective electronic mode with the characteristic soundlike dispersion in terms of the energy bands crossing the Fermi level with different group velocities. We find a correlation between the appearance of such low-energy electronic modes and the occupation of the graphite interlayer band.

We studied the magnetoresistivity and the Hall effect of the type-I Weyl semimetal TaAs to address the controversy surrounding its anomalous transport properties in relation to its bulk topological character. For fields and currents along the basal plane, we observe a very pronounced planar Hall effect (PHE) upon field rotation with respect to the crystallographic axes at temperatures as high as $T=100\mathrm{K}$. Parametric plots of the PHE signal as a function of the longitudinal magnetoresistivity (LMR) collected at $T=10\mathrm{K}$ lead to concentric traces as reported for ${\mathrm{Na}}_3\mathrm{Bi}$ and GdBiPt. This would suggest that the negative LMR and the PHE observed in TaAs are intrinsically associated with the axial anomaly among its Weyl nodes. For fields nearly along the $a$ axis we observe hysteresis as one surpasses the quantum limit, where the magnetic torque indicates a change in regime as the field increases, i.e., from paramagnetism and diamagnetism due to Weyl fermions above and below the Weyl node(s), respectively, to a paramagnetic one associated with the field-independent $n=0$ Landau level. Hysteresis coupled to the overall behavior of the torque would be consistent with a topological phase transition associated with the suppression of the Weyl dispersion at the quantum limit. This transition leads to the suppression of the negative LMR confirming that it is intrinsically associated with the Weyl dispersion. The Hall effect for fields along the $c$ axis reveals two successive changes in slope, or two successive decrements in carrier mobility, one at the quantum limit and a second one at the critical field where a phase transition toward an insulating state was recently reported. This suggests the possibility of two successive phase transitions as function of the field with the higher-field one involving solely the $n=0$ Landau level. Finally, for both field orientations we observe Shubnikov–de Haas like oscillations beyond the quantum limit hence involving quasiparticles at fractional filling factors.

Weyl semimetals (WSMs) are intriguing topological states of matter that support various anomalous magnetotransport phenomena. One such phenomenon is a positive longitudinal magnetoconductivity and the associated planar Hall effect, which arise due to an effect known as chiral anomaly which is nonzero in the presence of electric and magnetic fields $(\mathbf{E}\ne 0,\mathbf{B}\ne 0$, and $\mathbf{E}·\mathbf{B}\ne 0)$. In this paper we show that another fascinating effect is the planar thermal Hall effect (PTHE), associated with positive longitudinal magnetothermal conductivity (LMTC), which arise even in the absence of chiral anomaly $\left(\mathbf{E}=0,\mathbf{B}\ne 0\right)$. This effect is a result of chiral magnetic effect (CME) and involves the appearance of an in-plane transverse temperature gradient when the current due to a nonzero temperature gradient $\left(\mathbf{\nabla }T\right)$ and the magnetic field $\left(\mathbf{B}\right)$ are not aligned with each other. Using semiclassical Boltzmann transport formalism in the relaxation time approximation we compute both longitudinal magnetothermal conductivity and planar thermal Hall conductivity (PTHC) for a time-reversal symmetry-breaking WSM. We find that both LMTC and PTHC are quadratic in $B$ in type-I WSM whereas each follows a linear-$B$ dependence in type-II WSMs in a configuration where $\nabla T$ and B are applied along the tilt direction. In addition, we investigate the Wiedemann-Franz law for an inversion symmetry-broken WSM (e.g., ${\mathrm{WTe}}_2)$ and find that this law is violated in these systems due to both chiral anomaly and CME.

Muon spin rotation $\left(\mu \text{SR}\right)$ with low-energy muons is a powerful nuclear method where electrical and magnetic properties of thin films can be investigated in a depth-resolved manner. Here, we present a study on proton-irradiated Si and 4H-SiC where the formation of the hydrogen-like muonium (Mu) is analyzed as a function of the proton dose. While the Mu formation is strongly suppressed in the highly defective region of the shallow proton stopping profile, the Mu signal quickly recovers for higher muon energies where the muons reach the untreated semiconductor bulk. A lower sensitivity limit of low-energy $\mu \text{SR}$ to crystal defects of around ${10}^{17}$ to ${10}^{18}{\mathrm{cm}}^{-3}$ is estimated. Our results demonstrate the high potential of this technique to nondestructively probe near-surface regions without the need for electronic device fabrication and to provide valuable complementary information when investigating defects in semiconductors.

Orbital angular momentum (OAM) light possesses in addition to its usual helicity ($s=\pm \hslash$, depending on its circular polarization) an orbital angular momentum $l$. This means that in principle one can transfer more than a single quantum of $\hslash$ during an optical transition from light to a quantum system. However, quantum objects are usually so small (typically in the nanometer range) that they only locally probe the dipolar character of the local electric field. In order to sense the complete macroscopic electric field, we utilize Rydberg excitons in the semiconductor cuprite (${\mathrm{Cu}}_2\mathrm{O}$), which are single quantum objects of up to micrometer size. Their interaction with focused OAM light allows for matching the focal spot size and the wave-function diameter. Here, the common dipole selection rules ($\mathrm{\Delta }j=\pm 1$) should be broken, and transitions of higher $\mathrm{\Delta }j$ with higher-order OAM states should become more probable. Based on group theory, we analyze in detail the optical selection rules governing this process. Then we are able to predict what kind of alternative exciton transitions (quantum number $n$ and $l^{\mathrm{exc}}$) one would expect in absorption spectroscopy on ${\mathrm{Cu}}_2\mathrm{O}$ using different kinds of OAM light.

We investigate the mutual influence of tilt, disorder, and Coulomb interaction in a type-I Dirac semimetal (DSM) with $x$-direction tilt by performing a renormalization group analysis. The interplay between disorder and ordinary tilt generates an effective tilt along the $x$ direction, which is the physically observable one. There exist two types of disorder which increase the effective tilt and drive a phase transition from the DSM phase to the diffusive metal phase. The diffusive phase transition stops the increase of the effective tilt and the surface of the original Dirac cone in the diffusive metal phase is just slightly tilted. Surprisingly, the Dirac point is replaced by a bulk nodal arc in the diffusive metal phase. The Coulomb interaction suppresses the diffusive phase transition and therefore is harmful to the formation of bulk nodal arc. In contrast, there also exist other two types of disorder which reduce the effective tilt and induce no phase transition. For these two types of disorder, the Coulomb interaction enhances their low-energy relevances. Coexistence of Coulomb interaction with any of them leads to a stable infrared fixed point where the coupling strengths for two kinds of interaction are identical and the effective tilt vanishes. The original tilted Dirac semimetal now reacts like an untilted and interaction-free Dirac semimetal. Our results show that interplay between tilt, disorder, and Coulomb interaction results in rich low-energy properties for the tilted Dirac fermions.

We report the giant plasmonically induced (PI) circular conversion dichroism in transmission $\left(\mathrm{CC}{\mathrm{D}}_T\right)$ and PI-unconverted CD effect $\left(\mathrm{unCC}{\mathrm{D}}_R\right)$ in reflection from the golden slit grating filled by chiral medium (GSGCM). The $\mathrm{CC}{\mathrm{D}}_T$ and $\mathrm{unCC}{\mathrm{D}}_R$ are the differences between the off-diagonal elements in transmission matrix and diagonal elements in reflection matrix, respectively, and originate from both the chirality and anisotropy. Our research indicates that the $\mathrm{PI}-\mathrm{CC}{\mathrm{D}}_T$ and PI-$\mathrm{unCC}{\mathrm{D}}_R$ in the plasmonic resonance wave band come from the coupling between the inherent chiroptical resonance of chiral medium and the plasmonic resonance of the golden slit grating, where the real part of chiral parameter $\xi$ from chiral medium plays a main role in the coupling process. The isotropic chiral medium slab, the anisotropic chiral medium grating, the golden slit grating, and the GSGCM have been rigorously investigated in this work. Our research clearly reveals the difference of PI-chiroptical effect between the anisotropic and isotropic system, and will find potential applications in biosensor and nanophotonic devices.

The high-frequency conductivity of the two dimensional electron system formed at a ZnO/MgZnO heterojunction was investigated in the 4–18 GHz frequency range at high Landau level fillings and in the regime of both integer quantum Hall effect. The conductivity was probed by measuring the transmission of the broadband coplanar waveguide deposited on the sample surface. At low magnetic fields in case the sample was additionally subjected to the exciting radiation of the 60–140 GHz frequencies, the high-frequency conductivity exhibited well developed microwave-induced oscillations. Remarkably, these oscillations were detected in the whole range of probe frequencies studied—up to 18 GHz—in contrast to GaAs/AlGaAs heterostructures, where microwave-induced oscillations were reported to smear out at probe frequencies around 1 GHz. Furthermore, for each exciting frequency the phase and the period of the microwave-induced oscillations revealed no dependence on the probe frequency and coincided with the period and phase of the microwave-induced resistance oscillations studied independently by dc transport measurements in the same sample. The characteristic probe frequency at which the oscillations start to degrade corresponds to the characteristic scattering time in the region between quantum scattering rates deduced from the cyclotron resonance linewidth and that extracted from the magnetic field dependence of the microwave-induced resistance oscillations.

We develop a theoretical framework to study the influence of coupling asymmetry on the thermoelectrics of a strongly coupled $\mathrm{SU}(N)$ Kondo impurity based on a local Fermi liquid theory. Applying a nonequilibrium Keldysh formalism, we investigate a charge current driven by the voltage bias and temperature gradient in the strong coupling regime of an asymmetrically coupled $\mathrm{SU}(N)$ quantum impurity. The thermoelectric characterizations are made via nonlinear Seebeck effects. We demonstrate that the beyond particle-hole (PH) symmetric $\mathrm{SU}(N)$ Kondo variants are highly desirable with respect to the corresponding PH-symmetric setups in order to have significantly improved thermoelectric performance. The greatly enhanced Seebeck coefficients by tailoring the coupling asymmetry of beyond PH-symmetric $\mathrm{SU}(N)$ Kondo effects are explored. Apart from presenting the analytical expressions of asymmetry-dependent transport coefficients for general $\mathrm{SU}(N)$ Kondo effects, we make a close connection of our findings with the experimentally studied SU(2) and SU(4) Kondo effects in quantum dot nanostructures. Seebeck effects associated with the theoretically proposed SU(3) Kondo effects are discussed in detail.