Using the Maxwell-Bloch equations for resonantly absorbing and amplifying media, we study the temporal dynamics of light propagation through the $\mathcal{PT}$-symmetric structures with alternating loss and gain layers. This approach allows us to precisely describe the response of the structure near the exceptional points of $\mathcal{PT}$ symmetry breaking phase transition and, in particular, take into account the nonlinear effect of loss and gain saturation in the $\mathcal{PT}$ symmetry broken state. We reveal that in this latter state the multilayer system possesses a lasinglike behavior, releasing the pumped energy in the form of powerful pulses. We predict locking of pulse direction due to the $\mathcal{PT}$ symmetry breaking, as well as saturation-induced irreversibility of phase transition and nonreciprocal transmission.

Ferroic domain walls (DWs) create different symmetries and ordered states compared with those in single-domain bulk materials. In particular, the DWs of an antiferromagnet with noncoplanar spin structure have a distinct symmetry that cannot be realized in those of their ferromagnet counterparts. In this paper, we show that an unconventional anomalous Hall effect (AHE) can arise from the DWs of a noncoplanar antiferromagnet, $\mathrm{N}{\mathrm{d}}_2\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$. Bulk $\mathrm{N}{\mathrm{d}}_2\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ has a cubic symmetry; thus, its Hall signal should be zero without an applied magnetic field. The DWs generated in this material break the twofold rotational symmetry, which allows for finite anomalous Hall conductivity. A strong $f\text{−}d$ exchange interaction between the Nd and Ir magnetic moments significantly influences antiferromagnetic (AFM) domain switching. Our epitaxial $\mathrm{N}{\mathrm{d}}_2\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ thin film showed a large enhancement of the AHE signal when the AFM domains switched, indicating that the AHE is mainly due to DWs. Our paper highlights the symmetry-broken interface of AFM materials as a means of exploring topological effects and their relevant applications.

Most electronic properties of metals are determined solely by the low-energy states around the Fermi level and, for topological metals/semimetals, these low-energy states become distinct because of their unusual energy dispersion and emergent pseudospin degree of freedom. Here, we propose a class of materials which are termed as quadratic contact point (QCP) semimetals. In these materials, the conduction and valence bands contact at isolated points in the Brillouin zone, around which the band dispersions are quadratic along all three directions. We show that in the absence/presence of spin-orbit coupling, there may exist triply/quadruply degenerate QCPs that are protected by the crystalline symmetry. We construct effective models to characterize the low-energy fermions near these QCPs. Under strong magnetic field, unlike the usual 3D electron gas, there appear unconventional features in the Landau spectrum. The QCP semimetal phase is adjacent to a variety of topological phases. For example, by breaking symmetries via Zeeman field or lattice strain, it can be transformed into a Weyl semimetal with Weyl and double Weyl points, a ${\mathbb{Z}}_2$ topological insulator/metal, or a Dirac semimetal. Via first-principles calculations, we identify realistic materials ${\mathrm{Cu}}_2\mathrm{Se}$ and ${\mathrm{RhAs}}_3$ as candidates for QCP semimetals.

Deconfined quantum critical points are characterized by the presence of an emergent gauge field and exotic fractionalized particles, which exist as well-defined excitations only at the critical point. We here demonstrate the existence of quantum critical points described by an emergent tensor gauge theory featuring subdimensional excitations, in close relation to fracton theories. We begin by reexamining a previously studied Lifshitz transition between two valence bond solid (VBS) phases on a bilayer honeycomb lattice. We show that the critical theory maps onto a rank-2 tensor gauge theory featuring one-dimensional particles. In a slightly different context, the same tensor gauge theory also describes a Lifshitz quantum critical point between a two-dimensional superfluid and a finite-momentum Bose condensate, both of which are dual to rank-1 gauge theories. This represents an entirely new class of deconfined quantum criticality, in which a critical tensor gauge theory arises on top of a stable conventional gauge theory. Furthermore, we propose that this quantum critical point gives rise to a new finite-temperature phase of bosons, behaving as an exciton Bose condensate, in which excitons (boson-hole pairs) are condensed but individual bosons are not. We discuss how small modifications of this theory give rise to the stable quantum “exciton Bose liquid” phase studied in Paramekanti, Balents, and Fisher, Phys. Rev. B 66, 054526 (2002).

We investigate theoretically the dynamical conductivity in the nodal-line semimetal ZrSiS by using a multi-orbital tight-binding model based on a first-principles band calculation. We find that the dynamical conductivity in the clean limit is not frequency independent unlike the ideal Dirac model, while the nearly flat dependence on frequency is achieved by introducing the disorder-induced energy broadening. The result also applies to similar compounds such as ZrSiSe, ZrSiTe, and HfSiS.

We investigate the full counting statistics of extreme-near-field radiative heat transfer using nonequilibrium Green's function formalism. In the extreme near field, the electron-electron interactions between two metallic bodies dominate the heat transfer process. We start from a general tight-binding electron Hamiltonian and obtain a Levitov-Lesovik-like scaled cumulant-generating function (SCGF) using random-phase approximation to deal with electron-electron interaction. The expressions of heat current and its fluctuation (second cumulant) are obtained from the SCGF. The fluctuation symmetry relation of the SCGF is verified. In the linear response limit (small-temperature gradient), we express the heat current cumulant by a linear combination of lower-order cumulants. The heat current fluctuation is $2k_B{T}^2$ times the thermal conductance with $T$ being the average temperature in the linear response limit, and this provides an evaluation of heat current fluctuation by measuring the thermal conductance in extreme-near-field radiative heat transfer.

The lack of inversion symmetry in semiconducting transition metal dichalcogenide monolayers (TMDMs) enables a considerable intrinsic piezoelectricity, which opens prospects for atomically thin piezotronics and optoelectronics. Here, based on the tight-binding (TB) approach and Berry phase expression for electronic polarization difference, we establish an atomic-scale TB theory for demonstrating piezoelectric physics in TMDMs. Using the TB piezoelectric theory, we predict the electronic Grüneisen parameter (EGP), which measures the electron-phonon couplings for TMDMs. By virtue of the constructed analytical piezoelectric model, we further reveal the correlation between the electronic contribution to piezoelectric coefficients and strain-induced pseudomagnetic gauge field (PMF). These predicted EGP and PMF for TMDMs are experimentally testable, and hence the TB piezoelectric model is an alternative theoretical framework for calculating electron-phonon interactions and PMF.

We investigate the spectral and transport properties of a diatomic square lattice with hopping to the next-nearest-neighbors and broken time-reversal symmetry, which behaves as a Chern insulator. In a finite-size approach, the attention is paid to the formation of chiral edge states in the topological insulating phase, but also in the semimetallic one. The edge states are revealed in the ribbon and plaquette geometries by analytical and numerical methods, significant differences being produced by the specific atomic connectivity at the boundary. The Hall resistance $R_H$ is calculated in the plaquette geometry using the Landauer-Büttiker approach. The chiral edge states located in the unique gap of the energy spectrum manifest themselves by quantized values $R_H=\pm h/e^2$ specific to the Chern insulator. The semimetallic system containing chiral edge states embedded in the quasicontinuum of bulk states shows a disorder-driven AQHE as a consequence of the Anderson localization process.

We consider the effect of disorder on the tight-binding Hamiltonians with a flat band and derive a common mathematical formulation of the average density of states and inverse participation ratio applicable for a wide range of them. The system information in the formulation appears through a single parameter which plays an important role in search of the critical points for disorder driven transitions in flat bands [P. Shukla, arXiv:1807.02436]. In a weak disorder regime, the formulation indicates an insensitivity of the statistical measures to disorder strength, thus confirming the numerical results obtained by our as well as previous studies.

This paper presents an extensive study of the structural, magnetic, and thermodynamic properties of the hulsite ${\mathrm{Ni}}_{5.15}{\mathrm{Sn}}_{0.85}{\left({\mathrm{O}}_2{\mathrm{BO}}_3\right)}_2$. The crystal structure of the hulsite has two planar substructures formed by Ni and Sn atoms: one with rectangular configuration and the other with a triangular arrangement. These substructures are linked by the boron ions and by Ni in another site closer to the rectangular arrangement, resulting in a quasi-two-dimensional character. Thus, this system literally adds a new dimension to the study of oxyborates. Our results point to a complex magnetic behavior consistent with these substructures. The planes with rectangular arrangement form a complex magnetic ordering at 180 K (one of the highest magnetic transitions among the oxyborates). The other subsystem, formed by Ni atoms located in a two-dimensional triangular lattice, does not order down to temperatures as low as 3 K. The experimental results suggest a spin-liquid behavior for this subsystem. The magnetic moments of the ions between these planes also freeze at low temperatures. The two magnetic planes coexist as independent subsystems down to the lowest temperatures of our experiments.

The theory of $\tilde{J}$ pseudospin for the $f$ element in a cubic environment is developed. By fulfilling the symmetry requirements and the adiabatic connection to the atomic limit, the crystal-field states are uniquely transformed into $\tilde{J}$-pseudospin states. In terms of the pseudospin operators, both the total angular momentum and the crystal-field Hamiltonian contain higher-rank tensor terms than the traditional ones do, which means the present framework naturally includes effects such as covalency and $J$ mixing beyond the $f$-shell model. Combining the developed theory with ab initio calculations, the $\tilde{J}$-pseudospin states for ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Np}}^{4+}$ ions in octahedral sites of insulators are derived.

We study the fully gapped chiral Mott insulator (CMI) of frustrated Bose-Hubbard models on ladders and two-dimensional lattices by perturbative strong-coupling analysis and density matrix renormalization group (DMRG). First we show the existence of a low-lying exciton state on several geometries carrying the correct quantum numbers responsible for the condensation of excitons and formation of the CMI in the intermediate interaction regime. Then we perform systematic DMRG simulations on several two-leg ladder systems with $\pi$ flux and carefully characterize the two quantum phase transitions. We discuss the possibility to extend the generally very small CMI window by including repulsive nearest-neighbor interactions or changing density and coupling ratios.

We investigate the stability of the spiral spin liquid phase in ${\mathrm{MnSc}}_2{\mathrm{S}}_4$ against thermal and quantum fluctuations as well as against perturbing effects of longer-range interactions. Employing ab initio density functional theory (DFT) calculations we propose a realistic Hamiltonian for ${\mathrm{MnSc}}_2{\mathrm{S}}_4$, featuring second $\left(J_2\right)$ and third $\left(J_3\right)$ neighbor Heisenberg interactions on the diamond lattice that are considerably larger than previously assumed. We argue that the combination of strong $J_2$ and $J_3$ couplings reproduces the correct magnetic Bragg peak position measured experimentally. Calculating the spin-structure factor within the pseudofermion functional-renormalization group technique, we find that close to the magnetic phase transition the sizable $J_3$ couplings induce a strong spiral selection effect, in agreement with experiments. With increasing temperature the spiral selection becomes weaker such that in a window around three to five times the ordering temperature an approximate spiral spin liquid is realized in ${\mathrm{MnSc}}_2{\mathrm{S}}_4$.

Single molecule magnets comprising rare earth metals are of high interest due to the unquenched orbital moments of the rare earth ions that result in a large energy barrier for magnetization reversal. We investigate the magnetic properties of polynuclear $3d\text{−}4f15$-MC-5 metallacrowns using x-ray magnetic circular dichroism of powder samples at a temperature of 7 K in a magnetic field of 7 T. The sum rule analysis reveals element-specific spin and orbital moments. The magnetic moments of the $3d$ transition metal Ni(II) ions are coupled antiferromagnetically to each other and contribute only little to the total molecular moment. The spin and orbital moments of the rare earth ions are unexpectedly smaller than the ionic values resulting from Hund's rules. We explain the reduction of the orbital magnetic moment by a finite magnetic anisotropy. Considering an energy functional including magnetic anisotropy and Zeeman energy the powder average reveals a magnetic anisotropy of 28 meV (340 K) in the case of Dy(III) and 7 meV (85 K) in the case of Tb(III). The spin moments agree with the ionic value, too, when the expectation values of the dipole operator are considered.

The application of a 3.5 GPa pressure on Fe in a ${\mathrm{H}}_2$ environment leads to the formation of iron hydride FeH. Using a combination of high pressure x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at the Fe K edge, we have investigated the modification of electronic and magnetic properties induced (i) by the transition from bcc-Fe to dhcp (double hexagonal)-FeH under pressure and (ii) by the compression of FeH up to 28 GPa. XAS and XMCD spectra under pressure have been computed in bcc-Fe and dhcp-FeH within a monoelectronic framework. Our approach is based on a semirelativistic density-functional theory (DFT) calculation of the electron density in the presence of a core hole using plane waves and pseudopotentials. Our method has been successful to reproduce the experimental spectra and to interpret the magnetic and electronic structure of FeH. In addition, we have identified a transition around 28 GPa, which is a purely magnetic transition from a ferromagnetic state to a paramagnetic state.

A Majorana-based qubit can form within the fermion parity subspace of a two-island topological superconductor setup consisting of four Majorana zero modes. We study the relaxation time of this Majorana qubit induced by the intrinsic Majorana-phonon interaction in combination with single-electron tunneling between the two islands. The theory is developed for both the spinless $p$-wave Kitaev chain model and the spin-orbit-coupled semiconductor nanowire with induced superconductivity and Zeeman splitting. We systematically discuss the dependence of the relaxation rate on Majorana charge distribution (varying with the chemical potential, magnetic field, superconducting pairing, spin-orbit coupling, and island length) as well as on phonon wavelength and the length of the tunnel barrier. Importantly, the accompanied phonon energy can be tuned by the voltage bias over the tunnel barrier, leading to relatively easy manipulation and suppression of the relaxation rate.

We report numerical and analytical studies of the donor-acceptor recombination in compensated semiconductors. Our calculations take into account random electric fields of charged impurities that are important in the nonzero compensation case. We show that the donor-acceptor optical spectrum can be described as a sum of two components: monomolecular and bimolecular. In the low compensation limit, we develop two analytical models for both types of recombination. Also our numerical simulation predicts that these two components of the photoluminescence spectra can be resolved under certain experimental conditions.

The electronic structure of interstitial hydrogen in a compound semiconductor ${\mathrm{FeS}}_2$ (naturally $n$ type) is inferred from a muon study. An implanted muon (Mu, a pseudohydrogen) forms electronically different defect centers discerned by the hyperfine parameter ${\omega }_{\mathrm{hf}}$. A body of evidence indicates that one muon is situated at the center of an iron-cornered tetrahedron with nearly isotropic ${\omega }_{\mathrm{hf}}$ (${\mathrm{Mu}}_{\mathrm{p}}$) and that the other exists as a diamagnetic state (${\mathrm{Mu}}_{\mathrm{d}}$, ${\omega }_{\mathrm{hf}}\simeq 0$). Their response to thermal agitation indicates that the ${\mathrm{Mu}}_{\mathrm{d}}$ center accompanies a shallow level (donor or acceptor) understood by the effective mass model, while the electronic structure of the ${\mathrm{Mu}}_{\mathrm{p}}$ center is more isolated from the host than ${\mathrm{Mu}}_{\mathrm{d}}$, forming a deeper donor level. These observations suggest that interstitial hydrogen also serves as an electronically active impurity in ${\mathrm{FeS}}_2$. Based on earlier reports on the hydrogen diffusion in ${\mathrm{FeS}}_2$, the possibility of fast diffusion for ${\mathrm{Mu}}_{\mathrm{p}}$ leading to the formation of a complex defect state (${\mathrm{Mu}}_{\mathrm{d}}^{*}$, $T\le 100$ K) or to a motional narrowing state (${\mathrm{Mu}}_{\mathrm{p}}^{*}$, $T\ge 150$ K) is also discussed.