We study nonperturbatively the anomalous Hall current and its high harmonics generated in Weyl and Dirac semimetals by strong elliptically polarized laser fields in the context of kinetic theory. We find a crossover between perturbative and nonperturbative regimes characterized by the electric field strength ${\mathcal{E}}^{*}=\frac{\mu \omega }{2e{v}_F}$ ($\omega$, laser frequency; $\mu$, Fermi energy; $v_F$, Fermi velocity). In the perturbative regime, the anomalous Hall current depends quadratically on the field strength $\left(\mathcal{E}\right)$, whereas the higher-order corrections, as well as high harmonics, vanish at zero temperature. In the nonperturbative regime, the anomalous Hall current saturates and decays as $(ln\mathcal{E})/\mathcal{E}$, while even-order high harmonics are generated when in-plane rotational symmetry is broken. Based on the analytical solution of the Boltzmann equation, we reveal the topological origin of the sharp crossover: the Weyl monopole stays inside or moves outside of the Fermi sphere, respectively, during its fictitious motion in the perturbative or nonperturbative regimes. Our findings establish a nonlinear response intrinsically connected to topology, characteristic of Weyl and Dirac semimetals.

The geminate annihilation of two triplet excitons created by singlet exciton fission is affected by the dimensionality of transport as determined by typically anisotropic triplet exciton mobilities in organic molecular crystals. We analyze this process using a random-walk model where the time dynamics of the geminate annihilation probability is determined by the average exciton hopping times along the crystallographic directions. The model is then applied to the geminate fluorescence dynamics in rubrene, where the main channel for triplet-triplet annihilation is via triplet fusion and subsequent photon emission, and we identify the transitions between transport in one, two, and three dimensions.

The Caldeira-Leggett model of quantum Brownian motion is generalized using a generic velocity-dependent coupling. That leads to the description of a set of models able to capture Markovian and non-Markovian versions of Brownian and Lévy statistics, depending on the functional form of the coupling and on the spectral function of the reservoir. One specific coupling force is found that establishes a connection with Lévy statistics of cold atoms in Sisyphus laser cooling. In the low-velocity limit, this also gives rise to additional inertia of the Brownian particle, reproducing the Abraham-Lorentz equation from first principles for a superohmic bath. Through path-integral quantization in Euclidean time, the environment is integrated out, leaving a set of nonlocal effective actions. These results further serve as starting points for several numerical calculations, particularly decoherence properties of nonohmic baths.

We report the magnetic ordering and structural distortion in PrFeAsO crystals, the basis compound for one of the oxypnictide superconductors, using high-resolution x-ray diffraction, neutron diffraction, and x-ray resonant magnetic scattering (XRMS). We find the structural tetragonal-to-orthorhombic phase transition at $T_{\mathrm{S}}=147\mathrm{K}$, the AFM phase transition of the Fe moments at $T_{\mathrm{Fe}}=72\mathrm{K}$, and the Pr AFM phase transition at $T_{\mathrm{Pr}}=21\mathrm{K}$. Combined high-resolution neutron diffraction and XRMS show unambiguously that the Pr moments point parallel to the longer orthorhombic $\mathit{a}$ axis and order antiferromagnetically along the $\mathit{a}$ axis but ferromagnetically along the $\mathit{b}$ and $\mathit{c}$ directions in the stripelike AFM order. The temperature-dependent magnetic order parameter of the Pr moments shows no evidence for a reorientation of moments.

Low-energy magnon excitations in magnetoelectric ${\mathrm{LiFePO}}_4$ have been investigated by high-frequency–high-field electron spin resonance spectroscopy in magnetic fields up to $B=58\mathrm{T}$ and frequencies up to $f=745\mathrm{GHz}$. For magnetic fields applied along the easy magnetic axis, the excitation gap softens and vanishes at the spin-flop field of $B_{\mathrm{SF}}=32\mathrm{T}$ before hardening again at higher fields. In addition, for $B\lesssim B_{\mathrm{SF}}$ we observe a resonance mode assigned to excitations due to Dzyaloshinskii-Moriya (DM) interactions, thereby evidencing a sizable DM interaction of $\approx 150\mu \mathrm{eV}$ in ${\mathrm{LiFePO}}_4$. Both the magnetization and the excitations up to high magnetic fields are described in terms of a mean-field theory model which extends recent zero-field inelastic neutron scattering results. Our results imply that magnetic interactions as well as magnetic anisotropy have a sizable quadratic field dependence which we attribute to significant magnetostriction.

The quantum spin liquid (QSL) is an exotic phase of magnetic materials where the spins continue to fluctuate without any symmetry breaking down to zero temperature. Among the handful reports of QSL with spin $S\ge$ 1, examples with magnetic ions on a three-dimensional (3D) magnetic lattice are extremely rare since both larger spin and higher dimension tend to suppress quantum fluctuations. In this work, we offer a new strategy to achieve 3D QSL with high spin by utilizing two types of transition metal ions; both are magnetically active but located at crystallographically inequivalent positions. We design a 3D magnetic system ${\mathrm{Ba}}_3{\mathrm{NiIr}}_2{\mathrm{O}}_9$ consisting of interconnected corner-shared ${\mathrm{NiO}}_6$ octahedra and face-shared ${\mathrm{Ir}}_2{\mathrm{O}}_9$ dimer, both having triangular arrangements in a-b plane. X-ray absorption spectroscopy measurements confirm the presence of ${\mathrm{Ni}}^{2+}$ ($S=1)$. Our detailed thermodynamic and magnetic measurements reveal that this compound is a realization of gapless QSL state down to at least 100 mK. Ab initio calculations find a strong magnetic exchange between Ir and Ni sublattices and in-plane antiferromagnetic coupling between the dimers, resulting in dynamically fluctuating magnetic moments.

We consider quantum phase transitions in a system of bright dipolar excitons which can form bound pairs (dipolar biexcitons). The biexciton energy is tuned from negative to positive values through the scattering threshold. At sufficiently large density an exciton superfluid transforms into a superfluid of biexcitons. With the average relative momenta of excitons in the pairs being beyond the light cone, the transition is accompanied by a reduction of the photoluminescence intensity. Effective magnetic fields due to the long-range exchange splitting of exciton states shift the position of the gap in the elementary excitation spectrum to a circle of degenerate minima in the $\mathit{k}$ space. Closing the gap results in the formation of exciton stripes polarized linearly along the direction of their translational symmetry. In the biexciton energy vs density phase diagram the novel phase intervenes between the dark biexciton and radiative exciton superfluids. We conclude that formation of a BCS-like biexciton condensate induces correlated alignment of the effective magnetic fields and excitonic spins. We outline important differences in the predicted mechanism from the phenomenon of spin-orbit-coupled Bose-Einstein condensation.

We analyze the phase diagram of the spin-$\frac{1}{2}$ Yukawa–Sachdev-Ye-Kitaev model, which describes complex spin-$\frac{1}{2}$ fermions randomly interacting with real bosons via a Yukawa coupling, at finite temperatures and varying fermion density. A recent work [Wang and Chubukov, Phys. Rev. Res. 2, 033084 (2020).] showed that, upon varying the filling or chemical potential, a first-order quantum phase transition exists between a non-Fermi-liquid (NFL) phase and an insulating phase. Here we show that in such a model with time-reversal symmetry this quantum phase transition is preempted by a pairing phase that develops as a low-temperature instability. As a remnant of the would-be NFL-insulator transition, the superconducting critical temperature rapidly decreases beyond a certain chemical potential. On the other hand, depending on the parameters, the first-order quantum phase transition extends to finite temperatures and terminates at a thermal critical point, beyond which the NFL and the insulator become the same phase, similar to that of the liquid-gas and metal-insulator transition in real materials. We determine the pairing phase boundary and the location of the thermal critical point via combined analytic and quantum Monte Carlo numeric efforts. Our results provide a model realization of the transition of NFLs towards superconductivity and insulating states and therefore offer a controlled platform for future investigations of the generic phase diagram that hosts a NFL, insulator, and superconductor and their phase transitions.

Single photons from solid-state quantum emitters are playing a crucial role in the development of photonic quantum technologies and, by extension, higher order states, such as N-photon Fock states, allow for applications, e.g., in quantum-enhanced sensing. To verify the applicability of these states in future quantum technological implementations involving photon-atom interactions (i.e., storage of a quantum state in alkali vapor and photon delay) we utilize in the present study the dispersion of a hot cesium vapor at the $D_1$ line to realize a temporal delay for two-photon Fock states as a result of the slow-light effect. Single photons are generated on demand from an InGaAs quantum dot, while their quantum interference at a beam splitter is used to generate a two-photon Fock state. We verify the successful propagation and the preservation of the two-photon Fock states after the interaction with the slow-light medium, while a significant temporal delay (five times the initial photon length) is achieved with a high vapor transmission of $90\%$.

The recently discovered two-dimensional (2D) layered semiconductor $\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{N}}_4$ has aroused great interest due to its unique 2D material characteristics. In this study, we found that differences in the structural details for $\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{N}}_4$ may lead to differences in the intensity of second harmonic generation (SHG) and its response to strain. Accordingly, SHG can be used as a simple technique to identify the structural details of this system. We further calculated the SHG effects of $\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{N}}_4$ derivatives and investigated their strain-regulation mechanism, especially including the anomalous SHG responses under strain for $\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{P}}_4$ and $\mathrm{Mo}{\mathrm{Ge}}_2{\mathrm{P}}_4$, differing from other known 2D materials. The studies may have forward-looking significance for the research of nonlinear optics and optoelectronics in this novel 2D material system.

Twisted atomic bilayers are emerging platforms for manipulating chiral light-matter interaction at the extreme nanoscale, due to their inherent magnetoelectric responses induced by the finite twist angle and quantum interlayer coupling between the atomic layers. Recent studies have reported the direct correspondence between twisted atomic bilayers and chiral metasurfaces, which features a chiral surface conductivity, in addition to the electric and magnetic surface conductivities. However, far-field chiral optics in light of these constitutive conductivities remains unexplored. Within the framework of the full Maxwell equations, we find that the chiral surface conductivity can be exploited to realize perfect polarization transformation between linearly polarized light. Remarkably, such an exotic chiral phenomenon can occur either for the reflected or transmitted light. Moreover, we reveal that all transmitted light through the judiciously designed chiral surface conductivity can always have the polarization different from the incident light, irrespective of the incident angle.

A new state of matter, an excitonic insulator (EI) state, was predicted to emerge from Bose-Einstein condensation of electron-hole pairs. Some candidate materials were suggested but it has been elusive to confirm its existence. Recent works gave renewed support for the EI picture of the charge density wave (CDW) state below the critical temperature $T_c\approx 200$ K of $1T\text{−}{\mathrm{TiSe}}_2$. Yet, an important link to its establishment is to show that a majority fraction of the measured $T_c$ indeed follows from the Coulomb interaction alone, while a quantitative match of the $T_c$ may require assistance from the electron-lattice coupling. This will establish that the CDW is formed predominantly by the Coulomb interaction and help confirm the EI view for ${\mathrm{TiSe}}_2$. Here, we provide such calculations by solving the exciton gap equation with material-specific electronic structures. We obtain, with no fitting parameters, $T_c\approx 135\pm 27$ K for the normal state gap of $E_g\approx 74\pm 15$ meV. It seems that the calculated $T_c$ from Coulomb interaction gives a majority fraction of experimental $T_c$ for recently determined values of $E_g$. The measured doping dependence of $T_c$ was satisfactorily reproduced as well. Also in agreement with experiments is the same set of calculations of the photoemission spectroscopy and density of states. The semiconducting state above and EI below $T_c$ together should give a coherent picture of $1T\text{−}{\mathrm{TiSe}}_2$.

The Euler number is a topological number recently debuted in the topological physics. It is defined for a set of bands unlike the Chern number defined for a single band. We propose a simple model realizing the topological Euler insulator. We utilize the fact that the Euler number in a three-band model in two dimensions is reduced to the Pontryagin number. A skyrmion structure appears in momentum phase, yielding a nontrivial Euler number. Topological edge states emerge when the Euler number is nonzero. We discuss how to realize this model in electric circuits. We show that topological edge states are well signaled by impedance resonances.

In the present work, we report different interlayer excitonic states of an aligned $\mathrm{Mo}{\mathrm{Se}}_2/\mathrm{Mo}{\mathrm{S}}_2$ incommensurate van der Waals (vdW) heterostructure (HS). The HS was fabricated by stacking chemical vapor deposited monolayers, and it was studied using photoluminescence (PL) measurements. We observed the emergence of two low-energy peaks in the PL spectrum of the HS measured at 100 K, which were absent in the constituent monolayers. The orbital resolved electronic band structure and the optical absorption considering the electron-hole interaction for these HSs were calculated using first-principles density functional theory simulations, which showed energy band hybridization and the presence of interlayer excitons (ILEs). Based on these observations, the peak at $\sim 1.57$ eV was assigned to a momentum direct ILE, while the other peak at $\sim 1.35$ eV showed feeble emission intensity and was assigned to a momentum indirect ILE. The emergence of both of these excitonic peaks in the HS PL spectrum can be attributed to the formation of a spatially periodic moiré potential at a nanometer length scale resulting in hybridization. Our results can help to understand the physics of ILEs and to engineer vdW HSs for efficient optoelectronic devices.

We use magnetoconductivity to study the magnetic proximity effect on surface states of doped topological insulators. Our bilayers consist of a layer of ${\mathrm{Fe}}_7{\mathrm{Se}}_8$, which is a metallic ferrimagnet, and a layer of ${\mathrm{Bi}}_{0.8}{\mathrm{Sb}}_{1.2}{\mathrm{Te}}_3$, which is a highly hole-doped topological insulator. Using transport measurements and a modified Hikami-Larkin-Nagaoka model, we show that the ferromagnet shortens significantly the effective coherence length of the surface states, suggesting that a gap is opened at the Dirac point. We show that the magnetically induced gap persists on surface states which are separated from the magnet by a topological insulator layer as thick as 170 nm. Furthermore, the size of the gap is found to be proportional to the magnetization that we extract from the anomalous Hall effect. Our results give information on the ties between carrier density, induced magnetization, and magnetically induced gap in topological-insulator–ferromagnet bilayers. This information is important both for a theoretical understanding of magnetic interactions in topological insulators and for the practical fabrication of such bilayers, which are the basis of various suggested technologies, such as spintronic devices, far-infrared detectors, etc.

We report on low-temperature susceptibility and magnetization measurements made on single crystals of the recently discovered heavy-fermion superconductor ${\mathrm{UTe}}_2$ and compare the results with the two ambient pressure ferromagnetic superconductors URhGe and UCoGe. Hysteresis curves in the superconducting phase show a familiar diamond shape superimposed on a large paramagnetic background. The Meissner state was measured by zero-field cooling in small fields of a few Oe as well as ac susceptibility measurements in small fields and resulted in 100% shielding, with a sharp transition. However, the field-cooling Meissner-Ochsenfeld effect (expulsion of flux) was negligible in fields greater than just a few Oe, but becomes nearly 30% of the perfect diamagnetic signal when the field was reduced to 0.01 Oe. The critical current due to flux pinning was studied by ac susceptibility techniques. Over the range in fields and temperature of this study, no signature of a ferromagnetic transition could be discerned. The lower critical field $H_{\mathrm{c}1}$ has been measured along the three crystallographic axes, and surprisingly, the anisotropy of $H_{\mathrm{c}1}$ contradicts that of the upper critical field. We discuss this discrepancy and show that it may provide additional support for a magnetic field-dependent pairing mediated by ferromagnetic fluctuations in ${\mathrm{UTe}}_2$.

The antiferromagnetic quantum Ising chain has a quantum critical point which belongs to the universality class of the transverse Ising model (TIM). When a longitudinal field ($h$) is switched on, the phase transition is preserved, which turns to first order for $h/\mathrm{\Gamma }\to \infty , \mathrm{\Gamma }$ being the strength of the transverse field. Here we will re-examine the critical properties along the phase transition line. During a quantum block renormalization group calculation, the TIM fixed point for $h/\mathrm{\Gamma }>0$ is found to be unstable. Using DMRG techniques, we calculated the entanglement entropy and the spin-spin correlation function, both of which signaled a divergent correlation length at the transition point with the TIM exponents. At the same time, the bulk correlation function has a jump and the end-to-end correlation function has a discontinuous derivative at the transition point. Consequently for finite $h/\mathrm{\Gamma }$ the transition is of mixed order.

The ${\mathrm{Th}}_7{\mathrm{Fe}}_3$ family of superconductors provides a rich playground for unconventional superconductivity. ${\mathrm{La}}_7{\mathrm{Ni}}_3$ is the latest member of this family, which we here investigate by means of thermodynamic and muon spin rotation and relaxation measurements. Our specific heat data provides evidence for two distinct and approximately isotropic superconducting gaps. The larger gap has a value slightly higher than that of weak-coupling BCS theory, indicating the presence of significant correlations. These observations are confirmed by transverse-field muon spin rotation/relaxation measurements. Furthermore, zero-field measurements reveal small internal fields in the superconducting state, which occur close to the onset of superconductivity and indicate that the superconducting order parameter breaks time-reversal symmetry. We discuss two possible microscopic scenarios—an unconventional $E_2(1,i)$ state and an $s+is$ superconductor, which is reached by two consecutive transitions—and illustrate which interactions will favor these phases. Our results establish ${\mathrm{La}}_7{\mathrm{Ni}}_3$ as the first member of the ${\mathrm{Th}}_7{\mathrm{Fe}}_3$ family displaying both time-reversal-symmetry-breaking and multigap superconductivity.

Motivated by the realization of the chiral superfluid phase of liquid ${}^3\mathrm{He}$ stabilized at lower pressures and over a wide temperature range in planar aerogels, we examine equilibrium properties of the superfluid $A$ phase suffering from impurity scatterings due to planar correlated defects. In the specular limit of scattering due to the planar defects, an analog of Anderson's theorem for the $s$-wave Fermi superfluid is well satisfied in the chiral $A$ phase in the planar aerogels just like in the polar phase in nematic aerogels, and the chiral $A$ phase region is extended by such a planar anisotropy even with no strong coupling correction. It is also pointed out that the vortex energy in the chiral $A$ phase depends on the relative sense of the vorticity to the chirality. Based on this result, it is argued that, due to the presence of chiral domains induced by the randomness, the vortex lattice structure shows broken time-reversal symmetry and may include vortex loops.