We have studied the magneto-optical properties of photoexcitations in $\mathrm{CsPb}{\mathrm{Br}}_3$ single crystal using the technique of picosecond time-resolved quantum beatings (QBs) in the circularly polarized photoinduced reflection as well as steady state magnetocircular dichroism (MCD) in $\mathrm{CsPb}{\mathrm{Br}}_3$ film. In the Voigt geometry at magnetic field strength $B$ > 0, we observed fast and slow QB oscillations that we attribute to the Larmor precession frequency of electrons and holes, respectively. From the linear frequency dependence on $B$, we extract the carrier anisotropic Landé $g$ factors for applied B along [010] and [001]; for electrons $|g_{\left[001\right]}^{\mathrm{e}}|=1.95\pm 0.04$ and $|g_{\left[010\right]}^{\mathrm{e}}|=1.82\pm 0.04$, whereas for holes $|g_{\left[001\right]}^{\mathrm{h}}|=0.69\pm 0.02$ and $|g_{\left[010\right]}^{\mathrm{h}}|=0.76\pm 0.02$. These values are in excellent agreement with a k·p model calculation applied to $\mathrm{CsPb}{\mathrm{Br}}_3$. Surprisingly, at $B=0$, we still observed QB oscillation of ∼500 MHz that we interpret as due to the Overhauser field that originates from the spin-aligned nuclei caused by the Knight field. From the measured MCD spectrum vs $B$, we obtained the $g$ factor of the bright excitons $g_{\mathrm{ex}}=2.18\pm 0.08$ showing that the $g$ value of holes in $\mathrm{CsPb}{\mathrm{Br}}_3$ is positive. We also measured the temperature and magnetic field dependencies of the electron and hole spin dephasing times which support the Elliot-Yafet spin-relaxation mechanism.

A detailed analysis of the magnetic properties of a vanthoffite-type mineral ${\mathrm{Na}}_6\mathrm{Mn}{\left({\mathrm{SO}}_4\right)}_4$ based on dc magnetization, low-temperature neutron powder diffraction, and theoretical calculations is reported. The mineral crystallizes in a monoclinic system with space group $P{2}_1/c$, where ${\mathrm{MnO}}_6$ octahedra are linked via ${\mathrm{SO}}_4$ tetrahedra, forming a two-dimensional (2D) sheet structure in the $bc$ plane of the crystal. This gives rise to superexchange interaction between two ${\text{Mn}}^{2+}$ ions mediated by two nonmagnetic bridging anions (Mn-O-O-Mn) and leads to an antiferromagnetic ordering below 3 K. The magnetic structure derived from neutron powder diffraction at 1.7 K depicts an antiferromagnetic spin arrangement in the $bc$ plane. The magnetic properties are modeled by numerical calculations using an exact diagonalization technique, which fits the experimental results and also provides the antiferromagnetic ground state of ${\mathrm{Na}}_6\mathrm{Mn}{\left({\mathrm{SO}}_4\right)}_4$. Both experimental and theoretical calculation reveal a quasi-2D type of magnetic interaction in this polyanionic system, where the dominant antiferromagnetic interaction exists in the plane. The determined collinear antiferromagnetic ground state is consistent with the theoretical predictions for a $J_1-J_2$ Heisenberg triangular antiferromagnetic model.

Motivated by the observation of nematic superconductivity in several systems, we revisit the problem of the leading pairing instability of two-component unconventional superconductors on the triangular lattice—such as $(p_x,p_y)$-wave and $(d_{x^2-y^2},d_{xy})$-wave superconductors. Such a system has two possible superconducting states: the chiral state (e.g., $p+ip$ or $d+id$), which breaks time-reversal symmetry, and the nematic state (e.g., $p+p$ or $d+d$), which breaks the threefold rotational symmetry of the lattice. Weak-coupling calculations generally favor the chiral over the nematic superconducting state, raising the question of what mechanism can stabilize the latter. Here, we show that the electromagnetic field fluctuations can play a crucial role in selecting between these two states. Specifically, we derive and analyze the effective free energy for the two-component superconducting order parameter after integrating out the gauge-field fluctuations, which is formally justified if the spatial order parameter fluctuations can be neglected. A nonanalytic cubic term arises, as in the case of a conventional $s$-wave superconductor. However, unlike the latter, the cubic term depends on the relative phase and on the relative amplitudes between the two order parameter components, in such a way that it generally favors the nematic state. This result is a direct consequence of the fact that the stiffness of the superconducting order parameter is not isotropic. Competition with the quartic term, which favors the chiral state, leads to a renormalized phase diagram in which the nematic state displaces the chiral state over a wide region in the parameter space. We analyze the stability of the fluctuation-induced nematic phase, generalize our results to tetragonal lattices, and discuss their applicability to candidate nematic superconductors, including twisted bilayer graphene.

We study generalized multifractality characterizing fluctuations and correlations of eigenstates in disordered systems of symmetry classes AII, D, and DIII. Both metallic phases and Anderson-localization transitions are considered. By using the nonlinear sigma-model approach, we construct pure-scaling eigenfunction observables. The construction is verified by numerical simulations of appropriate microscopic models, which also yield numerical values of the corresponding exponents. In the metallic phases, the numerically obtained exponents satisfy Weyl symmetry relations as well as generalized parabolicity (proportionality to eigenvalues of the quadratic Casimir operator). At the same time, the generalized parabolicity is strongly violated at critical points of metal-insulator transitions, signaling violation of local conformal invariance. Moreover, in classes D and DIII, even the Weyl symmetry breaks down at critical points of metal-insulator transitions. This last feature is related to a peculiarity of the sigma-model manifolds in these symmetry classes: they consist of two disjoint components. Domain walls associated with these additional degrees of freedom are crucial for ensuring Anderson localization and, at the same time, lead to the violation of the Weyl symmetry.

We study the emission of femtosecond spin current pulses at Fe/Au interfaces in well-defined epitaxial Fe/Au(001) and Fe/Au/Fe(001) structures. Using the exceptional sensitivity of optical second harmonic generation to spin currents and transient spin density at interfaces, we perform direct measurements of the ultrafast spin transport. To link the optical data to microscopic material parameters, we use a simple model of electron emission from ultrathin Fe layers, which ignores the diffusive transport in Fe, and develop a description of superdiffusive spin transport in Au. This allows us to extract the electron velocity and scattering rates in Au, estimate the dynamics of secondary carrier population in Fe, and retrieve the spin current pulse shape from the experimental data. We study the variation of spin current pulse shape with increasing thickness of the Fe emitter layer. The observed dependency is well explained by including the diffusive spin transport in Fe. This example shows the potential of our approach to develop spin current emitters delivering desired pulse shapes.

The nature of the superconducting state in kagome metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ is a key issue in need of experimental clarification. Here, we report on a study of the superconducting order parameter in the kagome superconductor ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ through simultaneous “soft” point-contact spectroscopy and resistivity measurements under both ambient and a hydrostatic pressure. Signatures of two-gap superconductivity are resolved in the soft point-contact spectra, accompanied by an asymmetric excess conductance above $T_c$. Quantitative analysis based on the two-dimensional Blonder-Tinkham-Klapwijk model reveals an ($s+s$)-wave superconducting gap with $2{\mathrm{\Delta }}_0/k_B{T}_c\simeq 7.2$, placing ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ in the strong-coupling regime. The strong-coupling two-gap feature indicates a high electronic density of states (DOS) and possible existence of flat-band-driven multiple van Hove singularities (VHSs) at the Fermi level. The presence of asymmetric excess spectral conductance above $T_c$ hints at a modest electronic correlation in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. Under a hydrostatic pressure of 2.1 kbar, the nodeless multigap nature of the superconducting state remains, whereas both the larger gap and the excess spectral conductance are greatly suppressed, accompanied by an enhanced $T_c$. An estimate of the spectral-weighted gap ratio reveals a weakened coupling strength, indicative of a reduced total superconducting DOS upon pressure. Our results point to key roles of both flat-band-associated VHSs and electronic correlation in the onset of kagome superconductivity and shed some light on the interplay between charge-density-wave order and superconductivity in vanadium-based kagome superconductors.

We consider a chiral spin liquid constructed using the parton theory. This state supports non-Abelian anyons and neutral fermions, which share some similarities with the celebrated Moore-Read state. The edge physics of the parton state and the Moore-Read state is very different. Based on conformal field theory (CFT) analysis, it is proposed that the edge states exhibit an emergent SO(5) symmetry. The counting of edge states in the low-lying SO(5)${}_1$ CFT towers is computed. The chiral spin liquid is generated using tensor network methods, which allows us to confirm the SO(5)${}_1$ counting using entanglement spectrum. An additional feature of multiple branches in the entanglement spectrum is observed and analyzed.

Charge transport in topological insulators is primarily characterized by so-called topologically projected helical edge states, where charge carriers are correlated in spin and momentum. In principle, dissipationless current can be carried by these edge states as backscattering from impurities and defects is suppressed as long as time-reversal symmetry is not broken. However, applied magnetic fields or underlying nuclear spin defects in the substrate can break this time-reversal symmetry. In particular, magnetic impurities lead to backscattering by spin-flip processes. We have investigated the effects of pointwise magnetic impurities on the transport properties of helical edge states in the Bernevig, Hughes, and Zhang model using the nonequilibrium Green's function formalism and compared the results to a semianalytic approach. Using these techniques we study the influence of impurity strength and spin impurity polarization. We observe a secondary effect of defect-defect interaction that depends on the underlying material parameters which introduces a nonmonotonic response of the conductance to defect density. This in turn suggests a qualitative difference in magnetotransport signatures in the dilute and high-density spin impurity limits.

In this paper, we investigate the resistance fluctuations near the Lifshitz transition in ${\mathrm{WTe}}_2$ devices. We identify the Lifshitz transition from electrical and thermal transport studies. The band structure obtained at low temperatures using quantum oscillation measurements consists of two hole pockets and two electron pockets at the Fermi energy, the hole pockets vanish above the Lifshitz transition temperature. The electrical noise shows a conspicuous peak near the Lifshitz transition temperature; we establish this noise to be arising from charge carrier scattering between the Weyl nodes and proximate areas of the bands. Our comparative study of the noise on ${\mathrm{WTe}}_2$ devices fabricated on substrates with different scattering mechanisms elucidates the effect of interband scattering on the physics of Weyl semimetals.

The timescale of electronic cooling is an important parameter controlling the performance of devices based on quantum materials for optoelectronic, thermoelectric, and thermal management applications. In most conventional materials, cooling proceeds via the emission of phonons, a process that can bottleneck the carrier relaxation dynamics, thus degrading the device performance. Here we present the theory of near-field radiative heat transfer that occurs when a two-dimensional electron system is coupled via the nonretarded Coulomb interaction to a three-dimensional bulk that can behave as a very efficient electronic heat sink. We apply our theory to study the cooling dynamics of surface states of three-dimensional topological insulators and of graphene in proximity to small-gap bulk materials. The “Coulomb cooling” we introduce is alternative to the conventional phonon-mediated cooling, can be very efficient, and can dominate the cooling dynamics under certain circumstances. We show that this cooling mechanism can lead to a sub-picosecond timescale, significantly faster than the cooling dynamics normally observed in Dirac materials.

Tight-binding single-particle models on simple Bravais lattices in space dimension $d\ge 2$, when exposed to commensurate DC fields, result in the complete absence of transport due to the formation of Wannier-Stark flatbands [Phys. Rev. Res. 3, 013174 (2021)]. The single-particle states localize in a factorial manner, i.e., faster than exponential. Here, we introduce interaction among two such particles that partially lifts the localization and results in metallic two-particle bound states that propagate in the directions perpendicular to the DC field. We demonstrate this effect using a square lattice with Hubbard interaction. We apply perturbation theory in the regime of interaction strength $\left(U\right)\ll \text{hopping strength}\left(\mathfrak{h}\right)\ll \text{field strength}\left(\mathcal{F}\right)$, and obtain estimates for the group velocity of the bound states in the direction perpendicular to the field. The two-particle group velocity scales as $U{(\mathfrak{h}/\mathcal{F})}^{\nu }$. We calculate the dependence of the exponent $\nu$ on the DC field direction and on the dominant two-particle configurations related to the choices of unperturbed flatbands. Numerical simulations confirm our predictions from the perturbative analysis.

We study the phase diagram and the dynamical spin structure factor of the spin-1/2 $J_1–J_3$ Heisenberg model on the square lattice using density matrix renormalization group, exact diagonalization (ED), and cluster perturbation theory (CPT). By extrapolating the order parameters and studying the level crossings of the low-lying energy and entanglement spectra, we obtain the phase diagram of this model and identify a narrow region of quantum spin liquid (QSL) phase followed by a plaquette valence-bond solid (PVBS) state in the intermediate region, whose nature has been controversial for many years. More importantly, we use CPT and ED to study the dynamical spin structure factor in the QSL and the PVBS phase. In the QSL phase, the high-energy magnon mode completely turns into some dispersive weak excitations around the $X$ and $M$ points. For the PVBS phase, the low-energy spectrum is characterized by a gapped triplet excitation, and at the high energy, we find another branch of dispersive excitation with broad continua, which is unlike the plaquette phase in the $2\times 2$ checkerboard model. In the latter case, the second branch of excitation is nearly flat due to the weak effective interactions between the local excitations of the plaquettes. And in the $J_1–J_3$ Heisenberg model, the uniform interactions and the spontaneously translational symmetry breaking of the PVBS phase make the difference in the excitation spectra.

We study ultrafast relaxation of photoinjected electrons confined within the Γ valley of InP, based on the transient electron distribution functions determined using time- and angle-resolved photoemission spectroscopy. To elucidate fundamental processes that lead to the quasithermalization of electron subsystem at the nonthermal regime of relaxation, the dynamics are investigated in two $p$-type samples with different doping levels, which exhibit different energy-relaxation rates of energetic electrons. In both samples, the photoinjected nonthermal-electron distributions are quasithermalized only at a finite time delay of several hundreds of femtoseconds. Although the magnitude of the time delay for quasithermalization depends on the initially prepared nascent electron distribution and on the energy-relaxation rates in different samples, the quasithermalization is established at the specific electron distribution of ensembles; electrons with a density of $\sim 1\times {10}^{17}\mathrm{c}{\mathrm{m}}^{-3}$ are condensed in the phase space characterized by the maximum energy of 0.27 eV and by the maximum wave vector of $0.09{\AA }^{-1}$. The essential features of quasithermalization observed in InP are the same as those recently reported for GaAs [Phys. Rev. B 104, 245201 (2021)], showing that the delayed quasithermalization is general for electrons photoinjected by ultrashort-laser excitation in the Γ valley of direct-gap semiconductors.

The new Kondo material ${\mathrm{YbIr}}_3{\mathrm{Si}}_7$, similar to other Kondo insulators, has been reported to exhibit charge-neutral fermionic excitations through measurements of specific heat and thermal conductivity at low temperatures. We performed ${}^{29}\mathrm{Si}\text{−}\mathrm{NMR}$ on ${\mathrm{YbIr}}_3{\mathrm{Si}}_7$ to investigate the magnetic response of charge-neutral fermions from a microscopic perspective. In low magnetic fields parallel to the $c$ axis, a single NMR peak in the paramagnetic state splits into three peaks below $T_{\mathrm{N}}$. In contrast, only a slight shift of the single NMR peak was observed in high magnetic fields. This spectral change as a function of the $c$-axis magnetic field is interpreted as a spin-flop transition, at which the magnetic moments oriented along the $c$ axis antiferromagnetic (AFM-I) phase are rotated to the $ab$ plane with a ferromagnetic component along the $c$-axis (AFM-II phase). In the vicinity of the spin-flop magnetic field $H_{\mathrm{M}}$, the nuclear spin-lattice relaxation rate $1/T_1$ was found to be proportional to temperature at low temperatures, indicating the existence of charge-neutral fermions. Furthermore, a peak of $1/T_1$ versus the $c$-axis magnetic field suggests that the charge-neutral fermions in ${\mathrm{YbIr}}_3{\mathrm{Si}}_7$ are closely related to its magnetic properties. Our findings shed light on the origin of charge-neutral fermions in insulators.

We use molecular dynamics simulation to study the effect of initial temperature on the shock compression of porous silica with densities of 50%, 75%, and full density. We find that the response is strongly influenced by temperature for shocks in a relatively narrow pressure range. Within this range, near the Hugoniot elastic limit, initial preheating from 300 to 1000 K can increase the final shock density by as much as 30%. However, this enhanced densification effect with preheating temperature is negligible at lower pressures (elastic compression), and is equally negligible at higher pressures (strong shock regime). For vitreous silica, the effect of initial temperature is greatest in the compaction regime (pressures between 1 and 3 GPa) where material strength plays a significant role in the mechanical response. Here, preheating can dramatically increase the final density in silica for a given pressure and porosity. Microstructure was found to influence the behavior, with aerogel structures more strongly impacted than nanopowder packings—likely due to the fact that their strengths are more susceptible to thermal softening. For similar reasons, the effect of temperature is greatest in porosities between 50% and 75% density. In some particular cases, the effect of preheating on the Hugoniot can be comparable to increasing porosity by 25%.

A charge-$4e$ superconductor forms due to the condensation of quartets of electrons. While in previous works, the mechanism for the formation of charge-$4e$ superconductivity has been analyzed in terms of the binding of Cooper pairs in unconventional superconductors, its properties in the fermionic sector have not been studied systematically due to its inherently interacting nature even at the mean-field level. Here we propose a solvable model for a charge-$4e$ superconductor—a spinful version of the Sachdev-Ye-Kitaev model with an anomalous quartic term. We show that the ground state is gapless and resembles a heavy Fermi liquid. We analytically solve for the superfluid density and show that it is perturbative in the strength of the charge-$4e$ order parameter, in sharp contrast with a regular (charge-$2e$) superconductor. Upon lowering the temperature, we show that the correlation between charge-$4e$ order and regular interaction terms can drive a first-order phase transition to a charge-$2e$ superconducting state.

The rapid progress in the engineering of topological edge/interface states has attracted considerable interest in both photonic and phononic systems originating from their electronic equivalent systems. However, most classical wave types, such as electromagnetic/acoustic waves in topological engineering, are described by two-dimensional (2D) partial differential equations (PDEs), which are not suitable for thin plate motions. We present a 1D elastic topological interface state induced by symmetry breaking to bridge a knowledge gap in 4D PDE, particularly for flexural waves on thin-plate frameworks. We also leverage the localized state at the interface for energy harvesting, which can provide a sufficiently high voltage feeder by piezoelectric effects.