We present a generic procedure for quantifying the interplay of electronic and lattice degrees of freedom in photodoped insulators through a comparative analysis of theoretical many-body simulations and time- and angle-resolved photoemission spectroscopy (TR-ARPES) of the transient response of the candidate excitonic insulator ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$. Our analysis demonstrates that the electron-electron interactions dominate the electron-phonon ones. In particular, a detailed analysis of the TR-ARPES spectrum enables a clear separation of the dominant broadening (electronic lifetime) effects from the much smaller band-gap renormalization. Theoretical calculations show that the observed strong spectral broadening arises from the electronic scattering of the photoexcited particle-hole pairs and cannot be accounted for in a model in which electron-phonon interactions are dominant. The competing interactions were quantified using the scaling analysis in the weak fluence regime. We demonstrate that the magnitude of the weaker subdominant band-gap renormalization sensitively depends on the distance from the semiconductor/semimetal transition in the high-temperature state, which could explain the apparent contradictions between various TR-ARPES experiments. The analysis presented here indicates that electron-electron interactions play a vital role (albeit not the sole one) in stabilizing the insulating state, and establishes the comparison between lifetime and gap evolution as an important probe of correlated insulators.

Lithographic quantum dots in strained-$\mathrm{Ge}/\mathrm{SiGe}$ have rapidly progressed from the realization of a single-hole quantum dot to multiqubit devices. We present a measurement of the out-of-plane $g$ factor for a single-hole quantum dot in this material. This experiment avoids the strong orbital effects present in the higher $p$-like orbitals in this configuration and validates the expected $g$-factor anisotropy. The results are in good agreement with calculations of the $g$ factor using the heavy- and light-hole spaces of the Luttinger Hamiltonian, showing strong tunability through both the $B$ field and the charge state.

Using a pair of $p$- and $n$-type semiconductors separated by a nanoscale vacuum gap, we introduce the concept of a prototype optoelectronics element, the “photonic $p\text{−}n$ junction,” as an analog of the electronic $p\text{−}n$ junction, and demonstrate that the cooperation of $p$-type and $n$-type carriers is ideal for tuning of photon tunneling and modulating near-field heat flux electrically. Three fundamental modes of photon-carrier interaction specific in the photonic $p\text{−}n$ junction tunable under a bias voltage are revealed, namely, surface plasmon polariton mode, symmetric depleted internal plasmon polariton mode, and symmetric accumulated surface plasmon polariton mode. The switch among these three modes explains the high-modulation performance. The presented concept opens an avenue in the design of smart thermal management devices and tunable or programmable integrated optoelectronic devices.

Impurities usually affect the transmission of acoustic signals, and how to effectively suppress their adverse effects is an important content in the research of acoustic materials. In a recent theoretical proposal, it was pointed out that perfect transmission through impurities can be achieved by adding a gain-loss distribution respecting parity-time ($\mathcal{PT}$) symmetry. In this paper, we demonstrate a similar but not identical way to realize the extraordinary physical property of impurity-shielding, which leads to perfect transmission at the so-called exceptional points. By systematically probing into the system composed of an equivalent medium slab sandwiched by a pair of $\mathcal{PT}$-symmetric metasurfaces, we obtain the two complementary solutions of exceptional points corresponding to perfect transmission. Interestingly, the two solutions of exceptional points coalesce into one when the mass density of the slab approaches zero. At such a critical point, we find that the impurity-shielding effect can be perfectly demonstrated, irrespective of embedded impurities of almost any shape and materials, whether they are Hermitian or non-Hermitian. In addition, the proposed system is frequency independent, indicating that the prototype can work well even in the subwavelength range. Our paper shows that exceptional points can be used to eliminate scattering of impurities in a density-near-zero medium, revealing their potential applications in acoustic sensing, directional imaging, and other related wave disciplines.

We investigate the one-dimensional longer-range Kitaev chain with an on-site complex Aubry-André-Harper (AAH) potential. For the incommensurate (commensurate) case, by calculating the real-space (Bloch) winding number and energy spectrum we discover the AAH potential-induced phases and present the related phase diagrams. We also find that the imaginary phase shift in AAH potentials can suppress these topological phases for both cases. For the incommensurate case, the inverse participate ratios of the eigenstates with the smallest $\left|E\right|\mathrm{s}$ indicate the existence of the incommensurate AAH potential-induced phases, and the result of the mean inverse participate ratio shows that these incommensurate AAH potential-induced phases are not always strongly localized.

To stabilize the long-range magnetic ordering in two-dimensional (2D) materials is a challenge from both fundamental and application points of view. It is difficult to be realized due to the delicate competition between thermal energy, magnetic exchange energy, and spin frustration. Here we study the temperature and field effects on the spin structure of two-dimensional van der Waals system ${\mathrm{MnPS}}_3$ using dc magnetic susceptibility and electron spin resonance (ESR) techniques. Clear development of three-step transitions was observed in the in-plane susceptibility. A low-temperature plateau from 78 to 38 K can be related to the Heisenberg antiferromagnetic (HAFM) ordering with spins pointing to the $z$ direction. Following, a sharp rising after the plateau suggests a transition into the $XY$ system. The rising slope slows down at 30 K indicating a transition into another phase, possibly a vortex-antivortex state. By fitting the temperature-dependent ESR parameters with the Berezinskii-Kosterlitz-Thouless (BKT) model in the range 100–300 K, we propose a spin 2D system with the vortex-antivortex structure below the HAFM transition. Susceptibility and ESR data show that the application of a high field along the $z$ direction destroys both the $XY$ phase and the BKT transition, reconciling to the scenario of a topological phase transition at zero field.

We develop an explicitly gauge-invariant semiclassical approach to investigate the nonlinear response of superconductors to monochromatic terahertz radiation. We demonstrate that in clean superconductors charge conservation forbids nonlinear response to a uniform field. We apply our approach to quasi-two-dimensional films and obtain an explicit expression for the photoinduced current. We find that the photoinduced current exhibits a strong dependence on polarization and the incidence angle of the radiation. Our predictions may be directly verified in experiments with quasi-two-dimensional superconducting films.

${\mathrm{Mg}}_2{\mathrm{Si}}_{1\text{−}x}{\mathrm{Sn}}_x$ solid solutions show enhanced thermoelectric performance when the Sn mole fraction $x$ is approximately $x=0.7$. This has been discussed in terms of complexity of the electronic structure arising from the crossover of two bottom conduction bands, but direct detailed understanding of the origins and the precise nature of this band convergence is limited. Here, we report first-principles calculations of the band edge changes analyzed by band unfolding and crystal orbital Hamilton population techniques. We find that strain is particularly important at this crossing. ${\mathrm{Mg}}_2\mathrm{Si}$ and ${\mathrm{Mg}}_2\mathrm{Sn}$ show opposite trends in the conduction band edge shifts leading to a band crossover at $x\sim 0.625$. However, there are also important effects due to disorder. Transport calculations show that enhancement of the figure of merit $ZT$ value owes to the combined effects of lattice strain, band edge overlap, composition change, and disorder.

In a magnetic superlattice composed of kinks in a ferromagnetic spin array, the change in the kink number requires the movement of the kinks to and from the crystal surface. Namely, the kinks must have a velocity, and the superlattice must be nonequilibrium. Evidence of the nonequilibrium state has never been observed in previous model compounds. In ${\mathrm{MnNb}}_3{\mathrm{S}}_6$, a long magnetization relaxation was observed, and the nature of the nonequilibrium state was more pronounced in the kink annihilation process rather than the kink creation process. The annihilation process can be phenomenologically reproduced using the unfrustrated magnetic clusters model. The nonequilibrium state in the annihilation process has a longer relaxation time than that in the nucleation process, since an energy barrier exists only in the latter.

In this work, we employ a two-dimensional square lattice model hosting both magnetic and ferroelectric orders to analyze the role of the Dzyaloshinskii-Moriya interaction in the ground state configuration and quasiparticle excitations. The inverse Dzyaloshinskii-Moriya mechanism stabilizes a spin spiral accompanying an enhanced ferroelectric polarization. As the bosonic excitations of the magnetic and ferroelectric orders, magnons and ferrons are coupled via the dynamical Dzyaloshinskii-Moriya interaction. We derive explicitly the magnon-ferron coupling and investigate the resulting magnon-ferron hybridization in different spin-textured phases driven by an external magnetic field, from which a field-associated nonreciprocal magnon-ferron coupling is predicted in the spin conical phase.

The optical conductivity of the new Dirac semimetal candidate ${\mathrm{Ir}}_2{\mathrm{In}}_8\mathrm{Se}$ is measured in a frequency range from 40 to 30 000 ${\mathrm{cm}}^{-1}$ at temperatures from 300 K down to 10 K. The measurement reveals that the compound is a low carrier density metal. We find that the real part of the conductivity ${\sigma }_1\left(\omega \right)$ is linear in frequency over a broad range from 500 to 4000 ${\mathrm{cm}}^{-1}$ at 300 K and varies slightly with cooling. This linearity strongly suggests the presence of three-dimensional linear electronic bands with band crossings near the Fermi level. Band-structure calculations indicate the presence of type-II Dirac points. By comparing our data with the optical conductivity computed from the band structure, we conclude that the observed linear dependence mainly originates from the Dirac cones and the transition between the Dirac cones and the next lower bands. In addition, a weak energy gap feature is resolved below the charge density wave phase transition temperature in the reflectivity spectra. An enhanced structure arising from imperfect Fermi surface nesting is identified in the electronic susceptibility function, suggesting a Fermi surface nesting driven instability.

A recent study has shown that it is possible to have enhancement, in contrast to an expected suppression, in tunneling density of states (TDOS) in a Luttinger liquid (LL) which is solely driven by the nonlocal density-density interactions. Also, it is well known that a LL in proximity to a superconductor (SC) shows enhancement in TDOS in the vicinity of the junction in the zero-energy limit. In this paper, we study the interplay of nonlocal density-density interaction and superconducting correlations in the TDOS in the vicinity of the SC-LL junction, where the LL may be realized on the edge of an integer or a fractional quantum Hall state. We show that the interplay of superconducting proximity effect and nonlocal interactions can give rise to enhancement in TDOS in the weak interaction limit, beyond what was previously observed. We also show that, in the full parameter regime comprising both the local and the nonlocal interactions, the region of enhanced TDOS for LL junction with “superconducting” boundary condition and that of “nonsuperconducting charge-conserving” boundary condition [discussed in A. Ratnakar and S. Das, Phys. Rev. B 104, 045402 (2021)] are mutually exclusive. We show that this fact can be understood in terms of symmetry relation established between the superconducting and nonsuperconducting sectors of the theory. We compare the dependence of the proximity-induced pair potential and TDOS as a function of distance $x$ from the junction. We demonstrate that the dependence of the spatial power-law exponent for the “$\mathrm{TDOS}\left(x\right)/\mathrm{TDOS}(x\to 0)$” and the “pair potential($x$)/pair potential($x\to 0$)” are distinct function of the various local and nonlocal interaction parameters, which implies that the TDOS enhancement can not be directly attributed to the proximity-induced pair potential in the LL.

We develop a general theory to study strong random quenched disorder effects in systems of experimental relevance in the search for Majorana zero modes in topological superconductors. Using the random matrix theory in a class D ensemble, we simulate the transport properties of random quantum dots by attaching leads, and calculating the differential conductance in the $S$-matrix formalism. To add the concept of the length to the random system so that disordered Majorana nanowires can be simulated by the random matrix theory, we generalize the model of a single quantum dot to a chain of quantum dots by analogy with the superconductor-semiconductor nanowire Majorana platform. We first define a new concept, the robustness of zero-bias conductance peaks, in terms of an effective random Hamiltonian considering the self-energy of leads. We then study the joint distribution for the robustness and zero-bias conductance peaks, and find a strong correlation that the zero-bias conductance peak with stronger robustness is also prone to carry a larger conductance peak near $2e^2/h$. This trend is more prominent in shorter chains than in longer chains. This is consistent with experimentally observed zero-bias conductance associated with disorder-induced trivial Andreev bound states (the so-called ugly zero-bias peaks). Finally, we study the end-to-end correlation of the disorder-induced zero-bias conductances from two leads by calculating the normalized mutual information, which estimates the degrees of the correlation arising from the trivial zero-bias conductance peaks. Our work provides an estimate of several important metrics used in superconductor-semiconductor experiments to determine the nature of zero-bias conductance peaks, including the robustness, the quantization, and the end-to-end correlation of the trivial zero-bias peaks. Therefore, in order to claim any evidence for the Majorana zero modes, one must establish the observed zero-bias conductance peaks to have considerable statistical significance well beyond what we find in this work to exist for the trivial peaks.

We perform first-principles density functional theory calculations to systematically investigate electronic structures of a quasi-one-dimensional $M\mathrm{O}{X}_4$ ($M=\mathrm{Cr},\mathrm{Mo},\mathrm{W}$, and $X=\mathrm{F},\mathrm{Cl},\mathrm{Br}$) materials consisting of weakly coupled one-dimensional chains. The compositional dependence on the ferroelectric and piezoelectric properties such as switching polarization, energy barriers, Born effective charges, and piezoelectric coefficients are investigated. We find that the majority of the $M\mathrm{O}{X}_4$ systems exhibit sizable polarization and large piezoelectric coefficients comparable to conventional ferroelectrics. More importantly, we find that the energy cost of forming a ${180}^{\circ }$ domain walls is negligibly small due to the weak interchain coupling, having an atomically thin domain wall with almost no reduction in the local polarization. The robust ferroelectric switching, atomically thin ferroelectric domain wall, and large piezoelectric coefficients make the $M\mathrm{O}{X}_4$ family of materials potential candidates for applications for efficient electronic devices such as high-density ferroelectric memories and high-resolution pressure sensors.

We present a detailed study of the temperature-dependent electron spin relaxation rate in $n$-type bulk GaAs in the regime of the metal-to-insulator transition at vanishing magnetic fields. The high-accuracy measurements reveal the longest spin relaxation time for a doping concentration slightly below the metal-to-insulator transition at a finite temperature of $\sim 7\mathrm{K}$. This global minimum of the electron spin relaxation rate results from a delicate interplay of hyperfine interaction, variable range hopping, and the Dyakonov-Perel mechanism. At higher doping densities, the Dyakonov-Perel mechanism becomes dominant at all temperatures changing with temperature gradually from the degenerate to the nondegenerate regime. A theoretical model including temperature-dependent transport data yields not only quantitative agreement with the experimental data but reveals additionally the gradual change from percolation-based large angle momentum scattering to ionized impurity small angle scattering. A simple interpolation of all available data allows to extract a maximal-possible spin relaxation time in $n$-doped, bulk GaAs for negligible external magnetic fields of $\approx 1\mathrm{\mu }\mathrm{s}$.

We calculate the thermal decay in the one-dimensional transient thermal grating (TTG) experiment by solving the transient Boltzmann-Peierls transport equation (BPTE) within the framework of the single-mode relaxation time approximation and using modified Debye-Callaway model in which both longitudinal and transverse phonon modes are included explicitly. We consider surface heating of an opaque thick semiconductor (SC) crystal film that we assume to have a cubic symmetry and is treated as a continuum, elastic, isotropic, and dispersionless medium. We obtain a nonuniversal spectral suppression function (SSF) in the integrand of the effective apparent thermal conductivity that is similar to the one obtained by Chiloyan et al. [Phys. Rev. B 93, 155201 (2016)] using the standard single-mode relaxation time approximation (RTA) model. Therefore, the nonuniversal character of the SSF in the TTG experiment does not depend on the form of the collision operator approximation in the BPTE: Callaway's or standard. Moreover, the analysis of the behavior of the thermal decay rate shows how the peculiar crystal momentum shuffling effect of phonon-phonon scattering Normal processes (N processes) that is captured by Callaway's model influences the onset of the nondiffusive (quasiballistic) regime in the phonon transport process in SC crystals. This effect tends independently from the other phonon scattering processes to favor the maintenance of the phonon diffusive regime over a large length-scale range, a remarkable feature that cannot be put into light with the standard RTA model used in previous works. Hence, the implicit effect of N processes has certainly an important impact on the extraction of the phonon mean-free path spectrum distribution, especially in the high-temperature regime.

We study theoretically the effects of an anisotropic elastic strain on the exciton energy spectrum fine structure and optical selection rules in atomically thin crystals based on transition-metal dichalcogenides. The presence of strain breaks the chiral selection rules at the $\mathit{K}$ points of the Brillouin zone and makes optical transitions linearly polarized. The orientation of the induced linear polarization is related to the main axes of the strain tensor. Elastic strain provides an additive contribution to the exciton fine structure splitting, in agreement with experimental evidence obtained from the uniaxially strained ${\mathrm{WSe}}_2$ monolayer. The applied strain also induces momentum-dependent Zeeman splitting. Depending on the strain orientation and magnitude, Dirac points with a linear dispersion can be formed in the exciton energy spectrum. We provide a symmetry analysis of the strain effects and develop a microscopic theory for all relevant strain-induced contributions to the exciton fine structure Hamiltonian.

The electronic structure of ${\mathrm{FePS}}_3$ was resolved using x-ray absorption spectroscopy with linear dichroism of x ray between in-plane and out-of-plane geometries. By integrating the measurements with semiempirical calculations, the observed nonvanishing feature in the dichroism spectra demands a site symmetry lower than cubic to be taken into account. These effects cause an energy splitting between $a_{1g}$ and $e_g^{\pi }$ orbitals of around 0.350 eV, contrary to previous reports purporting a smaller and insignificant energy splitting. Furthermore, reduction in the Slater integral was needed to establish agreement with experiment and theory suggesting metal-ligand covalency in the system. These findings shed light on how the local crystalline environment affects the electronic structure and the anisotropy in ${\mathrm{FePS}}_3$ and will prove imperative in its potential spintronic applications.

We study classical dimers on two-dimensional quasiperiodic Ammann-Beenker (AB) tilings. Using the discrete scale-symmetry of quasiperiodic tilings, we prove that each infinite tiling admits “perfect matchings”, where every vertex is touched by one dimer. We show the appearance of so-called monomer pseudomembranes. These are sets of edges, which collectively host exactly one dimer, which bound certain eightfold-symmetric regions of the tiling. Regions bounded by pseudomembranes are matched together in a way that resembles perfect matchings of the tiling itself. These structures emerge at all scales, suggesting the preservation of collective dimer fluctuations over long distances. We provide numerical evidence, via Monte Carlo simulations, of dimer correlations consistent with power laws over a hierarchy of different lengthscales. We also find evidence of rich monomer correlations, with monomers displaying a pattern of attraction and repulsion to different regions within pseudomembranes, along with signatures of deconfinement within certain annular regions of the tiling.

We show how the tracer motion of tagged, distinguishable particles can effectively describe transport in various homogeneous quantum many-body systems with constraints. We consider systems of spinful particles on a one-dimensional lattice subjected to constrained spin interactions, such that some or even all multipole moments of the effective spin pattern formed by the particles are conserved. On the one hand, when all moments—and thus the entire spin pattern—are conserved, dynamical spin correlations reduce to tracer motion identically, generically yielding a subdiffusive dynamical exponent $z=4$. This provides a common framework to understand the dynamics of several constrained lattice models, including models with XNOR or $t{J}_z$ constraints. We consider random unitary circuit dynamics with such a conserved spin pattern and use the tracer picture to obtain asymptotically exact expressions for their late-time dynamical correlations. Our results can also be extended to integrable quantum many-body systems that feature a conserved spin pattern but whose dynamics is insensitive to the pattern, which includes for example the folded XXZ spin chain. On the other hand, when only a finite number of moments of the pattern are conserved, the dynamics is described by a convolution of the internal hydrodynamics of the spin pattern with a tracer distribution function. As a consequence, we find that the tracer universality is robust in generic systems if at least three multipole moments of the spin pattern (its total charge, dipole moment and quadrupole moment) remain conserved. In cases where only total magnetization and dipole moment of the pattern are constant, we uncover an intriguing coexistence of two processes with an equal dynamical exponent but different scaling functions, which we relate to phase coexistence at a first-order transition.