Motivated by recent interest in exploring excitonic condensate as the ground state of some narrow-band-gap semiconductors, such as transition metal dichalcogenides and layered chalcogenide material ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$, in this paper, we theoretically study the dynamics of condensate in response to periodically driven laser fields with different polarizations and intensities. In particular, we consider laser light beams with bicircular and circular polarizations breaking the time-reversal symmetry, and linear polarization. We show that the amplitude of the condensate oscillates in time during irradiating by light with a magnitude depending on the light intensity. The dynamics survives even after the light is switched off. The phase mode, however, changes linearly with time for a condensate originating from purely electronic correlations. We further show that in the presence of electron-phonon coupling, the linear-in-time behavior is replaced by a harmonically oscillating behavior, a manifestation of gapped phase modes due to relative band charge symmetry breaking. Furthermore, we show that the primarily electronic and primarily lattice cases corresponding to strong and weak electron-phonon coupling, respectively, reveal distinct dynamics of the condensate, an observation which can modify the optical response of an excitonic insulator by stimulating amplitude and phase modes in the former case.

The work of Ahn et al. [S. Ahn, S. Das Sarma, and J. P. Kestner, Phys. Rev. B 103, L041304 (2021)] derives the noise power spectrum of a two-level fluctuator (TLF) in the case where it interacts only with a subregion of a full electron bath and thus is subject to a fluctuating temperature. However, Eq. (1), which gives the variance of the subbath temperature in terms of the heat capacity, in that work carries the implicit assumption that the heat capacity of this subbath may be taken to be a constant, which is a good approximation at higher temperatures, but breaks down at lower temperatures. We thus extend this work to the case in which the fact that the electronic heat capacity of a two-dimensional electron gas (2DEG) $C_V\propto T_0$, where $T_0$ is the temperature of the full 2DEG, rather than constant in temperature, is fully taken into account. We show that, at low temperatures, the resulting power spectrum of the noise $S(\omega ,T_0)\propto e^{-C/T_0^{3/8}}$, in contrast to $S(\omega ,T_0)\propto e^{-C^{\prime }/T_0^{1/3}}$ as found previously, where $C$ and $C^{\prime }$ are constants. We also compare the numerical results that one would obtain from the two models and find that our results for $S(\omega ,T_0)$ can differ from those of Ahn et al. by several orders of magnitude at low temperatures. Finally, we perform a fit of the spectra of two TLFs to experimental data from Connors et al. [E. J. Connors, J. J. Nelson, H. Qiao, L. F. Edge, and J. M. Nichol, Phys. Rev. B 100, 165305 (2019)] using our results and find excellent agreement with the data over most of the temperature range considered in the experiment.

The photon-mediated radiative heat transfer (RHT) in the near-field regime decays dramatically with the increase of the separation distance. It is an open problem in applied physics to realize super-Planckian transport behaviors over long distances. This work aims to theoretically explore RHT in a many-body system consisting of nanoparticles (NPs) and a dielectric nanowire (NW). The results show that the coupling effect of localized surface phonon polaritons (SPhPs) with propagating surface modes can enhance the RHT between two interacting NPs. The maximum enhancement ratio in the presence of a NW can reach up to five orders of magnitude, higher than that of the thin film and semi-infinite slab under the same conditions. In addition, we construct a dual waveguide system consisting of a one-dimensional linear NP array and a NW. Due to the synergistic effect between the strong interactions of localized SPhPs and the propagating surface modes along the NW, the dual waveguide mode can compensate for the decrease in RHT due to the increase in spacing, thus achieving ultrastrong, long-range energy transport between NPs at both ends. The present work has potential applications in the contactless energy conversion and management of optoelectronic devices at the micro- and nanoscale.

Bilayer moiré structures have attracted significant attention recently due to their spatially modulated layer degrees of freedom. However, the layer-dependent transport mechanism in the moiré structures is still a problem to be explored. Here we investigate the layer-dependent transport properties regulated by the strain, the interlayer bias, and the number of moiré periods in a strained moiré homobilayer transition metal dichalcogenide's nanoribbon based on low-energy efficient models. The charge carriers can perfectly pass through the scattering region with the moiré potential, while it is noted that the overall transmission coefficient is mainly contributed from either intralayer or interlayer transmissions. The transition of the transport mechanism between intralayer and interlayer transmissions can be achieved by adjusting the strain. The intralayer transmissions are suppressed and one of the interlayer transmissions can be selected by a vertical external electric field, which can cause a controllable layer polarization. Moreover, the staggered intralayer and interlayer minigaps are formed as the number of moiré periods increases in the scattering region due to the overlap of the wave functions in two adjacent moiré periods. Our finding points to an opportunity to realize layer functionalities by the strain and electric field.

Two-dimensional amorphous materials can be regarded as electronic systems with a combination of amorphous and dimensional confinement effects that contributes to the discrete localized tail states near the Fermi level. In general, the magnetic properties of magnetic atoms may not be preserved upon doping into nonmagnetic metals or adsorbing onto their surfaces, e.g., Ni atoms adsorbed onto nonmagnetic metal or semiconductor surfaces. Earlier experimental and theoretical studies showed that as a Ni atom adsorbed on pristine graphene, the magnetic moment completely disappeared. Here, using first-principles calculations, we investigate the structural and magnetic properties of Ni single atom and dimer adsorbed on amorphous graphene (AG). We find that the carbon eight-member ring in AG gives rise to the most stable adsorption site for Ni adatoms and that the spin moments of adsorbed Ni single atom and dimer can be partially preserved. The mechanism of this spin preservation of Ni adatoms can be understood in terms of the disorder-induced localization and its tight-binding model. Such a localization effect due to the disordered lattice of AG weakens the hybridization between the Ni d- and graphene π orbitals, thereby leading to a preservation of spin moment in Ni adatoms. Our findings offer promise for many applications such as single-atom catalysis and spintronic devices.

We present the results of hard x-ray angle-resolved photoemission spectroscopy and photoemission diffraction measurements performed on high-quality single crystals of the valence transition compound ${\mathrm{EuPd}}_2{\mathrm{Si}}_2$ for temperatures $25\text{K}\le T\le 300$ K. At low temperatures, we observe a Eu $4f$ valence $v=2.5$, which decreases to $v=2.1$ for temperatures above the valence transition around $T_V\approx 160$ K. The experimental valence numbers resulting from an evaluation of the Eu(III)/Eu(II) $3d$ core levels, are used for calculating band structures using density functional theory. The valence transition significantly changes the band structure as determined by angle-resolved photoemission spectroscopy. In particular, the Eu $5d$ valence bands are shifted to lower binding energies with increasing Eu $4f$ occupancy. To a lesser extent, bands derived from the Si $3p$ and Pd $4d$ orbitals are also affected. This observation suggests a partial charge transfer between Eu and Pd/Si sites. Comparison with ab initio theory shows a good agreement with experiment, in particular concerning the unequal band shift with increasing Eu $4f$ occupancy.

Spin- and valley-resolved transport properties of a double beam off-resonant circular polarized light-modulated (CPL-modulated) junction based on monolayer transition-metal dichalcogenides (TMDc) are studied. We find that tuning between a high resistance configuration and a low resistance one can be realized with pure CPL modulation. Numerical results of spin- and valley-resolved transport show the tunneling resistance induced by CPL is different from previously reported tunneling magnetoresistance (TMR) effect based on two-dimensional materials. Out of analogy, we name it as the tunneling optoresistance (TOR) effect, and introduced TOR ratio to depict the difference between configurations with high and low tunneling resistance. Our results imply that ${\mathrm{TOR}}_{\max }=1$ can be realized when the CPL-induced gap is beyond a certain limit. We also give an intuitive explanation under near the $K/K^{\prime }$ valley approximation which matches numerical results well. Our results show optically modulated quantum structures based on TMDc materials may dynamically tune between a high resistance configuration and a low resistance configuration, indicating its promising potential for applications in high-speed storage and spintronic or valleytronic devices.

Combining the microscopic calculation of superlattice minibands and macroscopic real density matrix approach, one can obtain electric susceptibilities of the superlattice system irradiated by an electromagnetic wave. It is shown how to calculate the dispersion relation, excitonic resonance positions, and susceptibility of a ${\mathrm{Cu}}_2\mathrm{O}/\mathrm{MgO}$-based superlattice (SL), when Rydberg exciton polaritons appear, including the effect of the coherence between the electron-hole pair and the electromagnetic field and the polaritonic effect. Using the Kronig-Penney model for computing miniband SL parameters, analytical expressions for optical functions are obtained and numerical calculations for a ${\mathrm{Cu}}_2\mathrm{O}/\mathrm{MgO}$ SL are performed.

We theoretically study the photogalvanic effect in a strained two-dimensional transition-metal dichalcogenide monolayer due to deformation-induced lowering of the monolayer symmetry and electron-density difference in opposite valleys. This effect arises as a system response to a scalar nonequilibrium perturbation (electron-density difference in the valleys), which is in contrast with the conventional photogalvanic effect, represents the second-order response to an external electromagnetic radiation. Using the description of linear and nonlinear interband recombination, we develop a theory for a $\mathit{p}$-type and an intrinsic monolayer semiconductor. We show that at low temperatures, the photogalvanic current is caused by the impurity relaxation processes controlling the valley population imbalance.

The temporal behavior of the lower polariton (LP) distribution in the reciprocal space, $n_{\mathrm{LP}}\left(k\right)$, and formation of long-range spatial coherence are investigated in a nonequilibrium incoherent LP fluid generated resonantly with picosecond optical pulses at 2 K in a high-$Q$ planar GaAs/AlAs microcavity with 12 InGaAs quantum wells. The dynamics of $n_{\mathrm{LP}}\left(k\right)$ is found to be independent of excitation density and well described within the framework of linear Schrödinger equations taking into account random potential disorder, $\delta E_{\mathrm{LP}}$, and finite lifetime of LPs, up to LP density $n_{\mathrm{LP}}(t=0)=7\times {10}^{10}{\mathrm{cm}}^{-2}$ (3.5 orders of magnitude greater than the threshold density of Bose-Einstein condensation for LPs). This is explained by the smallness of the ratios of LP interaction energy to both the mean kinetic energy and potential disorder. The contribution of interparticle interaction to the formation of spatial coherence in the LP fluid is insignificant at $E_{\mathrm{int}}\ll \delta E_{\mathrm{LP}}$, but becomes noticeable already at $E_{\mathrm{int}}\approx 0.2\delta E_{\mathrm{LP}}$, despite the fact that its effect on the $k$ distribution of LPs remains insignificant. Coherence length $L_c$ in LP fluid with $n_{\mathrm{LP}}(t=0)=2$ and $7\times {10}^{10}{\mathrm{cm}}^{-2}$ in the region with $\delta E_{\mathrm{LP}}=0.15$ meV at $t=160$ ps increases to 4.1 and $5.3\mu \mathrm{m}$, respectively, whereas in an incoherent Bose gas with the same $n_{\mathrm{LP}}\left(k\right)$ it is equal to $3.6\mu \mathrm{m}$.

Collisions between electrons and holes can dominate the carrier scattering in clean graphene samples in the vicinity of the charge neutrality point. While electron-hole limited resistance in pristine gapless graphene is well studied, its evolution with induction of band gap $E_g$ is less explored. Here, we derive the functional dependence of electron-hole collision limited resistance of gapped graphene ${\rho }_{eh}$ on the ratio of gap and thermal energy $E_g/kT$. At low temperatures and large band gaps, the resistance grows linearly with $E_g/kT$ and possesses a minimum at $E_g\approx 2.5kT$. This contrasts to the Arrhenius activation-type behavior for intrinsic semiconductors. Introduction of impurities restores the Arrhenius law for resistivity at low temperatures and/or high doping densities. The hallmark of electron-hole collision effects in graphene resistivity at charge neutrality is thus the crossover between exponential and power-law resistivity scalings with temperature.

Transition metal (TM) defects in silicon carbide (SiC) are a promising platform for applications in quantum technology as some of these defects, e.g., vanadium (V), allow for optical emission in one of the telecom bands. For other defects, it was shown that straining the crystal can lead to beneficial effects regarding the emission properties. Motivated by this, we theoretically study the main effects of strain on the electronic-level structure and optical electric-dipole transitions of the V defect in SiC. In particular, we show how strain can be used to engineer the $g$ tensor, electronic selection rules, and the hyperfine interaction. Based on these insights, we discuss optical Lambda systems and a path forward to initializing the quantum state of strained TM defects in SiC.

Non-Hermitian quasicrystal constitutes a unique class of disordered open system with $\mathcal{PT}$-symmetry breaking, localization, and topological triple phase transitions. In this paper, we uncover the effect of quantum correlation on phase transitions and entanglement dynamics in non-Hermitian quasicrystals. Focusing on two interacting bosons in a Bose-Hubbard lattice with quasiperiodically modulated gain and loss, we find that the on-site interaction between bosons could drag the $\mathcal{PT}$ and localization transition thresholds towards weaker disorder regions compared with the noninteracting case. Moreover, the interaction facilitates the expansion of the critical point of a triple phase transition in the noninteracting system into a critical phase with mobility edges, whose domain could be flexibly controlled by tuning the interaction strength. Systematic analyses of the spectrum, inverse participation ratio, topological winding number, wavepacket dynamics, and entanglement entropy lead to consistent predictions about the correlation-driven phases and transitions in our system. Our findings pave the way for further studies of the interplay between disorder and interaction in non-Hermitian quantum matter.

Nonequilibrium dynamics in spin systems is a topic currently under intense investigation as it provides fundamental insights into thermalization, universality, and exotic transport phenomena. While most of the studies have been focused on ideal closed quantum many-body systems such as ultracold atomic quantum gases and one-dimensional spin chains, driven-dissipative Bose gases in steady states away from equilibrium in classical systems also lead to intriguing nonequilibrium physics. In this work, we theoretically investigate out-of-equilibrium dynamics of magnons in a gapped Heisenberg quantum antiferromagnet based on Boltzmann transport theory. We show that, by treating scattering terms beyond the relaxation-time approximation in the Boltzmann transport equation, energy and particle number conservation mandate that nonequilibrium magnons cannot relax to equilibrium, but decay to other nonequilibrium stationary states. The only decay channel for these stationary states back to equilibrium is through the nonconserving interactions (i.e., changing particle number and/or energy within the magnon system) such as boundary or magnon-phonon scattering. At low temperatures, these nonconserving interactions are much slower processes than intrinsic magnon-magnon interaction in a gapped spin system. Using magnon-phonon interaction as a quintessential type of nonconserving interaction, we then propose that nonequilibrium steady states of magnons can be maintained and tailored using periodic driving at frequencies faster than relaxation due to phonon interactions. These findings reveal a class of classical material systems that are suitable platforms to study nonequilibrium statistical physics and macroscopic phenomena such as classical Bose-Einstein condensation of quasiparticles and magnon supercurrents that are relevant for spintronic applications.

A fundamental instability in the nonequilibrium conduction band under an electric field bias is proposed via the spontaneous emission of coherent phonons. Analytic theory, supported by numerical calculations, establishes that the quantum avalanche, an abrupt nonequilibrium occupation of excited bands, results from the competition between the collapse of the band minimum via the phonon emission and the dephasing of the electron with the environment. The continuous avalanche transition is a quantum phase transition with the nonequilibrium phase diagram determined by the avalanche parameter $\beta$, with peculiar reentrant avalanche domes close to the phase boundary. We further confirm the nature of the quantum avalanche with the temperature dependence.

We establish a general Langevin dynamics model of interacting, single-domain magnetic nanoparticles (MNPs) in liquid suspension at finite temperature. The model couples the Landau-Lifshitz-Gilbert equation for the moment dynamics with the mechanical rotation and translation of the particles. Within this model, we derive expressions for the instantaneous transfer of energy, linear, and angular momentum between the particles and with the environment. We demonstrate by numerical tests that all conserved quantities are fully accounted for, thus validating the model and the transfer expressions. The energy transfer expressions derived here are also useful analysis tools to decompose the instantaneous, nonequilibrium power loss at each MNP into different loss channels. To demonstrate the model capabilities, we analyze simulations of MNP collisions and high-frequency hysteresis in terms of power and energy contributions.

We investigate the stability of the skyrmion crystal phase in a tetragonal polar system with the Dzyaloshinskii-Moriya interaction by focusing on the symmetry of ordering wave vectors forming the skyrmion crystal. Our analysis is based on numerical simulations for an effective spin model, which is derived from the weak-coupling regime in the Kondo lattice model on a polar square lattice. We show that a hybrid square skyrmion crystal consisting of Bloch and Néel spin textures emerges even under polar $C_{4\mathrm{v}}$ symmetries when the ordering wave vectors correspond to low-symmetric wave vectors in momentum space, which is in contrast to the expectation from the Lifshitz invariants. We also show the instability toward the antiskyrmion crystal and rhombic skyrmion crystal depending on the direction of the Dzyaloshinskii-Moriya vector in momentum space. Furthermore, we show that the regions of the skyrmion crystal phases are affected by taking into account the symmetric anisotropic exchange interaction. Our results open the potential direction of engineering the hybrid skyrmion crystal and antiskyrmion crystal phases in polar magnets.

Motivated by the recently developed classical spin model for the Migdal-Eliashberg theory, we develop numerical and analytical methods based on this spin-chain representation and apply these methods to the Bogoliubov-Tomachov-Morel-Anderson pairing potential, which incorporates the phonon-mediated attraction and Coulomb repulsion. We show that the Monte Carlo method with heat bath updates can efficiently obtain the gap functions even for the situations challenging for the iterative solvers, suggesting an unprecedented robust approach for solving the full nonlinear Migdal-Eliashberg theory. Moreover, we derive the renormalization of all the couplings by tracing out the high-frequency spins in the partition function. The derived analytical renormalization equations produce the well-known ${\mu }^{*}$ effect for the Bogoliubov-Tomachov-Morel-Anderson pairing potential and can be generalized to other superconductivity problems. We further point out that several interesting features (e.g., sign changing in the frequency-dependent gap function) can be intuitively understood using the classical spin-chain representation for Migdal-Eliasherg theory. Our results show the advantage of using the spin-chain representation for solving Migdal-Eliashberg theory and provide new ways for tackling general superconductivity problems.