The luminescence yield of transition metal dichalcogenide monolayers frequently suffers from the formation of long-lived dark states, which include excitons with intervalley charge carriers, spin-forbidden transitions, and a large center-of-mass momentum located outside the light cone of dispersion relations. Efficient relaxation from bright exciton states to dark states suppresses the quantum yield of photon emission. In addition, the radiative recombination of excitons is heavily influenced by Auger-type exciton-exciton scattering, which yields another nonradiative relaxation channel at room temperature. Here, we show that Auger-type scattering is promoted not only between (bright) excitons but also between excitons and long-lived dark states. We studied the luminescence dynamics of monolayer ${\mathrm{WS}}_2$ capped with hexagonal BN over broad time ranges of picoseconds to milliseconds using carefully designed pump-and-probe techniques. We observed that luminescence quenching associated with Auger-type scattering occurs on 1–100-$\mu \mathrm{s}$ timescales, which thus correspond to the lifetimes of the relevant dark states. The broad distribution of the measured lifetimes implies the impact of various types of long-lived states on the exciton annihilation process.

Based on first-principles calculations, we predict a stable two-dimensional semiconductor, namely tin dioxide ${\mathrm{SnO}}_2$. By investigating its dynamical, thermal, and mechanical properties, we find that ${\mathrm{SnO}}_2$ monolayer is an auxetic material with a large in-plane negative Poisson's ratio. Furthermore, our results show that ${\mathrm{SnO}}_2$ is an indirect-gap semiconductor with a band gap in the region of 3.7 eV and an extremely high electron mobility, $\sim {10}^3{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. Interestingly, the band structure of ${\mathrm{SnO}}_2$ presents double Mexican-hat-like band edges in the valence bands near the Fermi level. Due to such a unique band feature, a ferromagnetic phase transition takes place with a half-metallic ground state that can be induced by hole doping within a very wide concentration range. Such a magnetic phase can be well explained by the Stoner mechanism. A peculiar feature of the magnetic state is the presence of large magnetocrystalline anisotropy that can switch from in-plane to out-of-plane upon hole doping. Hence, ${\mathrm{SnO}}_2$ monolayer can be tuned to be either an $XY$ magnet or an Ising one, with a magnetic critical temperature above room temperature at proper hole concentrations. These findings demonstrate that the predicted phase of ${\mathrm{SnO}}_2$ is a rare example of $p$-type magnetism and a possible candidate for spintronic applications.

The non-Hermitian skin effect, i.e., eigenstate condensation at the edges in lattices with open boundaries, is an exotic manifestation of non-Hermitian systems. In Bloch theory, an effective non-Hermitian Hamiltonian is generally used to describe dissipation, which, however, is not norm preserving and neglects quantum jumps. Here it is shown that in a self-consistent description of the dissipative dynamics in a one-band lattice, based on the stochastic Schrödinger equation or Lindblad master equation with a collective jump operator, the skin effect and its dynamical features are washed out. Nevertheless, both short- and long-time relaxation dynamics provide a hidden signature of the skin effect found in the semiclassical limit. In particular, relaxation toward a maximally mixed state with the largest von Neumann entropy in a lattice with open boundaries is a manifestation of the semiclassical skin effect.

We theoretically study a generalized Hubbard model on Moiré superlattices of twisted bilayers, and find very rich filling-factor-dependent quantum phase diagrams tuned by interaction strength and twist angle. Strong long-range Coulomb interaction in the Moiré-Hubbard model induces Wigner crystals at a series of fractional filling factors. The effective lattice of the Wigner crystal is controlled by the filling factor, and can be triangle, rectangle, honeycomb, kagome, etc., providing a single platform to realize many different spin models on various lattices by simply tuning carrier density. In addition to Wigner crystals that are topologically trivial, interaction-induced Chern insulators emerge in the phase diagram. This finding paves a way for engineering interaction-induced quantum anomalous Hall effect in Moiré-Hubbard systems where the corresponding single-particle Moiré band is topologically trivial.

The interplay between light and matter attracts tremendous interest for exploring novel topological quantum states and their phase transitions. Here we show by first-principles calculations and the Floquet theorem that a carbon allotrope body-centered tetragonal ${\text{C}}_{16}$ $\left(\mathrm{bct}\text{−}{\mathrm{C}}_{16}\right)$, a typical nodal-line semimetal, exhibits exotic photoinduced Floquet mixed-Weyl semimetallic features. Under the irradiation of a linearly polarized light, $\mathrm{bct}\text{−}{\mathrm{C}}_{16}$ undergoes a topological phase transition from a nodal-line semimetal to a Weyl semimetal with two pairs of tunable Weyl points. With increasing the light intensity, left-handed Weyl points evolve from type I into type II while right-handed ones are always preserved to be type I, giving rise to light-induced unconventional Weyl pairs composed of distinct types of Weyl points. Importantly, a special Weyl pair formed by type-I and type-III Weyl points is present at the critical transition point. The photon-dressed Fermi arcs connecting the projections of two different types of Weyl points are clearly visible, further revealing their unique topological features. Our work not only realizes promising unconventional Weyl pairs but also paves a reliable avenue for investigating light-induced topological phase transitions.

We study the effect of random potentials created by different types of impurities on the transverse magnetoresistance of Weyl semimetals. We show that the magnetic field and temperature dependence of the magnetoresistance is strongly affected by the type of impurity potential. We analyze in detail two limiting cases: (i) the ultraquantum limit, when the applied magnetic field is so high that only the zeroth and first Landau levels contribute to the magnetotransport, and (ii) the semiclassical situation, for which a large number of Landau levels come into play. A formal diagrammatic approach allowed us to obtain expressions for the components of the electrical conductivity tensor in both limits. In contrast to the oversimplified case of the $\delta$-correlated disorder, the long-range impurity potential (including that of Coulomb impurities) introduces an additional length scale, which changes the geometry and physics of the problem. We show that the magnetoresistance can deviate from the linear behavior as a function of magnetic field for a certain class of impurity potentials.

In this work, we investigate hole spin-flip transitions in a single self-assembled InGaAs/GaAs quantum dot. We find the hole wave functions using the eight-band $\mathit{k}·\mathit{p}$ model, and we calculate phonon-assisted spin relaxation rates for the ground-state Zeeman doublet. We systematically study the importance of various admixture and direct spin-phonon mechanisms giving rise to the transition rates. We show that the biaxial and shear strain constitute dominant spin-admixture coupling mechanisms. Then, we demonstrate that hole spin lifetime can be increased if a quantum dot is covered by a strain-reducing layer. Finally, we show that the spin relaxation can be described by an effective model.

The phonon Boltzmann transport equation (BTE) with the relaxation time approximation (RTA) scattering model is used to calculate transient temperature profiles in graphene, and the results are compared to analogous molecular dynamics (MD) simulations. For the BTE calculations, the phonon dispersion relation and frequency-dependent scattering rates are obtained from a combination of MD data and semi-empirical power-law expressions for the normal and Umklapp phonon lifetimes. The dimensions and initial temperature conditions of graphene are varied to study the size and temperature dependence of thermal transport physics at the mesoscopic scale. Good quantitative agreement to within 5% is found between the BTE and MD results, over a wide range of temperatures and lengthscales of the temperature variation in the graphene sheet. Small differences are attributed to the inaccuracy of the RTA as applied to graphene, and to neglecting four-phonon scattering in the BTE simulations. The present results may further understanding in applications such as the transient heating of nanoelectronics.

The low-energy physics of two-dimensional quantum anomalous Hall insulators like (Hg,Mn)Te quantum wells or magnetically doped (Bi,Sb)Te thin films can be effectively described by two Chern insulators, including a Dirac as well as a momentum-dependent mass term. Each of those Chern insulators is directly related to the parity anomaly of planar quantum electrodynamics. In this work, we analyze the finite-temperature Hall conductivity of a single Chern insulator in $2+1$ space-time dimensions under the influence of a chemical potential and an out-of-plane magnetic field. At zero magnetic field, this nondissipative transport coefficient originates from the parity anomaly of planar quantum electrodynamics. We show that the parity anomaly itself is not renormalized by finite-temperature effects. However, it induces two terms of different physical origin in the effective action of a Chern insulator, which is proportional to the Hall conductivity. The first term is temperature and chemical potential independent, and solely encodes the intrinsic topological response. The second term specifies the nontopological thermal response of conduction and valence band states. In particular, we show that the relativistic mass of a Chern insulator counteracts finite-temperature effects, whereas its nonrelativistic mass enhances these corrections. Moreover, we extend our analysis to finite magnetic fields and relate the thermal response of a Chern insulator therein to the spectral asymmetry, which is a measure of the parity anomaly in orbital fields.

The Johari-Goldstein secondary $\left(\beta \right)$ relaxation is an intrinsic feature of glasses, which is crucial to many properties of disordered materials. One puzzling feature of $\beta$-relaxation is its wide relaxation peak, which could imply a critical role of entropy. Here we quantify the activation entropy related to the $\beta$-relaxation in metallic glass via well-tempered metadynamics simulations. The activation free energy of the $\beta$-relaxation drastically decreases with increasing temperature, indicating a strongly entropic effect that may contribute a multiplication prefactor up to several orders of magnitude to the frequency. We further argue the entropic effect by linear extrapolation of the temperature-dependent activation free energy to 0 K, which gives rise to activation energy, in agreement with the barrier spectrum explored by the activation-relaxation technique. The entropic effect signifies the multiplicity of activation pathways which agrees with the experimentally found wide frequency domain of the $\beta$-relaxation.

The recent exploration of the valley degree of freedom in photonic systems has enriched the topological phases of light and brought the robust transport of edge states around sharp bends. The two and more simultaneous band gaps in valley-Hall systems have attracted researchers’ attention for enlarging the working bandwidth. However, band gaps with frequency range dependent topology were not reported and the demonstrated flow of electromagnetic waves is limited to the robust transport of edge states. Here, the frequency range degree of freedom is introduced into valley photonic crystals with dual band gaps. Based on the high-order plane-wave expansion model, we derive an effective Hamiltonian which characterizes dual band gaps. Metallic valley photonic crystals are demonstrated as examples in which all four topological phases are found. At the domain walls between topologically distinct valley photonic crystals, frequency range dependent edge states are demonstrated and a broadband photonic detouring is proposed. Our findings provide the guidance for designing the frequency range dependent property of topological structures and show its potential applications in wavelength division multiplexers.

Since experimental verification of the predicted chiral phonon (CP), CPs have become a rising topic. The previous observation was a two-step process, which identified the CP by intervalley transfer of valley holes created from direct optical transition. It is highly desirable to simplify the creation and detection of CPs. Moreover, while indirect optical transition in transition metal dichalcogenide heterostructures has been observed, it is important to promote the understanding of the interplay between the phonon, electron, and photon in the indirect transition. Here, we take the ${\mathrm{MoS}}_2$/${\mathrm{WS}}_2$ heterostructure as a prototype and study CPs in the indirect optical transition. By first-principles calculations, the heterostructure with an indirect electronic gap exhibits CPs at the high-symmetry $\pm \mathit{K}$ points of the phonon Brillouin zone, which have sizable circular polarization and discrete pseudoangular momentum. CPs with distinct chiralities can be selectively and directly generated in indirect optical transitions by tuning the optical chirality and frequency. These findings provide microscopic understanding of the indirect optical transition by interacting chiral fermions and chiral bosons and enable more convenient detection and control of CPs.

As a potential high-efficiency XUV light source, high-order harmonic generation (HHG) in solids contains the dynamic information of electrons. The interactions among many particles in solids are ubiquitous and, to some extent, modulate the motion of electrons. However, the role of electron-electron interaction (EEI) effect in solid HHG has not been well understood. In this paper, we study the laser-intensity-dependent EEI effect under the given laser pulse durations. By comparing the finite chain model with the infinite extension model, we find that, if the electric field is strong enough, the induced electric field caused by EEI is nontrivial for the finite chain model. We also find that EEI only can enhance HHG spectra in a limited frequency range under some specific intensities of the laser field in both models, and the EEI effect is related to the configuration of the system. This study can provide a reference for whether EEI can be ignored when simulating the interaction between one-dimensional solids and lasers.

Magnets with broken local inversion symmetries are interesting candidates for chiral magnetic textures, such as skyrmions and spin spirals. The property of these magnets is that each subsequent layer can possess a different Dzyaloshniskii-Moriya interaction (DMI) originating from the local inversion symmetry breaking. Given that new candidates of such systems are emerging, with the Van der Waals crystals and magnetic multilayer systems, it is interesting to investigate how the chiral magnetic textures depend on the number of layers and the coupling between them. In this article, we model the magnetic layers with a classical Heisenberg spin model where the sign of the DMI alternates for each consecutive layer. We use Monte Carlo simulations to examine chiral magnetic textures and show that the pitch of magnetic spirals is influenced by the interlayer coupling and the number of layers. We observe even-odd effects in the number of layers where we observe a suppression of the spin spirals for even layer numbers. We give an explanation for our findings by proposing a net DMI in systems with strongly coupled layers. Our results can be used to determine the DMI in systems with a known number of layers and for new technologies based on the tunability of the spiral wavelength.

States of matter that break time-reversal symmetry are invariably associated with magnetism or circulating currents. Recently, one of us proposed a phase, the directional scalar spin chiral order (DSSCO), as an exception: it breaks time-reversal symmetry via chiral ordering of spins along a particular direction, but is spin-rotation symmetric. In this work we prove the existence of this state via state-of-the-art density matrix renormalization group (DMRG) analysis on a spin-1 chain with nearest-neighbor bilinear-biquadratic interactions and additional third-neighbor ferromagnetic Heisenberg exchange. Despite the large entanglement introduced by the third-neighbor coupling, we are able to access system sizes up to $L=918$ sites. We find first-order phase transitions from the DSSCO into the famous Haldane phase as well as a spin-quadrupolar phase where spin nematic correlations dominate. In the Haldane phase we demonstrate a method for detecting the topological edge states using DMRG that could be useful for other topological phases too. This method can be understood as the well-known active operators applied only at the boundary.

We report the synthesis and physical properties studies of quasi-one-dimensional (quais-1D) iron chalcogenide ${\mathrm{BaFe}}_2{\mathrm{Se}}_4$ which shares the ${\mathrm{FeSe}}_4$ tetrahedra building motif commonly seen in the iron chalcogenide superconductors. A high-quality polycrystalline sample was achieved by solid-state reaction method and characterized by x-ray diffraction, electrical resistivity, magnetic susceptibility, and neutron diffraction measurements. ${\mathrm{BaFe}}_2{\mathrm{Se}}_4$ is a narrow gap semiconductor that magnetically orders at $\sim 310$ K. Both neutron powder diffraction results and isothermal $M-H$ loops suggest a canted antiferromagnetic structure where Fe sublattices are antiferromagnetically ordered along the $c$-axis quasi-1D chain direction, resulting in a net ferromagnetic moment in the perpendicular direction along the $a$ axis with tilted angle of $18.7^{\circ }$ towards the $b$ axis.

We investigate the effects of electron-phonon coupling, as well as of spin and charge fluctuations on the superconducting state in a single layer of FeSe on ${\mathrm{SrTiO}}_3$ substrate. These three bosonic mediators of Cooper pairing are treated on equal footing in a multichannel, full-bandwidth, multiband, and anisotropic Eliashberg theory of the interacting state. Our self-consistent calculations show that an $s$-wave symmetry of the superconducting gap is compatible only with a complete absence of spin fluctuations. When spin fluctuations are present, the sign-changing nodeless $d$-wave pairing symmetry is always obtained, yet the essential ingredient for explaining the gap magnitude and critical temperature is still the interfacial electron-phonon interaction.

We study quantum geometric contributions to the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature $T_{\text{BKT}}$ in the presence of fluctuations beyond BCS theory. Because quantum geometric effects become progressively more important with stronger pairing attraction, a full understanding of 2D multiorbital superconductivity requires the incorporation of preformed pairs. We find it is through the effective mass of these pairs that quantum geometry enters the theory and this suggests that the quantum geometric effects are present in the nonsuperconducting pseudogap phase as well. Increasing these geometric contributions tends to raise $T_{\text{BKT}}$, which then competes with fluctuation effects that generally depress it. We argue that a way to physically quantify the magnitude of these geometric terms is in terms of the ratio of the pairing onset temperature $T^{*}$ to $T_{\text{BKT}}$. Our paper calls attention to an experimental study demonstrating how both temperatures and, thus, their ratio may be currently accessible. They can be extracted from the same voltage-current measurements, which are generally used to establish BKT physics. We use these observations to provide rough preliminary estimates of the magnitude of the geometric contributions in, for example, magic angle twisted bilayer graphene.

We find that the best resonating valence bond (RVB) state of the spin-$\frac{1}{2}$ kagome antiferromagnetic Heisenberg model (spin-$\frac{1}{2}$ KAFH) is described by a $Z_2$ mean field ansatz belonging to the $Z_2[0,\pi ]\beta$ class, which hosts a mean field spinon dispersion very different from that of the widely studied $U\left(1\right)$ Dirac spin liquid state. However, we find that the physical spin fluctuation spectrum calculated from the Gutzwiller projected random phase approximation (RPA) (GRPA) theory above such an RVB state is actually gapless and is almost identical to that above the $U\left(1\right)$ Dirac spin liquid state. We find that such a peculiar behavior can be attributed to the unique flat band physics on the kagome lattice, which makes the mapping between the mean field ansatz and the RVB state noninjective. We find that the spin fluctuation spectrum of the spin-$\frac{1}{2}$ KAFH is not at all featureless, but is characterized by a prominent spectral peak at about $0.25J$ around the $\mathbf{M}$ point, which is immersed in a broad continuum extending to $2.7J$. Based on these results, we argue that the spectral peak below 2 meV in the inelastic neutron scattering spectrum of hebertsmithite ${\mathrm{ZnCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_2$, which has been attributed to the contribution of ${\mathrm{Cu}}^{2+}$ impurity spins occupying the ${\mathrm{Zn}}^{2+}$ site, should rather be understood as the intrinsic contribution from the kagome layer. We propose to verify such a picture by measuring the Knight shift on the Cu site, rather than the O site, which is almost blind to the spin fluctuation at the $\mathbf{M}$ point as a result of the strong antiferromagnetic correlation between nearest neighboring spins on the kagome lattice.

In this paper, we propose strongly correlated gapless states (or critical states) of spin-1/2 electrons in $1+1$ dimensions, such as the doped ferromagnetic and antiferromagnetic spin-1/2 Ising chains. We find that the metallic phases in the doped ferromagnetic and antiferromagnetic Ising chain are different strongly correlated gapless phases, despite the fact that the two phases have the same symmetry. The doped antiferromagnetic Ising chain has a finite energy gap for all charge-1 fermionic excitations even without pairing caused by the attractive interaction, resembling the pseudogap phase of underdoped high $T_c$ superconductors. Applying a transverse field to the ferromagnetic and antiferromagnetic metallic phase can restore the $Z_2$ symmetry, which gives rise to two distinct critical points despite the fact that the two transitions have exactly the same symmetry-breaking pattern. We also propose chiral metallic states. Some of these gapless states are strongly correlated in the sense that they do not belong to the usual Tomonaga-Luttinger phase of fermions, i.e., they cannot be smoothly deformed into noninteracting fermion systems with the same symmetry. Our nonperturbative results are obtained by noting that gapless quantum systems have emergent categorical symmetries (i.e., noninvertible gravitational anomalies), which are described by multicomponent partition functions that are modular covariant. This allows us to calculate the scaling dimensions and quantum numbers of all the low-energy operators for those strongly correlated gapless states. This demonstrates an application of emergent categorical symmetries in determining low-energy properties of strongly correlated gapless states, which are hard to obtain otherwise.