We present photoluminescence measurements in monolayer ${\mathrm{WSe}}_2$, which point to the importance of the interaction between charged particles and excitonic complexes. The theoretical analysis highlights the key role played by exchange scattering, referring to cases wherein the particle composition of the complex changes after the interaction. For example, exchange scattering renders bright excitonic complexes dark in monolayer ${\mathrm{WSe}}_2$ on accounts of the unique valley-spin configuration in this material. In addition to the ultrafast energy relaxation of hot excitonic complexes following their interaction with electrons or holes, our analysis sheds light on several key features that are commonly seen in the photoluminescence of this monolayer semiconductor. In particular, we can understand why the photoluminescence intensity of the neutral bright exciton is strongest when the monolayer is hole doped rather than charge neutral or electron doped. Similarly, we can understand the reason for the dramatic increase of the photoluminescence intensity of negatively charged excitons (trions) as soon as electrons are added to the monolayer. To self-consistently explain the findings, we further study the photoluminescence spectra at different excitation energies and analyze the behavior of the elusive indirect exciton.

Ensembles of gold nanoparticles present a magnetic behavior which is at odds with the weakly diamagnetic response of bulk gold. In particular, an unusual ferromagnetic order has been unveiled by several experiments. Here we investigate if the combined effect of orbital magnetism of conduction electrons and interparticle dipolar interaction can lead to magnetic ordering. Using different model systems of interacting mesoscopic magnetic dipoles, together with a microscopic description of the electron dynamics within the nanoparticles, we find that a spontaneous magnetic moment may arise in dimers of metallic nanoparticles when the latter are characterized by a large orbital paramagnetic susceptibility.

Thermoelectric materials have attracted significant interest owing to their wide range of applications, such as power generators and refrigerators. Along with the evaluation of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and its alloys, the search for materials that show good thermoelectric performance has risen considerably. The efficiency of thermoelectric conversion is quantified by the figure of merit, $ZT$, and $ZT$ greater than unity has been considered as a target. For a large $ZT$, acquiring a large Seebeck coefficient and a reduction of phonon thermal conductivity are significant given its definition. In this work, we propose that a thin film of topological nodal line semimetals can be a new and promising thermoelectric material. We also discuss the correlation effect and spin current induced by the temperature gradient. The obtained results give various insights into the quest for the best thermoelectric materials.

We theoretically investigate optical injection processes, including one- and two-photon carrier injection and two-color coherent current injection, in twisted bilayer graphene with moderate angles. The electronic states are described by a continuum model, and the spectra of injection coefficients are numerically calculated for different chemical potentials and twist angles, where the transitions between different bands are understood by the electron energy resolved injection coefficients. The comparison with the injection in monolayer graphene shows the significance of the interlayer coupling in the injection processes. For undoped twisted bilayer graphene, all spectra of injection coefficients can be divided into three energy regimes, which vary with the twist angle. For very low photon energies in the linear dispersion regime, the injection is similar to graphene with a renormalized Fermi velocity determined by the twist angle; for very high photon energies where the interlayer coupling is negligible, the injection is the same as that of graphene; and in the middle regime around the transition energy of the Van Hove singularity, the injection shows fruitful fine structures. Furthermore, the two-photon carrier injection diverges for the photon energy in the middle regime due to the existence of double resonant transitions.

The recent discovery of higher-order topology has largely enriched the classification of topological materials. Theoretical and experimental studies have unveiled various higher-order topological insulators that exhibit topologically protected corner or hinge states. More recently, higher-order topology has been introduced to topological semimetals. Thus far, realistic models and experimental verifications on higher-order topological semimetals are still very limited. Here we design and demonstrate a three-dimensional photonic crystal that realizes a higher-order Dirac semimetal phase. Numerical results on the band structure show that the designed three-dimensional photonic crystal is able to host two fourfold Dirac points, which are connected in the momentum-space projections via higher-order hinge states localized at the hinge. The higher-order topology can be characterized by the topological invariant ${\chi }^{\left(6\right)}$ at different values of $k_z$. An experiment at microwave frequencies is also presented to measure the hinge state dispersion. Our work demonstrates the physical realization of a higher-order Dirac semimetal phase and paves the way to explore higher-order topological semimetal phases in three-dimensional photonic systems.

Quantum many-body scars (QMBS) constitute a new quantum dynamical regime in which rare “scarred” eigenstates mediate weak ergodicity breaking. One open question is to understand the most general setting in which these states arise. In this work, we develop a generic construction that embeds a new class of QMBS, rainbow scars, into the spectrum of an arbitrary Hamiltonian. Unlike other examples of QMBS, rainbow scars display extensive bipartite entanglement entropy while retaining a simple entanglement structure. Specifically, the entanglement scaling is volume-law for a random bipartition, while scaling for a fine-tuned bipartition is subextensive. When internal symmetries are present, the construction leads to multiple, and even towers, of rainbow scars revealed through distinctive non-thermal dynamics. Remarkably, certain symmetries can lead rainbow scars to arise in translation-invariant models. To this end, we provide an experimental road map for realizing rainbow scar states in a Rydberg-atom quantum simulator, leading to coherent oscillations distinct from the strictly sub-volume-law QMBS previously realized in the same system.

We study the excitation spectrum of the spin-1 Kitaev model using the symmetric tensor network. By evaluating the virtual order parameters defined on the virtual Hilbert space in the tensor network formalism, we confirm the ground state is in a ${\mathbb{Z}}_2$ spin-liquid phase. Using the correspondence between the transfer matrix spectrum and low-lying excitations, we find that contrary to the dispersive Majorana excitation in the spin-1/2 case, the isotropic spin-1 Kitaev model has a dispersive bosonic charge excitation. The bottom of the gapped single-particle charge excitations is found at $\mathbf{K},{\mathbf{K}}^{\prime }=(\pm 2\pi /3,\mp 2\pi /3)$, with a corresponding correlation length of $\xi \approx 6.7$ unit cells. The lower edge of the two-particle continuum, which is closely related to the dynamical structure factor measured in inelastic neutron scattering experiments, is obtained by extracting the excitations in the vacuum superselection sector in the anyon theory language.

The Chandrasekhar-Clogston limit normally places stringent conditions on the magnitude of the magnetic field that can coexist with spin-singlet superconductivity, restricting the critical induced Zeeman shift to a fraction of the superconducting gap. Here, we consider a model system where the spin-singlet Cooper pairing in a dispersive band crossing the Fermi level is boosted by an additional flat-band located away from the Fermi level. The boosting of the pairing in the dispersive band allows for nontrivial solutions to the coupled gap equations for spin-splitting fields considerably larger than the superconducting gaps at zero field. Further, the additional Cooper pairing in the flat-band, away from the Fermi level, can increase the superconducting condensation energy without affecting the paramagnetic susceptibility of the system, making the free energy favor the superconducting state. This opens up the possibility for spin-singlet superconductivity beyond the standard Chandrasekhar-Clogston limit.

Two-dimensional quantum antiferromagnets host rich physics, including long-range ordering, high-$T_c$ superconductivity, quantum spin-liquid behavior, topological ordering, a variety of other exotic phases, and quantum criticalities. Frustrating perturbations in antiferromagnets may give rise to strong quantum fluctuations, challenging the theoretical understanding of the many-body ground state. Here we develop a method to describe the quantum antiferromagnets using fermionic degrees of freedom. The method is based on a formally exact mapping between spin-exchange models and theories describing fermionic matter with the emergent $\text{U}\left(1\right)$ Chern-Simons gauge field. For the planar Néel state, this mapping self-consistently generates the Chern-Simons superconductor mean-field ground state of introduced spinless fermions. We systematically compare the Chern-Simons superconductor state with the planar Néel state at the level of collective modes as well as order parameters. We reveal qualitative and quantitative correspondences between these two states. We demonstrate that such a construction using the fractionalized excitations and Chern-Simons gauge field can not only describe the Néel order, but can also be applied to study quantum spin liquids. Furthermore, we show that the confinement-deconfinement transitions from the Néel order to quantum spin liquids are signaled and characterized by the instabilities of Chern-Simons superconductors, driven by strong frustration. The results suggest observing and classifying the descendants of antiferromagnets, including other ordered states and unconventional superconductors, as well as emergent quantum spin liquids.

We introduce techniques for analyzing the structure of quantum states of many-body localized (MBL) spin chains by identifying correlation clusters from pairwise correlations. These techniques proceed by interpreting pairwise correlations in the state as a weighted graph, which we analyze using an established graph theoretic clustering algorithm. We validate our approach by studying the eigenstates of a disordered XXZ spin chain across the MBL to ergodic transition, as well as the nonequilibrium dynamics in the MBL phase following a global quantum quench. We successfully reproduce theoretical predictions about the MBL transition obtained from renormalization group schemes. Furthermore, we identify a clear signature of many-body dynamics analogous to the logarithmic growth of entanglement. The techniques that we introduce are computationally inexpensive and, in combination with matrix product state methods, allow for the study of large-scale localized systems. Moreover, the correlation functions we use are directly accessible in a range of experimental settings, including cold atoms.

The intrinsic dissipation of systems into a shared reservoir introduces coherence between two systems, enabling anti-parity-time (anti-$\mathcal{PT}$) symmetry. In this paper, we propose an anti-$\mathcal{PT}$ symmetric converter, consisting of a microwave cavity coupled dissipatively to a ferromagnetic sphere, which supports significant improvements in the conversion efficiency when compared to coherently coupled setups. In particular, when only the ferrite sample is driven, the strong coherence induced by the vacuum of the mediating channel leads to much stronger enhancements in the intended conversion. The enhancement is an inalienable artifact of the emergence of a long-lived, dark mode associated with a quasi-real singularity of the hybrid system. In addition, we observe considerable asymmetry in the efficiencies of microwave-to-optical and optical-to-microwave conversions, in spite of the symmetrical structure of the trilinear optomagnonic coupling stimulating both the transduction phenomena. The nonreciprocity stems from the intrinsic asymmetry in the couplings of the microwave and optical fields to the cavity-magnon network as well as the phase coupling entailed by the spatial separation.

We discuss the thermodynamic and magnetic properties of a competing high-spin/low-spin two-orbital two-electron model on a square lattice possibly relevant to the nickelates such as ${\mathrm{Ba}}_2{\mathrm{NiO}}_2{\left(\mathrm{AgSe}\right)}_2$. We focus on parameter regimes where a high-spin ($S=1$) and a low-spin ($S=0$) state are energetically close to each other and discuss various exchange processes in such a system. The model we study is motivated by, but different from, the Kondo-necklace model proposed for the system recently [Jin et al., Phys. Rev. Research 2, 033197 (2020)]. Although there are similarities between the two models in terms of the ground-state phases and their symmetries, the detailed properties of the different phases and the nature of phase transitions are entirely different and should be easy to distinguish from an experimental point of view.

In a Josephson junction mediated by a two-dimensional irradiated topological insulator, the current-phase relation can be electrically modulated, including the 0-$\pi$ phase transition and the anomalous phase shift of the current. Qualitatively combining analyses and numerical simulations, we find that the $0\text{−}\pi$ phase transition and anomalous phase shift can be controlled by a gate voltage and a transverse electric field, respectively. These possible electrical modulations result from the photoinduced anisotropic helical edge states. Due to the anisotropy and spin-momentum locking nature in the edge channels, an effective Zeeman field is caused by an electrical potential whose orientation automatically matches the spin alignment of the edge modes. The photoinduced anisotropy provides potential applications in Flouqet engineering helical supercurrent and electrically modulated topological superconducting devices.

A van der Waals coupled Weyl semimetal ${\mathrm{NbIrTe}}_4$ is investigated by combining scanning tunneling microscopy/spectroscopy and first-principles calculations. We observe a sharp peak in the tunneling conductance near the Fermi energy ($E_F$). Comparison with calculations indicates that the peak originates from a van Hove singularity (vHs) associated with a Lifshitz transition of the surface Fermi-arc state. Interestingly, our tunneling spectroscopy also shows signatures of strong electron-boson coupling. This is potentially due to an anomalously enhanced charge susceptibility coming from the near-$E_F$ vHs formation.

HoSbTe was predicted to be a weak topological insulator whose spin-orbit coupling (SOC) gaps are reported to be as large as hundreds of meV. Utilizing infrared spectroscopy, we find that the compound has a metallic nature from 350 down to 10 K. Particularly, both its itinerant carrier density and scattering rate are demonstrated to decrease with temperature cooling, which is responsible for the appearance of a broad hump feature in the temperature-dependent resistivity around 200 K. More importantly, we reveal the appearance of a charge-density-wave (CDW) gap whose energy scale is identified to shift from 364 meV at 10 K to 252 meV at 350 K. An SOC-induced band splitting is observed as well, and its coexistence with the CDW gap in the same compound paves the way to explore more intriguing physics.

Traditional maximum entropy and sparsity-based algorithms for analytic continuation often suffer from the ill-posed kernel matrix or demand tremendous computation time for parameter tuning. Here we propose a neural network method by convex optimization and replace the ill-posed inverse problem by a sequence of well-conditioned surrogate problems. After training, the learned optimizers are able to give a solution of high quality with low time cost and achieve higher parameter efficiency than heuristic fully connected networks. The output can also be used as a neural default model to improve the maximum entropy for better performance. Our methods may be easily extended to other high-dimensional inverse problems via large-scale pretraining.

Three-dimensional topological gapless phases have attracted significant attention due to their unique electronic properties. A flagship example is Weyl semimetals, which require breaking time-reversal or inversion symmetry. In two dimensions, the dimensionality reduction requires imposing an additional symmetry, thereby weakening the phase. Like its three-dimensional counterpart, these “two-dimensional Weyl semimetals” present edge states directly related to Weyl nodes. The direct comparison with the edge states in zigzag-like terminated graphene ribbons is unavoidable, offering the question of how robust these states are and their differences. Here we benchmark the robustness of the edge states in two-dimensional Weyl semimetals without inversion symmetry with those present in zigzag graphene ribbons. Our results show that, despite having a similar electronic band structure, the edge states of two-dimensional Weyl semimetals are more robust against vacancies than graphene ribbons. We attribute this enhanced robustness to a crucial role of the spin degree of freedom in the former case.

Single crystals of $R\mathrm{Cu}{\mathrm{As}}_2$ ($R=\mathrm{Yb}$ and Y) have been synthesized from high temperature ternary melts and their physical properties investigated by magnetization, electrical resistivity, specific heat, and $\mu \mathrm{SR}$ measurements. Magnetization measurements of $\mathrm{Yb}\mathrm{Cu}{\mathrm{As}}_2$ suggest that $4f$ electrons are almost localized and the observed magnetic properties are influenced by the Kondo and Ruderman-Kittel-Kasuya-Yosida interactions as well as crystalline electric field (CEF) effect. Electrical resistivity measurements show typical Kondo-lattice behavior with a broad local maximum around $\sim 125\mathrm{K}$ and a secondary hump around $\sim 20\mathrm{K}$, suggesting a strong hybridization between $4f$ and conduction electrons. A small residual resistivity $(\sim 0.68\mu \mathrm{\Omega }\mathrm{cm})$ and the large residual resistivity ratio ($\mathrm{RRR}\sim 150$) indicate a high quality single crystal. An analysis of the specific heat data indicates that the ground state doublet is well separated from excited CEF levels. Although an earlier neutron scattering experiment on powdered $\mathrm{Yb}\mathrm{Cu}{\mathrm{As}}_2$ samples observed no magnetic Bragg peaks associated with magnetic ordering, thermodynamic and transport property measurements in this study clearly indicate antiferromagnetic ordering below $T_N=3.7\mathrm{K}$ which is also supported by a muon spin relaxation investigation of $\mathrm{Yb}\mathrm{Cu}{\mathrm{As}}_2$ single crystals. The Kondo temperature ($T_K$) estimated from the magnetic entropy and magnetic susceptibility is in the range $5–7.5\mathrm{K}$, which is of similar magnitude as $T_N$. Interestingly, magnetization, specific heat, and electrical resistivity measurements as a function of magnetic field show a hysteresis loop both below and above $T_N$.

We explore the behavior of the disconnected entanglement entropy (DEE) across the topological phases of a long-range interacting Kitaev chain where the long-range interactions decay as a power law with an exponent $\alpha$. We show that while the DEE may not remain invariant deep within the topologically nontrivial phase when $\alpha <1$, it nevertheless shows a quantized discontinuous jump at the quantum critical point and can act as a strong marker for the detection of topological phase transition. We also study the time evolution of the DEE after a sudden quench of the chemical potential within the same phase. In the short-range limit of a finite chain, the DEE is expected to remain constant up to a critical time after the quench, which diverges in the thermodynamic limit. However, no such critical time is found to exist when the long-range interactions dominate (i.e., $\alpha <1$).

The material $\alpha \text{−}{\mathrm{RuCl}}_3$, with a two-dimensional Ru honeycomb sublattice, has attracted considerable attention because it may be a realization of the Kitaev quantum spin liquid. Recently, a new honeycomb material, $\alpha \text{−}{\mathrm{RuI}}_3$, was prepared under moderately high pressure, and it is stable under ambient conditions. However, different from $\alpha \text{−}{\mathrm{RuCl}}_3, \alpha \text{−}{\mathrm{RuI}}_3$ was reported to be a paramagnetic metal without long-range magnetic order down to 0.35 K. Here, the structural and electronic properties of the quasi-two-dimensional $\alpha \text{−}{\mathrm{RuI}}_3$ are theoretically studied. First, based on first-principles density functional theory calculations, the ABC stacking honeycomb-layer $R\overline{3}$ (No. 148) structure is found to be the most likely stacking order for $\alpha \text{−}{\mathrm{RuI}}_3$ along the $c$ axis. Furthermore, both $R\overline{3}$ and $P\overline{3}1c$ are dynamically stable because no imaginary frequency modes were obtained in the phononic dispersion spectrum without Hubbard $U$. Moreover, the different physical behavior of $\alpha \text{−}{\mathrm{RuI}}_3$ compared to $\alpha \text{−}{\mathrm{RuCl}}_3$ can be understood naturally. The strong hybridization between Ru $4d$ and I $5p$ orbitals decreases the “effective” atomic Hubbard repulsion, leading the electrons of ${\mathrm{RuI}}_3$ to be less localized than in ${\mathrm{RuCl}}_3$. As a consequence, the effective electronic correlation is reduced from Cl to I, leading to the metallic nature of $\alpha \text{−}{\mathrm{RuI}}_3$. Based on the $\mathrm{DFT}+U$ ($U_{\mathrm{eff}}=2$ eV) plus spin-orbital coupling, we obtained a spin-orbit Mott insulating behavior for $\alpha \text{−}{\mathrm{RuCl}}_3$ and, with the same procedure, a metallic behavior for $\alpha \text{−}{\mathrm{RuI}}_3$, in good agreement with experimental results. Furthermore, when introducing large (unrealistic) $U_{\mathrm{eff}}=6$ eV, the spin-orbit Mott gap opens in $\alpha \text{−}{\mathrm{RuI}}_3$ as well, supporting the physical picture we are proposing. Our results provide guidance to experimentalists and theorists working on two-dimensional transition metal tri-iodide layered materials.