We investigate the experimental signatures of Raman spectroscopy of bimagnon and trimagnon excitations in the distorted triangular lattice antiferromagnets $\alpha \text{−}L{\mathrm{Cr}}_2{\mathrm{O}}_4$ $(L=\mathrm{Sr}, \mathrm{Ca})$. Motivated by Raman scattering experiments, we utilize spin wave theory to analyze the nearly ${120}^{\circ }$ spin-3/2 spiral-ordered antiferromagnetic ground state to compute the single-magnon density of states, single-magnon dispersion, and bimagnon and trimagnon Raman spectra (polarized and unpolarized). We perform calculations of the Heisenberg antiferromagnetic Hamiltonian that incorporates magnetic interactions (exchange, anisotropy, and interlayer coupling) and lattice distortion within a four-sublattice unit cell. We investigate the Hamiltonian for both model parameter sets and experimentally proposed magnetic interactions for $\alpha \text{−}L{\mathrm{Cr}}_2{\mathrm{O}}_4$ $(L=\mathrm{Sr}, \mathrm{Ca})$. It is found that Raman scattering is capable of capturing the effect of the rotonlike $M$ and $M^{\prime }$ points on the bimagnon Raman spectrum. Our calculation confirms the connection between single-magnon rotonlike excitation energy and the bimagnon Raman excitation spectrum observed experimentally. The roton energy minimum in momentum space is half of the energy of a bimagnon excitation signal. The experimental magnetic Raman scattering result displays two peaks which have Raman shifts of 15 and 40 meV. Theoretical modeling and analysis of the experimental spectrum of $\alpha \text{−}{\mathrm{SrCr}}_2{\mathrm{O}}_4$ within our distorted Heisenberg Hamiltonian lattice suggest that the low-energy peak at 15 meV is associated with the bimagnon excitation, whereas the high-energy peak around 40 meV is primarily a trimagnon excitation. Based on our fitting procedure we propose a set of magnetic interaction parameters for $\alpha \text{−}{\mathrm{SrCr}}_2{\mathrm{O}}_4$. These parameters reproduce not only the experimental Raman spectrum but also the inelastic neutron scattering response (including capturing high-energy magnon branches). We also compute the unpolarized bimagnon and trimagnon Raman spectra for $\alpha \text{−}{\mathrm{CaCr}}_2{\mathrm{O}}_4$. In contrast to its Sr cousin the Ca-based material has an enhanced bimagnon response, with the high-energy peak still dominated by the trimagnon excitation. Furthermore, the polarization sensitivity of the Raman spectrum can be utilized to distinguish the bimagnon and trimagnon excitation channels.

Promoting the Curie temperature and tunning the magnetocrystalline anisotropy have been key issues in two-dimensional (2D) magnetic materials. Here, we address this issue by studying $\mathrm{Bi}/{\mathrm{CrI}}_3$ heterostructures using first-principles calculations. Results show that the easy-magnetization direction rotates from the easy axis in the ${\mathrm{CrI}}_3$ monolayer to the easy plane in the $\mathrm{Bi}/{\mathrm{CrI}}_3$ heterostructure. A built-in electric field in ${\mathrm{CrI}}_3$ introduced by the coupling between the ${\mathrm{CrI}}_3$ and Bi monolayers is found responsible for this spin reorientation. The Curie temperature of the $\mathrm{Bi}/{\mathrm{CrI}}_3$ heterostructure reaches 107 K, significantly larger than the Curie temperature (45 K) of the pristine ${\mathrm{CrI}}_3$ monolayer. The metal property of the heterostructure introduces a double exchangelike ferromagnetic (FM) coupling which promotes the Curie temperature. Moreover, in the Bi/bilayer ${\mathrm{CrI}}_3$ heterostructure, the coupling between ${\mathrm{CrI}}_3$ and Bi makes the interlayer ${\mathrm{CrI}}_3-{\mathrm{CrI}}_3$ exchange interaction become FM, and the ${\mathrm{CrI}}_3$ bilayer has a FM ground state. Our work provides a way to modulate the magnetic properties of 2D materials and may be used for designing 2D spintronic devices.

Two-band electronic structures with a valence and a conduction band separated by a tunable energy gap and with pairing of electrons in different channels can be relevant to investigate the properties of two-dimensional multiband superconductors and electron-hole superfluids, such as monolayer FeSe, recently discovered superconducting bilayer graphene, and double-bilayer graphene electron-hole systems. This electronic configuration also allows us to study the coexistence of superconductivity and charge-density waves in connection with underdoped cuprates and transition metal dichalcogenides. By using a mean-field approach to study the system mentioned above, we have obtained numerical results for superconducting gaps, chemical potential, condensate fractions, coherence lengths, and superconducting mean-field critical temperature, considering a tunable band gap and different fillings of the conduction band, for a parametric choice of the pairing interactions. By tuning these quantities, the electrons redistribute among valence and conduction band in a complex way, leading to a new physics with respect to single-band superconductors, such as density-induced and band-selective BCS-BEC crossover, quantum phase transitions, and hidden criticalities. At finite temperature, this phenomenon is also responsible for the nonmonotonic behavior of the superconducting gaps resulting in a superconducting-normal state reentrant transition, without the need of disorder or magnetic effects.

We show that nitrogen-vacancy (NV) centers could provide distinct signatures of emergent gauge fields in certain phase transitions in spin liquids. We consider the relaxation rate $1/T_1$ of a diamond NV center placed a distance $z_0$ above a crystal consisting of two-dimensional layers which hosts a U(1) quantum spin liquid with spinon Fermi surface. We show that, during a spinon pairing transition from a U(1) to a Z2 spin liquid, the relaxation rate exhibits a rapid fall just below the pairing transition. The drop is much faster than what can be accounted for by the opening of the energy gap. Instead, the rapid fall with a unique $z_0$ dependence of its height and width is the signature of the Meissner effect of the gauge field. We identify the organic ET compound which has a phase transition at 6 K as a candidate for the experimental observation of this phenomenon.

Owning to their high controllability, laser pulses have contributed greatly to our understanding of strongly correlated electron systems. However, typical multicycle pulses do not control the symmetry of systems, which plays an important role in the emergence of novel quantum phases. Here, we demonstrate that subcycle pulses whose oscillation is less than one period within a pulse envelope can control inversion and time-reversal symmetries in the electronic states of the one-dimensional extended Hubbard model. Using an ultrashort subcycle pulse, one can generate a steady electric current (SEC) in a photoexcited state due to an Aharonov-Bohm flux instantaneously introduced through the phase of an electric field. Consequently, time-reversal symmetry is broken. In contrast, a broad subcycle pulse does not induce SEC but instead generates electric polarization, thus breaking inversion symmetry. Both symmetry breakings in a photoexcited state can be monitored by second-harmonic generation. These findings provide a new methodology for designing the symmetries of electronic states and open up a new field of subcycle-pulse engineering.

Strong coupling between matter and quantized electromagnetic fields in a cavity has emerged as a possible route toward controlling the phase of matter in the absence of an external drive. We develop a faithful and efficient theoretical framework to analyze quantum geometry and topology in materials ultrastrongly coupled to cavity electromagnetic fields in two dimensions. The formalism allows us to accurately evaluate geometrical and topological quantities, such as Berry phase and Chern number, in ultrastrong and deep strong-coupling regimes. We apply our general framework to analyze a concrete model of massive Dirac fermions coupled to a circularly polarized cavity mode. Surprisingly, in addition to an ordinary transition to the topological phase, our analysis reveals a unique feature in deep strong-coupling regimes, namely the emergence of a reentrant transition to the topologically trivial phase. We also demonstrate its intuitive understanding by showing the unitary mapping between the low-energy effective theory of strongly coupled light-matter systems and the Haldane honeycomb model.

The layered transition metal dichalcogenide $1T\text{−}{\mathrm{TaS}}_2$ has evoked great interest owing to its particularly rich electronic phase diagram including different charge density wave (CDW) phases. However, few studies have focused on its hysteretic electronic phase transitions based on the in-depth discussion of the delicate interplay among temperature-dependent electronic interactions. Here, we report a sequence of spatial electronic phase transitions in the hysteresis temperature range (160–230 K) of $1T\text{−}{\mathrm{TaS}}_2$ via variable-temperature scanning tunneling microscopy. Several emergent electronic states are investigated at multiscale during the commensurate CDW–triclinic CDW (CCDW-TCDW) phase transitions: a spotty-CDW state above $\sim 160\mathrm{K}$, a network-CDW (NCDW) state above $\sim 180\mathrm{K}$ during the warmup process, a belt-TCDW state below $\sim 230\mathrm{K}$, a NCDW state below $\sim 200\mathrm{K}$, and finally a mosaic-CDW state below $\sim 160\mathrm{K}$ during cooldown from the TCDW phase. These emergent electronic states are associated with the delicate temperature-dependent competition and/or cooperation of stacking-dependent interlayer interactions, intralayer electron-electron correlations, and electron-phonon ($e\text{−}ph$) coupling of $1T\text{−}{\mathrm{TaS}}_2$. Our results not only provide insight to understand the hysteretic electronic phase transitions in the correlated CDW state, but also pave a way to realize more exotic quantum states by accurately and effectively tuning various interior interactions in correlated materials.

We report $c$-axis transport studies on magnetic topological insulators $\mathrm{Mn}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_4$. We performed systematic $c$-axis magnetoresistivity measurements under high magnetic fields (up to 35 T) on several representative samples. We find that the lightly hole- and lightly electron-doped samples, while both having the same order of magnitude of carrier density and similar spin-flop transitions, exhibit sharp contrast in electronic anisotropy and transport mechanism. The electronic anisotropy is remarkably enhanced for the lightly hole-doped sample relative to pristine $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ but not for the lightly electron-doped sample. The lightly electron-doped sample displays a giant negative longitudinal magnetoresistivity (LMR) induced by the spin-valve effect at the spin-flop transition field, whereas the lightly hole-doped sample exhibits remarkable negative LMR consistent with the chiral anomaly behavior of a Weyl semimetal. Furthermore, we find the large negative LMR of the lightly hole-doped sample extends to a wide temperature range above the Néel temperature $\left(T_{\mathrm{N}}\right)$ where the magnetoconductivity is proportional to $B^2$. This fact, together with the short-range intralayer ferromagnetic correlation revealed in isothermal magnetization measurements, suggests the possible presence of the Weyl state above $T_{\mathrm{N}}$. These results demonstrate that in the $c$-axis magnetotransport of $\mathrm{Mn}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_4$, the spin scattering is dominant in the lightly electron-doped sample but overwhelmed by the chiral anomaly effect in the lightly hole-doped sample due to the presence of the Weyl state. These findings extend the understanding of the transport properties of $\mathrm{Mn}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_4$.

By employing element-specific spectroscopy in the ultrafast time scale in ${\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x$ alloys, we find a composition-dependent effect in the demagnetization that we relate to electron-magnon scattering and changes in the spin-wave stiffness. In all six measured alloys of different composition, the demagnetization of Ni compared to Fe exhibits a delay, an effect which we find is inherent in alloys but not in elemental Fe and Ni. Using a model based on electron-magnon scattering, we extract a spin-wave stiffness from all alloys that show excellent agreement with values obtained from other techniques.

Higher-order topological insulators containing multidimensional boundary states have been extensively researched. Here, we characterize the zero-energy corner states generated in the honeycomb lattice by calculating the multiple chiral numbers topological invariant in real space with fewer symmetry constraints. By relating the concept of boundary-obstructed topological insulators which shows a gap in the bulk band, we analyze the edge states transition and the phase diagram of the zero-energy level of the hexagonal structure which are consistent with the characterization results of the sample. Crucially, we construct three samples hosting the nontrivial, phase-transition-point and trivial edge states in acoustic systems and demonstrate the higher-order band topology. Our paper demonstrates the usefulness of directly characterizing topological properties of chiral-symmetric systems by applying multiple chiral numbers.

A class of integrable models, such as the one-dimensional transverse-field Ising model, responds nonmonotonically to a periodic drive with respect to the driving parameters and freezes almost absolutely for certain combinations of the latter. In this paper, we go beyond the two-band structure of the Ising-like models studied previously and ask whether such an unusual nonmonotonic response and near-absolute freezing occur in integrable systems with a higher number of bands. To this end, we consider a tight-binding model for bilayer graphene subjected to an interlayer potential difference. We find that when the potential is driven periodically, the system responds nonmonotonically to variations in the driving amplitude $V_0$ and frequency $\omega$ and shows near-absolute freezing for certain values of $V_0/\omega$. However, the freezing occurs only in the presence of a constant bias in the driving, i.e., when $V=V^{\prime }+V_{0}cos\omega t$. When $V^{\prime }=0$, the freezing is switched off for all values of $V_0/\omega$. We support our numerical results with analytical calculations based on a rotating wave approximation. We also give a proposal to realize the driven bilayer system via ultracold atoms in an optical lattice, where the driving can be implemented by shaking the lattice.

Computing phonons from first principles is typically considered a solved problem, yet inadequacies in existing techniques continue to yield deficient results in systems with sensitive phonons. Here we circumvent this issue using the lone irreducible derivative (LID) and bundled irreducible derivative (BID) approaches to computing phonons via finite displacements, where the former optimizes precision via energy derivatives and the latter provides the most efficient algorithm using force derivatives. A condition number optimized basis for BID is derived which guarantees the minimum amplification of error. Additionally, a hybrid LID-BID approach is formulated, in which select irreducible derivatives computed using LID replace BID results. We illustrate our approach on two prototypical systems with sensitive phonons: the shape memory alloy AuZn and metallic lithium. Comparing our resulting phonons in the aforementioned crystals to calculations in the literature reveals nontrivial inaccuracies. Our approaches can be fully automated, making them well suited for both niche systems of interest and high-throughput approaches.

We propose stacked two-dimensional lattice designs of frustrated and SO(3)-symmetric spin models consisting of antiferromagnetic (AFM) triangular and ferromagnetic (FM) sixfold-symmetric sublattices that realize emergent ${\mathbb{Z}}_3$ Potts nematic order. Considering bilinear-biquadratic spin interactions, our models describe an SO(3)-symmetric triangular lattice AFM subject to a fluctuating magnetization arising from the FM coupled sublattice. We focus on the classical AFM-FM windmill model and map out the zero- and finite-temperature phase diagrams using Monte Carlo simulations and analytical calculations. We discover a state with composite Potts nematic order above the ferrimagnetic three-sublattice up-up-down ground state and relate it to Potts phases in SO(3)-broken Heisenberg and Ising AFMs in external magnetic fields. Finally, we show that the biquadratic exchange in our model is automatically induced by thermal and quantum fluctuations in the purely bilinear Heisenberg model, easing the requirements for realizing these lattice designs experimentally.

Describing a quantum impurity coupled to one or more noninteracting fermionic reservoirs is a paradigmatic problem in quantum many-body physics. While historically the focus has been on the equilibrium properties of the impurity-reservoir system, recent experiments with mesoscopic and cold-atomic systems enabled studies of highly nonequilibrium impurity models, which require novel theoretical techniques. We propose an approach to analyze impurity dynamics based on the matrix-product state (MPS) representation of the Feynman-Vernon influence functional (IF). The efficiency of such a MPS representation rests on the moderate value of the temporal entanglement (TE) entropy of the IF, viewed as a fictitious “wave function” in the time domain. We obtain explicit expressions of this wave function for a family of one-dimensional reservoirs, and analyze the scaling of TE with the evolution time for different reservoir's initial states. While for initial states with short-range correlations we find temporal area-law scaling, Fermi-sea-type initial states yield logarithmic scaling with time, closely related to the real-space entanglement scaling in critical $1d$ systems. Furthermore, we describe an efficient algorithm for converting the explicit form of general reservoirs' IF to MPS form. Once the IF is encoded by a MPS, arbitrary temporal correlation functions of the interacting impurity can be efficiently computed, irrespective of the form of impurity interactions and bath geometry. The approach introduced here can be applied to a number of experimental setups, including highly nonequilibrium transport via quantum dots and real-time formation of impurity-reservoir correlations, as well as in nonequilibrium dynamical mean-field theory.

We present a first-principles scheme for incorporating many-body interactions into the unified description of the quadratic optical response to light of noncentrosymmetric crystals. The proposed method is based on time-dependent current-density response theory and includes the electron-hole attraction via a tensorial long-range exchange-correlation kernel, which we calculate using the parameter-free bootstrap approximation. By bridging with the Wannier-interpolation of the independent-particle transition matrix elements, the resulting numerical scheme is very general and allows us to resolve narrow many-body spectral features at low computational cost. We showcase its potential by inspecting the second-harmonic generation in the benchmark zinc-blende semiconductor GaAs, the layered graphitic semiconductor $\mathrm{B}{\mathrm{C}}_2\mathrm{N}$, and the Weyl semimetal TaAs. Our results show that excitonic effects can give rise to large and sharply localized one- and two-photon resonances that are absent in the independent-particle approximation. We find overall good agreement with available experimental measurements, capturing the magnitude and peak structure of the spectrum as well as the angular dependence at fixed photon energy. The implementation of the method in Wannier-based code packages can serve as a basis for performing accurate theoretical predictions of quadratic optical properties in a vast pool of materials.

In this work we study the treatment of asymmetric open quantum systems with neural networks based on the restricted Boltzmann machine. In particular, we are interested in the nonequilibrium steady-state current in the boundary-driven (anisotropic) Heisenberg spin chain. We address previously published difficulties in treating asymmetric dissipative systems with neural-network quantum states and Monte Carlo sampling and present an optimization method and a sampling technique that can be used to obtain high-fidelity steady-state approximations of such systems. We point out some inherent symmetries of the Lindblad operator under consideration and exploit them during sampling. We show that local observables are not always a good indicator of the quality of the approximation and finally present results for the spin current that are in agreement with known results of simple open Heisenberg chains.

We study the stability of the fermionic quasiparticle in a fermion-boson model on a Bethe lattice, with fermions interacting with local bosons via polaronic-type coupling. We solve the problem by mapping it onto a noninteracting chain with site-dependent potential. We show that, despite a finite number of bosonic excitations costing zero energy, among the many analyzed cases, the occurrence of a complete collapse of the quasiparticle is rare. The quasiparticle disappearance becomes easier with an increase in (i) the total number of bosons with zero energy and (ii) the relative strength of the coupling between bosons and fermions. The postulated model can, among other things, be applied to study systems in which fermions are introduced into antiferromagnetic (or antiferro-orbital) domains surrounded by ferromagnetic (or ferro-orbital) ordered states. This might take place in the overdoped cuprates or upon doping manganese or vanadium oxides. Finally, we show how this model leads to an in-depth understanding of the onset of quasiparticles in the one-dimensional (1D) and 2D $t\text{−}{J}^z$ model.

Local quantum criticality in itinerant fermion systems has been extensively investigated through the soft-gap Anderson impurity model, wherein a localized, correlated impurity hybridizes with a broad conduction band with a singular, ${\left|\omega \right|}^r$, density of states. However, lattice models hosting quantum critical points (QCPs) do not appear to have such a spectrum emerging at the QCP. In this work, we report the emergence of such a singular form of the density of states in a three-orbital lattice model, within dynamical mean field theory, precisely at a quantum critical point, separating a gapless, Fermi liquid (FL) metallic phase from a gapped, Mott insulating phase. A temperature-dependent exponent, $\alpha$, defined using the corresponding Matsubara self-energy, is found to vary from $+1$ deep in the FL regime, to $-1$ in the Mott insulator regime. Interestingly, we find that $\alpha$ becomes temperature-independent, and hence isosbestic, precisely at the QCP. The isosbestic exponent is shown to lead to an emergent soft-gap spectrum, ${\left|\omega \right|}^r$, at the QCP, where $r=|{\alpha }_{\mathrm{iso}}|$. We discuss the implications of our findings for non-Fermi-liquid behavior in the quantum critical region of the phase diagram.

A resonance transition occurs between two states when the system is under an oscillatory driving field with a proper frequency. It is commonly believed that frequency must be matched at the resonance transition. Here we show that the above condition is not necessarily satisfied when the system under driving encloses nontrivial band degeneracies, such as a Dirac point. We consider a kagome lattice under the modulation of an artificial gauge field and show the inequivalence of the modulation frequency and energy difference between states at transitions. Meanwhile, our analytical formula reveals that the transitions exhibit chirality around the Dirac cone. When the system Hamiltonian under modulation circles around a higher-order charged band degeneracy, the fundamental one-photon transition is absent. These peculiar properties can be explained with the effective Hamiltonian under $n$-fold rotational symmetry.

By combining the density matrix renormalization group (DMRG) method with Gutzwiller projected wave functions, we study the SU(4) symmetric spin-orbital model on the honeycomb lattice. We find that the ground states can be well described by a Gutzwiller projected $\pi$-flux state with Dirac-type gapless excitations at one quarter filling. Although these Dirac points are gapped by emergent gauge fluxes on finite cylinders, they govern the critical behavior in the thermodynamic limit. By inserting a $\theta =\pi$ spin flux to twist the boundary condition, we can shift the gapless sector to the ground state, which provides compelling evidence for the presence of a gapless Dirac spin-orbital liquid.