Density functional theory calculations not only allow to predict the vibrational and optical properties of solids but also to understand and disentangle the mechanisms playing a key role in the generation of coherent optical phonons. Recent experiments performed on a ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ nanoscale thin film have shown that a terahertz (THz) pulse launches at least a coherent $A_{1g}^1$ phonon as the transient transmittance measured using an isotropic detection scheme displays oscillations with a frequency matching the frequency of the $A_{1g}^1$ mode measured in Raman experiments. Such an observation can be explained by invoking either a sum-frequency process or cubic or quartic phonon-phonon couplings as considered for ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, a parent compound of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. By resorting to group theory and calculating energy surfaces from first principles, the main phonon-phonon couplings can be identified. Furthermore, a minimal model can be built to compute the dynamics of the Raman active modes coupled to the infrared active mode driven by the experimental THz pulse. Our model firmly establishes that cubic phonon-phonon interactions are relevant as the agreement between the computed and experimental transmittance is noteworthy.

Spin reversal and magnetization hysteresis loop were studied in the mean-field approximation for the anisotropic model of the compensated ferrimagnet ${\mathrm{Ni}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$. A rigorous analysis was applied to determine the energy of exchange anisotropy in the $\mathit{ab}$ plane. Based on the experimental data on the single crystal [Phys. Rev. B 111, 014420 (2025)], the values of the main parameters of microscopic interactions were obtained. This made it possible to calculate the temperature dependence of magnetization, showing spin reversal, as well as the magnetization hysteresis loop, demonstrating the exchange bias. A good qualitative agreement between the experiment and theory was obtained. The effect of single-ion anisotropy on the magnetism of nickel niobate was analyzed and the importance of the contribution of antiferromagnetic exchange interactions with the second nearest neighbors to the stable ferrimagnetic state of the crystal is presented.

Rényi entanglement entropy is widely used to study quantum entanglement properties in strongly correlated systems, and its analytic continuation as the Rényi index $n\to 1$ is often believed to yield von Neumann entanglement entropy. However, earlier theoretical analysis indicated that this process exhibits a singularity for the colored Motzkin spin chain problem, leading to different system size $l$ scaling behaviors of $\sim \sqrt{l}$ and $\sim lnl$ for the von Neumann and Rényi entropies, respectively. Our analytical and numerical calculations confirm this transition, which can be explained by the exponentially increasing density of states in the entanglement spectrum we extract numerically. Moreover, disorder operators can be measured easily in numerics and experiments and always have area-law or volume-law scaling similar to entanglement entropies. We further explored disorder operators under various symmetries of such a system. Both analytical and numerical results demonstrate that the scaling behaviors of disorder operators also follow $lnl$ as the leading term, matching that of Rényi entropy. Moreover, we find that the coefficient of the term $lnl$ is a universal constant shared by both the Rényi entropy and disorder operators and propose that it can probe the underlying constraint physics of Motzkin walks.

Quantum entanglement can be an effective diagnostic tool for probing topological phases protected by global symmetries. Recently, the notion of nontrivial topology in critical systems has been proposed and is attracting growing attention. In this work, as a concrete example, we explore the quantum entanglement properties of fermionic symmetry-enriched quantum critical points by constructing exactly solvable models based on stacked multiple Kitaev chains. We first analytically establish the global phase diagram using entanglement entropy and reveal three topologically distinct gapped phases with different winding numbers, along with three topologically distinct transition lines separating them. Importantly, we unambiguously demonstrate that two transition lines exhibit fundamentally different topological properties despite sharing the same central charge. Specifically, they display nontrivial topological degeneracy in the entanglement spectrum under periodic boundary conditions, thereby generalizing the Li-Haldane bulk-boundary correspondence to a broader class of fermionic symmetry-enriched criticality. Additionally, we identify a novel Lifshitz multicritical point at the intersection of the three transition lines, which also exhibits nontrivial topological degeneracy. This work provides a valuable reference for investigating gapless topological phases of matter from the perspective of quantum entanglement.

We perform a multi-band exact diagonalization (ED) study of rhombohedral pentalayer graphene twisted on hexagonal boron nitride with a focus on fractional Chern insulators (FCI) in systems with weak moiré gaps, complementing the results of [Yu et al., arXiv:2407.13770]. We consider both the charge-neutrality (CN) and average (AVE) interaction schemes. Saliently and surprisingly, we now find using the particle entanglement spectrum that the FCI at filling factor 1/3 in the CN scheme predicted by single-(Hartree-Fock) band ED is unstable towards a transition to charge density wave once a small fraction of electrons is allowed to occupy the higher bands. Meanwhile, the FCI at filling 2/3 in the AVE scheme remains more robust under similar band mixing until being suppressed when increasing band mixing. To tackle truncation errors that arise from including multiple bands in larger system sizes, we propose an ED iteration method that iteratively optimizes the single-particle basis so that the particles in the ground state should reside mainly in the lowest band. Nevertheless, we find that the FCI gap remains absent after convergence when the mixing with higher bands is considered. These findings highlight the delicate sensitivity of FCIs to multi-band effects and the shortcoming of all of the current models to explain the experimental emergence of such phases.

The interplay between relativistic electrons and crystalline symmetry in solids has garnered growing attention owing to their potential applications in spintronics and quantum technologies. The intertwined nontrivial topology and Rashba effect with additional control over spin degrees of freedom can coexist under the composite quantum compounds (CQCs) platform. The synergy of high-energy Rashba spin splitting (RSS) and topological surface states offers a promising coupling between electron spins and electricity. In the present study, first-principles calculations and symmetry analysis using vasp and wien2k are performed to achieve the coexistence of giant RSS, ferroelectricity, and topological phases. We investigate $X\text{GeBi}, X\text{Sn}M$ ($X$ = K, Rb, Cs; $M$ = As, Sb), and $\mathrm{CsGeS}{\mathrm{b}}_{(1-x)}\mathrm{B}{\mathrm{i}}_x$ ($X$ = 0.125, 0.25, 0.375, 0.5) compounds for potential CQCs. The CsGeBi compound is termed as intrinsic CQC with α = 4.10 eV Å while functionalized $\mathrm{CsGeS}{\mathrm{b}}_{0.5}\mathrm{B}{\mathrm{i}}_{0.5}$ exhibits α = 6.89 eV Å which is a very large value among all topological materials. This work also elucidates the significant difference between vasp and wien2k relativistic band calculations such as Rashba spin splitting, unnoticed so far for surface asymmetric materials. The possible physical origin of different RSS is attributed to the different implementation methods of crystalline potential gradient in vasp (PAW) and wien2k (LAPW+lo) calculations. The findings underscore the enhanced reliability of full-potential codes in analyzing the electronic band structure of materials exhibiting surface-induced asymmetry. The spin-texture reversal corresponding to switching in polarization facilitates control and manipulation of spin degrees of freedom in these ferroelectric Rashba semiconductors. The synthesis of KSnSb, KSnAs, NaSnAs, and NaSnP offers a practical framework for guiding the synthesis of the investigated materials in current study. This research initiative unlock pathways for delving into the intertwined ferroelectricity, Rashba effect, and nontrivial topology with more reliable theoretical approaches.

We present here a comprehensive microscopic theory of a family of neutral excitations in the fractional quantum Hall fluids, related to the geometric fluctuations of the quantum Hall ground states. Many of the physical properties of such geometric modes can be inferred analytically. These include the chirality, multiplicity, and energy of these geometric modes, as well as the relationship to the density modulation of the ground states of both incompressible and compressible fluids, with or without translational symmetry (e.g., the bubble and stripe phases). With a particular focus on the recently experimentally measured graviton modes as the special case, we elucidate both the universal aspects of the geometric modes and the nonuniversal aspects dependent on the details of the microscopic Hamiltonians. The microscopic theory explains some of the phenomenological components in the effective field theory and composite fermion theory. It predicts how geometric or graviton modes of both chiralities can be measured in experiments for any topological or compressible phases at different energy scales. In particular, we show that gapped geometric modes can exist even for compressible FQH phases, though translational symmetry of the ground state is important for such modes to couple to external probes (e.g., Raman scattering) in the long-wavelength limit.

We investigate the properties of natural two-dimensional (2D) magnetoplasma modes in laterally confined electron systems, such as 2D materials, quantum wells, and inversion layers in semiconductors with an elliptic Fermi surface. The resistivity of the system is considered in a dynamical anisotropic Drude model. The problem is solved in the fully screened limit, i.e., under the assumption that the distance between the two-dimensional electron system and the nearby metal gate is small compared to all other lengths in the system, including the wavelength of the plasmons. Remarkably, in this limit plasma oscillations in an anisotropic 2D confined system are equivalent to plasma oscillations in an isotropic 2D electron system obtained with some stretching, even when the electromagnetic retardation is taken into account. Moreover, accounting for electromagnetic retardation leads only to a renormalization of the effective masses of carriers, somewhat like for relativity. As an example, we reduce the equations describing plasmons in a gated disk with an anisotropic two-dimensional electron gas to the equations describing oscillations in an ellipse with isotropic Drude resistivity. In the absence of a magnetic field, we solve them analytically and find the eigenfrequencies. To find a solution in a magnetic field, we expand the current of plasma oscillations in the complete set of Mathieu functions. Leaving the leading terms of the expansion, we approximately find and analyze the magnetodispersion of the lowest modes.

We investigate the phase diagram of a one-dimensional model of hardcore bosons or spinless fermions with tunable nearest-neighbor interactions. By introducing alternating repulsive and attractive interactions on consecutive bonds, we show that the system undergoes a quantum phase transition from a bond-ordered (BO) phase to a charge-density wave-II phase as the attractive interaction strength increases at a fixed repulsive interaction. For a specific interaction pattern, the BO phase exhibits topological properties, which vanish when the pattern is altered, leading to a transition from a topological BO phase to a trivial BO phase through a gap-closing point where both interactions vanish. We identify these phases using a combination of order parameters, topological invariants, edge-state analysis, and Thouless charge pumping. By extending our analysis beyond half filling, we explore the phase diagram across all densities and identify the superfluid and the pair-superfluid phases, characterized by single-particle and bound-pair excitations at incommensurate densities. The proposed model is experimentally realizable in platforms such as Rydberg excited or ultracold atoms in optical lattices, offering a versatile framework to study such interplay between topology and interactions in low-dimensional systems. Finally, we present the complete ground-state phase diagram for all possible signs of the interaction strengths, repulsive or attractive, on consecutive bonds.

Optical measurements reveal the charge-ordered nature of the insulating state in the chiral molecular conductors ${{\text{[(S,S/R,R)-DM-EDT-TTF]}}_2\mathrm{ClO}}_4$ (DM-Cl) and ${{\text{[(S,S/R,R)-DM-EDT-TTF]}}_2\mathrm{ReO}}_4$ (DM-Re). These enantiopure materials exhibit metallic conductivity at room temperature and a metal-insulator transition upon cooling below ${\mathrm{T}}_{\mathrm{MI}}=40$ and 105 K, respectively. Optical conductivity spectra display a polarization-independent in-plane response, attributed to the hexagonal structure of the conducting DM-EDT-TTF layer, featuring broadband electronic excitations, molecular vibrational modes, and a Drude component in the metallic phase. The gap opening at 150 and $250 {\mathrm{cm}}^{-1}$ for DM-Cl and DM-Re, respectively, signals the transition from a metallic to an insulating state upon cooling. Drude-Lorentz-Fano analysis of the optical spectra yields Hubbard parameters, the intersite Coulomb repulsion $V$ and the bandwidth $W$, supporting the presence of charge order in the insulating phase. Raman spectra of DM-Cl and DM-Re, focusing on charge-sensitive molecular vibrations, further confirm a charge-ordered insulating state with a charge disproportionation of $0.10e$ and $0.13e$, respectively. Strong DM-EDT-TTF molecular modes, activated in optical spectra through coupling with the electronic background, support the presence of lattice frustration.

In this work, we develop a nonperturbative theory to identify a light pseudo-Goldstone mode with a small mass, as well as a type of Goldstone mode with a tiny slope (termed the slow-Goldstone mode), which may not be obtained via traditional perturbative methods. We demonstrate our formalism in the context of superfluids formed by Rashba spin-orbit coupled spinor bosons in a square lattice weakly interacting with a spin-anisotropic interaction. The experimental detections of these two modes, especially their roles leading to the quantum information scramblings at a finite temperature are discussed. The slow-Goldstone mode is compared with the slow light and the soft mode in the Sachdev-Ye-Kitaev models. This nonperturbative formalism can be widely applied to study other emergent particles in various quantum matter.

Altermagnetism, combing advantages of ferromagnetism and antiferromagnetism, exhibits rich fundamental physics with great interest in both theory and experiment. Here, we explicitly demonstrated the emergence of quantum spin Hall effect (QSHE) in two-dimensional altermagnets, i.e., altermagnetic QSHE, and propose that stacked Chern insulators with opposite chiralities offers a rational way to achieve the altermagnetic QSHE. In particular, through spin space group analysis and first-principles calculations, ${\mathrm{RuCS}}_3$ is revealed as a suitable candidate for material realization, where chiral and helical edge states appear in the monolayer and stacked bilayer, characterized by Chern number $\mathcal{C}=1$ and spin Chern number ${\mathcal{C}}_S=-1$, respectively. Moreover, the robustness of both the altermagnetic spin-splitting and QSHE with vanishing net magnetic moments is confirmed by strain engineering. Our results greatly enrich the prominent physical phenomena and expand the domain of QSHE, which are expected to draw great experimental attentions.

We show how weak symmetry-protected topological (SPT) phases on systems with a boundary are not in one-to-one correspondence with weak SPT phases on fully periodic systems, breaking the standard anomaly inflow interpretation of SPT phases. We further discuss the implications for the crystalline equivalence principle.

Magnetic Weyl semimetals have emerged as a fertile ground for exploring quantum phases of matter and quasiparticles. The latter includes unconventional fermions with larger Chern numbers than those of conventional Weyl fermions and topologically nontrivial band structures in bosonic excitations. They hold potential for further experimental exploration of emergent quantum matter, magnon-polarons, which arise from hybridization between spin and lattice degrees of freedom. Such hybridization of distinct collective modes offers a pathway to control coupled degrees of freedom as well as quantum phases. Magnon-polarons are anticipated to mark their presence in heat transport. In our findings, we observed a significant rise in magnetothermal conductivity of the Weyl semimetal NdAlSi with complex magnetic order and a ferrimagnetic ground state. Furthermore, our theoretical results predict the existence of chiral phonons with nonvanishing pseudoangular momentum. Eventually, the huge increase in magnetothermal conductivity is attributed to the hybridization or strong coupling of magnons and chiral phonons. These charge-neutral excitations can drive significant advancements in the fields of magnonics, spintronics, and emerging topological quantum computing.

The structural phase transitions and thermal conductivity of minerals are crucial for exploration the physical and chemical properties of the Earths interior and its dynamic evolution. Understanding these properties allows scientists to build more accurate models of the Earth's interior and its geodynamic behavior. Here, we conduct extensively structural searches of ${\mathrm{CaSiO}}_3$ under high pressure by CALYPSO method and first-principles calculations. Several stable phases of ${\mathrm{CaSiO}}_3$ are uncovered, which are in good agreement with the previous experiments. The lattice thermal conductivities of ${\mathrm{CaSiO}}_3$ minerals are found to decrease with the increases of temperature and pressure. Under geothermal gradient conditions, the thermal conductivities of the $P\overline{1}$ (3 GPa, 900 K), $Pnma$ (14 GPa, 1800 K), $I4/mcm$ (24 GPa, 2000 K), and $I4/mcm$ (136 GPa, 3000 K) phases of ${\mathrm{CaSiO}}_3$ are calculated to be 0.94, 0.11, 0.05 and 0.03 $\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$, respectively. However, after considering the two-channel model corrections, these thermal conductivities are increased to 2.57, 1.01, 1.70 and 2.97 $\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$, respectively. Interestingly, the $I4/mcm$ phase exhibits an unexpected behavior, showing the enhancements in thermal conductivities with the increases of temperature and pressure. The anomalous increase of thermal conductivities are derived from dual in-plane vibrations of oxygen atoms in the Si-O octahedra. Our findings provide valuable insights to understand the heat transfer mechanisms and the mantle convection dynamics of the Earth.

We study the dynamics under continuous measurements for free fermions in a quasiperiodic potential by using the Aubry-André-Harper model with hopping rate $J$ and potential strength $V$. On the basis of the quantum trajectory method, we obtain the phase diagram for the steady-state entanglement entropy and demonstrate that robust logarithmic system-size scaling emerges up to a critical potential strength $V_c/J\sim 2.3$. Moreover, we find that the measurement induces entanglement phase transitions from the logarithmic-law phase to the area-law phase for the potential strength $V<V_c$, while any finite measurement stabilizes the area-law phase for $V>V_c$. This result is distinct from the entanglement scaling in the unitary limit, where volume-law and area-law phases undergo a transition at $V/J=2$. To further support the phase diagram, we analyze the connected correlation function and find that it shows algebraic decay in the logarithmic-law phase, while it decays quickly in the area-law phase. Our results can be tested in ultracold atoms by introducing quasiperiodic potentials and continuously monitoring the local occupation number with an off-resonant probe light.

Nanoscale topological spin textures in magnetic systems are emerging as promising candidates for scalable quantum architectures. Despite their potential as qubits, previous studies have been limited to semiclassical approaches, leaving a critical gap: the lack of a fully quantum demonstration. Here, we address this challenge by employing the density matrix renormalization group (DMRG) method to establish domain wall (DW) qubits in coupled quantum spin-1/2 chains. We calculate the ground-state energies and excitation gaps of the system and find that DWs with opposite chiralities form a well-defined low-energy sector, distinctly isolated from higher excited states in the presence of anisotropies. This renders the chirality states suitable for encoding quantum information, serving as robust qubits. Interestingly, when a magnetic field is applied, we observe tunneling between quantum DW states with opposite chiralities. Through quantum simulations, we construct an effective qubit Hamiltonian that exhibits strongly anisotropic $g$ factors, offering a way to implement single-qubit gates. Furthermore, we obtain an effective interacting Hamiltonian for two mobile DWs in coupled quantum spin chains from DMRG simulations, enabling the implementation of two-qubit gates. Single-qubit and two-qubit gates are also demonstrated in real-time simulations using the time-dependent variational principle. Our work represents a critical step from semiclassical constructions to a fully quantum demonstration of the potential of DW textures for scalable quantum computing, establishing a solid foundation for future quantum architectures based on topological magnetic textures.