${\mathrm{BiFeO}}_3$ is a technologically relevant multiferroic perovskite featuring ferroelectricity and antiferromagnetism. Its lattice, magnetic, and ferroelectric degrees of freedom are coupled to its optically active excitations and thus hold the potential to be reversibly probed and controlled by light. In this work, we combine ab initio density functional and many-body perturbation theory methods with an extensive symmetry and atomic-orbital analysis to describe and understand the electronic excited states spectrum and its imprint on the optical absorption spectrum with quantitative accuracy and qualitative insights. We find that the optical absorption spectrum of ${\mathrm{BiFeO}}_3$ contains several strongly bound and spatially localized electronic transitions in which the spin degree of freedom is flipped. We thoroughly characterize these localized spin-flip transitions in terms of the unusual crystal-field splitting of Fe-$3d$ single-electron orbitals. Our symmetry analysis further allows us to thoroughly explain how the spin content and the energetic fine structure of these strongly bound excitons are dictated by the interplay between crystal symmetry, electron-hole attraction, and the spin-orbit coupling.

Transition metal dichalcogenides (TMDs) constitute an intriguing platform for studying charge-ordered states including conventional and generalized Wigner crystals as well as Mott insulating states. In this work, we combine a phonon mode expansion of the electronic crystal vibrations with the Lindemann criterion to investigate the quantum and thermal stability of these strongly correlated phases in the exemplary materials of ${\mathrm{MoSe}}_2$ monolayers and twisted ${\mathrm{MoSe}}_2\text{−}{\mathrm{WSe}}_2$ heterostructures. We find that the moiré potential in heterobilayers acts as a harmonic trap, flattening the energy dispersion of phonon excitations and resulting in an order of magnitude larger melting temperatures compared to monolayer Wigner crystals. Furthermore, we explore the tunability of the correlated states with respect to dielectric environment and bilayer stacking. In particular, we show that the reduced screening in freestanding TMDs results in a tenfold increase in the melting temperature compared to hBN-encapsulated TMDs. Moreover, the deeper moiré potential in R-type stacked heterostructures makes generalized Wigner crystals more stable than in H-type stacking. Overall, our study provides important microscopic insights on the stability and tunability of charge-ordered states in TMD-based structures.

A first-principles computational method with self-consistent on-site and intersite Hubbard functionals is able to treat local and nonlocal Coulomb interactions on an equal footing. To apply the method to understand solids with strong spin-orbit coupling (SOC), we have extended a pseudohybrid functional approach developed by Agapito-Curtarolo-Buongiorno Nardelli to implement self-consistent extended Hubbard energy functionals for noncollinear spin states. With this, energy bands of semiconductors with various SOC strengths such as Si, Ge, GaAs, GaSb, CdSe, and PdO are obtained, agreeing with results from fully relativistic $GW$ approximation (FR-GWA) as well as experiments. We also compute energy gaps of HgTe, ${\mathrm{CuTlS}}_2$, and ${\mathrm{CuTlSe}}_2$ and assign them to be topological insulators correctly, unlike characteristic failures for judging topological properties from typical hybrid functionals. We demonstrate feasibility of our method to handle large systems by computing surface bands of topological insulators, ${\mathrm{Bi}}_{\text{2}}{\mathrm{Se}}_{\text{3}}$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, with varying thickness up to eight quintuple layers. Considering its low computational cost comparable to conventional ab initio methods and improved accuracy to FR-GWA, we expect that our method provides an opportunity to study large-scale correlated systems with the strong SOC efficiently and reliably.

Itinerant ferromagnetism is understood in terms of a quasiparticle picture with renormalized many-body effects. While the ferromagnetic ground state is destabilized by thermal and quantum fluctuations leading to exotic states such as unconventional superconductivity, how the quasiparticles evolve across the ferromagnetic transition is a target of intensive debate. Here, we present a type of ferromagnetic transition that is accompanied by a drastic reconstruction of quasiparticle spectrum in a layered ferromagnetic ruthenate, ${\mathrm{Sr}}_4{\mathrm{Ru}}_3{\mathrm{O}}_{10}$. Angle-resolved photoemission spectroscopy uncovered that the three-dimensional coherent states below the ferromagnetic transition temperature $\left(T_{\mathrm{C}}\right)$ turn into two-dimensional incoherent electronic states slightly above $T_{\mathrm{C}}$ characterized by the disappearance of trilayer band splitting, ferromagnetic exchange splitting, and long-lived quasiparticles. Our findings suggest that the electronic coherence strongly modifies the fermiology and magnetic order, pointing to an intriguing coupling between quasiparticles and magnetic properties.

We put forward a Monte Carlo algorithm that samples the Euclidean time operator growth dynamics at infinite temperature. Crucially, our approach is free from the numerical sign problem for a broad family of quantum many-body spin systems, allowing for numerically exact and unbiased calculations. We apply this methodological headway to study the high-frequency dynamics of the mixed-field quantum Ising model in one and two dimensions. The resulting quantum dynamics display rapid thermalization, supporting the recently proposed operator growth hypothesis. Physically, our findings correspond to an exponential falloff of generic response functions of local correlators at large frequencies. Remarkably, our calculations are sufficiently sensitive to detect subtle logarithmic corrections of the hypothesis in one dimension. In addition, in two dimensions, we uncover a nontrivial dynamical crossover between two large-frequency decay rates. Last, we reveal spatiotemporal scaling laws associated with operator growth, which are found to be strongly affected by boundary contributions.

Due to the hybridization of charged excitons with cavity photons, trion polaritons in microcavity semiconductors are a promising avenue for realizing strong polariton interactions and many-body polariton phases. We develop a quantum field theoretical formalism to study the formation of trion polaritons in a microcavity semiconductor doped with itinerant electrons within the ultrastrong light-matter coupling regime. We predict a new class of many-body states resulting from the intriguing interplay of polarons and virtual photons in an uncharted territory of light-matter interactions and Feshbach physics.

We determine the optical properties of gated bilayer graphene quantum dots with trigonal warping (TW) of single-particle energy spectra. The lateral structure of metallic gates confines electrons and holes in a quantum dot (QD) electrostatically. The gated bilayer graphene energy spectrum is characterized by two $K$ valleys surrounded by three minivalleys with energies depending on the applied vertical electric field. Employing an atomistic tight-binding model, we compute the single-particle QD states and analyze the influence of TW on the energy spectrum as the lateral confining potential depth varies. We find a regime where the QD levels are dominated by the presence of three minivalleys around each $K$ valley. Next, we compute dipole matrix elements and analyze the oscillator strengths and optical selection rules for optical valence to conduction band transitions. We then include electron-electron interactions by first computing the microscopic Coulomb matrix elements and electron self-energy and solving the Bethe-Salpeter equation to obtain the excitonic spectrum. Finally, we obtain the absorption spectrum for a shallow confining potential depth, which further amplifies the effects of TW on the optical properties. Our results predict the existence of two degenerate bright exciton states, each built of the three minivalley states that do not exist in the deep confinement regime, where the effects of TW are negligible.

Bulk transition metal dichalcogenides (TMDs) materials exhibit surprisingly high refractive index in visible to infrared range. Bulk TMD-based nanostructures can therefore serve as excellent resonant nanoresonators supporting various types of optical modes, which can in turn hybridize with TMD excitons in the same material. Such self-hybridizing structures are becoming attractive candidates for the study of intrinsic light-matter interactions due to its excellent mode overlap. However, very few studies have been reported on flexible tuning of intrinsic coupling in bulk TMD-based nanoresonators. Here, we propose a simple self-hybridizing nanoparticle-on-mirror (NPoM) system consisting of a single tungsten disulfide (${\mathrm{WS}}_2$) nanodisk placed on a metallic reflector deposited with a thin dielectric layer. We demonstrate a flexible tuning of self-hybridization of intrinsic TMD excitons with magnetic Mie mode and anapole, which is controlled by the size of the ${\mathrm{WS}}_2$ disk. Importantly, we show that the coupling strength of the system can be further modulated by tuning the thickness of the dielectric layer, enabling us to bring the system from weak to intermediate, and even to a strong coupling regime. Theoretical modeling combined with near-field analysis reveals that such an effective modulation on the coupling strength results from the variation of field confinement and number of excitons involved in the intrinsic coupling process. The proposed self-hybridizing system provides a promising platform for the study of highly tunable intrinsic light-matter interactions and may pave the way for the development of high-performance TMD-based tunable polaritonic devices.

Two-dimensional atomic monolayers of heavy elements grown on insulating substrates are promising for creating devices that exploit topological quantum states. Here, based on first-principles calculations and $k·p$ model, we systematically investigate the electronic structures and topological properties of the TlSb monolayer epitaxially grown on nonmagnetic BaTe(111) surface, TlSb/BaTe(111). We discover a non-Dirac quantum spin Hall gap of 331.0 meV at its $\mathrm{\Gamma }$ point, induced by strong spin-orbit coupling (SOC) of the Sb and Tl $p_x/p_y$ orbitals. Remarkably, nitrogen functionalization introduces ferromagnetic magnetization that breaks time-reversal symmetry and results in fully spin-polarized states, where the spin-up bands precisely degenerate at the Fermi energy with quadratic non-Dirac band dispersions. This spin-polarized non-Dirac bands, combined with SOC, opens a nontrivial gap of 258.0 meV with a significant Rashba effect, enabling the realization of a large-gap non-Dirac quantum anomalous Hall (QAH) effect, which is verified by a calculated Chern number of 1 and the observed topological edge states. We elucidate the underlying mechanism of this QAH state using the $k·p$ model. Furthermore, our findings suggest that this approach can be extended to other nonmagnetic substrates such as SrTe and BaSe, demonstrating a versatile strategy to achieve large-gap QAH effects without the need for magnetic doping or specialized substrates.

We investigate helical edge states emerging at the composite domain wall of spin and pseudospin degrees of freedom in a two-dimensional (2D) bulk governed by the Bernevig-Hughes-Zhang Hamiltonian which underwent quantum spin Hall to anomalous Hall transition. We numerically study the stability of Majorana bound states (MBSs) formed due to proximity-induced superconductivity in these helical edge states. We establish the robustness of MBSs against moderate chemical potential or magnetic disorder owing to the existence of the simultaneous orthogonality between the right and the left moving modes in both spin and pseudospin space. Hence our proposal could pave the way to realizing robust Majorana bound states on 2D platforms.

We show a transition from a bound state to a continuum resonance in a shallow quantum well (QW) by electrostatic gating to bend the conduction band edge. This bound state continuum resonance (BSCR) transition is particularly relevant in topological quantum computing platforms where shallow InAs QWs are used. We explain the observed capacitance jump and the parallel metal-insulator transition accompanying the BSCR transition. An experimental puzzle in shallow InAs QWs is the mobility drop at an electron density smaller than expected for the bound state second subband occupation. We explain this puzzle as a result of intersubband scattering involving a level-broadened continuum resonance, mediated by screened Coulomb impurities.

A theoretical study of the phonon-induced linewidths of the occupied electronic bands and the electron-phonon coupling (EPC) constant in graphene is presented. We propose an approximation in the spirit of the rigid ion approximation which considerably simplifies calculations of the EPC in metallic systems. We apply a tight-binding approach for the electrons and a force-constant model for the phonons with parameters obtained from first principles. A simple procedure is presented in order to estimate the influence of the graphene-substrate interaction on the phonon dispersion and thereby also on, e.g., the linewidths. The EPC is the strongest in the energy region where $\pi$ and $\sigma$ bands overlap. We find that the electron scattering is predominantly driven by the out-of-plane acoustic ZA and optical ZO phonon modes, while in general the high-energy optical phonon modes LO and TO are of secondary importance.

We classify the magnetic ground states of a 2D lattice of localized magnetic moments, which are coupled to a superconducting substrate with Rashba spin-orbit coupling. We discover a rich magnetic phase diagram with surprisingly complex structures including 2q-spin spirals, a 2×2-periodic pattern, and skyrmion lattices, self-consistently, using an effective classical spin Hamiltonian and show that the system hosts nonzero 4-spin interactions. Our in-depth analysis of about ten thousand magnetic configurations becomes feasible using contrastive clustering, a recent advanced unsupervised machine learning technique. This paper proposes simple few-band systems for noncollinear magnetic states and stimulates further research on topological effects in their self-consistent electronic structure.

We theoretically investigate spin pumping into a quantum easy-plane ferromagnetic spin chain system. This quantum spin chain is effectively described by the Tomonaga-Luttinger (TL) liquid despite the ferromagnetic exchange interaction because of the easy-plane magnetic anisotropy. This TL liquid state has an extremely strong interaction that is hardly realized in other quantum antiferromagnetic chain systems or weakly interacting electron systems. We show how the strongly interacting TL liquid affects the ferromagnetic resonance that occurs in the ferromagnetic insulator. In particular, we discuss the dependence of the Gilbert damping on the temperature and the junction length. The Gilbert damping allows us to extract information about the above-mentioned strong interaction within the quantum ferromagnetic spin chain. We also point out that a well-known compound ${\mathrm{CsCuCl}}_3$ will be suitable for the realization of our setup.

The Kibble-Zurek (KZ) mechanism describes the scaling behavior when driving a system across a continuous symmetry-breaking transition. Previous studies have shown that the KZ-like scaling behavior also lies in the topological transitions in the Qi-Wu-Zhang model (2D) and the Su-Schrieffer-Heeger model (1D), although symmetry breaking does not exist in these systems. Both models with linear band crossings yield $\nu \simeq 1$ and $z\simeq 1$. A natural question is whether different critical exponents can emerge in topological transitions beyond the linear band crossings. In this paper, we look into the KZ behavior in a topological 2D checkerboard lattice with a quadratic band crossing. We investigate from dual perspectives: momentum distribution of the Berry curvature in clean systems for simplicity, and real-space analysis of domain-like local Chern marker configurations in disordered systems, which is a more intuitive analog to conventional KZ description. In equilibrium, we find the correlation length diverges at the transition with a power $\nu \simeq 1/2$. Then, by slowly quenching the system across the topological phase transition, we find that the freeze-out time $t_{\mathrm{f}}$ and the unfrozen length scale $\xi \left(t_{\mathrm{f}}\right)$ both satisfy the KZ scaling, verifying $z\simeq 2$. We subsequently explore KZ behavior in topological phase transitions with other higher-order band crossings and find the relationship between the critical exponents and the order. Our results extend the understanding of the KZ mechanism and nonequilibrium topological phase transitions.

Inspired by recent experimental studies of local magnetic moments interacting with a metallic quasicrystal, we study the low-temperature fate of spins placed in two-dimensional tilings. In the diluted local moment limit, we calculate the spin relaxation rate $1/T_1$, as measured by electron spin resonance, and show that it displays a marked dependence on the system size $N$ and the filing $n$ of the electronic bath. For a finite concentration of spins, we integrate out the conduction electrons and generate an effective magnetic coupling between the local moments, which we treat as Ising spins. Despite the strongly frustrating nature of the magnetic couplings and the lack of periodicity in the problem, we find long-range orders for finite $N$ in our large-scale Monte Carlo simulations. However, the resulting magnetically state is fragile, as clusters of essentially free spins fluctuate down to very low temperatures.

We study numerically and analytically how the Dyakonov-Shur instability for a two-dimensional (2D) inviscid electronic fluid in a long channel can be affected by an external, out-of-plane static magnetic field. By linear stability analysis for a model based on the shallow-water equations, we describe the discrete spectrum of frequencies. When the fluid system is near the subsonic-to-supersonic transition point, a magnetically controlled gap between the stability and instability spectra of the complex eigenfrequencies is evident by our computations. This suggests that, within this model, the passage from stability to instability (and vice versa) is no longer continuous in the effective Mach parameter of the boundary conditions. We also demonstrate that the growth exponents are enhanced by the magnetic field. In a regime of weak magnetic fields, we derive a scaling law for the eigenfrequencies by perturbation theory. We discuss theoretical implications of our results in efforts to generate terahertz electromagnetic radiation by 2D electronic transport.

We study a system of hard-core bosons on a two-dimensional periodic honeycomb lattice subjected to an on-site potential with alternating signs along $y$ direction, using machine learning (ML) techniques. The model hosts a rich phase diagram consisting of six different phases including a charge density wave, a superfluid phase and two dimer insulator phases, one of which is also a weak topological insulator with zero Chern number but a nontrivial Berry phase [Ghosh et al., SciPost Phys. 10, 059 (2021)]. Using two distinct correlation functions computed via quantum Monte Carlo method, a relatively simple ML model is able to learn information from the various phases simultaneously and accurately predict their phase boundaries. By employing our ML model trained on the data set obtained from the Hamiltonian without explicit interactions, we determine the phase structure of the system in the presence of nearest-neighbor interactions. Additionally, we investigate the robustness of the weak topological insulator phase against interactions by predicting the topological invariant, which is otherwise difficult to obtain.

The charge density wave (CDW) instability often requires the participation of electron density of state at the Fermi level for the electronic energy gain in the structurally modulated phase, whether it is due to Fermi surface nesting (FSN) or the electron-phonon coupling mechanism. The physics of the CDW in the quasi-one-dimensional semimetal ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_7$ remains elusive for the lack of consistent Fermi surface response in the CDW state. Here, we investigate the electric-field effect in the CDW state in ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_7$ using ionic liquid gated field-effect transistors (iFETs) fabricated based on 10–20-nm-thick flakes. The strong electric field at the ionic liquid/flake interface and low carrier density in ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_7$ facilitate a finite electric-field response in the CDW state. The onset temperature of CDW shows a nearly negligible electric field dependence, whereas the resistivity shows a sizeable tunability: the resistivity increases at positive gating in the normal state but decreases in the CDW state instead. Our first-principles calculations revealed nested holelike Fermi surfaces with a nesting vector weakly coupled with electric-field gating and a nesting area increasing more significantly in a negative electric gating than in a positive electric gating. The calculated Fermi surfaces provide an adequate account for the intricate transport in ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_7$, which indicates the dominant role of FSN in the CDW mechanism under current debate in this system.