A strongly correlated system with competing ground states is often poised close to the quantum critical point. External perturbations such as pressure, strain, electric field, and chemical doping can stabilize its ground state with exotic physical properties. Cr-doping is the lone exception, which enhances the Curie temperature in one such correlated system, ${\mathrm{SrRuO}}_3$. To find the origin of $T_C$ enhancement, we investigate the temperature-dependent structure, spectroscopic, magnetic, and magnetotransport properties in ${\mathrm{SrRu}}_{1-x}{\mathrm{Cr}}_x{\mathrm{O}}_3$. Cr-doping squeezes the unit cell volume, which effectively enhances the stretching octahedral distortion nearly five times more than pure ${\mathrm{SrRuO}}_3$. The Curie temperature increment of $\sim 22$ K for $x=0.15$ is found to be intertwined with the structural modulation. Temperature-dependent neutron diffraction analysis indicates that the unit cell volume minima coincide exactly with the enhanced ferromagnetic ordering ($\sim 190$ K). Further analysis reveals that the effect of Cr-doping not only freezes the octahedral tilt below 100 K but also suppresses the complex magnetism responsible for exchange bias and the topological Hall effect in ${\mathrm{SrRuO}}_3$. The spectroscopic measurements find a reduction of itinerancy of $d$-electrons with Cr-doping. The magnetotransport measurements portray an evolution from itinerant to localized ferromagnetism.

We present a nonequilibrium steady-state implementation of the two-particle self-consistent method. This approach respects the Mermin-Wagner theorem and incorporates nonlocal spatial fluctuations through self-consistent static vertices. The real-frequency implementation allows one to compute spectral properties without analytical continuation in both equilibrium and nonequilibrium. As an interesting application, we investigate spin correlations in the bilayer square lattice Hubbard model under a perpendicular static electric field. In equilibrium, the result yields spin correlations which are in good agreement with recent optical lattice experiments. Under a large enough static electric field, the interlayer spin correlations switch from antiferromagnetic to ferromagnetic. We clarify how this phenomenon is linked to the nonequilibrium modifications of the spin excitation spectrum.

The recent constructions of flat moiré minibands in specifically twisted multilayer graphene and twisted transition metal dichalcogenides (TMDs) have facilitated the observation of strong correlations with convenient tunability. These correlations in flat bands result in band dispersion heavily influenced by carrier densities, leading to filling-dependent quasiparticle band renormalizations. Particularly, in magic-angle twisted bilayer graphene (MATBG), the band structure—including the quasiparticle energy and wave function—is crucial in understanding the correlated properties. Previous theoretical studies have demonstrated the presence of a time-reversal-even charge Hall counterflow in response to a direct current (DC) electric field in twisted bilayers as chiral structures. In this study, we show that such layer Hall counterflow can serve as a sensitive probe for MATBG model parameters, which are currently ambiguous as a result of unavoidable structural relaxation and twist-angle disorder. We present the layer Hall counterflow and the associated in-plane magnetization for three different MATBG continuum models, based on which many-body interacting models have been widely applied to study strong correlations in MATBG. At the single-particle level, our findings indicate notable differences in layer-projected Hall conductivity, both in magnitude and sign, between different MATBG continuum models. Furthermore, our self-consistent Hartree calculations, performed on each of these single-particle continuum models, reveal renormalized layer-projected Hall conductivity by the self-consistent Hartree field.

Monolayer transition metal dichalcogenides (TMDs) have a simple crystal structure, but they exhibit intricate physical phenomena that differ from their bulk counterparts. Recently, there has been significant interest in the electronic behavior of monolayer TMDs hosted in a natural van der Waals superlattice material, ${\mathrm{Ba}}_6{\mathrm{Nb}}_{11}{\mathrm{S}}_{28}$, consisting of alternating ${\mathrm{NbS}}_2$ monolayers and block layers. Here, we report the electronic structure study of ${\mathrm{Ba}}_6{\mathrm{Nb}}_{11}{\mathrm{S}}_{28}$ and ${\mathrm{Ba}}_6{\mathrm{Ta}}_{11}{\mathrm{S}}_{28}$. Using angle-resolved photoemission spectroscopy and density functional theory calculation, we show that the electronic structures of the superlattices are similar to those of monolayer TMDs. The two-dimensional characteristics indicate that the interlayer coupling of adjacent TMD layers is suppressed by the intercalation of the ${\mathrm{Ba}}_3{\mathrm{NbS}}_5$ or ${\mathrm{Ba}}_3{\mathrm{TaS}}_5$ block layer. A clear band splitting due to spin-orbital coupling is observed in ${\mathrm{Ba}}_6{\mathrm{Ta}}_{11}{\mathrm{S}}_{28}$, while no obvious splitting is found in ${\mathrm{Ba}}_6{\mathrm{Nb}}_{11}{\mathrm{S}}_{28}$. These observations are in qualitative agreement with the observation on monolayer films of ${\mathrm{NbS}}_2$ and ${\mathrm{TaS}}_2$. Based on our findings, these natural superlattices can serve as an effective model system for studying monolayer materials and their potential applications.

The dephasing time of spin-orbit qubits is limited by the coupling with electrical and charge noise. However, there may exist “dephasing sweet spots” where the qubit decouples (to first order) from the noise so that the dephasing time reaches a maximum. Here we discuss the nature of the dephasing sweet spots of a spin-orbit qubit electrically coupled to some fluctuator. We characterize the Zeeman energy $E_{\mathrm{Z}}$ of this qubit by the tensor $G$ such that $E_{\mathrm{Z}}={\mu }_B\sqrt{{\mathbf{B}}^{\mathrm{T}}G\mathbf{B}}$ (with ${\mu }_B$ the Bohr magneton and $\mathbf{B}$ the magnetic field), and its response to the fluctuator by the derivative $G^{\prime }$ of $G$ with respect to the fluctuating field. The geometrical nature of the sweet spots on the unit sphere describing the magnetic field orientation depends on the sign of the eigenvalues of $G^{\prime }$. We show that sweet spots usually draw lines on this sphere. We then discuss how to characterize the electrical susceptibility of a spin-orbit qubit with test modulations on the gates. We apply these considerations to a Ge/GeSi spin-qubit heterostructure, and discuss the prospects for the engineering of sweet spots.

Recently, chiral anomaly (CA) has been proposed to occur in spin-orbit coupled noncentrosymmetric metals (SOC-NCMs), motivating CA to be a Fermi surface property rather than a Weyl node property. Although the nature of the anomaly is similar in both SOC-NCMs and Weyl systems, here we point out significant fundamental differences between the two. We show that the different nature of the orbital magnetic moment (OMM) in the two systems leads to nontrivial consequences—particularly the sign of the longitudinal magnetoconductance always remains positive in a SOC noncentrosymmetric metal, unlike a Weyl metal that displays either sign. Furthermore, we investigate the planar Hall effect and the geometrical contribution to the Hall effect in the two systems and point out significant differences in the two systems. We conduct our analysis for magnetic and nonmagnetic impurities, making our study important in light of current and upcoming experiments in both SOC-NCMs and Weyl metals.

Loss of triplet excitons by triplet-triplet annihilation (TTA) and triplet-polaron quenching (TPQ) is a major problem in modern organic light-emitting diodes with phosphorescent host-guest emission layers. Modeling of TTA and TPQ in these emission layers is therefore important. However, this modeling is complicated by positional correlations among the triplets and polarons. Kinetic Monte Carlo (KMC) simulations can account for these correlations, but are computationally expensive. In a previous paper [Taherpour et al., Phys. Rev. B 105, 085202 (2022)] we developed a master equation approach to modeling of TTA that accurately accounts for correlations, and is at the same time fast. In the present work, we extend the approach to include modeling of TPQ. We calculate the influence of TTA and TPQ on transient photoluminescence experiments and on steady-state emission efficiency, using KMC simulations as benchmark. We show that our extended master equation modeling is an accurate and fast alternative to KMC simulations.

We investigate thickness-dependent transformation from the paramagnetic to the ferromagnetic phase in $\mathrm{C}{\mathrm{r}}_2\mathrm{G}{\mathrm{e}}_2\mathrm{T}{\mathrm{e}}_6$ (CGT) in bulk and few-layer flake forms. Two-dimensional (2D) Ising-like critical transition in bulk CGT occurs at $T_c=67$ K with out-of-plane magnetic anisotropy. Few-layer CGT on $\mathrm{hBN}/\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$ substrate displays the same $T_c$ but also exhibits a critical transition at $T_c^{\prime }=14.2$ K. Here, critical scaling analysis reveals the critical exponents differ significantly from those in bulk and do not align with the known universality classes. Our density functional theory and classical calculations indicate competition between magnetocrystalline and dipolar anisotropy emerges with reduced dimensions. The observed behavior is due to minor structural distortions in low-dimensional CGT, which modify the balance between spin-orbit coupling, exchange interactions, and dipolar anisotropy. This triggers a critical crossover at $T_c^{\prime }$. In this paper, we show the emergence of a complex interplay of short- and long-range interactions below $T_c^{\prime }$ as CGT approaches the 2D limit.

The notion of higher-order topological phases has endowed topological states of matter beyond the first order. In this work, we report the topological phase transition between the conventional ${\mathbb{Z}}_2$ topological insulator and second-order topological insulator in a kagome circuit. Such a phase transition emerges at the competition between the spin-orbit couplings and nonequivalent nearest-neighbor hoppings without breaking symmetry. The bulk topological invariants, the ${\mathbb{Z}}_2$ index and spin-polarized bulk polarizations, are calculated to describe the complete phase diagram. The one-dimensional gapless helical edge states of ${\mathbb{Z}}_2$ topological phase and zero-dimensional corner states of second-order topological phase are observed in the ribbon and finite-size circuit samples, respectively. Our findings in the electric circuits provide an experimental bridge to connect the first-order and higher-order topological phases.

We investigate the skin effect and topological properties in the one-dimensional semi–Su-Schrieffer-Heeger (semi–SSH) lattice, in which the intersite couplings in one direction are uniform but in the other direction the intersite couplings change alternately. Following the detailed theoretical demonstration, we find that in this system, the skin effect can be tunable by adjusting the magnitudes of the intersite couplings in the two directions. However, the change of the skin effect does not necessarily modulate the topological edge states, and such two phenomena are allowed to appear in a spatially separated way. Therefore, we propose one scheme to change the directionality of the topological edge states and the localized bulk states, by controlling the intersite couplings. This promotes the understanding of the skin effects and topological properties in the one-dimensional SSH lattices with nonreciprocal couplings.

Quantum transport in nonequilibrium settings plays a fundamental role in understanding the properties of systems ranging from quantum devices to biological systems. Dephasing—a key aspect of out-of-equilibrium systems—arises from interactions with a noisy environment and can profoundly modify transport properties. Here we investigate the impact of dephasing on the nonequilibrium steady-state transport properties of noninteracting fermions on a one-dimensional lattice with long-range hopping (proportional to $1/r^{\alpha }$), where $\alpha >1$. We demonstrate the emergence of distinct transport regimes as the long-range hopping parameter, $\alpha$, is tuned. In the short-range limit ($\alpha \gg 1$), transport is diffusive. Conversely, in the long-range limit $[\alpha \sim \mathcal{O}(1\left)\right]$, we observe a superdiffusive transport regime. Using numerical simulations of the Lindblad master equation and corroborating these with an analysis of the current-operator norm, we identify a critical long-range hopping parameter, ${\alpha }_c\approx 1.5$, below which superdiffusive transport becomes pronounced and rapidly becomes independent of the dephasing strength. Our results elucidate the intricate balance between dephasing and unitary dynamics, revealing steady-state transport features.

We perform an in-depth investigation of the phase diagram of the $J_1-J_2$ Heisenberg model on the square lattice. We take advantage of density matrix renormalization group and fully augmented matrix product states methods and reach unprecedented accuracy with large bond dimensions. We utilize excited-level crossing analysis to pinpoint the phase transition points. It was believed before that there exists a narrow spin liquid phase sandwiched by the Néel antiferromagnetic (AFM) and valence bond solid (VBS) phases. Through careful finite-size scaling of the level crossing points, we find a direct phase transition between the Néel AFM and VBS phases at $J_2/J_1=0.535\left(3\right)$, suggesting the absence of an intermediate spin liquid phase. We also provide accurate results for ground-state energies for a variety of sizes, from which we find that the transition between the Néel AFM and VBS phases is continuous. These results indicate the existence of a deconfined quantum critical point at $J_2/J_1=0.535\left(3\right)$ in the model. From the crossing of the first derivative of the energies with $J_2$ for different sizes, we also determine the precise location of the first-order phase transition between the VBS and stripe AFM phases at $J_2/J_1=0.610\left(5\right)$.

We investigate the role of a quasiperiodically driven electric field in a disordered fermionic chain. In the clean noninteracting case, we show the emergence of dynamical localization—a phenomenon previously known to exist only for a perfect periodic drive. In contrast, in the presence of disorder, where a high-frequency periodic drive preserves Anderson localization, we show that the quasiperiodic drive destroys it and leads to slow relaxation. Considering the role of interactions, we uncover the phenomenon of quasiperiodic driving-induced logarithmic relaxation, where a suitably tuned drive (corresponding to dynamical localization in the clean, noninteracting limit) slows down the dynamics even when the disorder is small enough for the system to be in the ergodic phase. This is in sharp contrast to the fast relaxation seen in the undriven model, as well as the absence of thermalization (drive-induced many-body localization) exhibited by a high-frequency periodic drive.

We study the interplay between quasiperiodic disorder and superconductivity in a one-dimensional tight-binding model with the quasiperiodic modulation of on-site energies that follow the Fibonacci rule, and all the eigenstates are multifractal. As a signature of multifractality, we observe the power-law dependence of the correlation between different single-particle eigenstates as a function of their energy difference. We numerically compute the mean-field superconducting transition temperature for every realization of a Fibonacci chain of a given size and find the distribution of critical temperatures, analyze their statistics, and estimate the mean value and variance of critical temperatures for various regimes of the attractive coupling strength and quasiperiodic disorder. We find an enhancement of the critical temperature compared to the analytical results that are based on strong assumptions of the absence of correlations and self-averaging of multiple characteristics of the system, which are not justified for the Fibonacci chain. For the very weak coupling regime, we observe a crossover where the self-averaging of the critical temperature breaks down completely and strong sample-to-sample fluctuations emerge.

We present an efficient method to compute the modular extension of both fermionic topological orders and ${\mathbb{Z}}_2$-symmetric bosonic topological orders in two spatial dimensions, from congruence representations of ${\mathrm{SL}}_2\left(\mathbb{Z}\right)$ and its subgroups. To demonstrate the validity of our approach, we provide explicit calculations for topological orders with ranks up to 10 for the fermionic cases and up to 6 for the bosonic cases. Along the way, we clarify the relation between fermionic rational conformal field theories, which live on the boundary of the corresponding fermionic topological orders, and modular extensions. We show that the ${\mathrm{SL}}_2\left(\mathbb{Z}\right)$ representation of the Ramond-Ramond sector can be determined from the Nuveu-Schwarz-Nuveu-Schwarz sector using the modular extensions.

Two-dimensional electron gas (2DEG) states at oxide interfaces between two ferroic materials have been fertile ground to realize controllable multiferroicity. Here, we investigate the 2DEG states at the interface of ferroelectric $\mathrm{BaTi}{\mathrm{O}}_3$ and a magnetic layer of iron using angle-resolved photoemission spectroscopy. Orbital-selective charge transfer occurs on the surprisingly robust 2DEG. Based on first-principles calculations, we show how the interfacial hybridization can give rise to the unexpected charge transfer in the magnetic 2DEG. Our study reveals a close interplay on a 2DEG between magnetic and ferroelectric interfaces, which sheds light on future design principles of multiferroic 2DEG states.

We study the quantum oscillations of interlayer capacitance in an excitonic insulating electron-hole double layer with the Hartree-Fock mean-field theory. Such oscillations could be simply understood from the physical picture of an exciton formed by electron and hole Landau levels, in which the direct gap between the electron-hole Landau levels will oscillate with the exciton chemical potential and the inverse of the magnetic field. We also find that the excitonic order parameters can be destroyed by a strong magnetic field. At this time, the system becomes two independent quantum Hall liquids, and the interlayer capacitance oscillates to zero at zero temperature.

The bulk electronic structure of ${A\mathrm{V}}_3{\mathrm{Sb}}_5$ $(A=\text{K}, \mathrm{Cs})$ has been investigated by means of hard x-ray photoemission spectroscopy (HAXPES). The asymmetric shape of V and Sb core level peaks indicates that the V $3d$ and Sb $5p$ electrons are involved in the conduction band. The absence of a satellite structure in the V $2p$ HAXPES spectra shows a weak electronic correlation in the V $3d$ states. Splitting of the V $2p$ peak is not observed in the density wave phase indicating the charge disproportionation between the V sites is undetectably small, consistent with the weakness of the on-site electronic correlation and the possibility of bond order. The Sb $4d_{5/2}$ binding energy agrees with that of the Cs-terminated surface, indicating that the electronic structure of the ${\mathrm{V}}_3{\mathrm{Sb}}_5$ layer just below the Cs surface is close to the bulk.