Using determinant quantum Monte Carlo, we compare three methods of evaluating the dc Hall coefficient $R_H$ of the Hubbard model: the direct measurement of the off-diagonal current-current correlator ${\chi }_{xy}$ in a system coupled to a finite magnetic field (FF), ${\chi }_{xy}^{\text{FF}}$; the three-current linear response to an infinitesimal field as measured in the zero-field (ZF) Hubbard Hamiltonian, ${\chi }_{xy}^{\text{ZF}}$; and the leading order of the recurrent expansion $R_H^{\left(0\right)}$ in terms of thermodynamic susceptibilities. The two quantities ${\chi }_{xy}^{\text{FF}}$ and ${\chi }_{xy}^{\text{ZF}}$ can be compared directly in imaginary time. Proxies for $R_H$ constructed from the three-current correlator ${\chi }_{xy}^{\text{ZF}}$ can be determined under different simplifying assumptions and compared with $R_H^{\left(0\right)}$. We find these different quantities to be consistent with one another, validating previous conclusions about the close correspondence between Fermi surface topology and the sign of $R_H$, even for strongly correlated systems. These various quantities also provide a useful set of numerical tools for testing theoretical predictions about the full behavior of the Hall conductivity for strong correlations.

Quantum spin liquids (QSLs), in which spins are highly entangled, have been considered a groundwork for generating exotic superconductivity. Despite numerous efforts, superconductivity emerging from QSLs has been unrealized in actual materials due to the difficulties in stabilizing QSL states with metallic conductivity. Recently, an organic compound, $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4{\mathrm{Hg}}_{2.89}{\mathrm{Br}}_8$, with a nearly regular triangular lattice of molecular dimers was recognized as a candidate for doped QSLs. In this study, we report an unusual superconducting phase of $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4{\mathrm{Hg}}_{2.89}{\mathrm{Br}}_8$: unexpectedly large ratios of the upper critical field to the critical temperature $H_{\mathrm{c}2}/T_{\mathrm{c}}$ in fields not only parallel but also perpendicular to the two-dimensional conducting layers and a very wide region of fluctuating superconductivity above $T_{\mathrm{c}}$. Our results reveal that these peculiarities arise from strong electron correlations and possible quantum criticality unique to the doped QSL state, leading to a heavy mass of itinerant carriers and a large superconducting energy gap.

In recent years, new phases of matter that are beyond the Landau paradigm of symmetry breaking have been accumulating, and to catch up with this fast development, new notions of global symmetry are introduced. Among them, the higher-form symmetry, whose symmetry charges are spatially extended, can be used to describe topologically ordered phases as the spontaneous breaking of the symmetry, and consequently unify the unconventional and conventional phases under the same conceptual framework. However, such conceptual tools have not been put into quantitative tests except for certain solvable models, therefore limiting their usage in the more generic quantum many-body systems. In this work, we study ${\mathbb{Z}}_2$ higher-form symmetry in a quantum Ising model, which is dual to the global (zero-form) Ising symmetry. We compute the expectation value of the Ising disorder operator, which is a nonlocal order parameter for the higher-form symmetry, analytically in free scalar theories and through unbiased quantum Monte Carlo simulations for the interacting fixed point in $(2+1)d$. From the scaling form of this extended object, we confirm that the higher-form symmetry is indeed spontaneously broken inside the paramagnetic, or quantum disordered phase (in the Landau sense), but remains symmetric in the ferromagnetic or ordered phase. At the Ising critical point, we find that the disorder operator also obeys a “perimeter” law scaling with possibly multiplicative power-law corrections. We discuss examples where both the global zero-form symmetry and the dual higher-form symmetry are preserved, in systems with a codimension-1 manifold of gapless points in momentum space. These results provide nontrivial working examples of higher-form symmetry operators, including the direct computation of one-form order parameter in an interacting conformal field theory, and open the avenue for their generic implementation in quantum many-body systems.

We propose an unconventional type of Hall effect in a topological Dirac semimetal with ferromagnetic electrodes. The topological Dirac semimetal itself has time-reversal symmetry, whereas attached ferromagnetic electrodes break it, causing the large Hall response. This induced Hall effect is a characteristic of the helical surface/edge states that arise in topological materials such as topological Dirac semimetals or quantum spin Hall insulators. We compute the Hall conductance/resistance and the Hall angle by using a lattice model with four-terminal geometry. For topological Dirac semimetals with four electrodes, the induced Hall effect occurs whether the current electrodes or the voltage electrodes are ferromagnetic. When the spins in electrodes are almost fully polarized, the Hall angle becomes as large as that of quantum Hall states or ideal magnetic Weyl semimetals. We show the robustness of the induced Hall effect against impurities and also discuss the spin injection and spin decay problems. This Hall response can be used to detect whether the magnetizations of the two ferromagnetic electrodes are parallel or antiparallel.

The development of non-Hermitian topological band theory has led to observations of novel topological phenomena in effectively classical, driven, and dissipative systems. However, for open quantum many-body systems, the absence of a ground state presents a challenge to define robust signatures of non-Hermitian topology. We show that such a signature is provided by crossings in the time evolution of the entanglement spectrum. These crossings occur in quenches from the trivial to the topological phase of a driven-dissipative Kitaev chain that is described by a Markovian quantum master equation in Lindblad form. At the topological transition, which can be crossed either by changing parameters of the Hamiltonian of the system or by increasing the strength of dissipation, the timescale at which the first entanglement spectrum crossing occurs diverges with a dynamical critical exponent of $\epsilon =1/2$. We corroborate these numerical findings with an exact analytical solution of the quench dynamics for a spectrally flat postquench Liouvillian. This exact solution suggests an interpretation of the topological quench dynamics as a fermion parity pump. Our work thus reveals signatures of non-Hermitian topology that are unique to quantum many-body systems and cannot be emulated in classical simulators of non-Hermitian wave physics.

For graphene nanoribbons with Rashba spin-orbit coupling, the peculiar magnetic response due to the presence of a magnetization and geometric confinement are analyzed within a tight-binding model. We observe a sizable transverse susceptibility that can be considered as a gate-voltage-induced magnetoelectric torque without the need of a bias voltage, with different directions for zigzag and armchair ribbons. The local torque generates noncollinear spin polarization between the two edges and/or along the ribbon, and the net torque averages to zero if the magnetization is homogeneous. Nevertheless, a nonzero net torque can appear in partially magnetized nanoribbons or in nanoflakes of irregular shapes. The equilibrium spin current produced by the spin-orbit coupling also appears in nanoribbons, but the component flowing in the direction of confinement is strongly suppressed. Even without the magnetization, an out-of-plane polarized helical edge spin current is produced, resembling that in the quantum spin Hall effect. Moreover, a magnetization pointing perpendicular to the edge produces a laminar flow of edge charge currents, whose flow direction is symmetric (nonchiral) or antisymmetric (chiral) between the two edges, depending on whether the magnetization points in-plane or out-of-plane.

Since their theoretical prediction by Peierls in the 1930s, charge density waves (CDWs have been one of the most commonly encountered electronic phases in low-dimensional metallic systems. The instability mechanism originally proposed combines Fermi surface nesting and electron-phonon coupling but is, strictly speaking, only valid in one dimension. In higher dimensions, its relevance is questionable as sharp maxima in the static electronic susceptibility $\mathit{\chi }\left(\mathit{q}\right)$ are smeared out, and is, in many cases, unable to account for the periodicity of the observed charge modulations. Here, we investigate the quasi-two-dimensional ${\mathrm{LaAgSb}}_2$, which exhibits two CDW transitions, by a combination of diffuse x-ray scattering, inelastic x-ray scattering, and ab initio calculations. We demonstrate that the CDW formation is driven by phonon softening. The corresponding Kohn anomalies are visualized in three dimensions through the momentum distribution of the x-ray diffuse scattering intensity. We show that they can be quantitatively accounted for by considering the electronic susceptibility calculated from a Dirac-like band, weighted by anisotropic electron-phonon coupling. This remarkable agreement sheds new light on the importance of Fermi surface nesting in CDW formation.

The interplay between nontrivial topological states of matter and strong electronic correlations is one of the most compelling open questions in condensed matter physics. Due to experimental challenges, there is an increasing desire to find more microscopic techniques to complement the results of more traditional experiments. In this work, we locally explore the Kondo insulator ${\mathrm{Sm}}_{1-x}{\mathrm{Gd}}_x{\mathrm{B}}_6$ by means of electron spin resonance (ESR) of ${\mathrm{Gd}}^{3+}$ ions at low temperatures. Our analysis reveals that the ${\mathrm{Gd}}^{3+}$ ESR line shape shows an anomalous evolution as a function of temperature, wherein for highly dilute samples $(x\approx 0.0002)$ the ${\mathrm{Gd}}^{3+}$ ESR line shape changes from a localized ESR local moment character to a diffusive-like character. Upon manipulating the sample surface with a focused ion beam we demonstrate, in combination with electrical resistivity measurements, that the localized character of the ${\mathrm{Gd}}^{3+}$ ESR line shape is recovered by increasing the penetration of the microwave in the sample. This provides compelling evidence for the contribution of surface or near-surface excitations to the relaxation mechanism in the ${\mathrm{Gd}}^{3+}$ spin dynamics. Our work brings new insights into the importance of nontrivial surface excitations in ESR, opening new routes to be explored both theoretically and experimentally.

Mesoscopic quantum dots (QDs) are ubiquitous in quantum devices as reliable sources of hot electrons. However, we have observed an unexpectedly significant energy relaxation of QD-emitted hot electrons up to $\approx 55\%$ of its excitation $\le 1.5\mathrm{meV}$ from the Fermi level. The energetics of hot electrons were obtained through transverse magnetic focusing over a few microns using both QD and quantum point contact (QPC) emitters. Unlike the QPC counterparts, QD emissions deviated substantially from Fermi gas predictions—the focusing peak appeared at lower magnetic fields, and excessive broadening was observed. The phenomenon was modeled by a capacitive interaction transferring energy from the hot electron to the QD. Model simulations reproduced the key experimental features, implying the presence of a strong yet overlooked relaxation mechanism that is intrinsic to QD emissions. Our observation calls for the prudent use of QDs as single electron sources.

Parafermionic zero modes, ${\mathbb{Z}}_n$-symmetric generalizations of the well-known ${\mathbb{Z}}_2$ Majorana zero modes, can emerge as edge states in topologically nontrivial strongly correlated systems displaying fractionalized excitations. In this paper, we investigate how signatures of parafermionic zero modes can be detected by its effects on the properties of a quantum dot tunnel-coupled to a system hosting such states. Concretely, we consider a strongly correlated one-dimensional fermionic model supporting ${\mathbb{Z}}_4$ parafermionic zero modes coupled to an interacting quantum dot at one of its ends. By using a combination of density matrix renormalization group calculations and analytical approaches, we show that the dot's zero-energy spectral function and average occupation numbers can be used to distinguish between trivial, ${\mathbb{Z}}_4$ and $2\times {\mathbb{Z}}_2$ phases of the system. The present work opens the prospect of using quantum dots as detection tools to probe nontrivial topological phases in strongly correlated systems.

Superfluid vortices are quantum excitations carrying a quantized amount of orbital angular momentum in a phase where global symmetry is spontaneously broken. We address a question whether magnetic vortices in superconductors with dynamical gauge fields carry nonzero orbital angular momentum or not. We provide the answer in view of the angular momentum conservation in several distinct classes of examples from interdisciplinary fields of physics across condensed matter, dense nuclear systems, and cosmology. The angular momentum carried by gauge field configuration around the magnetic vortex plays a crucial role in satisfying the principle of the angular momentum conservation. Based on various ways on how the angular momentum conservation is realized, we provide a general scheme of classifying magnetic vortices in different phases of matter.

We investigate proximity effects in a spin-triplet superconductor (TSC) interfaced with a ferromagnet (FM), assuming different types of magnetic profiles and chiral or helical pairings. The region of the coexistence of spin-triplet superconductivity and magnetism is significantly influenced by the orientation and spatial extension of the magnetization with respect to the spin configuration of the Cooper pairs, resulting in clear-cut anisotropy signatures. A characteristic mark of the inverse proximity effect arises in the induced spin polarization at the TSC-FM interface. This is unexpectedly stronger when the magnetic proximity is weaker, thus unveiling immediate detection signatures for spin-triplet pairs. We show that an anomalous magnetic proximity can occur at the interface between the itinerant ferromagnet $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ and the unconventional superconductor ${\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$. Such scenario indicates the potential to design characteristic inverse proximity effects in experimentally available $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3\text{−}{\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$ heterostructures and to assess the occurrence of spin-triplet pairs in the highly debated superconducting phase of ${\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$.

Single-atom magnets, the ultimate limit of high-density magnetic storage, have attracted widespread attention over recent decades. However, they are far from being applicable as a storage medium because of their low magnetic stability. In this paper, we describe a strategy to induce huge magnetic anisotropy in “bimetal magnets” on a benzene (Bz) substrate based on the electron filling of $d$ orbitals. Our first-principles calculations reveal that OsX-Bz ($X=\mathrm{Fe}$, Ru, and Os) exhibits high structural stability, large unquenched orbital moments ($1.55–1.59{\mu }_B$), and huge perpendicular magnetic anisotropy energy (MAE) above 54.8 meV. The synergistic effect of two transition metal components toward this huge MAE and preserved orbital moment is discussed in depth, which is mainly attributed to the d-d coupling induced energy level rearrangement. Our work may provide insights into the underlying physical mechanism and bimetal molecule magnet design.

The ratio between the shear viscosity and the entropy $\eta /s$ is considered a universal measure of the strength of interactions in quantum systems. This quantity was conjectured to have a universal lower bound $(1/4\pi )\hslash /k_B$, which indicates a very strongly correlated quantum fluid. By solving the quantum kinetic theory for a nodal-line semimetal in the hydrodynamic regime, we show that $\eta /s\propto T$ violates the universal lower bound, scaling toward zero with decreasing temperature $T$ in the perturbative limit. We find that the hydrodynamic scattering time between collisions is nearly temperature independent, up to logarithmic scaling corrections, and can be extremely short for large nodal lines, near the Mott-Ragel-Ioffe limit. Our finding suggests that nodal-line semimetals can be very strongly correlated quantum systems.