We calculate the spin correlations that result from the motion of a single dopant in the hard-core Fermi-Hubbard model, as the geometry evolves from a square to a triangular lattice. In particular, we consider the square lattice with an additional hopping along one diagonal, whose strength is continuously varied. We use a high-temperature expansion which expresses the partition function as a sum over closed paths taken by the dopant. We sample thousands of diagrams in the space of closed paths using the quantum Monte Carlo approach of Raghavan and Elser [Phys. Rev. Lett. 75, 4083 (1995)], which is free of finite-size effects and allows us to simulate temperatures as low as $T\sim 0.3\left|t\right|$, even in cases where there is a sign problem. For the case of a hole dopant, we find a crossover from kinetic ferromagnetism to kinetic antiferromagnetism as the geometry is tuned from square to triangular, which can be observed in current quantum gas microscopes.

Understanding the coherent properties of electron spins driven by electric fields is crucial for their potential application in quantum-coherent nanoscience. In this work, we address two distinct driving mechanisms in electric-field-driven electron spin resonance as implemented in scanning tunneling spectroscopy. We study the origin of the driving field using a single-orbital Anderson impurity, connected to polarized leads and biased by a voltage modulated on resonance with a spin transition. By mapping the quantum master equation into a system of equations for the impurity spin, we identify two distinct driving mechanisms. Below the charging thresholds of the impurity, electron spin resonance is dominated by a magnetically exchange-driven mechanism or field-like torque. Conversely, above the charging threshold spin-transfer torque caused by the spin-polarized current through the impurity drives the spin transition. Only the first mechanism enables coherent quantum spin control, while the second one leads to fast decoherence and spin accumulation towards a non-equilibrium steady state. The electron spin resonance signals and spin dynamics vary significantly depending on which driving mechanism dominates, highlighting the potential for optimizing quantum-coherent control in electrically driven quantum systems.

Bulk topology and criticality can both lead to nontrivial boundary effects. Topological orders are often characterized by their robust edge states, while bulk critical points can have different boundary scalings. The interplay between these two different boundary effects is an intriguing problem. The boundary of the deconfined quantum critical point (DQCP) is the ideal platform for the interplay of the two boundary effects, as the DQCP is also an intrinsically gapless symmetry protected topological (igSPT) state. In this work we discuss the boundary of the DQCP and several related models. We demonstrate that the fluctuation of the bulk order parameters and their various boundary conditions lead to a rich possibility of the edge states, including a “pseudogapped” (or super power-law decay) behavior as opposed to the conventional “pristine” (or power-law) behavior. We also briefly discuss the quantum information aspect of our work.

The charge density wave (CDW), a translational symmetry breaking electronic liquid, plays a pivotal role in correlated quantum materials, such as high-$T_c$ superconductors and topological semimetals. Recently, CDWs that possibly intertwine with superconductivity and magnetism are observed in various kagome metals. However, the nature of CDWs and the role of the Fermi-surface (FS) topology in these materials remain an unresolved challenge. In this paper, we reveal the formation of CDWs in the newly discovered kagome metal ${\mathrm{LuNb}}_6{\mathrm{Sn}}_6$. We observe a ‘‘yield sign’’-like hollow triangular diffuse scattering pattern and nearly complete softening of a flat optical phonon band near ${\mathbf{Q}}_H=(1/3,1/3,1/2)$. The scattering intensity of the diffuse scattering displays divergent behavior as decreasing temperature until $T_{\text{CDW}}=70$ K, where a competing CDW at $Q_{\text{CDW}}=(1/3,1/3,1/3)$ emerges. Combined with scanning tunneling microscopy/spectroscopy and first-principles calculations, our observations support a Fermi-surface driven frustration that suppresses the leading CDW instability at ${\mathbf{Q}}_H$. This frustration is relieved by the emergence of a subleading CDW at $Q_{\text{CDW}}$. These results provide insights into the interplay between the Fermi surface, strong electron-phonon coupling, and charge density wave formation in quantum materials.

The flexophotovoltaic effect (FPVE) in two-dimensional (2D) materials offers a promising route for optoelectronic energy conversion, yet the interplay between flexoelectric polarization and electronic/optical properties remains elusive. Here, we reveal that strain-gradient engineering in bent ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ nanoribbons with varying widths not only induces self-doping and a nonzero Berry curvature dipole that drives shift current, but also enables an outstanding flexoelectric power conversion efficiency (PCE) of $9.05\%$, leading among 2D FPVE systems. Moreover, we demonstrate a self-powered FPVE photodetector that generates spin-polarized photocurrent, including pure spin current, without external bias. These findings establish strain-tailored ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ as a unified platform for high-efficient photovoltaics, optoelectronics, and spintronics, driven solely by mechanical bending.

A comprehensive ${}^{31}\mathrm{P}$ nuclear magnetic resonance (NMR) study, combined with thermodynamic measurements and first-principles band-structure calculations, has been conducted to explore the ground state of the $S=5/2$ double trillium lattice antiferromagnet ${\mathrm{KSrFe}}_2{\left({\mathrm{PO}}_4\right)}_3$. Our experimental results indicate that the magnetic ground state is neither a conventional three-dimensional (3D) long-range order (LRO) nor a pure gapless spin-liquid state, as conjectured previously [Boya et al., APL Mater. 10, 101103 (2022)]. Specifically, the observation of a nearly field-independent NMR linewidth below $T^{*}=(3.5\pm 0.4)\mathrm{K}$, and a significant enhancement of spin-spin relaxation rate $1/T_2$ below $2T^{*}$ (where $T^{*}$ is the characteristic temperature identified from the magnetic susceptibility), indicate a complex magnetic ground state where spin freezing coexists with persistent dynamics. Furthermore, we argue that the lack of magnetic LRO and the persistence of strong magnetic fluctuations in ${\mathrm{KSrFe}}_2{\left({\mathrm{PO}}_4\right)}_3$ are unlikely to originate from intersite K/Sr disorder, rather they arise due to intrinsic magnetic frustration. Our findings position ${\mathrm{KSrFe}}_2{\left({\mathrm{PO}}_4\right)}_3$ into a broader family of geometrically frustrated magnets characterized by coexisting spin freezing and pronounced antiferromagnetic fluctuations, marking it as a promising platform for investigating exotic phenomena in 3D frustrated magnets.

Altermagnetism, a recently proposed and experimentally confirmed class of magnetic order, features collinear compensated magnetism with unconventional spin-split bands. Here, we show that in a metallic 2D $d$-wave altermagnet with $\left[C_2\right|\left|C_4\right]$ symmetry, secondary instabilities can arise. Using an unbiased functional renormalization group approach, we analyze the weak-coupling instabilities of a 2D Hubbard model with a preexisting altermagnetic order inspired by our ab initio electronic structure calculations of realistic material candidates from ${\mathrm{V}}_2{X}_2\mathrm{O}$ ($X$ = Te, Se) family. We identify two distinct spin-density-wave (SDW) states that break the underlying altermagnetic $\left[C_2\right|\left|C_4\right]$ symmetry. Additionally, we find spin-fluctuation-induced instabilities leading to a singlet $d$-wave superconducting state and an unconventional commensurate pair-density-wave (PDW) state with extended $s$-wave and spin-triplet symmetry. We analyze the pairing mechanism and characterize the excitation spectrum, which exhibits Bogoliubov Fermi surfaces or nodal points depending on the gap size.

The suitability of transition metal dichalcogenides (TMDCs) for the use as scattering targets in electron spin-polarization detectors is put to a paradigmatic test: a case study of ${\mathrm{MoS}}_2$ is performed. Therefore, the electron reflectivity, the Sherman function, and the figure of merit are measured for the following samples: a single layer of ${\mathrm{MoS}}_2$ on Au(111) and cleaved ${\mathrm{MoS}}_2$ single-crystal surfaces. ${\mathrm{MoS}}_2$ offers promising features for the use in spin-polarization detection: working points with a reversed sign of the Sherman function and a long-term target stability. To overcome the low figure of merit of ${\mathrm{MoS}}_2$, the closely-related 2D material ${\mathrm{WS}}_2$ with higher spin-orbit interaction is proposed, which may resemble the promising features with higher Sherman function. This study demonstrates the potential suitability of 2D materials such as TMDCs as scattering targets in spin-polarization detectors.

The Boltzmann distribution encodes our subjective knowledge of the configuration in a classical lattice model, given only its Hamiltonian. If we acquire further information about the configuration from measurement, our knowledge is updated according to Bayes' theorem. We examine the resulting “conditioned ensembles,” finding that they show many new phase transitions and new renormalization-group fixed points. (Similar conditioned ensembles also describe “partial quenches” in which some of the system's degrees of freedom are instantaneously frozen, while the others continue to evolve.) After describing general features of the replica field theories for these problems, we analyze the effect of measurement on illustrative critical systems, including: critical Ising and Potts models, which show surprisingly rich phase diagrams, with RG fixed points at weak, intermediate, and infinite measurement strength; various models involving free fields, XY spins, or flux lines in 2D or 3D; and geometrical models such as polymers or clusters. We also give a formalism for measurement of classical stochastic processes. We use this to make connections with quantum dynamics, in particular with “charge sharpening” in 1D, for which we give a purely hydrodynamic derivation of the known effective field theory. We discuss qualitative differences between RG flows for the above measured systems, described by $N\to 1$ replica limits, and those for disordered systems, described by $N\to 0$ limits. In addition to discussing measurement of critical states, we give a unifying treatment of a family of inference problems for noncritical states. These are related to the Nishimori line in the phase diagram of the random-bond Ising model, and are relevant to various quantum error correction problems. We describe distinct physical interpretations of conditioned ensembles and note interesting open questions.

In fractional quantum Hall fluids, the quasiparticle excitations are anyons with fractional charges and statistics. Effective interactions among the anyons can be induced by either model or realistic electron-electron (e-e) interactions. Without losing the generality, we investigate such phenomena for the Laughlin $1/3$ and Moore-Read (MR) non-Abelian phases. Anyons display rich internal dynamics in both cases that will lead to interesting experimental consequences. In particular, bound states of two Laughlin anyons are preferred under short-range e-e interactions, leading to $2e/3$—instead of $e/3$—effective charge carriers at low temperatures, which have been seen in several experiments. MR phases host two topologically distinct fusion channels: “1” and “$\psi$.” The different effective interactions of $e/4$ anyons in the two sectors suggest the vanishing of the degeneracy of fusion channels when the e-e interaction is no longer its model Hamiltonian, in which case different bound states could also appear. This indicates the possibility of energetically manipulating the two types of anyons by tuning the bare e-e interactions. We point out that the results of recently developed high-resolution scanning tunneling microscopy measurements will be affected by the effective anyon interactions, where anyons are clustered together after the tunneling of electrons. The low-lying parts of the local density of states affected by various anyon clusters are simulated for both Abelian and non-Abelian systems with (screened) Coulomb interactions.

Altermagnets constitute a class of collinear magnets with momentum-dependent spin splitting and vanishing net magnetization. Direct observation of the characteristic altermagnetic spin splitting, however, remains challenging. Indirect signatures can be obtained via transport studies, which so far have only considered homogeneous driving fields. We propose to leverage nonuniform electric fields and spin density gradients to probe the shape and the spin polarization of altermagnetic Fermi surfaces via transport measurements. By using both a semiclassical Boltzmann approach and a lattice Keldysh formalism, we show that altermagnets excite swirling electric and spin currents whose profiles depend on the relative orientation of altermagnetic lobes with respect to the sample boundaries. These currents can be measured via magnetometry techniques. Unlike previous proposals considering the hydrodynamic regime of transport, swirling currents are observed even in the Ohmic regime and rely exclusively on the altermagnetic spin splitting, with no swirls observed in ferromagnets. The electric and spin current vortices predicted here provide a different altermagnetic signature in an experimentally accessible setup.

Physical systems defined on hyperbolic lattices may exhibit phases of matter that only emerge due to negative curvature. We focus on the case of the Ising model under open boundary conditions and show that an intermediate phase emerges in addition to standard (high-temperature) paramagnetic and (low-temperature) ferromagnetic phases. When performing the Kramers-Wannier duality, the fact that it alters boundary conditions becomes crucial since a finite fraction of lattice sites lie on the boundary. We propose characterizing this intermediate phase by its sensitivity to boundary conditions, wherein bulk ordering is not spontaneous but rather induced by boundary effects, setting it apart from the Landau paradigm of spontaneous symmetry breaking. By developing a ${\mathbb{Z}}_2$ symmetry-restricted extension of the corner transfer matrix renormalization group method, we provide numerical evidence for the existence of all three distinct phases and their corresponding two-stage phase transitions, thereby establishing the complete phase diagram. We also establish how the (spontaneous) intermediate-to-ferromagnetic and the (induced) paramagnetic-to-intermediate transition points are related by the Kramers-Wannier duality relation. We discuss a holographic correspondence between boundary and bulk behaviors and derive exact expressions for boundary correlation functions on Cayley trees.

We present results of a Raman-scattering study of the Kagome ferromagnet ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$, with a focus on electronic and phononic excitations and their interplay. We provide a theoretical analysis of the electronic-band structure, enabling a semiquantitative explanation of the spectra. A prominent feature in the electronic spectra is a redistribution of spectral weight from low to high energies in all polarization configurations starting at the Curie temperature $T_{\mathrm{C}}$. In the symmetry-resolved spectra, the suppression of the $A_{1\mathrm{g}}$ continuum in the ferromagnetic state arises from the redistribution of electronic states below $T_{\mathrm{C}}$, while a strong enhancement of the $A_{2\mathrm{g}}$ continuum is linked to the dynamics of fermions near the Fermi level $E_{\mathrm{F}}$ being characterized by spin-momentum locking near Weyl points. The $A_{1\mathrm{g}}$ phonon modulates the position of these Weyl points and couples strongly to the related fermions close to $E_{\mathrm{F}}$. These results allow a comprehensive understanding of the bulk band-structure evolution as a function of temperature in ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$, offering key insights for further studies of the driving force behind the long-range magnetic order and novel topological states in this compound.

Inspired by “fracton hydrodynamic” universality classes of dynamics with unusual conservation laws, we present a dynamical universality class that arises out of local area-preserving dynamics in the noncommutative plane. On this symplectic manifold, the area-preserving spatial symmetry group $\mathrm{SL}(2,\mathbb{R})⋊{\mathbb{R}}^2$ is a symmetry group compatible with nontrivial many-body dynamics. The conservation laws associated with this symmetry group correspond to the dipole and quadrupole moments of the particles. We study the unusual dynamics of a crystal lattice subject to such symmetries, and argue that the hydrodynamic description of lattice dynamics breaks down due to relevant nonlinearities. Numerical simulations of classical Hamiltonian dynamical systems with this symmetry are largely consistent with a tree-level effective field theory estimate for the endpoint of this instability.

We investigate the ultrafast dynamics of exciton formation in (Ga,In)As quantum wells using optical pump–terahertz probe spectroscopy, complemented by time-resolved photoluminescence. Terahertz spectroscopy directly probes intraexcitonic transitions and thus, in principle, distinguishes between unbound electron-hole plasma and Coulomb-bound exciton populations. At low excitation densities, this separation is effective: the Drude response of the electron-hole plasma remains spectrally distinct from narrow intraexcitonic resonances. However, at higher excitation densities, spectral broadening and overlap obscure this distinction.

Following low-density nonresonant excitation, we initially observe a pure Drude-like plasma response that gradually transitions into an intraexcitonic resonance. In contrast, at higher excitation densities, conventional analysis based on the Drude-Lorentz model suggests questionably high exciton fractions up to 32% immediately after excitation. It remains uncertain whether these fractions represent genuine instantaneous exciton formation or arise from spectral overlap and analytical ambiguity. To resolve this, we introduce a differential probing technique utilizing exciton ionization induced by strong THz fields. By comparing responses obtained at high and low THz field strengths, we isolate the exciton contribution from the pure plasma contributions. This approach reveals intrinsic exciton formation dynamics governed by two characteristic timescales: a fast component in the vicinity of $10\mathrm{ps}$ and a slower process around $250\mathrm{ps}$. Notably, these timescales remain largely independent of excitation density, contrasting sharply with the strong density dependence suggested by conventional methods.

Our results establish a robust method to disentangle exciton and plasma dynamics, providing clearer insight into exciton formation under nonresonant excitation in semiconductor quantum wells.

In this study, we explore the optical properties of quantum dots doped with a ${\mathrm{Ni}}^{2+}$ ion that interacts with a charged exciton. The carriers-${\mathrm{Ni}}^{2+}$ exchange interaction, which is dominated by coupling with holes, enables optical interaction with an individual localized spin. Systematic magneto-optical analysis reveals that the strain distribution at the ${\mathrm{Ni}}^{2+}$ site significantly influences its spin structure. In positively charged dots dominated by in-plane biaxial strain, the three spin states of ${\mathrm{Ni}}^{2+}$ ($S_z=0, S_z=\pm 1$) can be observed, and the magneto-optical spectra enable local strain anisotropy to be determined. However, in most of the dots, lower-symmetry strain mixes all the ${\mathrm{Ni}}^{2+}$ spin states, thereby increasing the number of observed optical transitions. In charged dots, we identify optical transitions that share a common excited state. They form a series of $\mathrm{\Lambda }$-level systems that can be individually addressed optically to determine the energy-level structure. Magneto-optical measurements demonstrate that the hole-${\mathrm{Ni}}^{2+}$ exchange interaction is antiferromagnetic and considerably stronger than the electron-${\mathrm{Ni}}^{2+}$ interaction. A spin-effective model that incorporates local strain orientation can successfully reproduce key experimental results. Furthermore, we demonstrate that low-symmetry terms in the hole-${\mathrm{Ni}}^{2+}$ exchange interaction must be considered to accurately describe the emission spectra details in a magnetic field.

Non-Hermitian (NH) systems exhibit intricate spectral topology and feature biorthogonal left and right eigenstates, resulting in distinct NH phenomena, such as the NH skin effect and ultraspectral sensitivity. However, conventional techniques fall short in directly measuring complex-valued spectra and biorthogonal eigenstates, particularly in higher-dimensional NH systems where geometry and boundary conditions become pivotal. Here, we present a Green's function-based method, which enables direct measurement of these fundamental quantities in arbitrary wave-based NH lattices. By capturing amplitude and phase responses for all pump-probe configurations experimentally, we acquire the full Green's function in two-dimensional non-Hermitian acoustic crystals. Utilizing this technique, we first demonstrate eigenstate skewness and spectral collapse in nonreciprocal lattices. Additionally, by controlling geometry and boundary conditions in reciprocal lattices, we observe spectral transitions—a unique hallmark of higher-dimensional NH systems. Our results not only render complex spectral topology and left eigenstates experimentally accessible and practically meaningful, but also establish a universal and versatile framework for exploring complex spectral features and NH dynamics across diverse physical systems.

In conventional fluids, it is well known that Euler-scale equations are plagued by ambiguiti es and instabilities. In one dimension smooth initial conditions may develop shocks, and weak solutions, such as for domain wall initial conditions (the paradigmatic Riemann problem of hydrodynamics), are not unique (and must be supplemented by physically motivated conditions such as non-negative entropy production). The absence of shock formation experimentally observed in quasi-one-dimensional cold-atomic gases, which are described by the Lieb-Liniger model, provides perhaps the strongest pointer to a modification of the hydrodynamic equation due to integrability. Generalized hydrodynamics (GHD) is the required hydrodynamic theory, taking into account the infinite number of conserved quantities afforded by integrability. We provide a new quadrature for the GHD equation—a solution in terms of a Banach fixed-point problem where time has been explicitly integrated. The quadrature is an efficient numerical solution tool; and it allows us, in the Lieb-Liniger model, to rigorously show that no shock may appear at all times, and, when combined with recent hydrodynamic fluctuation theories, to obtain new expressions for correlations in nonstationary states, establishing for the first time the presence of discontinuities that are fundamental to the nonequilibrium physics of integrable systems.

The observation of Josephson current in the quantum Hall regime has attracted considerable attention, revealing the coexistence of two seemingly incompatible phases: the quantum Hall and superconducting states. However, the mechanism underlying the Josephson current remains unclear because of the observed $h/2e$ magnetic interference period and the absence of precisely quantized Hall plateaus. To address this issue, we investigate the edge dependence of the Josephson current in graphene Josephson junctions operating in the quantum Hall regime. By systematically comparing devices with native, etched, edge-free, and gate-defined edges, we demonstrate that the Josephson current is confined to the physical edges and is highly sensitive to specific edge configurations. Our findings provide direct evidence that counter-propagating quantum Hall edge states mediate Andreev bound states, enabling Josephson coupling. These results clarify the underlying mechanism of Josephson current in the quantum Hall regime and offer new strategies for engineering superconducting hybrid devices.

Recent experiments in twisted bilayer ${\mathrm{MoTe}}_2$ ($\mathrm{t}{\mathrm{MoTe}}_2$) have uncovered a rich landscape of correlated phases. In this work, we investigate the reentrant integer quantum anomalous Hall (RIQAH) states reported by F. Xu et al. [arXiv:2504.06972] which display a notable mismatch between the Hall conductivity measured via transport and that inferred from the Streda formula. We argue that this discrepancy can be explained if the RIQAH state is a quantum Hall bubble or Wigner crystal phase, analogous to similar well-established phenomena in two-dimensional (2D) GaAs quantum wells. While this explains the RIQAH state at filling $\nu =-0.63$, F. Xu et al. report that the other RIQAH state at $\nu =-0.7$ has a smaller slope, necessitating a different interpretation. We propose and substantiate with analysis of the experimental data that this discrepancy arises due to a nearby resistive peak masking the true slope. Furthermore, we identify this resistive peak as a signature of a phase transition near $\nu =-0.75$, possibly driven by a van Hove singularity. The anomalous Hall response and Landau fan evolution across this transition suggest a change in Fermi-surface topology and a metallic phase with a nonquantized Hall response. These observations offer new insights into the nature of the RIQAH states and raise the possibility that the nearby superconducting phase may have a valley-imbalanced metal parent state.