We show that the standard textbook description of (restricted) Hartree-Fock theory for (Fermi) Hubbard-like models is in need of an update, and we present such an update allowing us to correct basic and established results in the condensed matter physics literature that are qualitatively wrong. Our update amounts to adding a test that reliably checks the thermodynamic stability of solutions of Hartree-Fock equations. This stability test makes it possible to detect, by simple means and with certainty, regions in phase space where the model exhibits mixed phases where two conventional phases coexist and translation invariance is broken in complicated ways; in such a mixed phase, unconventional physics is to be expected. Our results show that mixed phases are ubiquitous in Hubbard-like models in arbitrary dimensions.

The bulk band structure of an in situ prepared ferromagnetic ${\mathrm{Fe}}_4\mathrm{N}$ thin film has been investigated using spin- and angle-resolved photoelectron spectroscopy with synchrotron radiation. It was observed that most of the bands near the Fermi level are of minority spin. The experimental results are well reproduced by first-principles calculations. Our orbital analysis of the band structure has revealed that the electron pocket near the $\mathrm{\Gamma }$ point is primarily attributed to the Fe $3d_{xy}$ orbital of the Fe(II) site, while the electron pocket near the M point is associated with the Fe(II) $3d_{z^2}$ orbital. Our observation of real band structure in ${\mathrm{Fe}}_4\mathrm{N}$ thin films could provide valuable insight into the properties of anisotropic magnetoresistance and anomalous Nernst effect.

We numerically investigate wake dynamics in a superfluid flowing past a penetrable obstacle. The penetrable obstacle does not fully deplete the density, and we define an effective diameter $D_{\mathrm{eff}}$ from the Mach-1 contour of the time-averaged irrotational flow around the obstacle, which delineates the local supersonic region where quantized vortices nucleate. Using this flow-defined length, we construct a superfluid Reynolds number ${\mathrm{Re}}_{\mathrm{s}}\equiv (v_0-v_c)D_{\mathrm{eff}}/(\hslash /m)$, with $v_0$ being the flow speed, $v_c$ the critical velocity, and $m$ the particle mass, and show that ${\mathrm{Re}}_{\mathrm{s}}$ organizes the wake dynamics across obstacle sizes and strengths: the transition from dipole-row emission to alternating vortex cluster shedding occurs for ${\mathrm{Re}}_{\mathrm{s}}\approx 2$ and the Strouhal number and drag coefficient collapse onto universal curves versus ${\mathrm{Re}}_{\mathrm{s}}$. These results extend the notion of dynamic similarity in superfluid flows to penetrable obstacles and show that the dynamically relevant length scale is determined by the supersonic region rather than by the geometric obstacle size.

The Kondo effect is a paradigmatic model of strongly correlated physics, where a magnetic impurity forms a many-body singlet with a fermionic environment. The properties of this singlet depend sensitively on correlations within the environment, which are particularly pronounced in one dimension, where interactions give rise to Luttinger liquid behavior. A promising platform to study these effects is provided by cold gases of ytterbium (Yb) atoms, where an atomic impurity is coupled to an interacting one-dimensional fermionic environment via both magnetic and potential scattering. These two scattering mechanisms compete with each other, raising the question of how robust Kondo screening remains in such systems. Here, we show that potential scattering can suppress Kondo screening in one-dimensional Yb gases. Remarkably, however, Kondo physics persists in well-defined regimes determined by the interactions in the environment. Combining analytical renormalization-group methods with density-matrix renormalization-group simulations, we identify a transition from a strongly to a weakly entangled impurity as potential scattering increases. The two approaches are in excellent agreement across the parameter regimes considered. Our results provide a quantitative criterion for the emergence of Kondo screening in one-dimensional Yb gases and delineate experimentally accessible regimes for its realization in cold-atom platforms.

Topological bound states emerging at domain walls of dimerized chains provide a robust platform for exploring correlation effects beyond single-particle physics. When such a soliton state is coupled to a metallic substrate, local Coulomb interactions can give rise to Kondo screening. Here, we demonstrate analytically and numerically that, in an interacting Su-Schrieffer-Heeger chain, the Kondo temperature is directly controlled by the topological mass that governs the bulk gap. Near the topological transition, the Kondo scale collapses linearly with the mass parameter while retaining its exponential sensitivity to hybridization. This establishes a minimal mechanism by which a bulk topological parameter quantitatively determines an emergent many-body energy scale. Our results clarify the strong configuration dependence of soliton-induced Kondo signatures observed in graphene nanoribbon systems on Au(111) and provide experimentally testable predictions for scanning tunneling spectroscopy.

Topological gauge theories constitute a framework for understanding strongly correlated quantum matter in terms of weakly interacting composite degrees of freedom. Their topological properties become evident when these theories are realized on a space of nontrivial topology. Here, we propose a scheme to realize a one-dimensional topological gauge theory, the so-called chiral BF theory, on a ring geometry. We obtain such a theory by dimensionally reducing Chern-Simons theory on a disk to the chiral BF theory defined on the ring. Then, we encode the theory into a Hamiltonian with a coupling between angular momentum and density, and we propose and numerically benchmark its realization in an optically dressed Bose gas confined in a ring-shaped trap. There, the topological properties of the underlying theory manifest themselves through a magnetic flux variable that is density dependent. We quantify such density-dependent magnetic flux in terms of the ground-state angular momentum and the chiral properties of the system through a Bogoliubov analysis. Our proposal enables the observation of topological features of the chiral BF theory that become manifest due to the nontrivial topology of the ring geometry.

The conversion between spin and orbital currents is at the origin of the orbital torque and its Onsager reciprocal, the orbital pumping. Here, we propose a phenomenological model to describe the orbital torque in magnetic bilayers composed of an orbital source (i.e., a light metal such as Ti, Ru, ${\mathrm{CuO}}_x$, etc.) and a spin-orbit coupled magnet (i.e., typically Ni, (Co/Pt)${}_n$, etc.). This approach accounts for spin-to-orbit and orbit-to-spin conversion in the ferromagnet and at the interface. We show that the orbital torque arises from a compromise between orbital current injection from the orbital source to the ferromagnet and spin current backflow from the ferromagnet back to the orbital source. We also discuss the concept of orbital-mixing conductance and introduce the “orbit-spin-mixing” and “spin-orbit-mixing” conductances that govern the orbital torque and orbital pumping, respectively.

The coherence characteristics of a tin-vacancy (SnV) color center in diamond are investigated through optical means, including linewidth broadening effects and coherent population trapping (CPT) between the ground-state orbital levels. Spectral analysis is required as due to the large spin-orbit splitting of the orbital ground states, thermalization between the ground states occurs at rates that are impractical to measure directly in the time domain. First, by implementing a temperature-dependent linewidth broadening measurement, including the challenging-to-measure D transition, phononic coupling coefficients are determined. These measurements are performed on an emitter with a lifetime-limited linewidth and atomlike properties, making the measurement representative for high-quality SnVs. Next, a CPT-type experiment is carried out to independently analyze thermal decoherence processes at 4 K. The spectral information is transformed into its conjugate variable time, providing picosecond resolution and revealing an orbital depolarization timescale of $\sim 30\mathrm{ps}$. Consequences of the investigated dynamics are then used to estimate spin dephasing times limited by thermal effects.

The impact of disorder on wave transport has been extensively studied in Hermitian systems, where static randomness gives rise to Anderson localization. In non-Hermitian lattices, static disorder can lead to peculiar transport features, including jumpy wave evolution. In contrast, much less is known about how transport is modified when the on-site disorder evolves during propagation. Here, we address this problem by investigating two pertinent non-Hermitian lattice models with disorder altered at regular intervals, characterized by a finite disorder period. In lattices with symmetric couplings and complex on-site disorder, short disorder periods suppress localization and give rise to diffusionlike spreading, while longer periods allow the emergence of jumps. In Hatano-Nelson lattices with real on-site disorder, the non-Hermitian skin effect asymptotically dominates regardless of the disorder strength, while the disorder period reshapes the drift velocity and modulates its competition with Anderson localization. These results establish evolving disorder as a way of tuning non-Hermitian transport.

The interplay of twist and strain in bilayer graphene enables the formation of moiré patterns and narrow bands that host correlated and topological phases. While magic-angle twisted bilayer graphene has been widely studied, strain provides an additional and realistic control knob for band engineering. In this work, we first generate a global method to construct commensurate supercells for arbitrary twist and heterostrain. Then, using atomistic tight-binding and strain-extended continuum models to study the commensurate structures, we identify configurations that minimize the bandwidth beyond the magic angle. The results reveal a strong dependence of band narrowing and topology on strain type, magnitude, direction, and lattice relaxation. Particularly, shear strain produces a stronger distortion than uniaxial strain. Including electron-electron interactions through a self-consistent Hartree potential shows that strain broadens the bare bands while reducing electrostatic renormalization. Strain also drives topological transitions as the narrow and remote bands hybridize, establishing twisted and strained bilayer graphene as a tunable platform for flat-band and topological phenomena.

Generalized parity measurements are instrumental for the preparation of nontrivial quantum states and the detection of errors in error-correction codes. However, realizing such measurements in current experimental platforms becomes prohibitively difficult with growing system size. Here, using algorithmic methods of quantum signal processing, we detail a simple and efficient control protocol for implementing generalized parity measurements. Crucially, our measurement protocol does not depend on the size of the system and implements the desired measurement in constant time. We benchmark the performance of the proposed measurement protocol in preparing large multicomponent cat states on superconducting circuit quantum electrodynamics platforms. We show that a 20-component cat state with 400 photons can be prepared with success probability $>2\%$ and fidelity $\approx 90\%$ limited by the cavity decay rate and higher-order qubit-cavity coupling. Our results demonstrate the feasibility of generalized parity measurements in current bosonic and spin systems, paving the way to improvements in state-preparation and error-correction capabilities.

Using time-dependent density functional theory, we study how density and size of impurities govern dissipation of persistent currents of fermionic superfluid rings in the Bardeen-Cooper-Schrieffer regime. The critical winding number for vortex emission increases with impurity density, but this enhancement is impurity size dependent and capped by the pair-breaking threshold. Below this vortex-emission threshold, the winding number remains constant while flow energy dissipates through impurity-enhanced pair breaking. Above the threshold, vortex-impurity interactions produce distinct mobility regimes—deflected trajectories, individual pinning, collective pinning, and intersite hopping, controlled by the impurity size and density, which determine the dominant dissipation channel. These findings provide design principles for ultracold-atom experiments and insights into vortex-pinning dynamics in neutron-star crusts and superconductors.

Topological phenomena in quantum critical systems have recently attracted growing attention, as they go beyond the traditional paradigms of condensed matter and statistical physics. However, a general framework for identifying such nontrivial phenomena, particularly in higher-dimensional systems, remains insufficiently explored. In this work, we propose a universal fingerprint for detecting nontrivial topology in critical free-fermion systems protected by global on-site symmetries. Specifically, we analytically establish an exact relation between the bulk entanglement spectrum and the boundary energy spectrum at topological criticality in arbitrary dimensions, demonstrating that the degeneracy of edge modes can be extracted from the bulk entanglement spectrum. These findings, further supported by numerical simulations of lattice models, provide a universal fingerprint for identifying nontrivial topology in critical free-fermion systems.

We investigate Josephson transport in a fully closed, two-dimensional superfluid circuit formed by a ring-shaped ${}^{87}\mathrm{Rb}$ Bose-Einstein condensate that contains two optical barriers acting as movable weak links. Translating these barriers at controlled speeds imposes a steady bias current, enabling direct mapping of the current-chemical-potential ($I\text{−}\mathrm{\Delta }\mu$) characteristics. For narrow junctions ($w\approx 1\mu \text{m}$), the circuit exhibits a pronounced dc branch that terminates at a critical current $I_c=9\left(1\right)\times {10}^3{\text{s}}^{-1}$; above this threshold, the system switches to an ac, resistive regime. Classical-field simulations that include the moving barriers quantitatively reproduce both the nonlinear $I\text{−}\mathrm{\Delta }\mu$ curve and the measured $I_c$, validating the underlying microscopic picture. Analysis of the ensuing phase dynamics shows that dissipation is mediated by the nucleation and traversal of vortex-antivortex pairs through the junctions, while the bulk condensate remains globally phase locked—direct evidence of the ring's topological constraint enforcing quantized circulation. These results establish a cold-atom analog of a superconducting quantum interference device in which Josephson dynamics can be resolved at the single-vortex level, providing a versatile platform for atomtronic circuit elements, nonreciprocal Josephson devices, and on-chip Sagnac interferometers for multiaxis rotation sensing.

We provide a quantum mechanical description of phonon amplification in a heterostructure consisting of a two-dimensional electron gas (2DEG) stacked on top of a piezoelectric material. An applied drift voltage effectively creates a population inversion in the momentum states of the 2DEG electrons, giving rise to spontaneous emission of phonons. Once an acoustic wave is launched, the pumped electrons release phonons via stimulated emission, returning to depleted ground states before being pumped back to the excited states. We show that whereas efficient amplification using a one-dimensional electron gas requires the acoustic wavelength to roughly equal the mean distance between electrons, a 2DEG enables efficient amplification for any wavelength greater than this distance. We derive the imaginary and real parts of the 2DEG’s first-order acoustic susceptibility as functions of electronic drift velocity in specific limits and derive the gain per unit length for the signal and the quantum noise, with the gain matching the classical result in the short-electronic-lifetime (low-mobility) regime. Moreover, we analyze the gain clamping due to pump depletion and calculate the maximum achievable intensity. Our results provide a framework for designing acoustic devices including a quantum phononic laser and phase-insensitive quantum phononic amplifiers.

The Remeika series superconductor, ${\left({\text{Ca}}_x{\text{Sr}}_{1-x}\right)}_3{\text{Rh}}_4{\text{Sn}}_{13}$, shows a rare nonmagnetic quantum critical point (QCP) associated with the continuous charge-density wave (CDW) and structural transition under the “dome” of superconductivity achieved by tuning composition and applying pressure. Here, we use a nonmagnetic pointlike disorder induced by 2.5 MeV electron irradiation to suppress the CDW and drive the system to and even beyond the QCP. This conclusion is based on a clear evolution of temperature-dependent resistivity, $\rho \left(T\right)$, from the Fermi liquid to the non-Fermi liquid regime with increasing amount of disorder. Starting on the CDW side, below the suggested QCP concentration of $x_c=0.9$, added disorder resulted in a progressively larger linear term and a reduced quadratic term in $\rho \left(T\right)$. Nearly perfect $T\text{−}\mathrm{linear}$ dependence is observed at the dose at which long-range CDW order is suppressed to $T=0$, consistent with the expectations. We refine the QCP location in this system and place it in the interval between $x=0.75$ and 0.85. Our results strongly support the concept that the disorder can tune the system to the quantum critical regime and even beyond. It follows from the argument by Imry and Ma that any ordered phase is unstable toward quenched disorder. Introduced in a controlled way, this disorder becomes a nonthermal tuning parameter likely applicable to a variety of different systems.