Understanding of bonding is key to modeling materials and predicting properties thereof. A widely adopted indicator of bonds and atomic shells is the electron localization function (ELF). The building blocks of the ELF are also used in the construction of modern density functional approximations. Here, we demonstrate that the ELF breaks down when applied beyond regular nonrelativistic quantum states. We show that for tackling general noncollinear open-shell solutions, it is essential to address both the U(1) gauge invariance, i.e., invariance under a multiplication by a position dependent phase factor, and SU(2) gauge invariance, i.e., invariance under local spin rotations, conjointly. Remarkably, we find that the extended ELF also improves the description of paradigmatic collinear states.

We propose a new avenue in which percolation, which has been much associated with critical phase transitions, can also dictate the asymptotic dynamics of non-Hermitian systems by breaking $\mathcal{P}\mathcal{T}$ symmetry. Central to it is our newly designed mechanism of topologically guided gain, where chiral edge wave packets in a topological system experience non-Hermitian gain or loss based on how they are topologically steered. For sufficiently wide topological islands, this leads to irreversible growth due to positive feedback from interlayer tunneling. As such, a percolation transition that merges small topological islands into larger ones also drives the edge spectrum across a real to complex transition. Our discovery showcases intriguing dynamical consequences from the triple interplay of chiral topology, directed gain, and interlayer tunneling, and suggests new routes for the topology to be harnessed in the control of feedback systems.

Motivated by the experiments on the triangular lattice bilayer colbaltate ${\mathrm{K}}_2{\mathrm{Co}}_2({\mathrm{SeO}}_3{)}_3$, we consider an extended $XXZ$ model to explore the underlying physics. The model is composed of interacting ${\mathrm{Co}}^{2+}$ dimers on the triangular lattice, where the ${\mathrm{Co}}^{2+}$ ion provides an effective spin-$1/2$ local moment via the spin-orbit coupling and the crystal field effect. The intradimer interaction is dominant and would simply favor the local spin singlet, and the interdimer interactions compete with the interdimer interaction, leading to rich behaviors. With the easy-axis anisotropy, it is shown that, in the ground state manifold of the intradimer Ising interaction, the system realizes an effective quantum Ising model, where the ground state is either a three-sublattice order with a mixture of antiferromagnetic Ising order and a valence-bond spin singlet or Ising disordered. The finite temperature regime realizes the Berezinskii-Kosterlitz-Thouless physics. To explore the full excitations, we incorporate the excited state manifold of the intradimer Ising interaction and establish the emergent Kugel-Khomskii physics. Thus, the triangular lattice bilayer colbaltate is an excellent platform to explore the interplay between geometrical frustration and anisotropic interactions as well as the emergent effective models and the resulting physics.

Microscopic origins of charge currents and electromagnetic (EM) radiation generated by them in spintronic THz emitters—such as, femtosecond laser pulse-driven single magnetic layer or its heterostructures with a nonmagnetic layer hosting strong spin-orbit coupling (SOC)—remain poorly understood despite nearly three decades since the discovery of ultrafast demagnetization. We introduce a first-principles method to compute these quantities, where the dynamics of charge and current densities is obtained from real-time time-dependent density functional theory, which are then fed into the Jefimenko equations for properly retarded electric and magnetic field solutions of the Maxwell equations. By Fourier transforming different time-dependent terms in the Jefimenko equations, we unravel that in the 0.1–30 THz range the electric field of far-field EM radiation by the Ni layer, chosen as an example, is dominated by charge current pumped by demagnetization, while often invoked magnetic dipole radiation from the time-dependent magnetization of a single magnetic layer is a negligible effect. Such an effect of charge current pumping by a time-dependent quantum system, whose magnetization is shrinking while its vector does not rotate, does not require any spin-to-charge conversion via SOC effects. In the $\mathrm{Ni}/\mathrm{Pt}$ bilayer, EM radiation remains dominated by the charge current within the Ni layer, whose magnitude is larger than in the case of a single Ni layer due to faster demagnetization, while often invoked spin-to-charge conversion within the Pt layer provides an additional but smaller contribution. By using the Poynting vector and its flux, we also quantify the efficiency of conversion of light into emitted EM radiation, and its angular distribution.

Decoupling systems into independently evolving components has a long history of simplifying seemingly complex systems. They enable a better understanding of the underlying dynamics and causal structures while providing more efficient means to simulate such processes on a computer. Here we outline a variational decoupling algorithm for decoupling unitary quantum dynamics—allowing us to decompose a given $n$-qubit unitary gate into multiple independently evolving sub-components. We apply this approach to quantum circuit synthesis—the task of discovering quantum circuit implementations of target unitary dynamics. Our numerical studies illustrate significant benefits, showing that variational decoupling enables us to synthesize general two- and four-qubit gates to fidelity that conventional variational circuits cannot reach.

Gravitational-wave signals detected to date are commonly interpreted under the paradigm that they originate from pairs of black holes or neutron stars. Here, we explore the alternative scenario of boson-star signals being present in the data stream. We perform accurate and long ($\sim 20$ orbits) numerical simulations of boson-star binaries and inject the resulting strain into LIGO noise. Our Bayesian inference reveals that some boson-star signals are degenerate with current approximants, albeit with biased parameters, while others exhibit smoking-gun signatures leaving behind conspicuous residuals.

We measure the complete set of angular coefficients $J_i$ for exclusive $\overline{B}\to D^{*}\ell {\overline{\nu }}_{\ell }$ decays ($\ell =e$, $\mu$). Our analysis uses the full $711{\mathrm{fb}}^{-1}$ Belle dataset with hadronic tag-side reconstruction. The results allow us to extract the form factors describing the $\overline{B}\to D^{*}$ transition and the Cabibbo-Kobayashi-Maskawa matrix element $|V_{\mathrm{cb}}|$. Using recent lattice QCD calculations for the hadronic form factors, we find $|V_{\mathrm{cb}}|=(40.7\pm 0.7)\times {10}^{-3}$ using the Boyd-Grinstein-Lebed parametrization, compatible with determinations from inclusive semileptonic decays. We search for lepton flavor universality violation as a function of the hadronic recoil parameter $w$ and investigate the differences of the electron and muon angular distributions. We find no deviation from standard model expectations.

We demonstrate the generation of a persistent planar molecular alignment by subjecting a relatively warm gas sample to a resonant femtosecond laser pulse. By optically probing ${\mathrm{I}}_2$ molecules in their vibronic ground states, we observe a persistent delocalization of their axes near the plane orthogonal to the field direction. This phenomenon is attributed to the one-photon resonant excitation, primarily removing molecules from the thermal ground-state distribution that are initially aligned along the field, i.e., those with small projection of their rotational angular momentum along the field.

X-ray ablation dynamics of the planar foil with preimposed sinusoidal ripples is investigated at the SG 100 kJ Laser Facility. A significant fraction of the second harmonics is observed and identified at the beginning of the ablative drive when the amplitude of the perturbation is within the linear regime. With radiation-hydrodynamic simulations and a developed simple model, we can reveal that such a novel phenomenon is due to the fact that a sustained deformation of the ablation front is initiated since the ablation pressure is directed to the normal direction of the perturbation surface. We find that this deformation dominates the early-time perturbation evolution and results in a specific stage in addition to the traditional ablative Richtmyer-Meshkov instability phase. Our results can be applicable to various regions such as implosions in inertial confinement fusion and the dynamics of molecular clouds in astrophysics.

Diffusion coefficients for crystallized Coulomb plasmas are essential microphysics input for modeling white dwarf cores and neutron star crusts but are poorly understood. In this work we present a model for diffusion in Coulomb crystals. We show that melting and diffusion follow the same universal scaling such that diffusion is independent of screening. Our simulations show, contrary to prevailing wisdom, that the formation of vacancies is not suppressed by the large pressure. Rather, vacancy formation and hole diffusion is the dominant mode of self-diffusion in Coulomb crystals.

The mechanism of superconductivity in materials with aborted ferroelectricity and its emergence out of a dilute metallic phase in systems like doped ${\mathrm{SrTiO}}_3$ is an outstanding issue in condensed matter physics. This dilute metal has anomalous properties that are both similar and different to those found in the normal state of other unconventional superconductors. For instance, ${\mathrm{T}}^2$ resistivity can be found at densities that are too small to allow current decay through electron-electron scattering. We have investigated the optical properties of the dilute metallic phase in doped ${\mathrm{SrTiO}}_3$ using THz time-domain spectroscopy. At low frequencies the THz response exhibits a Drude-like form as expected for typical metal-like conductivity. We observed the frequency and temperature dependencies to the low energy scattering rate $\mathrm{\Gamma }(\omega ,T)\propto (\hslash \omega {)}^2+(p\pi k_{B}T{)}^2$ expected in a conventional Fermi liquid. However, we find the lowest known $p$ values of 0.39–0.72. As $p$ is 2 in a canonical Fermi liquid and existing models based on energy dependent elastic scattering bound $p$ from below to 1, our observation lies outside current explanation. Our data also give insight into the high temperature regime and show that the temperature dependence of the resistivity derives in part from strong $T$ dependent mass renormalizations.

The phase diagrams of quasi two-dimensional organic superconductors display a plethora of fundamental phenomena associated with strong electron correlations, such as unconventional superconductivity, metal-insulator transitions, frustrated magnetism and spin liquid behavior. We analyze a minimal model for these compounds, the Hubbard model on an anisotropic triangular lattice, using cutting-edge quantum embedding methods respecting the lattice symmetry. We demonstrate the existence of unconventional superconductivity by directly entering the symmetry-broken phase. We show that the crossover from the Fermi liquid metal to the Mott insulator is associated with the formation of a pseudogap. The predicted momentum-selective destruction of the Fermi surface into hot and cold regions provides motivation for further spectroscopic studies. Our theoretical results agree with experimental phase diagrams of $\kappa$-BEDT organics.

Nonreciprocity in acoustics is of paramount importance in many practical applications and has been experimentally realized using nonlinear media, moving fluids, or time modulation, which regrettably suffer from large volumes and high-power consumption, difficulty in integration, and inevitable vibrations or phase noise. In modern Hamiltonian theory, the violation of system’s reciprocity can be achieved via asymmetric Peierls phases, which typically involves with non-Hermiticity or time-reversal symmetry breaking. Here, we propose a framework for designing nonreciprocal acoustic devices based on the asymmetric Peierls phases that can be fully controlled via active acoustic components. The fully controlled Peierls phases enable various high-performance acoustic devices, including non-Hermitian extensions of isolators, gyrators, and circulators, which are otherwise impossible in previous approaches that are bound by Hermiticity or passivity. We reveal that the transmission phases in isolators are equivalent to the Peierls phase plus a constant. The nonreciprocal phase delay in gyrators and the unirotational transmission behavior in circulators result from the gauge-invariant Aharonov-Bohm phases determined by Peierls phases. Our work not only uncovers multiple intriguing physics related to Peierls phases but also provides a general approach to compact, integratable, nonreciprocal acoustic devices.

Magnetic resonance dynamics has been studied for a polar magnet ${\mathrm{VOSe}}_2{\mathrm{O}}_5$, which hosts several nontrivial magnetic phases including Néel-type skyrmion lattice (SkL). In both cycloidal and SkL spin states, two excitation modes active to oscillating magnetic field $B_{\nu }\perp c$ and one mode active to $B_{\nu }\parallel c$ are identified. The subsequent micromagnetic simulations well reproduce the observed selection rules and relative resonance frequencies, which allows the unambiguous assignment of the spin oscillation manner for each mode. Interestingly, the IC-2 phase with a potential double-$q$ character was found to host similar excitation modes as the SkL state. We also discovered the existence of the novel B’ phase with four modes active to $B_{\nu }\perp c$. The present results provide a fundamental basis for the comprehensive understanding of resonant spin dynamics in polar magnets, and highlight ${\mathrm{VOSe}}_2{\mathrm{O}}_5$ as a unique material platform to host a rich variety of nontrivial spin excitations.

We derive a systematic approach to the thermodynamics of quantum systems based on the underlying symmetry groups. We first show that the entropy of a system can be described in terms of group-theoretical quantities that are largely independent of the details of its density matrix. We then apply our technique to generic $N$ identical interacting $d$-level quantum systems. Using permutation invariance, we find that, for large $N$, the entropy displays a universal asymptotic behavior in terms of a function $s(\mathit{x})$ that is completely independent of the microscopic details of the model, but depends only on the size of the irreducible representations of the permutation group ${\mathrm{S}}_N$. In turn, the equilibrium state of the system and macroscopic fluctuations around it are shown to satisfy a large deviation principle with a rate function $f(\mathit{x})=e(\mathit{x})-{\beta }^{-1}s(\mathit{x})$, where $e(\mathit{x})$ only depends on the ground state energy of particular subspaces determined by group representation theory, and $\beta$ is the inverse temperature. We apply our theory to the transverse-field Curie-Weiss model, a minimal model of phase transition exhibiting an interplay of thermal and quantum fluctuations.