In this work we develop the theory of the Loschmidt echo and dynamical phase transitions in noninteracting strongly disordered Fermi systems after a quench. In finite systems the Loschmidt echo displays zeros in the complex time plane that depend on the random potential realization. Remarkably, the zeros coalesce to form a 2D manifold in the thermodynamic limit, atypical for 1D systems, crossing the real axis at a sharply defined critical time. We show that this dynamical phase transition can be understood as a transition in the distribution function of the smallest absolute value of the eigenvalues of the Loschmidt matrix and develop a finite-size scaling theory. Contrary to expectations, the notion of dynamical phase transitions in disordered systems becomes decoupled from the equilibrium Anderson localization transition. Our results highlight the striking qualitative differences of quench dynamics in disordered and nondisordered many-fermion systems.

With the help of scanning tunneling microscopy (STM) it has become possible to address single magnetic impurities on superconducting surfaces and to investigate the peculiar properties of the in-gap states known as Yu-Shiba-Rusinov (YSR) states. These systems are an ideal playground to investigate multiple aspects of superconducting bound states, such as the occurrence of quantum phase transitions or the interplay between Andreev transport physics and the spin degree of freedom, with profound implications for disparate topics like Majorana modes or Andreev spin qubits. However, until very recently YSR states were only investigated with conventional tunneling spectroscopy, missing the crucial information contained in other transport properties such as shot noise. In this paper we adapt the concept of full counting statistics (FCS) to provide the deepest insight thus far into the spin-dependent transport in these hybrid atomic-scale systems. We illustrate the power of FCS by analyzing different situations in which YSR states show up including single-impurity junctions with a normal and a superconducting STM tip, as well as double-impurity systems where one can probe the tunneling between individual YSR states [Nat. Phys. 16, 1227 (2020)]. The FCS concept allows us to unambiguously identify every tunneling process that plays a role in these situations and to classify them according to the charge transferred in them. Moreover, FCS provides all the relevant transport properties, including current, shot noise, and all the cumulants of the current distribution. In particular, our approach is able to reproduce the experimental results recently reported on the shot noise of a single-impurity junction with a normal STM tip [Phys. Rev. Lett. 128, 247001 (2022)]. We also predict the signatures of resonant (and nonresonant) multiple Andreev reflections in the shot noise and Fano factor of single-impurity junctions with two superconducting electrodes and show that the FCS approach allows us to understand conductance features that have been incorrectly interpreted in the literature. In the case of double-impurity junctions we show that the direct tunneling between YSR states is characterized by a strong reduction of the Fano factor that reaches a minimum value of $7/32$, a significant result in quantum transport. The FCS approach presented here can be naturally extended to investigate the spin-dependent superconducting transport in a variety of situations, such as atomic spin chains on surfaces or superconductor-semiconductor nanowire junctions, and it is also suitable to analyze multiterminal superconducting junctions, irradiated contacts, and many other basic situations.

We study an atom-cavity hybrid system where fermionic atoms in a one-dimensional lattice are subject to a cavity-induced dynamic gauge potential. The gauge coupling leads to highly degenerate steady states in which the fermions accumulate to one edge of the lattice under an open boundary condition. Such a phenomenon originates from the many-body Liouvillian superoperator of the system, which, being intrinsically non-Hermitian, is unstable against boundary perturbations and manifests the non-Hermitian skin effect. Contrary to the single-body case, the steady state of a multiatom system is approached much slower under the open boundary condition, as the long-time damping of the cavity mode exhibits distinct rates at different times. This stagewise slowdown is attributed to the competition between light-assisted hopping and the dynamic gauge coupling, which significantly reduces the steady-state degeneracy under the open boundary condition, as distinct hosts of quasisteady states dominate the dynamics at different timescales.

Waves propagating in certain one-dimensional quasiperiodic lattices are known to exhibit a sharp localization transition. We theoretically predict and experimentally observe that the localization of light in one-dimensional photonic quasicrystals is followed by a second delocalization transition for some states on increasing quasiperiodic modulation strength—an example of a reentrant transition. We further propose that this phenomenon can be qualitatively captured by a dimerized tight-binding model with long-range couplings. This work furthers our understanding of localization physics in complex systems and the impact of quasiperiodicity on light transport in photonic devices.

Inspired by the progress on the spin liquid candidates ${\mathrm{Ce}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$ and ${\mathrm{Ce}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$ where the Ce ions carry the dipole-octupole doublets, we analyze the distinction between the thermodynamic and spectroscopic measurements for the octupolar U(1) spin liquid. Due to the peculiar properties of octupolar U(1) spin liquid and the selective Zeeman coupling, both uniform susceptibility and the spectroscopic measurements only probe the spinon excitations, while the specific heat measures all excitations. After clarifying the contents in each measurement, we observe that, thermodynamics carries the information of single spinon, while the spectroscopy is associated with spinon pairs. Such distinction immediately leads to the multiplicity relation of the excitation gaps from different measurements that include the uniform susceptibility, Knight shift, and $1/T_1$ spin lattice relaxation of NMR and $\mu \mathrm{SR}$, and the inelastic neutron scattering measurements. We hope this work provides a useful recipe for the identification of fractionalization in the octupolar U(1) spin liquid and other spin liquids.

Layered trihalides, mainly formed by metal elements and halogens, are promising candidates for spintronic devices and van der Waals heterostructures, but are rarely found in nonmetal halides. Motivated by the purpose of searching for the compelling layered configuration in a covalent system, we performed systemically high-pressure experiments to explore the crystal structures in typical ${\mathrm{PBr}}_3$ with diamond anvil cell technique. At room temperature, ${\mathrm{PBr}}_3$ crystallizes into a molecular crystal with orthorhombic symmetry above 0.9 GPa and is found to persist up to at least 43.9 GPa as confirmed by in situ Raman and x-ray diffraction measurements. Strikingly, a novel layered phase with a space group $P{2}_1/c$ is observed at $\sim 27.0\mathrm{GPa}$ and $\sim 1800$ K, which is characterized by quasi-six-coordination of P atoms with nearby Br atoms. In addition, first-principles calculations indicate that the unusual coordination of the P atom in $P{2}_1/c$ phase is highly correlated with the pressure-induced generation of pnictogen bonds. Our current findings not only expand the understanding of the phase diagram of ${\mathrm{PBr}}_3$, but also show an interesting transformation from molecular crystal to an extended layered phase achieved in a broad range of nonmetal trihalides.

We present a study on the transport and material properties of aluminum spanning from ambient to warm dense matter conditions using a machine-learned interatomic potential (ML-IAP). Prior research has utilized ML-IAPs to simulate phenomena in warm dense matter, but these potentials have often been calibrated for a narrow range of temperatures and pressures. In contrast, we train a single ML-IAP over a wide range of temperatures, using density functional theory molecular dynamics (DFT-MD) data. Our approach overcomes the computational limitations of DFT-MD simulations, enabling us to study the transport and material properties of matter at higher temperatures and longer time scales. We demonstrate the ML-IAP transferability across a wide range of temperatures using molecular dynamics by examining the ionic part of thermal conductivity, shear viscosity, self-diffusion coefficient, sound velocity, and structure factor of aluminum up to about 60000 K, where we find good agreement with previous theoretical data.

Fermi gases and liquids display an excitation spectrum that is simply connected, ensuring closed Fermi surfaces. In strongly correlated systems such as the cuprate superconductors, the existence of open sheets of Fermi surface known as Fermi arcs indicate a distinctly different topology of the spectrum with no equivalent in Fermi-liquid theory. Here, we demonstrate a generic mechanism by which correlation effects in fermionic systems can change the topology of the spectrum. Using diagrammatic Monte Carlo simulations, we demonstrate the existence of disconnected and multiply connected excitation spectra in the attractive Hubbard model in the BCS-BEC crossover regime. These topologically nontrivial spectra are a prerequisite for Fermi arcs.

Recently, intriguing physical properties have been unraveled in anisotropic semiconductors in which the in-plane electronic band structure anisotropy often originates from the low crystallographic symmetry. The atomic chain is the ultimate limit in material downscaling for electronics—a frontier for establishing an entirely new field of one-dimensional quantum materials. Electronic and structural properties of chain-like InTe are essential for a better understanding of device applications such as thermoelectrics. Here, we use scanning tunneling microscopy/scanning tunneling spectroscopy (STS) measurements and density functional theory (DFT) calculations to image the in-plane structural anisotropy directly in tetragonal InTe. As results, we report the direct observation of one-dimensional ${\mathrm{In}}^{1+}$ chains in InTe. We demonstrate that InTe exhibits a bandgap of about $0.40\pm 0.02$ eV located at the M point of the Brillouin zone. Additionally, line defects are observed in our sample and were attributed to ${\mathrm{In}}^{1+}$ chain vacancy along the $c$-axis—a general feature in many other TlSe-like compounds. Our STS and DFT results prove that the presence of ${\mathrm{In}}^{1+}$ induces a localized gap state, located near the valence band maximum. This acceptor state is responsible for the high intrinsic p-type doping of InTe that we also confirm using angle-resolved photoemission spectroscopy.

Fast and accurate current-driven manipulation of magnetic domain walls (DWs) is crucial for the realization of high-performance spintronic devices. Conventional methods to observe current-induced DW motion (CIDWM) require the accumulative measurement of magnetization dynamics, which is in general incompatible with stochastic magnetization dynamics. We demonstrate stroboscopic time-resolved imaging of CIDWM in a Pt/GdFeCo wire by electrically synchronizing the optical probe pulse and current injection pulse. The observed CIDWM dynamics unveil a time-varying DW velocity during the pulse, which can largely be attributed to the gradual temperature change due to Joule heating and an enhanced DW velocity at the angular momentum compensation temperature. The stroboscopic observation of DW dynamics was also utilized to investigate the contributions of inertial forces in CIDWM. Our method provides access to the spatial and stochastic magnetization dynamics in sophisticated structures driven by external stimuli and should be applicable to a wide range of current-driven phenomena.

We address magnetic neutron scattering in the presence of local noncentrosymmetric asymmetries of the magnetization density. Such inversion-symmetry breaking, combined with the absence of time-reversal symmetry, can be described in terms of magnetoelectric multipoles which form the second term after the magnetic dipole in the multipole expansion of the magnetization density. We provide a pedagogical review of the theoretical formalism of magnetic neutron diffraction in terms of the multipole expansion of the scattering cross-section. In particular, we show how to compute the contribution of magnetoelectric multipoles to the scattering amplitude starting from ab initio calculations. We also provide general guidelines on how to experimentally detect long-range order of magnetoelectric multipoles using either unpolarized or polarized neutron scattering. We apply the formalism to CuO, and we discuss first-principles predictions and experimental spherical neutron polarimetry measurements that are consistent with the presence of magnetoelectric multipoles in CuO.

For decades transition-metal oxides have generated a huge interest due to the multitude of physical phenomena they exhibit. In this class of materials, the rare-earth nickelates, $R{\mathrm{NiO}}_3$, stand out for their rich phase diagram stemming from complex couplings between the lattice, electronic, and magnetic degrees of freedom. Here, we present a first-principles study of the low-temperature phase for two members of the $R{\mathrm{NiO}}_3$ series, with $R=$ Pr, Y. We employ density-functional theory with Hubbard corrections accounting not only for the onsite localizing interactions among the Ni-$3d$ electrons ($U$), but also the intersite hybridization effects between the transition metals and the ligands ($V$). All the $U$ and $V$ parameters are calculated from first principles using density-functional perturbation theory, resulting in a fully ab initio methodology. Our simulations show that the inclusion of the intersite interaction parameters $V$ is necessary to simultaneously capture the features well-established by experimental characterizations of the low-temperature state: insulating character, antiferromagnetism, and bond disproportionation. On the contrary, for some magnetic orderings the inclusion of onsite interaction parameters $U$ alone completely suppresses the breathing distortion occurring in the low-temperature phase and produces an erroneous electronic state with a vanishing band gap. In addition—only when both the $U$ and $V$ are considered—we predict a polar phase with a magnetization-dependent electric polarization, supporting recent experimental observations that suggest a possible occurrence of type-II multiferroicity for these materials.

Phantom relaxation is relaxation with a rate that is not given by a finite spectral gap. Studying the average purity dynamics in a staircase random Haar circuit and the spectral decomposition of a nonsymmetric matrix describing the underlying Markovian evolution, we explain how that can arise out of an ordinary-looking spectrum. Crucial are alternating expansion coefficients that diverge in the thermodynamic limit due to the non-Hermitian skin effect in the matrix describing the average purity dynamics under an overall unitary evolution. The mysterious phantom relaxation emerges out of localized generalized eigenvectors describing the Jordan normal form kernel, and, independently, also out of interesting trigonometric sums due to localized true eigenvectors. All this shows that when dealing with non-Hermitian matrices it can happen that the spectrum is not the relevant object; rather, it is the pseudospectrum or, equivalently, a delicate cancellation enabled by localized eigenvectors.

Robust and tunable topological Josephson junctions (TJJs) are highly desirable platforms for investigating the anomalous Josephson effect and topological quantum computation applications. Experimental demonstrations have been done in hybrid superconducting-two dimensional topological insulator (2DTI) platforms, sensitive to magnetic disorder and interactions with phonons and other electrons. In this work, we propose a robust and electrostatically tunable TJJ by combining the physics of the integer quantum Hall (IQH) regime and of superconductors. We provide analytical insights about the corresponding Andreev bound state spectrum, the Josephson current and the anomalous current. We demonstrate the existence of protected zero-energy crossings, that can be controlled through electrostatic external gates. This electrostatic tunability has a direct advantage to compensate for non-ideal interfaces and undesirable reflections that may occur in any realistic samples. TJJs in the IQH regime could be realized in graphene and other 2D materials. They are of particular relevance towards scalable and robust Andreev-qubit platforms, and also for efficient phase batteries.

It is a general notion that disorder, introduced by either chemical substitution or intercalation as well as by electron irradiation, is detrimental to the realization of long-range charge density wave (CDW) order. We study the disorder-induced suppression of in-plane CDW orders in two-dimensional Pd-intercalated ${\mathrm{ErTe}}_3$ compositions by exploring the real part of the optical conductivity with light polarized along the in-plane $a$ and $c$ axes. Our findings reveal an anisotropic charge dynamics with respect to both incommensurate unidirectional CDW phases of ${\mathrm{ErTe}}_3$, occurring within the $ac$ plane. The anisotropic optical response gets substantially washed out with Pd intercalation, hand in hand with the suppression of both CDW orders. The spectral weight analysis, though, advances the scenario, for which the CDW phases evolve from a (partially) depleted Fermi surface already above their critical onset temperatures. We therefore argue that the long-range CDW orders of ${\mathrm{ErTe}}_3$ tend to be progressively dwarfed by Pd intercalation, which favors the presence of short-range CDW segments for both crystallographic directions persisting in a broad temperature $\left(T\right)$ interval up to the normal state, and being suggestive of precursor effects of the CDW orders as well as possibly coexisting with superconductivity at low $T$.

Here, by making use of medium and high resolution autocorrected off-axis electron holography, we directly probe the electrostatic potential as well as in-plane structural reconstruction at edges and steps in multilayer hexagonal boron nitride. In combination with ab initio calculations, the data allows revealing the formation of folded zigzag edges at steps comprising two monolayers and their absence at monolayer steps.

We propose a way to identify strongly Hund-correlated materials by unveiling a key signature of Hund correlations at the two-particle level. The defining feature is the sign of the response of the frozen spin ratio (the long-time local spin-spin correlation function divided by the instantaneous value) under variation of electron density. The underlying physical reason is that the sign is closely related to the strength of charge fluctuations between the dominant atomic multiplets and higher-spin ones in a neighboring charge subspace. It is the predominance of these fluctuations that promotes Hund metallicity. The temperature dependence of the frozen spin ratio can further reveal a non-Fermi-liquid behavior and thus the Hund metal states. We analyze both degenerate and nondegenerate multiorbital Hubbard models and corroborate our argument by taking doped ${\mathrm{La}}_2{\mathrm{CuO}}_4$ and LaFeAsO as representative material examples, respectively, of Mott and Hund metals. Our proposal should be applicable to systems with non-half-filled integer electron fillings and their doped cases provided the doping drove the electron density toward the half filling.