The triple-layer ${\mathrm{Sr}}_4{\mathrm{Ru}}_3{\mathrm{O}}_{10}$ is a spin-orbit-coupled metal featuring an intriguing combination of ferromagnetic and metamagnetic states. Here we report a comprehensive study of single-crystal neutron and x-ray diffraction on ${\mathrm{Sr}}_4{\mathrm{Ru}}_3{\mathrm{O}}_{10}$ with 4% iridium doping. The system crystallizes in an orthorhombic $Bbem$ structure in which two triple layers are related by a base centering symmetry operation within one unit cell. The 4% Ir doping at the Ru site sensitively alters the structural balance between the ${\mathrm{RuO}}_6$ layers, as such the two outer-layer octahedra rotate $6.7^{\circ }$ about the $c$ axis, while the central-layer one rotates in the opposite direction by $11.0^{\circ }$. The increased lattice distortion stabilizes a collinear ferromagnetic state with magnetic moments fully aligned along the $c$ axis. The emerging perpendicular magnetic anisotropy naturally increases the critical field for the inplane metamagnetic transition to 4.5 Tesla. All results highlight the critical linkage between the magnetic properties and underlying lattice through orbital degrees of freedom, as well as the enhanced spin orbit coupling due to heavier $5d$ ions. Our observation provides a viable route for an efficient manipulation of the magnetic states with functionalities.

We characterize perovskite ${\mathrm{TiF}}_3$, a material which is reported to display significant negative thermal expansion at elevated temperatures above its cubic-to-rhombohedral structural phase transition at 330 K. We find the optical response favors an insulating state in both structural phases, which we show can be produced in density functional theory calculations only through the introduction of an on-site Coulomb repulsion. Analysis of the magnetic susceptibility data gives a $S=\frac{1}{2}$ local moment per ${\mathrm{Ti}}^{+3}$ ion and an antiferromagnetic exchange coupling. Together, these results show that ${\mathrm{TiF}}_3$ is a strongly correlated electron system, a fact which constrains possible mechanisms of strong negative thermal expansion in the ${\mathrm{Sc}}_{1-x}{\mathrm{Ti}}_x{\mathrm{F}}_3$ system. We consider the relative strength of the Jahn-Teller and electric dipole interactions in driving the structural transition.

We analyze the electronic structure of twisted bilayer graphene (TBG) nanoribbons close to the magic angle. We describe a transition from an incomplete to a complete moiré structure. By considering zigzag and armchair edge terminations, the low-energy bands are strongly modified, and thus, the edge flat-band localization is sensitive to the type of boundary. By means of a scaled tight-binding model, we calculate the band structure and find that, for an armchair configuration, an incomplete moiré edge suppresses the edge localization, while for a zigzag configuration, we find a strong interference of the edge states with the moiré bands. In particular, for the armchair termination, we observe a competition between the ribbon periodicity and the graphene monolayers, which we describe with a potential well toy model. Furthermore, for ribbons with widths of multiple moiré cells, the flat bands of the moirés in the bulk are unperturbed as we change the borders. These results are explained in terms of the strong electronic localization, nearly Gaussian, in the $\mathit{AA}$ stacking regions, as confirmed by an inverse participation ratio analysis. Our results demonstrate that the electronic structure of TBG nanoribbons is sensitive to the edge termination, offering an explanation for recent experimental results.

Employing an unbiased sign-problem-free quantum Monte Carlo approach, we investigate the effects of long-range Coulomb forces on Bose-Einstein condensation of bipolarons using a model of bond phonon-modulated electron hopping. In the absence of long-range repulsion, this model was recently shown to give rise to small-size light-mass bipolarons that undergo a superfluid transition at high values of the critical transition temperature $T_{\mathrm{c}}$. We find that $T_{\mathrm{c}}$ in our model, even with the long-range Coulomb repulsion, remains much larger than that of Holstein bipolarons and can be on the order of or greater than the typical upper bounds on phonon-mediated $T_{\mathrm{c}}$ based on the Migdal-Eliashberg and McMillan approximations. Our work points to a physically simple mechanism for superconductivity in the low-density regime that may be relevant to current experiments on dilute superconductors.

We study a one-dimensional ladder of two coupled XXZ spin chains and identify several distinct gapless symmetry-enriched critical phases. These have the same unbroken symmetries and long-wavelength description, but cannot be connected without encountering either a phase transition or other intermediate phases. Using bosonizaion, we analyze the nature of their distinction by determining how microscopic symmetries are manifested in the long-wavelength fields, the behavior of charged local and nonlocal operators, and identify the universality class of all direct continuous phase transitions between them. One of these phases is a gapless topological phase with protected edge modes. We characterize its precise nature and place it within the broader classification. We also find the occurrence of “multiversality” in the phase diagram, wherein two fixed phases are separated by continuous transitions with different universality classes in different parameter regimes. We determine the phase diagram and all its aspects, as well as verify our predictions numerically using density matrix renormalization group and a mapping onto an effective spin-1 model.

Non-Hermitian topological systems have attracted much interest due to their unique topological properties when the non-Hermitian skin effect (NHSE) appears. However, the experimental realization of NHSE conventionally requires nonreciprocal couplings, which are compatible with limited systems. Here, we propose a mechanism of loss-induced Floquet NHSE, where the loss provides the basic source of non-Hermicity and the Floquet engineering brings about Floquet-induced complex next-nearest-neighbor couplings. We also extend the generalized Brillouin zone theory to nonequilibrium systems to describe the Floquet NHSE. Furthermore, we show that this mechanism can realize the second-order NHSE when generalized to two-dimensional systems. Our proposal can be realized in photonic lattices with helical waveguides and other related systems, which opens the door for the study of topological phases in Floquet non-Hermitian systems.

We report local structure measurements for ${\mathrm{CsPbBr}}_3$ and $\left({\left({\mathrm{NH}}_2\right)}_2\mathrm{CH}\right){\mathrm{PbBr}}_3$ $\left({\mathrm{FAPbBr}}_3\right)$ and compare them with recent results for $\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right){\mathrm{PbBr}}_3$ $\left({\mathrm{MAPbBr}}_3\right)$. The Pb-Br bonding is similar for all three systems; the effective spring constants, $\kappa$, are comparable (ranging from 1.20 to 1.95 eV/${\AA }^2$), but small in magnitude indicating very soft materials. However, there are also important differences between the three systems. Static disorder is very small for ${\mathrm{CsPbBr}}_3$ but increases somewhat with the size of the organic molecular ions ${\mathrm{MA}}^{+}$ and ${\mathrm{FA}}^{+}$. At room temperature, dynamic disorder dominates in all compounds. The thermal disorder of the Pb-Br pair distribution function (PDF), i.e., the Debye-Waller factor ${\sigma }^2$ follows a correlated Debye or Einstein model up to 300 K in ${\mathrm{CsPbBr}}_3$ (orthorhombic phase), but for ${\mathrm{FAPbBr}}_3$ and ${\mathrm{MAPbBr}}_3$, there is a break in the ${\sigma }^2\left(T\right)$ curve at the orthorhombic-tetragonal transition (o-t) near 150 K, indicating a small change in the spring constant $\kappa$. $\kappa$ increases for ${\mathrm{MAPbBr}}_3$ but decreases in ${\mathrm{FAPbBr}}_3$ at this transition. These changes are attributed to changes in the H-bonding between ${\mathrm{Br}}^{-}$ and ${\mathrm{MA}}^{+}$ or ${\mathrm{FA}}^{+}$ at this transition, as a result of librations or rotations of these molecular cations. In addition, the Pb-Br PDF becomes asymmetric at a relatively low temperature for ${\mathrm{FAPbBr}}_3$ and ${\mathrm{MAPbBr}}_3$, while this effect is significantly smaller for ${\mathrm{CsPbBr}}_3$. Finally, we address the question of a model to explain the asymmetric PDF. Two main models are discussed in the literature, an anharmonic pair potential and a split-pair distribution, possibly driven by the presence of a lone pair on the Pb ion. We show that the fourth cumulant $C_4$ can differentiate between these two models and other possible models. Experimentally $C_4$ is positive at 250 K and above, for all three systems and that is inconsistent with a split-peak model, for which $C_4$ is negative for splittings larger than 0.12 Å.

We propose a scheme for generating and controlling entangled coherent states (ECSs) of magnons, i.e., the quanta of the collective spin excitations in magnetic systems, or phonons in mechanical resonators. The proposed hybrid circuit architecture comprises a superconducting transmon qubit coupled to a pair of magnonic yttrium iron garnet spherical resonators or mechanical beam resonators via flux-mediated interactions. Specifically, the coupling results from the magnetic/mechanical quantum fluctuations modulating the qubit inductor, formed by a superconducting quantum interference device. We show that the resulting radiation-pressure interaction of the qubit with each mode can be employed to generate maximally entangled states of magnons or phonons. In addition, we numerically demonstrate a protocol for the preparation of magnonic and mechanical Bell states with high fidelity including realistic dissipation mechanisms. Furthermore, we have devised a scheme for reading out the prepared states using standard qubit control and resonator field displacements. In this paper, we demonstrate an alternative platform for quantum information using ECSs in hybrid magnonic and mechanical quantum networks.

The ability to transport single electrons on a quantum dot array dramatically increases the freedom in designing quantum computation schemes that can be implemented on solid-state devices. So far, however, routing schemes to precisely control the transport paths of single electrons have yet to be established. Here, we propose a silicon single-electron router that transports pumped electrons along the desired route on the branches of a T-shaped quantum dot array by inputting a synchronous phase-controlled signal to multiple gates. Notably, we show that it is possible to achieve a routing accuracy above 99% by boosting the accuracy of the electron-transport timing with an assist gate in front of the branching paths. We also evaluated the minimum error rate of routing by the model of electron transport based on the Wigner representation in an energy-time space. The results suggest possibilities for fast and accurate transport of single electrons on the two-dimensional quantum dot arrays.

${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Au}}_3$ crystallizes in the same noncentrosymmetric cubic structure as the prototypical Kondo insulator ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Pt}}_3$. Here we report the physical properties of ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Au}}_3$ single crystals using magnetization, thermodynamic, and electrical-transport measurements. Magnetic-susceptibility and heat-capacity data reveal antiferromagnetic order below $T_N=3.2\mathrm{K}$. The magnetic entropy $S_{\mathrm{mag}}$ reaches $R\ln 2$ slightly above $T_N$, which suggests localized $4f$ moments in a doublet ground state. Multiple field-induced magnetic transitions are observed at temperatures below $T_N$, which indicate a complex spin structure with competing interactions. ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Au}}_3$ shows semimetallic behavior in electrical resistivity in contrast to the majority of reported cerium-based 343 compounds which are semiconducting. Electrical-resistivity measurements under hydrostatic pressure reveal a slight enhancement of $T_N$ under pressures up to 2.3 GPa, which supports a scenario wherein ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Au}}_3$ belongs to the far left of the Doniach phase diagram dominated by Ruderman-Kittel-Kasuya-Yosida interactions. Using realistic many-body simulations, we confirm the semimetallic electronic structure of ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Au}}_3$ and quantitatively reproduce its local moment behavior in the paramagnetic state.

In a series of recent reports, doped lead apatite (LK-99) has been proposed as a candidate ambient temperature and pressure superconductor. However, from both an experimental and theoretical perspective, these claims are largely unsubstantiated. To this end, our synthesis and subsequent analysis of an LK-99 sample reveals a multiphase material that does not exhibit high-temperature superconductivity. We study the structure of this phase with single-crystal x-ray diffraction (SXRD) and find a structure consistent with doped ${\mathrm{Pb}}_{10}{\left({\mathrm{PO}}_4\right)}_6{\left(\mathrm{OH}\right)}_2$. However, the material is transparent which rules out a superconducting nature. From ab initio defect formation energy calculations, we find that the material likely hosts ${\mathrm{OH}}^{-}$ anions, rather than divalent ${\mathrm{O}}^{2-}$ anions, within the hexagonal channels and that Cu substitution is highly thermodynamically disfavored. Phonon spectra on the equilibrium structures reveal numerous unstable phonon modes. Together, these calculations suggest it is doubtful that Cu enters the structure in meaningful concentrations, despite initial attempts to model LK-99 in this way. However, for the sake of completeness, we perform ab initio calculations of the topology, quantum geometry, and Wannier function localization in the Cu-dominated flat bands of four separate doped structures. In all cases, we find they are atomically localized by irreps, Wilson loops, and the Fubini-Study metric. It is unlikely that such bands can support strong superfluidity, and instead are susceptible to ferromagnetism (or out-of-plane antiferromagnetism) at low temperatures, which we find in ab initio studies. In sum, ${\mathrm{Pb}}_9\mathrm{Cu}{\left({\mathrm{PO}}_4\right)}_6{\left(\mathrm{OH}\right)}_2$ could more likely be a magnet, rather than an ambient temperature and pressure superconductor.

Materials with mesoscopic scales have provided an excellent platform for quantum-mechanical studies. Among them, the periodic oscillations of electrical resistivity against the direct and the inverse of magnetic fields, such as the Aharonov-Bohm effect and the Shubnikov-de Haas effect, manifest the interference of the wave function relevant to the electron motion perpendicular to the magnetic field. In contrast, the electron motion along the magnetic field also leads to the magnetic-field periodicity, which is the so-called Sondheimer effect. However, the Sondheimer effect has been understood only in the framework of the semiclassical picture, and thereby its interpretation at the quasiquantum limit was not clear. Here, we show that thin-film graphite exhibits clear sinusoidal oscillations over a wide range of the magnetic fields, where conventional quantum oscillations are absent. In addition, the sample with a designed step in the middle for eliminating the stacking disorder effect verifies that the period of the oscillations is inversely proportional to the thickness, which supports the emergence of the Sondheimer oscillations in the quasiquantum limit. These findings suggest that the Sondheimer oscillations can be reinterpreted as inter-Landau-level resonances. Our results expand the quantum oscillation family and pave the way for the exploration of the out-of-plane wavefunction motion.

We examine graphene quantum dots in back-gated devices on hexagonal boron nitride (hBN) and visualize their merger using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy. These dots are formed by the combination of nanoscale potential wells created by pulsing the voltage of an STM tip above charged defects in the hBN underlayer, and strong magnetic fields gapping the density of states, which add insulating rings in the potential wells. Control of the charge state is achieved via the back gate and sample bias voltages, and the position of the STM tip which serves as a mobile top gate. The individual quantum dots present a distinct phenomenology of single-electron charging due to multiple Landau levels crossing the Fermi energy concentrically. Here, we study side by side pairs of these quantum dots via STM, where we observe a tunable interdot coupling and mergeability. Specifically, with increasing charge filling, the quantum dots formed by electrons belonging to one Landau level merge into a single quantum dot, while the electrons in the next-higher Landau level remain spatially separated into two charge pockets. Using the probe tip as a multifunction tool, we visualize the evolution, growth, and merger of this unique double quantum dot system as a function of tip position and gate voltages.

We explore a class of “open” quantum circuit models with local decoherence (“noise”) and local projective measurements, each respecting a global ${\mathbb{Z}}_2$ symmetry. The model supports a spin-glass phase where the ${\mathbb{Z}}_2$ symmetry is spontaneously broken [not possible in an equilibrium one-dimensional (1D) system], a paramagnetic phase characterized by a divergent susceptibility, and an intermediate “trivial” phase. All three phases are also stable to ${\mathbb{Z}}_2$-symmetric local unitary gates, and the dynamical phase transitions between the phases are in the percolation universality class. The open circuit dynamics can be purified by explicitly introducing a bath with its own “scrambling” dynamics, which does not change any of the universal physics. Within the spin-glass phase the circuit dynamics can be interpreted as a quantum repetition code, with each stabilizer of the code measured stochastically at a finite rate, and the decoherences as effective bit-flip errors. Motivated by the geometry of the spin-glass phase, we devise a decoding algorithm for recovering an arbitrary initial qubit state in the code space, assuming knowledge of the history of the measurement outcomes, and the ability of performing local Pauli measurements and gates on the final state. For a circuit with $L^d$ qubits running for time $T$, the time needed to execute the decoder scales as $O\left(L^{d}T\right)$ (with dimensionality $d$). With this decoder in hand, we find that the information of the initial encoded qubit state can be retained (and then recovered) for a time logarithmic in $L$ for a 1D circuit, and for a time at least linear in $L$ in two dimensions below a finite-error threshold. For both the repetition and toric codes, we compare and contrast our decoding algorithm with earlier algorithms that map the error model to the random bond Ising model.

In positive muon spin rotation and relaxation (${\mu }^{+}\mathrm{SR}$) spectroscopy, positive muons (${\mu }^{+}$) implanted into solid oxides are conventionally treated as immobile spin probes at interstitial sites below room temperature. This is because each ${\mu }^{+}$ is thought to be tightly bound to an oxygen atom in the host lattice to form a muonic analog of the hydroxy group. On the basis of this concept, anomalies in ${\mu }^{+}\mathrm{SR}$ spectra observed in oxides have been attributed in most cases to the intrinsic properties of host materials. On the other hand, global ${\mu }^{+}$ diffusion with an activation energy of $\sim 0.1\mathrm{eV}$ has been reported in some chemically substituted perovskite oxides at cryogenic temperatures, although the reason for the small activation energy despite the formation of the strong $\mathrm{O}\mu$ bond has not yet been quantitatively understood. In this study, we investigated interstitial ${\mu }^{+}$ diffusion in the perovskite oxide lattice using ${\mathrm{KTaO}}_3$ cubic perovskite as a model system. We used the ${\mu }^{+}\mathrm{SR}$ method and density functional theory calculations along with the harmonic transition state theory to study this phenomenon both experimentally and theoretically. Experimental activation energies for global ${\mu }^{+}$ diffusion obtained below room temperature were less than a quarter of the calculated classical potential barrier height for a bottleneck ${\mu }^{+}$ transfer path. The reduction in the effective barrier height could be explained by the harmonic transition state theory with a zero-point energy correction; a significant difference in zero-point energies for ${\mu }^{+}$ at the positions in the $\mathrm{O}\mu$ bonding equilibrium state and a bond-breaking transition state was the primary cause of the reduction. This suggests that the assumption of immobile ${\mu }^{+}$ in solid oxides is not always satisfied since such a significant decrease in diffusion barrier height can also occur in other oxides.

Excitons are prevalent in semiconductors and insulators, and their binding energies are critical for optoelectronic applications. The state-of-the-art method for first-principles calculations of excitons in extended systems is the ab initio $GW$-Bethe-Salpeter equation (BSE) approach, which can require a fine sampling of reciprocal space to accurately resolve solid-state exciton properties. Here we show, for a range of semiconductors and insulators, that the commonly employed approach of uniformly sampling the Brillouin zone can lead to underconverged exciton binding energies, as impractical grid sizes are required to achieve adequate convergence. We further show that nonuniform sampling of the Brillouin zone, focused on the region of reciprocal space where the exciton wave function resides, enables efficient rapid numerical convergence of exciton binding energies at a given level of theory. We propose a well-defined convergence procedure, which can be carried out at relatively low computational cost and which in some cases leads to a correction of previous best theoretical estimates by almost a factor of 2, qualitatively changing the predicted exciton physics. These results call for the adoption of nonuniform sampling methods for ab initio $GW$-BSE calculations and for revisiting previously computed values for exciton binding energies of many systems.

Motivated by the recently reported high-temperature superconductivity in the bilayer ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ (LNO) under pressure, here we comprehensively study this system using ab initio techniques. The Ni $3d$ orbitals have a large bandwidth at ambient pressure, increasing by $\sim 22\%$ at 29.5 GPa. Without electronic interactions, the Ni $d_{3z^2-r^2}$ orbitals form a bonding-antibonding molecular orbital state via the O $p_z$ inducing a “dimer” lattice in the LNO bilayers. The Fermi surface consists of two-electron sheets with mixed $e_g$ orbitals and a hole pocket defined by the $d_{3z^2-r^2}$ orbital, suggesting a Ni two-orbital minimum model. Different from the infinite-layer nickelate, we obtained a large interorbital hopping between $d_{3z^2-r^2}$ and $d_{x^2-y^2}$ states in LNO, caused by the ligand “bridge” of in-plane O $p_x$ or $p_y$ orbitals connecting those two orbitals, inducing $d\text{−}p$ $\sigma$-bonding characteristics. The competition between the intraorbital and interorbital hoppings leads to an interesting dominant spin stripe $(\pi ,0)$ order because of bond ferromagnetic tendencies via the recently discussed “half-empty” mechanism.

We provide a prescription to construct Rényi and von Neumann entropy of a system of interacting fermions from a knowledge of its correlation functions. We show that Rényi entanglement entropy of interacting fermions in arbitrary dimensions can be represented by a Schwinger-Keldysh free energy on replicated manifolds with a current between the replicas. The current is local in real space and is present only in the subsystem which is not integrated out. This allows us to construct a diagrammatic representation of entanglement entropy in terms of connected correlators in the standard field theory with no replicas. This construction is agnostic to how the correlators are calculated, and one can use calculated, simulated, or measured values of the correlators in this formula. Using this diagrammatic representation, one can decompose entanglement into contributions which depend on the one-particle correlator, two-particle correlator, and so on. We provide an analytic formula for the one-particle contribution and a diagrammatic construction for higher-order contributions. We show how this construction can be extended for von Neumann entropy through analytic continuation. For a practical implementation of a quantum state, where one usually has information only about few-particle correlators, this provides an approximate way of calculating entanglement commensurate with the limited knowledge about the underlying quantum state.

We propose another class of magnetic proximity effects based on the spin-dependent hybridization between the electronic states at the Fermi energy in a nonmagnetic conductor and the narrow spin-split bands of a ferromagnetic insulator. Unlike conventional exchange proximity, we show this hybridization proximity effect has a very strong influence on the nonmagnetic layer and can be further modulated by application of an electric field. We use density functional theory calculations to illustrate this effect in graphene placed next to a monolayer of ${\mathrm{CrI}}_3$, a ferromagnetic insulator. We find strong hybridization of the graphene bands with the narrow conduction band of ${\mathrm{CrI}}_3$ in one spin channel only. We show that our results are robust with respect to lattice mismatch and twist angle variations. Furthermore, we show that an out-of-plane electric field can be used to modulate the hybridization strength, paving the way for applications.

We study the effects of quantum fluctuations on the transport properties of multiband superconductors near a pair-breaking quantum critical point. For this purpose, we consider a minimal model of the quantum phase transition in a system with two nested two-dimensional Fermi surfaces. Under the assumption that doping the system adds nonmagnetic impurities but does not change the densities of carriers, we include disorder potentials that render both intra- and interband collisions. Interband scattering leads to full suppression of the unconventional $s^{\pm }$ superconducting order similar to the effect of paramagnetic impurities in isotropic single-band superconductors. We use the diagrammatic technique of quantum field theory to compute the corrections to electrical conductivity in a normal state due to superconducting fluctuations in the entire low-temperature quantum regime. We show that the sign of the conductivity correction depends on how the quantum critical point is approached in the phase diagram. We contrast our findings to existing approaches to this problem based on the renormalization group, time-dependent Ginzburg-Landau phenomenology, and effective bosonic action field theories.