Recently, an 80 K superconductor was discovered in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ under high pressure. Density function theory calculations identify $d_{x^2-y^2},d_{z^2}$ as the active orbitals on the bilayer square lattice with a $d^{8-x}$ configuration of Ni per site. Here, $x$ is the hole doping level. One naive expectation is to describe this system in terms of a two-orbital $t\text{−}J$ model. However, we emphasize the importance of Hund's coupling $J_H$ and the $x=0$ limit should be viewed as a spin-one Mott insulator. Especially, the significant Hund's coupling shares the interlayer superexchange $J_{\perp }$ of the $d_{z^2}$ orbital to the $d_{x^2-y^2}$ orbital, an effect that cannot be captured by conventional perturbation or mean-field approaches. This study first explores the limit where the $d_{z^2}$ orbital is Mott localized, dealing with a one-orbital bilayer $t\text{−}J$ model focused on the $d_{x^2-y^2}$ orbital. Notably, we find that strong interlayer pairing survives up to $x=0.5$ hole doping driven by the transmitted $J_{\perp }$, which explains the existence of a high Tc superconductor in the experiment at this doping level. Next, we uncover the more realistic situation where the $d_{z^2}$ orbital is slightly hole-doped and cannot be simply integrated out. We take the $J_H\to +\infty$ limit and propose a type II $t\text{−}J$ model with four spin-half singlon $\left(d^7\right)$ states and three spin-one doublon $\left(d^8\right)$ states. Employing a parton mean-field approach, we recover similar results as in the one-orbital $t\text{−}J$ model, but now with the effect of the $J_{\perp }$ automatically generated.

Fractional Chern insulators realize the remarkable physics of the fractional quantum Hall effect (FQHE) in crystalline systems with Chern bands. The lowest Landau level (LLL) is known to host the FQHE, but not all Chern bands are suitable for realizing fractional Chern insulators (FCI). Previous approaches to stabilizing FCIs focused on mimicking the LLL through momentum-space criteria. Here, instead, we take a real-space perspective by introducing the notion of vortexability. Vortexable Chern bands admit a fixed operator that introduces vortices into any band wavefunction while keeping the state entirely within the same band. Vortexable bands admit trial wavefunctions for FCI states, akin to Laughlin states. In the absence of dispersion and for sufficiently short-ranged interactions, these FCI states are the ground state—independent of the distribution of Berry curvature. Vortexable bands are much more general than the LLL, and we showcase a recipe for constructing them. We exhibit diverse examples in graphene-based systems with or without magnetic field, and with any Chern number. A special class of vortexable bands is shown to be equivalent to the momentum-space “trace condition” or “ideal band condition”. In addition, we also identify a more general form of vortexability that goes beyond this criterion. We introduce a modified measure that quantifies deviations from general vortexability, which can be applied to generic Chern bands to identify promising FCI platforms.

We theoretically and experimentally study the universal scaling property of the spin-$\frac{1}{2}$ Kondo state in the magnetic field dependence of bias-voltage linear conductance through a quantum dot at low temperatures. We discuss an efficient and reliable procedure to evaluate the Kondo temperature defined at the ground state from experimental or numerical data sets of the magnetic field dependence of the linear conductance or the magnetization of the quantum dot. This procedure is helpful for quantitative comparison of the theory and the experiment, and useful in Kondo-correlated systems where temperature control over a wide range is difficult, such as for cold atoms. We demonstrate its application to experimentally measure electric current through a carbon nanotube quantum dot.

Recent observations of Kitaev interactions in $d^7$ electron configurations have generated research interest in ${\mathrm{Co}}^{2+}$ based honeycomb magnets. Here, we report the synthesis of a ${\mathrm{K}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ single crystal. X-ray diffraction and electron microscopy experiments show that ${\mathrm{K}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ crystallizes in a $P{6}_3/mcm$ space group with undistorted honeycomb layers of ${\mathrm{Co}}^{2+}$ ions. Anisotropic magnetic and thermodynamic characterizations demonstrate an effective $S=\frac{1}{2}$ state of ${\mathrm{Co}}^{2+}$ at low temperature, competing magnetic phases, and weak magnetic transitions that can be suppressed by applying fields. The interlayer K disorder level is found to be tunable by thermal treatments, and it plays detectable effects on magnetic behaviors. Our discoveries offer a promising platform for exploring intricate frustrated magnetism, intertwined structural orderings, and Kitaev interactions.

Over the past decade, there has been great interest in topological effects, with concepts originally developed in the context of electron transport in condensed matter platforms now being extended to optical systems. While topological properties in electronic systems are often linked to the quantization of electric conductivity observed in the integer quantum Hall effect, a direct analog in optics remains elusive. In this paper, we bridge this gap by demonstrating that the response of the Poynting vector (which may be regarded as a photon current) to mechanical acceleration of a medium provides a precise photonic analog of the electric conductivity. It is shown that the photonic conductivity determines the energy irreversibly transferred from periodic mechanical driving of the medium to the electromagnetic field. Furthermore, it is demonstrated that, for nonreciprocal systems enclosed in a cavity, constant acceleration of the system induces a flow of photons along a direction perpendicular to acceleration, analogous to the Hall effect but for light. The spectral density of the photonic conductivity is quantized in the band gaps of the bulk region with the conductivity quantum determined by the gap Chern number.

We investigated the in-plane thermal conductivity of surface plasmon polaritons (SPPs) propagating along thin Au and Ag films on a ${\mathrm{SiO}}_2$ substrate with a Ti adhesive layer. To determine the propagation length and skin depth of SPPs along Au and Ag thin films, we numerically solved the dispersion relation while considering the size effect of the permittivity of metal. Additionally, we derived the spatial distribution of SPPs along the film thickness to analyze the effect of the Ti adhesive layer on the plasmon thermal conductivity of Au and Ag thin films. Our theoretical predictions revealed a decrease of approximately 30% in plasmon thermal conductivity when considering the size effect of the permittivity of thin metal films. Furthermore, this causes the film thickness at which maximum thermal conductivity occurs to increase by about 30%. Taking these factors into account, we calculated the optimal thickness of Au and Ag films, along with Ti adhesive layers, on ${\mathrm{SiO}}_2$ substrates to be approximately 20 nm. By fabricating a sample with the optimal thickness of Au and Ag films, we experimentally demonstrated that the plasmon thermal conductivity of Au and Ag films can be as high as about 20% of their electron contribution. This research will broaden the thermal design applications of ballistic thermal transport by SPPs propagating along thin metal coatings in microelectronics.

There is a heated debate on the Josephson effect in twisted ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+x}$ flakes. Latest results suggest the presence of either anomalously isotropic pairing or exotic $d+\mathrm{i}d$-wave pairing, in addition to the commonly believed $d$-wave one. Here, we address this controversy by fabricating ultraclean junctions with uncompromised crystalline quality at the junction interface. In the optimally doped regime, we obtain prominent Josephson coupling (2–4 mV) in multiple junctions with the twist angle of ${45}^{\circ }$, in sharp contrast to a recent report that shows two orders of magnitude suppression around ${45}^{\circ }$ from the value at $0^{\circ }$. We further extend this study to the previously unexplored overdoped regime and observe pronounced Josephson tunneling at ${45}^{\circ }$ together with Josephson diode effect up to 50 K. Our study helps establish the persistent presence of an isotropic pairing component across the entire superconducting phase diagram.

The Ising chain realizes the fundamental paradigm of spin fractionalization, where locally flipping a spin creates two domain walls (spinons) that can separate apart at no energy cost. In a quasi-one-dimensional system, the mean-field effects of the weak three-dimensional couplings confine the spinons into a Zeeman ladder of two-spinon bound states. Here, we experimentally tune the confinement potential between spinons in the quasi-one-dimensional Ising ferromagnet $\mathrm{Co}{\mathrm{Nb}}_2{\mathrm{O}}_6$ by means of an applied magnetic field with a large component along the Ising direction. Using high-resolution single crystal inelastic neutron scattering, we directly observe how the spectrum evolves from the limit of very weak confinement at low field (with many closely spaced bound states with energies scaling as the field strength to the power 2/3) to very strong confinement at high field (where it consists of a magnon and a dispersive two-magnon bound state, with a linear field dependence). At intermediate fields, we explore how the higher-order bound states disappear from the spectrum as they move to higher energies and overlap with the two-particle continuum. By performing a global fit to the observed spectrum in zero field and high field applied along two orthogonal directions, combined with a quantitative parametrization of the interchain couplings, we propose a refined single-chain and interchain Hamiltonian that quantitatively reproduces the dispersions of all observed modes and their field dependence.

Inspired by coupled-layer constructions of fracton orders, we introduce the $X$-cube Floquet code, a dynamical quantum error-correcting code where the number of encoded logical qubits grows with system size. The $X$-cube Floquet code is defined on a three-dimensional lattice, built from intersecting two-dimensional layers in the $xy$, $yz$, and $xz$ directions, and consists of a periodic sequence of two-qubit measurements which couple the layers together. Within a single Floquet cycle, the code space switches between that of the $X$-cube fracton order and layers of entangled, two-dimensional toric codes. The encoded logical qubits' dynamics are analyzed, and we argue that the new code has a nonzero error threshold.

Conduction electrons interacting with a dynamic impurity can give rise to a local Fermi liquid. The latter has the same low-energy spectrum as an ideal Fermi gas containing a static impurity. The Fermi liquids's elementary excitations are, however, not bare electrons. In the vicinity of the impurity, they are dressed by virtual particle-hole pairs. Here we study this dressing. Among other things, we construct a mode-resolved measure of dressing. To evaluate it in position representation, we have to circumvent the limitations of the numerical renormalization group, which discretizes the conduction band logarithmically. We therefore extend natural orbital methods that successfully characterize the ground state, to describe excitations. We demonstrate that the dressing profile shows nontrivial power-law decay at large distances. Our natural orbital methodology could lay the foundation for calculating the properties of local Fermi-liquid quasiparticles in nontrivial geometries such as disordered hosts or mesoscopic devices.

Reflecting crystal and electronic structures, high harmonic generation (HHG) in solids show intriguing behaviors that are absent in isotropic gases, and are expected to be novel light sources and spectroscopic tools. Recent studies reported that the efficiency of HHG in some materials can be enhanced by using elliptically polarized excitations, which we call “elliptical enhancement”, while the mechanism underlying the phenomenon remains unknown. Here, we unambiguously demonstrate that the elliptical enhancement occurs in an archetypal direct-gap semiconductor GaAs and is accompanied by the emergence of nonlinear optical activity, despite that GaAs hosts no magnetization or linear birefringence. Remarkably, the elliptical enhancement becomes pronounced only at particular intermediate excitation strengths. The excitation-strength dependence and polarization state of the harmonics indicate that interference among multiple nonlinear emission processes plays a key role. Such an interference of nonlinear processes can be controlled by changing the ellipticity of excitation field when the HHG occurs in the region of anisotropic band structure, and hence, elliptical enhancement occurs effectively in higher-order HHG. Further, microscopic calculations imply that such interfering nonlinear processes are rooted in the interband transitions among multiple bands of a solid (in the case of GaAs, transitions between the conduction and light-, heavy-, split-off hole bands). Our study reveals that, besides the material properties, an optimal-strength excitation that can be used to fine tune the nonlinear dynamics is especially important for controlling HHG properties, providing an alternative perspective for ultrafast light-control of solid-state materials.

In the cuprate superconductors, the spatial coherence of the charge density wave (CDW) state grows below a temperature $T_{\mathrm{CDW}}$, the origin of which is debated. Using x-ray photon correlation spectroscopy, we have studied the temporal atomic relaxation dynamics in ${\mathrm{La}}_{1.88}{\mathrm{Sr}}_{0.12}{\mathrm{CuO}}_4$ to shed light on this question. Cooling within an emergent structurally distorted phase, which favors the CDW modulation in symmetry and develops in two stages between 180 and 120 K, we observe a crossover from cooperativelike to incoherentlike relaxation dynamics at $T_{\mathrm{CDW}}=75\left(10\right) \mathrm{K}$ . We argue that, if the CDW is hosted by this distortion, the concomitant relaxational crossover and enhancement of CDW spatial coherence supports the interplay between relaxational atomic fluctuations and CDWs in materials of this class on quasistatic times cales.

${\mathrm{CsO}}_2$ is a member of the family of alkali superoxides (formula ${A\mathrm{O}}_2$ with $A=\mathrm{Na}, \mathrm{K}, \mathrm{Rb}, \mathrm{and} \mathrm{Cs}$) that exhibit magnetic behavior arising from open $p$-shell electrons residing on ${\mathrm{O}}_2^{-}$ molecules. We use neutron diffraction to solve the crystal and magnetic structures of ${\mathrm{CsO}}_2$ and observe a complex series of structures on cooling from room temperature to 1.6 K. These include an incommensurate modulation along the $a$ axis of the structure at intermediate temperatures, which then locks into a commensurate modulation that doubles the unit cell compared to the previously supposed orthorhombic unit cell. In both incommensurate and commensurate phases, our structural solution involves a staggering of the cesium ion positions along the $b$ axis, in contrast to studies of other alkali superoxides in which staggered tilts of the ${\mathrm{O}}_2^{-}$ dimers relative to the $c$ axis are seen. Below $T\simeq 10 \mathrm{K}$, we observe magnetic Bragg reflections arising from an antiferromagnetically ordered structure with a wave vector of $\mathbf{k}=(0,0,0)$ (relative to the doubled crystallographic unit cell), with moments that point predominantly along the $b$ axis with a small component along the $a$ axis that hints at possible anisotropic exchange coupling (consistent with the crystal structure). Measurements of the magnetic Bragg reflections in an applied magnetic field suggest a spin-flop transition takes place between 2 T and 4 T in which moments likely flop to point along the crystallographic $a$ axis. Our measurements indicate that ${\mathrm{CsO}}_2$ is an interesting example of magnetic properties being inherently linked to the crystal structure, in that the staggered displacement of the cesium ions activates antisymmetric exchange, which then permits the observed spin canting.

Under time-reversal symmetry, quantum spin Hall edge channels are protected against elastic backscattering. However, even for samples which exhibit conductance quantization due to the quantum spin Hall effect, reproducible fluctuations shape the quantization plateau when the chemical potential is tuned through the bulk gap. Here, we examine those fluctuations in micron-sized HgTe quantum well devices. By performing temperature- and gate-dependent measurements, we conclude that “charge puddles” in the narrow-gap material have a Kondo-type interaction with the edge channels resulting in the observed conductance fluctuations. Our results provide insight into the underlying mechanisms of scattering in quantum spin Hall edge channels.

The temperature dependence of the zero-field splitting (ZFS) between the $|m_s=0\rangle$ and $|m_s=\pm 1\rangle$ levels of the nitrogen-vacancy (NV) center's electronic ground-state spin triplet can be used as a robust nanoscale thermometer in a broad range of environments. However, despite numerous measurements of this dependence in different temperature ranges, to our knowledge no analytical expression has been put forward that captures the scaling of the ZFS of the NV center across all relevant temperatures. Here we present a simple, analytical, and physically motivated expression for the temperature dependence of the NV center's ZFS that matches all experimental observations, in which the ZFS shifts in proportion to the occupation numbers of two representative phonon modes. In contrast to prior models our expression does not diverge outside the regions of fitting. We show that our model quantitatively matches experimental measurements of the ZFS from 15 to 500 K in single NV centers in ultrapure bulk diamond, and we compare our model and measurements to prior models and experimental data.

We report the formation of a pinned vortex liquid spanning a very large region of the magnetic field-temperature parameter space in a 5-nm-thick amorphous superconducting ${\mathrm{Re}}_x\mathrm{Zr}$ ($x\approx 6$) ($a$-ReZr) thin film, using a combination of low-temperature scanning tunneling spectroscopic (STS) imaging and magnetotransport measurements. The nature of the vortex liquid differs significantly from a regular liquid. Analyzing series of STS images captured as a function of time, we observe that the interplay of pinning and intervortex interactions produces a very inhomogeneous state, where some vortices remain static, whereas others move forming a percolating network along which vortices are mobile. With increase in temperature or magnetic field this network becomes denser, eventually encompassing all vortices. Our results provide key insight on the nature of a pinned vortex liquid and some of the peculiarities in the transport properties of ultrathin superconducting films.

External monitoring of quantum many-body systems can give rise to a measurement-induced phase transition characterized by a change in behavior of the entanglement entropy from an area law to an unbounded growth. In this paper, we show that this transition extends beyond bipartite correlations to multipartite entanglement. Using the quantum Fisher information, we investigate the entanglement dynamics of a continuously monitored quantum Ising chain. Multipartite entanglement exhibits the same phase boundaries observed for the entropy in the postselected no-click trajectory. Instead, quantum jumps give rise to a more complex behavior that still features the transition, but adds the possibility of having a third phase with logarithmic entropy but bounded multipartiteness.

Theoretical studies suggest that Su-Schrieffer-Heeger-like electron-phonon ($e$-ph) interactions can mediate high-temperature bipolaronic superconductivity that is robust against repulsive electron-electron interactions. Here we present a comparative analysis of the pairing and competing charge/bond correlations in the two-dimensional Holstein and optical Su-Schrieffer-Heeger (SSH) models using numerically exact determinant quantum Monte Carlo. We find that the SSH interactions support light bipolarons and strong superconducting correlations out to relatively large values of the $e$-ph coupling $\lambda$ and densities near half-filling, while the Holstein interaction does not due to the formation of heavy bipolarons and competing charge-density-wave order. We further find that the Holstein and SSH models have comparable pairing correlations in the weak coupling limit for carrier concentrations $\langle n\rangle \ll 1$, where competing orders and polaronic effects are absent. These results support the proposal that SSH (bi)polarons can support superconductivity to larger values of $\lambda$ in comparison to the Holstein polaron, but that the resulting $T_{\mathrm{c}}$ gains are small in the weak coupling limit. We also find that the SSH model's pairing correlations are suppressed after including a weak on-site Hubbard repulsion. These results have important implications for identifying and engineering bipolaronic superconductivity.

Two-dimensional materials offer a unique range of magnetic, electronic, and mechanical properties which can be controlled by external stimuli. Pressure is a particularly important stimulus, as it can be achieved readily and can produce large responses, especially in low-dimensional materials. In this paper, we explore the pressure dependence of the structural and magnetic properties of a two-dimensional van der Waals (vdW) molecular framework antiferromagnet with ferromagnetic layers, $\mathrm{Ni}{\left(\mathrm{NCS}\right)}_2$, up to 8.4 kbar. Through a combination of x-ray and neutron diffraction analysis, we find that $\mathrm{Ni}{\left(\mathrm{NCS}\right)}_2$ is significantly more compressible than comparable vdW metal halides, and its response is anisotropic not only out of the plane, but also within the layers. Using bulk magnetization and neutron diffraction data, we show that the ambient layered antiferromagnetic phase is maintained up to the largest investigated pressure, but with an enhanced Néel temperature, $T_{\mathrm{N}}$ ($\mathrm{\Delta }{T}_{\mathrm{N}}/T_{\mathrm{N}}=+19\%$), and a large pressure sensitivity ($Q=\frac{1}{T_{\mathrm{N}}}\frac{d{T}_{\mathrm{N}}}{dP}=+2.3\%{\mathrm{kbar}}^{-1}$), one of the larger values of magnetic pressure responsiveness for a vdW material. Density functional theory calculations suggest that this is due to increasing three dimensionality. These results provide insights into the pressure response of molecular framework vdW magnets and suggest that the investigation of other molecular framework vdW magnets might uncover contenders for future pressure-switchable devices.

Magnetic skyrmions are nano-sized topologically nontrivial spin textures that can be moved by external stimuli such as spin currents and internal stimuli such as spatial gradients of a material parameter. Since the total energy of a skyrmion depends linearly on most of these parameters, like the perpendicular magnetic anisotropy, the exchange constant, or the Dzyaloshinskii-Moriya interaction strength, a skyrmion will move uniformly in a weak parameter gradient. In this paper, we show that the linear behavior changes once the gradients are strong enough so that the magnetic profile of a skyrmion is significantly altered throughout the propagation. In that case, the skyrmion experiences acceleration and moves along a curved trajectory. Furthermore, we show that when spin-orbit torques and material parameter gradients trigger a skyrmion motion, it can move on a straight path along the current or gradient direction. We discuss the significance of suppressing the skyrmion Hall effect for spintronic and neuromorphic applications of skyrmions. Lastly, we extend our discussion and compare it to a gradient generated by the Dzyaloshinskii-Moriya interaction.