Recent studies have highlighted the potential for achieving room-temperature superconductivity in the clathrate hydride prototype $A{B}_2{\mathrm{H}}_{24}$ with the $P6/mmm$ space group symmetry. To investigate the underlying superconducting mechanism for this class of frameworks, we performed comprehensive first-principles calculations by systematically substituting metal elements from groups I to IV of the periodic table. The results identify 21 dynamically stable compounds at 300 GPa, exhibiting an extraordinary variation in superconducting critical temperatures ($T_{\mathrm{c}}$), ranging from 276 K in $\mathrm{AcS}{\mathrm{c}}_2{\mathrm{H}}_{24}$ to 2 K in ${\mathrm{ThCe}}_2{\mathrm{H}}_{24}$. Through detailed structural analysis, we proposed two key model parameters, $S_{\text{couple}}$ and coupling strength (CS), to elucidate the strong correlation with superconducting properties (showing correlation coefficients of 0.87 and 0.93 with respect to $T_{\mathrm{c}}$). These model parameters are further demonstrated to be extendable for other clathrate-type hydrides, such as $\mathrm{L}{\mathrm{i}}_{2}M{\mathrm{H}}_{16/17}$ and binary superhydrides ${M\mathrm{H}}_6$, ${M\mathrm{H}}_9$ and ${M\mathrm{H}}_{10}$. The proposed CS shows strong predictive capability, achieving an overall correlation coefficient of 0.85 across different clathrate hydride systems. These findings provide insights into the fundamental mechanisms governing the superconducting properties of clathrate hydrides, offering useful guidance for future superconductor design.

Measurements can qualitatively alter correlations and entanglement emerging in gapless quantum matter. We show how a single round of measurements on gapless quantum systems can, upon rotating the measurement basis, induce nontrivial transitions separating regimes displaying universal characteristics governed by distinct boundary conformal field theories. We develop the theory of such “measurement-induced boundary transitions” by investigating a gapless parent of the one-dimensional cluster state, obtained by appropriately symmetrizing a commuting projector Hamiltonian for the latter. Projective measurements on the cluster state are known to convert the wave function, after post-selection or decoding, into a long-range-ordered Greenberger-Horne-Zeilinger (GHZ) state. Similar measurements applied to the gapless parent (i) generate long-range order coexisting with power-law correlations when post-selecting for uniform outcomes, and (ii) yield power-law correlations distinct from those in the pre-measurement state upon decoding. In the post-selection scenario, rotating the measurement basis preserves long-range order up until a critical tilt angle marking a measurement-induced boundary transition to a power-law-ordered regime. Such a transition—which does not exist in the descendant cluster state—establishes new connections between measurement effects on many-body states and nontrivial renormalization-group flows. We extend our analysis to tricritical Ising and three-state Potts critical theories, which also display measurement-induced boundary transitions, and propose general criteria for their existence in other settings.

The layered nature and simple structure of FeSe reveal this iron-based superconductor as a unique building block for the design of artificial heterostructure materials. While superconductivity develops in ultrathin films of FeSe on $\mathrm{SrTi}{\mathrm{O}}_3$ substrates, it remains unclear whether it can be developed on more chemically inert, layered materials such as graphene. Here, we report on the characterization of the structural, chemical, and electronic properties of few-layer and multilayer FeSe on bilayer graphene grown on SiC(0001) using low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) and x-ray photoelectron spectroscopy (XPS). STM imaging of our FeSe films with thicknesses up to three layers exhibits the tetragonal crystal structure of bulk FeSe, which is supported by XPS spectra consistent with the FeSe bulk counterpart. While our STS measurements at 340 mK reveal a metallic character for few-layer FeSe on BLG/SiC(0001), they show an absence of superconductivity, as the low-lying electronic structure exhibits a spatially anisotropic dip-like feature robust against magnetic fields. However, we find that superconductivity in FeSe/BLG/SiC(0001) starts to emerge from a thickness of five layers. Cooper pairing strength increases with film thickness, reaching critical temperatures up to 6 K above 20 layers. Our results underscore the significance of the substrate and thickness as key factors driving the suppression of superconductivity in FeSe in the 2D limit.

We study the excitation spectrum of the $S=1/2$ Heisenberg model on the randomly diluted square lattice by stochastic analytic continuation of imaginary-time correlations obtained by quantum Monte Carlo simulations in the low-temperature limit. Focusing on relatively low dilution fractions, $p=1/16$ and $p=1/8$, the dynamic structure factor $S(\mathit{q},\omega )$ exhibits a strongly damped magnon peak with anomalous dispersion near $\mathit{q}=(0,0)$ and $(\pi ,\pi )$, a nondispersive low-energy peak from localized excitations, and a second peak within the continuum connecting these two features. A magnon with anomalous logarithmic dispersion, close to our result, was predicted in spin wave and $T$-matrix theory [Chernyshev et al., Phys. Rev. B 65, 104407 (2002)], with the quasiparticle description breaking down at the low localization energy. However, no intermediate mode was predicted. Analyzing spectral functions in real space for individual vacancy realizations by energy tomography, we find that these excitations are concentrated on a subset of the spins adjacent to vacancies, with fewer spins involved as the energy is lowered. We argue that the low-energy excitations are those of a sparse random network of effective moments at a fraction of the vacancies, leading to a damped diffusive-like behavior with localization at the lowest energies (the localized mode). In the case of the magnon, there is a shift in the real-space spectral weight distribution, from the spins away from vacancies at high energy to those adjacent to vacancies at lower energy. We also analyze the Anderson quantum rotor excitation at $\omega \propto N^{-1}$ (with $N=L^2$ the system size), which in the clean system is visible in $S(\mathit{q},\omega )$ only at $\mathit{q}=(\pi ,\pi )$ but spreads through the entire Brillouin zone due to the lack of translational invariance when $p>0$. Beyond weight close to $\mathit{q}=(0,0)$ and $(\pi ,\pi )$, which we explain by local sublattice imbalance within a classical dimer-monomer model, there is also intricate structure arising from correlated singlet fluctuations, which we demonstrate by enhancing said fluctuations with four-spin couplings in the Hamiltonian. The features in momentum space reflect spatially nonuniform breaking of the spin rotation symmetry and should be experimentally observable with elastic neutron scattering on layered quantum antiferromagnets doped with nonmagnetic impurities. All the $\omega >0$ excitations should be observable by inelastic neutron scattering. Technically, our work demonstrates that surprisingly complex spectral information can be obtained from quantum Monte Carlo data, despite the “ill-posed” numerical analytic continuation problem.

We perform a systematic study of the signatures of fragile topology in over 50 nonmagnetic two-dimensional materials with formula ${\mathrm{AB}}_2$, belonging to space group $P\overline{3}m1$. Using group theory analysis in the framework of topological quantum chemistry, we find fragile bands near the Fermi level for all the materials studied. Since stable topological bands are also present in these systems, the interplay of both phases is discussed, showing that corner charges appear in over 80% of the materials and are linked to fragile topology. Using first-principles calculations, we predict fractionally charged corner charges protected by $C_3$ symmetry. Our work aims to broaden the scope of materials with experimentally accessible fragile bands.

The Chern vector is a vectorial generalization of the scalar Chern number, being able to characterize the topological phases of three-dimensional (3D) Chern insulators. Such a vectorial generalization extends the applicability of Chern-type bulk-boundary correspondence from one-dimensional edge states to two-dimensional surface states, whose unique features, such as robust chiral surface states and forming nontrivial torus knots or links in the surface Brillouin zone, have been demonstrated recently in 3D photonic crystals. However, to date, the Chern vectors have been limited to the simplest form with only one nonzero component, resulting in the chiral surface states only propagating along four side surfaces parallel to the Chern vector, and the surface-state torus knots or links emerging, not on the surface of a single crystal as in other 3D topological phases, but only along an internal domain wall between two crystals with perpendicular Chern vectors. Here, we extend the 3D Chern insulator phase to acoustic crystals for sound waves and propose a scheme to construct an arbitrary Chern vector with multiple nonzero components that allow the chiral surface states to propagate along all six surfaces and the emergence of surface-state torus knots or links on the surface of a single crystal. These results provide a complete picture of bulk-boundary correspondence for Chern vectors and may have potential applications in designing topological acoustic devices.

Josephson junctions (JJs) with nonsinusoidal current-phase relations (CPRs) have gathered increasing attention, partly due to growing interest in topological two-dimensional materials. Understanding how CPRs and inhomogeneities in JJs influence their response is crucial for accurate interpretation of experimental observations. This work reports that a nonsinusoidal CPR, combined with asymmetries in the JJ, can break spatial symmetry and give rise to the Josephson diode effect (JDE) in the short-junction regime. This nonreciprocity is shown to emerge as an intrinsic mechanism related to the maximization of the supercurrent, rather than being solely driven by geometric or material asymmetries. Further analysis shows that JDE efficiency is strongly influenced by the CPR shape but is largely insensitive to junction asymmetry, making the observed nonreciprocity not only a potential experimental signature of unconventional CPRs but also a possible method for probing their properties.

Spin qubits based on valence band hole states are highly promising for quantum information processing due to their strong spin-orbit coupling and ultrafast operation speed. As these systems scale up, achieving high-fidelity single-qubit operations becomes essential. However, mitigating crosstalk effects from neighboring qubits in larger arrays, particularly for anisotropic qubits with strong spin-orbit coupling, presents a significant challenge. We investigate the impact of crosstalk on qubit fidelities during single-qubit operations and derive an analytical equation that serves as a synchronization condition to eliminate crosstalk in anisotropic media. Our analysis proposes optimized driving field conditions that can robustly synchronize Rabi oscillations and minimize crosstalk, showing a strong dependence on qubit anisotropy and the orientation of the external magnetic field. Taking experimental data into our analysis, we identify a set of parameter values that enable nearly crosstalk-free single-qubit gates, thereby paving the way for scalable quantum computing architectures.

In this work, we consider superconductor/flat band material/superconductor (S/FB/S) Josephs on junctions (JJs) where the flat band material possesses isolated flat bands with exactly zero Fermi velocity. Contrary to conventional S/N/S JJs where the critical Josephson current vanishes when the Fermi velocity goes to zero, we show in this work that the critical current in the S/FB/S junction is controlled by the quantum metric length ${\xi }_{\mathrm{QM}}$ of the flat bands. Microscopically, when ${\xi }_{\mathrm{QM}}$ of the flat band is long enough, the interface bound states originally localized at the two S/FB, FB/S interfaces can penetrate deeply into the flat band material and hybridize to form Andreev bound states (ABSs). These ABSs are able to carry long range and sizable supercurrents. Importantly, ${\xi }_{\mathrm{QM}}$ also controls how far the proximity effect can penetrate into the flat band material. This stands in sharp contrast to the de Gennes' theory for S/N junctions which predicts that the proximity effect is expected to be zero when the Fermi velocity of the normal metal is zero. We further suggest that the S/FB/S junctions would give rise to a new type of resonant Josephson transistor which can carry a sizable and highly gate-tunable supercurrent.

There are no clean samples in nature. Therefore, when we come to discuss the entanglement properties of quantum materials, the effects of disorder must be taken into account. This question is of particular interest for highly entangled phases, such as quantum spin liquids, which lie outside the Landau paradigm for classifying phases of matter. In this work, we explore what experimentally accessible measures, in the form of concurrence, residual tangle, and quantum Fisher information can teach us about the entanglement in the presence of disorder. As a representative example, we consider the Tomonaga-Luttinger liquids (TLL) and disorder-driven random singlet (RS) phases found in antiferromagnetic quantum spin chains. Using quantum Fisher information and residual tangle, we demonstrate that both TLL and RS phases exhibit multipartite entanglement. In the case of the RS phase, we attribute this to entanglement localized below a crossover length scale. We further show that the order of disorder average matters in calculating measures like concurrence and that this can lead to false inferences when interpreting experiments. Nonetheless, correctly interpreted, these witnesses provide useful information about the effects of disorder. We explore how information about the central charge of the TLL can be extracted from the low-temperature behavior of concurrence and conjecture that this analysis can be extended to the effective central charge of the RS phase. Finally, we establish how RS and TLL phases can be distinguished through the growth of multipartite entanglement, as witnessed by the equal-time structure factor. These results establish that, used carefully, experiments based on entanglement witnesses can provide important information about quantum spin systems in the presence of disorder.

Plasmon resonances of supported metal nanostructures are affected by the substrate in an anisotropic way, which gives rise to mode splitting and inhomogeneous energy shifts. We provide experimental evidence that the commonly observed and intuitive case of a weaker influence at larger distances to the substrate does not apply to the plasmon mode structure of small, quasispherical particles. Instead, for the latter, we observe that the energy shift induced by a carbon substrate is strongest if the plasmon is excited farthest from the support. This nonlocal behavior results from the plasmon mode geometry in conjunction with carbon-induced symmetry breaking, which is revealed by electron energy loss spectroscopy (EELS) of silver clusters deposited at the rim of a carbon film. Decomposing the experimental maps based on spectra obtained from the boundary element method (BEM) uncovers the full mode structure. These results strongly suggest that some of the persisting literature controversy on plasmon energies of small, spherical nanoparticles is due to unconsidered spectral effects of an anisotropic local environment.

Materials with a kagome lattice naturally host topological electronic structures, giving rise to many intriguing quantum states within this family. In this study, we performed calculations of the electronic structure and Fermi surface, and measured the longitudinal resistivity ${\rho }_{xx}(\mathit{T},\mathit{B})$, Hall resistivity ${\rho }_{yx}(\mathit{T},\mathit{B})$, and quantum oscillations of the magnetization as a function of temperature and magnetic field for ${\mathrm{Pd}}_3{\mathrm{Tl}}_2{\mathrm{S}}_2$, a kagome-lattice material. We observed an unsaturated extremely large magnetoresistance of 14 500% at 2 K and 9 T, the highest reported in kagome lattice materials, which was attributed to the carrier compensation supported by the Hall resistivity measurements. The band structure calculations, along with the observation of nontrivial Berry phases in the de Haas–van Alphen oscillations, demonstrate that ${\mathrm{Pd}}_3{\mathrm{Tl}}_2{\mathrm{S}}_2$ is a topological nodal-net (Hopf link) semimetal.

Ultracold quantum-gas mixtures of fermionic atoms with resonant control of interactions offer a unique test-bed to explore few- and many-body quantum states with unconventional properties. The emergence of such strongly correlated systems, for instance, symmetry-broken superfluids, is usually accompanied by hydrodynamic collective behavior. Thus experimental progress in this field naturally requires a deep understanding of hydrodynamic regimes. Here, we report on experiments employing a tunable Fermi-Fermi mixture of ${}^{161}\mathrm{Dy}$ and ${}^{40}\mathrm{K}$ near quantum degeneracy. We investigate the full spectrum of dipole modes across a Feshbach resonance and characterize the crossover from collisionless to deep hydrodynamic behavior in measurements of frequencies and damping rates. We compare our results with a theoretical model that considers the motion of the mass centers of the two species, and we identify the contributions of friction and mean-field interaction. We show that one oscillating mode exists over the whole range of interactions, exhibiting striking changes of frequency and damping in the deep hydrodynamic regime. We observe the second oscillating mode to split into two purely exponential damping modes. One of these exponential modes shows very fast damping, faster than any other relevant timescale, and is largely insensitive to experimental imperfections. It provides an accurate measure for the interspecies drag effect, which generalizes the concept of spin drag explored in other experiments. We finally characterize the interspecies drag in terms of a microscopic friction coefficient, and we discuss its unitarity-limited universal behavior on top of the resonance.