We evaluate the sound attenuation in a Weyl semimetal subject to a magnetic field or a pseudomagnetic field associated with a strain. Due to the interplay of intra- and internode scattering processes as well as screening, the fields generically reduce the sound absorption. A nontrivial dependence on the relative direction of the magnetic field and the sound wave vector, i.e., the magnetic sound dichroism, can occur in materials with nonsymmetric Weyl nodes (e.g., different Fermi velocities and/or relaxation times). It is found that the sound dichroism in Weyl materials can also be activated by an external strain-induced pseudomagnetic field. In view of the dependence on the field direction, the dichroism may lead to a weak enhancement of the sound attenuation compared with its value at vanishing fields.

We utilized high-energy-resolution resonant inelastic x-ray scattering (RIXS) at both the Ta and Ni $L_3$ edges to map out element-specific particle-hole excitations in ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$ across the phase transition. Our results reveal a momentum-dependent gaplike feature in the low-energy spectrum, which agrees well with the band gap in element-specific joint density of states calculations based on ab initio estimates of the electronic structure in both the low-temperature monoclinic and high-temperature orthorhombic structures. Below $T_c$, the RIXS energy-momentum map shows a minimal gap at the Brillouin zone center $(\sim 0.16 \mathrm{eV})$, confirming that ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$ possesses a direct band gap in its low-temperature ground state. However, inside the gap, no signature of anticipated collective modes with an energy scale comparable to the gap size can be identified. Upon increasing the temperature to above $T_c$, whereas the gap at the zone center closes, the RIXS map at finite momenta still possesses the gross features of the low-temperature map, suggesting a substantial mixing between the Ta and Ni orbits in the conduction and valence bands, which does not change substantially across the phase transition. Our experimental observations and comparison to the theoretical calculations lend further support to the phase transition and the corresponding gap opening in ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$ being largely structural by nature, with a possible minor contribution from the putative exciton condensate.

Harnessing the properties of vortices in superconductors is crucial for fundamental science and technological applications; thus, it has been an ongoing goal to locally probe and control vortices. Here, we use a scanning probe technique that enables studies of vortex dynamics in superconducting systems by leveraging the resonant behavior of a raster-scanned, magnetic-tipped cantilever. This experimental setup allows us to image and control vortices, as well as extract key energy scales of the vortex interactions. Applying this technique to lattices of superconductor island arrays on a metal, we obtain a variety of striking spatial patterns that encode information about the energy landscape for vortices in the system. We interpret these patterns in terms of local vortex dynamics and extract the relative strengths of the characteristic energy scales in the system, such as the vortex-magnetic field and vortex-vortex interaction strengths, as well as the vortex chemical potential. We also demonstrate that the relative strengths of the interactions can be tuned and show how these interactions shift with an applied bias. The high degree of tunability and local nature of such vortex imaging and control not only enable new understanding of vortex interactions, but also have potential applications in more complex systems such as those relevant to quantum computing.

We study spin density wave quantum critical points in two-dimensional metals with a quenched disorder potential coupling to the electron density. Adopting an $\epsilon$ expansion around three spatial dimensions, where both disorder and the Yukawa-type interaction between electrons and bosonic order parameter fluctuations are marginal, we present a perturbative, one-loop renormalization group analysis of this problem, where the interplay between fermionic and bosonic excitations is fully incorporated. Considering two different Gaussian disorder models restricted to small-angle scattering, we show that the non-Fermi liquid fixed point of the clean spin density wave (SDW) hot spot model is generically unstable and the theory flows to strong coupling due to a mutual enhancement of interactions and disorder. We study properties of the asymptotic flow towards strong coupling, where our perturbative approach eventually breaks down. Our results indicate that disorder dominates at low energies, suggesting that the ground state in two dimensions is Anderson-localized.

We perform a scanning tunneling microscopy/spectroscopy study on the layered iron-based superconductor ${\mathrm{KCa}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$ with a critical temperature of about 33.5 K. Two types of terminated surfaces are generally observed after cleaving the samples in vacuum. On one commonly obtained surface, we observe a full gap feature with energy gap values close to 4.6 meV. This type of spectrum shows a clean and uniform full gap in space, which indicates the absence of gap nodes in this superconductor. Quasiparticle interference patterns have also been measured which show an intraband scattering pattern possibly due to the holelike $\alpha$ pocket. The Fermi energy of this band is only about $24\pm 6$ meV, as derived from the energy dispersion result. Meanwhile, impurity-induced bound-state peaks can be observed at about $\pm 2.2$ meV on some spectra, and the peak value seems to be independent of magnetic field. On the second type of surface, which is rarely obtained, the fully gapped feature can still be observed in the tunneling spectra, although multiple gaps are obtained from either a single spectrum or separate ones, and the gap values determined from coherence peaks locate mainly in the range from 4 to 7 meV. Our results clearly indicate the multiple and nodeless superconducting gap nature of the layered superconductor ${\mathrm{KCa}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$, and the superfluid is mainly contributed by the holelike Fermi surfaces near the $\mathrm{\Gamma }$ point. This result should inspire further consideration of the effect of the shallow and incipient bands near the $M$ point and help us to understand the pairing mechanism in this highly layered iron-based superconductor.

We report a polarization-resolved Raman spectroscopy study of the orbital dependence of the quasiparticle properties in the prototypical multiband Fermi liquid ${\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$. We show that the quasiparticle scattering rate displays ${\omega }^2$ dependence, as expected for a Fermi liquid. In addition, we observe a clear polarization dependence in the energy and temperature dependence of the quasiparticle scattering rate and mass, with the $d_{xz/yz}$ orbital derived quasiparticles showing significantly more robust Fermi liquid properties than the $d_{xy}$ orbital derived ones. The observed orbital dichotomy of the quasiparticles is consistent with the picture of ${\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$ as a Hund's metal. Our study establishes Raman scattering as a powerful probe of Fermi liquid properties in correlated metals.

The high pressure phases of Rb have previously been investigated to 101 GPa, above which Rb is predicted to adopt a double-hexagonal close-packed (dhcp, Pearson hP4) structure similar to that already observed in cesium at 72 GPa. Previous ab initio structure searches have indicated that the hP4 phase should become stable in rubidium at $143\mathrm{GPa}$. We present data from static compression experiments on Rb up to $264\left(8\right)\mathrm{GPa}$, showing the onset of the hP4 phase at $207\left(6\right)\mathrm{GPa}$. The $V/V_0$ of $\sim 0.121$ measured at $264\mathrm{GPa}$ constitutes the highest compression ratio (more than eightfold) at which structural information has been obtained from a metal using x-ray diffraction methods and is second only to x-ray measurements performed on hydrogen at $V/V_0\sim 0.094$ at $190\mathrm{GPa}$. At these extreme compression ratios, the compressive behavior of rubidium shifts from that of a free electron metal to that of a regular $d$-block metal.

Recent discoveries in topological physics hold promise for disorder-robust quantum systems and technologies. Topological states provide the crucial ingredient of such systems featuring increased robustness to disorder and imperfections. Here we use an array of superconducting qubits to engineer a one-dimensional topologically nontrivial quantum metamaterial. By performing microwave spectroscopy of the fabricated array, we experimentally observe the spectrum of elementary excitations. We reveal not only the single-photon topological states but also the bands of exotic bound photon pairs arising due to the inherent anharmonicity of qubits. Furthermore, we discuss the formation of the two-photon bound edge-localized state and confirm the topological origin of our model demonstrating disorder-robust behavior of photon-photon correlation function for the topological edge state. Our work provides an experimental implementation of the topological model with attractive photon-photon interaction in a quantum metamaterial.

We characterize experimentally and theoretically the high-energy dielectric screening properties of the prototypical correlated metal ${\mathrm{SrVO}}_3$. The dynamical structure factor measured by inelastic x-ray scattering spectroscopy as a function of momentum transfer is in very good agreement with first-principles calculations in the adiabatic local-density approximation to time-dependent density-functional theory. Our results reveal the crucial importance of crystal local fields in the charge response function of correlated materials: They lead to depolarization effects for localized excitations and couple spectra from different Brillouin zones.

The past years have seen rapid progress in the classification of topological materials. These diagnostical methods are increasingly getting explored in the pertinent context of magnetic structures. We report on a general class of electronic configurations within a set of antiferromagnetic-compatible space groups that are necessarily topological. Interestingly, we find a systematic correspondence between these antiferromagnetic phases to necessarily nontrivial topological ferro/ferrimagnetic counterparts that are readily obtained through physically motivated perturbations. Addressing the exhaustive list of magnetic space groups in which this mechanism occurs, we also verify its presence on planes in 3D systems that were deemed trivial in existing classification schemes. This leads to the formulation of the concept of subdimensional topologies, featuring nontriviality within part of the system that coexists with stable Weyl points away from these planes, thereby uncovering novel topological materials in the full 3D sense that have readily observable features in their bulk and surface spectrum.

Two new recently proposed classes of topological phases, namely, fractons and higher order topological insulators (HOTIs), share at least superficial similarities. The wide variety of proposals for these phases calls for a universal field theory description that captures their key characteristic physical phenomena. In this work, we construct topological multipolar response theories that capture the essential features of some classes of fractons having subsystem symmetries and higher order topological insulators. Remarkably, we find that despite their distinct symmetry structure, some classes of fractons and HOTIs can be connected through their essentially identical topological response theories. More precisely, we propose a topological quadrupole response theory that describes both a 2D symmetry-protected fracton phase and a related bosonic quadrupolar HOTI with strong interactions. Such a topological quadrupole term encapsulates the protected corner charge modes and, for the HOTI, also determines an anomalous edge with a fractional dipole moment. In 3D, we propose a dipolar Chern-Simons theory with a quantized coefficient as a description of the response of both second-order HOTIs harboring chiral hinge currents and of a related fracton phase. This theory correctly predicts chiral currents on the hinges and anomalous dipole currents on the surfaces. We generalize these results to higher dimensions to reveal a family of multipolar Chern-Simons terms and related $\theta$-term actions that can be reached via dimensional reduction or extension from the Chern-Simons theories.

Raman heterodyne spectroscopy is a powerful tool for characterizing the energy and dynamics of spins. The technique uses an optical pump to transfer coherence from a spin transition to an optical transition where the coherent emission is more easily detected. Here Raman heterodyne spectroscopy is used to probe an isotopically purified ensemble of erbium dopants in a yttrium orthosilicate $\left({\mathrm{Y}}_2{\mathrm{SiO}}_5\right)$ crystal coupled to a microwave cavity. Because the erbium electron spin transition is strongly coupled to the microwave cavity, we observed Raman heterodyne signals at the resonant frequencies of the hybrid spin-cavity modes (polaritons) rather than the bare erbium spin-transition frequency. Using the coupled system, we made saturation recovery measurements of the ground-state spin relaxation time $T_1=10\pm 3$ s and also observed Raman heterodyne signals using an excited state spin transition. We discuss the implications of these results for efforts toward converting microwave quantum states to optical quantum states.

A heterostructure consisting of the Mott insulator ${\mathrm{LaVO}}_3$ and the band insulator ${\mathrm{SrTiO}}_3$ is considered a promising candidate for future photovoltaic applications. Not only does the (direct) excitation gap of ${\mathrm{LaVO}}_3$ match well the solar spectrum, but its correlated nature and predicted built-in potential, owing to the nonpolar/polar interface when integrated with ${\mathrm{SrTiO}}_3$, also offer remarkable advantages over conventional solar cells. However, experimental data beyond the observation of a thickness-dependent metal-insulator transition are scarce and a profound, microscopic understanding of the electronic properties is still lacking. By means of soft and hard x-ray photoemission spectroscopy as well as resistivity and Hall effect measurements we study the electrical properties, band bending, and band alignment of ${\mathrm{LaVO}}_3/{\mathrm{SrTiO}}_3$ heterostructures. We find a critical ${\mathrm{LaVO}}_3$ thickness of five unit cells, confinement of the conducting electrons to exclusively Ti $3d$ states at the interface, and a potential gradient in the film. From these findings we conclude on electronic reconstruction as the driving mechanism for the formation of the metallic interface in ${\mathrm{LaVO}}_3/{\mathrm{SrTiO}}_3$.

We describe the large-$N$ saddle point, and the structure of fluctuations about the saddle point, of a theory containing a sharp, critical Fermi surface in two spatial dimensions. The theory describes the onset of Ising order in a Fermi liquid, and closely related theories apply to other cases with critical Fermi surfaces. We employ random couplings in flavor space between the fermions and the bosonic order parameter, but there is no spatial randomness: consequently, the $G\text{−}\mathrm{\Sigma }$ path integral of the theory is expressed in terms of fields bilocal in spacetime. The critical exponents of the large-$N$ saddle point are the same as in the well-studied nonrandom random-phase-approximation theory; in particular, the entropy density vanishes in the limit of zero temperature. We present a full numerical solution of the large-$N$ saddle-point equations, and the results agree with the critical behavior obtained analytically. Following analyses of Sachdev-Ye-Kitaev models, we describe scaling operators which descend from fermion bilinears around the Fermi surface. This leads to a systematic consideration of the role of time reparametrization symmetry, and the scaling of the Cooper pairing and $2k_F$ operators which can determine associated instabilities of the critical Fermi surface. We find no violations of scaling from time reparametrizations. We also consider the same model but with spatially random couplings: this provides a systematic large-$N$ theory of a marginal Fermi liquid with Planckian transport.

The phase diagram of ${\mathrm{Pr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ (PCMO) is modified for $x\le 0.3$, which suggests a reevaluation of the phase diagram of other manganites in that doping region. Rather than an orbital-ordered phase reaching up to high temperatures of ∼800–1100 K, we propose a loss of spontaneous orbital order near room temperature. Above this temperature, the phase is characterized by a finite orbital polarization and octahedral tilt pattern. The tilt pattern couples to the Jahn-Teller distortion and thus induces a remaining orbital order, which persists up to high temperatures, where the tilt order is lost as well. This explains the experimental observation of orbital order up to high temperatures. The reevaluation of the orbital-order transition is based on observed anomalies of various physical properties at temperatures of 220–260 K in epitaxial thin films of PCMO $x=0.1$, i.e., in the photovoltaic effect, electric transport, magnetization, optical, and ultrafast transient pump probe studies. Finite-temperature simulations based on a tight-binding model with carefully adjusted parameters from first-principles calculations exhibit an orbital-order phase transition at $T_{\mathrm{OO}}\approx 300\mathrm{K}$ for PCMO $x=0.1$. This is consistent with the experimental observation of temperature-dependent changes in lattice parameter for bulk samples of the same doping at 300 K for $x=0.1$ and 350 K for $x=0$, typical for second-order phase transitions. Since our reassignment of the orbital-order phase transition toward lower temperature challenges a well-established and long-accepted picture, we provide results of multiple complementary measurements as well as a detailed discussion.

Nd-based pyrochlore oxides of the form ${\mathrm{Nd}}_2{B}_2{\mathrm{O}}_7$ have garnered a significant amount of interest owing to the moment fragmentation physics observed in ${\mathrm{Nd}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$ and speculated in ${\mathrm{Nd}}_2{\mathrm{Hf}}_2{\mathrm{O}}_7$. Notably this phenomenon is not ubiquitous in this family, as it is absent in ${\mathrm{Nd}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$, which features a smaller ionic radius on the $B$ site. Here, we explore the necessary conditions for moment fragmentation in the Nd pyrochlore family through a detailed study of the mixed $B$-site pyrochlore ${\mathrm{Nd}}_2{\mathrm{GaSbO}}_7$. The $B$ site of this system is characterized by significant disorder and an extremely small average ionic radius. Similarly to ${\mathrm{Nd}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$, we find no evidence for moment fragmentation through our bulk characterization and neutron scattering experiments, indicating that chemical pressure (and not necessarily the $B$-site disorder) plays a key role in the presence or absence of this phenomenon in this material family. Surprisingly, the presence of significant $B$-site disorder in ${\mathrm{Nd}}_2{\mathrm{GaSbO}}_7$ does not generate a spin-glass ground state and instead the same all-in-all-out magnetic order identified in other Nd pyrochlores is found here.

Quasi-two-dimensional kagome metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{K}$, Rb, and Cs) have attracted much recent interest due to exotic quantum phenomena such as unconventional superconductivity, topological charge order, and giant anomalous Hall effect. Here we report pressure-induced reemergent superconductivity in $\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$ by electrical transport measurements under high pressures up to 47.9 GPa. We show that the superconducting critical temperature $T_c$ is first enhanced by pressure and reaches its first maximum ∼8.9 K at 0.8 GPa; then the $T_c$ is suppressed by pressure and cannot be detected above 7.5 GPa, forming a dome-shaped superconducting phase diagram. Remarkably, upon further compression above 16.5 GPa, a new superconducting state arises, of which $T_c$ is enhanced by pressure to a second maximum ∼5.0 K and the reemergent superconductivity keeps robust up to 47.9 GPa. Combined with high-pressure synchrotron x-ray-diffraction measurements that demonstrate the stability of the pristine hexagonal phase up to 43.1 GPa, we suggest that the reemergence of superconductivity in the V-based superconductor could be attributed to a pressure-induced Lifshitz transition.

We present conductance-matrix measurements in long, three-terminal hybrid superconductor-semiconductor nanowires, and compare with theoretical predictions of a magnetic-field-driven, topological quantum phase transition. By examining the nonlocal conductance, we identify the closure of the excitation gap in the bulk of the semiconductor before the emergence of zero-bias peaks, ruling out spurious gap-closure signatures from localized states. We observe that after the gap closes, nonlocal signals and zero-bias peaks fluctuate strongly at both ends, inconsistent with a simple picture of clean topological superconductivity.

Interest in layered two-dimensional materials, particularly stacked heterostructures of transition-metal dichalcogenides, has led to the need for a better understanding of the structural and electronic changes induced by stacking. Here, we investigate the effects of idealized heterostructuring, with periodic commensurate stacking, on the structural, electronic, and vibrational properties, when compared to the counterpart bulk transition-metal dichalcogenide. We find that in heterostructures with dissimilar chalcogen species there is a strong compression of the interlayer spacing, compared to the bulk compounds. This compression of the heterostructure is caused by an increase in the strength of the induced polarization interaction between the layers, but not a full charge transfer. We argue that this effect is real, not due to the imposed commensurability, and should be observable in heterostructures combining different chalcogens. Interestingly, we find that incommensurate stacking of Ti-based dichalcogenides can lead to the stabilization of the charge-density wave phonon mode, which is unstable in the 1T phase at low temperature. Mixed Ti- and Zr-based heterostructures are still dynamically unstable, but $\mathrm{Ti}{\mathrm{S}}_2/\mathrm{Zr}{\mathrm{S}}_2$ becomes ferroelectric.

The silver ruthenium oxide $\mathrm{Ag}\mathrm{Ru}{\mathrm{O}}_3$ consists of honeycomb ${\mathrm{Ru}}_2^{5+}{\mathrm{O}}_6^{2-}$ layers and can be considered an analogue of $\mathrm{Sr}{\mathrm{Ru}}_2{\mathrm{O}}_6$ with a different intercalation. We present measurements of magnetic susceptibility and specific heat on $\mathrm{Ag}\mathrm{Ru}{\mathrm{O}}_3$ single crystals, which reveal a sharp antiferromagnetic transition at 342(3) K. The electrical transport in single crystals of $\mathrm{Ag}\mathrm{Ru}{\mathrm{O}}_3$ is determined by a combination of activated conduction over an intrinsic semiconducting gap of $\approx 100$ meV and carriers trapped and thermally released from defects. From powder neutron diffraction data a Néel-type antiferromagnetic structure with the Ru moments along the $c$ axis is derived. Raman spectroscopy on $\mathrm{Ag}\mathrm{Ru}{\mathrm{O}}_3$ single crystals and muon spin rotation spectroscopy on powder samples indicate a further weak phase transition or a crossover in the temperature range 125–200 K. The transition does not show up in the magnetic susceptibility, and its origin is argued to be related to defects but cannot be fully clarified. The experimental findings are complemented by density-functional-theory-based electronic structure calculations. It is found that the magnetism in $\mathrm{Ag}\mathrm{Ru}{\mathrm{O}}_3$ is similar to that in $\mathrm{Sr}{\mathrm{Ru}}_2{\mathrm{O}}_6$, however, with stronger intralayer and weaker interlayer magnetic exchange interactions.