We propose that phonons can intrinsically mediate topological superconductivity on the surface of Weyl semimetals. Weyl semimetals are gapless topological materials with nondegenerate zero-energy-surface states known as Fermi arcs. We derive the phonon spectrum and electron-phonon coupling in an effective model of a Weyl semimetal and apply weak-coupling Bardeen-Cooper-Schrieffer theory of superconductivity. In a slab geometry, we find that surface superconductivity dominates over bulk superconductivity in a range of chemical potentials around the Weyl nodes. The superconducting gap function realizes spinless chiral $p$-wave Cooper pairing in the Fermi arcs, leading to Majorana bound states in the core of vortices. Furthermore, we find a suppression of the absolute value of the gap in the center of the Fermi arcs, which is not captured by a local Hubbard attraction. The suppression is due to the nonlocal origin of electron-phonon coupling, leading to a layer dependence which has important consequences for topological surface states.

One fascinating aspect of correlated materials is that minor changes in system parameters may lead to huge variations in material properties. Here, utilizing small-spot angle-resolved photoemission spectroscopy (ARPES), we studied the diverse electronic phases that emerge on the surface of a correlated two-dimensional (2D) material $1T\text{−}{\mathrm{TaSe}}_2$. Three electronic phases were resolved including a large-gap insulating phase, a small-gap insulating phase, and a metallic phase. We characterized both the in-plane and out-of-plane band dispersions for three different phases. We found that the large- and small-gap insulating phases exhibit 2D properties. While the large-gap insulating phase could be attributed to a Mott insulator where the hopping of electrons across the two nearest central Ta atoms is prohibited due to Coulomb repulsion, the small-gap insulating phase could be attributed to a band insulator where the highest occupied band is fully filled due to an even number stacking of ${\mathrm{TaSe}}_2$ layers. For the metallic phase, we observed a dispersive band along the out-of-plane direction, indicating its three-dimensional property. The in-plane band dispersion is highly anisotropic, suggesting that an anisotropic electron hopping is turned on in the metallic phase across different ${\mathrm{TaSe}}_2$ layers. Our detailed characterization of three different phases of $1T\text{−}{\mathrm{TaSe}}_2$ indicates that the formation of star-of-David superstructures in the charge-density-wave (CDW) state of $1T\text{−}{\mathrm{TaSe}}_2$ not only leads to a correlated insulating phase within one ${\mathrm{TaSe}}_2$ layer but also constructs a distinct interlayer coupling that is highly sensitive to the CDW stacking order across different ${\mathrm{TaSe}}_2$ layers. As the stacking order changes, the interlayer coupling changes drastically, which directly determines how electrons hop in $1T\text{−}{\mathrm{TaSe}}_2$. Our observation explains the emergence of diverse electronic phases in $1T\text{−}{\mathrm{TaSe}}_2$. It also highlights that the dimensionality and electronic transport properties of $1T\text{−}{\mathrm{TaSe}}_2$ are highly tunable. They can be tuned by manipulating the phase difference between two adjacent CDW layers. Such manipulation does not break the lattice continuity and could be potentially used in developing nanoscale functional devices.

Recently, it has been established that Chern insulators possess an intrinsic two-dimensional electric polarization, despite having gapless edge states and nonlocalizable Wannier orbitals. This polarization, ${\vec{P}}_{\text{o}}$, can be defined in a many-body setting from various physical quantities, including dislocation charges, boundary charge distributions, and linear momentum. Importantly, there is a dependence on a choice of real-space origin o within the unit cell. In contrast, Coh and Vanderbilt extended the single-particle Berry phase definition of polarization to Chern insulators by choosing an arbitrary point in momentum space, ${\vec{k}}_0$. In this paper, we unify these two approaches and show that when the real-space origin o and momentum-space point ${\vec{k}}_0$ are appropriately chosen in relation to each other, the Berry phase and many-body definitions of polarization are equal.

In the realm of condensed-matter physics, the quantum metric, a pivotal component of the quantum geometric tensor, has been overshadowed by the Berry curvature in terms of research attention. This study investigates the contribution of the quantum metric to the longitudinal conductivity in dispersive two-band systems. We discover that when the density of states vanishes at semimetallic band crossing points, the quantum metric dominates the longitudinal conductivity. By constructing three gapless examples in one-dimensional (1D), 2D, and 3D systems, respectively, we calculate the quantum-metric-induced longitudinal conductivity. In the 1D system with two intersecting bands, the quantum metric successfully explains the discontinuity of intraband conductivity when the band gap opens. For the 2D Dirac system, it accounts for the minimum conductivity at the Dirac point. In the 3D system with a degenerate nodal ring, the longitudinal conductivity is entirely contributed by the quantum metric and is proportional to the length of the nodal ring. Significantly, our results are independent of the scattering rate and are valid in the clean limit, offering a solid theoretical basis for experimental observation of quantum metric effects in longitudinal conductivity, and propelling forward the research on quantum geometry in condensed-matter physics.

Electron spin qubits in silicon-germanium (SiGe) heterostructures are a major candidate for the realization of scalable quantum computers. A critical challenge in strained Si/SiGe quantum wells (QWs) is the existence of two nearly degenerate valley states at the conduction-band minimum that can lead to leakage of quantum information. To address this issue, various strategies have been explored to enhance the valley splitting (i.e., the energy gap between the two low-energy conduction-band minima), such as sharp interfaces, oscillating germanium concentrations in the QW (known as wiggle wells), and shear strain engineering. In this work, we develop a comprehensive envelope-function theory augmented by an empirical nonlocal pseudopotential model to incorporate the effects of alloy disorder, strain, and nontrivial resonances arising from interactions between valley states across neighboring Brillouin zones. We apply our model to analyze common epitaxial profiles studied in the literature with a focus on wiggle-well-type structures and compare our results with previous work. Our framework provides an efficient tool for quantifying the interplay of these effects on the valley splitting, enabling complex epitaxial profile optimization in future work.

We present a unique perspective on the $p$-string condensation procedure for constructing (3+1)-dimensional [(3+1)D] fracton phases by implementing this process via the gauging of higher-form symmetries. Specifically, we show that gauging a 1-form symmetry in (3+1)D that is generated by Abelian anyons in isotropic stacks of (2+1)-dimensional [(2+1)D] topological orders naturally results in a (3+1)D $p$-string condensed phase, providing a controlled nonperturbative construction that realizes fracton orders. This approach clarifies the symmetry principles underlying $p$-string condensation and generalizes the familiar connection between anyon condensation and 1-form gauging in two spatial dimensions. We demonstrate this correspondence explicitly in both field theories and lattice models: In field theory, we derive the foliated field theory description of the ${\mathbb{Z}}_N$ X-cube model by gauging a higher-form symmetry in stacks of (2+1)D ${\mathbb{Z}}_N$ gauge theories; on the lattice, we show how gauging a diagonal 1-form symmetry in isotropic stacks of $G$-graded string-net models leads to string membrane nets hosting restricted mobility excitations. This perspective naturally generalizes to spatial dimensions $d\ge 2$ and provides a step towards building an algebraic theory of $p$-string condensation.

A wide family of two-dimensional (2D) systems, including stripe-phase superconductors, sliding Luttinger liquids, and anisotropic 2D materials, can be modeled by an array of coupled one-dimensional (1D) electron channels or nanowire arrays. Here we report experiments in arrays of conducting nanowires with gate and field tunable interwire coupling, that are programmed at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface. We find a magnetically tuned metal-to-insulator transition in which the transverse resistance of the nanowire array increases by up to four orders of magnitude, which can be further tuned with a gate voltage. To explain this behavior, we develop a minimal model of a coupled two-wire system where a Wenzel-Kramers-Brillouin–based approach is used to estimate the transverse tunneling conductance. We demonstrate the existence of distinct conductance features and highlight the crucial role played by the field dependence of the interwire potential barrier on transport properties. Since our model makes minimal assumptions, we expect our predictions to hold for a wide class of coupled 1D systems. The nanowire arrays can serve as model systems to understand the origin of exotic behavior in correlated materials via analog quantum simulation.

The discovery of superconductivity in infinite-layer nickelates has ignited stark interest within the scientific community, particularly regarding its likely unconventional origin. Conflicting magnetotransport measurements report either isotropic or anisotropic suppression of superconductivity in an external magnetic field, with distinct implications for the nature of superconducting order. In order to ensure a most suited model subject to subsequent many-body analysis, we develop a first-principles-guided minimal theory including Ni $d_{x^2-y^2}$, La $d_{3z^2-r^2}$, and La $d_{xy}$ orbitals. Amended by the consideration of orbital-selective pairing formation, which emphasises the correlation state of the Ni $3d_{x^2-y^2}$ orbital, we calculate the superconducting ordering susceptibility mediated by spin fluctuations. We find a parametric competition between even-parity $\mathrm{d}$-wave and, in contrast to previous studies, odd-parity $\mathrm{p}$-wave pairing, which becomes favorable through a large quasiparticle weight renormalization for Ni $3d_{x^2-y^2}$ electrons. Our findings not only shed light on the distinctiveness of ${\mathrm{LaNiO}}_2$ as compared to cuprate superconductors or nickelates of different rare-earth composition but also suggest similarities to other candidate odd-parity superconductors.

We demonstrate that the electronic Hamiltonian of a solid may not strictly share the exact symmetry of its host lattice, even though the wave functions generally do. Based on this observation, we argue that the conventional space-group description of a solid's energy bands may be incomplete; the Hamiltonian itself can possess more fundamental properties than the wave function, challenging the traditional understanding of the role of symmetry. We introduce this emergent symmetry as subspace symmetry, which, in certain cases, remains exact across the full Hilbert space. General proofs of its existence are provided, along with calculations on a 1D chain model, showing that topology can arise within otherwise trivial band structures. Finally, we explore its connections to other symmetries and present a concrete example of a “fake” altermagnet. Subspace symmetry offers new insights into our understanding of symmetry, topology, and condensed matter physics.

Recent studies have demonstrated that photoexcitation can induce structural phase transitions locally under a weak excitation, significantly altering material properties in an ultrahigh-speed and energy-efficient manner. However, the underlying mechanisms cannot be fully explained by the collective atomic motions of photoexcited coherent phonons. In this work, using real-time time-dependent density functional theory (rt-TDDFT), we show that local Nb-Nb dimers are broken in ${\mathrm{NbO}}_2$ due to the atomic driving forces amplified by the trapping of local photoexcited holes. More trapped photoholes are subsequently transferred to adjacent Nb-Nb dimers along the atomic chain direction, and finally forming a conductive channel for carriers, yielding a transition from a semiconductor phase to a metallic phase. Furthermore, we discover that stability of the local phase transition depends on the initial lattice temperature, persisting for a longer timescale in the range of 300 K to 600 K. Our work highlights the directional nature of local phase transitions in photoexcited ${\mathrm{NbO}}_2$ and paves the way for future experimental investigations.

The magnetic properties of the Tb-honeycomb lattice compound ${\mathrm{TbPt}}_6{\mathrm{Al}}_3$, which crystallizes in the ${\mathrm{NdPt}}_6{\mathrm{Al}}_3$-type trigonal structure, have been studied by the measurements of electrical resistivity, magnetization $M\left(T,B\right)$, and specific heat on single-crystalline samples. The magnetic susceptibility, $M\left(T\right)/B$, for $B\parallel c=0.1\mathrm{T}$ shows a cusp at $T_{\mathrm{N}}=3.5\mathrm{K}$, in which temperature decreases with increasing the magnitude of $B\parallel c$, while $M\left(T\right)/B$ for $B\parallel a=0.1$ T remains constant with decreasing temperature below $T_{\mathrm{N}}$. This anisotropic behavior suggests a collinear antiferromagnetic (AFM) order of the $\mathrm{T}{\mathrm{b}}^{3+}$ moments pointing along the $c$ axis. The data of $M\left(T\right)/B$ for $T>10\mathrm{K}$ on the single crystal and that of inelastic neutron scattering from powdered samples have been simultaneously analyzed using the crystal field model. The analysis reveals the non-Kramers doublet ground state for the $\mathrm{T}{\mathrm{b}}^{3+}$ ion under the trigonal crystal field. The neutron powder diffraction measurement shows that the collinear AFM structure with a magnetic propagation vector $\mathbf{k}=\left[0,0,0\right]$ is associated with moments of $5.1{\mu }_{\mathrm{B}}/\mathrm{Tb}$ pointing along the $c$ axis. Comparison of the magnetic point group with the nontrivial spin Laue group indicates that ${\mathrm{TbPt}}_6{\mathrm{Al}}_3$ is classified into bulk $g$-wave altermagnets.

High-entropy alloy (HEA) superconductors are known to exhibit a robust superconducting state with adaptable thermomechanical properties. In this study, we have investigated the electronic structure of HEA-type $R{\mathrm{OBiS}}_2$ (R = rare earth) layered superconductors with mixing entropy using soft x-ray absorption (XAS) and x-ray photoemission spectroscopy (XPS) measurements. The electronic structure evolution differs between equivalent and nonequivalent R mixing as a function of mixing entropy. Ce ${\mathrm{M}}_{4,5}$-edge XAS and Ce $3d$ XPS reveal clear evidence of ${\mathrm{Ce}}^{3+}$/${\mathrm{Ce}}^{4+}$ mixed valence in the HEA-type R${\mathrm{OBiS}}_2$, similar to the pristine ${\mathrm{CeOBiS}}_2$ compound, suggesting a similar charge transfer mechanism responsible for self-doping in these materials. Unlike Ce, all other R atoms exhibit a nominal R${}^{3+}$ valence state across samples with varying mixing entropy. The self-doped charge is consistent with the Luttinger volume estimated by previous angle-resolved photoemission spectroscopy (ARPES) studies. The distinct behaviors observed in equivalent and nonequivalent mixing are discussed in terms of differing potential energy landscapes of the Bi sites manifested in the Bi $4f$ XPS spectra, where intrinsic disorder is expected to have different characteristics of the superconductivity.

Topological Weyl semimetals represent a novel class of quantum materials that exhibit remarkable properties arising from their unique electronic structure. In this work, we employ state-of-the-art ab initio methods to investigate the role of the electron-phonon interactions on the charge transport properties of TaAs. Our calculations of the temperature-dependent electrical conductivity with the iterative Boltzmann transport equation show excellent agreement with experimental measurements above 100 K. Extending the analysis to doped systems, we demonstrate that even small shifts in the Fermi level can lead to substantial changes in conductivity, driven by the complex topology of the Fermi surface. In particular, modifications in Fermi surface nesting emerge as a key factor influencing scattering processes and carrier lifetimes. These findings offer critical insights into the microscopic mechanisms that govern transport in TaAs and highlight the sensitivity of Weyl semimetals to doping and carrier dynamics.

Antiferromagnetic magnonics has become the focus of intense scientific research because of the advantages of these materials compared to ferromagnets. However, ferrimagnetic materials have received much less attention despite exhibiting similar dynamical features at the angular momentum compensation point. In this paper, we present analytical expressions describing the dispersion relation of forward volume spin waves in ferrimagnetic materials near the angular momentum compensation point. We benchmark the derived dispersion relations against full micromagnetic simulations showing an excellent agreement between both approaches. We predict two different branches for forward volume spin waves in ferrimagnetic materials merging into a single branch at the angular momentum compensation point. Our results can assist in the design of magnonic devices built on ferrimagnetic materials.

The emergence of two-dimensional magnetic carbon materials offers a promising platform for spintronic applications. However, pristine carbon materials typically exhibit nonmagnetic behavior. In this work, we investigate the magnetic properties of holey graphyne (HGY) doped with nonmetal sp-impurity atoms (B, N, P, and S) via adsorption and substitution. First-principles calculations reveal that specific doping configurations exhibit spontaneous ferromagnetism driven by narrow impurity bands at the Fermi level. Among them, the Curie temperatures of the P-adsorbed HGY (H2 site) and P-substituted HGY (T2 site) reach 200 K and 104 K, respectively. Notably, for P-substituted HGY (T2 site), the magnetic moments primarily originate from the sp-hybridized carbon atoms adjacent to the impurity rather than from the impurity itself, thereby enhancing the chemical reactivity of the carbon atoms. Furthermore, reducing the substitutional doping concentration to approximately 1% enhances $\pi$-electron localization and narrows the impurity bandwidth, forming isolated and nonoverlapping impurity bands, which promotes spin polarization of the system. Under low doping conditions, all doped structures exhibit ferromagnetic ground states. These findings propose a tunable strategy for achieving magnetism and bipolar magnetic semiconducting behavior in carbon-based 2D semiconductors, paving the way for lightweight carbon-based spintronic devices.

The van der Waals (vdW) semiconductor black phosphorus (BP) has been extensively studied, especially since the discovery of phosphorene. In contrast, its sister compound violet phosphorus (VP), another vdW semiconductor, has received far less attention. Here we report the pressure-induced superconductivity in VP up to $\sim 40$ GPa. The zero-resistance superconducting transition emerges at 3.57 GPa, due to a structural phase transition from monoclinic $\left(M\right)$ to rhombohedral $\left(R\right)$ structure. The superconducting transition temperature $\left(T_{\mathrm{c}}\right)$ shows a plateau of $\sim 7$ K from 3.6–15 GPa in the $R$ structure, then jumps to another plateau of $\sim 10$ K above 15 GPa in the simple cubic $\left(C\right)$ structure. We establish and compare the temperature-pressure superconducting phase diagram of VP with that of BP. For BP, the superconductivity appears with an orthorhombic $\left(O\right)\text{−}R$ structural transition at a critical pressure of 5 GPa, with a lower $T_{\mathrm{c}}$ than VP. The pressure-induced structural phase transition and superconductivity in VP demonstrate its pressure-sensitive and easily tunable electronic properties. These properties suggest promising prospects for further electronic and optoelectronic applications, particularly given its stability as a vdW semiconductor at ambient conditions.

When hard-core bosons on a two-leg ladder get frustrated by ring exchange interactions, the elusive $d$-wave Bose liquid (DBL) can be stabilized, a bosonic analog of a correlated metal. Here, we analyze the effect of extended Hubbard interactions on the DBL phase. Strikingly, these interactions are found to act in favor of the exotic Bose liquid. This observation is of immediate relevance for physical systems in which nonlocal exchange processes occur as a consequence of extended range density-density interactions. Our observation also helps to achieve DBL physics in a synthetic-dimension ladder, where on-site interactions translate into nonlocal interactions along a synthetic rung. In this context, we also consider the extreme limit, in which the local hard-core constraint is elevated to an effective rung blockade. In addition to the enhancement of DBL physics due to extended range density-density interactions, we also find signatures of an interesting intermediate phase between the superfluid and the DBL regime. This phase, called the density-modulated $s$-wave pairing phase, combines features of density wave and $s$-wave pairing. Our results offer insights into the physics of frustrated bosons by highlighting the influence of density-density interaction and rung blockade.

Majorana zero modes (MZMs) have been proposed as a promising basis for Majorana qubits offering great potential for topological quantum computation. Such modes may form at the ends of a magnetic atomic chain on a superconductor. Here, we study the topological properties of Yu-Shiba-Rusinov (YSR) bands of excitations in Mn chains constructed on a Nb(110) and on a Ta(110) substrate using first-principles calculations and scanning tunneling microscopy and spectroscopy experiments. We demonstrate that even and odd YSR states with respect to mirroring on the symmetry plane containing the chain have different dispersions. On the Nb substrate, a wide but topologically trivial minigap is opened in the even states by the spin–orbit coupling, while the odd states display signatures of precursors of MZMs with a smaller minigap. On the Ta substrate, the stronger spin–orbit coupling increases the hybridization between the bands, but no clear minigap can be observed in either of them. These findings highlight the influence of symmetries on interpreting the spectroscopic signatures of candidates for MZMs.