Topological superconductors harbor, at their boundaries and vortex cores, zero-energy Majorana bound states, which can be the building blocks in fault-tolerant topological quantum computing. Planar Josephson junctions host such topological superconducting phases, highly tunable by an external magnetic field or phase difference between the superconducting leads. Despite many theoretical and experimental studies, the signatures of the transition to a topological superconducting phase, based on minima in the critical supercurrent $I_c$ flowing across the junction, the 0-$\pi$ transition in the ground state junction phase and their anisotropic magnetic-field response have remained unsettled. Using rigorous numerical calculations with several experimentally relevant parameter settings, we show that $I_c$ and ${\phi }_{\mathrm{GS}}$ cannot indicate unambiguously a topological transition in any realistic planar junctions. Furthermore, the anisotropic variations of $I_c$ and ${\phi }_{\mathrm{GS}}$ with an in-plane magnetic field appear in junctions that are undoubtedly in a trivial superconducting phase, raising concerns on the effectiveness of these probes in identifying topological transitions in planar junctions. We discuss possible strategies to confirm a topological superconducting phase in these platforms.

We present a combined growth and transport study of superconducting single-crystalline ${\mathrm{NdO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$. Evidence of two-dimensional superconductivity with significant phase fluctuations of preformed Cooper pairs preceding the superconducting transition is reported. This result is based on three key observations. (1) The resistive superconducting transition temperature $T_c$ (defined by resistivity $\rho \to 0$) increases with increasing disorder. (2) As $T\to T_c$, the conductivity diverges significantly faster than what is expected from Gaussian fluctuations in two and three dimensions. (3) Non-Ohmic resistance behavior is observed in the superconducting state. Altogether, our observations are consistent with a temperature regime of phase-fluctuating superconductivity. The crystal structure with magnetic ordering tendencies in the ${\mathrm{NdO}}_{0.5}{\mathrm{F}}_{0.5}$ layers and (super)conductivity in the ${\mathrm{BiS}}_2$ layers is likely responsible for the two-dimensional phase fluctuations. As such, ${\mathrm{NdO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ falls into the class of unconventional “laminar” bulk superconductors that include cuprate materials and $4\mathrm{Hb}\text{−}{\mathrm{TaS}}_2$.

The ternary van der Waals (vdW) pentagonal semiconductor PdPSe has gathered significant attention due to its unique anisotropic structure and exotic optoelectronic properties originating from the distinct Cairo pentagonal tiling topology. Here, we report pressure tuning of the photoelectronic and electronic properties of pentagonal PdPSe. We show that the photocurrent is enhanced by over two orders of magnitude at 25.9 GPa and suddenly disappears upon further compression. Meanwhile, metallization and superconductivity simultaneously occur at around 25 GPa. The superconducting critical temperature $T_{\mathrm{c}}$ initially increases and then becomes robust up to ∼60 GPa. Synchrotron x-ray diffraction and Raman experiments consistently indicate the disappearance of photocurrent and the occurrence of superconductivity are both correlated to the formation of a high-pressure amorphous phase. As a result of the irreversible nature of the amorphization, metallic conductivity retains near ambient pressure in depressurized PdPSe. The synchronous evolution of structural, photoelectronic, and electronic properties elucidates a deep understanding of the structure-property relationship and emergent functionalities in a vdW pentagonal semiconductor via pressure modulation. This finding further provides an opportunity for tuning, understanding, and exploring the related vdW pentagonal materials under extreme conditions.

We generalize the odd elasticity of planar materials to thermoelasticity, admitting spatially inhomogeneous properties. First, we show that for active systems breaking Onsager relations thermal evolution is given by an odd generalization of the Maxwell-Cattaneo relation. Next, three different heat conduction models of odd solids are considered, leading, respectively, to a classical coupled thermoelasticity with Fourier law, thermoelasticity with relaxation times of the Maxwell-Cattaneo type, and thermoelasticity with two relaxation times. Governing equations are established in terms of either displacement-temperature pair, stress-heat flux pair, or stress-temperature pair. Next, we establish a form of the stiffness tensor, ensuring its inversion to a compatibility tensor, and write equations of elasticity in the presence of eigenstrains, such as thermal strains, where we find that the stress field remains unchanged for a specific additive change of the compliance tensor field. This so-called stress invariance gives an equivalence class of a wide range of odd materials with different values of material properties. Effectively, within each class, the elastic compliances may be modified by a field linear in the plane without affecting the stress field. Finally, we study hydrodynamic modes in an odd thermoelastic solid with Fourier heat conduction and argue that contrary to even elastic solids, the temperature can affect both dilatational and shear waves. We present odd corrections to sound attenuation and diffusion coefficients.

The recently introduced mean-field anharmonic bond model has shown remarkable accuracy in predicting finite temperature free energies for certain potential models of fcc crystals without thermodynamic sampling. In this work, we extend the model to treat modern machine learning potentials in both isochoric and isobaric ensembles while preserving existing vibrational correlations and ensuring thermodynamic self-consistency. Testing against molecular dynamics simulations of bulk fcc Al and Cu, we find free energies with an accuracy of a few meV/atom up to the melting temperature under typical operating pressures, with similar accuracy for thermal expansion. Our sampling-free estimation is universally superior to the quasiharmonic approximation for less than ten percent of the computational cost and many orders of magnitude more efficient than thermodynamic integration. We discuss applications of the method in modern computational materials science workflows.

We investigate the dynamical properties of the XY spin-1/2 chain with infinite-range transverse interactions and find a dynamical phase transition with a chaotic dynamical phase. In the latter, we find nonvanishing finite-time Lyapunov exponents and intermittent behavior signaled by fast and slow entropy growth periods. Further, we study the XY chain with a local self-consistent transverse field and observe a localization phase transition. We show that localization stabilizes the chaotic dynamical phase.

Magnetic van der Waals materials are often proposed for use in future spintronic devices, aiming to leverage the combination of long-range magnetic order and near-atomic thinness to produce energy-efficient components. One class of material that has been discussed in this context are the iron halides ${\mathrm{FeCl}}_2$ and ${\mathrm{FeBr}}_2$, which are $A$-type antiferromagnets with strong uniaxial magnetocrystalline anisotropy. However, despite characterization of the bulk materials, the possibility for sustaining the magnetic behaviors that would underpin such applications in thin flakes has not been investigated. In this work, we use nitrogen-vacancy center microscopy to quantitatively image magnetism in individual exfoliated flakes of these iron halides, revealing the absence of magnetic remanence, a weak induced magnetization under bias field, and variable behavior versus temperature. We show that our results are consistent with the antiferromagnetic behavior of the bulk material with a soft ferromagnetic uncompensated layer, indicating that extended ($>1\mu \mathrm{m}$) ferromagnetic domains are not sustained even at low temperatures (down to 4 K). Finally, we find that the magnetic order is strongly affected by the sample preparation, with a surprising diamagnetic behavior observed in a thin, hydrated sample.

A notable characteristic of ${\mathrm{PbTaSe}}_2$, a prototypical noncentrosymmetric superconductor (NCS), is that its superconductivity can be modulated through a structural transition under hydrostatic pressure [Kaluarachchi et al. Phys. Rev. B 95, 224508 (2017)]. Here we report on a combined pressure-sensitive point-contact Andreev reflection (PCAR) spectroscopy with bulk resistance measurements on ${\mathrm{PbTaSe}}_2$, to elucidate the nature of the surface and bulk superconductivity and their evolution with hydrostatic pressure. It is found that in a high-pressure region the superconducting gap opening temperature $T_c^A$ is significantly lower than the bulk resistive transition temperature $T_c^R$, revealing a clear experimental signature of surface-bulk separation associated with enhanced antisymmetric spin-orbit coupling (ASOC). The PCAR spectra, reflecting the superconducting surface state, are analyzed with two-dimensional Blonder-Tinkham-Klapwijk theory, yielding an isotropic $s$-wave full BCS gap in the strong-coupling regime. These results suggest the coexistence of full gap $s$-wave superconductivity and topological surface states in ${\mathrm{PbTaSe}}_2$, indicating that this NSC superconductor with significantly enhanced ASOC may offer a solid platform to investigate the topological aspect in the superconducting condensate.

We explore Weyl and Dirac semimetals with tilted nodes as platforms for realizing an intrinsic superconducting diode effect. Although tilting breaks sufficient spatial and time-reversal symmetries, we prove that — at least for conventional $s$-wave singlet pairing — the effect is forbidden by an emergent particle-hole symmetry at low energies if the Fermi level is tuned to the nodes. Then, as a stepping stone to the three-dimensional semimetals, we analyze a minimal one-dimensional model with a tilted helical node using Ginzburg-Landau theory. While one might expect a drastic enhancement of the effect when the node turns from type-I to type-II, we find that the presence of multiple Fermi pockets is more important as it enables multiple pairing amplitudes with independent contributions to supercurrents in opposite directions. Equipped with this insight, we construct minimal lattice models of Weyl and Dirac semimetals and study the superconducting diode effect in them. Once again, we see a enhancement when the normal state has multiple Fermi pockets per node that can accommodate more than one pairing channel. In summary, this study sheds light on the key factors governing the intrinsic superconducting diode effect in systems with asymmetric band structures and paves the way for realizing it in topological semimetals.

Atoms deposited on atomically thin substrates are a playground for exotic quantum many-body physics due to the highly tunable, atomic-scale nature of the interaction potentials. The ability to engineer strong interparticle interactions can lead to the emergence of collective states of matter, not possible in the context of dilute atomic gases confined in optical lattices. While it is known that the first layer of adsorbed helium on graphene is permanently locked into a solid phase, we motivate, with a physically intuitive mean-field calculation, and confirm, with quantum Monte Carlo simulations, that simple isotropic graphene lattice expansion unlocks a large variety of two-dimensional ordered commensurate, incommensurate, cluster atomic solid, and superfluid states for adsorbed atoms. It is especially significant that an atomically thin superfluid phase of matter emerges under experimentally feasible strain values, with potentially supersolid phases in close proximity on the phase diagram.

Yttrium hexahydride is a top superconducting superhydride, but its measured $T_c$ data are widely scattered and well below theoretical predictions. These prominent anomalies remain unresolved despite major efforts, impeding the understanding of this distinct class of superb superconductors. Here, we show that nonhydrostatic stresses, which are omnipresent in high-pressure experiments but omitted in prior theoretical studies, cause large deviations of $T_c$ from hydrostatic values, accounting for the strong scattering of experimental data and, joint with an anharmonic correction, reconciling the diverging theoretical and experimental $T_c$ results. Analysis of stress-induced phonon and charge responses unveils the underlying microscopic mechanisms. These findings offer a viable solution to the notable superconductivity anomalies in ${\mathrm{YH}}_6$, and the insights have broad implications for other superconducting superhydrides and, more generally, superconductors under extreme compression.

Parity-violating superconductors can support a low-dimension local interaction that becomes, upon condensation, a purely spatial Chern-Simons term. Solutions to the resulting generalized London equations can be obtained from solutions of the ordinary London equations with a complex penetration depth, and suggest several remarkable physical phenomena. The problem of flux exclusion by a sphere brings in an anapole moment, the problem of current-carrying wires brings in an azimuthal magnetic field, and the problem of vortices brings in currents along the vortices. We demonstrate that interactions of this kind, together with a conceptually related dimensionally reduced Chern-Simons interaction, can arise from physically plausible microscopic interactions.

The structural modifications associated with magnetic and multiferroic transitions in ${\mathrm{FeVO}}_4$ polycrystalline samples have been investigated using complementary techniques, including synchrotron radiation x-ray diffraction (XRD), Raman spectroscopy, and x-ray absorption fine-structure spectroscopy (XAFS). Temperature-dependent measurements reveal unveiled details of the local structural modifications intimately related to the spin-lattice coupling. This study is particularly focused on illuminating the changes occurring crossing the two distinct antiferromagnetic transition temperatures at 21.8 $(\pm 0.5) \mathrm{K}$ $\left(T_{N1}\right)$ and 15.6 $(\pm 0.5) \mathrm{K}$ $\left(T_{N2}\right)$ in the ${\mathrm{FeVO}}_4$. The combined analysis of XRD, Raman, and XAFS spectra rules out major modification of the Fe valence state or the average $\mathrm{Fe}\text{−}{\mathrm{O}}_n$ coordination geometry related to the transitions at $T_{N1}$ and $T_{N2}$. On the contrary, structural modifications are revealed in the Fe-Fe, Fe-V, and Fe-O-V path distances as a function of temperature, attributable to changes in the tilting and rotation of $\mathrm{Fe}\text{−}{\mathrm{O}}_n$ and/or $\mathrm{V}\text{−}{\mathrm{O}}_n$ polyhedra, likely representing the microstructural fingerprint of changes in the super-super-exchange interactions along the ${\mathrm{Fe}}^{3+}\text{−}\mathrm{O}\text{−}{\mathrm{V}}^{5+}\text{−}\mathrm{O}\text{−}{\mathrm{Fe}}^{3+}$ pathway.

This work is an analytic theoretical study of a two-dimensional (2D) semiconductor with a Fermi surface that is split by the Zeeman coupling of electron spins to an external magnetic field in the presence of electron-electron interactions. We calculate the spin susceptibility for long-range and finite-range interactions diagrammatically, and we find a resonant peak structure at the Kohn anomaly already in first-order perturbation theory. In contrast to the density-density correlator that is suppressed due to the large electrostatic energy required to stabilize charge density order, the spin susceptibility does not suffer from electrostatic screening effects, thus favoring spin density wave order in 2D semiconductors. Our results impose significant consequences for determining magnetic phases in 2D semiconductors. For example, a strongly enhanced Kohn anomaly may result in helical ordering of magnetic impurities due to the Ruderman-Kittel-Kasuya-Yosida interaction. Furthermore, the spin degree of freedom can equally represent a layer pseudospin in the case of bilayer materials. In this case, the external “magnetic field” is a combination of layer bias and interlayer hopping. The sharp peak of the 2D static spin susceptibility may then be responsible for dipole-density-wave order in bilayer materials at large enough electron-phonon coupling.

The algebraic tools used to study topological phases of matter are not clearly suited to studying processes in which the bulk energy gap closes, such as phase transitions. We describe an elementary two edge thought experiment, which reveals the effect of an anyon condensation phase transition on the robust edge properties of a sample, bypassing a limitation of the algebraic description. In particular, the two edge construction allows some edge degrees of freedom to be tracked through the transition, despite the bulk gap closing. The two edge model demonstrates that bulk anyon condensation induces symmetry breaking in the edge model. Further, the construction recovers the expected result that the number of chiral current carrying modes at the edge cannot change through anyon condensation. We illustrate the construction through detailed analysis of anyon condensation transitions in an achiral phase, the toric code, and in chiral phases, the Kitaev spin liquids.

The TiSiCO-family monolayer $X_{2}Y{\mathrm{CO}}_2$ ($X=\mathrm{Ti}$, Zr, Hf; $Y=\mathrm{Si}$, Ge) is a two-dimensional second-order topological insulator with unique valley-layer coupling in equilibrium condition. In this work, based on the four-band tight-binding (TB) model of monolayer ${\mathrm{Ti}}_2{\mathrm{SiCO}}_2$ (ML-TiSiCO) and the Floquet theory, we study the nonequilibrium properties of the ML-TiSiCO under a periodic field of laser and a gate-electric field. We find the interaction between the time-periodic polarized light and the electric field can lead to a variety of intriguing topological phase transitions. By driving the system with only circularly polarized light (CPL), a photoinduced topological phase transition occurs from a second-order topological insulator to a Chern insulator with a Chern number of $C=\pm 2$, and the sign of the Chern number $C$ is determined by the chirality of the incident light. Further adding a perpendicular electric field, we find that the ML-TiSiCO exhibits a rich phase diagram, consisting of Chern insulators with different Chern numbers and various topological semimetals. In contrast, since the linearly polarized light (LPL) does not break time-reversal symmetry, the Chern number of the system would not be changed under the irradiation of LPL. However, there still exist many topological phases, including second-order topological insulator, topological semi-Dirac, Dirac, and valley-polarized Dirac semimetals under the interaction between the LPL and the electric field. Our results not only enhance the understanding of the fundamental properties of ML-TiSiCO but also broaden the potential applications of such materials in optoelectronic devices.

We report the results of our calculations of the lattice parameters and fundamental band gaps in the rocksalt structured quasibinary random alloy aluminum scandium nitride (${\mathrm{Al}}_x{\mathrm{Sc}}_{1-x}\mathrm{N}$) in the range of $0\le x\le 1$. ${\mathrm{Al}}_x{\mathrm{Sc}}_{1-x}\mathrm{N}$ is a random alloy with very promising properties for the introduction of novel functionalities in (opto)electronic devices. The lattice parameters for the cubic rocksalt ${\mathrm{Al}}_x{\mathrm{Sc}}_{1-x}\mathrm{N}$ deviate positively from Vegard's law with a small bowing. The fundamental band gap shows a more complex behavior and contrary to previous reports, the fundamental band gap in rocksalt ${\mathrm{Al}}_x{\mathrm{Sc}}_{1-x}\mathrm{N}$ is indirect. Furthermore, rocksalt ${\mathrm{Al}}_x{\mathrm{Sc}}_{1-x}\mathrm{N}$ allows band-gap engineering in the range from $1.4$ to $7.1\mathrm{eV}$.

The Kane-Mele model is known to show a quantized spin Hall conductivity at zero temperature. Including Hubbard interactions at each site leads to a quantum phase transition to an XY antiferromagnet at sufficiently high interaction strength. Here, we use the two-particle self-consistent approach (TPSC), which we extend to include spin-orbit coupling, to investigate the Kane-Mele-Hubbard model at finite temperature and half filling. TPSC is a weak to intermediate coupling approach capable of calculating a frequency- and momentum-dependent self-energy from spin and charge fluctuations. We present results for the spin Hall conductivity and correlation lengths for antiferromagnetic spin fluctuations for different values of temperature, spin-orbit coupling, and Hubbard interaction. The vertex corrections, which here are analogs of Maki-Thompson contributions, show a strong momentum dependence and give a large contribution in the vicinity of the phase transition at all temperatures. Their inclusion is necessary to observe the quantization of the spin Hall conductivity for the interacting system in the zero-temperature limit. At finite temperature, increasing the Hubbard interaction leads to a decrease of the spin Hall conductivity. This decrease can be explained by band-gap renormalization from scattering of electrons on antiferromagnetic spin fluctuations.

We examine topological phases and symmetry-protected electronic edge states in the context of a Rydberg composite: a Rydberg atom interfaced with a structured arrangement of ground-state atoms. We show that the spectrum of the electronic Hamiltonian of such a composite possesses a mapping to that of a tight-binding Hamiltonian, which can exhibit nontrivial topology depending on the arrangement of the ground-state atoms and the principal quantum number of the Rydberg state. The Rydberg electron moves in a combined potential including the long-ranged Coulomb interaction with the Rydberg core and short-ranged interactions with each neutral atom; the effective hopping amplitudes between sites are determined by this combination. We first confirm the existence of topologically-protected edge states in a Rydberg composite by mapping it to the paradigmatic Su-Schrieffer-Heeger dimer model. Following that, we show that more complicated systems with trimer unit cells can be studied in a Rydberg composite.

We apply the adaptive variational quantum imaginary time evolution (AVQITE) method to prepare ground states of one-dimensional spin $S=1$ models. We compare different spin-to-qubit encodings (standard binary, Gray, unary, and multiplet) with regard to the performance and quantum resource cost of the algorithm. Using state-vector simulations, we study two well-known spin-1 models: the Blume-Capel model of transverse-field Ising spins with single-ion anisotropy, and the XXZ model with single-ion anisotropy. We consider system sizes of up to 20 qubits, which corresponds to spin-1 chains up to length 10. We determine the dependence of the number of cnot gates in the AVQITE state preparation circuit on the encoding, the initial state, and the choice of operator pool in the adaptive method. Independent of the choice of encoding, we find that the cnot gate count scales cubically with the number of spins for the Blume-Capel model and quartically for the anisotropic XXZ model. However, the multiplet and Gray encodings present smaller prefactors in the scaling relations. These results provide useful insights for the implementation of AVQITE on quantum hardware.