Superconducting transition metal dichalcogenides have emerged as a platform for hosting novel and nontrivial physical phenomena. We report a detailed investigation of superconducting and transport properties on $2H\text{−}\mathrm{TaSeS}$ single crystals. It suggests that TaSeS is a multigap anisotropic superconductor with the upper critical field, breaking the Pauli limiting field in both in-plane and out-of-plane directions. The angle dependence of the upper critical field suggests the two-dimensional superconducting nature in bulk $2H\text{−}\mathrm{TaSeS}$.

Muon spin rotation ($\mu \mathrm{SR}$) and inelastic x-ray scattering were used to investigate the superconducting properties of the filled-skutterudite compound ${\mathrm{LaRu}}_4{\mathrm{As}}_{12}$. A two-gap isotropic ($s+s$)-wave model can explain the temperature dependence of the superfluid density. Zero-field $\mu \mathrm{SR}$ measurements confirm that the time-reversal symmetry does not break upon entering the superconducting state. The measurements of lattice dynamics at 2, 20, and 300 K revealed temperature dependences of the phonon modes that do not strictly follow a hardening of phonon frequencies upon cooling as expected within the quasiharmonic picture. The 20 K data rather mark a turning point for the majority of the phonon frequencies. Indeed, a hardening is observed approaching 20 K from above, while for a few branches a weak softening is visible upon further cooling to 2 K. The observed dispersion relations of phonon modes throughout the Brillouin zone matches with the density functional theory prediction quite closely. Our results point out that cubic ${\mathrm{LaRu}}_4{\mathrm{As}}_{12}$ is a good reference material for studying multiband superconductivity, including those with lower crystallographic symmetries such as iron arsenide–based superconductors.

We report the discovery of structural phase transitions and superconductivity in the full Heusler compounds $X{\mathrm{Pd}}_2\mathrm{Sn}$ ($X=\text{Ti}$, Zr, Hf), by means of electrical transport, magnetic susceptibility, specific heat, and x-ray diffraction measurements. $\mathrm{Ti}{\mathrm{Pd}}_2\mathrm{Sn}$, $\mathrm{Zr}{\mathrm{Pd}}_2\mathrm{Sn}$, and $\mathrm{Hf}{\mathrm{Pd}}_2\mathrm{Sn}$ undergo structural phase transitions from the room-temperature cubic $\mathrm{Mn}{\mathrm{Cu}}_2\mathrm{Al}$-type structure (space group $Fm\overline{3}m$) to a low-temperature tetragonal structure at around 160, 110, and 90 K, respectively, which are likely related to charge density wave (CDW) instabilities. Low-temperature single-crystal x-ray diffraction measurements of $\mathrm{Zr}{\mathrm{Pd}}_2\mathrm{Sn}$ demonstrate the emergence of a superstructure with multiple commensurate modulations below $T_s$. $\mathrm{Zr}{\mathrm{Pd}}_2\mathrm{Sn}$ and $\mathrm{Hf}{\mathrm{Pd}}_2\mathrm{Sn}$ have bulk superconductivity with transition temperatures $T_c\sim 1.2$ and 1.3 K, respectively. Density functional theory calculations reveal evidence for structural and electronic instabilities which can give rise to CDW formation, suggesting that these $X{\mathrm{Pd}}_2\mathrm{Sn}$ systems are good candidates for examining the interplay between CDW and superconductivity.

We study a three-leg spin-1/2 ladder with geometrically frustrated interleg interactions. We call this model an anisotropic triangular-strip (ATS) model. We numerically and field-theoretically show that its ground state belongs to a gapless symmetry-protected topological (SPT) phase. The numerical approach is based on density-matrix renormalization group analyses of the entanglement entropy and the entanglement spectrum. Whereas the entanglement entropy exhibits a critical behavior, the entanglement spectrum is nontrivially degenerate. These entanglement properties imply that the ground state is a gapless topological phase. We investigate the ATS model using a quantum field theory to support the numerical findings. When the frustrated interchain interaction is deemed a perturbation acting on the three spin chains, the frustrated interchain interaction almost isolates the second chain from the other two chains. However, at the same time, the second chain mediates a ferromagnetic interaction between the first and third chains. Therefore, the ground state of the ATS model is a gapless Tomonaga-Luttinger liquid weakly coupled to a spin-1 Haldane chain with irrelevant interactions. Last but not least, we show that the gapless SPT phase of the ATS model is a symmetry-protected critical phase. We point out that the symmetry protection of criticality is essential in characterization of the gapless SPT phase.

When two layers of two-dimensional materials are assembled with a relative twist, moiré patterns arise, inducing a tremendous wealth of exotic phenomena. In this work, we consider twisting two triangular lattices hosting Dirac quantum spin liquids. A single decoupled layer is described by compact quantum electrodynamics in $2+1$ dimensions (${\mathrm{QED}}_3$) with an emergent $\mathrm{U}\left(1\right)$ gauge field, which is assumed to flow to a strongly interacting fixed point in the IR with conformal symmetry. We use recent results for the quantum numbers of monopole operators, which tunnel $2\pi$ fluxes of the compact gauge field. It is found that, in the bilayer system, interlayer monopole tunneling is a symmetry-allowed relevant perturbation which induces an (ordering) instability. We show using perturbation theory that upon twisting the two layers the system remains unstable under the interlayer interaction, but any finite twist angle softens this instability compared to the untwisted case. To analyze the resulting phase induced by the (twisted) interlayer tunneling, we use “conformal mean-field theory”, which reduces the interacting bilayer system to two copies of ${\mathrm{QED}}_3$ coupled to background fields which are to be determined self-consistently. In the weak-coupling regime, where the interlayer coupling is weak compared to the energy scale set by the moiré lattice constant, we solve the self-consistency equations perturbatively. In the limit of strongly coupled layers, a local scaling approximation is used, and we find that the magnetically ordered state exhibits a lattice of magnetic vortices, with the lattice constant tunable through the twisting angle.

Counterdiabatic (CD) protocols enable fast driving of quantum states by invoking an auxiliary adiabatic gauge potential (AGP) that suppresses transitions to excited states throughout the driving process. Usually, the full spectrum of the original unassisted Hamiltonian is a prerequisite for constructing the exact AGP, which implies that CD protocols are extremely difficult for many-body systems. Here, we apply a variational CD protocol recently proposed by Claeys et al. [Phys. Rev. Lett. 123, 090602 (2019)] to a two-component fermionic Hubbard model in one spatial dimension. This protocol engages an approximated AGP expressed as a series of nested commutators. We show that the optimal variational parameters in the approximated AGP satisfy a set of linear equations whose coefficients are given by the squared Frobenius norms of these commutators. We devise an exact algorithm that escapes the formidable iterative matrix-vector multiplications and evaluates the nested commutators and the CD Hamiltonian in analytic representations. We then examine the CD driving of the one-dimensional Hubbard model up to $L=14$ sites with driving order $l⩽3$. Our results demonstrate the usefulness of the variational CD protocol to the Hubbard model and permit a possible route towards fast ground-state preparation for many-body systems.

A number of exactly solvable spin models, including the Kitaev toric code in two and three dimensions and the X-cube model in three dimensions, can be related to their respective parent lattice gauge theories (LGTs) through the mathematical process of “Higgsing.” Field theories of the low-energy excitations of these spin models can be developed subsequently, building upon the symmetry of the parent LGTs. Recently, two of the present authors proposed a variant of the three-dimensional toric code which we now call the F3 model, whose elementary excitations consist of freeon and fluxon excitations of the three-dimensional toric code and fracton excitations of the X-cube model. In this work, we identify the parent LGT of the F3 model as the hybrid rank-1 and rank-2 U(1) LGT and develop the corresponding field theory. The resulting Lagrangian of the F3 model is that of a three-dimensional toric code with an extra term, which ties the dynamics of fractons to that of fluxons. The matter part of the effective action for the F3 model can be derived as well by carefully keeping track of the gauge invariance of the F3 model. Hydrodynamic equations of motion of the quasiparticles are derived which properly reflect their constrained dynamics. Finally, we present a tight-binding model for the quasiparticle motion in the F3 model.

Notable non-Fermi liquid and quantum critical behaviors are observed in rare-earth metallic systems with non-Kramers local moments supporting a number of different multipolar moments. A prominent example is Pr(Ti, V)${}_2{\mathrm{Al}}_{20}$, where the non-Kramers doublet of the ${\mathrm{Pr}}^{3+}$ ion allows quadrupolar and octupolar moments, but lacks a dipolar moment. Previous theoretical studies show that a single impurity Kondo problem with such an unusual local moment leads to exotic non-Fermi liquid states. In this work, we investigate possible quantum critical behaviors arising from the competition between non-Fermi liquid states and multipolar-ordered phases induced by the RKKY interaction. We consider a local version of the corresponding Kondo lattice model, namely the Bose-Fermi Kondo model. Here, the multipolar local moments are coupled to fermionic and bosonic bath degrees of freedom representing the multipolar Kondo effect and RKKY interactions. Using a perturbative renormalization group (RG) study up to two loop order, we find critical points between non-Fermi liquid Kondo fixed points and a quadrupolar ordered fixed point. The critical points describe quantum critical behaviors at the corresponding phase transitions and can be distinguished by higher order corrections in the octupolar susceptibility that can be measured by ultrasound experiments. Our results imply the existence of a rich expansion of the phases and quantum critical behaviors in multipolar heavy fermion systems.

Density functional theory (DFT) within the local or semilocal density approximations, i.e., the local density approximation (LDA) or generalized gradient approximation (GGA), has become a workhorse in the electronic structure theory of solids, being extremely fast and reliable for energetics and structural properties, yet remaining highly inaccurate for predicting band gaps of semiconductors and insulators. The accurate prediction of band gaps using first-principles methods is time consuming, requiring hybrid functionals, quasiparticle GW, or quantum Monte Carlo methods. Efficiently correcting DFT-LDA/GGA band gaps and unveiling the main chemical and structural factors involved in this correction is desirable for discovering novel materials in high-throughput calculations. In this direction, we use DFT and machine learning techniques to correct band gaps and band-edge positions of a representative subset of $AB{\mathrm{O}}_3$ perovskite oxides. Relying on the results of HSE06 hybrid functional calculations as target values of band gaps, we find a systematic band-gap correction of $\sim 1.5$ eV for this class of materials, where $\sim 1$ eV comes from downward shifting the valence band and $\sim 0.5$ eV from uplifting the conduction band. The main chemical and structural factors determining the band-gap correction are determined through a feature selection procedure.

Due to the rise of applications in several optoelectronic and spintronic platforms, theoretically engineering the propagation of light in low-dimensional systems dealing with both charge and spin degrees of freedom has recently triggered considerable interest. By breaking the $C_{3v}$ symmetry through external exchange fields, we engineer the propagation of an incident circularly polarized light in a noncentrosymmetric quantum spin Hall insulator, monolayer PbBiI, that is active in the near-infrared region of the electromagnetic spectrum. The Kubo formalism is ideal for optical properties. We endeavor to thoroughly demonstrate that a critical-breaking regime of three types of fields leads to various anisotropic electronic phases and optical interband transitions. We find various near-to-far infrared shifts for the optical activity of the system when the $C_{3v}$ symmetry breaking fields are turned on. This is proven in the ensuing two ways: The spectrum of optical conductivity components and the intensities of scattered/absorbed light. We further find the criteria under which the system is transparent. Finally, depending on the exchange fields, we engineer the eccentricities for the reflected and transmitted lights in monolayer PbBiI to see how the polarization of incident circular light becomes linear and elliptical. The applicability of results in industry and medicine is also briefly discussed.

Interactions between elementary excitations are of great interest from a fundamental aspect and for novel applications. While plasmon-excitons have been extensively studied, the interaction mechanisms between acoustic vibrations (phonons) and localized surface plasmons (LSPs) remain quite unexplored. Here, we present a theoretical framework for the investigation of the interaction between confined acoustic vibrations and LSPs involved in resonant acoustic phonons Raman scattering. We express the Raman scattering process in the framework of Fermi's golden rule and present the concept of Raman energy density (RED). Similarly to the Raman-Brillouin electronic density (RBED) introduced for semiconductors, this physical quantity is used as a theoretical tool for the interpretation of resonant Raman scattering mediated by LSPs in metallic nanoparticles. The RED represents the electromagnetic energy density excited in the Raman scattering process and modulated by the acoustic vibrations of the nanoparticle. We show that, similarly to the LDOS and the RBED, the RED can be mapped in the near-field region, and provides a clear picture of the interaction between LSPs and acoustic vibrations giving rise to inelastic scattering measurable in the far field. We use RED concept to investigate elastic (an)isotropy effects and extract the Raman selection rules of spherical nanoparticles embedded in a dielectric environment.

In this paper, we investigate the spinless stationary Schrödinger equation for the electron when it is permanently bound to a generalized Ellis-Bronnikov bilayer graphene wormholelike surface. The curvature gives rise to a geometric potential affecting thus the electronic dynamics. The geometry of the wormhole's shape is controlled by the parameter $n$ which assumes even values. We discuss the role played by the parameter $n$ and the orbital angular momentum on bound states and probability density for the electron.

We report strong enhancement of spin-orbit torques by incorporating ${\mathrm{Si}}_3{\mathrm{N}}_4$ impurities into the dirty metal Pt. We find that the ${\mathrm{Si}}_3{\mathrm{N}}_4$ impurities lower the spin Hall conductivity and the charge conductivity of the Pt host at different rates, leading to a twofold increase in the dampinglike spin-orbit torque per unit current density for ferrimagnetic ${\mathrm{Pt}}_x{\left({\mathrm{Si}}_3{\mathrm{N}}_4\right)}_{1–x}/{\mathrm{Co}}_{0.65}{\mathrm{Tb}}_{0.35}$ bilayers. This torque enhancement is attributed to the optimized trade-off between the intrinsic spin Hall conductivity and the spin carrier lifetime in the dirty limit. We also find that only 58% of the angular momentum of the spin current entering the ferrimagnetic ${\mathrm{Co}}_{0.65}{\mathrm{Tb}}_{0.35}$ relaxes via exchange interaction and thus makes a contribution to spin torque generation. This work establishes ${\mathrm{Pt}}_{0.7}{\left({\mathrm{Si}}_3{\mathrm{N}}_4\right)}_{0.3}$ with a high spin Hall ratio of 0.8 and a high charge conductivity of $1.2\times {10}^6{\mathrm{\Omega }}^{-1}{\mathrm{m}}^{-1}$ as a compelling spin Hall metal for spin-orbitronics. This work also reaffirms the variation of the spin-orbit torque with relative spin relaxation rates within the magnetic layer.

We study an exactly solvable one-dimensional spin-$\frac{1}{2}$ model which can support weak zero modes in its ground-state manifold. The spin chain has staggered $XXZ$-type and $ZZ$-type spin interaction on neighboring bonds and is thus dubbed the ($XXZ,Z$) chain. The model is equivalent to an interacting fermionic chain by Jordan-Wigner transformation. We study the phase diagram of the system and work out the conditions and properties of its weak zero modes. In the fermion chain representation, these weak zero modes are given by even-order polynomials of Majorana fermion operators and are thus bosonic. The fermionic chain Hamiltonian contains only fermion hopping and interaction terms and may have potential realization in experiments.

In gated bilayer graphene, topological zero-line modes (ZLMs) appear along lines separating regions with opposite valley Hall topologies. Although it is experimentally difficult to design the electric gates to realize ZLMs due to the extremely challenging techniques, twisted bilayer graphene provides a natural platform to produce ZLMs in the presence of a uniform electric field. In this Letter, we develop a set of wave-packet dynamics for ZLMs in monolayer graphene, which can be utilized to characterize various gapless edge modes and can quantitatively reproduce the electronic transport properties at topological intersections. To our surprise, at a topological trifurcation, we show that the counterintuitive current partition (i.e., the direct transport propagation) originates from the microscopic mechanism “bypass jump” which is proved to exist in both monolayer and bilayer systems. Our method can be applied to understand the microscopic pictures of the electronic transport features of all kinds of topological states.

We report that complete suppression of a phonon-driven structural phase transition causes partial breakdown of a three-dimensional translation symmetry in a well-defined sublattice. This state is revealed for a dielectric compound, ${\mathrm{Ba}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Al}}_2{\mathrm{O}}_4$, that comprises an ${\mathrm{AlO}}_4$ network incorporated into a hexagonal Ba(Sr) sublattice. Pair distribution function analyses and inelastic neutron scattering experiments provide clear-cut evidence of the ${\mathrm{AlO}}_4$ network forming a continuum of Al-O short-range correlations similar to glasses, whereas the Ba(Sr) sublattice preserves the original translational symmetry. This glassy network significantly dampens the phonon spectrum and transforms it into the broad one resembling those typically observed in glass materials.

We present a one-dimensional coupled ring resonator lattice exhibiting a variant of the non-Hermitian skin effect (NHSE) that we call the anomalous Floquet NHSE. Unlike existing approaches to achieving the NHSE by engineering gain and loss on different ring segments, our design uses fixed on-site gain or loss in each ring. The anomalous Floquet NHSE is marked by the existence of skin modes at every value of the Floquet quasienergy, allowing for broadband asymmetric transmission. Varying the gain or loss induces a non-Hermitian topological phase transition, reversing the localization direction of the skin modes. An experimental implementation in an acoustic lattice yields good agreement with theoretical predictions, with a very broad relative bandwidth of around 40%.

Experimental investigations of the phase transition in GeTe provide contradictory conclusions regarding the nature of the phase transition. Considering growing interest in technological applications of GeTe, settling these disputes is of great importance. To that end, we present a molecular dynamics study of the structural phase transition in GeTe using a machine-learned interatomic potential with ab initio accuracy. First, we calculate the asymmetric shape of the radial distribution function of the nearest-neighbor bonds above the critical temperature, in agreement with previous studies. However, we show that this effect is not necessarily linked to the order-disorder phase transition and can occur as a result of large anharmonicity. Next, we study in detail the static and dynamic properties of the order parameter in the vicinity of the phase transition and find fingerprints of both order-disorder and displacive phase transition.

We present a large-scale simulation of the ground state interface properties of the diluted Sherrington-Kirkpatrick spin glass of Gaussian disorder for a broad range of the bond occupation probability $p$ using the strong disorder renormalization group and the population annealing Monte Carlo methods. The model is studied in the framework of the diluted one-dimensional Kotliar-Anderson-Stein spin glass with power-law interactions in the mean-field regime. We find that the interface is space-filling independent of $p$, i.e., the fractal dimension $d_s=1$. The stiffness exponent $\theta$ is likely also independent of $p$, despite that the energy finite-size correction exponent $\omega$ varies with $p$ as recently found. The energy finite-size scaling is also analyzed and compared with that of the $\pm J$ disorder, finding that the thermodynamic energy is universal in both $p$ and the disorder, and the exponent $\omega$ varies with $p$ but is universal in the disorder.