We propose and theoretically substantiate a method to study the nonlocal conductivity of two-dimensional electron systems (2DESs) using the tools of near-field microscopy. We show that the height-dependent polarizability of an illuminated near-field probe is substantially different for various transport regimes of charge carriers in a 2DES. For the hydrodynamic transport regime, the polarizability scales as $z_0^{-2}$, where $z_0$ is the elevation of the probe above the 2DES. Both for Drude and for classical ballistic regimes of conduction, the polarizability scales as $z_0^{-3}$. In the former case, the polarization is carrier density independent, while in the latter it largely depends on carrier density. More generally, we find that the polarizability of the probe is proportional to the Laplace transform of the wave-vector-dependent conductivity and the inverse dielectric function of the 2DES over the wave vectors $q$. Our results should provide a simple tool for studies of nonlocal conductivity in solids, which is challenging to address with other techniques.

Torsion can be realized as dislocation in the crystal lattice of material. It is particularly interesting if the material has fermions in the spectrum, such as graphene, topological insulators, and Dirac and Weyl semimetals, as it's transport properties can be affected by the torsion. In this paper, we find that, due to Weyl anomaly, torsion in Dirac and Weyl semimetals can induce novel chiral currents, either near a boundary or in a “conformally flat space.” We briefly discuss how to measure this interesting effect in experiment. It is remarkable that these experiments can help to clarify the theoretical controversy of whether an imaginary Pontryagin density could appear in the Weyl anomaly.

By means of first-principles calculations, we study the topological nontrivial electronic structures and lattice dynamics of bulk and monolayer ${\mathrm{W}}_2{\mathrm{O}}_3{\mathrm{I}}_4$. We find that both of them are nodal-line semimetals in the absence of spin-orbit coupling (SOC), in which the nodal lines are induced by the band inversion between conduction and valence bands contributed by W $d$ and I p orbitals. Moreover, ${\mathrm{W}}_2{\mathrm{O}}_3{\mathrm{I}}_4$ bulk and monolayer exhibit excellent mechanical and thermal stability. By studying the external field modulation of topological nodal lines without SOC effect in monolayer ${\mathrm{W}}_2{\mathrm{O}}_3{\mathrm{I}}_4$, we find that the nodal lines are extremely robust against the biaxial strain but destroyed under vertical electric field due to the breaking of horizonal mirror symmetry. It is studied that the SOC effect here is large and non-negligible, and after considering the SOC effect, ${\mathrm{W}}_2{\mathrm{O}}_3{\mathrm{I}}_4$ bulk and monolayer undergo change of topological state from a topological nodal-line semimetal to a topological insulator. Our work not only extends the member of inorganic three-dimensional and two-dimensional topological semimetals library but also provides a prototype material candidate to investigate the double nodal-line surface states.

The topological singularity of the Bloch states close to the Fermi level significantly enhances nonlinear electric responses in topological semimetals. Here, we systematically characterize this enhancement for a large class of topological nodal-point fermions, including those with linear, linear-quadratic, and quadratic dispersions. Specifically, we determine the leading power-law dependence of the nonlinear response functions on the chemical potential $\mu$ defined relative to the nodal point. We identify two characteristics that qualitatively improve nonlinear transports compared to those of conventional Dirac and Weyl fermions. First, the type-II (overtilted) spectrum leads to the $log\mu$ enhancement of nonlinear response functions having zero scaling dimension with respect to $\mu$, which is not seen in a type-I (moderately or not tilted) spectrum. Second, the anisotropic linear-quadratic dispersion increases the power of small-$\mu$ divergence for the nonlinear response tensors along the linearly dispersing direction. Our work reveals new experimental signatures of unconventional nodal points in topological semimetals as well as provides a guiding principle for giant nonlinear electric responses.

Elastic topological states have been receiving increased attention in numerous scientific and engineering fields due to their defect-immune nature, resulting in applications of vibration control and information processing. Here, we present the data-driven discovery of elastic topological states using dynamic mode decomposition (DMD). The DMD spectrum and DMD modes are retrieved from the propagation of the relevant states along the topological boundary, where their nature is learned by DMD. Applications such as classification and synthesis of wave propagation can be achieved by the underlying characteristics from DMD. We demonstrate the classification between topological and traditional metamaterials using DMD modes. Moreover, the model enabled by the DMD modes realizes the synthesis of topological state propagation along the given interface. Our approach to characterizing topological states using DMD can pave the way towards data-driven discovery of topological phenomena in material physics and more broadly lattice systems.

Two-dimensional Heisenberg honeycomb ferromagnets are expected to have interesting topological magnon effects, as their magnon dispersion can have Dirac points. The Dirac points are gapped with the finite second-nearest-neighbor Dzyaloshinskii-Moriya interaction, providing nontrivial Berry curvature with the finite magnon Hall effect. However, it is unknown how the topological properties are affected by magnon damping. We report the thermal Hall effect in ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ (CGT), an insulating two-dimensional honeycomb ferromagnet with a large Dirac magnon gap and significant magnon damping. Interestingly, the thermal Hall conductivity in CGT shows coexisting phonon and magnon contributions. Using an empirical two-component model, we successfully estimate the magnon contribution separate from the phonon part, revealing that the magnon Hall conductivity was 20 times smaller than the theoretical calculation. Finally, we suggest that such considerable suppression in the magnon Hall conductivity is due to the magnon damping effect in CGT.

The magnetic excitation spectrum and Hamiltonian of the quantum magnet ${\mathrm{BaCuTe}}_2{\mathrm{O}}_6$ is studied by inelastic neutron scattering (INS) and density functional theory (DFT). INS on powder and single crystal samples reveals overlapping spinon continua—the spectrum of an antiferromagnetic spin-1/2 spin chain—due to equivalent chains running along the a, b, and c directions. Long-range magnetic order onsets below $T_{\mathrm{N}}=6.3$ K due to interchain interactions, and is accompanied by the emergence of sharp spin-wave excitations, which replace the continua at low energies. The spin-wave spectrum is highly complex and was successfully modelled achieving excellent agreement with the data. The extracted interactions reveal an intrachain interaction, $J_3=2.9$ meV, while the antiferromagnetic hyperkagome interaction $J_2$ is the subleading interaction responsible for coupling the chains together in a frustrated way. DFT calculations reveal a similar picture for ${\mathrm{BaCuTe}}_2{\mathrm{O}}_6$ of dominant $J_3$ and subleading $J_2$ antiferromagnetic interactions and also indicate a high sensitivity of the interactions to small changes of structure, which could explain the very different Hamiltonians observed in the sister compounds ${\mathrm{SrCuTe}}_2{\mathrm{O}}_6$ and ${\mathrm{PbCuTe}}_2{\mathrm{O}}_6$.

Andreev bound states (ABSs) occur in Josephson junctions when the total phase of the Andreev and normal reflections is a multiple of $2\pi$. In ballistic junctions with an applied voltage bias, a quasiparticle undergoes multiple Andreev reflections before entering the leads, resulting in peaks in the current-voltage $I\left(V\right)$ curve. Here we present a model for a two-dimensional S/TI/MTI/TI/S junction, where S is a superconductor, TI is a topological insulator, and MTI is a magnetic topological insulator barrier. We show that the interplay of broken time-reversal symmetry and topology results in an asymmetric $I\left(V\right)$ curve. Such junctions are predicted to host chiral Majorana modes. We demonstrate that the peak positions in $I\left(V\right)$ are directly linked to ABSs. We use this to show how the angle-resolved $I\left(V\right)$ curve becomes a spectroscopic tool for the chirality and degeneracy of ABSs.

The propagation velocities of shock waves with Mach numbers in the range 1.1–1.5 have been directly measured in liquid lead over a wide region of fluid states in the specific volume and pressure plane across the metal-nonmetal transition range. The measured values are compared with those obtained from a caloric equation of state (EOS) constructed using the results of dynamic experiments. The comparison shows that the measured values are in good agreement within the values calculated by the EOS. These results suggest that, for the entire region of the fluid states investigated here (where the fluid is a one-phase system) the shock waves are stable. The values of the critical pressure, critical volume, and critical enthalpy were determined using the EOS and compared with literature data. Thus, the EOS of liquid lead is presented, describing its thermodynamic properties with known accuracy in wide ranges of specific volume and pressure.

Two-dimensional (2D) honeycomb ferromagnets offer the unprecedented opportunity to study interactions between collective modes that in standard bulk ferromagnets do not cross paths. Indeed, they harbor an optical spin-wave branch, i.e., a spin wave which disperses weakly near the Brillouin zone center. When doped with free carriers, they also host the typical gapless plasmonic mode of 2D itinerant electron/hole systems. When the plasmon branch meets the optical spin-wave branch, energy and momentum matching occurs, paving the way for interactions between the charge and spin sector. In this paper, we present a microscopic theory of such plasmon-magnon interactions, which is based on a double random phase approximation. We comment on the possibility to unveil this physics in recently isolated 2D honeycomb magnets such as ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$.

The negatively charged boron vacancy (${\text{VB}}^{-}$) spin defect in two-dimensional (2D) hexagonal boron nitride ($h$-BN) has attracted much attention for potential applications in quantum photonics recently. Its inherent van der Waals force mechanism guarantees convenient heterostructures for quantum sensing. By virtue of such materials, researchers not only can fabricate enough thin spin film naturally close to the sensing target but also can prepare an almost perfect spin ensemble with a uniform orientation. We here propose a setup with an ensemble of ${\text{VB}}^{-}$ spins strongly coupled to the superconducting coplanar waveguide resonator through the magnetic-dipolar interaction. The collective coupling strength is predicted to be $G/2\pi \sim 15$ MHz, which corresponds to the strong coupling region. This collective spin-photon interaction can mimic the one-axis twisting Lipkin-Meshkov-Glick model effectively and therefore guarantee the dynamic generation of the spin-squeezed state. In addition, we show the influence of inhomogeneous coupling caused by sample thickness on squeezing, which proves the validity of the homogeneity assumption in our scheme. This attempt not only explores the possibility and superiority of 2D ${\text{VB}}^{-}$ spins but also opens another avenue for quantum hybridization.

We theoretically investigate the thermoelectric properties (electronic contribution) of a hybrid structure comprising an inversion symmetry broken Weyl semimetal (WSM) and intrinsic Weyl superconductor (WSC) with $s$-wave pairing, employing the Blonder-Tinkham-Klapwijk formulation for noninteracting electrons. Our study unfolds interesting features for various relevant physical quantities such as thermal conductance, the thermoelectric coefficient, and the corresponding figure of merit. We also explore the effects of an interfacial insulating (I) barrier (WSM-I-WSC setup) on the thermoelectric response in the thin barrier limit. Further, we compute the ratio of the thermal to the electrical conductance in different temperature regimes and find that the Wiedemann-Franz law is violated for small temperatures (below critical temperature $T_c$) near the Weyl points while it saturates to the Lorentz number, away from the Weyl points, at all temperatures irrespective of the barrier strength. We compare and contrast this behavior with other Dirac material heterostructures and provide a detailed analysis of the thermal transport. Our study can facilitate the fabrication of mesoscopic thermoelectric devices based on WSMs.

Symmetry defects, e.g., vortices in conventional superfluids, play a critical role in a complete description of symmetry breaking phases. In this paper, we develop the theory of symmetry defects in fractonic superfluids, i.e., spontaneously higher-rank symmetry (HRS) breaking phases initially proposed by Yuan et al. [Phys. Rev. Res. 2, 023267 (2020)] and by Chen et al. [Phys. Rev. Res. 3, 013226 (2021)]. By Noether's theorem, HRS is associated with the conservation law of higher moments, e.g., dipoles, quadrupoles, and angular moments. We establish finite-temperature phase diagrams by identifying a series of topological phase transitions via the renormalization group flow equations and Debye-Hückel approximation. Accordingly, a series of Kosterlitz-Thouless topological transitions are found to occur successively at different temperatures, which are triggered by proliferation of defects, defect bound states, and so on. Such a hierarchical proliferation brings rich phase structures. Meanwhile, a screening effect from sufficiently high density of defect bound states leads to instability and collapse of the intermediate temperature phases, which further enriches the phase diagrams. For concreteness, we consider a fractonic superfluid in which “angular moments” are conserved. We then present the general theory, in which other types of HRS can be analyzed in a similar manner. Further directions are present at the end of the paper.

The zero-temperature phase diagram of the $J_1\text{−}{J}_2\mathrm{SU}\left(N\right)$ antiferromagnetic Heisenberg spin chain is investigated by means of complementary field theory and numerical approaches for general $N$. A fully gapped $\mathrm{SU}\left(N\right)$ valence bond solid made of $N$ sites is formed above a critical value of $J_2/J_1$ for all $N$. We find that the extension of this $N$-merized phase for larger values of $J_2$ strongly depends on the parity of $N$. For even $N$, the phase smoothly interpolates to the large $J_2$ regime where the model can be viewed as a zigzag $\mathrm{SU}\left(N\right)$ two-leg spin ladder. The phase exhibits both a $N$-merized ground state and incommensurate spin-spin correlations. In stark contrast to the even case, we show that the $N$-merized phase with odd $N$ has only a finite extent with no incommensuration. A gapless phase in the $\mathrm{SU}{\left(N\right)}_1$ universality class is stabilized for larger $J_2$ that stems from the existence of a massless renormalization group flow from $\mathrm{SU}{\left(N\right)}_2$ to $\mathrm{SU}{\left(N\right)}_1$ conformal field theories when $N$ is odd.

We report on ultrasonic and magnetocaloric-effect measurements of the hexagonal antiferromagnet ${\mathrm{Dy}}_3{\mathrm{Ru}}_4{\mathrm{Al}}_{12}$ having a distorted kagome lattice. We observed a pronounced softening of the transverse modulus, $C_{44}$, above 30 T applied along [100], indicating an as-yet-unidentified magnetic field induced phase transition. We determined the field-temperature phase diagrams using pulsed magnetic fields up to 55.2 T, applied along [100] and [001]. Optimizing crystal-electric-field parameters within the mean-field approximation, we conclude that the enhancement of the field induced phase-transition temperature is due to electric quadrupolar and magnetic octupolar mediated interactions. We propose that the order parameter of the magnetic field induced phase transitions is the electric quadrupole $O_{zx}$, facilitated by octupole-mediated interactions.

A recent work [Phys. Rev. Lett. 126, 208002 (2021)] has explored how thermal noise-induced randomness in a self-assembled photonic crystal affects photonic band gaps (PBGs). For the system of a two-dimensional photonic crystal composed of a self-assembled array of rods with square cross sections, it was found that its PBGs can exist over an extensive range of packing densities. Counterintuitively, at intermediate packing densities, the transverse magnetic (TM) band gap of the self-assembled system can be larger than that of its corresponding perfect system (rods arranged in a perfect square lattice and having identical orientations). Due to shape anisotropicity, the randomness in the self-assembled system contains two kinds of randomness, i.e., positional and orientational randomness of the particles. In this paper, we further investigate how PBGs are influenced solely by positional or orientational randomness. We find that compared to the perfect situation, the introduction of only orientational randomness decreases the transverse electric (TE) band gap whereas having no obvious effects on the TM band gap. In contrast, the introduction of only positional randomness decreases the TE band gap significantly, whereas it can widen or narrow the TM band gap, depending on the packing density. We also discuss the thermal (i.e., self-assembled) system where two kinds of randomness are present. Our paper contributes to a better understanding of the role orientational randomness and positional randomness play on PBGs and may benefit the PBG engineering of photonic crystals through self-assembly approaches.

We investigate the spectral properties of polaronic excitations within the framework of an analog quantum simulator based on inductively coupled superconducting transmon qubits and microwave resonators. This system emulates a lattice model that describes a nonlocal coupling of an itinerant spinless-fermion excitation to dispersionless (Einstein-type) phonons through the Peierls and breathing-mode interaction mechanisms. The model is characterized by a sharp, level-crossing transition at a critical value of the effective excitation-phonon coupling strength; above the transition point, the ground state of this model corresponds to a heavily dressed (small-polaron) excitation. Using the kernel-polynomial method, we evaluate the momentum-frequency resolved spectral function of this system for a broad range of parameters. In particular, we underscore the ramifications of the fact that the zero-quasimomentum Bloch state of a bare excitation represents the exact eigenstate of the Hamiltonian of this system for an arbitrary excitation-phonon coupling strength. We also show that—based on the numerically evaluated spectral function and its well-known relation with the survival probability of the initial, bare-excitation Bloch state (the Loschmidt echo)—one can make predictions about the system dynamics following an excitation-phonon interaction quench. To make contact with anticipated experimental realizations, we utilize a previously proposed method for extracting dynamical-response functions in systems with local (single-qubit) addressability using the multiqubit (many-body) version of the Ramsey interference protocol.

The atomic motion controls important properties of materials, such as thermal transport, phase transitions, and vibrational spectra. However, simulating the ionic dynamics is exceptionally challenging when quantum fluctuations are relevant (e.g., at low temperatures or with light atoms) and the energy landscape is anharmonic. In this paper, we present the time-dependent self-consistent harmonic approximation (TDSCHA) [L. Monacelli and F. Mauri, Phys. Rev. B 103, 104305 (2021)] in the Wigner framework, paving the way for the efficient computation of the nuclear motion in systems with sizable quantum and thermal anharmonic fluctuations. Besides the improved numerical efficiency, the Wigner formalism unveils the classical limit of TDSCHA and provides a link with the many-body perturbation theory of Feynman diagrams. We further extend the method to account for the nonlinear couplings between phonons and photons, responsible, e.g., for a nonvanishing Raman signal in high-symmetry Raman inactive crystals, firstly discussed by Rasetti and Fermi. We benchmark the method in phase III of high-pressure hydrogen ab initio. The nonlinear photon-phonon coupling reshapes the IR spectra and explains the high-frequency shoulder of the ${\mathrm{H}}_2$ vibron observed in experiments. The Wigner TDSCHA is computationally cheap and derived from first principles. It is unbiased by assumptions on the phonon-phonon and phonon-photon scattering and does not depend on empirical parameters. Therefore, the method can be adopted in unsupervised high-throughput calculations.