We put forward a two-dimensional multipolar higher-order topological insulator whose properties originate from the interference of the degenerate modes of the meta-atoms. This system features disorder-robust corner modes and cannot be reduced to the known crystalline topological phases, providing the example of multipolar topology in a $C_3$-symmetric lattice. The multimode nature of the lattice gives rise to flat bands and corner states with extreme localization. We assemble the designed structure, observe multipolar topological corner states experimentally, and demonstrate their coherent control.

From the vantage point of an incompressible fractional quantum Hall (FQH) state, an electron injected into it is a complex bound state of an odd number of composite fermions (CFs) dressed by a cloud of particle-hole pairs of CFs, where each CF itself is the bound state of an electron and an even number of quantized vortices. Recent scanning tunneling microscopy experiments provide a spectroscopic probe of the internal energy levels of this bound state, yielding unique information on the short-distance correlations in the FQH liquids. We present detailed calculations showing that the internal energy levels of this bound state, as manifested in the energy-resolved local density of states, depend sensitively on the filling factor and on whether an electron is added or removed. In general, multiple, approximately equally spaced peaks are predicted, with their separation providing a measure of a renormalized CF cyclotron energy. We discuss what aspects of experiments are explained by our model and which ones remain to be explained.

We here introduce the concept of fragile dislocation modes, which are localized only in a fraction of a topological phase while otherwise leaking into the bulk continuum. As we demonstrate here, such dislocation modes are hosted in an obstructed atomic topological phase in the two-dimensional Su-Schrieffer-Heeger model but only in a finite region with an indirect gap at high energy. They are realized as chiral pairs at finite energies with protection stemming from a combination of the chiral (unitary particle-hole) and the point group $\left(C_{4v}\right)$ symmetries, but only when the indirect gap is open. In this regime, we corroborate the stability of the defect modes by following their localization and also by explicitly adding a weak chemical potential disorder. Our findings should, therefore, be consequential for the experimental observation of such modes in designer topological crystals and classical metamaterials.

We theoretically study the conditions under which an intrinsic spin Nernst effect–a transverse spin current induced by an applied temperature gradient–can occur in a canted-antiferromagnet insulator, such as ${\mathrm{LaFeO}}_3$ and other materials of the same family. The spin Nernst effect may provide a microscopic mechanism for an experimentally observed anomalous thermovoltage in ${\mathrm{LaFeO}}_3/\mathrm{Pt}$ heterostructures, where spin is transferred across the insulator/metal interface when a temperature gradient is applied to ${\mathrm{LaFeO}}_3$ parallel to the interface [W. Lin et al., Nat. Phys. 18, 800 (2022)]. We find that ${\mathrm{LaFeO}}_3$ exhibits an intrinsic spin Nernst effect when inversion symmetry is broken on the axes parallel to both the applied temperature gradient and the direction of spin transport, which can result in a spin injection across the insulator/metal interface. Our paper provides a general derivation of a symmetry-breaking-induced spin Nernst effect, which may open a path to engineering a finite spin Nernst effect in systems where it would otherwise not arise.

The effect of pressure on the low-temperature states of the ${\mathrm{Re}}_3{\mathrm{Ge}}_7$ is investigated by both electrical and Hall resistance and magnetization measurements. At ambient pressure, the temperature-dependent resistance of ${\mathrm{Re}}_3{\mathrm{Ge}}_7$ behaves quasilinearly from room temperature down to 60 K, then undergoes a two-step metal-to-insulator transition (MIT) at temperatures $T_1=59.4\mathrm{K}$ and $T_2=58.7\mathrm{K}$, which may be related to a structural phase transition or occurrence of charge-density wave ordering. Upon applying pressure, the two-step ($T_1,T_2)$ MIT splits into three steps ($T_1,T_2$. and $T_3$) above 1 GPa, and all traces of MITs are fully suppressed by ∼8 GPa. Subsequently, the onset of bulk superconductivity (SC) occurs between 10.8 and 12.2 GPa and persists to our highest pressure of 26.8 GPa. At 12.2 GPa the superconducting transition temperature, $T_c$, and upper critical field, $H_{c}2$ reach the maximum of ${\mathrm{T}}_{\mathrm{c}}$ (onset) ∼5.9 K and $H_{c}2$ (1.8 K) ∼14 kOe. Our results not only present the observation of SC under high pressure in ${\mathrm{Re}}_3{\mathrm{Ge}}_7$ but also delineate the interplay between SC and other competing electronic states by creating a T-p phase diagram for this potentially topologically nontrivial system ${\mathrm{Re}}_3{\mathrm{Ge}}_7$.

The magnetic and structural properties of the recently discovered pnictogen/chalcogen-free superconductor LaFeSiH have been investigated by ${}^{57}\mathrm{Fe}$ synchrotron Mössbauer source spectroscopy, X-ray and neutron powder diffraction, and ${}^{29}\mathrm{Si}$ nuclear magnetic resonance spectroscopy. In contrast with earlier work suggesting the presence of an orthorhombic and magnetic ground state as in underdoped Fe-based pnictides, our results unambiguously establish that LaFeSiH is in fact similar to strongly overdoped Fe-based pnictides: there is no magnetic order (including under hydrostatic pressure up to 18.8 GPa), nor even fluctuating local moments and the system remains tetragonal down to 2 K. This raises the prospect of enhancing the $T_c$ of LaFeSiH by reducing its carrier concentration through appropriate chemical substitutions.

We present a comparative study of the temperature-dependent Raman scattering of pyrochlore iridates $A_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, where $A=\mathrm{Gd}$, Dy, and Er, spanning the temperature range from 295 to 5 K. Our investigation covers the metal-insulator phase transition, coinciding with the paramagnetic-to-all-in/all-out (AIAO) spin ordering at the magnetic transition temperature $T_N$. The crystal symmetry remains unchanged across this magnetic transition. Below $T_N$, two additional modes ($N_1$ and $N_2$) emerge, associated with single magnon excitations within the AIAO-ordered state. Analysis of magnon energies enables us to estimate the isotropic exchange $(J)$ and Dzyaloshinskii-Moriya interaction parameter $\left(D\right)$. The resulting $D/J$ ratios are $\sim 0.15$ for ${\mathrm{Gd}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ and ${\mathrm{Dy}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$. In all the three systems, the Ir-O-Ir bond bending vibrations, $A_{1g}$ and $E_g$, exhibit anomalous frequency softening in the magnetically ordered AIAO state, primarily originating from the spin-phonon interaction induced by the phonon modulation of the Dzyaloshinskii-Moriya spin-exchange interaction. The two stretching modes $T_{2g}^2$ and $T_{2g}^4$ experience significant hardening in the magnetic insulating phase. Additionally, substantial anomalous linewidth broadening of the $E_g$ mode is observed in all samples, indicating strong phonon renormalization below $T_N$. Our experimental findings establish experimental evidence of magnon excitations and strong spin-phonon coupling in the AIAO-ordered state for these iridates.

We theoretically investigate the thermal Hall transport of magnon-polarons in a two-dimensional honeycomb antiferromagnetic insulator under the influence of a perpendicular magnetic field, varying in strength. The application of a perpendicular magnetic field induces a magnetic phase transition from the collinear antiferromagnetic phase to the spin-flop phase, leading to a significant alteration in Hall transport across the transition point. In this paper, our focus is on the intrinsic contribution to thermal Hall transport arising from the magnetoelastic interaction. To facilitate experimental verification of our theoretical results, we present the dependence of thermal Hall conductivity on magnetic field strength and temperature.

The magnetic interactions that determine magnetic order and magnon energies typically involve only two spins. While rare, multispin interactions can also appear in quantum magnets and be the driving force in the ground-state selection and in the nature of its excitations. By performing time-domain terahertz and magneto-Raman spectroscopy measurements combined with theoretical modeling, we determine the origin of the magnon excitation gap in Dirac antiferromagnet ${\mathrm{CoTiO}}_3$. By adding a ring-exchange interaction in a hexagonal plaquette of the honeycomb lattice to both an XXZ spin model and to a low-energy spin-orbital flavor wave model, a gap is generated in the magnon spectrum at the Brillouin zone center. With this addition, the flavor wave model reproduces a large swath of experimental results including terahertz, Raman, inelastic neutron scattering, and magnetization experiments.

We study the response of critical resonating valence bond (RVB) spin liquids to doping with longer-range singlets, and more generally of $\mathrm{U}\left(1\right)$-symmetric tensor networks to nonsymmetric perturbations. Using a field theory description, we find that in the RVB, doping constitutes a relevant perturbation that immediately opens up a gap, contrary to previous observations. Our analysis predicts a very large correlation length even at significant doping, which we verify using high-accuracy numerical simulations. This emphasizes the need for careful analysis, but it also justifies the use of such states as a variational ansatz for critical systems. Finally, we give an example of a projected entangled pair state where nonsymmetric perturbations do not open up a gap and the $\mathrm{U}\left(1\right)$ symmetry reemerges.

We investigate the wavelike dynamics for high harmonic generation (HHG) in solids. Our results indicate that the wavelike behavior of electrons plays a crucial role in HHG process of solids. In solids, the group velocity dispersion, i.e., the second derivative of energy band, can lead to substantial spatial spread for the evolution of the electron wave packet. This kind of wavelike effect is embedded in the interference of electron-hole wavelets, and allows electrons and holes to be separated spatially when emitting HHG. Incorporating the wavelike effect, we show how one can understand the time-frequency properties of HHG from the perspective of wavelike picture. Furthermore, taking the gapped graphene as a prototype, we also demonstrate that the wavelike picture can well describe the two-dimensional electron dynamics and the emission intensity of HHG. Our analysis provides a comprehensive understanding of the HHG emission properties in solids, and shows how to comprehend the underlying electron dynamics from the perspective of wavelike picture.

In this paper we discuss the topological transition between trivial and nontrivial phases of a quasiperiodic (Aubry-André like) mechanical Su-Schrieffer-Heeger model. We find that there exists a nontrivial boundary separating the two topological phases, and an analytical expression for this boundary is found. We discuss the localization of the vibrational modes using the calculation of the inverse participation ratio and access the localization nature of the states of the system. We find three different regimes: extended, localized, and critical, depending on the intensity of the Aubry-André spring. We further study the energy-dependent mobility edge (ME) separating localized from extended eigenstates and find its analytical expression for both commensurate and incommensurate modulation wavelengths, thus enlarging the library of models possessing analytical expressions for the ME. Our results extend previous results for the theory of fermionic topological insulators and localization theory in quantum matter to the classical realm.

Only a few magnetic kagome materials exhibit lattice instabilities among the large kagome material array. In this work, we find that kagome magnets $R{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ ($R$ = rare-earth elements) and their nonmagnetic counterparts ${R\mathrm{V}}_6{\mathrm{Sn}}_6$ exhibit an intriguing interplay between magnetism and lattice dynamics on their surfaces. Notably, ${R\mathrm{V}}_6{\mathrm{Sn}}_6$ surfaces terminated by kagome layers demonstrate pronounced lattice instabilities, manifesting as $\sqrt{3}\times \sqrt{3}$ and $2\times 2$ surface charge orders (SCOs). These instabilities are absent on corresponding magnetic materials $R{\mathrm{Mn}}_6{\mathrm{Sn}}_6$. Here, the SCO is suppressed by magnetism. Otherwise, surface distortions would significantly reduce the spin polarization, elevating the energy via Hund's rule. Thus, SCOs are energetically unfavorable on magnetic surfaces. The competition of magnetism and lattice instability is further substantiated by observing SCOs on V-substituted $R{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ kagome surfaces, contrasting with their absence on Mn-substituted ${R\mathrm{V}}_6{\mathrm{Sn}}_6$ surfaces. Our findings reveal unexpected surface instability and profound spin-lattice coupling in these kagome magnets, highlighting the complex dynamics hidden within magnetic materials.

We theoretically investigate topological features of a one-dimensional Su-Schrieffer-Heeger lattice with modulating non-Hermitian on-site potentials containing four sublattices per unit cell. The lattice can be either commensurate or incommensurate. In the former case, the entire lattice can be mapped by supercells completely. While in the latter case, there are two extra lattice points, thereby making the last cell incomplete. We find that an anti-$\mathcal{PT}$ transition occurs at exceptional points of edge states at certain parameters, which does not coincide with the conventional topological phase transition characterized by the Berry phase, provided the imaginary on-site potential is large enough. Interestingly, when the potential exceeds a critical value, edge states appear even in the regime with a trivial Berry phase. To characterize these novel edge states we present topological invariants associated with the system's parity. Finally, we analyze the dynamics for initial states with different spatial distributions, which exhibit distinct dynamics for the commensurate and incommensurate cases, depending on the imaginary part of edge state energy.

We present a comparative analysis of $1s$ and $2p$ core-level hard x-ray photoelectron spectroscopy (HAXPES) spectra of metallic ${\mathrm{VO}}_2$ and ${\mathrm{CrO}}_2$. Even though the V $1s$ and $2p$ spectra in ${\mathrm{VO}}_2$ show similar line shapes except for the absence/presence of a spin-orbit splitting in the core levels, the Cr $1s$ and $2p$ spectra exhibit distinct main-line shapes. The experimental HAXPES spectra are analyzed by the Anderson impurity model based on the density functional theory $+$ dynamical mean-field theory and a conventional ${\mathrm{MO}}_6$ cluster model. We examine the interplay between the formation of the intra-atomic multiplet between a core hole and valence electrons and the charge-transfer effect on the chemical bonding followed by the $1s$ and $2p$ core electron excitations. We demonstrate the advantage of $1s$ HAXPES to the routinely employed $2p$ one for distinguishing contributions of a metal-ligand and metallic screening from a state near the Fermi level in metallic early $3d$ transition-metal oxides.

We conduct a theoretical investigation of the topological phenomena associated with chiral superconducting pairing states induced in a doped Kane-Mele model on a honeycomb lattice. Through numerical analysis, we obtain exotic phase diagrams for both the $d+id$ and $p+ip$ superconducting states. In the case of the $d+id$ pairing state, high Chern number states with $C=\pm 4$ emerge. The Chern number decreases as the spin-orbit coupling is introduced. For the $p+ip$ pairing state, additional phase transition lines are present in the overdoped region near the Van Hove singularity point, leading to the emergence of high Chern number phases with $C=\pm 6$. We further verify these high Chern number phases through the bulk-edge correspondence. To understand the origin of the exotic topological phase diagrams in the chiral superconducting state, we examine the electronic structure at the phase transition lines. This investigation provides insight into the complex interplay between chiral superconductivity and topological properties, potentially paving the way for the discovery of new materials with unique topological properties.