The discovery of graphene has stimulated enormous interest in two-dimensional (2D) electron gas with linear band dispersion. However, to date, 2D Dirac semimetals are still very rare due to the fact that 2D Dirac states are generally fragile against perturbations such as spin-orbit coupling. Nonsymmorphic crystal symmetries can enforce the formation of Dirac nodes, providing another route to establishing symmetry-protected Dirac states in 2D materials. Here, we report the symmetry-protected Dirac states in nonsymmorphic $\alpha$-antimonene (Sb monolayer). The antimonene was synthesized by the method of molecular beam epitaxy. 2D Dirac states with large anisotropy were observed by angle-resolved photoemission spectroscopy. The Dirac states in $\alpha$-antimonene are spin-orbit coupled in contrast to the spinless Dirac states in graphene. The result extends the graphene physics into a family of 2D materials where spin-orbit coupling is present.

Electron valleys in transition-metal dichalcogenide monolayers drive novel physics and allow designing multifunctional architectures for applications. We propose to manipulate the electron valleys in these systems for spin/valley filter and valve devices through band engineering. Instead of the magnetic proximity effect that has been extensively used in previous studies, in our strategy, the electron valleys are directly coupled to the spin-polarized states of two-dimensional ferromagnets. We find that this coupling results in a valley-selective gap opening due to the spin-momentum locking in the transition-metal dichalcogenide monolayers. This physics gives rise to a variety of unexpected electronic properties and phenomena including half-metallicity, electrically switchable valley polarization, spin/valley filter, and valve effects in the transition-metal dichalcogenide monolayers. We further demonstrate our idea in ${\mathrm{MoTe}}_2/{\mathrm{CoCl}}_2$ and ${\mathrm{CoCl}}_2/{\mathrm{MoTe}}_2/{\mathrm{CoCl}}_2$ van der Waals heterojunctions based on first-principles calculations. Thus, our study provides a way of engineering the electron valleys in transition-metal dichalcogenide monolayers for contemporary devices.

Microwave measurements have recently been successfully applied to measure ferroelectric materials on the nanoscale, including detection of polarization switching and ferroelectric domain walls. Here we discuss the question of whether scanning probe microscopy operating at microwave frequency can identify the changes associated with the soft phonon dynamics in a ferroic. The analytical expressions for the electric potential, complex impedance, and dielectric losses are derived and analyzed, since these physical quantities are linked to experimentally measurable properties of the ferroic. As a ferroic we consider virtual or proper ferroelectric with an optic phonon mode that softens at a Curie point. We also consider a decay mechanism linked to the conductance of the ferroic, thus manifesting itself as the dielectric loss in the material. Our key finding is that the influence of the soft phonon dispersion on the surface potential distribution, complex impedance, and dielectric losses are evidently strong in the vicinity (∼10–30 K) of the Curie temperature. Furthermore, we quantified how the spatial distribution and frequency spectra of the complex impedance and the dielectric losses react on the dynamics of the soft phonons near the Curie point. These results set the stage for characterization of polar phase transitions with nanoscale microwave measurements, providing a complementary approach to well established electromechanical measurements for fundamental understanding of ferroelectric properties as well as their applications in telecommunication and computing.

We theoretically study the magnon spin thermal transport using a strong-coupling approach in pyrochlore iridate trilayer thin films grown along the [111] direction. As a result of the Dzyaloshinskii-Moriya interaction, the spin configuration of the ground state is an all-in/all-out ordering on neighboring tetrahedra of the pyrochlore lattice. In such a state, the system has an inversion symmetry and a Nernst-type thermal spin current response is well defined. We calculate the temperature dependence of the magnon Nernst response with respect to the magnon band topology controlled by the spin-orbit coupling parameters and observe topologically protected chiral edge modes over a range of parameters. Our study complements prior work on the magnon thermal Hall effect in thin-film pyrochlore iridates and suggests that the [111] grown thin-film pyrochlore iridates are a promising candidate for thermal spin transport and spin caloritronic devices.

Two-dimensional (2D) ferrovalley materials that exhibit spontaneous valley polarization are both fundamentally intriguing and practically appealing to be used in valleytronic devices. Usually, the research on 2D ferrovalley materials is mainly focused on inorganic systems, severely suffering from in-plane magnetization. Here, we alternatively show by k·p model analysis and high-throughput first-principles calculations that ideal spontaneous valley polarization is present in 2D organometallic lattice. We explore the design principle for organic 2D ferrovalley materials composed of (quasi-)planer molecules and transition-metal atoms in hexagonal lattice, and identify 12 promising candidates. These systems have a ferromagnetic or antiferromagnetic semiconducting state, and importantly they exhibit robust out-of-plane magnetization. The interplay between spin and valley, together with strong spin-orbit coupling of transition-metal atoms, guarantee the spontaneous valley polarization in these systems, facilitating the anomalous valley Hall effect. Our findings significantly broaden the scientific and technological impact of ferrovalley physics.

We report the temperature and composition dependence of static magnetic susceptibility and Raman spectroscopic measurements on van der Waals antiferromagnets ${\mathrm{Ni}}_{1-x}{\mathrm{Fe}}_x{\mathrm{PS}}_3$. The end members ${\mathrm{NiPS}}_3$ and ${\mathrm{FePS}}_3$ feature $XY$- and Ising-like magnetism, respectively, enabling chemical tuning of magnetic anisotropy and spin correlations. ${\mathrm{Ni}}_{1-x}{\mathrm{Fe}}_x{\mathrm{PS}}_3$ shows a turnover from the $XY$ to Ising anisotropy through $x\approx 0.1$. Although the $XY$ anisotropy is rapidly suppressed on introducing Fe content, two-magnon scattering evidences the slow repression of short-range magnetic correlations deep inside the Fe-rich side. Counterintuitively, the two-magnon signal undergoes less renormalization of its energy with increasing $x$ despite the larger spin number and enhanced classical magnetism. The disparate static and dynamic magnetic behaviors indicate the emergence of an exotic spin state in alloy van der Waals magnets.

NbMnP is a metallic material, which consists of the zigzag chains of Mn moments along the $b$ axis. The magnetic susceptibility as well as the resistivity shows an anomaly at 233 K, which indicates an antiferromagnetic phase transition. Our neutron powder diffraction experiment reveals that the magnetic structure is a $Q=0$ structure and noncollinear with an easy plane anisotropy perpendicular to the $b$ axis, and the $a$- and $c$-axis magnetic components align antiferromagnetically and ferromagnetically along the zigzag chain direction, respectively. The ordered moment is $1.2{\mu }_{\mathrm{B}}$, which is reduced probably due to the itineracy of the Mn moments. A localized picture model suggests that the $Q=0$ magnetic structure is formed by the frustration among several exchange couplings. A weak ferromagnetic component is also present in the antiferromagnetic phase, which is considered to be caused by a Dzyaloshinskii-Moriya interaction.

The kagome lattice is host to flat bands, topological electronic structures, Van Hove singularities, and diverse electronic instabilities, providing an ideal platform for realizing highly tunable electronic states. Here, we report soft and mechanical point-contact spectroscopy (SPCS and MPCS) studies of the kagome superconductors ${\mathrm{KV}}_3{\mathrm{Sb}}_5$ and $\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$. Compared to the superconducting transition temperature $T_{\mathrm{c}}$ from specific heat and electrical resistance measurements, significantly enhanced values of $T_{\mathrm{c}}$ are observed via the zero-bias conductance of SPCS, which become further enhanced in MPCS measurements. While the differential conductance curves from SPCS can be described by a two-gap $s$-wave model, a single $s$-wave gap reasonably captures the MPCS data, likely due to a diminishing spectral weight of the other gap. The enhanced superconductivity probably arises from local strain caused by the point contact, which also leads to two-gap or single-gap behaviors observed in different point contacts. Our results demonstrate highly strain-sensitive superconductivity in kagome metals $\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$ and ${\mathrm{KV}}_3{\mathrm{Sb}}_5$, which may be harnessed in the manipulation of possible Majorana zero modes.

The dominant mechanism of high-transition-temperature $\left(T_{\text{c}}\right)$ superconductivity in cuprates remains an agonizing puzzle in condensed matter physics. The more recent discoveries of FeSe-based superconductors provide ideal new platforms to explore the high-$T_{\text{c}}$ superconducting mechanism. Here we develop a generic mechanism of superconductivity centered at the commonality shared by many high-$T_{\text{c}}$ materials, namely, their effective carrier densities are universally low, and are therefore necessarily accompanied by low energy plasmons. We first show that the excitations of such plasmons can largely suppress the Coulomb repulsion of the electrons. Furthermore, the electron-phonon and electron-plasmon couplings can inherently join force in mediating electron pairing, and when applied to the monolayered FeSe on ${\mathrm{SrTiO}}_3$, the plasmon-enhanced $T_{\text{c}}$ is one order of magnitude higher than that due to phonon alone, to the experimentally observed range. The present “phonon $+$ plasmon” mechanism also embodies characteristic dependences of $T_{\text{c}}$ on the carrier density and isotope substitution, and may find broad applicability in many superconducting systems with low carrier densities, including, most notably, the cuprates.

Josephson devices with ferromagnetic barriers have been widely studied. Much less is known when the ferromagnetic layer is insulating. In this paper we investigate the transport properties of superconductor/ferromagnetic insulator/superconductor (SFIS) junctions with particular attention paid to the temperature behavior of the critical current that may be used as a fingerprint of the junction. We investigate the specific role of impurities as well as of possible spin-mixing mechanisms due to the spin-orbit coupling. The transition between the 0 and the $\pi$ phases can be properly tuned, thus achieving stable $\pi$ junctions over the entire temperature range that may be possibly employed in superconducting quantum circuits.

We present a series of models of three-dimensional rotation-symmetric fragile topological insulators in class AI (time-reversal symmetric and spin-orbit-free systems), which have gapless surface states protected by time-reversal ($T$) and $n$-fold rotation ($C_n$) symmetries ($n=2,4,6$). Our models are generalizations of Fu's model of a spinless topological crystalline insulator, in which orbital degrees of freedom play the role of pseudospins. We consider minimal surface Hamiltonian with $C_n$ symmetry in class AI and discuss possible symmetry-protected gapless surface states, i.e., a quadratic band touching and multiple Dirac cones with linear dispersion. We characterize topological structure of bulk wave functions in terms of two kinds of topological invariants obtained from Wilson loops: ${\mathbb{Z}}_2$ invariants protected by $C_n$ ($n=4,6$) and time-reversal symmetries, and $C_{2}T$-symmetry-protected $\mathbb{Z}$ invariants (the Euler class) when the number of occupied bands is two. Accordingly, our models realize two kinds of fragile topological insulators. One is a fragile $\mathbb{Z}$ topological insulator whose only nontrivial topological index is the Euler class that specifies the number of surface Dirac cones. The other is a fragile ${\mathbb{Z}}_2$ topological insulator having gapless surface states with either a quadratic band touching or four (six) Dirac cones, which are protected by time-reversal and $C_4$ ($C_6$) symmetries. Finally, we discuss the instability of gapless surface states against the addition of $s$-orbital bands and demonstrate that surface states are gapped out through hybridization with surface-localized $s$-orbital bands.

We study the Kitaev-Heisenberg-$\mathrm{\Gamma }$ model with antiferromagnetic Kitaev exchanges in the strong anisotropic (toric code) limit to understand the phases and the intervening phase transitions between the gapped $Z_2$ quantum spin liquid and the spin-ordered (in the Heisenberg limit) as well as paramagnetic phases (in the pseudodipolar, $\mathrm{\Gamma }$, limit). We find that the paramagnetic phase obtained in the large $\mathrm{\Gamma }$ limit has no topological entanglement entropy and is proximate to a gapless critical point of a system described by equal superposition of differently oriented stacked one-dimensional $Z_2\times Z_2$ symmetry protected topological phases. Using a combination of exact diagonalization calculations and field theoretic analysis we map out the phases and phase transitions to reveal the complete phase diagram as a function of the Heisenberg, the Kitaev, and the pseudodipolar interactions. Our work shows a rich plethora of unconventional phases and phase transitions and provides a comprehensive understanding of the physics of anisotropic Kitaev-Heisenberg-$\mathrm{\Gamma }$ systems along with our recent paper [Phys. Rev. B 102, 235124 (2020)] where the ferromagnetic Kitaev exchange was studied.

Exciton-quasiparticle interactions govern the optical nonlinear response of heretostructures at the exciton transitions. We theoretically analyze the exciton-exciton, exciton-electron, and exciton-hole interactions in heterostructures with GaAs/AlGaAs quantum wells (QWs), however, our theoretical approach can be applied to other types of heterostructures. The exciton-quasiparticle scattering amplitudes are calculated in the framework of the two-particle Hartree-Fock approximation. The exchange interaction, which is mainly responsible for the scattering, is modeled using microscopically calculated exciton wave functions. The corresponding exchange interaction constants are determined as a function of quantum well width. The spin-dependent exciton-quasiparticle elastic scattering matrix elements are used to obtain the collisional broadening of the exciton transition within the Born approximation as a function of the areal density of the respective quasiparticles for QWs of various widths. The obtained density dependences of the collisional nonradiative broadening can be used to estimate the exciton and charge carrier densities in optical experiments.

We determined the structure of epitaxial $2H\text{−}\mathrm{Ta}{\mathrm{S}}_2$ on Au(111) using the method of x-ray standing waves (XSW), supported by density functional theory (DFT) calculations and scanning tunneling microscopy (STM). The lattice mismatch between substrate and overlayer gives rise to a moiré superstructure, which modulates the structural and electronic properties. For a specific registry (S atoms directly above Au substrate atoms), local covalentlike bonds form, whereas globally weak van der Waals bonding prevails. Still, the $\mathrm{Ta}{\mathrm{S}}_2$ layer remains rather flat. Significant charge transfer from Au(111) into the conduction band of the two-dimensional material is found.

We present a combined scanning tunneling microscopy (STM), noncontact atomic force microscopy (nc-AFM), and differential reflectance spectroscopy (DRS) study aiming at the characterization of the relationship between structural and optical properties of a supramolecular assembly of a curcuminoid derivative adsorbed on two kinds of substrates: bilayers of KCl on Au(111) [2 ML KCl/Au(111)] and KCl(001) bulk. The molecule features a difluoroboron (${\mathrm{BF}}_2$) complex as well as cyano (CN) end groups, which induces an internal dipole moment of a few Debyes. The comparative study of the structural properties of the molecules adsorbed on 2 ML KCl/Au(111), which allows for STM characterization, and KCl bulk, which allows for nc-AFM and DRS characterization, shows that the molecular adsorption is similar on both substrates. STM and nc-AFM give evidence that the on-surface condensation of the molecules into supramolecular assemblies is steered by two main interactions: the molecule-substrate interaction between the CN end groups of the molecule and the cationic ${\mathrm{K}}^{+}$ species, as well as an in-plane intermolecular interaction involving ${\mathrm{BF}}_2$ and phenyl groups. Based on density functional theory calculations, we propose an epitaxial molecular structure described by a two-molecule unit cell forming an $8\times 2$ supercell on the KCl surface that grows in form of ribbons along the $\langle 100\rangle$ or $\langle 010\rangle$ directions of the substrate, which are polar. Among several possible stereoisomers for the molecule, it is found that the balance between vertical and in-plane interactions selects the one whose size matches the distance between ${\mathrm{K}}^{+}$ species to condense on the substrate and not the most stable one stemming from the gas phase. Finally, optical spectroscopy in solvent and DRS measurements performed on the supramolecular assembly exhibit similar features identified as three absorption peaks tracing a $\pi \to {\pi }^{*}$ electronic transition followed by a vibronic progression, plus a broader band featuring a set of higher-energy transitions. Due to both the weak electrostatic interactions and a lack of $\pi \text{−}\pi$ interactions in the condensed molecular phase, DR spectra exhibit no major excitonic effect, but a nonrigid redshift of the molecular absorption band compared to the situation in solvent. This observation is found to be consistent with the isomer selection due to the transition from the gas phase to the on-surface condensed phase.

Light-induced acoustic waves can be used as sensitive probes, providing a pathway toward microscopic imaging and metrology in optically inaccessible media. The ability to detect such waves depends on the interaction of an optical probe pulse with the acoustic waves in the topmost layers of the structure. Therefore, the interplay between optoacoustic coupling and material boundaries, combined with the properties of acoustic waves near free surfaces is of prime importance. Here we show an approach toward optimized optical detection of such laser-excited acoustic waves. We explore the physics underlying this detection, finding that the presence of a free surface actually reduces the optoacoustic interaction, and subsequently enhancing this interaction via adding transparent nanolayers on the free surface. Our work uncovers an important yet rarely explored aspect in optical detection of strain waves via free surfaces and may lead to strategies for signal enhancement in the imaging and characterization of subsurface structures using laser-excited strain waves.

Although fragile topology has been intensely studied in static crystals in terms of Wannier obstruction, it is not clear how to generalize the concept to dynamical systems. In this work we generalize the concept of fragile topology, and provide a definition of fragile topology for noninteracting Floquet crystals, which we refer to as dynamical fragile topology. In contrast to the static fragile topology defined by Wannier obstruction, dynamical fragile topology is defined for the nontrivial quantum dynamics characterized by the obstruction to static limits (OTSL). Specifically, the OTSL of a Floquet crystal is fragile if and only if it disappears after adding a symmetry-preserving static Hamiltonian in a direct-sum way preserving the relevant gaps (RGs). We further present a concrete $2+1\mathrm{D}$ example for dynamical fragile topology, based on a model that is qualitatively the same as the dynamical model with anomalous chiral edge modes in Rudner et al. [Phys. Rev. X 3, 031005 (2013)]. The fragile OTSL in the $2+1\mathrm{D}$ example exhibits anomalous chiral edge modes for a natural open boundary condition, and does not require any crystalline symmetries besides lattice translations. Our work paves the way to study fragile topology for general quantum dynamics.

Higgs spectroscopy, the study of Higgs bosons of a superconductor, is an emerging field in studying superconductivity. Here we show that the Berry curvature and the quantum metric of bands play a central role in the Higgs mode generation. They allow the detection of Higgs bosons even when the conventional contribution from the band curvature vanishes. Furthermore, we show that the Higgs mode can couple to the external electromagnetic field linearly when mediated by the Berry connection. As a result, we predict the existence of a second-harmonic generation, in addition to the well-known third-harmonic generation. We elucidate our theory further by considering a flat band superconductor realized by the Harper-Hubbard model and show that the geometric Higgs mode is lower bounded by the band Chern number. We demonstrate in twisted bilayer graphene the existence of the geometrically induced Higgs modes when superconductivity is realised in the nearly flat band at the magic angle.

Two-dimensional atomic layers composed of metallic atoms with a high principal quantum number in valence shells are promising materials for applications in state-of-the-art magnetic devices due to the presence of a strong spin-orbit coupling. In this work we experimentally explored the effects of triggering perpendicular magnetic anisotropy (PMA) of ferromagnetic (FM) films by applying a series of $6p$-Pb, Bi and $4d$-Pd (reference) atomic layers with high-level atomic orbital and low-level lattice strain. Our research indicates that, compared with Pd atoms, Pb and Bi atoms can produce higher-strength PMA on adjacent FM films through orbital hybridization at the interface. Moreover, we demonstrate that the PMA induced by Pb and Bi atoms is highly sensitive to the ordering degree of the wetting layer which is determined by the interplay of surface free, cohesive, and strain energies within the grown materials; the Pb(Bi) atoms added on the wetting layer can enhance the PMA of the adjacent FM film only when the wetting layer maintains an ordered structure. This work clarifies the critical interface effects of $6p$-HMs on the FM layer, thus providing important clues to increase control over the spin-orbit interaction engendered by HMs through the interface.