We theoretically investigated the Floquet states of bilayer graphene heterostructure under the irradiation by horizontally incident circular polarized light. The in-plane and out-of-plane electric field of the light periodically perturbs the intralayer and interlayer hopping, respectively. For circular polarized light, the two components of the electric field have $\pi /2$ phase difference, so that the two types of hopping are periodically perturbed with the $\pi /2$ phase difference, which modify the effective interlayer hopping. We focus on the model of bilayer graphene in the heterostructure of antiferromagnetic van der Walls spin valve. The amplitude of the irradiation can tune the band gap and topological properties of the bulk state. The spin-polarized quantum anomalous Hall phase with Chern number being one is predicted. The incident angle of the irradiation can tune the band gap and dispersion of the edge states in zigzag nanoribbons.

We report on the synthesis, crystal structure, magnetic, thermodynamic, and electron-spin-resonance properties of the coordination complex $\left[{\mathrm{Cu}}_2{\left(\mathrm{pz}\right)}_3{\left(4\text{−}\mathrm{HOpy}\right)}_4\right]{\left({\mathrm{ClO}}_4\right)}_4$ (pz=pyrazine; 4-HOpy=4-hydroxypyridine). This material is identified as a spin-1/2 Heisenberg ladder system with exchange-coupling parameters $J_{rung}/k_B=12.1\left(1\right)\mathrm{K}$ and $J_{leg}/k_B=10.5\left(3\right)\mathrm{K}$ $[J_{rung}/J_{leg}=1.15\left(4\right)]$. For single crystals our measurements revealed two critical fields, ${\mu }_0{H}_{c1}=4.63\left(5\right)\mathrm{T}$ and ${\mu }_0{H}_{c2}=22.78\left(5\right)\mathrm{T}$ (for $H\parallel a^{*}$), separating the gapped spin-liquid, gapless Tomonaga-Luttinger-liquid, and fully spin-polarized phase. No signature of a field-induced transition into a magnetically ordered phase was found at temperatures down to 450 mK. The material bridges an important gap by providing an excellent physical realization of an almost isotropic spin-1/2 strong-rung Heisenberg ladder system with modest exchange-coupling energy and critical-field scales.

We analyze two different fermionic systems that defy Mott localization showing a metallic ground state at integer filling and very large Coulomb repulsion. The first is a multiorbital Hubbard model with a Hund's coupling (this physics has been widely studied, and the new metallic state is called a Hund's metal), and the second is a SU(3) Hubbard model with a patterned single-particle potential designed to display a similar interaction-resistant metal in a setup which can be implemented with $\mathrm{SU}\left(N\right)$ ultracold atoms. With simple analytical arguments and exact numerical diagonalization of the Hamiltonians for a minimal three-site system, we demonstrate that the interaction-resistant metal emerges in both cases as a compromise between two different insulating solutions which are stabilized by different terms of the models. This provides strong evidence that the Hund's metal is a specific realization of a more general phenomenon which can be realized in various strongly correlated systems.

The recently developed theory of topological quantum chemistry (TQC) has built a close connection between band representations in momentum space and orbital characters in real space. It provides an effective way to diagnose topological materials, leading to the discovery of lots of topological materials after the screening of all known nonmagnetic compounds. On the other hand, it can also efficiently reveal spatial orbital characters, including average charge centers and site-symmetry characters. By using TQC theory with the computed irreducible representations in the first-principles calculations, we demonstrate that the electrides with excess electrons serving as anions at vacancies can be well identified by analyzing band representations (BRs), which cannot be expressed as a sum of atomic-orbital-induced band representations (aBRs). In fact, the floating bands (formed by the excess electrons) belong to the BRs induced from the “pseudo-orbitals” centered at vacancies. In other words, the electrides are proved to be unconventional ionic crystals, where a set of occupied bands is not a sum of aBRs, but necessarily contains a BR from vacancies. The TQC theory provides a promising avenue to pursue more electride candidates in ionic crystals.

The temperature-dependent evolution pattern of $5f$ electrons helps to elucidate the long-standing itinerant-localized dual nature in plutonium-based compounds. In this work, we investigate the correlated electronic states of $\mathrm{Pu}{\mathrm{In}}_3$ dependence on temperature by using a combination of the density functional theory and the dynamical mean-field theory. Not only is the experimental photoemission spectroscopy reproduced, but also a $5f$ itinerant-localized crossover is identified. Moreover, it is found that the quasiparticle multiplets from the many-body transitions gradually enhance with decreasing temperature, accompanied by the hybridizations with $5f$ electrons and conduction bands. The temperature-induced variation of Fermi surface topology suggests a possible electronic Lifshitz transition and the onset of magnetic order at low temperature. Finally, the ubiquitous existence orbital selective $5f$ electronic correlation is also discovered in $\mathrm{Pu}{\mathrm{In}}_3$. These illuminating results shall enrich the understanding on Pu-based compounds and serve as critical predictions for ongoing experimental research.

We demonstrate an experimental approach to determining the excited-state interatomic forces using femtosecond x-ray pulses from an x-ray free-electron laser. We determine experimentally the excited-state interatomic forces that connect photoexcited carriers to the nonequilibrium lattice dynamics in the prototypical Peierls-distorted material, bismuth. The forces are obtained by a constrained least-squares fit of a pairwise interatomic force model to the excited-state phonon dispersion relation as measured by the time- and momentum-resolved x-ray diffuse scattering. We find that photoexcited carriers weaken predominantly the nearest-neighbor forces, which drives the measured softening of the transverse acoustic modes throughout the Brillouin zone as well as the zone-center $A_{1g}$ optical mode. This demonstrates a bond-selective approach to measuring electron-phonon coupling relevant to a broad range of photoinduced phase transitions and transient light-driven states in quantum materials.

Several numerical studies have shown that the electronic properties of twisted bilayers of graphene (TBLG) and transition metal dichalcogenides (TMDs) are tunable by strain engineering of the stacking layers. In particular, the flatness of the low-energy moiré bands of the rigid and the relaxed TBLG was found to be, substantially, sensitive to the strain. However, to the best of our knowledge, there are no full analytical calculations of the effect of strain on such bands. We derive, based on the continuum model of moiré flat bands, the low-energy Hamiltonian of twisted homobilayers of graphene and TMDs under strain at small twist angles. We obtain the analytical expressions of the strain-renormalized Dirac velocities and explain the role of strain in the emergence of the flat bands. We discuss how strain could correct the twist angles and bring them closer to the magic angle ${\theta }_m=1.{05}^{\circ }$ of TBLG and how it may reduce the widths of the lowest-energy bands at charge neutrality of the twisted homobilayer of TMDs. The analytical results are compared with numerical and experimental findings and also with our numerical calculations based on the continuum model.

In this study, an order-by-disorder mechanism has been proposed in a two-dimensional PXP model, where the extensive degeneracy of the classical ground-state manifold is due to strict occupation constraints instead of geometrical frustrations. By performing an unbias large-scale quantum Monte Carlo simulation, we find that local quantum fluctuations, which usually work against long-range ordering, lift the macroscopic classical degeneracy and give rise to a compressible ground state with charge-density-wave long-range order. A simple trial wave function has been proposed to capture the essence of the ground-state of the two-dimensional PXP model. The finite-temperature properties of this model have also been studied, and we find a thermal phase transition with a universality class of a two-dimensional Ising model.

It has recently been realized that the first-order moment of the Berry curvature, namely, the Berry curvature dipole (BCD), can give rise to nonlinear current in a wide variety of time-reversal invariant and non-centrosymmetric materials. While the BCD in two-dimensional Dirac systems is known to be finite only in the presence of either substantial spin-orbit coupling where low-energy Dirac quasiparticles form tilted cones or higher order warping of the Fermi surface, we argue that the low-energy Dirac quasiparticles arising from the merging of a pair of Dirac points without any tilt or warping of the Fermi surface can lead to a nonzero BCD. Remarkably, in such systems, the BCD is found to be independent of Dirac velocity as opposed to the Dirac dispersion with a tilt or warping effects. We further show that the proposed systems can naturally host helicity-dependent photocurrent due to their linear momentum-dependent Berry curvatures. Finally, we discuss an important byproduct of this work, i.e., nonlinear anomalous Nernst effect as a second-order thermal response.

We measure the spin-charge interconversion by the spin Hall effect in various ferromagnetic/Pt nanodevices. The extracted effective spin Hall angles of Pt evolve drastically with the ferromagnetic (FM) materials (CoFe, Co, and NiFe), when assuming transparent interfaces and a bulk origin of the spin injection/detection by the FM elements. By carefully estimating the interface resistance, we show that it is quite large and cannot be neglected. We then evidence that the spin injection/detection at the FM/Pt interfaces are dominated by the spin polarization of the interfaces. We show that interfacial asymmetric spin scattering becomes the driving mechanism of the spin injection in our samples.

Zr-Te compounds are an ideal platform to investigate novel electrical transport properties. Here we report the Kondo effect in single-crystalline ${\mathrm{ZrTe}}_2$. When $T<8$ K, ${\mathrm{ZrTe}}_2$ exhibits a $\log T$ dependence of resistivity, which may derive from three different mechanisms, including electron-electron interaction, weak localization, and Kondo effect. By measuring transport properties with magnetic field along different directions, the former two mechanisms were excluded. Isotropic negative magnetoresistance was extracted by subtracting different quadratic background signals caused by Lorentz force, which reveals the existence of a weak Kondo effect in this material.

The noncollinear magnetic state of epitaxial Mn monolayers on tungsten (110) crystal surfaces is investigated by means of soft x-ray absorption spectroscopy to complement earlier spin-polarized scanning tunneling microscopy experiments. X-ray absorption spectra (XAS), x-ray linear dichroism (XLD) and field-induced x-ray magnetic circular dichroism (XMCD) Mn $L_{2,3}$-edge spectra were measured in the temperature range from 8 to 300 K and compared to results of fully relativistic ab initio calculations. We show that antiferromagnetic (AFM) helical and cycloidal spirals give rise to significantly different Mn $L_{2,3}$-edge XLD signals, enabling, thus, to distinguish between them. It follows from our results that the magnetic ground state of a Mn monolayer on W(110) is an AFM cycloidal spin spiral. Based on temperature-dependent XAS, XLD, and field-induced XMCD spectra we deduce that magnetic properties of the Mn monolayer on W(110) vary with temperature, but this variation lacks a clear indication of a phase transition in the investigated temperature range up to 300 K—even though a crossover exists around 170 K in the temperature dependence of XAS branching ratios and in XLD profiles.

We study voltage controllable superconducting state in a multiterminal bridge composed of the dirty superconductor/pure normal metal (SN) bilayer and pure normal metal. In the proposed system small control current $I_{\mathit{ctrl}}$ flows via normal bridge, creates voltage drop $V$, and modifies the distribution function of electrons in a connected SN bilayer. In the case of a long normal bridge the voltage-induced nonequilibrium effects could be interpreted in terms of increased local electron temperature. In this limit we experimentally find the large sensitivity of critical current $I_c$ of Cu/MoN/Pt-Cu bridge to $I_{\mathit{ctrl}}$ and relatively large current gain which originate from steep dependence of $I_c$ on temperature and large $I_c$ (comparable with theoretical depairing current of the superconducting bridge). In the short normal bridge deviation from equilibrium cannot be described by a simple increase in local temperature, but we also theoretically find large sensitivity of $I_c$ to control current/voltage. In this limit we predict existence at finite $V$ of the so-called in-plane Fulde-Ferrell state with spontaneous currents in the SN bilayer. We argue that its appearance is connected with voltage-induced paramagnetic response in the N layer.

$\mathrm{TaIr}{\mathrm{Te}}_4$ is a recently discovered type-II Weyl semimetal, hosting only four Weyl points. Here, we study the cleaved $\mathrm{TaIr}{\mathrm{Te}}_4$ crystal using scanning tunneling microscopy/spectroscopy and find that it also hosts a superconducting state with a transition temperature of 3.9 K. From Dynes function fitting, the superconducting phase is consistent with anisotropic $s$-wave pairing, with a superconducting gap of 1.31 meV. This value leads to a value of $2{\mathrm{\Delta }}_{\max }/k_B{T}_C=7.81$, much larger than the 3.53 predicted by Bardeen-Cooper-Schrieffer theory for weak-coupling superconductors. The critical field is found to be 0.7 T based on the analysis of tunneling conductance as a function of magnetic field. Two types of nonmagnetic defects on the $\mathrm{TaIr}{\mathrm{Te}}_4$ surface are observed, neither of which induce bound states inside the superconducting gap, further supporting conventional $s$-wave superconductivity in the $\mathrm{TaIr}{\mathrm{Te}}_4$ system.

Acoustic vortices (AVs) based on artificial metamaterials have attracted intense research efforts for their potential use in subdiffraction imaging, particle manipulation, and acoustic communication. However, to date, major strategies to generate AVs have an intrinsic limitation of fundamentally depending on complex phase modulations, which require a higher number of multiple elements as the desired topological charge increases, let alone the elaborate dephasing between the individual microstructured elements in the switching process of topological charges. In this paper, we propose an annulus anisotropic structure coupled with a pair of eccentric asynchronous monopole sources that can realize multiple-order AV modes at different frequencies. The generated AVs originate from the interactions of two superimposed multipoles characterized with orthogonal emission patterns and $\pi /2$-phase differences. The AVs with topological charges from 1 to 5 are achieved simultaneously at dipole, quadrupole, hexapole, octupole, and decapole resonances, respectively. Numerical simulations and experimental results clearly confirm the feasibility and practical applicability of the proposed design strategy in realizing topologically diverse AVs. Our design, exempted from complex phase modulations, possesses flexible topological charge modes at the expense of frequency, which may foresee potential capabilities in multiplexing and demultiplexing of AVs.

We present measurements of spin-orbit torques generated by Ir as a function of film thickness in sputtered Ir/CoFeB and Ir/Co samples. We find that Ir provides a dampinglike component of spin-orbit torque with a maximum spin-torque conductivity ${\sigma }_{\mathrm{DL}}^{\mathrm{eff}}=(1.4\pm 0.1)\times {10}^5\frac{\hslash }{2e}{\mathrm{\Omega }}^{-1}{\mathrm{m}}^{-1}$ and a maximum spin-torque efficiency of ${\xi }_{DL}=0.042\pm 0.005$, which is sufficient to drive switching in a 0.8 nm film of CoFeB with perpendicular magnetic anisotropy. We also observe a surprisingly large fieldlike spin-orbit torque (FLT). Measurements as a function of Ir thickness indicate a substantial contribution to the FLT from an interface mechanism, so that in the ultrathin limit there is a nonzero FLT with a maximum torque conductivity ${\sigma }_{\mathrm{FL}}^{\mathrm{eff}}=-(5.0\pm 0.5)\times {10}^4\frac{\hslash }{2e}{\mathrm{\Omega }}^{-1}{\mathrm{m}}^{-1}$. When the Ir film thickness becomes comparable to or greater than its spin diffusion length, $1.6\pm 0.3\mathrm{nm}$, there is also a smaller bulk contribution to the fieldlike torque.

Room-temperature electric-field control of magnetism is actively sought to realize electric-field assisted changes in perpendicular magnetic anisotropy (PMA), which is important to magnetic random access memories (MRAMs) and future spin-orbit based logic technologies. Traditional routes to achieve such control rely on heterostructures of ferromagnets and/or ferroelectrics, exploiting interfacial effects, including strain generated by the substrate, or electric-field induced changes in the interfacial electronic structures. Here we present design rules based on $d$-orbital splitting in an octahedral field and crystallographic symmetries for electric-field control of PMA utilizing hybrid improper ferroelectricity by scaffolding simple perovskite oxides into ultrashort period superlattices, ${\left(\mathrm{AB}{\mathrm{O}}_3\right)}_1/{\left({\mathrm{A}}^{\prime }\mathrm{B}{\mathrm{O}}_3\right)}_1$, and in multiferroic ${\mathrm{AA}}^{\prime }{\mathrm{BB}}^{\prime }{\mathrm{O}}_6$ double perovskites. We validate the strategy using first principles calculations and a single-ion anisotropic model. We find a change in the magnetic anisotropy from the in-plane to the out of plane direction in ${\left(\mathrm{BiFe}{\mathrm{O}}_3\right)}_1/{\left(\mathrm{LaFe}{\mathrm{O}}_3\right)}_1$ and a 50% decrease of the magnetization along the out of plane direction in $\mathrm{LaYNiMn}{\mathrm{O}}_6$, when a polar to nonpolar phase transition occurs with strain. The origin of the PMA control is due to the structural tunable competitions among the $t_{2g}$ and $e_g$ orbital interactions on the magnetic ions arising from relativistic spin-orbital interactions that are susceptible to changes in the oxygen octahedral tilts across the field-tunable transition. Our results allow us to search rapidly for other promising multiferroics materials with voltage-controlled magnetic anisotropy for applications in low-energy information storage and logic devices.

The bulk polarization is a ${\mathbb{Z}}_2$ topological invariant characterizing noninteracting systems in one dimension with chiral or particle-hole symmetries. We show that the bulk polarization can always be determined from the single-particle entanglement spectrum, even in the absence of symmetries that quantize it. In the symmetric case, the known relation between the bulk polarization and the number of virtual topological edge modes is recovered. We use the bulk polarization to compute Chern numbers in one and two dimensions, which illuminates their known relation to the entanglement spectrum. Furthermore, we discuss an alternative bulk polarization that can carry more information about the surface spectrum than the conventional one and can simplify the calculation of Chern numbers.

Nonlinear responses in condensed matter are intensively studied because they provide rich information about the materials and hold the possibility of being applied in diodes or high-frequency optical devices. While nonlinear responses in noninteracting models have been explored widely, the effect of strong correlations on the nonlinear response is still poorly understood, even though it has been suggested that correlations can enhance the nonlinear response. In this work, we first give an analytical derivation of nonlinear responses using Green's function methods at finite temperature. Then, we discuss the difficulties of considering dissipation using conventional methods, such as the reduced density matrix method. We reveal that the relaxation time approximation leads to severe limitations when considering optical responses. Finally, we demonstrate that correlation effects, such as the renormalization of the band structure and different lifetimes in orbitals or sublattices, can significantly enhance nonlinear responses and even change the sign of the nonlinear conductivity.

We show that mutual statistics between quantum particles can be tuned to generate emergent novel few-particle quantum mechanics for the boundary modes of symmetry-protected topological phases of matter. As a concrete setting, we study a system of pseudofermions, defined as quantum particles with tunable algebra, which lie on two distinct Su-Schrieffer-Heeger (SSH) chains. We find that as the mutual statistics of the particles are tuned—the boundary modes present in the two chains gets nontrivially entangled showing a sudden jump in their mutual entanglement entropy. We further show that such tuning of statistics engenders a first-order transition between two topologically nontrivial phases which differ in the behavior of interchain entanglement. Using a combination of analytical and numerical techniques and effective modeling, we uncover the rich physics that this system hosts. The results are of particular relevance in context of the study of the effective low-energy quantum mechanics of topological edge modes in one hand and their recent realization in ultracold atoms on the other. This then provides for controlled manipulation of such low-energy modes.