We experimentally study the microwave generation characteristics of a spin Hall nano-oscillator driven by a local spin current injected into a nanoscale region of extended Pt/Ni/Fe trilayers due to the spin Hall effect in the Pt layer and the interfacial Rashba effect at the Pt/Ni interface. The dependence of the generated microwave spectra on the exciting current and on the magnitude and angle of the magnetic field indicates that multiple spin-wave modes with either higher frequencies than the ferromagnetic resonance frequency $f_{\mathrm{FMR}}$ at low out-of-plane tilting angles and small fields or lower frequencies than $f_{\mathrm{FMR}}$ at high tilting angles and large fields are excited by local spin currents. Furthermore, the temperature dependence of the generated spectra in the extended Pt/Ni/Fe trilayers demonstrates that the thermally activated mode transition between these distinct dynamical modes is significantly suppressed. These results suggest that the observed multiple modes are physically separated in different local potential wells created by the spatial inhomogeneity of the internal field in the asymmetric ferromagnetic bilayer with strong interfacial exchange coupling and anisotropy fields. Finally, the very weak dependence of the minimum linewidth on temperature further confirms the lack of thermal-fluctuation-induced mode coupling and mode transition between these individual dynamical modes.

Rare-earth transition-metal ferrimagnets have two strongly coupled sublattices of distinct chemical nature, which give rise to complex and fast dynamics of great interest to spintronics. However, the dynamics of ferrimagnets remains less understood than ferromagnets. We measure the spin wave (SW) spectra of a GdFeCo film by Brillouin light scattering spectroscopy (BLS) across its compensation temperatures—temperatures at which either the sublattices' magnetizations or their angular moments cancel out, mimicking an antiferromagnet. We find two SW modes per wave vector with complex thermal dependencies, which cross at a field-dependent temperature. We develop an analytical model based on two sublattices corresponding to the rare earth and the transition metal, which reproduces quantitatively the SW spectra and their evolution with temperature and field. This validates the proposed energy and dynamical model of the ferrimagnet, and demonstrates the usefulness of BLS in the study of this promising class of materials.

We studied a mechanism of magnon absorption from a magnon waveguide to adjacent metals. Magnon propagation in the yttrium-iron-garnet magnon waveguide induced/detected via coplanar waveguides was reduced by attaching metals to the magnon waveguide. In particular, the propagating magnon at the surface of the magnon waveguide was largely suppressed by attaching metal strips whereas the uniform or volume mode showed only a very slight decrease. A systematic study on the reduction of magnon propagation was performed by changing the material of metals, changing their thickness, and inserting $\mathrm{Si}{\mathrm{O}}_2$ between the magnon waveguide and metals. The reduction in magnon propagation by $\mathrm{Si}{\mathrm{O}}_2$ insertion indicated that the eddy-current contribution played a key role in magnon propagation with the magnon gate system with metal electrodes. Our study revealed the relationship between the magnon loss and metallic electrodes, which is indispensable in the fabrication of magnonic devices.

A small number of superconductors are believed to exhibit intrinsic spin-triplet pairing, and they are often discussed in terms of a simple, ${}^3\mathrm{He}$-like picture where ferromagnetic spin fluctuations provide the “glue.” However, in some cases in which reliable inelastic neutron scattering measurements are available, spin excitations are found to be peaked at finite momentum $\mathbf{q}$ rather than $\mathbf{q}=\mathit{0}$. Here we investigate some simple models that exhibit triplet pairing arising from antiferromagnetic spin fluctuations. We show that a strong peak at larger $\mathbf{q}$ in the magnetic susceptibility can drive such states and can give rise to pairing states with nodes in the $k_z$ plane even in the presence of a pure two-dimensional Fermi surface. In these situations, dominant pair scattering processes occur between Fermi surface segments with like signs of the superconducting order parameter, yet they are consistent with an overall odd-parity state. We examine the applicability of these scenarios to the putative triplet superconductor $\mathrm{U}{\mathrm{Te}}_2$.

Many strongly correlated materials are characterized by deeply intertwined charge and spin order. Besides their high superconducting transition temperatures, one of the central features of these complex patterns in cuprates is a phase shift which occurs across lines of decreased hole density. That is, when doped away from their antiferromagnetic phase, the additional charge is not distributed uniformly, but rather in “stripes.” The sublattices preferentially occupied by up and down spin are reversed across these stripes, a phenomenon referred to as a “$\pi$ phase shift.” Many of the spin-charge patterns, including the $\pi$ phase shift, are reproduced by density matrix renormalization group and quantum Monte Carlo calculations of simplified tight binding (repulsive Hubbard) models. In this paper we demonstrate that this sublattice reversal is generic by considering the corresponding phenomenon in the attractive Hubbard Hamiltonian, where a charge density wave phase forms at half filling. We introduce charge stripes via an appropriate local chemical potential; measurements of charge correlation across the resulting lines of lowered density reveal a clear $\pi$ phase.

We construct an example of a 1d quasiperiodically driven spin chain whose edge states can coherently store quantum information, protected by a combination of localization, dynamics, and topology. In a sharp departure from topological phases in static and periodically driven (Floquet) spin chains, this model does not rely upon microscopic symmetry protection: Instead, the edge states are protected purely by emergent dynamical symmetries. We explore the dynamical signatures of this emergent dynamical symmetry-protected topological (EDSPT) order through exact numerics, time evolving block decimation, and analytic high-frequency expansion, finding evidence that the EDSPT is a stable dynamical phase protected by bulk many-body localization up to (at least) stretched-exponentially long timescales, and possibly beyond. We argue that EDSPTs are special to the quasiperiodically driven setting, and cannot arise in Floquet systems. Moreover, we find evidence of a type of boundary critical with no known static or Floquet analogue, in which the edge spin dynamics transition from quasiperiodic to chaotic, leading to bulk thermalization.

Weyl nodes can be classified into zero-dimensional (0D) Weyl points, 1D Weyl nodal lines, and 2D Weyl nodal surfaces (WNS), which possess finite Chern numbers. Up to date, the largest Chern number of WPs identified in Weyl semimetals is 4, which is thought to be a maximal value for linearly crossing points in solids. On the other hand, whether the Chern numbers of nonzero-dimensional linear crossing Weyl nodal objects have one upper limit is still an open question. In this work, combining angle-resolved photoemission spectroscopy with density-functional theory calculations, we show that the chiral crystal AlPt hosts a cube-shaped charged WNS which is formed by the linear crossings of two singly degenerate bands. Different from conventional Weyl nodes, the cube-shaped nodal surface in AlPt is enforced by nonsymmorphic chiral symmetries and time-reversal symmetry rather than accidental band crossings, and it possesses a giant Chern number $\left|C\right|=26$. Moreover, our results and analysis prove that there is no upper limit for the Chern numbers of such kind of 2D Weyl nodal object.

We give a bulk-hinge correspondence for higher-order topological phases protected by rotoinversion $C_4\mathcal{I}$ symmetry in magnetic systems. Our approach allows us to show the emergence of the chiral hinge modes only from the information of the $C_4\mathcal{I}$ eigenvalues at the high-symmetry points in the Brillouin zone. In addition, based on the bulk-hinge correspondence, we propose a class of higher-order Weyl semimetals (HOWSMs) being Weyl semimetals with hinge modes and Fermi-arc surface states. The HOWSM is characterized by topological invariants for three-dimensional higher-order topological insulators, and the topological invariants are determined by the $C_4\mathcal{I}$-symmetry eigenvalues at the high-symmetry points. This HOWSM has chiral hinge modes as a direct consequence of the three-dimensional higher-order topology in the bulk.

We study the topological gap labeling of general three-dimensional (3D) quasicrystals and we find that every gap in the spectrum is characterized by a set of third Chern numbers. We show that a quasiperiodic structure has multiple Brillouin zones defined by redundant wave vectors, and the number of states below a gap is quantized as an integer linear combination of volumes of these Brillouin zones. The associated quantum numbers to characterize energy gaps can be expressed as third Chern numbers by considering a formal relationship between an adiabatic charge pumping under cyclic deformation of the quasiperiodic potential and a topological nonlinear electromagnetic response in 6D band insulators.

We numerically investigate the second-order nonlinear Hall transport properties of a four-terminal system with time-reversal symmetry and broken inversion symmetry. Within the nonequilibrium Green's function formalism, the second-order nonlinear conductances are derived, where the internal Coulomb potential in response to external voltages is explicitly included to guarantee the gauge invariance. For the system with single mirror symmetry ${\mathcal{M}}_x$, nonlinear Hall properties are only observable in the $y$ direction and contributed solely from the second-order nonlinear effect. From the symmetry point of view, the observed nonlinear Hall transport phenomena have one-to-one correspondence with the Berry curvature dipole induced nonlinear Hall effect semiclassically obtained for the same Hamiltonian. In addition to the nonlinear Hall effect originated from symmetries of the system, it is found that the internal Coulomb potential has the same symmetry of the four-terminal system, which gives rise to an extra nonlinear Hall response. Moveover, the phase relaxation mechanism modeled by virtual probes leads to the dephasing-induced nonlinear Hall effect.

Solid-state thermionic structures made from layered van der Waals heterostructures have shown promising thermal-to-electrical energy conversion efficiencies theoretically. In this paper, we further study these structures using first-principles calculations combined with the Green's function method. By calculating the electron-phonon relaxation length, we confirm ballistic transport in these structures. We study the effect of the number of layers, the energy barrier, and the asymmetry of the contacts on the performance of ${\mathrm{MoSe}}_2$-based thermionic converters. We show that the key to high-performance thermionic diodes is to make a low-energy barrier and low-resistance metallic contacts, and we identify copper as the optimum metallic contact to ${\mathrm{MoSe}}_2$-based devices. We further show that, unlike the vacuum-based thermionic diodes, asymmetry does not result in improved performance within the linearized transport theory.

The atomic scale control of interface geometry and lateral width of in-plane lateral heterostructures based on 2D materials can pave the way for many nanotechnological applications. In this work, we propose a device architecture composed of spatially patterned lateral domains of ${\mathrm{MoSe}}_2$ embedded in graphene (graphene-${\mathrm{MoSe}}_2$). We have considered four structures that combine different interface geometries. By using first-principles calculations based on density functional theory, we explored how the energetic, structural, electronic, and magnetic properties of each structure depend on the graphene and ${\mathrm{MoSe}}_2$ stripe widths. We find a variety of electronic characteristics, including spin-polarized semiconductor, nonmagnetic semiconductor, spin-polarized metallic, nonmagnetic metallic, and half-metallic behaviors, which indicate promising applications in spintronics. In addition, we investigate the adsorption of nitrogen-based molecules (${\mathrm{NO}}_2$, NO, and ${\mathrm{NH}}_3$) for the most stable heterostructure under Mo-rich conditions. For this case, we find that the interfaces of the hybrid material play an important role on the sensing properties, leading to improved adsorption characteristics in comparison with the isolated pure graphene and ${\mathrm{MoSe}}_2$ systems. In that sense, we hope that future investigations exploring the effects of different interfaces, geometries and adsorbant concentrations may further elucidate the potential of graphene/TMD lateral heterostructures for sensing applications.

We report a systematic study of the magnetoresistance, Hall effect, and the electronic structure in an iodine-intercalated tungsten ditelluride (${\mathrm{WTe}}_2\mathrm{I}$) single crystal, which crystallizes in a centrosymmetric structure. ${\mathrm{WTe}}_2\mathrm{I}$ presents significant anisotropic magnetotransport behavior with the magnetic field rotated in the $xz$ and $yz$ planes and violation of the Kohler's rule. Unexpectedly, nonsaturating linear magnetoresistance (LMR) was observed under the magnetic field parallel to the $z$ axis and the current along the $x$ axis. The analysis of the Hall effect reveals that the carrier mobility in ${\mathrm{WTe}}_2\mathrm{I}$ is notably lower than that of ${\mathrm{WTe}}_2$ and meets the picture of the Parrish and Littlewood model, suggesting that the LMR is likely caused by disorder effects. First-principles calculations show that the electronic structure undergoes a topological phase transition from a Weyl state to a Dirac state due to iodine intercalation. This finding provides a platform to search for physical phenomena related to ${\mathrm{WTe}}_2$ and study the topological phase transition.

Calculating the ground state properties of a Hamiltonian can be mapped to the problem of finding the ground state of a smaller Hamiltonian through the use of embedding methods. These embedding techniques have the ability to drastically reduce the problem size, and hence the number of qubits required when running on a quantum computer. However, the embedding process can produce a relatively complicated Hamiltonian, leading to a more complex quantum algorithm. In this paper we carry out a detailed study into how density matrix embedding theory (DMET) could be implemented on a quantum computer to solve the Hubbard model. We consider the variational quantum eigensolver (VQE) as the solver for the embedded Hamiltonian within the DMET algorithm. We derive the exact form of the embedded Hamiltonian and use it to construct efficient ansatz circuits and measurement schemes. We conduct detailed numerical simulations up to 16 qubits, the largest to date, for a range of Hubbard model parameters and find that the combination of DMET and VQE is effective for reproducing ground state properties of the model.

The discovery of monolayer FeSe on ${\mathrm{SrTiO}}_3$ (STO) with the boosting superconducting transition temperature has prompted an intense increase in experimental and theoretical researches on interfacial superconductivity enhancement. One of the most concerning research subjects is to explore the nature of the interfacial coupling. Here, we performed a comparative study between the isostructural CoSe/STO and FeSe/STO by measuring the phonons and plasmons via high-resolution electron energy loss spectroscopy. With the increasing thickness of the epitaxial film, the measured intensity of the Fuchs-Kliewer (F-K) phonons and the plasmon from STO decay much faster in the CoSe film than those in FeSe, owing to the higher carrier concentration in CoSe. By comparing the temperature-dependent behaviors of the F-K phonons and plasmons, we discovered that the interfacial coupling in CoSe/STO is adiabatic because of its large Fermi energy. In contrast, the interfacial electron-phonon coupling in FeSe/STO is nonadiabatic, resulting in the formation of interfacial polarons that plays an important role in the superconductivity enhancement.

Ionic contributions to the van der Waals interactions of typical polar compounds have been investigated based on density-functional-theory-calculated dielectric functions. Derived from the Lifshitz theory, the contribution of ionic polarizabilities to the dispersion energy between parallel half-spaces can be up to 3.4%. Using the Clausius-Mossotti relationship with a Lorentz factor of 4π/3, it is found that the ionic effect on $C_6$ is consistent with that on energy. In parallel slabs separated distantly, the ionic polarizability plays a substantially enhanced role, contributing up to 25% of the energy. The significant ionic effect in the latter case is due to a reduction in the screening of low-frequency terms, and a zero Lorentz factor should be assumed to predict a consistent ionic effect on $C_6$.

We develop an approximate quasistatic theory describing the low-frequency plasmonic resonances of slender nanometallic rings and configurations thereof. First, we use asymptotic arguments to reduce the plasmonic eigenvalue problem governing the geometric (material- and frequency-independent) modes of a given ring structure to a one-dimensional periodic integrodifferential problem in which the eigenfunctions are represented by azimuthal voltage and polarization-charge profiles associated with each ring. Second, we obtain closed-form solutions to the reduced eigenvalue problem for azimuthally invariant rings (including torus-shaped rings but also allowing for noncircular cross-sectional shapes), as well as coaxial dimers and chains of such rings. For more general geometries, involving azimuthally nonuniform rings and noncoaxial structures, we solve the reduced eigenvalue problem using a semianalytical scheme based on Fourier expansions of the reduced eigenfunctions. Third, we used the asymptotically approximated modes, in conjunction with the quasistatic spectral theory of plasmonic resonance, to study and interpret the frequency response of a wide range of nanometallic slender-ring structures under plane-wave illumination.

Layered transition metal dichalcogenides have a rich phase diagram and they feature two-dimensionality in numerous physical properties. ${\mathrm{Co}}_{1/3}{\mathrm{NbS}}_2$ is one of the newest members of this family where Co atoms are intercalated into the van der Waals gaps between ${\mathrm{NbS}}_2$ layers. We study the three-dimensional electronic band structure of ${\mathrm{Co}}_{1/3}{\mathrm{NbS}}_2$ using both surface and bulk sensitive angle-resolved photoemission spectroscopy. We show that the electronic bands do not fit into the rigid band shift picture after the Co intercalation. Instead, ${\mathrm{Co}}_{1/3}{\mathrm{NbS}}_2$ displays a different orbital character near the Fermi level compared to the pristine ${\mathrm{NbS}}_2$ compound and has a clear band dispersion in the $k_z$ direction despite its layered structure. Our photoemission study demonstrates the out-of-plane electronic correlations introduced by the Co intercalation, thus offering a different perspective on this compound. Finally, we propose how Fermi level tuning could lead to exotic phases such as spin density wave instability.

We develop a theoretical description of the Raman spectroscopy in the spin-phonon-coupled Kitaev system and show that it can provide observable signatures of fractionalized excitations characteristic of the underlying spin-liquid phase. In particular, we obtain the explicit form of the phonon modes and construct the coupling Hamiltonian based on the $D_{3d}$ symmetry. We then systematically compute the Raman intensity and show that the spin-phonon coupling renormalizes phonon propagators and generates the salient Fano lineshape. We find that the temperature evolution of the Fano lineshape displays two crossovers, and the low-temperature crossover shows pronounced magnetic-field dependence. We thus identify the observable effect of the Majorana fermions and the $Z_2$ gauge fluxes encoded in the Fano lineshape. Our results are consistent with the phonon Raman scattering experiments in the candidate material $\alpha -{\mathrm{RuCl}}_3$.

The electron-electron interaction in the Landau levels of bilayer graphene is markedly different from that of conventional semiconductors such as GaAs. We show that in the zeroth Landau level of bilayer graphene, in the orbital which is dominated by the nonrelativistic second Landau level wave function, by tuning the magnetic field, a topological quantum phase transition from an Abelian composite fermion to a non-Abelian parton fractional quantum Hall state can be induced at filling factors $1/2$, 2/5, and $3/7$. The parton states host exotic anyons that can potentially be utilized to store and process quantum information. Intriguingly, some of these transitions may have been observed in a recent experiment (K. Huang et al., arXiv:2105.07058).