Recently, the topological states in electric circuits have attracted a great deal of attention, and they have formed the basis of the field of so-called topoelectrical circuits. We realize a second-order topological electric circuit with capacitors and inductors, and we observe the corner states that exist in the bulk gap, the continuum of bulk modes, and the dependence on the corner shapes by measuring the resonance peaks of the corner impedance. More importantly, we broaden the second-order topological circuit to the non-Hermitian region by adding resistors. Interestingly, the non-Hermiticity tunes the amplitude of the corner impedance; however, it has no effect on the topological properties. Our system provides a highly tunable platform in which to explore the intriguing topological properties of higher-order topological insulators.

Molecular dynamics simulations of liquid and glassy selenium have been carried out using density functional (400–773 K, 600 atoms) and classical force field (290–500 K, 5488 atoms) methods. Structural features (structure factors, pair distribution functions, bond lengths, bond and dihedral angles, cavities) and dynamical properties (diffusion coefficients, power spectra, sound velocity, collective excitations, bond lifetimes) agree well with experimental data where available. The structures are predominantly chainlike, with a small fraction of rings with a range of sizes, and large cavity volumes lead to flexible chains. It is striking that the density functional simulations show very few ${\mathrm{Se}}_8$ rings at 600 K and below.

We consider a driven, non-Hermitian generalization of the Aubry-Andre-Harper model. We show that the introduction of periodic driving allows us to obtain fully real quasienergy spectra in configurations where the corresponding static system has complex energy eigenvalues for any non-Hermitian degree. We illustrate how generalized parity-time ($\mathcal{PT}$) symmetry can be applied within the Floquet formalism and show that our model's fully real quasienergy spectrum corresponds to an unbroken symmetry. This model exhibits a Floquet topological phase, few examples of have been found in non-Hermitian potentials.

All-optical helicity-dependent manipulations of magnetism have attracted broad attention in the context of ultrafast control of magnetic units. Here, we investigate the spin dynamics in time reversal symmetric insulators induced by strong circularly polarized light. We perform real-time time-dependent density functional theory calculations together with model Hamiltonian analyses for $\mathrm{Mo}{\mathrm{S}}_2$ and $\mathrm{W}{\mathrm{S}}_2$ monolayers, which are exemplary spin-orbit-coupled time reversal symmetric insulators. We trace the evolution of dynamical spin states, starting from the Kramers-paired electronic ground state, and find that the induced magnetization exhibits a sharp resonance peak when the applied light frequency is close to half the spin-flipping energy gap. The resonance condition is secondarily affected by the field strength and the pulse width. We suggest that low-energy time reversal broken excitations of insulators can be pursued with a sharp frequency selection as another class of ultrafast phenomena.

A magnetic bimeron composed of two merons is a topological counterpart of a magnetic skyrmion in in-plane magnets, which can be used as the nonvolatile information carrier in spintronic devices. Here we analytically and numerically study the dynamics of ferromagnetic bimerons driven by spin currents and magnetic fields. Numerical simulations demonstrate that two bimerons with opposite signs of topological numbers can be created simultaneously in a ferromagnetic thin film via current-induced spin torques. The current-induced spin torques can also drive the bimeron and its speed is analytically derived, which agrees with the numerical results. Since the bimerons with opposite topological numbers can coexist and have opposite drift directions, two-lane racetracks can be built to accurately encode the data bits. In addition, the dynamics of bimerons induced by magnetic field gradients and alternating magnetic fields are investigated. It is found that the bimeron driven by alternating magnetic fields can propagate along a certain direction. Moreover, combining a suitable magnetic field gradient, the Magnus-force-induced transverse motion can be completely suppressed, which implies that there is no skyrmion Hall effect. Our results are useful for understanding the bimeron dynamics and may provide guidelines for building future bimeron-based spintronic devices.

The nature of the parent compound is intimately connected to the doping-induced emergence of superconductivity and the pairing mechanism in high-temperature superconductors. In high-temperature cuprate superconductors, the parent phase is believed to be a Mott insulator and superconductivity is realized by doping the Mott insulator. In iron-based superconductors, on the other hand, the parent compounds discovered so far are all bad metals which are either antiferromagnetic or nematic. Here we report direct spectroscopic evidence that the parent phase of the single-layer $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$ (FeSe/STO) films is insulating. It represents a case in the iron-based superconductors in which the parent compound is an insulator. We also find that the single-layer FeSe/STO films exhibit a distinct doping evolution to superconductivity that is similar to doping a Mott insulator in cuprate superconductors. These observations provide key insights in understanding the record high- $T_C$ superconductivity in single-layer FeSe/STO films.

Multiple mechanisms for extremely large magnetoresistance (XMR) found in many topologically nontrivial/trivial semimetals have been theoretically proposed, but experimentally it is unclear which mechanism is responsible in a particular sample. In this paper, by the combination of band structure calculations, numerical simulations of magnetoresistance (MR), Hall resistivity, and de Haas-van Alphen (dHvA) oscillation measurements, we studied the MR anisotropy of $\mathrm{Si}{\mathrm{P}}_2$ which is verified to be a topologically trivial, incomplete compensation semimetal. It was found that as magnetic field $H$ is applied along the $a$ axis, the MR exhibits an unsaturated nearly linear $H$ dependence, which was argued to arise from incomplete carriers compensation. For the $H\parallel \left[101\right]$ orientation, an unsaturated nearly quadratic $H$ dependence of MR up to $5.88\times {10}^4\%$ (at 1.8 K, 31.2 T) and field-induced up-turn behavior in resistivity were observed, which was suggested due to the existence of hole open orbits extending along the $k_x$ direction. Good agreement of the experimental results with the simulations based on the calculated Fermi surface (FS) indicates that the topology of FS plays an important role in its MR.

The dualities that map hard-to-solve, interacting theories to free, noninteracting ones often trigger a deeper understanding of the systems to which they apply. However, simplifying assumptions such as Lorentz invariance, low dimensionality, or the absence of axial gauge fields, limit their application to a broad class of systems, including topological semimetals. Here we derive several axial field theory dualities in $2+1$ and $3+1$ dimensions by developing an axial slave-rotor approach capable of accounting for the axial anomaly. Our $2+1$-dimensional duality suggests the existence of a dual, critical surface theory for strained three-dimensional nonsymmorphic topological insulators. Our $3+1$-dimensional duality maps free Dirac fermions to Dirac fermions coupled to emergent U(1) and Kalb-Ramond vector and axial gauge fields. Upon fixing an axial field configuration that breaks Lorentz invariance, this duality maps free to interacting Weyl semimetals, thereby suggesting that the quantization of the nonlinear circular photogalvanic effect can be robust to certain interactions. Our paper emphasizes how axial and Lorentz-breaking dualities improve our understanding of topological matter.

The chiral anomaly is one of the unusual effects of Weyl fermions and has attracted much interest. Experimental observation of this effect has largely been focused on a negative longitudinal magnetoresistance, which, however, has been proven to be insufficient. Thermoelectric experiment, a technique complimentary to the electrical resistivity and able to the capability to reveal the energy dependence of electron transport, has not been reported in many relevant semimetals. Here, we study the electrical and thermoelectric properties of a topological semimetal, $\mathrm{Zr}{\mathrm{Te}}_5$. Besides a previously reported negative magnetoresistance (NMR), a negative magnetothermopower (MTEP) is observed when the magnetic field is parallel to the temperature gradient. Both the magnetoconductivity and MTEP follow a quadratic dependence on the magnetic field. Moreover, the quadratic coefficient of MTEP is about twice as large as that of magnetoconductivity. These observations are consistent with the effect of the chiral anomaly. We also discuss other possible origins of these observations, such as current jetting, ionized impurity scattering, and spin-related effects.

Usually, in optical spectra of double-walled carbon nanotubes (DWCNTs), weak van der Waals coupling between the layers leads only to a small shift of transition energies with respect to their values in pristine single-walled nanotubes. However, recent results have shown that the Rayleigh spectrum of the DWCNT (12,11)@(17,16) contains additional peaks. Using the tight-binding approximation, we demonstrate that in specific DWCNTs the interlayer coupling can slightly modify the band structure of pristine nanotubes in such a way that the unconventional inter-tube electronic transitions become possible and additional peaks in the DWCNT optical spectrum appear. Using the known experimental data on 118 optical transitions in DWCNTs, in addition to the recently published case, we reveal six more DWCNTs with inter-tube transitions and obtain geometrical selection rules permitting them. In a few dozen of DWCNTs our approach yields the energies of electronic transitions close to the experimentally observed ones and may be useful for structural identification of this type of nanotubes.

Using the first principles calculations, we analyze the structural and electronic properties of a ${\mathrm{PtSe}}_2$ monolayer on Pt substrate, obtained by direct selenization of the Pt(111) surface [Wang et al., Nano Lett. 15, 4013 (2015)]. We demonstrate that in order to reproduce the experimental result that ${\mathrm{PtSe}}_2$ is physisorbed on the surface, the surface must be passivated. We propose that this passivation is most likely due to intercalation of Se atoms between ${\mathrm{PtSe}}_2$ and Pt surface during the selenization process. In this case the mean distance between the Se-passivated surface and ${\mathrm{PtSe}}_2$ is found to be $3.24\text{Å}$, which is consistent with the distance that can be extracted from the scanning transmission electron microscopy image of the hybrid system, and the adsorption energy is found to fall into physisorption range. Therefore, our findings provide an insight into the synthesis of ${\mathrm{PtSe}}_2$ by direct selenization: A realistic structural model should include a Se-passivated surface and not a clean one.

It has been well established that the presence of quantum-well states in metal thin films plays an essential role in determining the thickness dependence of many physical properties. Oscillatory features in these properties are often observed and attributed to the energy variations of these quantum-well states near the Fermi level. Modern film growth capability has made it possible to create composite metal thin films in which one metal thin film is stacked on top of a dissimilar one with precise control of individual thickness. How the original quantum-well states evolve in the composite film becomes a critical issue in understanding the electronic structure of these new complex thin-film systems. In this paper we present first-principles calculations and measurements by angle-resolved photoemission spectroscopy for electronic states in a bimetallic film composed of ten layers of Pb and nine layers of Ag in the [111] direction on a Si substrate. It is found that the original quantum-well states in individual Pb and Ag films evolve into a new set of states in the bimetallic film by extending into the additional space, instead of directly coupling with each other as one would have expected. The new set of quantum-well states therefore has modified effective masses and energy values compared with the parent ones. Even though the Pb/Ag interface is incommensurate, the coherently coupled electronic states across the whole bimetallic film are verified by supercell configurations with different rotational arrangements in the calculation. The excellent agreement between theory and experiment in the energy dispersion of the quantum-well states in this bimetallic film confirms the physical picture proposed in this work, which could form the basis for exploring the electronic structure in multiple stacked thin films with more complicated designs.

The interplay between band topology and magnetic order could generate a variety of time-reversal-breaking gapped topological phases with exotic topological quantization phenomena, such as quantum anomalous Hall (QAH) insulators and axion insulators (AxI). Here by combining analytic models and first-principles calculations, we find extremely rich magnetic topological quantum states in a van der Waals layered material ${\mathrm{Mn}}_2{\mathrm{Bi}}_2{\mathrm{Te}}_5$, including a dynamic axion field in the antiferromagnetic bulk, an ideal magnetic Weyl semimetal with a single pair of Weyl points, as well as QAH and AxI phases in thin films. The phase transition between QAH and AxI is tuned by the layer magnetization, which would provide a promising platform for chiral superconducting phases and Majorana fermion. We further present a simple and unified continuum model that captures the magnetic topological features, and is generic for ${\mathrm{Mn}}_2{\mathrm{Bi}}_2{\mathrm{Te}}_5$ and ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ family materials.

Numerous angle-resolved photoemission spectroscopy (ARPES) studies of a wide class of low-density metallic systems, ranging from doped transition metal oxides to quasi-two-dimensional interfaces between insulators, exhibit phonon sidebands below the quasiparticle peak as a unique hallmark of polaronic correlations. Here, we single out properties of ARPES spectra that can provide a robust estimate of the effective range (screening length) of the electron-phonon interaction, regardless of the limited experimental resolution, dimensionality, and particular features of the electronic structure, facilitating a general methodology for an analysis of a whole class of materials.

We study the linear response of a spin current to a small electric field in a two-dimensional crystalline insulator with nonconserved spin. We adopt the spin current operator proposed in J. Shi et al. [Phys. Rev. Lett. 96, 076604 (2006)], which satisfies a continuity equation and fits the Onsager relations. We use the time-independent perturbation theory to present a formula for the spin Hall conductivity, which consists of a “Chern-type” term, reminiscent of the Kubo formula obtained for the quantum Hall systems, and a correction term that accounts for the nonconservation of spin. We illustrate our findings on the Bernevig-Hughes-Zhang model and the Kane-Mele model for time-reversal-symmetric topological insulators and show that the correction term scales quadratically with the amplitude of the spin-conservation-breaking terms. In both models, the spin Hall conductivity deviates from the quantized value when spin is not conserved.

We investigate the low-temperature thermoelectric properties of three-dimensional nodal-line semimetals within the semiclassical Boltzmann formalism. Considering short-range interaction between electrons and scattering agents, we calculate the anisotropic relaxation times and then obtain the charge conductivity and thermopower along the radial and the axial directions with respect to the nodal-line plane. Increasing the carrier concentration, a sharp change in the thermopower signals topological transition in the shape of the Fermi surface from a torus into an ellipsoid. An adequate treatment of the energy and direction dependence of the relaxation time is necessary for the observation of the topological transition of the Fermi surface in the thermoelectric properties.

We perform extensive classifications of ${\mathbb{Z}}_2$ quantum spin liquids on the simple cubic, body centered cubic, and face centered cubic (fcc) lattices using a spin-rotation-invariant fermionic projective symmetry group approach. Taking into account that all three lattices share the same point group $O_h$, we apply an efficient gauge where the classification for the simple cubic lattice can be partially carried over to the other two lattices. We identify hundreds of projective representations for each of the three lattices, however, when constructing short-range mean-field models for the fermionic partons (spinons) these phases collapse to only very few relevant cases. We self-consistently calculate the corresponding mean-field parameters for frustrated Heisenberg models on all three lattices with up to third-neighbor spin interactions and discuss the spinon dispersions, ground-state energies, and dynamical spin structure factors. Our results indicate that phases with nonuniform spinon hopping or pairing amplitudes are energetically favored. An unusual situation is identified for the fcc lattice where the spinon dispersion minimizing the mean-field energy features a network of symmetry-protected linelike zero modes in reciprocal space. We further discuss characteristic fingerprints of these phases in the dynamical spin structure factor which may help to identify and distinguish them in future numerical or experimental studies.

We study the effect of anisotropy (strain) on dynamical gap generation in graphene. We work with a low energy effective theory obtained from a tight-binding Hamiltonian expanded around the Dirac points in momentum space. We use a nonperturbative Schwinger-Dyson approach and calculate a coupled set of five momentum dependent dressing functions. Our results show that the critical coupling depends only weakly on the anisotropy parameter, and increases with greater anisotropy.