Transition-metal dipnictides (TMDs) have recently been identified as possible candidates to host a topology-protected electronic band structure. These materials belong to an isostructural family and show several exotic transport properties. Especially, the large values of magnetoresistance (MR) and carrier mobility have drawn significant attention from the perspective of technological applications. In this paper, we investigate the magnetotransport and Fermi surface properties of single-crystalline ${\mathrm{MoAs}}_2$, another member of this group of compounds. A field-induced resistivity plateau and a large MR have been observed, which are comparable to those in several topological systems. Interestingly, in contrast to other isostructural materials, the carrier density in ${\mathrm{MoAs}}_2$ is quite high and shows single-band-dominated transport. The Fermi pockets, which have been identified from the quantum oscillation, are the largest among the members of this group and have significant anisotropy with crystallographic direction. Our first-principles calculations reveal a substantial difference between the band structures of ${\mathrm{MoAs}}_2$ and that of other TMDs. The calculated Fermi surface consists of one electron pocket and another “open-orbit” hole pocket, which has not been observed in TMDs so far.

We report a study of Co-doped berthierite ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{\mathrm{Sb}}_2{\mathrm{S}}_4$ ($x=0$, 0.1, and 0.2). The alloy series of ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{\mathrm{Sb}}_2{\mathrm{S}}_4$ crystallize in an orthorhombic structure with the Pnma space group, similar to ${\mathrm{FeSb}}_2$, and show semiconducting behavior. The large discrepancy between activation energy for conductivity, $E_{\rho }$ (146 $\sim 270\mathrm{meV}$), and thermopower, $E_S$ (47 $\sim 108\mathrm{meV}$), indicates the polaronic transport mechanism. Bulk magnetization and heat-capacity measurements of pure ${\mathrm{FeSb}}_2{\mathrm{S}}_4$ ($x=0$) exhibit a broad antiferromagnetic transition ($T_N=46\mathrm{K}$) followed by an additional weak transition ($T^{*}=50\mathrm{K}$). Transition temperatures ($T_N$ and $T^{*}$) slightly decrease with increasing Co content $x$. This is also reflected in the thermal conductivity measurement, indicating strong spin-lattice coupling. ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{\mathrm{Sb}}_2{\mathrm{S}}_4$ shows relatively high value of thermopower (up to $\sim 624\mu \mathrm{V}{\mathrm{K}}^{-1}$ at 300 K) and thermal conductivity much lower when compared to ${\mathrm{FeSb}}_2$, a feature desired for potential applications based on ${\mathrm{FeSb}}_2$ materials.

Assembling large-scale quantum networks is a key goal of modern physics research with applications in quantum information and computation. Quantum wires and waveguides in which massive particles propagate in tailored confinement is one promising platform for realizing a quantum network. In the literature, such networks are often treated as quantum graphs, that is, the wave functions are taken to live on graphs of one-dimensional edges meeting in vertices. Hitherto, it has been unclear what boundary conditions on the vertices produce the physical states one finds in nature. This paper treats a quantum network from a physical approach, explicitly finds the physical eigenstates and compares them to the quantum-graph description. The basic building block of a quantum network is an X-shaped potential well made by crossing two quantum wires, and we consider a massive particle in such an X well. The system is analyzed using a variational method based on an expansion into modes with fast convergence and it provides a very clear intuition for the physics of the problem. The particle is found to have a ground state that is exponentially localized to the center of the X well, and the other symmetric solutions are formed so to be orthogonal to the ground state. This is in contrast to the predictions of the conventionally used so-called Kirchoff boundary conditions in quantum graph theory that predict a different sequence of symmetric solutions that cannot be physically realized. Numerical methods have previously been the only source of information on the ground-state wave function and our results provide a different perspective with strong analytical insights. The ground-state wave function has a spatial profile that looks very similar to the shape of a solitonic solution to a nonlinear Schrödinger equation, enabling an analytical prediction of the wave number. When combining multiple X wells into a network or grid, each site supports a solitonlike localized state. These localized solutions only couple to each other and are able to jump from one site to another as if they were trapped in a discrete lattice.

The structural, magnetic, and electrical properties of a ${\mathrm{Fe}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$ polycrystalline sample have been characterized. It is found that this compound crystallizes in the $P\overline{3}c1$ space group of the $\alpha \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ structure and is thus isostructural to ${\mathrm{Co}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$ and ${\mathrm{Mn}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$, two linear magnetoelectric oxides. But in marked contrast, its ${\varepsilon }^{\prime }\left(T\right)$ curve reveals two broad transitions at $T_{N1}\cong 90\mathrm{K}$ and $T_{N2}\cong 77\mathrm{K}$, the former corresponding to the antiferromagnetic ordering temperature. Below $T_{N1}$, the M$\left(H\right)$ magnetization curves reveal the existence of spin flop at about 6 T. In this temperature region, a H-induced electric polarization for ${\mu }_{0}H>6$ T is evidenced by both sets of $I_p{\left(T\right)}_H$ and $P{\left(H\right)}_T$ curves. All these results point towards ${\mathrm{Fe}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$ being a magnetoelectric member of the $A_4{B}_2{\mathrm{O}}_9$ family ($A=\mathrm{Mn}$, Fe, Co and $B=\mathrm{Nb}$, Ta).

We report the electronic band structures and concomitant Fermi surfaces for a family of exfoliable tetradymite compounds with the formula $T_{2}C{h}_{2}Pn$, obtained as a modification to the well-known topological insulator binaries ${\mathrm{Bi}}_2$(${\mathrm{Se},\mathrm{Te})}_3$ by replacing one chalcogen ($Ch$) with a pnictogen ($Pn$) and Bi with the tetravalent transition metals $T=$ Ti, Zr, or Hf. This imbalances the electron count and results in layered metals characterized by relatively high carrier mobilities and bulk two-dimensional Fermi surfaces whose topography is well-described by first-principles calculations. Intriguingly, slab electronic structure calculations predict Dirac-like surface states. In contrast to ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, where the surface Dirac bands are at the $\mathrm{\Gamma }$ point, for (${\mathrm{Zr},\mathrm{Hf})}_2{\mathrm{Te}}_2$(P,As) there are Dirac cones of strong topological character around both the $\overline{\mathrm{\Gamma }}$ and $\overline{M}$ points, which are above and below the Fermi energy, respectively. For ${\mathrm{Ti}}_2{\mathrm{Te}}_2\mathrm{P}$, the surface state is predicted to exist only around the $\overline{M}$ point. In agreement with these predictions, the surface states that are located below the Fermi energy are observed by angle-resolved photoemission spectroscopy measurements, revealing that they coexist with the bulk metallic state. Thus this family of materials provides a foundation upon which to develop novel phenomena that exploit both the bulk and surface states (e.g., topological superconductivity).

We introduce a systematic construction of a gapless symmetry-protected topological phase in one dimension by “decorating” the domain walls of Luttinger liquids. The resulting strongly interacting phases provide a concrete example of a gapless symmetry-protected topological (gSPT) phase with robust symmetry-protected edge modes. Using boundary conformal field theory arguments, we show that while the bulks of such gSPT phases are identical to conventional Luttinger liquids, their boundary critical behavior is controlled by a different, strongly coupled renormalization group fixed point. Our results are checked against extensive density matrix renormalization group calculations.

We develop a vortex metal theory for a partially filled Landau level at $\nu =\frac{1}{2n}$ whose ground state contains a composite Fermi surface formed by the vortex of electrons. In the projected Landau-level limit, the composite Fermi surface contains a $\frac{-\pi }{n}$ Berry phase. Such a fractional Berry phase is a consequence of Landau-level projection which produces the Girvin-MacDonald-Platzman [S. M. Girvin, A. H. MacDonald, and P. M. Platzman, Phys. Rev. B 33, 2481 (1986)] guiding center algebra and embellishes an anomalous velocity to the equation of motion for the vortex metal. Further, we investigate a particle-hole symmetric bilayer system with ${\nu }_1=\frac{1}{2n}$ and ${\nu }_2=1-\frac{1}{2n}$ at each layer, and demonstrate that the $\frac{-\pi }{n}$ Berry phase on the composite Fermi surface leads to the suppression of $2k_f$ backscattering between the particle-hole partner bilayer, which could be a smoking gun to detect the fractional Berry phase. We also mention various instabilities and competing orders in such bilayer systems, including a $Z_{4n}$ topological order phase driven by quantum criticality.

We study spin control for an electron confined in a flake of silicene. We find that the lowest-energy conduction-band levels are split by the diagonal intrinsic spin-orbit coupling into Kramers doublets with a definite projection of the spin on the orbital magnetic moment. We study the spin control by AC electric fields using the nondiagonal Rashba component of the spin-orbit interactions with the time-dependent atomistic tight-binding approach. The Rashba interactions in AC electric fields produce Rabi spin-flip times of the order of a nanosecond. These times can be reduced to tens of picoseconds provided that the vertical electric field is tuned to an avoided crossing opened by the Rashba spin-orbit interaction. We demonstrate that the speedup of the spin transitions is possible due to the intervalley coupling induced by the armchair edge of the flake. The study is confronted with the results for circular quantum dots decoupled from the edge with well defined angular momentum and valley index.

Acoustic-phonon Raman scattering, as a defect-induced second-order Raman scattering process (with incident photon scattered by one acoustic phonon at the Brillouin-zone edge and the momentum conservation fulfilled by defect scattering), is used as a sensitive tool to study the defects of transition-metal dichalcogenides (TMDs). Moreover, second-order Raman scattering processes are closely related to the valley depolarization of single-layer TMDs in potential valleytronic applications. Here, the layer dependence of second-order Raman intensity of $\mathrm{Mo}{\mathrm{S}}_2$ and $\mathrm{WS}{\mathrm{e}}_2$ is studied. The electronic band structures of $\mathrm{Mo}{\mathrm{S}}_2$ and $\mathrm{WS}{\mathrm{e}}_2$ are modified by the layer thicknesses; hence, the resonance conditions for both first-order and second-order Raman scattering processes are tuned. In contrast to the first-order Raman scattering, second-order Raman scattering of $\mathrm{Mo}{\mathrm{S}}_2$ and $\mathrm{WS}{\mathrm{e}}_2$ involves additional intervalley scattering of electrons by phonons with large momenta. As a result, the electron states that contribute most to the second-order Raman intensity are different from that to first-order process. A weaker layer-tuned resonance enhancement of second-order Raman intensity is observed for both $\mathrm{Mo}{\mathrm{S}}_2$ and $\mathrm{WS}{\mathrm{e}}_2$. Specifically, when the incident laser has photon energy close to the optical band gap and the Raman spectra are normalized by the first-order Raman peaks, single-layer $\mathrm{Mo}{\mathrm{S}}_2$ or $\mathrm{WS}{\mathrm{e}}_2$ has the strongest second-order Raman intensity. This layer-dependent second-order Raman intensity can be further utilized as an indicator to identify the layer number of $\mathrm{Mo}{\mathrm{S}}_2$ and $\mathrm{WS}{\mathrm{e}}_2$.

The recently emerging two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have been a fertile ground for exploring abundant exotic physical properties. Critical points, the extrema or saddle points of electronic bands, are the cornerstone of condensed-matter physics and fundamentally determine the optical and transport phenomena of the TMDCs. However, for bilayer $\mathrm{Mo}{\mathrm{S}}_2$, a typical TMDC and the unprecedented electrically tunable venue for valleytronics, there has been a considerable controversy on its intrinsic electronic structure, especially for the conduction band-edge locations. Moreover, interlayer hopping and layer polarization in bilayer $\mathrm{Mo}{\mathrm{S}}_2$ which play vital roles in valley-spintronic applications have remained experimentally elusive. Here, we report the experimental observation of intrinsic critical points locations, interlayer hopping, layer-spin polarization, and their evolution with temperature in bilayer $\mathrm{Mo}{\mathrm{S}}_2$ by performing temperature-dependent photoluminescence. Our measurements confirm that the conduction-band minimum locates at the $K_{\mathrm{c}}$ instead of $Q_{\mathrm{c}}$, and the energy splitting between $Q_{\mathrm{c}}$ and $K_{\mathrm{c}}$ redshifts with a descent of temperature. Furthermore, the interlayer hopping energy for holes and temperature-dependent layer polarization are quantitatively determined. Our observations are in good harmony with density-functional theory calculations.

We theoretically study the electronic structure and spin properties of one-dimensional nanostructures of the prototypical bulk topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Realistic models of experimentally observed ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ nanowires and nanoribbons are considered using the tight-binding method. At low energies, the band structures are composed of a series of evenly spaced degenerate subbands resulting from circumferential confinement of the topological surface states. The direct band gaps due to the nontrivial $\pi$ Berry phase show a clear dependence on the circumference. The spin-momentum locking of the topological surface states results in a pronounced $2\pi$ spin rotation around the circumference with the degree of spin polarization dependent on the momentum along the nanostructure. Overall, the band structures and spin textures are more complicated for nanoribbons, which expose two distinct facets. The effects of reduced dimensionality are rationalized with the help of a simple model that considers circumferential quantization of the topological surface states. Furthermore, the surface spin density induced by an electric current along the nanostructure shows a pronounced oscillatory dependence on the charge-carrier energy, which can be exploited in spintronics applications.

A single gold nanorod dropped on the surface of a silica substrate is used as a transient optoacoustic source of gigahertz hypersounds. We demonstrate the all-optical detection of the as-generated acoustic wave front propagating in the silica substrate. For this purpose, time-resolved femtosecond pump-probe experiments are performed in a reflection configuration. The fundamental breathing mode of the nanorod is detected at 23 GHz by interferometry, and the longitudinal acoustic wave radiated in the silica substrate is detected by time-resolved Brillouin scattering. By tuning the optical probe wavelength from 750 to 900 nm, hypersounds with wavelengths of 260–315 nm are detected in the silica substrate, with corresponding acoustic frequencies in the range of 19–23 GHz. To confirm the origin of these hypersounds, we theoretically analyze the influence of the acoustic excitation spectrum on the temporal envelope of the transient reflectivity. This analysis proves that the acoustic wave detected in the silica substrate results from the excitation of the breathing mode of the nanorod. These results pave the way for performing local in-depth elastic nanoscopy.

We investigate the band structure of image potential states in quasi-free-standing graphene (QFG) monolayer islands using angle-resolved two-photon-photoemission spectroscopy. Direct probing by low-energy electron diffraction shows that QFG is formed following oxygen intercalation into the graphene-Ir(111) interface. Despite the apparent decoupling of the monolayer graphene from the Ir substrate, we find that the binding energy of the $\mathit{n}=1$ image potential state on these QFG islands increases by 0.17 eV, as compared to the original Gr/Ir(111) interface. We use calculations based on density-functional theory to construct an empirical, one-dimensional potential that quantitatively reproduces the image potential state binding energy and links the changes in the interface structure to the shift in energy. Specifically, two factors contribute comparably to this energy shift: a deeper potential well arising from the presence of intercalated oxygen adatoms and a wider potential well associated with the increase in the graphene-Ir distance. While image potential states have not been observed previously on QFG by photoemission, our paper now demonstrates that they may be strongly excited in a well-defined QFG system produced by oxygen intercalation. This opens an opportunity for studying the surface electron dynamics in QFG systems, beyond those found in typical nonintercalated graphene-on-substrate systems.

We develop a general theory to model the angle-resolved photoemission spectroscopy (ARPES) of commensurate and incommensurate van der Waals (vdW) structures, formed by lattice mismatched and/or misaligned stacked layers of two-dimensional materials. The present theory is based on a tight-binding description of the structure and the concept of generalized umklapp processes, going beyond previous descriptions of ARPES in incommensurate vdW structures, which are based on continuous, low-energy models, being limited to structures with small lattice mismatch/misalignment. As applications of the general formalism, we study the ARPES bands and constant energy maps for two structures: twisted bilayer graphene and twisted bilayer ${\mathrm{MoS}}_2$. The present theory should be useful in correctly interpreting experimental results of ARPES of vdW structures and other systems displaying competition between different periodicities, such as two-dimensional materials weakly coupled to a substrate and materials with density wave phases.

We report the anomalous Hall effect (AHE) in single crystals of a quasi-two-dimensional ${\mathrm{Fe}}_{3-x}{\mathrm{GeTe}}_2$ ($x\approx 0.36$) ferromagnet grown by the flux method which induces defects on the Fe site and bad metallic resistivity. Fe $K$-edge x-ray absorption spectroscopy was measured to provide information on the local atomic environment in such crystals. The dc and ac magnetic susceptibility measurements indicate a second-stage transition below 119 K in addition to the paramagnetic to ferromagnetic transition at 153 K. A linear scaling behavior between the modified anomalous Hall resistivity ${\rho }_{xy}/{\mu }_0{H}_{\mathrm{eff}}$ and longitudinal resistivity ${\rho }_{xx}^{2}M/{\mu }_0{H}_{\mathrm{eff}}$ implies that the AHE in ${\mathrm{Fe}}_{3-x}{\mathrm{GeTe}}_2$ should be dominated by the intrinsic Karplus-Luttinger mechanism rather than the extrinsic skew-scattering and side-jump mechanisms. The observed deviation in the linear-$M$ Hall conductivity ${\sigma }_{xy}^A$ below 30 K is in line with its transport characteristic at low temperatures, implying the scattering of conduction electrons due to magnetic disorder and the evolution of the Fermi surface induced by a possible spin-reorientation transition.

We investigate the photonic and phononic surface and edge modes in finite-size three-dimensional phoxonic crystals. By appropriately terminating the phoxonic crystals, the photons and phonons can be simultaneously guided at the two-dimensional surface and/or the one-dimensional edge of the terminated crystals. The Bloch surface and edge modes show that the electromagnetic and acoustic waves are highly localized near the surface and edge, respectively. The surface and edge geometries play important roles in tailoring the dispersion relations of the surface and edge modes, and dual band gaps for the surface or edge modes can be simultaneously achieved by changing the geometrical configurations. Furthermore, as the band gaps for the bulk modes are the essential prerequisites for the realization of dual surface and edge modes, the photonic and phononic bulk-mode band gap properties of three different types of phoxonic crystals with six-connected networks are revealed. It is found that the geometrical characteristic of the crystals with six-connected networks leads to dual large bulk-mode band gaps. Compared with the conventional bulk modes, the surface and edge modes provide a new approach for the photon and phonon manipulation and show great potential for phoxonic crystal devices and optomechanics.

Using the Green functions technique, we study the subgap spectrum of short three-dimensional superconductor–normal metal–superconductor junctions containing one or two point impurities in the normal layer. We find that a single nonmagnetic or magnetic defect induces two quasibound Shiba-like states. If the defect is located close to the junction edge, the energies of these states oscillate as functions of the distance between the impurity and the edge. In the case of two nonmagnetic impurities, there are generally four quasibound states (two per spin projection). Their energies oscillate as functions of the distance between the impurities, and reach their asymptotic values when this distance becomes much larger than the Fermi wavelength. The contributions of the impurities to the Josephson current, local density of states, and to the normal-state conductance of the junction are analyzed.

The pressure effect up to 24.0 kbar on superconducting and normal-state properties of $\beta -\mathrm{B}{\mathrm{i}}_2\mathrm{Pd}$ single crystal ($T_{\mathrm{c}}\approx 4.98\mathrm{K}$ at ambient pressure) has been investigated by measurements of the electrical resistivity. In addition, we have performed the heat capacity measurements in the temperature range 0.7–300 K at ambient pressure. The recent calculations of electronic density of states, electron-phonon interaction spectral function, and phonon density of states of $\beta -\mathrm{B}{\mathrm{i}}_2\mathrm{Pd}$ [Zheng and Margine, Phys. Rev. B 95, 014512 (2017)], are used to fit the resistivity and the heat capacity data. In the superconducting state we have focused on the influence of pressure on the superconducting transition temperature $T_{\mathrm{c}}$ and upper critical field $H_{\mathrm{c}2}$ and a negative effect with $d{T}_{\mathrm{c}}/dp=–0.025\mathrm{K}/\mathrm{kbar}$ and $d{H}_{\mathrm{c}2}/dp=–8\mathrm{mT}/\mathrm{kbar}$ is found. A simplified Bloch-Grüneisen model was used to analyze the pressure effect on the temperature dependence of the normal-state resistivity. The obtained results point to a decrease of the electron-phonon coupling parameter λ and to a shift of phonon frequencies to higher values with pressure. Moreover, the temperature dependence of the normal-state resistivity follows a $T^2$ dependence above $T_{\mathrm{c}}$ up to about 25 K. Together with the enhanced value of Sommerfeld coefficient $\gamma =13.23\mathrm{mJ}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{K}}^{-2}$ these results point to a certain role of the electron-electron interaction in the superconducting pairing mechanism in $\beta -\mathrm{B}{\mathrm{i}}_2\mathrm{Pd}$.