Dimensional crossover is of high relevance to understanding real-world quasi-one-dimensional (1D) systems. Here we study the collective excitations, measured as plasmon dispersions in an electron energy loss experiment, in a tunable family of model systems, namely, Au chains on stepped Si($hhk$) substrates, that allow variations of chain widths and interchain spacings. We indeed observe 1D-like dispersions, but with a significant influence of higher dimensions. Surprisingly, we find that it is not the interchain coupling but the width of the conduction channel, as confirmed by tunneling spectroscopy, that dominates the excitations.

We report a systematic angle-resolved photoemission spectroscopy on topological insulator (TI) ${\mathrm{TlBi}}_{1-x}{\mathrm{Sb}}_x{\mathrm{Te}}_2$ which is bulk insulating at $0.5\lesssim x\lesssim 0.9$ and undergoes a metal-insulator-metal transition with the Sb content $x$. We found that this transition is characterized by a systematic hole doping with increasing $x$, which results in the Fermi-level crossings of the bulk conduction and valence bands at $x\sim 0$ and $x\sim 1$, respectively. The Dirac point of the topological surface state is gradually isolated from the valence-band edge, accompanied by a sign reversal of Dirac carriers. We also found that the Dirac velocity is the largest among known solid-solution TI systems. The ${\mathrm{TlBi}}_{1-x}{\mathrm{Sb}}_x{\mathrm{Te}}_2$ system thus provides an excellent platform for Dirac-cone engineering and device applications of TIs.

We report on sliding charge density waves (CDWs) in the quasi-two-dimensional ${\mathrm{TbTe}}_3$ system probed by coherent x-ray diffraction combined with in situ transport measurements. We show that the non-Ohmic conductivity in ${\mathrm{TbTe}}_3$ is made possible thanks to a strong distortion of the CDW. Our diffraction experiment versus current shows first that the CDW remains undeformed below the threshold current $I_S$ and then suddenly rotates and reorders by motion above threshold. Contrary to quasi-one-dimensional systems, the CDW in ${\mathrm{TbTe}}_3$ does not display any phase shifts below $I_S$ and tolerates only slow spatial variations of the phase above. This is the first observation of CDW behavior in the bulk in a quasi-two-dimensional system allowing collective transport of charges at room temperature.

The adsorption structure and orientation of a metal-free phthalocyanine $\left({\mathrm{H}}_2\mathrm{Pc}\right)$ and a magnesium phthalocyanine (MgPc) on a bilayer of NaCl films were investigated both theoretically and experimentally by means of first-principles calculations based on density functional theory and by scanning tunneling microscopy. ${\mathrm{H}}_2\mathrm{Pc}$ is adsorbed with its center over the sodium cation, and H-N bonds in the molecule are aligned with the [100] or [010] surface direction of a bilayer (001)-terminated NaCl film. The most stable structures of MgPc on the NaCl film show two kinds of orientations corresponding to the molecule rotated by $\pm 7^{\circ }$ relative to the [110] surface direction, with the Mg cation positioned over the chlorine anion in both cases. The energetic barrier for switching between these orientations is as low as 9.0 meV, and during an STM measurement, an orientational change of MgPc can be observed. The interaction between the adsorbed molecule and the NaCl film were analyzed in terms of dispersion interaction, Mg-Cl chemical bonding, and electrostatic interaction. It is found that the small electrostatic interaction between the molecule and the film gives a dominant contribution to determining the molecular orientation. Our detailed and comprehensive studies of the molecule-insulator interaction will provide knowledge to understand and control the properties of molecules on an insulating material.

In this paper, we numerically study the magnetic transport properties of disordered Weyl semimetals (WSM) in the ultraquantum limit. We find a positive magnetic conductivity for the long-range disorder, although the system tends to have negative magnetic conductivity for the weak short-range disorder. Remarkably, for long-range disorder, such a positive magnetic conductivity usually cannot be described by the semiclassical Boltzmann transport theory even in the weak disorder limit, and the backscattering assisted by the high Landau levels is always important. Our results have significant implications for the positive magnetic conductivity recently discovered in the WSM systems, and point out two physical mechanisms: (i) the long-range correlated disorder suppresses the backscattering among the zeroth Landau level modes; (ii) with increasing the magnetic field, the backscattering assisted by the high Landau levels will also be suppressed.

We investigate theoretically the spectral and dynamical effects of the short-range exchange interaction between a single manganese (Mn) atom hosted by cylindrical CdTe quantum dots and its light-hole excitons or biexcitons. Our approach is based on the Kohn-Luttinger $k·p$ theory and configuration interaction method, the dynamics of the system in the presence of intraband relaxation being derived from the von Neumann–Lindblad equation. The complex structure of the light-hole exciton absorption spectrum reveals the exchange-induced exciton mixing and depends strongly on the Mn position. In particular, if the Mn atom is closer to the edges of the cylinder, the bright and dark light-hole excitons are mixed by the hole-Mn exchange alone. Consequently, their populations exhibit exchange-induced Rabi oscillations which can be viewed as optical signatures of light-hole spin reversal. Similar results are obtained for mixed biexcitons, in this case the exchange-induced Rabi oscillations being damped by the intraband hole relaxation processes. The effect of light-hole heavy-hole mixing is also discussed.

The electronic structure of epitaxial single-layer ${\mathrm{MoS}}_2$ on Au(111) is investigated by angle-resolved photoemission spectroscopy, scanning tunneling spectroscopy, and first-principles calculations. While the band dispersion of the supported single layer is close to a free-standing layer in the vicinity of the valence-band maximum at $\overline{K}$ and the calculated electronic band gap on Au(111) is similar to that calculated for the free-standing layer, significant modifications to the band structure are observed at other points of the two-dimensional Brillouin zone: at $\overline{\mathrm{\Gamma }}$, the valence-band maximum has a significantly higher binding energy than in the free ${\mathrm{MoS}}_2$ layer and the expected spin-degeneracy of the uppermost valence band at the $\overline{M}$ point cannot be observed. These band structure changes are reproduced by the calculations and can be explained by the detailed interaction of the out-of-plane ${\mathrm{MoS}}_2$ orbitals with the substrate.

We study the ground-state phase diagram and dynamics of the one-dimensional cluster model with several competing interactions. Paying particular attention to the relation between the entanglement spectrum (ES) and the bulk topological (winding) number, we first map out the ground-state phases of the model and determine the universality classes of the transitions from the exact solution. We then investigate the dynamical properties during interaction sweeps through the critical points of topological phase transitions. When the sweep speed is slow, the correlation functions and the entanglement entropy exhibit spatially periodic structures. On top of this, the levels in the ES oscillate temporally during the dynamics. By explicitly calculating the above quantities for excited states, we attribute these behaviors to the Bogoliubov quasiparticles generated near the critical points. We also show that the ES reflects the strength of the Majorana correlation even for the excited states.

We study the effect of the Rashba spin-orbit interaction in the quantum transport of carbon nanotubes with arbitrary chiralities. For certain spin directions, we find a strong spin-polarized electrical current that depends on the diameter of the tube, the length of the Rashba region, and on the tube chirality. Predictions for the spin-dependent conductances are presented for different families of achiral and chiral tubes. We have found that different symmetries acting on spatial and spin variables have to be considered in order to explain the relations between spin-resolved conductances in carbon nanotubes. These symmetries are more general than those employed in planar graphene systems. Our results indicate the possibility of having stable spin-polarized electrical currents in absence of external magnetic fields or magnetic impurities in carbon nanotubes.

We explore the inelastic electron-scattering cross section off a metallic quantum dot close to the Stoner instability. We focus on the regime of strong Coulomb blockade in which the scattering cross section is dominated by the cotunneling processes. For large enough exchange interaction, the quantum dot acquires a finite total spin in the ground state. In this so-called mesoscopic Stoner instability regime, we find that at low enough temperatures, the inelastic scattering cross section (including the contribution due to an elastic electron spin flip) for an electron with an energy close to the chemical potential is different from the case of a magnetic impurity with the same spin. This difference stems from (i) the presence of low-lying many-body states of a quantum dot and (ii) the correlations of the tunneling amplitudes. Our results provide a possible explanation for the absence of the dephasing rate saturation at low temperatures in a recent experiment [N. Teneh, A. Yu. Kuntsevich, V. M. Pudalov, and M. Reznikov, Phys. Rev. Lett. 109, 226403 (2012)] in which the existence of local spin droplets in disordered electron liquid has been unraveled.

We discuss the influence of site occupancy on the absorption of the hydrogen isotopes H and D in thin V(001) layers. By growing V(001) under biaxial compressive strain in Fe/V(001) superlattices, the hydrogen (H as well as D) is forced to reside exclusively in octahedral (${\mathrm{O}}_z$) sites, even at the lowest concentrations. A weakening of the isotope effects is observed when hydrogen resides in octahedral as compared to tetrahedral sites.

Polycrystalline samples of four intermetallic compounds: $M{\mathrm{V}}_2\mathrm{A}{\mathrm{l}}_{20}$ ($M=\mathrm{Sc}$, Y, La, and Lu) were synthesized using an arc-melting technique. The crystal structures were analyzed by means of powder x-ray diffraction and Rietveld analysis, and the physical properties were studied by means of heat capacity, electrical resistivity, and magnetic susceptibility measurements down to 0.4 K. For $\mathrm{Sc}{\mathrm{V}}_2\mathrm{A}{\mathrm{l}}_{20}, \mathrm{Lu}{\mathrm{V}}_2\mathrm{A}{\mathrm{l}}_{20}$, and $\mathrm{Y}{\mathrm{V}}_2\mathrm{A}{\mathrm{l}}_{20}$, superconductivity was observed with critical temperatures $T_{\mathrm{c}}=1.00$, 0.57, and 0.60 K, respectively. Superconductivity for the Lu compound is reported. Theoretical calculations of the electronic and phonon structures were conducted in order to analyze the superconductivity and dynamics in $\mathrm{Sc}{\mathrm{V}}_2\mathrm{A}{\mathrm{l}}_{20}, \mathrm{Y}{\mathrm{V}}_2\mathrm{A}{\mathrm{l}}_{20}$, and $\mathrm{Lu}{\mathrm{V}}_2\mathrm{A}{\mathrm{l}}_{20}$ and to explain the lack of a superconducting transition in $\mathrm{La}{\mathrm{V}}_2\mathrm{A}{\mathrm{l}}_{20}$ down to 0.4 K. The results of the experimental and theoretical studies show that all the compounds are weakly coupled type-II BCS superconductors, and reveal the importance of the $M$-atom anharmonic “rattling” modes for the superconductivity in these materials, which seem to enhance $T_{\mathrm{c}}$, especially for $\mathrm{Sc}{\mathrm{V}}_2\mathrm{A}{\mathrm{l}}_{20}$.

The face-centered-cubic (fcc) lattice of ${\mathrm{Ru}}^{5+}$ spins in the double perovskite ${\mathrm{Sr}}_2{\mathrm{YRuO}}_6$ shows a delicate, three-dimensional antiferromagnetic (AFM) ground state composed of stacked square AFM layers. Inelastic neutron scattering data taken on this state reveal a gapped low-energy excitation band emerging from [001] with spin excitations extending to 8 meV. These magnetic excitations are modeled by a simple $J_1\text{−}{J}_2$ interaction scheme allowing quantitative comparisons with similar materials. At higher temperatures, the low-energy excitation spectrum is dominated by a quasielastic component associated with size fluctuations of two-dimensional AFM clusters that exhibit asymmetric correlations even at low temperatures. Thus, the fcc lattice in general and the double-perovskite structure in particular emerge as hosts of both two-dimensional and three-dimensional dynamics resulting from frustration.

By means of low-temperature scanning tunneling microscopy, we report on the electronic structures of the BiO and SrO planes of the ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CuO}}_{6+\delta }$ (Bi-2201) superconductor prepared by argon-ion bombardment and annealing. Depending on post annealing conditions, the BiO planes exhibit either a pseudogap (PG) with sharp coherence peaks and an anomalously large gap magnitude of 49 meV or van Hove singularity (vHS) near the Fermi level, while the SrO is always characteristic of a PG-like feature. This contrasts with the ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ (Bi-2212) superconductor where vHS occurs solely on the SrO plane. We disclose the interstitial oxygen dopants ($\delta$ in the formulas) as a primary cause for the occurrence of vHS, which are located dominantly around the BiO and SrO planes, respectively, in Bi-2201 and Bi-2212. This is supported by the contrasting structural buckling amplitude of the BiO and SrO planes in the two superconductors. Our findings provide solid evidence for the irrelevance of PG to the superconductivity in the two superconductors, as well as insights into why Bi-2212 can achieve a higher superconducting transition temperature than Bi-2201, and by implication, the mechanism of cuprate superconductivity.

To explore the microscopic response of a metallic glass (MG) to an applied hydrostatic load we performed a high-pressure extended x-ray absorption fine structure (EXAFS) study of the ${\mathrm{Zr}}_{66.7}{\mathrm{Cu}}_{33.3}$ amorphous alloy. EXAFS fitting revealed that on compression, Zr-Zr pairs are strained preferentially. Strong Zr-Cu interactions contribute to the stiffness of dominant Cu-centered clusters and the overall compressibility of the MG is mostly determined by the softer bonds between the Zr atoms belonging to the clusters' outer shell. Stress accommodation is accompanied by a variation of bonding preferences, which is consistent with the observed hierarchy of elastic constants of different atomic pairs.

The problem of interacting bosons in frustrated lattices is an intricate one due to the absence of a unique minimum in the single-particle dispersion where macroscopic number of bosons can condense. Here, we consider a family of tight-binding models with macroscopically degenerate lowest energy bands, separated from other bands by a gap. We predict the formation of exotic states that spontaneously break rotational symmetry at relatively low filling. These states belong to three nematic phases: Wigner crystal, supersolid, and superfluid. The Wigner crystal phase is established exactly at low filling. Supersolid and superfluid phases, at larger filling, are obtained by making use of a projection onto the flat band, construction of an appropriate Wannier basis, and subsequent mean-field treatment. The nematic superfluid that we predict is uniform in real space but has an anisotropic momentum distribution, providing a novel scenario for Bose condensation with an additional nematic order. Our findings open up a promising direction of studying microscopic quantum liquid crystalline phases of bosons.

High formal valence Os-based double perovskites are a focus of current interest because they display strong interplay of large spin-orbit coupling and strong electronic correlation. Here we present the electronic and magnetic characteristics of a sequence of three cubic Os based double perovskites ${\mathrm{Ba}}_{2}A{\mathrm{OsO}}_6$ ($A=\text{Na}$, Ca, Y), with formal valences of $\mathrm{Os}{}^{+7}\left(d^1\right),\mathrm{Os}{}^{+6}\left(d^2\right)$, and $\mathrm{Os}{}^{+5}\left(d^3\right)$. For these first principles based calculations we apply an “exact exchange for correlated electrons” functional, with exact exchange applied in a hybrid fashion solely to the Os $\left(5d\right)$ states. While ${\mathrm{Ba}}_2{\mathrm{NaOsO}}_6$ is a reported ferromagnetic Dirac-Mott insulator studied previously, the other two show antiferromagnetic ordering while all retain the undistorted cubic structure. For comparison purposes we have investigated only the ferromagnetic ordered phase. A metal-insulator transition is predicted to occur upon rotating the direction of magnetization in all three materials, reflecting the central role of spin-orbit coupling in these small gap osmates. Surprises arising from comparing formal charge states with the radial charge densities are discussed. Chemical shielding factors and orbital susceptibilities are provided for comparison with future nuclear magnetic resonance data.