We show that spin-polarized electron transport can be achieved by the substitutional doping of only one sublattice of graphene by nitrogen or boron atoms. The bipartite character via two sublattices remains persistent in the electronic structures of graphene doped with low concentrations of nitrogens (borons). The delocalized spin-densities induced by the unpaired electrons at substitutional sites permeate only through the sublattice where the nitrogen (boron) atoms belong. For interacting nitrogen (boron) atoms located along the “zigzag” direction and in the same sublattice, the ferromagnetic spin-ordering is favorable, and substitution-induced localized impurity states selectively disturb the spin-polarized $\pi$ orbital of that same sublattice. The bipartite character of graphene lattice governs the unique properties of two-dimensional hybrid graphene-boron nitride nanostructures.

We report experimental evidence of surface-dominated transport in single crystalline nanoflake devices of topological insulator Bi${}_{1.5}$Sb${}_{0.5}$Te${}_{1.8}$Se${}_{1.2}$ (BSTS). The resistivity measurements show dramatic differences between the nanoflake devices and bulk single crystal. Based on a two-channel model, the analysis on the resistivity and Hall resistance indicates that ∼99$\%$ surface transport contribution can be realized in 200 nm-thick BSTS nanoflake devices. Using a standard back gate with SiO${}_2$ as a dielectric layer, a pronounced ambipolar electric field effect was observed in devices fabricated with 100–200 nm thick flakes. Moreover, angle-dependent magnetoresistances of a nanoflake device with a thickness of 596 nanometers are fitted to a universal curve for the perpendicular component of the applied magnetic field. The value of phase coherence length obtained from two-dimensional weak antilocalization fitting further confirmed the surface dominated transport. Our results open a path for realization of electric and spintronic devices based on the topological helical surface states.

We calculate magnetotransport oscillations in current through a triple-quantum-dot molecule, accounting for higher harmonics (having flux period $h/ne$, with $n$ an integer). For a reflection-symmetric triple quantum dot, we find that harmonics with $n$ odd can dominate over those with $n$ even. This is opposite to the behavior theoretically predicted due to “dark-state” localization, but has been observed in recent experiments [L. Gaudreau et al., Phys. Rev. B 80, 075415 (2009)], albeit in a triple dot that may not exhibit reflection symmetry. This feature arises from a more general result; in the weak-coupling limit, we find that the current is flux-independent for an arbitrary reflection-symmetric Aharonov-Bohm network. We further show that these effects are observable in nanoscale systems even in the presence of typical dephasing sources.

Ab initio calculations within the density-functional theory formalism are performed to investigate the chemical functionalization of a graphene-like monolayer of silicon—silicene—with B, N, Al, or P atoms. The structural, electronic, magnetic, and vibrational properties are reported. The most preferable adsorption sites are found to be valley, bridge, valley and hill sites for B, N, Al, and P adatoms, respectively. All the relaxed systems with adsorbed/substituted atoms exhibit metallic behavior with strongly bonded B, N, Al, and P atoms accompanied by an appreciable electron transfer from silicene to the B, N, and P adatom/substituent. The Al atoms exhibit opposite charge transfer, with $n$-type doping of silicene and weaker bonding. The adatoms/substituents induce characteristic branches in the phonon spectrum of silicene, which can be probed by Raman measurements. Using molecular dynamics, we found that the systems under study are stable up to at least $T=500$ K. Our results demonstrate that silicene has a very reactive and functionalizable surface.

We present investigations of the superconductor-to-insulator transition (SIT) of uniform a-Bi films using a technique sensitive to Cooper pair phase coherence. The films are perforated with a nanohoneycomb array of holes to form a multiply connected geometry and subjected to a perpendicular magnetic field. Film magnetoresistances on the superconducting side of the SIT oscillate with a period dictated by the superconducting flux quantum and the areal hole density. The oscillations disappear close to the SIT critical point to leave a monotonically rising magnetoresistance that persists in the insulating phase. These observations indicate that the Cooper pair phase coherence length, which is infinite in the superconducting phase, collapses to a value less than the interhole spacing at this SIT. This behavior is inconsistent with the gradual reduction of the phase coherence length expected for a bosonic phase-fluctuation-driven SIT. This result starkly contrasts with previous observations of oscillations persisting in the insulating phase of other films implying that there must be at least two distinct classes of disorder-tuned SITs.

We have carried out a comprehensive study of the UCo${}_{1-x}$Fe${}_x$Ge series across the entire range of compositions $0\le x\le 1$, and report the results of x-ray diffraction, magnetization, specific heat, and electrical resistivity measurements to uncover the $T$-$x$ phase diagram. Substitution of Fe into UCoGe initially results in an increase in the Curie temperature and a rapid destruction of the superconductivity. Near $x=0.22$, the ferromagnetic transition is suppressed to zero temperature at an apparent ferromagnetic itinerant electron quantum critical point, where the temperature dependence of the electrical resistivity and specific heat in this region reveal non-Fermi liquid behavior.

The method proposed by Tkatchenko and Scheffler [Phys. Rev. Lett. 102, 073005 (2009)] to correct density functional calculations for the missing van der Waals interactions is implemented in the Vienna ab initio simulation package (vasp) code and tested on a wide range of solids, including noble-gas crystals, molecular crystals ($\alpha$-N${}_2$, sulfur dioxide, benzene, naphthalene, cytosine), layered solids (graphite, hexagonal boron nitride, vanadium pentoxide, MoS${}_2$, NbSe${}_2$), chain-like structures (selenium, tellurium, cellulose I), ionic crystals (NaCl, KI), and metals (nickel, zinc, cadmium). In addition to the original formulation expressing the van der Waals (vdW) corrections as pairwise potentials whose strength is derived from the rescaled polarizabilities of the neutral free atoms, the self-consistently screened ($\text{TS}+\text{SCS}$) [Phys. Rev. Lett. 108, 236402 (2012)] variant of the method involving electrodynamic response effects has been examined. Analytical expressions for the forces acting on the atoms and for the components of the stress tensor needed for the relaxation of the volume and shape of the unit cell using the $\text{TS}+\text{SCS}$ method are derived. While the calculated structures are reasonably close to experiment, the van der Waals corrections to the binding energies are often found to be overestimated in comparison with experimental data. The $\text{TS}+\text{SCS}$ approach leads to significantly better results in some problematic cases, such as the binding energy of graphite. However, there is room for further improvements, in particular for strongly ionic systems.

Soft x-ray angle-resolved photoemission experiments have been performed to investigate the band structure and Fermi surface of UPd${}_3$. We have observed three U 5$f$ derived bands whose bandwidths are very narrow. Two of these bands appear around 0.8 eV and are slightly separated by about 0.2 eV. The presence of two bands at nearly the same energy might be due to the presence of the two inequivalent U sites. The other U 5$f$ band appears around 1.8 eV. The energy separation between this band and the two nearly degenerate bands agrees with the 5$f^1$-final-state multiplet model. In the vicinity of Fermi level, several highly dispersive bands, which form a holelike Fermi surface centered at the $\Gamma$ point and an electronlike Fermi surface at the K point, were observed. These dispersive valence bands are qualitatively explained by a band structure calculation that assumes a localized 5$f^2$ configuration. All these experimental observations are consistent with the localized U 5$f$ nature.

Using ab initio electronic structure calculations we investigate the change of the band structure and the ${\nu }_0$ topological invariant in HgSe (noncentrosymmetric system) under two different types of uniaxial strain along the [001] and [110] directions, respectively. Both compressive [001] and [110] strain lead to the opening of a (crystal field) band gap (with a maximum value of about 37 meV) in the vicinity of $\Gamma$, and the concomitant formation of a camel-back- (inverse camel-back-) shaped valence (conduction) band along the direction perpendicular to the strain with a minimum (maximum) at $\Gamma$. We find that the ${\mathbb{Z}}_2$ invariant ${\nu }_0=1$ which demonstrates conclusively that HgSe is a strong topological insulator (TI). With further increase of the strain the band gap decreases, vanishing at a critical strain value (which depends on the strain type) where HgSe undergoes a transition from a strong TI to a trivial (normal) insulator. HgSe exhibits a similar behavior under a tensile [110] uniaxial strain. On the other hand, HgSe remains a normal insulator by applying a [001] tensile uniaxial strain. Complementary electronic structure calculations of the nonpolar (110) surface under compressive [110] tensile strain show two Dirac cones at the $\Gamma$ point whose spin chiral states are associated with the top and bottom slab surfaces.

Single-phase high-entropy alloys are investigated using the exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA). Choosing the paramagnetic face-centered-cubic NiCoFeCr alloy as an example, we compare the CPA results with those obtained using the supercell (SC) method. For the equilibrium Wigner-Seitz radius and elastic properties, the single-site mean-field approximation turns out to yield consistent results with the SC approach. Next, we employ the EMTO-CPA method to study the bulk properties of CuNiCoFeCrTi${}_x$ ($x=0.0–0.5,1.0$) and NiCoFeCrTi high-entropy alloys. A detailed comparison between the theoretical results and the available experimental data demonstrates that ab initio theory can properly describe the fundamental properties of this important class of engineering alloys. Theory predicts NiCoFeCr and CuNiCoFeCr to be more isotropic and less ductile than the Ti-containing single-phase alloys (CuNiCoFeCrTi${}_x$ with $x\gtrsim 0.4$ and NiCoFeCrTi).

We study a pseudogap Anderson-Holstein model of a magnetic impurity level that hybridizes with a conduction band whose density of states vanishes in power-law fashion at the Fermi energy, and couples, via its charge, to a nondispersive bosonic mode (e.g., an optical phonon). The model, which we treat using poor-man's scaling and the numerical renormalization group, exhibits quantum phase transitions of different types depending on the strength of the impurity-boson coupling. For weak impurity-boson coupling, the suppression of the density of states near the Fermi energy leads to quantum phase transitions between strong-coupling (Kondo) and local-moment phases. For sufficiently strong impurity-boson coupling, however, the bare repulsion between a pair of electrons in the impurity level becomes an effective attraction, leading to quantum phase transitions between strong-coupling (charge Kondo) and local-charge phases. Even though the Hamiltonian exhibits different symmetries in the spin and charge sectors, the thermodynamic properties near the two types of quantum phase transition are closely related under spin-charge interchange. Moreover, the critical responses to a local magnetic field (for small impurity-boson coupling) and to an electric potential (for large impurity-boson coupling) are characterized by the same exponents, whose values place these quantum-critical points in the universality class of the pseudogap Anderson model. One specific case of the pseudogap Anderson-Holstein model may be realized in a double-quantum-dot device, where the quantum phase transitions manifest themselves in the finite-temperature linear electrical conductance.

First-principles density functional calculations were used to calculate surface properties of the LiMn${}_2$O${}_4$ spinel. The calculations were benchmarked to obtain the correct semiconducting, Jahn-Teller distorted electronic ground state of bulk LiMn${}_2$O${}_4$ and, using the same parameters, the predominant low-index polar surface facets (100), (110), and (111) were calculated to study their structure and stability. Following an investigation of possible surface terminations as well as surface layer reconstructions we find that the (111) LMO surface stabilizes through a targeted site exchange of the undercoordinated surface Mn cations with fully coordinated tetrahedral subsurface Li cations, effectively creating a partial inverse spinel arrangement at the surface. This reconstruction renders the (111) facet the most stable among the investigated facets. The equilibrium (Wulff) shape of a LiMn${}_2$O${}_4$ particle was constructed and exhibits a cubo-octahedral shape with predominant (1 1 1) facets, in agreement with common experimental findings for the spinel structure.

It is found that 1/3 monolayer (ML) of Sn forms a surface alloy with 2/3 ML of Ag on Ag(111). This highly ordered alloy layer shows a $\sqrt{3}\times \sqrt{3}$ structure. By employing experimental and theoretical tools (scanning tunneling microscopy [STM], angle resolved photoelectron spectroscopy, low-energy electron diffraction, and density functional theory), an atomic model has been obtained that reproduces the experimental electronic structure in both real and reciprocal space. Detailed surface band dispersions, constant energy contours, and STM images, obtained experimentally and theoretically, are compared in order to verify the model. Similar, 1-layer-thick alloys on Ag(111) with Pb, Bi, or Sb exhibit measurable spin–orbit interactions. However, no such spin split could be detected in the case of Sn in this study.

Heterostructures of Dirac materials such as graphene and topological insulators provide interesting platforms to explore exotic quantum states of electrons in solids. Here we study the electronic structure of the graphene-Sb${}_2$Te${}_3$ heterostructure using density functional theory and tight-binding methods. We show that the epitaxial graphene on Sb${}_2$Te${}_3$ turns into the quantum spin-Hall phase due to its proximity to the topological-insulating Sb${}_2$Te${}_3$. It is found that the epitaxial graphene develops a giant spin-orbit gap of about ∼20 meV, which is about three orders of magnitude larger than that of pristine graphene. We discuss the origin of such enhancement of the spin-orbit interaction and possible outcomes of the spin-Hall phase in graphene.

We study the mobility of Dirac fermions in monolayer graphene on a GaAs substrate, limited by the combined action of the extrinsic potential of piezoelectric surface acoustical phonons of GaAs (PA) and of the intrinsic deformation potential of acoustical phonons in graphene (DA). In the high-temperature ($T$) regime, the momentum relaxation rate exhibits the same linear dependence on $T$ but different dependencies on the carrier density $n$, corresponding to the mobility $\mu \propto 1/\sqrt{n}$ and $1/n$, respectively for the PA and DA scattering mechanisms. In the low-$T$ Bloch-Grüneisen regime, the mobility shows the same square-root density dependence $\mu \propto \sqrt{n}$, but different temperature dependencies $\mu \propto T^{-3}$ and $T^{-4}$, respectively for PA and DA phonon scattering.

Using density functional theory, we study atomic diffusion on the Si(553)-Au surface. We have calculated the potential energy surfaces for atoms of different chemical elements (Ag, Au, Mg, In, and Si). By tracing the adatom positions and the energetics of a fully relaxed surface, we have calculated energy barriers, diffusion paths, and hopping rates along and across the steps. The results show that in most cases, the surface diffusion is strongly anisotropic, while the energy barriers and hopping rates have a strong dependency on the chemical species of the adatom. In the case of Au, Ag, and Mg atoms, the diffusion can be realized in two separate one-dimensional channels, featuring different mechanisms of the diffusion.

We show how to use polar molecules in an optical lattice to engineer quantum spin models with arbitrary spin $S\ge 1/2$ and with interactions featuring a direction-dependent spin anisotropy. This is achieved by encoding the effective spin degrees of freedom in microwave-dressed rotational states of the molecules and by coupling the spins through dipolar interactions. We demonstrate how one of the experimentally most accessible anisotropies stabilizes symmetry protected topological phases in spin ladders. Using the numerically exact density matrix renormalization group method, we find that these interacting phases—previously studied only in the nearest-neighbor case—survive in the presence of long-range dipolar interactions. We also show how to use our approach to realize the bilinear-biquadratic spin-1 and the Kitaev honeycomb models. Experimental detection schemes and imperfections are discussed.

We consider the relaxation of a spin qubit in a quantum dot propagating as a whole in a one-dimensional semiconductor with hyperfine coupling. We show that motion leads to qualitatively new features in this process compared to static quantum dots. For a fast straightforward motion, the initial spin density decreases to zero with the relaxation rate independent of the spatial spread of the electron wave function and inversely proportional to the electron speed. However, for the oscillatory motion, the qubit acquires memory, and the dephasing becomes Gaussian rather than exponential. After some time, one-third of the initial spin polarization is restored, as it happens for static qubits. This revival can occur either through periodic peaks or through a monotonous increase in the polarization, after a minimum, until a plateau has been reached. Our results can be useful for the understanding of the spin dynamics and decoherence in quantum wires.