The Tao-Perdew-Staroverov-Scuseria (TPSS) meta-generalized-gradient-approximation (MGGA) and its revised version, the revTPSS, are implemented self-consistently within the framework of the projector-augmented-wave (PAW) method, using a plane wave basis set. Both TPSS and revTPSS yield accurate atomization energies for the molecules in the AE6 set, better than those of the standard Perdew-Burke-Ernzerhof (PBE) generalized-gradient-approximation. For lattice constants and bulk moduli of 20 diverse solids, revTPSS performs much better than PBE, and on average as well as PBEsol and Armiento-Mattsson (AM05), GGAs designed for solids. The latter two overestimate the atomization energies for molecules to an unacceptable degree. However, the revTPSS presents only a slight improvement over PBEsol for the prediction of cohesive energies for solids, and some deterioration with respect to PBE. We also study the magnetic properties of Fe, for which both TPSS and revTPSS predict the right ground-state solid phase, the ferromagnetic body-centered-cubic (bcc) structure, with an accurate magnetic moment.

We present a detailed study of the local density of states (LDOS) associated with the surface-state band near a step edge of the strong topological insulator Bi${}_2$Te${}_3$ and reveal a one-dimensional bound state that runs parallel to the step edge and is bound to it at some characteristic distance. This bound state is clearly observed in the bulk gap region, while it becomes entangled with the oscillations of the warped surface band at high energy [Alpichshev et al., Phys. Rev. Lett. 104, 016401 (2010)], and with the valence-band states near the Dirac point. We obtain excellent fits to theoretical predictions [Alpichshev et al., 2011] that properly incorporate the three-dimensional nature of the problem to the surface state [Zhang et al., Nat. Phys. 5, 438 (2009)]. Fitting the data at different energies, we can recalculate the LDOS originating from the Dirac band without the contribution of the bulk bands or incoherent tunneling effects.

Impurity-based, premelting-like, grain boundary (GB) “phases” (complexions) can form in alloys and influence sintering, creep, and microstructural development. Calculation of Phase Diagrams (CalPhaD) methods and Miedema-type statistical interfacial thermodynamic models are combined to forecast the formation and stability of subsolidus quasiliquid GB phases in binary alloys. This work supports a long-range scientific goal of developing “GB (phase) diagrams” as a new materials science tool to help controlling the materials fabrication processing and resultant materials properties. Using nickel-doped molybdenum as a model system, a type of GB diagram (called “λ diagram”) is computed to represent the temperature- and composition-dependent thermodynamic tendency for general GBs to disorder. Subsequently, controlled sintering experiments are conducted to estimate the GB diffusivity as a function of temperature and overall composition, and the experimental results correlate well with the computed GB diagram. Although they are not yet rigorous GB-phase diagrams with well-defined transition lines, the predictability and usefulness of such λ diagrams are demonstrated. Related interfacial thermodynamic models and computational approaches are discussed.

The Nernst coefficient of the cuprate superconductor YBa${}_2$Cu${}_3$O${}_y$ was recently shown to become strongly anisotropic within the basal plane when cooled below the pseudogap temperature $T^{\star }$, revealing that the pseudogap phase breaks the fourfold rotational symmetry of the CuO${}_2$ planes. Here we report on the evolution of this Nernst anisotropy at low temperature, once superconductivity is suppressed by a magnetic field. We find that the anisotropy drops rapidly below 80 K, to vanish in the $T=0$ limit. We show that this loss of anisotropy is due to the emergence of a small high-mobility electronlike pocket in the Fermi surface at low temperature, a reconstruction attributed to a low-temperature state that breaks the translational symmetry of the CuO${}_2$ planes. We discuss the sequence of broken symmetries—first rotational, then translational—in terms of an electronic nematic-to-smectic transition such as could arise when unidirectional spin or charge modulations order. We compare YBa${}_2$Cu${}_3$O${}_y$ with iron-pnictide superconductors where the process of (unidirectional) antiferromagnetic ordering gives rises to the same sequence of broken symmetries.

Motivated by recent density-matrix renormalization group (DMRG) calculations [Yan, Huse, and White, Science 332, 1173 (2011)], which claimed that the ground state of the nearest-neighbor spin-$1/2$ Heisenberg antiferromagnet on the kagome lattice geometry is a fully gapped spin liquid with numerical signatures of ${\mathbb{Z}}_2$ gauge structure, and a further theoretical work [Lu, Ran, and Lee, Phys. Rev. B 83, 224413 (2011)], which gave a classification of all Schwinger-fermion mean-field fully symmetric ${\mathbb{Z}}_2$ spin liquids on the kagome lattice, we have thoroughly studied Gutzwiller-projected fermionic wave functions by using quantum variational Monte Carlo techniques, hence implementing exactly the constraint of one fermion per site. In particular, we investigated the energetics of all ${\mathbb{Z}}_2$ candidates (gapped and gapless) that lie in the neighborhood of the energetically competitive U(1) gapless spin liquids. By using a state-of-the-art optimization method, we were able to conclusively show that the U(1) Dirac state is remarkably stable with respect to all ${\mathbb{Z}}_2$ spin liquids in its neighborhood, and in particular for opening a gap toward the so-called ${\mathbb{Z}}_2[0,\pi ]\beta$ state, which was conjectured to describe the ground state obtained by the DMRG method. Finally, we also considered the addition of a small second nearest-neighbor exchange coupling of both antiferromagnetic and ferromagnetic type, and obtained similar results, namely, a U(1) Dirac spin-liquid ground state.

We report on imaging of the nonuniform superconducting states in a Pb thin-film bridge on top of a ferromagnetic BaFe${}_{12}$O${}_{19}$ single crystal with a single straight domain wall along the center of the bridge by low-temperature scanning laser microscopy. We have visualized domain-wall superconductivity (DWS) close to the critical temperature of Pb, when the Pb film above the domain wall acts as a superconducting path for the current. The evolution of the DWS signal with temperature and the external-field-driven transition from DWS to reverse-domain superconductivity was visualized.

Nonthermal fixed points of the dynamics of a dilute degenerate Bose gas are analyzed in two and three spatial dimensions. For such systems, universal power-law distributions, previously found within a nonperturbative quantum-field theoretic approach, are shown to be related to vortical dynamics and superfluid turbulence. The results imply an interpretation of the momentum scaling at the nonthermal fixed points in terms of independent vortex excitations of the superfluid. Long-wavelength acoustic excitations on the top of these are found to follow a nonthermal power law. The results shed light on fundamental aspects of superfluid turbulence and have strong potential implications for related phenomena, e.g., in early-universe inflation or quark-gluon plasma dynamics.

We have obtained the first-principles electronic structure of solid coronene, which has been recently reported to exhibit superconductivity with potassium doping. Since coronene, along with picene, the first aromatic superconductor, now provides a class of superconductors as solids of aromatic compounds, here we compare the two cases by examining the electronic structures. In the undoped coronene crystal, where the molecules are arranged in a herringbone structure with two molecules in a unit cell, the conduction band above an insulating gap is found to comprise four bands, which basically originate from the lowest two unoccupied molecular orbitals (doubly-degenerate, reflecting the high symmetry of the molecular shape) in an isolated molecule but the bands are entangled as in solid picene. The Fermi surface for a candidate of the structure of ${\mathrm{K}}_x$coronene with $x=3$, for which superconductivity is found, comprises multiple sheets, as in doped picene but exhibiting a larger anisotropy with different topology.

We report on the direct observation of magnetoplasmon modes that are excited in a laterally confined two-dimensional electron gas (2DEG) by microwave radiation. The magnetoplasmon polaritons were imaged using spatially resolved photoluminescence (PL) spectroscopy of GaAs heterostructures, and their dependence on an external magnetic field and 2DEG density were studied. The PL pattern formation originates in the PL spectrum modulation by electrons that are heated by the local electric field of the magnetoplasmons. We revealed remarkable spatial oscillations of the microwave electric field in the samples containing a high mobility 2DEG.

We present a large-scale density functional theory (DFT) investigation of the $\mathit{AB}{\mathrm{O}}_3$ chemical space in the perovskite crystal structure, with the aim of identifying those that are relevant for forming piezoelectric materials. Screening criteria on the DFT results are used to select 49 compositions, which can be seen as the fundamental building blocks from which to create alloys with potentially good piezoelectric performance. This screening finds all the alloy end points used in three well-known high-performance piezoelectrics. The energy differences between different structural distortions, deformation, coupling between the displacement of the $A$ and $B$ sites, spontaneous polarization, Born effective charges, and stability is analyzed in each composition. We discuss the features that cause the high piezoelectric performance of the well-known piezoelectric lead zirconate titanate (PZT), and investigate to what extent these features occur in other compositions. We demonstrate how our results can be useful in the design of isovalent alloys with high piezoelectric performance.

We have studied spectral diffusion of the photoluminescence of a single CdSe quantum dot inserted in a ZnSe nanowire. We have measured the characteristic diffusion time as a function of pumping power and temperature using a recently developed technique [G. Sallen et al., Nat. Photon. 4, 696 (2010)] that offers subnanosecond resolution. These data are consistent with a model where only a single carrier wanders around in traps located in the vicinity of the quantum dot.

Far-infrared diagonal and Hall conductivities of multilayer epitaxial graphene on the C face of SiC were measured using magneto-optical absorption and Faraday rotation in magnetic fields up to 7 T and temperatures between 5 and 300 K. Multiple components were identified in the spectra, including (i) a quasiclassical cyclotron resonance, originating from the highly doped graphene layer closest to SiC, (ii) transitions between low-index Landau levels (LLs), which stem from weakly doped layers, and (iii) a broad optical absorption background. Electron- and hole-type LL transitions are optically distinguished and shown to coexist. An electron-hole asymmetry of the Fermi velocity of about 2% was found within one graphene layer, while the Fermi velocity varies by about 10% across the layers. The optical intensity of the LL transitions is several times smaller than is theoretically expected for isolated graphene monolayers without electron-electron and electron-phonon interactions.

Reduced density-matrix functional theory (RDMFT) has become an appealing alternative to density-functional theory to describe electronic properties of strongly correlated systems. Here we derive exact conditions for the suitability of RDMFT to describe the two-dimensional homogeneous electron gas, which is the base system for semiconductor quantum dots and quantum Hall devices, for example. Following the method of Cioslowski and Pernal [J. Chem. Phys. 111, 3396 (1999)] we focus on the properties of power functionals of the form $f(n,n^{\prime })={\left(n{n}^{\prime }\right)}^{\alpha }$ for the scaling function in the exchange-correlation energy. We show that in order to have stable and analytic solutions, and for $f$ to satisfy the homogeneous scaling constraint, the power is restricted to $1/4⩽\alpha ⩽3/4$. Applying a reasonable ansatz for the momentum distribution and the lower bound for the exchange-correlation energy tightens the physical regime further to $0.64\lesssim \alpha ⩽0.75$.

We show that topology of the low-energy band structure in bilayer graphene critically depends on mechanical deformations of the crystal which may easily develop in suspended graphene flakes. We describe the Lifshitz transition that takes place in strained bilayers upon splitting the parabolic bands at intermediate energies into several Dirac cones at the energy scale of a few meV. Then, we show how this affects the electron Landau level spectra and the quantum Hall effect.

Among the broad spectrum of systems predicted to exhibit topological superconductivity and Majorana fermions, one-dimensional wires with strong spin-orbit coupling provide one of the most promising experimental candidates. Here we investigate the fate of the topological superconducting phase in such wires when repulsive interactions are present. Using a combination of density matrix renormalization group, bosonization, and Hartree–Fock techniques, we demonstrate that while interactions degrade the bulk gap—consistent with recent results of Gangadharaiah et al.—they also greatly expand the parameter range over which the topological phase arises. In particular, we show that with interactions this phase can be accessed over a broader chemical potential window, thereby leading to greater immunity against disorder-induced chemical potential fluctuations in the wire. We also suggest that in certain wires strong interactions may allow Majorana fermions to be generated without requiring a magnetic field.

Thin-film perovskite cobaltites have been found to exhibit coercivity values enhanced by almost 2 orders of magnitude in comparison to bulk. In this paper, we have investigated this unexplained coercivity enhancement in detail, focusing on epitaxial SrTiO${}_3$(001)/Nd${}_{0.5}$Sr${}_{0.5}$CoO${}_3$ [SrTiO${}_3=\mathrm{STO}$] films, which display coercivity values up to 40 kOe at low temperatures. Thickness-dependent (10–800 Å) magnetometry and magnetotransport studies demonstrate that nanoscopic magnetic phase separation occurs in the interface region of Nd${}_{0.5}$Sr${}_{0.5}$CoO${}_3$ [consistent with recent work on SrTiO${}_3$(001)/La${}_{1-x}$Sr${}_x$CoO${}_3$], which is responsible for the degradation in magnetic and electronic properties in the very-thin-film limit. The coercivity is shown to be intimately related to the existence of this (70-Å-thick) interfacial phase-separated layer, leading us to advance an explanation for the coercivity enhancement in terms of the pinning of domain walls by interfacial nanoscopic ferromagnetic clusters and a crossover to single domain clusters at very low thickness. Simple estimates of the magnetocrystalline anisotropy (from the maximum coercivity), cluster dimensions (from the superparamagnetic blocking temperature), multidomain to single domain crossover point, and domain-wall width, provide quantitative support for this picture.

We report on molecular beam epitaxy growth of stoichiometric and superconducting FeSe crystalline thin films on double-layer graphene. Layer-by-layer growth of high-quality films has been achieved in a well-controlled manner by using Se-rich condition, which allow us to investigate the thickness-dependent superconductivity of FeSe. In situ low-temperature scanning tunneling spectra reveal that the local superconducting gap in the quasiparticle density of states is visible down to two triple layers for the minimum measurement temperature of 2.2 K, and that the transition temperature $T_{\mathrm{c}}$ scales inversely with film thickness.

We demonstrated extreme hole reduction and electron accumulation in YBa${}_2$Cu${}_3$O${}_y$ films by an electrochemical technique. Following the transition from superconductor to insulator, we succeeded in doping electrons as evidenced by the decrease in resistivity and sign change of the Hall coefficient. Further doping resulted in metallic $n$-type YBa${}_2$Cu${}_3$O${}_y$ with a carrier density of ∼2.5 × ${10}^{20}$ cm${}^{-3}$, but without any sign of superconductivity. The oxygen vacancies electrochemically induced in the CuO${}_2$ planes played a major role in the $p$-$n$ transition.

We give a self-consistent theory of the scale-dependent effective mass enhancement $m^{*}/m$ of quasiparticles by three-dimensional (3D) antiferromagnetic (AFM) spin fluctuations in the presence of disorder at an AFM quantum critical point. The coupling of fermionic and bosonic degrees of freedom in the critical regime is described in terms of a critical quasiparticle theory. Using the fact that even in the “non-Fermi liquid” regime the quasiparticle width does not exceed the quasiparticle energy, we adopt relations from Fermi liquid theory to determine the dependence of the spin fluctuation spectrum on $m^{*}/m$, from which the self-energy and hence $m^{*}/m$ may be calculated. The self-consistent equation for $m^{*}/m$ has a strong coupling solution provided the initial value is sufficiently large. We argue that in YbRh${}_2$Si${}_2$, quasi-2D AFM and/or 3D ferromagnetic Gaussian fluctuations existing over a wide range drive the system into the 3D strongly coupled fluctuation regime. We find critical exponents of the temperature dependence of the specific heat coefficient $\gamma \propto T^{-1/4}$ and of the resistivity $\rho (T)=\rho (0)+{\mathit{AT}}^{3/4}$ in good agreement with experiments on YbRh${}_2$Si${}_2$ in the temperature range $T<0.3\hspace{0.5em}\text{K}$.

We investigate the tunnel rates and energies of excited states of small numbers of electrons in a quantum dot fabricated in a Si/SiGe heterostructure. Tunnel rates for loading and unloading electrons are found to be strongly energy dependent, and they vary significantly between different excited states. We show that this phenomenon enables charge sensing measurements of the average electron occupation that are analogous to Coulomb diamonds. Excited-state energies can be read directly from the plot, and we develop a rate model that enables a quantitative understanding of the relative sizes of different electron tunnel rates.