Strain glassy behavior (SGB) in correlation with premartensitic tweed (PMT) morphology was observed in an Au${}_7$Cu${}_5$Al${}_4$ alloy, which is different from previously observed SGB that is incompatible with PMT in doped Ti-Ni alloys. Additionally, an “inverse freezing” transition from martensite to premartensitic strain glass upon heating was experimentally discovered, confirming a recent theoretical prediction. Investigations of rich phenomena in nonmagnetic Au-Cu-Al alloys are expected to shed light on the origin of both premartensitic transition and glasses.

We study the relation between the spin fluctuation and superconductivity in a heavily hole doped end material KFe${}_2$As${}_2$. We construct a five orbital model by approximately unfolding the Brillouin zone of the three-dimensional ten-orbital model obtained from first-principles calculation. By applying the random-phase approximation, we obtain the spin susceptibility and solve the linearized Eliashberg equation. The incommensurate spin fluctuation observed experimentally is understood as originating from interband interactions, where the multiorbital nature of the band structure results in an electron-hole asymmetry of the incommensurability in the whole iron-based superconductor family. As for superconductivity, $s$-wave and $d$-wave pairings are found to be in close competition, where the sign change in the gap function in the former is driven by the incommensurate spin fluctuations. We raise several possible explanations for the nodes in the superconducting gap of KFe${}_2$As${}_2$ observed experimentally.

High-resolution spin- and angle-resolved photoemission spectroscopy (spin-ARPES) was performed on the three-dimensional topological insulator Bi${}_2$Se${}_3$ using a recently developed high-efficiency spectrometer. The topological surface state's helical spin structure is observed, in agreement with theoretical prediction. Spin textures of both chiralities, at energies above and below the Dirac point, are observed, and the spin structure is found to persist at room temperature. The measurements reveal additional unexpected spin polarization effects, which also originate from the spin-orbit interaction, but are well differentiated from topological physics by contrasting momentum and photon energy and polarization dependencies. These observations demonstrate significant deviations of photoelectron and quasiparticle spin polarizations. Our findings illustrate the inherent complexity of spin-resolved ARPES and demonstrate key considerations for interpreting experimental results.

We introduce an extension of the density functional perturbation theory (DFPT) that allows self-consistent linear-response calculations from a DFT + $U$ ground state. Using this scheme, the full phonon dispersion of strongly correlated materials, whose ground state can be captured with Hubbard-corrected functionals, can be accessed with unprecedented accuracy and numerical efficiency. The tool is applied to the study of MnO and NiO in their antiferromagnetic (AFII) ground state. Our results confirm the highly noncubic behavior of these systems and show a strong interplay between features of the phonon spectrum and the occupation of specific $d$ states, suggesting the possibility to investigate the electronic structure of these materials through the analysis of their phonon spectrum.

It is observed that the magnetoconductance of bilayer films of copper (Cu) and copper monoxide (CuO) has distinct features compared to that of Cu films on conventional band insulator substrates. We analyze the data above 2 K by the theory of weak antilocalization in two-dimensional metals and suggest that spin-flip scattering by magnetic impurities inside Cu is suppressed in Cu/CuO samples. Plausibly the results imply a proximity effect of antiferromagnetism inside the Cu layer, which can be understood in the framework of Ruderman-Kittel-Kasuya-Yoshida interactions. The data below 1 K, which exhibit slow relaxation reminiscent of spin glass, are consistent with this interpretation.

The general problem of representing the Greens function $G(k,z)$ in terms of self-energy in field theories lacking Wick's theorem is considered. A simple construction shows that a Dyson-like representation with a self-energy $\Sigma (k,z)$ is always possible, provided we start with a spectral representation for $G(k,z)$ for finite-sized systems and take the thermodynamic limit. The self-energy itself can then be iteratively expressed in terms of another higher order self-energy, capturing the spirit of Mori's formulation. We further discuss alternative and more general forms of $G(k,z)$ that are possible. In particular, a recent theory, by the author, of extremely correlated Fermi liquids at density $n$, for Gutzwiller projected noncanonical Fermi operators, obtains a new form of the Greens function: $G(k,z)=[(1-\frac{n}{2})+\Psi (k,z)]/[z-{\widehat{E}}_k-\Phi (k,z)],$ with a pair of self-energies $\Phi \left(z\right)$ and $\Psi \left(z\right)$. Its relationship with the Dyson form is explored. A simple version of the two-self-energies model was shown recently to successfully fit several data sets of photoemission line shapes in cuprates. We provide details of the unusual spectral line shapes that arise in this model, with the characteristic skewed shape depending upon a single parameter. The energy distribution curve (EDC) and momentum distribution curve (MDC) line shapes are shown to be skewed in opposite directions, and provide a testable prediction of the theory.

The topological superfluid ${}^3$He-$B$ supports massless Dirac spectrum of surface bound states which can be described in terms of the self-conjugated Majorana field operators. We discuss here the possible signature of surface bound states in nuclear magnetic resonance absorption spectrum in a ${}^3$He-$B$ film. It is shown that transitions between different branches of the surface states spectrum lead to the nonzero absorption signal in longitudinal NMR scheme when the frequency is larger than the Larmour one.

We demonstrate multipulse quantum control of a single electron charge qubit. The qubit is manipulated by applying nonadiabatic voltage pulses to a surface depletion gate and readout is achieved using a quantum point-contact charge sensor. We observe Ramsey fringes in the excited-state occupation in response to a $\pi /2$-$\pi /2$ pulse sequence and extract $T_2^{*}\sim 60$ ps away from the charge degeneracy point. Simulations suggest these results may be extended to implement a charge echo by reducing the interdot tunnel coupling and pulse rise time, thereby increasing the nonadiabaticity of the pulses.

We report temperature-dependent surface x-ray scattering studies of the orbital ordered surface in La${}_{0.5}$Sr${}_{1.5}$MnO${}_4$. We find that as the bulk ordering temperature is approached from below the thickness of the interface between the electronically ordered and electronically disordered regions at the surface grows, though the bulk correlation length remains unchanged. Close to the transition, the surface is so rough that there is no well-defined electronic surface, despite the presence of bulk electronic order. That is, the electronic ordering at the surface has melted. Above the bulk transition, long-range ordering in the bulk is destroyed but finite-sized isotropic fluctuations persist, with a correlation length roughly equal to that of the low-temperature in-plane surface correlation length.

To optimize the bulk-insulating behavior in the topological insulator materials having a tetradymite structure, we have synthesized and have characterized single-crystal samples of ${\mathrm{Bi}}_{2\text{−}x}{\mathrm{Sb}}_x{\mathrm{Te}}_{3\text{−}y}{\mathrm{Se}}_y$ solid solution at various compositions. We have elucidated that there are a series of intrinsic compositions where the acceptors and donors compensate each other and present a maximal bulk-insulating behavior. At such compositions, the resistivity can become as large as several $\Omega$ cm at low temperatures, and one can infer the role of the surface-transport channel in the nonlinear Hall effect. In particular, the composition of ${\mathrm{Bi}}_{1.5}{\mathrm{Sb}}_{0.5}{\mathrm{Te}}_{1.7}{\mathrm{Se}}_{1.3}$ achieves the lowest bulk carrier density and appears to be best suited for surface-transport studies.

Magnetoelectric phase diagrams of the rare-earth ($R$) Mn perovskites $R$MnO${}_3$ are theoretically studied by focusing on crucial roles of the symmetric magnetostriction or the Peierls-type spin-phonon coupling through extending our previous work [M. Mochizuki et al., Phys. Rev. Lett. 105, 037205 (2010)]. We first construct a microscopic classical Heisenberg model for $R$MnO${}_3$ including the frustrated spin exchanges, single-ion anisotropy, and Dzyaloshinskii-Moriya interaction. We also incorporate the lattice degree of freedom coupled to the Mn spins via the Peierls-type magnetostriction. By analyzing this model using the replica-exchange Monte Carlo technique, we reproduce the entire phase diagram of $R$MnO${}_3$ in the plane of temperature and magnitude of the orthorhombic lattice distortion. Surprisingly it is found that in the $ab$-plane spiral spin phase, the ($\mathit{S}·\mathit{S}$)-type magnetostriction plays an important role for the ferroelectric order with polarization $\mathit{P}$$\parallel$$\mathit{a}$ whose contribution is comparable to or larger than the contribution from the ($\mathit{S}\times \mathit{S}$)-type magnetostriction, whereas in the $bc$-plane spiral phase, the ferroelectric order with $\mathit{P}$$\parallel$$\mathit{c}$ is purely of ($\mathit{S}\times \mathit{S}$) origin. This explains much larger $\mathit{P}$ in the $ab$-plane spiral phase than the $bc$-plane spiral phase as observed experimentally and gives a clue how to enhance the magnetoelectric coupling in the spin-spiral-based multiferroics. We also predict a noncollinear deformation of the $E$-type spin structure resulting in the finite ($\mathit{S}\times \mathit{S}$) contribution to the ferroelectric order with $\mathit{P}$$\parallel$$\mathit{a}$, and a wide coexisting regime of the commensurate $E$ and incommensurate spiral states, which resolve several experimental puzzles.

We develop the theory of the scaling of the anomalous Hall effect (AHE) in the insulating regime, which is observed in experiments to relate the anomalous Hall and diagonal conductivities by ${\sigma }_{xy}^{\text{AH}}\propto {\sigma }_{xx}^{1.40\sim 1.75}$ for a large range of materials. This scaling is qualitatively different from that observed in metals. Basing our theory on the phonon-assisted hopping mechanism and percolation theory in random networks, we derive a general formula for the anomalous Hall conductivity, in which the percolation theory averaging of the random-linked triad clusters is a key aspect that captures the correct observed physics. We show that it scales with the longitudinal conductivity as ${\sigma }_{xy}^{\text{AH}}\sim {\sigma }_{xx}^{\gamma }$ with $\gamma$ predicted to be $1.33⩽\gamma ⩽1.76$, quantitatively in agreement with the experimental observations. Our theory predicts that this scaling remains similar regardless of whether the hopping process is of long-range (variable-range hopping) or short-range type (activation hopping), or is influenced by interactions, i.e., the Efros-Shklovskii regime. Our theory completes the understanding of the AHE phase diagram in the insulating regime.

Ionization processes in thin films are central to organic electronics. The ionization potential $I\left(p\right)$ or electron affinity $A\left(p\right)$ of any molecule $p$ depends on the electronic polarization of the surrounding molecules and on electrostatic interactions $W\left(p\right)$ that are evaluated in films using the potential ${\Phi }^{\left(g\right)}($r$)$ due to gas-phase charge densities. $W\left(p\right)$ is combined with a self-consistent treatment of electronic polarization to obtain $I\left(p\right)$ and $A\left(p\right)$ using molecular quantum theory and the film's structure. $I\left(p\right)$ and $A\left(p\right)$ are not additive but contain cross terms in electronic polarization and electrostatics. The procedure accounts quantitatively for $I\left(p\right)$ of pentacene and perfluoropentacene films with standing molecules in bilayers or lying molecules in monolayers. Surface or subsurface dopants in pentacene films are modeled as ion pairs with Coulomb interactions between a fixed anion and an adjacent cation. Variations of ${\Phi }^{\left(g\right)}($r$)$ due to an ion pair modulate $I\left(p\right)$ and $A\left(p\right)$ locally and rationalize observed changes for tunneling into occupied and unoccupied pentacene states, respectively. As in molecular exciton theory, intermolecular overlap is neglected in the computation of $I\left(p\right)$ or $A\left(p\right)$. Electrostatic interactions are conveniently quantified by ${\Phi }^{\left(g\right)}\left(0\right)$ at the center of molecules.

The competition between the substrate-induced strain and the electrostatic polarization in improper ferroelectric films (hexagonal YMnO${}_3$) has been studied using optical second-harmonic generation techniques. Deposited on a (111) surface of yttria-stabilized zirconia substrate, the films exhibit (1) strong strain with negligible polarization, (2) relaxation accompanied by the nonlinear buildup of the polarization, and (3) bulklike polar behavior, in the order of increasing film thickness. The role of the substrate is distinctively different from that in proper ferroelectrics.

We consider the Chalker-Coddington network model for the integer quantum Hall effect, and examine the possibility of solving it exactly. In the supersymmetric path integral framework, we introduce a truncation procedure, leading to a series of well-defined two-dimensional loop models with two loop flavors. In the phase diagram of the first-order truncated model, we identify four integrable branches related to the dilute Birman-Wenzl-Murakami braid-monoid algebra and parameterized by the loop fugacity $n$. In the continuum limit, two of these branches (1,2) are described by a pair of decoupled copies of a Coulomb-gas theory, whereas the other two branches (3,4) couple the two loop flavors, and relate to an $\mathrm{SU}{\left(2\right)}_r\times \mathrm{SU}{\left(2\right)}_r/\mathrm{SU}{\left(2\right)}_{2r}$ Wess-Zumino-Witten (WZW) coset model for the particular values $n=-2\cos [\pi /(r+2\left)\right]$, where $r$ is a positive integer. The truncated Chalker-Coddington model is the $n=0$ point of branch 4. By numerical diagonalization, we find that its universality class is neither an analytic continuation of the WZW coset nor the universality class of the original Chalker-Coddington model. It constitutes rather an integrable, critical approximation to the latter.

We study the zero-temperature phase diagram and fractional excitations when a thin film of 3D topological insulator has two competing masses: $T$-symmetric exciton condensation and the $T$-breaking Zeeman effect. Two topologically distinct phases are identified: in one, the quasiparticles can be viewed as in a quantum spin Hall phase and in the other a quantum anomalous Hall phase. The vortices of the exciton order parameter can carry fractional charge and statistics of electrons in both phases. When the system undergoes the quantum phase transition between these two phases, the charges, statistics, and the number of fermionic zero mode of the excitonic vortices are also changed. We derive the effective field theory for vortices and external gauge field and present an explicit wave function for the fermionic zero mode localized at the excitonic vortices with or without orbital magnetic field. The quantum phase transition can be measured by optical Faraday or Kerr effect experiments, and, in closing, we discuss the conditions required to create the excitonic condensate.

An important question in the physics of superconducting nanostructures is the role of thermal fluctuations (TFs) on superconductivity in the zero-dimensional limit. Here, we probe the evolution of superconductivity as a function of temperature and particle size in single, isolated Pb nanoparticles. Accurate determination of the size and shape of each nanoparticle makes our system a good model to quantitatively compare the experimental findings with theoretical predictions. In particular we study the role of TFs on the tunneling density of states (DOS) and the superconducting energy gap (Δ) in these nanoparticles. For the smallest particles $h$ ≤ 13 nm, we clearly observe a finite energy gap beyond $T_{\mathrm{c}}$ giving rise to a “critical region.” We show explicitly through quantitative theoretical calculations that these deviations from mean-field predictions are caused by TFs. Moreover, for $T\ll T_{\mathrm{c}}$, where TFs are negligible and typical sizes below 20 nm, we show that Δ gradually decreases with reduction in particle size. This result is described by a theoretical model that includes finite size effects and zero temperature leading order corrections to the mean-field formalism.

We analyze the deformation of the potential energy surface (PES) due to the incorrect description of fractional electron systems (the nonlinearity of the energy with electron number) within a (semi)local density functional theory (DFT). Particularly sensitive to this failure are polaronic systems where charge localization is strongly coupled to lattice distortion. As an example we calculate the adiabatic PES for the hole transfer process in rutile and anatase TiO${}_2$. (Semi)local DFT leads to qualitatively wrong, barrierless curves. Removal of the nonlinearity improves the PES shape and allows us to calculate hole mobilities.

We study the nonlocal response of a confined electron gas within the hydrodynamical Drude model. We address the question as to whether plasmonic nanostructures exhibit nonlocal resonances that have no counterpart in the local-response Drude model. Avoiding the usual quasistatic approximation, we find that such resonances do indeed occur, but only above the plasma frequency. Thus the recently found nonlocal resonances at optical frequencies for very small structures, obtained within quasistatic approximation, are unphysical. As a specific example we consider nanosized metallic cylinders, for which extinction cross sections and field distributions can be calculated analytically.

We report inelastic neutron-scattering experiments on single crystals of Nd${}_{1-x}$Sr${}_x$MnO${}_3$ with $x=0.5$ and $x=0.49$. The spin-wave dispersion in the charge-, orbital-, and spin-ordered state in Nd${}_{0.5}$Sr${}_{0.5}$MnO${}_3$ exhibits a strongly anisotropic stiffness. The sign of the anisotropy is characteristic for the site-centered model for charge and orbital ordering in half doped manganites. Within this model, linear spin-wave theory yields a perfect description of the experimental dispersion. In the ferromagnetic metallic state of Nd${}_{1-x}$Sr${}_x$MnO${}_3$ with $x=0.49$ and $x=0.50$ magnetic excitations exhibit nearly the same magnon dispersion. High intense signals near the zone boundary over a wide energy level overlap with a sharp spin-wave dispersion, which can be described with a Heisenberg model including nearest-neighbor interactions.