The electronic interlayer transport of the lightly doped antiferromagnet ${\text{La}}_{1.79}{\text{Eu}}_{0.2}{\text{Sr}}_{0.01}{\text{CuO}}_4$ has been studied by means of magnetoresistance measurements. The central problem addressed concerns the differences between the electronic interlayer coupling in the tetragonal low-temperature (LTT) phase and the orthorhombic low-temperature (LTO) phase. The key observation is that the spin-flip-induced drop in the $c$-axis magnetoresistance of the LTO phase, which is characteristic for pure ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$, dramatically decreases in the LTT phase. The results show that the transition from orthorhombic to tetragonal symmetry and from collinear to noncollinear antiferromagnetic spin structure eliminates the strain dependent anisotropic interlayer hopping as well as the concomitant spin-valve-type transport channel. Implications for the stripe ordered LTT phase of ${\text{La}}_{2-x}{\text{Ba}}_x{\text{CuO}}_4$ are briefly discussed.

We report spatially resolved charge noise measurements on semiconductor samples by atomic force microscopy. We observed charge noise induced by light illumination on an InP/InGaAs heterostructure with surface InAs quantum dots and a buried two-dimensional electron gas. The observed noise exhibits generation-recombination noise or random telegraph noise depending on light intensity and bias voltage. A spatial resolution better than 20 nm was demonstrated by comparing the noise on and off the InAs quantum dots. The approach enables the localization of individual traps and will aid in understanding noise mechanisms.

An antiphased magnetoplasma (MP) mode in a two-dimensional electron gas has been studied by means of inelastic light-scattering spectroscopy. Unlike the cophased MP mode, it is purely quantum excitation which has no classic plasma analog. It is found that zero-momentum degeneracy for the antiphased and cophased modes predicted by the first-order perturbation approach in terms of the electron-electron interaction is lifted. The zero-momentum energy gap is determined by a negative correlation shift of the antiphased mode. This shift, observed experimentally and calculated theoretically within the second-order perturbation approach, is proportional to the effective Rydberg constant in a semiconductor material.

We calculate the electronic band structure of ABA-stacked trilayer graphene in the presence of external gates, using a self-consistent Hartree approximation to take account of screening. In the absence of a gate potential, there are separate pairs of linear and parabolic bands at low energy. A gate field perpendicular to the layers breaks mirror reflection symmetry with respect to the central layer and hybridizes the linear and parabolic low-energy bands, leaving a chiral Hamiltonian essentially different from that of monolayer or bilayer graphene. Using the self-consistent Born approximation, we find that the density of states and the minimal conductivity in the presence of disorder generally increase as the gate field increases, in sharp contrast with bilayer graphene.

We used spectroscopic photoemission and low-energy electron microscopy to investigate the electronic properties of epitaxial few-layer graphene grown on 6H-SiC(0001). Photoelectron emission microscopy (PEEM) images using secondary electrons (SEs) and $\text{C}1s$ photoelectrons can discriminate areas with different numbers of graphene layers. The SE emission spectra indicate that the work function increases with the number of graphene layers and that unoccupied states in the few-layer graphene promote SE emission. The $\text{C}1s$ PEEM images indicate that the $\text{C}1s$ core level shifts to lower binding energies as the number of graphene layers increases, which is consistent with the reported thickness dependence of the Dirac point energy.

We used density functional theory calculations to examine the initial stages of oxidation of the Pt(111) surface. Consistent with prior studies, our calculations predict that oxygen atoms adsorb on fcc sites and form $p\left(2\times 2\right)$ and $p\left(2\times 1\right)$ structures at coverages of 0.25 and 0.50 ML, respectively. In addition to various surface configurations of oxygen on fcc sites, we examined subsurface oxygen and clustering of oxygen atoms on the surface. We find that subsurface oxygen is not the precursor to the oxidation of the Pt(111) surface. Instead, we predict a strong preference for the formation and growth of one-dimensional Pt oxide chains within the $p\left(2\times 1\right)$ structure. In particular, at coverages above 0.50 ML, additional oxygen atoms prefer to aggregate between the close-packed oxygen rows formed by the $p\left(2\times 1\right)$ structure and induce large buckling $\left(\sim 1.8\text{Å}\right)$ and modification of the charge of the surface Pt atoms. The result is an oxide compound with threefold and fourfold Pt-O coordination that grows as a one-dimensional chain running parallel to the oxygen rows of the $p\left(2\times 1\right)$ structure. Furthermore, half of the oxygen atoms in the Pt oxide chains reside near hcp sites, contrary to some reports that oxygen atoms reside only on the fcc sites on Pt(111). Our results agree well with a recent scanning tunneling microscopy study and suggest a precursor mechanism to the oxidation of metal surfaces involving Pt oxide chain formation and growth on terraces at moderate oxygen coverages. Our results should have important implications to current models of NO and CO oxidation on Pt(111) and potentially on studies of the initial oxidation of other transition-metal and bimetallic surfaces.

Structural differences in the so-called M polymorphs of ${\text{AgNbO}}_3$ were analyzed using combined high-resolution x-ray diffraction, neutron total scattering, electron diffraction, and x-ray absorption fine-structure measurements. These polymorphs all crystallize with $Pbcm$ symmetry and lattice parameters $\surd 2a_c\times \surd 2a_c\times 4a_c$ (where $a_c\approx 4\text{Å}$ corresponds to the lattice parameter of an ideal cubic perovskite) which are determined by a complex octahedral tilt system $\left(a^{-}{b}^{-}{c}^{-}\right)/\left(a^{-}{b}^{-}{c}^{+}\right)$ involving a sequence of two in-phase and two antiphase rotations around the $c$ axis. Our results revealed that, similar to ${\text{KNbO}}_3$, the Nb cations in ${\text{AgNbO}}_3$ exhibit local off-center displacements correlated along Nb-Nb-Nb chains. The displacements appear to be present even in the high-temperature ${\text{AgNbO}}_3$ polymorphs where the Nb cations, on average, reside on the ideal fixed-coordinate sites. The onset of the $\left(a^{-}{b}^{-}{c}^{-}\right)/\left(a^{-}{b}^{-}{c}^{+}\right)$ tilting in the M polymorphs lifts the symmetry restrictions on the Nb positions and promotes ordering of the local Nb displacements into a long-range antipolarlike array. This ordering preserves the average $Pbcm$ symmetry but is manifested in electron diffuse scattering and corroborated by other local-structure sensitive techniques. Structural states previously identified as the ${\text{M}}_3$ and ${\text{M}}_2$ phases represent different stages of displacive ordering rather than distinct thermodynamic phases. Rietveld refinements indicated intimate coupling between the displacive behavior on the oxygen, Nb, and Ag sublattices. The $Pbcm$ symmetry of the octahedral framework precludes a complete ordering of Nb displacements so that some positional disorder is retained. This partial disorder likely gives a source to the dielectric relaxation which, according to previous spectroscopic studies, is the origin of the diffuse dielectric response exhibited by M-type ${\text{AgNbO}}_3$ at $\approx 250°\text{C}$.

We show in this paper that thermopower is enhanced in non-Abelian quantum Hall liquids under appropriate conditions. This is because thermopower measures entropy per electron in the clean limit, while the degeneracy and entropy associated with non-Abelian quasiparticles enhance entropy when they are present. Thus thermopower can potentially probe non-Abelian nature of the quasiparticles, and measure their quantum dimension.

The parent compounds of high-$T_c$ cuprates have long been considered to be antiferromagnetic insulators. In this Rapid Communication, however, we report that superconductivity is achieved in $T^{\prime }\text{−}{R}_2{\text{CuO}}_4$ ($R=\text{Pr}$, Nd, Sm, Eu, Gd). Superconducting $T^{\prime }\text{−}{R}_2{\text{CuO}}_4$ films were synthesized by using metal-organic decomposition. The highest $T_c$ of $T^{\prime }\text{−}{R}_2{\text{CuO}}_4$ is over 30 K, substantially higher than “electron-doped” analogs. Remarkably, ${\text{Gd}}_2{\text{CuO}}_4$, even the derivatives of which have not shown superconductivity so far, gets superconducting with $T_c^{\text{onset}}$ as high as $\sim 20\text{K}$. The implication of our results is briefly discussed.

We analyze the form of the fermionic propagator for two-dimensional fermions interacting with massless overdamped bosons. Examples include a nematic and Ising ferromagnetic quantum-critical points and fermions at a half-filled Landau level. Fermi-liquid behavior in these systems is broken at criticality by a singular self-energy, but the Fermi surface remains well defined. These are strong-coupling problems with no expansion parameter other than the number of fermionic species, $N$. The two known limits, $N⪢1$ and $N=0$, show qualitatively different behavior of the fermionic propagator $G\left({ϵ}_k,\omega \right)$. In the first limit, $G\left({ϵ}_k,\omega \right)$ has a pole at some ${ϵ}_k$; in the other it is analytic. We analyze the crossover between the two limits. We show that the pole survives for all $N$, but at small $N$ it only exists in a range $O\left(N^2\right)$ near the mass shell. At larger distances from the mass shell, the system evolves and $G\left({ϵ}_k,\omega \right)$ becomes regular. At $N=0$, the range where the pole exists collapses and $G\left({ϵ}_k,\omega \right)$ becomes regular everywhere.

Resistivity, magnetic susceptibility, and heat-capacity measurements are reported for single crystals of ${\text{Fe}}_{1+y}{\text{Te}}_x{\text{Se}}_{1-x}$ grown via a modified Bridgeman method with $0<y<0.15$ and $x=1$, 0.9, 0.75, 0. 67, 0.55, and 0.5. Although resistivity measurements show traces of superconductivity near 14 K for all $x$ except $x=1$, only crystals grown with compositions near $x=0.5$ exhibit bulk superconductivity. The appearance of bulk superconductivity correlates with a reduction in the magnitude of the magnetic susceptibility at room temperature and smaller values of $y$, the concentration of Fe in the Fe(2) site.

We report physical properties of the conductive magnet ${\text{PdCrO}}_2$ consisting of a layered structure with a triangular lattice of ${\text{Cr}}^{3+}$ ions $\left(S=3/2\right)$. We confirmed an antiferromagnetic transition at $T_{\text{N}}=37.5\text{K}$ by means of specific heat, electrical resistivity, magnetic susceptibility, and neutron-scattering measurements. The critical behavior in the specific heat persists in an unusually wide temperature range above $T_{\text{N}}$. This fact implies that spin correlations develop even at a much higher temperature than $T_N$. The observed sublinear temperature dependence of the resistivity above $T_{\text{N}}$ is also attributed to the short-range correlations among the frustrated spins. While the critical exponent for the magnetization agrees reasonably with the prediction of the relevant model, that for the specific heat evaluated in the wide temperature range differs substantially from the prediction.

We report inelastic neutron-scattering measurements of the spin dynamics in the layered hexagonal magnet $2H{\text{-AgNiO}}_2$, which has stacked triangular layers of antiferromagnetically coupled ${\text{Ni}}^{2+}$ spins $\left(S=1\right)$ ordered in a collinear alternating stripe pattern. We observe a broad band of magnetic excitations above a small gap of 1.8 meV and extending up to 7.5 meV, indicating strongly dispersive excitations. The measured dispersions of the boundaries of the powder-averaged spectrum can be quantitatively explained by a linear spin-wave dispersion for triangular layers with antiferromagnetic nearest- and weak next-nearest-neighbor couplings, a strong easy-axis anisotropy, and additional weak interlayer couplings. The resulting dispersion relation has global minima not at magnetic Bragg wave vectors but at symmetry-related soft points and we attribute this anomalous feature to the strong competition between the easy-axis anisotropy and the frustrated antiferromagnetic couplings. We have also calculated the quantum corrections to the dispersion relation to order $1/S$ in spin-wave theory by extending the work of Chubukov and Jolicoeur [Phys. Rev. B 46, 11137 (1992)] and find that the presence of easy-axis anisotropy significantly reduces the quantum renormalizations predicted for the isotropic model.

The motion of simple domain walls and of more complex magnetic textures in the presence of a transport current is described by the Landau-Lifshitz-Slonczewski (LLS) equations. Predictions of the LLS equations depend sensitively on the ratio between the dimensionless material parameter $\beta$ which characterizes nonadiabatic spin-transfer torques and the Gilbert damping parameter $\alpha$. This ratio has been variously estimated to be close to zero, close to one, and large compared to one. By identifying $\beta$ as the influence of a transport current on $\alpha$, we derive a concise, explicit, and relatively simple expression which relates $\beta$ to the band structure and Bloch state lifetimes of a magnetic metal. Using this expression we demonstrate that intrinsic spin-orbit interactions lead to intraband contributions to $\beta$ which are often dominant, and can be (i) estimated with some confidence and (ii) interpreted using the “breathing Fermi-surface” model.

A fast and stable numerical algorithm for solving the symmetric eigenvalue problem is presented. The technique deviates fundamentally from the traditional Krylov subspace iteration based techniques (Arnoldi and Lanczos algorithms) or other Davidson-Jacobi techniques and takes its inspiration from the contour integration and density-matrix representation in quantum mechanics. It will be shown that this algorithm—named FEAST—exhibits high efficiency, robustness, accuracy, and scalability on parallel architectures. Examples from electronic structure calculations of carbon nanotubes are presented, and numerical performances and capabilities are discussed.

Double quantum dots can provide an experimental realization of the two-impurity Kondo model which exhibits a non-Fermi-liquid quantum critical point (QCP) at a special value of its parameters. We generalize our recent study of double quantum dots in series [E. Sela and I. Affleck, Phys. Rev. Lett. 102, 047201 (2009)] to a parallel configuration with an Aharonov-Bohm flux. We present an exact universal result for the finite temperature and finite voltage conductance $G\left[V,T\right]$ along the crossover from the QCP to the low energy Fermi-liquid phase. Compared to the series configuration, here generically $G\left[V,T\right]\ne G\left[-V,T\right]$, leading to current rectification.

Although ultraviolet photoelectron emission spectroscopy and microscopy have served as powerful tools for electronic structure analysis in both reciprocal and real space, the integration of spectroscopy into microscopy still remains a challenge due to the technical difficulty of achieving both high spatial and energy resolution with slow electrons. We demonstrate photoelectron emission spectroscopy and microscopy covering the real, reciprocal, and energy spaces efficiently, enabled by a recently developed energy-filtered microscope. Observation of photoemission from micron-sized silver islands using $\text{He}\text{I}$ radiation reveals the importance of the unreduced final-state wave vectors, which determine the photoelectron angular distribution.

Using first-principles calculations, we demonstrate that an Fe monolayer can assume very different magnetic phases on hcp (0001) and fcc (111) surfaces of $4d$- and $5d$-transition metals. Due to the substrates’ $d$-band filling, the nearest-neighbor exchange coupling of Fe changes gradually from antiferromagnetic (AFM) for Fe films on Tc, Re, Ru, and Os to ferromagnetic on Rh, Ir, Pd, and Pt. In combination with the topological frustration on the triangular lattice of these surfaces the AFM coupling results in a $120°$ Néel structure for Fe on Re and Ru and an unexpected double-row-wise AFM structure on Rh, which is a superposition of left- and right-rotating $90°$ spin spirals.

I consider electrodynamics and the problem of knotted solitons in two-component superconductors. Possible existence of knotted solitons in multicomponent superconductors was predicted several years ago. However, their basic properties and stability in these systems remain an outstandingly difficult question both for analytical and numerical treatment. Here I propose a special perturbative approach to treat self-consistently all the degrees of freedom in the problem. I show that there exists a length scale for a Hopfion texture where the electrodynamics of a two-component superconductor is dominated by a self-induced Faddeev term, which is in stark contrast to the Meissner electrodynamics of single-component systems. I also show that at certain short length scales knotted solitons in the two-component Ginzburg-Landau model are not described by a Faddeev-Skyrme-type model and are unstable. However, these solitons can be stable at some intermediate length scales. I argue that configurations with high topological charge may be more stable in these systems than low-charge configurations. In the second part of the paper I discuss qualitatively different physics of the stability of knotted solitons in a more general Ginzburg-Landau model and point out the physically relevant terms which enhance or suppress the stability of knotted solitons. With this argument it is demonstrated that Ginzburg-Landau models possess stable knotted solitons.

We demonstrate that microscale tailoring of surfaces of magnetophotonic crystals provides an approach to engineer responses of known materials. In particular, we study states that are spatially localized at the interface between magnetophotonic and photonic crystals. Such states are an optical analog of the Tamm states long known in solid-state physics. The optical Tamm states are spectrally located inside band gaps and are associated with a sharp transmission peak in spectra of the system. Substantial enhancement of the Faraday rotation for the wavelength of the optical Tamm state is experimentally observed and attributed to strong light coupling to magnetic constituents of the magnetophotonic crystal. The experimental results are in excellent agreement with the theoretical predictions.