The broadband optical spectra on the layered manganites $R_{1-x}{\text{Sr}}_{1+x}{\text{MnO}}_4$ ($R=\text{Nd}$ and La) are investigated to reveal the active role of the orbital degree of freedom on the collective charge/orbital density-wave dynamics. A collective density-wave excitation is identified around 9 meV in the charge/orbital ordered phase, when the nominal $e_g$-electron filling $\left(1-x\right)$ of Mn ion is less than around 1/3. By contrast, such a collective mode almost vanishes at $x=1/2$, which is explained in terms of the enhanced Jahn-Teller interaction cooperative with the electron-correlation effect.

The non-Abelian topological order for superconductors is characterized by the existence of zero-energy Majorana fermions in edges of systems and in a vortex of a macroscopic condensate, which obey the non-Abelian statistics. This paper is devoted to an extensive study on the non-Abelian topological phase of spin-singlet superconductors with the Rashba spin-orbit interaction proposed in our previous paper [M. Sato, Y. Takahashi, and S. Fujimoto, Phys. Rev. Lett. 103, 020401 (2009)]. We mainly consider the $s$-wave pairing state and the $d+id$ pairing state. In the case of $d+id$-wave pairing, Majorana fermions appear in almost all parameter regions of the mixed state under an applied magnetic field, provided that the Fermi level crosses $k$ points in the vicinity of the $\Gamma$ point or the M point in the Brillouin zone while in the case of $s$-wave pairing, a strong magnetic field, the Zeeman energy of which is larger than the superconducting gap is required to realize the topological phase. We clarify that Majorana fermions in Rashba spin-singlet superconductors are much more stable than those realized in spin-triplet $p+ip$ superconductors in certain parameter regions. We also investigate the topological number which ensures the topological stability of the phase in detail. Furthermore, as a by-product, we found that topological order is also realized in conventional spin (or charge)-density wave states with the Rashba spin-orbit interaction, for which massless Dirac fermions appear in the edge of the systems and charge fractionalization occurs.

We have investigated the spin/orbital phase diagram in the perovskite orthovanadate $R{\text{VO}}_3$ ($R=\text{Eu}$, Y, Dy, and Ho) by measurements of magnetization, dielectric constant, specific heat, Raman scattering spectra, and x-ray diffraction, focusing on the interplay between the $\text{V}3d$ spin and the $4f$ moment of the $R$ ion. The thermally induced phase transition between the C-type spin/G-type orbital ordered state and the G-type spin/C-type orbital ordered state is observed for ${\text{Eu}}_{1-x}{\text{Y}}_x{\text{VO}}_3$ $\left(x=0–0.52\right)$ without $4f$ moment. By comparing this phase diagram with the spin/orbital ordering in ${\text{TbVO}}_3$, it is evident that the critical competition between the C-type spin/G-type orbital ordered phase and the G-type spin/C-type orbital ordered one depends not only on the ${\text{GdFeO}}_3$-type lattice distortion but also on the presence of the $4f$ moment of the $R$ ion. The magnetic field induced phase transition of the spin/orbital ordering is achieved concomitantly with polarizing $R4f$ moments for ${\text{DyVO}}_3$ and ${\text{HoVO}}_3$. For ${\text{DyVO}}_3$, the G-type spin/C-type orbital ordered phase is switched to the C-type spin/G-type orbital ordered one by applying a moderate magnetic field around 3 T. By contrast, the G-type spin/C-type orbital ordering is rather favored under the magnetic field in ${\text{HoVO}}_3$. The results cannot be uniquely explained in terms of the exchange interaction between the $\text{V}3d$ spin and the $R$-ion $4f$ moment. The coupling of the $R4f$ moment polarization with the lattice distortion tied with the orbital ordering of the $\text{V}3d$ sublattice may also be relevant to this field induced phase transition.

We derive and investigate the microscopic model of the quantum magnet ${\text{BiCu}}_2{\text{PO}}_6$ using band-structure calculations, magnetic susceptibility and high-field magnetization measurements, as well as exact diagonalization (ED) and density-matrix renormalization group (DMRG) techniques. The resulting quasi-one-dimensional spin model is a two-leg antiferromagnetic ladder with frustrating next-nearest-neighbor couplings along the legs. The individual couplings are estimated from band-structure calculations and by fitting the magnetic susceptibility with theoretical predictions, obtained using full diagonalizations. The nearest-neighbor leg coupling $J_1$, the rung coupling $J_4$, and one of the next-nearest-neighbor couplings $J_2$ amount to 120–150 K while the second next-nearest-neighbor coupling is $J_2^{\prime }\simeq J_2/2$. The spin ladders do not match the structural chains, and although the next-nearest-neighbor interactions $J_2$ and $J_2^{\prime }$ have very similar superexchange pathways, they differ substantially in magnitude due to a tiny difference in the O-O distances and in the arrangement of nonmagnetic ${\text{PO}}_4$ tetrahedra. An extensive ED study of the proposed model provides the low-energy excitation spectrum and shows that the system is in the strong rung coupling regime. The strong frustration by the next-nearest-neighbor couplings leads to a triplon branch with an incommensurate minimum. This is further corroborated by a strong-coupling expansion up to second order in the inter-rung coupling. Based on high-field magnetization measurements, we estimate the spin gap of $\Delta \simeq 32\text{K}$ and suggest the likely presence of antisymmetric Dzyaloshinskii-Moriya anisotropy and interladder coupling $J_3$. We also provide a tentative description of the physics of ${\text{BiCu}}_2{\text{PO}}_6$ in magnetic field, in the light of the low-energy excitation spectra and numerical calculations based on ED and DMRG. In particular, we raise the possibility for a rich interplay between one- and two-component Luttinger liquid phases and a magnetization plateau at 1/2 of the saturation value.

We study quantum transport and scattering of massless Dirac fermions by spatially localized static magnetic fields. The employed model describes in a unified manner the effects of orbital magnetic fields, Zeeman and exchange fields in topological insulators, and the pseudomagnetic fields caused by strain or defects in monolayer graphene. The general scattering theory is formulated, and for radially symmetric fields, the scattering amplitude and the total and transport cross sections are expressed in terms of phase shifts. As applications, we study ring-shaped magnetic fields (including the Aharanov-Bohm geometry) and scattering by magnetic dipoles.

We analyze the static and dynamic properties of two Ising-coupled quantum spins embedded in a common bosonic bath as an archetype of dissipative quantum mechanics. First, we elucidate the ground-state phase diagram for an Ohmic and a sub-Ohmic bath using a combination of bosonic numerical renormalization group (NRG), analytical techniques, and intuitive arguments. Second, by employing the time-dependent NRG we investigate the system’s rich dynamical behavior arising from the complex interplay between spin-spin and spin-bath interactions. Interestingly, spin oscillations can synchronize due to the proximity of the common non-Markovian bath and the system displays highly entangled steady states for certain nonequilibrium initial preparations. We complement our nonperturbative numerical results by exact analytical solutions when available and provide quantitative limits on the applicability of the perturbative Bloch-Redfield approach at weak coupling.

Inelastic neutron scattering from superconducting (SC) $\text{Ba}{\left({\text{Fe}}_{0.926}{\text{Co}}_{0.074}\right)}_2{\text{As}}_2$ reveals anisotropic and quasi-two-dimensional (2D) magnetic excitations close to ${\mathbf{Q}}_{\text{AFM}}=\left(\frac{1}{2}\frac{1}{2}\right)$ - the 2D antiferromagnetic (AFM) wave vector of the parent ${\text{BaFe}}_2{\text{As}}_2$ compound. The correlation length anisotropy of these low-energy fluctuations is consistent with spin nematic correlations in the $J_1\text{−}{J}_2$ model with $J_1/J_2\sim 1$. The spin resonance at $\sim 8.3\text{meV}$ in the SC state displays the same anisotropy. The anisotropic fluctuations experimentally evolve into two distinct maxima only along the direction transverse to ${\mathbf{Q}}_{\text{AFM}}$ above $\sim 80\text{meV}$ indicating unusual quasipropagating excitations.

First-principles calculations demonstrate the evolution of the band alignment at ${\text{La}}_{0.7}{A}_{0.3}{\text{MnO}}_3\left|{\text{La}}_{1-x}{A}_x\text{O}\right|{\text{TiO}}_2\mid {\text{SrTiO}}_3\left(001\right)$ heterointerfaces, where $A=\text{Ca}$, Sr, or Ba, as the interfacial $A$-site composition, ${\text{La}}_{1-x}{A}_x$, is varied from $x=0.5$ to $x=1.0$. This variation leads to a linear change in the ${\text{SrTiO}}_3$ valence-band offset with respect to the Fermi level of the ${\text{La}}_{0.7}{A}_{0.3}{\text{MnO}}_3$ metal electrode and hence to a linear change in the Schottky barrier height at this interface. The effect arises due to electrostatic screening of the polar interface, altering the interfacial dipole and hence the electrostatic potential step at this interface. We find that both the ${\text{La}}_{0.7}{A}_{0.3}{\text{MnO}}_3$ and ${\text{SrTiO}}_3$ layers contribute to screening with both electronic and ionic screening being important for the change in the interface dipole. The results are in agreement with the recent experimental data.

Using a two-orbital model of the superconducting phase of the pnictides, we compute the spectrum of the Leggett mode—a collective excitation of the phase of the superconducting gap known to exist in multigap superconductors—for different possible symmetries of the superconducting order parameter. Specifically, we identify the small regions of parameter space where the Leggett mode lies below the two-particle continuum, and hence should be visible as a sharp resonance peak. We discuss the possible utility of the Leggett mode in distinguishing different momentum dependencies of the superconducting gap. We argue that the observation of a sharp Leggett mode would be consistent with the presence of strong electron-electron correlations in iron-based superconductors. We also emphasize the importance of the orbital character of the Leggett mode, which can result in an experimental observation of the mode in channels other than $A_{1g}$.

A spin projection scheme is presented which allows the decomposition of the electric conductivity into two different spin channels within fully relativistic ab initio transport calculations that account for the impact of spin-orbit coupling. This is demonstrated by calculations of the spin-resolved conductivity of ${\text{Fe}}_{1-x}{\text{Cr}}_x$ and ${\text{Co}}_{1-x}{\text{Pt}}_x$ disordered alloys on the basis of the corresponding Kubo-Greenwood equation implemented using the Korringa-Kohn-Rostoker coherent-potential approximation band-structure method. In addition, results for the residual resistivity of diluted Ni-based alloys are presented that are compared to theoretical and experimental ones that rely on Mott’s two-current model for spin-polarized systems. The application of the scheme to deal with the spin-orbit induced spin-Hall effect is discussed in addition.

We demonstrate the stabilization of a magnetic ground state in epitaxial ${\text{NiMn}}_2{\text{O}}_4$ (NMO) thin films not observed in their bulk counterpart. Bulk NMO exhibits a magnetic transition from a paramagnetic phase to a collinear ferrimagnetic moment configuration below 110 K and to a canted moment configuration below 70 K. By contrast, as-grown NMO films exhibit a single magnetic transition at 60 K and annealed films exhibit the magnetic behavior found in bulk. Cation inversion and epitaxial strain are ruled out as possible causes for the new magnetic ground state in the as-grown films. However, a decrease in the octahedral ${\text{Mn}}^{4+}:{\text{Mn}}^{3+}$ concentration is observed and likely disrupts the double exchange that produces the magnetic state at intermediate temperatures. X-ray magnetic circular dichroism and bulk magnetometry indicate a canted ferrimagnetic state in all samples at low temperature. Together these results suggest that the collinear ferrimagnetic state observed in bulk NMO at intermediate temperatures is suppressed in the as grown NMO thin films due to a decrease in octahedral ${\text{Mn}}^{4+}$ while the canted moment ferrimagnetic ordering is preserved below 60 K.

We study the recently introduced ${\mathbb{Z}}_2$ pump consisting of a family of one-dimensional bulk insulators with time-reversal restriction on the pumping cycle. We find that the scattering matrices of these pumps are dichotomized by a topological index. We show that the class of pumps characterized by a nontrivial topological index allows, in contrast to its topologically trivial counterpart, for the noiseless pumping of quantized spin, even in the absence of spin conservation. This distinction sheds light on the ${\mathbb{Z}}_2$ classification of two-dimensional time-reversal invariant insulators.

We use scanning tunneling spectroscopy to show directly that the conduction band density of states (DOS) of ${\text{GaAs}}_{1-x}{\text{N}}_x$ with low nitrogen (N) content $x$ is enhanced about 0.5 eV above the band edge, followed by a decrease at higher energy. The structure of the measured DOS is in excellent agreement with calculations based on a Green’s-function formalism taking into account different N environments. This analysis highlights the inclusion of N-N pairs and the validity of the Green’s-function approach to describe the band structure of dilute nitride and related extreme semiconductor alloys.

In electronic devices where a two-dimensional electron gas (2DEG) comprises one or both sides of a plane capacitor, the resulting capacitance $C$ can be larger than the “geometric capacitance” $C_g$ determined by the physical separation $d$ between electrodes. This larger capacitance is known to result from the Coulomb correlations between individual electrons within the low-density 2DEG, which lead to a negative thermodynamic density of states. Experiments on such systems generally operate in the regime where the average spacing between electrons $n^{-1/2}$ in the 2DEG is smaller than $d$ and these experiments observe $C>C_g$ by only a few percent. A recent experiment [L. Li, C. Richter, S. Paetel, T. Kopp, J. Mannhart, and R. Ashoori, arXiv:1006.2847 (unpublished)], however, has observed $C$ larger than $C_g$ by almost 40% while operating in the regime $n{d}^2⪡1$. In this paper we argue that at $n{d}^2⪡1$ correlations between the electronic charge of opposite electrodes become important. We develop a theory of the capacitance for the full range of $n{d}^2$. We show that, in the absence of disorder, the capacitance can be $4d/a$ times larger than the geometric value, where $a⪡d$ is the electron Bohr radius. Our results compare favorably with the experiment of Li et al. [arXiv:1006.2847 (unpublished)] without the use of adjustable parameters.

We have performed in situ spin- and angle-resolved photoemission and ex situ magnetotransport measurements on ultrathin topological insulator ${\text{Bi}}_2{\text{Se}}_3$ films. The surface states deviate from a simple isotropic Dirac fermion due to hexagonal warping effects and their spin helical nature has been verified. In addition, the bulk states also cross the Fermi level, showing that the films are actually $n$ doped. However, the temperature dependence of the film resistivity was insulating and saturated below 35 K. Furthermore, the magnetoresistivity changed drastically as the sample was cooled down and showed an anomaly near zero field below 20 K. The possible origin of this peculiar behavior and the nature of the carriers involved are discussed in comparison with the results for nonmetallic bulk samples.

We comprehensively study nonequilibrium relaxation and aging processes in the two-dimensional random-site Ising model through numerical simulations. We discuss the dynamical correlation length as well as scaling functions of various two-time quantities as a function of temperature and of the degree of dilution. For already modest values of the dynamical correlation length $L$ deviations from a simple algebraic growth, $L\left(t\right)\sim t^{1/z}$, are observed. When taking this nonalgebraic growth properly into account, a simple aging behavior of the autocorrelation function is found. This is in stark contrast to earlier studies where, based on the assumption of algebraic growth, a superaging scenario was postulated for the autocorrelation function in disordered ferromagnets. We also study the scaling behavior of the space-time correlation as well as of the time integrated linear response and find again agreement with simple aging. Finally, we briefly discuss the possibility of superuniversality in the scaling properties of space- and time-dependent quantities.

We report pure electric-field effects on the excitonic absorbance of pentacene thin films as measured by unipolar field-effect devices that allowed us to separate the charge accumulation effects. The field-modulated spectra between 1.8 and 2.6 eV can be well fitted with the first derivative curve of Frenkel exciton absorption and its vibronic progression, and at higher energy a field-induced feature appears at around 2.95 eV. The results are in sharp contrast to the electroabsorption spectra reported by Sebastian et al. in previous studies [Chem. Phys. 61, 125 (1981)], and leads us to reconsider the excitonic structure including the location of charge-transfer excitons. Nonlinear $\pi$-electronic response is discussed based on second-order electro-optic (Kerr) spectra.

We study the symmetry of spin excitation spectra in 122-ferropnictide superconductors by comparing the results of first-principles calculations with inelastic neutron-scattering (INS) measurements on ${\text{BaFe}}_{1.85}{\text{Co}}_{0.15}{\text{As}}_2$ and ${\text{BaFe}}_{1.91}{\text{Ni}}_{0.09}{\text{As}}_2$ samples that exhibit neither static magnetic phases nor structural phase transitions. In both the normal and superconducting (SC) states, the spectrum lacks the three-dimensional $4_2/m$ screw symmetry around the $\left(\frac{1}{2}\frac{1}{2}L\right)$ axis that is implied by the $I4/mmm$ space group. This is manifest both in the in-plane anisotropy of the normal- and SC-state spin dynamics and in the out-of-plane dispersion of the spin-resonance mode. We show that this effect originates from the higher symmetry of the magnetic Fe sublattice with respect to the crystal itself, hence the INS signal inherits the symmetry of the unfolded Brillouin zone (BZ) of the Fe sublattice. The in-plane anisotropy is temperature independent and can be qualitatively reproduced in normal-state density-functional-theory calculations without invoking a symmetry-broken (“nematic”) ground state that was previously proposed as an explanation for this effect. Below the SC transition, the energy of the magnetic resonant mode ${\omega }_{\text{res}}$, as well as its intensity and the SC spin gap inherit the normal-state intensity modulation along the out-of-plane direction $L$ with a period twice larger than expected from the body-centered-tetragonal BZ symmetry. The amplitude of this modulation decreases at higher doping, providing an analogy to the splitting between even and odd resonant modes in bilayer cuprates. Combining our and previous data, we show that at odd $L$ a universal linear relationship $\hslash {\omega }_{\text{res}}\approx 4.3k_{\text{B}}{T}_{\text{c}}$ holds for all the studied Fe-based superconductors, independent of their carrier type. Its validity down to the lowest doping levels is consistent with weaker electron correlations in ferropnictides as compared to the underdoped cuprates.

We report a quantum oscillation study of ${\text{Tl}}_2{\text{Ba}}_2{\text{CuO}}_{6+\delta }$ for two different doping levels ($T_c=10$ and 26 K) and determine the Fermi-surface size and topology in considerable detail. Our results show that Fermi-liquid behavior is not confined to the edge of the superconducting dome and is robust up to at least $0.3T_c^{\text{max}}$. Superconductivity is found to survive up to a larger doping $p_c=0.31$ than in ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$. Our data imply that electronic inhomogeneity does not play a significant role in the loss of superconductivity and superfluid density in overdoped cuprates, and point toward a purely magnetic or electronic pairing mechanism.

A three-dimensional topological insulator manifests gapless surface modes, described by two-dimensional Dirac equation. We study magnetic textures, such as domain walls and vortices, in a ferromagnetic thin film deposited on a three-dimensional topological insulator. It is shown that these textures can be electrically charged, ascribed to the proximity effect with the Dirac surface states. We derive a general relation between the electric and the magnetic charges. As a physical consequence, we discuss domain-wall motion driven by an applied electric field, which promises magnetic devices with high thermal efficiency.