We report the crystal structure and unconventional magnetic ordering of ${\text{Cs}}_2{\text{Cu}}_3{\text{CeF}}_{12}$, which is composed of buckled kagome lattice of ${\text{Cu}}^{2+}$ ions. The exchange network in the buckled kagome lattice is fairly anisotropic, so that the present spin system can be divided into two subsystems: alternating Heisenberg chains with strong antiferromagnetic exchange interactions and dangling spins. Although the direct exchange interactions between neighboring spins were found to be all antiferromagnetic, ferromagnetic magnetic ordering of the dangling spins was observed. Magnetization exhibits a plateau at one-third of the saturation magnetization. These observations can be understood in terms of the indirect interaction between dangling spins mediated by the chain spin.

We propose a model for the pair of electromagnon excitations observed in the class of multiferroic materials $R{\text{MnO}}_3$ ($R$ is a rare-earth ion). The model is based on a harmonic cycloid ground state interacting with a zone-edge magnon and its twin excitation separated in momentum space by two times the cycloid wave vector. The pair of electromagnons is activated by cross coupling between magnetostriction and spin-orbit interactions. Remarkably, the spectral weight of the twin electromagnon is directly related to the presence of a magnetically induced oscillatory polarization in the ground state. This leads to the surprising prediction that ${\text{TbMnO}}_3$ has an oscillatory polarization with amplitude 50 times larger than its uniform polarization.

A series of new ${\text{Sr}}_{2-x}{\text{Ba}}_x{\text{CuO}}_{3+\delta }$ $\left(0\le x\le 0.6\right)$ superconductors were prepared using high-pressure and high-temperature synthesis. A Rietveld refinement based on powder x-ray diffraction confirms that the superconductors crystallize in the ${\text{K}}_2{\text{NiF}}_4$-type structure of a space group $I4/mmm$ similar to that of ${\text{La}}_2{\text{CuO}}_4$ but with partially occupied apical oxygen sites. It is found that the superconducting transition temperature $T_c$ of this Ba substituted ${\text{Sr}}_2{\text{CuO}}_{3+\delta }$ superconductor with constant carrier-doping level, i.e., constant $\delta$ is controlled not only by order/disorder of apical-O atoms but also by Ba content. $T_c^{\text{max}}=98\text{K}$ is achieved in the material with $x=0.6$ that reaches the record value of $T_c$ among the single-layer copper oxide superconductors and is higher than $T_c=95\text{K}$ of ${\text{Sr}}_2{\text{CuO}}_{3+\delta }$ with optimally ordered apical-O atoms. There is Sr-site disorder in ${\text{Sr}}_{2-x}{\text{Ba}}_x{\text{CuO}}_{3+\delta }$ which might lead to a reduction in $T_c$. The result indicates that another effect surpasses the disorder effect that is related either to the increased in-plane Cu-O bond length or to elongated apical-O distance due to Ba substitution with larger cation size. The present experiment demonstrates that the optimization of local geometry out of the Cu-O plane can dramatically enhance $T_c$ in the cuprate superconductors.

${\text{PbZr}}_{1-x}{\text{Ti}}_x{\text{O}}_3$ (PZT) ceramics made by mixed-oxide and sol-gel routes within the rhombohedral region of the phase diagram have been prepared, and their structures determined by Rietveld refinement using high-resolution time-of-flight neutron diffraction. Different structural models have been used but the best fits have been found using a mixture of rhombohedral $R3c$ $\left(C_{3v}^6\right)$ and monoclinic $Cm$ $\left(C_s^3\right)$ across the whole rhombohedral range with the proportion of monoclinic phase increasing continuously with composition $x$ toward the morphotropic phase boundary. Some differences are found between the ceramics prepared by the two routes, especially with respect to the diffraction peak widths, suggesting that the precise structural arrangements in these materials depend on the method of preparation. The results show that PZT ceramics made in this region always consist of mixed phases and this accounts for the fact that no phase boundary has been discovered in the PZT phase diagram between the monoclinic and rhombohedral phases.

We present a first-principles density-functional study of the structural, electronic, and magnetic properties of the ferroelectric domain walls in multiferroic ${\text{BiFeO}}_3$. We find that domain walls in which the rotations of the oxygen octahedra do not change their phase when the polarization reorients are the most favorable and of these, the $109°$ domain wall centered around the BiO plane has the lowest energy. The $109°$ and $180°$ walls have a significant change in the component of their polarization perpendicular to the wall; the corresponding step in the electrostatic potential is consistent with a recent report of electrical conductivity at the domain walls. Finally, we show that changes in the Fe-O-Fe bond angles at the domain walls cause changes in the canting of the Fe magnetic moments which can enhance the local magnetization at the domain walls.

In this paper we explore the braiding properties of the Moore-Read fractional Hall sequence, which amounts to computing the adiabatic evolution of the Hall liquid when the anyons are moved along various trajectories. In this work, the anyons are pinned to precise spatial configurations by using specific external potentials. Such external potentials break the translational symmetry and it appears that one will be forced to simulate the braidings on the entire many-body Hilbert space, an absolutely prohibitive scenario. We demonstrate how to overcome this difficulty and obtain the exact braidings for fairly large Hall systems. For this, we show that the incompressible state of a general $\left(k,m\right)$ fractional Hall sequence can be viewed as the unique zero mode of a specific Hamiltonian $H^{\left(k,m\right)}$, whose form is explicitly derived by using $k$-particle creation operators. The compressible Hall states corresponding to $n\times k$ anyons fixed at $w_1,\dots ,w_{nk}$ are shown to be the zero modes of a pinning Hamiltonian $H_{w_1,\dots ,w_{nk}}^{\left(k,m\right)}$, which is also explicitly derived. The zero modes of $H_{w_1,\dots ,w_{nk}}^{\left(k,m\right)}$ are shown to be contained in the space of the zero modes of $H^{\left(k,m\right)}$. Therefore, the computation of the braidings can be done entirely within this space, which we map out for a number of Hall systems. Using this efficient computational method, we study various properties of the Moore-Read states. In particular, we present direct confirmation of their topological and non-Abelian properties that were previously predicted using the underlying conformal field theory structure of the Moore-Read state.

It is generally believed that in spatial dimension $d>1$, the leading contribution to the entanglement entropy $S=-\text{tr}{\rho }_A\text{log}{\rho }_A$ scales as the area of the boundary of subsystem $A$. The coefficient of this “area law” is nonuniversal. However, in the neighborhood of a quantum critical point $S$ is believed to possess subleading universal corrections. In the present work, we study the entanglement entropy in the quantum $O\left(N\right)$ model in $1<d<3$. We use an expansion in $ϵ=3-d$ to evaluate (i) the universal geometric correction to $S$ for an infinite cylinder divided along a circular boundary; (ii) the universal correction to $S$ due to a finite correlation length. Both corrections are different at the Wilson-Fisher and Gaussian fixed points, and the $ϵ\to 0$ limit of the Wilson-Fisher fixed point is distinct from the Gaussian fixed point. In addition, we compute the correlation length correction to the Renyi entropy $S_n=\frac{1}{1-n}\text{log}\text{tr}{\rho }_A^n$ in $ϵ$ and large-$N$ expansions. For $N\to \infty$, this correction generally scales as $N^2$ rather than the naively expected $N$. Moreover, the Renyi entropy has a phase transition as a function of $n$ for $d$ close to 3.

Electronic, magnetic, or structural inhomogeneities ranging in size from nanoscopic to mesoscopic scales seem endemic and are possibly generic to colossal magnetoresistance manganites and other transition metal oxides. They are hence of great current interest and understanding them is of fundamental importance. We show here that an extension, to include long-range Coulomb interactions, of a quantum two-fluid $\ell \text{−}b$ model proposed recently for manganites [Phys. Rev. Lett. 92, 157203 (2004)] leads to an excellent description of such inhomogeneities. In the $\ell \text{−}b$ model two very different kinds of electronic states, one localized and polaronic $\left(\ell \right)$ and the other extended or broad band $\left(b\right)$ coexist. For model parameters appropriate to manganites and even within a simple dynamical mean-field theory (DMFT) framework, it describes many of the unusual phenomena seen in manganites, including colossal magnetoresistance (CMR), qualitatively and quantitatively. However, in the absence of long-ranged Coulomb interaction, a system described by such a model would actually phase separate, into macroscopic regions of $l$ and $b$ electrons, respectively. As we show in this paper, in the presence of Coulomb interactions, the macroscopic phase separation gets suppressed and instead nanometer scale regions of polarons interspersed with band electron puddles appear, constituting a kind of quantum Coulomb glass. We characterize the size scales and distribution of the inhomogeneity using computer simulations. For realistic values of the long-range Coulomb interaction parameter $V_0$, our results for the thresholds for occupancy of the $b$ states are in agreement with, and hence support, the earlier approach mentioned above based on a configuration averaged DMFT treatment which neglects $V_0$; but the present work has features that cannot be addressed in the DMFT framework. Our work points to an interplay of strong correlations, long-range Coulomb interaction, and dopant ion disorder, all inevitably present in transition metal oxides as the origin of nanoscale inhomogeneities rather than disorder frustrated phase competition as is generally believed. As regards manganites, it argues against explanations for CMR based on disorder frustrated phase separation and for an intrinsic origin of CMR. Based on this, we argue that the observed micrometer (meso) scale inhomogeneities owe their existence to extrinsic causes, e.g., strain due to cracks and defects. We suggest possible experiments to validate our speculation.

The investigation of the electronic properties of semiconductor surfaces using scanning tunneling spectroscopy (STS) is often hindered by nonequilibrium transport of the injected charge carriers. We propose a correction method for the resulting systematic errors in STS data, which is demonstrated for the well-known Si(111)-$\left(7\times 7\right)$ surface. The surface has an odd number of electrons per surface unit cell and is metallic above 20 K. We observe an energy gap in the ground state of this surface by STS at 0.3 K. After the correction, the measured width of the gap is $70\pm 15\text{meV}$, which is compatible with previous less precise estimates. No sharp peak of the density of states at the Fermi level is observed, in contrast to proposed models for the Si(111)-$\left(7\times 7\right)$ surface.

Spectral moment sum rules are presented for the inhomogeneous many-body problem described by the fermionic Falicov-Kimball or Hubbard models. These local sum rules allow for arbitrary hoppings, site energies, and interactions. They can be employed to quantify the accuracy of numerical solutions to the inhomogeneous many-body problem such as strongly correlated multilayered devices, ultracold atoms in an optical lattice with a trap potential, strongly correlated systems that are disordered, or systems with nontrivial spatial ordering such as a charge-density wave or a spin-density wave. We also show how the spectral moment sum rules determine the asymptotic behavior of the Green function, self-energy, and dynamical mean field when applied to the dynamical mean-field theory solution of the many-body problem. In particular, we illustrate in detail how one can dramatically reduce the number of Matsubara frequencies needed to solve the Falicov-Kimball model while still retaining high precision, and we sketch how one can incorporate these results into Hirsch-Fye quantum Monte Carlo solvers for the Hubbard (or more complicated) models. Since the solution of inhomogeneous problems is significantly more time consuming than periodic systems, efficient use of these sum rules can provide a dramatic speed up in the computational time required to solve the many-body problem. We also discuss how these sum rules behave in nonequilibrium situations as well, where the Hamiltonian has explicit time dependence due to a driving field or due to the time-dependent change in a parameter such as the interaction strength or the origin of the trap potential.

Hall effect of two-dimensional holes that are spin-polarized by a strong parallel magnetic field was explored with a small tilt angle. The Hall resistivity increases nonlinearly with the magnetic field and exhibits negative corrections. The anomalous negative corrections increase with the perpendicular magnetization of the two-dimensional hole system. We attribute this to the anomalous Hall effect of spin-polarized carriers in a nonmagnetic system. The anomalous corrections to the conductivity exhibit nonmonotonic dependence on the magnetic field.

The magnetic properties and the spin dynamics of two molecular magnets have been investigated by magnetization and dc susceptibility measurements, electron paramagnetic resonance and proton nuclear magnetic resonance (NMR) over a wide range of temperatures (1.6–300K) at applied magnetic fields $H=0.5$ and 1.5 T. The two molecular magnets consist of $\phantom{[}{\phantom{\{}{\text{Cu}}^{\text{II}}\left(\text{saldmen}\right)\left({\text{H}}_2\text{O}\right)\}}_6\left\{{\text{Fe}}^{\text{III}}{\left(\text{CN}\right)}_6\right\}]{\left({\text{ClO}}_4\right)}_3\cdot 8{\text{H}}_2\text{O}$ in short ${\text{Cu}}_6\text{Fe}$ (here H saldmen is the Schiff base resulted by reacting salicylaldehyde with N,N-dimethylethylenediamine) and the analog compound with cobalt, ${\text{Cu}}_6\text{Co}$. It is found that in ${\text{Cu}}_6\text{Fe}$, whose magnetic core is constituted by six ${\text{Cu}}^{2+}$ ions and one ${\text{Fe}}^{3+}$ ion all with $s=1/2$, a weak ferromagnetic interaction between ${\text{Cu}}^{2+}$ moments through the central ${\text{Fe}}^{3+}$ ion with $J=0.14\text{K}$ is present, while in ${\text{Cu}}_6\text{Co}$ the ${\text{Co}}^{3+}$ ion is diamagnetic and the weak interaction is antiferromagnetic with $J=-1.12\text{K}$. The NMR spectra show the presence of nonequivalent groups of protons with a measurable contact hyperfine interaction consistent with a small admixture of $s$ wave function with the $d$ function of the magnetic ion. The NMR relaxation results are explained in terms of a single-ion $\left({\text{Cu}}^{2+},{\text{Fe}}^{3+},{\text{Co}}^{3+}\right)$ uncorrelated spin dynamics with an almost temperature-independent correlation time due to the weak magnetic exchange interaction. We conclude that the two molecular magnets studied here behave as single-molecule paramagnets with a very weak intramolecular interaction, almost on the order of the dipolar intermolecular interaction. Thus they represent a separate class of molecular magnets which differ from the single-molecule magnets investigated up to now, where the intramolecular interaction is much larger than the intermolecular one.

The discovery of high-$T_c$ superconductivity in the pnictides, materials with a Fermi surface determined by several bands, highlights the need to understand how superconductivity arises in multiband systems. In this effort, using symmetry considerations and mean-field approximations, we discuss how strong hybridization among orbitals may lead to both intraband and interband pairings, and we present calculations of the spectral functions to guide the experimental search for this kind of state.

We calculate frequency and temperature dependence of the anomalous ac Hall conductivity induced by impurity scattering in a chiral $p_x+i{p}_y$ superconductor, such as ${\text{Sr}}_2{\text{RuO}}_4$, with spontaneous time-reversal-symmetry breaking in the absence of an external magnetic field. We consider two models of disorder, Gaussian and non-Gaussian, characterized by the second and third moments of the random impurity potential, respectively. Within both models, we find that the anomalous Hall conductivity has a finite real value at zero frequency, exhibits singularities at the threshold of photon absorption across the superconducting gap, and decays as some power of the high frequency $\omega$. The Hall conductivity increases linearly with the decrease in temperature below the superconducting transition and saturates at zero temperature. Using our results for the high-frequency Hall conductivity, we estimate the polar Kerr angle for light reflection from the material and compare it with the experimental measurements in ${\text{Sr}}_2{\text{RuO}}_4$ by Xia et al., Phys. Rev. Lett. 97, 167002 (2006).

We report single layer resistivities of two-dimensional electron and hole gases in an electron-hole bilayer with a 10 nm barrier. In a regime where the interlayer interaction is stronger than the intralayer interaction, we find that an insulating state $\left(d\rho /dT<0\right)$ emerges at $T\sim 1.5\text{K}$ or lower, when both the layers are simultaneously present. This happens deep in the “metallic” regime, even in layers with $k_{F}l>500$, thus making conventional mechanisms of localization due to disorder improbable. We suggest that this insulating state may be due to a charge-density-wave phase, as has been expected in electron-hole bilayers from the Singwi-Tosi-Land-Sjölander approximation-based calculations of Liu et al. [Phys. Rev. B 53, 7923 (1996)]. Our results are also in qualitative agreement with recent path-integral Monte Carlo simulations of a two component plasma in the low-temperature regime [Ludwig et al., Contrib. Plasma Phys. 47, 335 (2007)]

Recent experiment [S. I. Dorozhkin et al., Phys. Rev. Lett. 102, 036602 (2009)] on quantum Hall structures with strongly asymmetric contact configuration discovered microwave-induced photocurrent and photovoltage magneto-oscillations in the absence of dc driving. We show that in an irradiated sample the Landau quantization leads to violation of the Einstein relation between the dc conductivity and diffusion coefficient. Then, in the presence of a built-in electric field in a sample, the microwave illumination causes photogalvanic signals which oscillate as a function of magnetic field with the period determined by the ratio of the microwave frequency to the cyclotron frequency, as observed in the experiment.

We study correlated two-level quantum dots, coupled in effective one-channel fashion to metallic leads, with electron interactions including on-level and interlevel Coulomb repulsions, as well as the interorbital Hund’s rule exchange favoring the spin-1 state in the relevant sector of the free dot. For arbitrary dot occupancy, the underlying phases, quantum phase transitions (QPTs), thermodynamics, single-particle dynamics, and electronic transport properties are considered, and direct comparison is made to conductance experiments on lateral quantum dots. Two distinct phases arise generically, one characterized by a normal Fermi liquid fixed point (FP) and the other by an underscreened (USC) spin-1 FP. Associated QPTs, which occur in general in a mixed valent regime of nonintegral dot charge, are found to consist of continuous lines of Kosterlitz-Thouless transitions, separated by first-order level-crossing transitions at high symmetry points. A “Friedel-Luttinger sum rule” is derived and, together with a deduced generalization of Luttinger’s theorem to the USC phase (a singular Fermi liquid), is used to obtain a general result for the $T=0$ zero-bias conductance, expressed solely in terms of the dot occupancy and applicable to both phases. Relatedly, dynamical signatures of the QPT show two broad classes of behavior, corresponding to the collapse of either a Kondo resonance, or antiresonance, as the transition is approached from the Fermi liquid phase, the latter behavior being apparent in experimental differential conductance maps. The problem is studied using the numerical renormalization group method, combined with analytical arguments.

We present a first-principles theoretical approach to calculate the lattice thermal conductivity of diamond based on an exact solution of the Boltzmann transport equation. Density-functional perturbation theory is employed to generate the harmonic and third-order anharmonic interatomic force constants that are required as input. A central feature of this approach is that it provides accurate representations of the interatomic forces and at the same time introduced no adjustable parameters. The calculated lattice thermal conductivities for isotopically enriched and naturally occurring diamond are both in very good agreement with experimental data. The role of the scattering of heat-carrying acoustic phonons by optic branch phonons is also investigated. We show that inclusion of this scattering channel is indispensable in properly describing the thermal conductivity of semiconductors and insulators. The accurate adjustable-parameter-free results obtained herein highlight the promise of this approach in providing predictive descriptions of the lattice thermal conductivity of materials.

We use microprobe angle-resolved photoemission spectroscopy to study the Fermi surface and band dispersion of the ${\text{CuO}}_2$ planes in the high-temperature superconductor, ${\text{YBa}}_2{\text{Cu}}_4{\text{O}}_8$. We find a strong in-plane asymmetry of the electronic structure between directions along $a$ and $b$ axes. The saddle point of the antibonding band lies at a significantly higher energy in the $a$ direction $\left(\pi ,0\right)$ than the $b$ direction $\left(0,\pi \right)$, whereas the bonding band displays the opposite behavior. We demonstrate that the abnormal band shape is due to a strong asymmetry of the bilayer band splitting, likely caused by a nontrivial hybridization between the planes and chains. This asymmetry has an important implication for interpreting key properties of the Y-Ba-Cu-O family, especially the superconducting gap, transport, and results of inelastic neutron scattering.

A user friendly scheme based on the quantum kinetic equation is developed for studying thermal transport phenomena in the presence of interactions and disorder. We demonstrate that this scheme is suitable for both a systematic perturbative calculation as well as a general analysis. We believe that we present an adequate alternative to the Kubo formula, which for the thermal transport is rather cumbersome.