We use a surface-integral method to determine the polarization potential in nitride-based quantum dots (QDs) grown on a nonpolar substrate. There is uncertainty in the literature regarding the sign of the piezoelectric constant $e_{15}$. We find that only a negative $e_{15}$ can give the reduced electrostatic built-in field found experimentally in nonpolar GaN/AlN QDs. Our analysis of nonpolar InN/GaN QDs indicates that a significant built-in field remains in these structures. We calculate that despite the reduced polarization potential, ground-state electrons and holes can remain spatially separated in GaN/AlN QDs.

We present a general approach to understanding the quantum phases and phase transitions of quantum antiferromagnets in two spatial dimensions. We begin with the simplest spin-liquid state, the $Z_2$ spin liquid, whose elementary excitations are spinons and visons, carrying $Z_2$ electric and magnetic charges, respectively. Their dynamics is expressed in terms of a doubled U(1) Chern-Simons theory, which correctly captures the “topological” order of the $Z_2$ spin-liquid state. We show that the same theory also yields a description of the variety of ordered phases obtained when one or more of the elementary excitations condense. Field theories for the transitions and multicritical points between these phases are obtained. We survey experimental results on antiferromagnets on the anisotropic triangular lattice, and make connections between their phase diagrams and our results.

Exact (Hartree-Fock) exchange is needed to overcome some of the limitations of local and semilocal approximations of density-functional theory. So far, however, computational cost has limited the use of exact exchange in plane-wave calculations for extended systems. We show that this difficulty can be overcome by performing a unitary transformation from Bloch to maximally localized Wannier functions in combination with an efficient technique to compute real-space Coulomb integrals. The resulting scheme scales linearly with system size. We validate the scheme with representative applications.

We investigate the effect of disorder at various lattice sites outside the ${\text{CuO}}_2$ plane on the superconducting critical temperature $T_c$ of the cuprate superconductor ${\text{Bi}}_2{\text{Sr}}_2{\text{CaCu}}_2{\text{O}}_{8+\delta }$ (Bi2212). The most effective disorder turns out to be that at the Sr site in the neighboring blocks, which contain apical-oxygen atoms. The Sr-site disorder reduces $T_c$ and also produces a residual component $\left({\rho }_0\right)$ in the in-plane resistivity, the magnitude of which is found to be proportional to the magnitude of $T_c$ reduction. We demonstrate that both the $T_c$-degradation rate and ${\rho }_0$ decrease as the number of the ${\text{CuO}}_2$ planes increases in the unit cell, that is, as the number of the neighboring SrO blocks decreases. In real crystals of Bi-based cuprates, main source of disorder is Bi atoms randomly occupying the Sr sites. We have succeeded in reducing the Bi content at Sr sites as much as possible and achieved $T_c=98\text{K}$, a high-$T_c$ record in Bi2212.

An analytic theory of ballistic electron transport in disordered graphene in “short-and-wide” geometry is developed. We consider a sample of a large width $W$ and analyze the evolution of the conductance, the shot noise, and the full statistics of the charge transfer with increasing length $L$ both at the Dirac point and at a finite gate voltage. The transfer-matrix approach combined with the disorder perturbation theory and the renormalization group is used. We also discuss the crossover to the diffusive regime and construct a “phase diagram” of various transport regimes in graphene.

Polarized Raman scattering has been performed on ${\text{CaC}}_6$ superconductor. We identify two of the three Raman-active $E_g$ phonon modes at 440 and $1508{\text{cm}}^{-1}$ expected for the $R\overline{3}m$ space group of ${\text{CaC}}_6$. These first-order scattering modes appear along with the $D$ and $G$ bands around 1300 and $1600{\text{cm}}^{-1}$ that are similar in origin to the corresponding bands in plain graphite. The intensities of the $D$ and $G$ bands in ${\text{CaC}}_6$ correlate with degree of disorder. The $D$ band arises from the double resonant Raman-scattering process; its frequency shifts as a function of excitation energy with $\sim 35{\text{cm}}^{-1}/\text{eV}$. The double resonant Raman scattering probes phonon excitations with finite wave vector $\mathit{q}$. We estimate the characteristic spacing of structural defects to be on the scale of about $100\text{Å}$ by comparing the intensity of the $D$ band and the $1508{\text{cm}}^{-1}$ $E_g$ mode in ${\text{CaC}}_6$ to calibrated intensity ratio of analogous bands in disordered graphites. A sharp superconducting coherence peak at $24{\text{cm}}^{-1}$ is observed below $T_c$.

The effective magnetoelastic coefficients $B_i^{\text{eff}}$ are measured for epitaxial layers of Fe, Co, and Ni on Ir(100). Mechanical stress during film growth and stress during magnetization processes are measured directly by the optical cantilever curvature technique. Our results indicate that the effective magnetoelastic coupling coefficients $B_1^{\text{eff}}$ and $B_2^{\text{eff}}$ of Fe, Ni, and Co deviate from the respective bulk values, and we propose that strain may play an important role for this nonbulklike magnetoelastic behavior. The impact of the magnetoelastic coupling on the magnetic anisotropy is explored by magneto-optic Kerr effect measurements. The measurements reveal that the easy magnetization axis is in plane for Fe and Co layers on Ir(100). It changes from out of plane to in plane for Ni at about 15 monolayer with increasing film thickness. This spin reorientation transition can be well described by the measured magnetoelastic coupling coefficients. The result of nonbulklike magnetoelastic coupling is discussed in view of recent theoretical work on the calculation of magnetoelastic coupling in strained systems.

The ferroelectric degenerate semiconductor ${\text{Sn}}_{1-\delta }\text{Te}$ exhibits superconductivity with critical temperatures, $T_c$, of up to 0.3 K for hole densities of order ${10}^{21}{\text{cm}}^{-3}$. When doped on the tin site with greater than $x_c=1.7\left(3\right)\%$ indium atoms, however, superconductivity is observed up to 2 K, though the carrier density does not change significantly. We present specific-heat data showing that a stronger pairing interaction is present for $x>x_c$ than for $x<x_c$. By examining the effect of In dopant atoms on both $T_c$ and the temperature of the ferroelectric structural phase transition, $T_{\text{SPT}}$, we show that phonon modes related to this transition are not responsible for this $T_c$ enhancement, and discuss a plausible candidate based on negative $U$ pairing associated with mixed valency on the indium impurity sites.

We present an efficient diagrammatic method to describe nonlocal correlation effects in lattice fermion Hubbard-type models, which is based on a change of variables in the Grassmann path integrals. The new fermions are dual to the original ones and correspond to weakly interacting quasiparticles in the case of strong local correlations in the Hubbard model. The method starts with dynamical mean-field theory as a zeroth-order approximation and includes nonlocal effects in a perturbative way. In contrast to cluster approaches, this method utilizes an exact transition to a dual set of variables. It therefore becomes possible to treat vertices of an effective single-impurity problem as small parameters. This provides a very efficient interpolation between bandlike weak-coupling and atomic limits. The method is illustrated on the two-dimensional Hubbard model. The antiferromagnetic pseudogap, Fermi-arc formations, and non-Fermi-liquid effects due to the Van Hove singularity are correctly reproduced by the lowest-order diagrams. Extremum properties of the dual fermion approach are discussed in terms of the Feynman variational principle.

In this paper, we address ourselves to the nonmagnetic disorder effects onto the quantum critical point, which intervenes the three-dimensional $Z_2$ quantum spin Hall insulator (topological insulator) and an ordinary insulator. The minimal model describing this type of the quantum critical point is the single copy of the $3+1$ Dirac fermion, whose topological mass $m$ induces the phase transition between the topological insulator and an ordinary one. We first derive the phase diagram spanned by the mass term $m$, chemical potential $\mu$, and strength of the disorder within the self-consistent Born approximation. By way of this, we find a finite density of state appears even at zero energy and at the phase-transition point, i.e., $m=\mu =0$, if the strength of the disorder potential exceeds some critical value. To infer the structure of the low-energy effective theory around these zero-energy states, we further calculated the weak-localization correction to the conductivity. To be more specific, we have found that the diffuson is dominated by the charge diffusion mode and parity diffusion mode. While the charge diffusion mode always carries the diffusion pole, the parity diffusion mode becomes massless only at $m=0$, but suffers from the infrared cutoff for nonzero $m$. Corresponding to this feature of the diffuson, the Cooperon is also composed of two quasidegenerate contributions. We found that these two give rise to the same magnitude of the anti-weak-localization (AWL) correction with each other at $m=0$. As a result, when the topological mass $m$ is fine tuned to be zero (but for generic $\mu$), the AWL correction becomes doubled (quantum correction doubling). Based on this observation, we will discuss the possible microscopic picture of the “levitation and pair-annihilation” phenomena, recently discovered by Onoda et al. [Phys. Rev. Lett. 98, 076802 (2007)].

The recently discovered iron arsenide superconductors appear to display a universal set of characteristic features, including proximity to a magnetically ordered state and robustness of the superconductivity in the presence of disorder. Here we show that superconductivity in ${\text{Fe}}_{1+\delta }\text{Se}$, which can be considered the parent compound of the superconducting arsenide family, is destroyed by very small changes in stoichiometry. Further, we show that nonsuperconducting ${\text{Fe}}_{1+\delta }\text{Se}$ is not magnetically ordered down to 5 K. These results suggest that robust superconductivity and immediate instability against an ordered magnetic state should not be considered as intrinsic characteristics of iron-based superconducting systems.

We report a comprehensive study of stochastic electron spin fluctuations—spin noise—in lightly doped ($n$-type) bulk GaAs, which are measured using sensitive optical magnetometry based on off-resonant Faraday rotation. Frequency spectra of electron spin noise are studied as a function of electron density, magnetic field, temperature, probe-laser wavelength and intensity, and interaction volume. Electron spin lifetimes ${\tau }_s$ are inferred from the width of the spin noise spectra and are compared to direct measurements of ${\tau }_s$ using conventional Hanle-effect methods. Both methods reveal a strong and similar dependence of ${\tau }_s$ on the wavelength and intensity of the probe laser, highlighting the undesired influence of sub-bandgap absorption effects on the nominally “nonperturbative” spin noise measurements. As a function of temperature, the spin noise power increases approximately linearly from 1.5 to 30 K, as expected for degenerate electrons obeying Fermi-Dirac statistics, but with an additional zero-temperature offset. Finally, as the cross-sectional area of the probe laser shrinks and fewer electrons are probed, the measured Faraday rotation fluctuations due to electron spin noise are shown to increase, as expected.

Using different experimental techniques combined with density functional based theoretical methods we have explored the formation of interface-stabilized manganese oxide structures grown on Pd(100) at (sub)monolayer coverage. Among the multitude of phases experimentally observed we focus our attention on four structures which can be classified into two distinct regimes, characterized by different building blocks. Two oxygen-rich phases are described in terms of MnO(111)-like O-Mn-O trilayers, whereas the other two have a lower oxygen content and are based on a MnO(100)-like monolayer structure. The excellent agreement between calculated and experimental scanning tunneling microscopy images and vibrational electron-energy-loss spectra allows for a detailed atomic description of the explored models.

We present a procedure for finding the exact solution to the linear-response Boltzmann equation for two-dimensional anisotropic systems and demonstrate it on examples of noncrystalline anisotropic magnetoresistance in a system with spin-orbit interaction. We show that two decoupled integral equations must be solved in order to find the nonequilibrium distribution function up to linear order in the applied electric field. The examples are all based on the Rashba system with charged magnetic scatterers, a system where the nonequilibrium distribution function and anisotropic magnetoresistance can be evaluated analytically. Exact results are compared to earlier widely used approximative approaches. We find circumstances under which approximative approaches may become unreliable even on a qualitative level.

As a part of his studies explaining how an enclosed gas comes to equilibrium, Maxwell [Philos. Trans. R. Soc. Lond. 170, 231 (1879)] proposed what is now known as the “Maxwell assumption,” namely, that when gas particles collide with the walls of their container, a fraction is directly scattered with little change in state, while the remaining fraction becomes trapped at the surface and subsequently desorbs in a distribution at equilibrium with the surface temperature. In this paper a scattering theory is developed, using an iterative algorithm and classical mechanics for the collision process, which describes both direct scattering and trapping-desorption of the incident beam. That portion of an incident beam that is initially trapped in the physisorption potential well can be followed as the trapped atoms continue to make further interactions with the surface until they are all eventually ejected back into the continuum and leave the surface region. Several calculations show that this theory predicts when a system will obey the Maxwell assumption. Additional calculations show that the theory quantitatively explains recent experimental measurements [K. D. Gibson, N. Isa, and S. J. Sibener, J. Chem. Phys. 119, 13083 (2003)] of Ar scattering from a self-assembled monolayer on Ag(111) in which clear signals of both direct scattering and a trapping-desorption fraction are exhibited in the energy-resolved spectra.

We consider the instability of bilayer graphene with respect to a distorted configuration in the same spirit as the model introduced by Su et al. [Phys. Rev. Lett. 42, 1698 (1979)]. By computing the total energy of a distorted bilayer, we conclude that the ground state of the system favors a finite distortion. We explore how the equilibrium configuration changes with carrier density and an applied potential difference between the two layers.

Neutron-diffraction and magnetization measurements have been carried out on a series of samples of the magnetorefrigerant ${\text{Mn}}_{1+y}{\text{Fe}}_{1-y}{\text{P}}_{1-x}{\text{Ge}}_x$. The data reveal that the ferromagnetic and paramagnetic phases correspond to two very distinct crystal structures, with the magnetic-entropy change as a function of magnetic field or temperature being directly controlled by the phase fraction of this first-order transition. By tuning the physical properties of this system we have achieved a magnetic-entropy change [magnetocaloric effect (MCE)] for the composition ${\text{Mn}}_{1.1}{\text{Fe}}_{0.9}{\text{P}}_{0.80}{\text{Ge}}_{0.20}$ that has a similar shape for both increasing and decreasing field, with the maximum MCE exceeding $74\text{J}/\text{kg}\text{K}$—substantially higher than the previous record. The diffraction results also reveal that there is a substantial variation in the Ge content in the samples which causes a distribution of transition temperatures that reduces the MCE. It therefore should be possible to improve the MCE to exceed $100\text{J}/\text{kg}\text{K}$ under optimal conditions.

We investigate transitions between topologically ordered phases in two spatial dimensions induced by the condensation of a bosonic quasiparticle. To this end, we formulate an extension of the theory of symmetry-breaking phase transitions which applies to phases with topological excitations described by quantum groups or modular tensor categories. This enables us to deal with phases whose quasiparticles have noninteger quantum dimensions and obey braid statistics. Many examples of such phases can be constructed from two-dimensional rational conformal field theories, and we find that there is a beautiful connection between quantum group symmetry breaking and certain well-known constructions in conformal field theory, notably the coset construction, the construction of orbifold models, and more general conformal extensions. Besides the general framework, many representative examples are worked out in detail.

We report Hall-effect measurements of epitaxial Fe (001) thin films grown on MgO (001). We have focused on the dependence of the anomalous Hall effect (AHE) in heteroepitaxial structures $\text{MgO}\left(001\right)//\text{Fe}\left(t\right)/\text{MgO}$ with $t=10$, 2.5, 2, 1.8, and 1.3 nm. Our results have been interpreted in terms of a recent unified theory of the AHE. We have demonstrated that the thickness and roughness of the Fe layer are control parameters to tune both the longitudinal conductivity ${\sigma }_{xx}$ and anomalous Hall conductivity ${\sigma }_{xy}$. In this way, we report a crossover from the intrinsic moderately dirty region of conductivities where ${\sigma }_{xy}=\text{const}$ to the dirty region of poorly conducting materials $\left({\sigma }_{xx}<{10}^4\text{S}/\text{cm}\right)$ where we have found that the relation ${\sigma }_{xy}\propto {\sigma }_{xx}^n$ with $n=1.66\left(4\right)$ holds, in good agreement, with the expected universal scaling relationship reported in other ferromagnetic compounds.

We characterize a pair of Cooper-pair boxes coupled with a fixed capacitor using spectroscopy and measurements of the ground-state quantum capacitance. We use the extracted parameters to estimate the concurrence or degree of entanglement between the two qubits. We also present a thorough demonstration of a multiplexed quantum capacitance measurement technique, which is in principle scalable to a large array of superconducting qubits.