Marginal Fermi liquid was originally introduced as a phenomenological description of the cuprates in a part of the metallic doping range which appears to be governed by fluctuations due to a quantum-critical point. An essential result due to the form of the assumed fluctuation spectra is that the large inelastic quasiparticle relaxation rate near the Fermi surface is proportional to the energy measured from the chemical potential, ${\tau }_i^{-1}\propto \epsilon$. We present a microscopic long-wavelength derivation of the hydrodynamic properties in such a situation by an extension of the procedure that Eliashberg used for the derivation of the hydrodynamic properties of a Landau-Fermi liquid. In particular, the density-density and the current-current correlations and the relation between the two are derived, and the connection to microscopic calculations of the frequency dependence of the optical conductivity with an additional Fermi-liquid correction factor shown to follow. The method used here may be necessary, quite generally, for the correct hydrodynamic theory for any problem of quantum-critical fluctuations in fermions.

The observation of canonical spin-glass behavior in the pyrochlore oxide ${\text{Y}}_2{\text{Mo}}_2{\text{O}}_7$ has been a subject of considerable interest as the original structural studies were interpreted in terms of a well-ordered crystallographic model. It is widely held that the stabilization of the spin-glass state requires some level of positional disorder along with frustration. Recent reports from local probe measurements, extended x-ray-absorption fine structure (EXAFS) and ${}^{89}\text{Y}$ NMR, have been interpreted in terms of disorder involving the Mo-Mo distances (EXAFS) and multiple Y sites (NMR). This work reports results from temperature-dependent (15–300 K) neutron diffraction (ND) and neutron pair distribution function studies which can provide from the same data set information on both the average and local structures. The principal findings are that: (1) there is no crystallographic phase transition over the temperature region studied within the resolution of the ND data; (2) the diffraction data are well fitted using a fully ordered model but with large and anisotropic displacement parameters for three of the four atomic sites; (3) the pairwise real-space correlation function $G\left(r\right)$ shows clear evidence that the principal source of disorder is associated with the Y-O1 atom pairs rather than the Mo-Mo pairs, in disagreement with the interpretation of the EXAFS results; (4) fits to the $G\left(r\right)$ improve significantly when anisotropic displacements for all sites are included; (5) inclusion of a split-site position parameter for O1 improves, slightly, both the $G\left(r\right)$ fits and the Rietveld fits to the ND data; and (6) for all models the fits become worse as the temperature decreases and as the fitting range decreases. These results are qualitatively consistent with the ${}^{89}\text{Y}$ NMR observations and perhaps recent muon-spin-relaxation studies. The issue of static versus dynamic disorder is not resolved, definitively. An estimate of the distribution of exchange constants due to the disorder is made using spin-dimer analysis and compared with the Saunders-Chalker model for the generation of spin-glass behavior from “weak” disorder on geometrically frustrated lattices.

We investigated the formation of an AuSi eutectic from a silicon (100) surface cleaned under ultrahigh vacuum conditions and an evaporated 3-nm-thick gold layer using a suite of surface-sensitive x-ray techniques and Auger spectroscopy. The signature of the presence of the eutectic liquid came from its recently discovered surface-ordered state, whose diffraction pattern is confirmed. As expected, the eutectic started to form at its melting temperature $\left(380°\text{C}\right)$, but a small change in the film thickness was detected beforehand. The surface-ordered state was found to disappear at $T=473°\text{C}$ without the appearance of a second phase, but also to persist after resolidification of the liquid. Subsequent electron and x-ray microscopy after the sample was removed from vacuum showed the presence of phase-separated gold in the form of micronsized crystals on the surface.

The pinning force for an arbitrarily oriented individual vortex in a biaxial type-II superconductor is calculated from the collective pinning theory. It is shown that in anisotropic superconductors the critical force at which the vortex starts to move can be smaller than the pinning force since the vortex will move at an angle to the acting force. Explicit expressions for this angle and for the critical force and critical current density are given.

We investigate the dependence of the spin-wave resonance (SWR) spectra of ${\text{Ga}}_{0.95}{\text{Mn}}_{0.05}\text{As}$ thin films on the sample treatment. We find that for the external magnetic field perpendicular to the film plane, the SWR spectrum of the as-grown thin films and the changes upon etching and short-term hydrogenation can be quantitatively explained via a linear gradient in the uniaxial magnetic anisotropy field in growth direction. The model also qualitatively explains the SWR spectra observed for the in-plane easy-axis orientation of the external magnetic field. Furthermore, we observe a change in the effective surface spin pinning of the partially hydrogenated sample, which results from the tail in the hydrogen-diffusion profile. The latter leads to a rapidly changing hole concentration/magnetic anisotropy profile acting as a barrier for the spin-wave excitations. Therefore, short-term hydrogenation constitutes a simple method to efficiently manipulate the surface spin pinning.

We present a quantitative theoretical analysis of electron, hole, and exciton $g$ tensors of vertically coupled InAs/GaAs quantum dot pairs in external electric and magnetic fields. For magnetic fields lying in the growth plane, we predict a giant electrically tunable anisotropy of hole $g$ factors that is introduced by piezoelectric charges. This effect allows bias controlled $g$ factor switching and single-spin manipulations in a static magnetic field. We use a relativistic eight-band $\mathbf{k}\cdot \mathbf{p}$ envelope function method including strain, which accounts for magnetic fields in a gauge-invariant manner. In a regime where the molecular wave functions form bonding and antibonding orbitals and for vertical magnetic fields, our results reproduce experimentally observed resonant enhancements of exciton $g$ factors without any fitting parameters.

Many trial wave functions for fractional quantum Hall states in a single Landau level are given by functions called conformal blocks, taken from some conformal field theory. Also, wave functions for certain paired states of fermions in two dimensions, such as $p_x+i{p}_y$ states, reduce to such a form at long distances. Here we investigate the adiabatic transport of such many-particle trial wave functions using methods from two-dimensional field theory. One context for this is to calculate the statistics of widely separated quasiholes, which has been predicted to be non-Abelian in a variety of cases. The Berry phase or matrix (holonomy) resulting from adiabatic transport around a closed loop in parameter space is the same as the effect of analytic continuation around the same loop with the particle coordinates held fixed (monodromy), provided the trial functions are orthonormal and holomorphic in the parameters so that the Berry vector potential (or connection) vanishes. We show that this is the case (up to a simple area term) for paired states (including the Moore-Read quantum Hall state) and present general conditions for it to hold for other trial states (such as the Read-Rezayi series). We argue that trial states based on a nonunitary conformal field theory do not describe a gapped topological phase, at least in many cases. By considering adiabatic variation in the aspect ratio of the torus, we calculate the Hall viscosity, a nondissipative viscosity coefficient analogous to Hall conductivity, for paired states, Laughlin states, and more general quantum Hall states. Hall viscosity is an invariant within a topological phase and is generally proportional to the “conformal spin density” in the ground state.

We present time-resolved charge-sensing measurements on a GaAs double quantum dot with two proximal quantum point-contact (QPC) detectors. The QPC currents are analyzed with cross-correlation techniques, which enable us to measure dot charging and discharging rates for significantly smaller signal-to-noise ratios than required for charge detection with a single QPC. This allows us to reduce the current level in the detector and therefore the invasiveness of the detection process and may help to increase the available measurement bandwidth in noise-limited setups.

Recently it has been predicted that materials with exceptionally strong optical activity may also possess a negative refractive index, allowing the realization of superlenses for super-resolution imaging and data storage applications. Here we demonstrate experimentally and numerically that a chirality-induced negative index of refraction is possible. A negative index of refraction due to three-dimensional chirality is demonstrated for a bilayered metamaterial based on pairs of mutually twisted planar metal patterns in parallel planes, which also shows negative electric and magnetic responses and exceptionally strong optical activity and circular dichroism. Multilayered forms of the metamaterial are found to be suitable for use as ultrathin polarization rotators and circular polarizers for practical applications.

We present here an experimental study of Wigner islands formed by electrons floating over helium. Electrons are trapped electrostatically in a mesoscopic structure covered with a helium film, behaving as a quantum dot in the near-classical limit. By removing electrons one by one, we are able to find the addition spectrum, i.e., the energy required to add (or extract) one electron from the trap with occupation number $N$. Experimental addition spectra are compared with Monte Carlo simulations for the actual trap geometry, confirming the ordered state of electrons over helium in the island.

It is widely believed that in metals, unlike in the dilute magnetic semiconductors, the control of ferromagnetic ordering by external voltage is hardly achievable. We compare the two types of ferromagnets and show that there is no obvious reason why metals are less preferable for this phenomenon. A similar effect in metals, however, has a different physical picture and should be identified as a voltage-induced surface transition. We study its properties within the theory of the surface critical phenomena and discuss possible difficulties on the way to its experimental realization.

The models constructed by Affleck, Kennedy, Lieb, and Tasaki (AKLT) [Phys. Rev. Lett. 59, 799 (1987)] describe a family of quantum antiferromagnets on arbitrary lattices, where the local spin $S$ is an integer multiple $M$ of half the lattice coordination number. The equal-time quantum correlations in their ground states may be computed as finite temperature correlations of a classical $O\left(3\right)$ model on the same lattice, where the temperature is given by $T=1/M$. In dimensions $d=1$ and 2 this mapping implies that all AKLT states are quantum disordered. We consider AKLT states in $d=3$ where the nature of the AKLT states is now a question of detail depending upon the choice of lattice and spin; for sufficiently large $S$ some form of Néel order is almost inevitable. On the unfrustrated cubic lattice, we find that all AKLT states are ordered, while for the unfrustrated diamond lattice the minimal $S=2$ state is disordered while all other states are ordered. On the frustrated pyrochlore lattice, we find (conservatively) that several states starting with the minimal $S=3$ state are disordered. The disordered AKLT models we report here are a significant addition to the catalog of magnetic Hamiltonians in $d=3$ with ground states known to lack order on account of strong quantum fluctuations.

The electron-spin-resonance (ESR) linewidth for localized moments is studied within the framework of the Kondo impurity and Kondo lattice models. An ESR signal for an impurity can only be observed if the Kondo temperature is sufficiently small. For the Kondo lattice, short-range ferromagnetic correlations between the localized spins are necessary to obtain an observable signal. These results are very similar to those derived by Abrahams and Wölfle [Phys. Rev. B 78, 104423 (2008)] for the Anderson impurity and Anderson lattice models. It is concluded that from ESR data alone it is not possible to distinguish if the resonance is due to localized spins or conducting heavy electron spins.

Optical gain and stimulated emission processes in Si nanostructures are controlled by the dynamics of high-density carriers. Here, we report photoluminescence (PL) dynamics and multiexciton recombination in ${\text{Si}}_{1-x}{\text{Ge}}_x/\text{Si}$ superlattices (SLs) under high-density excitation. Saturation of the PL intensity and rapid PL decay are observed as the excitation laser intensity is increased. These phenomena occur due to nonradiative Auger recombination of the electron-hole pairs. We find that the Auger process in ${\text{Si}}_{1-x}{\text{Ge}}_x/\text{Si}$ SLs is less pronounced than that in the ${\text{Si}}_{1-x}{\text{Ge}}_x/\text{Si}$ single quantum wells. Our results show that coupled nanostructures have an advantage in efficient light emission and the control of many-body carrier dynamics.

We develop a strong-disorder renormalization group to study quantum phase transitions with continuous $O\left(N\right)$ symmetry order parameters under the influence of both quenched disorder and dissipation. For Ohmic dissipation, as realized in Hertz’s theory of the itinerant antiferromagnetic transition or in the superconductor-metal transition in nanowires, we find the transition to be governed by an exotic infinite-randomness fixed point in the same universality class as the (dissipationless) random transverse-field Ising model. We determine the critical behavior and calculate key observables at the transition and in the associated quantum Griffiths phase. We also briefly discuss the cases of super-Ohmic and sub-Ohmic dissipations.

We demonstrate the imaging capabilities of energy-filtered transmission electron microscopy at high-energy resolution in the low-energy-loss region, reporting the direct image of a surface plasmon of an elongated gold nanoparticle at energies around 1 eV. Using complimentary model calculations performed within the boundary element method approach we can assign the observed results to the plasmon eigenmodes of the metallic nanoparticle.

Microphotoluminescence spectroscopy at variable temperature, excitation intensity, and energy was performed on a single InAs/AlAs self-assembled quantum dot. The exciton emission line [zero-phonon line (ZPL)] exhibits a broad sideband due to exciton acoustic-phonon coupling by the deformation-potential mechanism. Additionally, narrow low-energy sidebands at about 0.25 meV of the ZPL are attributed to exciton acoustic-phonon piezoelectric coupling. In lowering the excitation energy or intensity these bands gradually dominate the emission spectrum of the quantum dot while the ZPL disappears. At high excitation intensity the sidebands due to piezoelectric coupling decrease strongly and the ZPL dominates the spectrum as a consequence of screening of the piezoelectric coupling by the photocreated free carriers.

The inelastic scattering of neutrons by nanoparticles due to acoustic vibrational modes (energy below 10 meV) confined in nanoparticles is calculated using the Zemach-Glauber formalism. Such vibrational modes are commonly observed by light-scattering techniques (Brillouin or low-frequency Raman scattering). We also report high-resolution inelastic neutron-scattering measurements for anatase ${\text{TiO}}_2$ nanoparticles in a loose powder. Factors enabling the observation of such vibrations are discussed. These include a narrow nanoparticle size distribution which minimizes inhomogeneous broadening of the spectrum and the presence of hydrogen atoms oscillating with the nanoparticle surfaces which enhances the number of scattered neutrons.

A generalized solar cell model for excitonic and classical bipolar solar cells describes the combined transport and interaction of electrons, holes, and excitons in accordance with the principle of detailed balance. Conventional inorganic solar cells, single-phase organic solar cells and bulk heterojunction solar cells, i.e., nanoscale mixtures of two organic materials, are special cases of this model. For high mobilities, the compatibility with the principle of detailed balance ensures that our model reproduces the Shockley-Queisser limit irrespective of how the energy transport is achieved. For less ideal devices distinct differences become visible between devices that are described by linear differential equations and those with nonlinear effects, such as a voltage-dependent collection in bipolar $p\text{−}i\text{−}n$-type devices. These differences in current-voltage characteristics are also decisive for the validity of the reciprocity theorem between photovoltaic quantum efficiency and electroluminescent emission. Finally, we discuss the effect of band offset at the heterointerface in a bulk heterojunction cell and the effect of the average distances between these heterointerfaces on the performance of a solar cell in order to show how our detailed balance model includes also these empirically important quantities.

We theoretically investigate the possibility of excitonic condensation in a system of two graphene monolayers separated by an insulator in which electrons and holes in the layers are induced by external gates. In contrast to the recent studies of this system, we take into account the screening of the interlayer Coulomb interaction by the carriers in the layers, and this drastically changes the result. Due to a large number of electron species in the system (two projections of spin, two valleys, and two layers) and to the suppression of backscattering in graphene, the maximum possible strength of the screened Coulomb interaction appears to be quite small, making the weak-coupling treatment applicable. We calculate the mean-field transition temperature for a clean system and demonstrate that its highest possible value $T_c^{\text{max}}\sim {10}^{-7}{ϵ}_F\lesssim 1\text{mK}$ is extremely small (${ϵ}_F$ is the Fermi energy). In addition, any sufficiently short-range disorder with the scattering time $\tau \lesssim \hslash /T_c^{\text{max}}$ would suppress the condensate completely. Our findings render experimental observation of excitonic condensation in the above setup improbable even at very low temperatures.