Space Charge Effects in Ferroelectric Thin Films

The effects of space charges on hysteresis loops and field distributions in ferroelectrics have been investigated numerically using the phenomenological Landau-Ginzburg-Devonshire theory. Cases with the ferroelectric fully and partially depleted have been considered. In general, increasing the number of charged impurities results in a lowering of the polarization and coercive field values. Squarer loops were observed in the partially depleted cases and a method was proposed to identify fully depleted samples experimentally from dielectric and polarization measurements alone. Unusual field distributions found for higher dopant concentrations have some interesting implications for leakage mechanisms and limit the range of validity of usual semiconductor equations for carrier transport.Comment: 23 pages, 11 figure

Multi-scale simulations of black hole accretion in barred galaxies: Self-gravitating disk models

Due to the non-axisymmetric potential of the central bar, barred spiral galaxies form, in addition to their characteristic arms and bar, a variety of structures within the thin gas disk, like nuclear rings, inner spirals and dust-lanes. These structures in the inner kiloparsec are most important to explain and understand the rate of black hole feeding. The aim of this work is to investigate the influence of stellar bars in spiral galaxies on the thin self-gravitating gas disk. We focus on the accretion of gas onto the central supermassive black hole and its time-dependent evolution. We conduct multi-scale simulations simultaneously resolving the galactic disk and the accretion disk around the central black-hole. We vary in all simulations the initial gas disk mass. As additional parameter we choose either the gas temperature for isothermal simulations or the cooling timescale in case of non-isothermal simulations. Accretion is either driven by a gravitationally unstable or clumpy accretion disk or by energy dissipation in strong shocks. Most simulations show a strong dependence of the accretion rate at the outer boundary of the central accretion disk ($r< 300~\mathrm{pc}$) on the gas flow at kiloparsec scales. The final black hole masses reach up to $\sim 10^9 M_\odot$ after $1.6~\mathrm{Gyr}$. Our models show the expected influence of the Eddington limit and a decline in growth rate at the corresponding sub-Eddington limit

Spin conductivity in almost integrable spin chains

The spin conductivity in the integrable spin-1/2 XXZ-chain is known to be infinite at finite temperatures T for anisotropies -1 < Delta < 1. Perturbations which break integrability, e.g. a next-nearest neighbor coupling J', render the conductivity finite. We construct numerically a non-local conserved operator J_parallel which is responsible for the finite spin Drude weight of the integrable model and calculate its decay rate for small J'. This allows us to obtain a lower bound for the spin conductivity sigma_s >= c(T) / J'^2, where c(T) is finite for J' to 0. We discuss the implication of our result for the general question how non-local conservation laws affect transport properties.Comment: 6 pages, 5 figure

Electrical Characterization of PbZr0.4Ti0.6O3 Capacitors

We have conducted a careful study of current-voltage (I-V) characteristics in fully integrated commercial PbZr0.4Ti0.6O3 thin film capacitors with Pt bottom and Ir/IrO2 top electrodes. Highly reproducible steady state I-V were obtained at various temperatures over two decades in voltage from current-time data and analyzed in terms of several common transport models including space charge limited conduction, Schottky thermionic emission under full and partial depletion and Poole-Frenkel conduction, showing that the later is the most plausible leakage mechanism in these high quality films. In addition, ferroelectric hysteresis loops and capacitance-voltage data were obtained over a large range of temperatures and discussed in terms of a modified Landau-Ginzburg-Devonshire theory accounting for space charge effects.Comment: 17 pages, 7 figure

Lower bounds for the conductivities of correlated quantum systems

We show how one can obtain a lower bound for the electrical, spin or heat conductivity of correlated quantum systems described by Hamiltonians of the form H = H0 + g H1. Here H0 is an interacting Hamiltonian characterized by conservation laws which lead to an infinite conductivity for g=0. The small perturbation g H1, however, renders the conductivity finite at finite temperatures. For example, H0 could be a continuum field theory, where momentum is conserved, or an integrable one-dimensional model while H1 might describe the effects of weak disorder. In the limit g to 0, we derive lower bounds for the relevant conductivities and show how they can be improved systematically using the memory matrix formalism. Furthermore, we discuss various applications and investigate under what conditions our lower bound may become exact.Comment: Title changed; 9 pages, 2 figure

Spectra and positions of galactic gamma-ray sources

The UCSD/MIT Hard X-Ray and Low Energy Gamma-Ray Experiment aboard HEAO-1 scanned the galactic center region during three epochs in 1977 and 1978 from 13 to 180 keV. The results are presented from the scanning epoch of 1978 September. Twenty-two known 2 to 10 keV source positions were necessary for an acceptable fit to the data. The spectra of the 16 strongest, least confused sources are all consistent with power laws with photon spectral indices ranging from 2.1 to 7.2. Acceptable fits to thermal bremsstrahlung models are also possible for most sources. No one source in this survey can be extrapolated to higher energy to match the intensity of the gamma-ray continuum as measured by HEAO-1 large field of view detectors, which implies that the continuum is a composite of contributions from a number of sources

11B^{11}B NMR and Relaxation in MgB2MgB_2 Superconductor

$^{11}B$ NMR and nuclear spin-lattice relaxation rate (NSLR) are reported at 7.2 Tesla and 1.4 Tesla in powder samples of the intermetallic compound $MgB_2$ with superconducting transition temperature in zero field $T_c$ = 39.2 K. From the first order quadrupole perturbed NMR specrum a quadrupole coupling frequency of 835 $\pm$ 5 kHz is obtained. The Knight shift is very small and it decreases to zero in the superconducting phase. The NSLR follows a linear law with $T_1T$ = 165 $\pm$ 10 (sec K) . The results in the normal phase indicate a negligible $s$-character of the wave function of the conduction electrons at the Fermi level. Below $T_c$ the NSLR is strongly field dependent indicating the presence of an important contribution related to the density and the thermal motion of flux lines. No coherence peak is observed at the lower field investigated (1.4 T)

Sub-wavelength focusing of high intensities in microfibre tips

Sub-wavelength efficient intensity confinement has been demonstrated in nanostructured optical microfibre tips. Focus Ion Beam (FIB) milling was used to nanostructure gold-coated optical microfibre tips and form apertures at the apex. Simulations were carried out to optimize the device design. Enhanced transmission efficiency (higher than 10-2) was achieved in spot sizes of ~λ/10. Nanostructured microfibre tips have the potential for a number of applications including optical recording, photolithography and scanning near-field optical microscopy (SNOM)

Quantitative measurements of the thermopower of Andreev interferometers

Using a new second derivative technique and thermometers which enable us to determine the local electron temperature in a mesoscopic metallic sample, we have obtained quantitative measurements of the low temperature field and temperature dependent thermopower of Andreev interferometers. As in previous experiments, the thermopower is found to oscillate as a function of magnetic field. The temperature dependence of the thermopower is nonmonotonic, with a minimum at a temperature of $\simeq0.5$ K. These results are discussed from the perspective of Andreev reflection at the normal-metal/superconductor interface.Comment: 6 pages, 4 figure