31 research outputs found
Characterisation of an n-type segmented BEGe detector
A four-fold segmented n-type point-contact "Broad Energy" high-purity
germanium detector, SegBEGe, has been characterised at the Max-Planck-Institut
f\"ur Physik in Munich. The main characteristics of the detector are described
and first measurements concerning the detector properties are presented. The
possibility to use mirror pulses to determine source positions is discussed as
well as charge losses observed close to the core contact
Magnetoconductance anisotropy of a polymer thin film at the onset of metallicity
Thin films of poly(2,5-bis(3-dodecyl-2-yl)-thieno[3,2-b] thiophene) (C12-PBTTT) polymer under electrolyte gating and doping are investigated as model systems for organic thin films devices approaching the metallic side of a metal-insulator (M-I) transition. For the most doped samples, with an estimated density reaching 8 x 10(20) cm(-3) holes and a conductivity exceeding 1000 S cm(-1), a positive high-field magnetoconductance is found in a limited temperature range window and only when the field is perpendicular to the sample plane. This signature of weak localization, combined with indications of finite zero-temperature conductivity, allows us to identify delocalized metallic-like transport in these thin films, even though the conductivity decreases when cooling down the samples
Structure optimization effects on the electronic properties of BiSrCaCuO
We present detailed first-principles calculations for the normal state
electronic properties of the high T superconductor
BiSrCaCuO, by means of the linearized augmented plane wave
(LAPW) method within the framework of density functional theory (DFT). As a
first step, the body centered tetragonal (BCT) cell has been adopted, and
optimized regarding its volume, ratio and internal atomic positions by
total energy and force minimizations. The full optimization of the BCT cell
leads to small but visible changes in the topology of the Fermi surface,
rounding the shape of CuO barrels, and causing both the BiO bands,
responsible for the pockets near the \textit{\=M} 2D symmetry point, to dip
below the Fermi level. We have then studied the influence of the distortions in
the BiO plane observed in nature by means of a
orthorhombic cell (AD-ORTH) with space group. Contrary to what has been
observed for the Bi-2201 compound, we find that for Bi-2212 the distortion does
not sensibly shift the BiO bands which retain their metallic character. As a
severe test for the considered structures we present Raman-active phonon
frequencies () and eigenvectors calculated within the frozen-phonon
approximation. Focussing on the totally symmetric A modes, we observe
that for a reliable attribution of the peaks observed in Raman experiments,
both - and a-axis vibrations must be taken into account, the latter being
activated by the in-plane orthorhombic distortion.Comment: 22 pages, 4 figure
Magnetoconductance anisotropy of a polymer thin film at the onset of metallicity
Thin films of poly(2,5-bis(3-dodecyl-2-yl)-thieno[3,2-b] thiophene) (C12-PBTTT) polymer under electrolyte gating and doping are investigated as model systems for organic thin films devices approaching the metallic side of a metal-insulator (M-I) transition. For the most doped samples, with an estimated density reaching 8 x 10(20) cm(-3) holes and a conductivity exceeding 1000 S cm(-1), a positive high-field magnetoconductance is found in a limited temperature range window and only when the field is perpendicular to the sample plane. This signature of weak localization, combined with indications of finite zero-temperature conductivity, allows us to identify delocalized metallic-like transport in these thin films, even though the conductivity decreases when cooling down the samples. (C) 2015 AIP Publishing LLC
Hole crystallization in the spin ladder of Sr14Cu24O41
One of the deepest questions in condensed matter physics concerns what other
phases compete with superconductivity in high-transition-temperature (high-Tc)
superconductors. One candidate is the "stripe" phase, in which the carriers
(holes) condense into rivers of charge separating regions of
antiferromagnetism. A related but lesser known system is the "spin ladder",
which consists of two coupled chains of magnetic ions forming an array of
rungs. A doped ladder can be thought of as a high-Tc material with lower
dimensionality, and has been predicted to exhibit both superconductivity and an
insulating "hole crystal" phase in which the carriers are localised through
many-body interactions. The competition between the two resembles that between
static stripes and superconductivity in high-Tc materials. Here we report
evidence, from resonant x-ray scattering, for the existence of a hole crystal
in the doped spin ladder of Sr14Cu24O41. This phase exists without a detectable
distortion in the structural lattice, indicating it arises from many-body
effects. Our measurements confirm theoretical predictions and support the
picture that proximity to charge ordered states is a general property of
superconductivity in copper-oxides.Comment: 10 pages, 4 figure
Low-frequency excitations in incommensurate biphenyl as studied by very low-temperature specific heat
We present the first specific-heat data of biphenyl in the
temperature range from 70Â mK to 7.3Â K. The extra phononic contribution is
principally attributed to the propagating phason branch in agreement with
neutron data. From these measurements, one can estimate a relatively large
weight of this branch confirming its anisotropic dispersion with weak
velocities. The shifting of the modulation boundaries in the metastable
incommensurate phase III can generate low-energy excitation contribution.
Contrary to the case of disordered crystals, no aging effects in the energy
relaxation and quasi-linear T term were detected
35 Cl NQR and 2H NMR analysis of the low-frequency dynamics close to the displacive normal-incommensurate phase transition in an organic crystal
Nuclear magnetic resonance (NMR) experiments, 35Cl nuclear quadrupole resonance (NQR) and 2H NMR, have been performed close to the displacive normal-incommensurate phase transition in the organic crystal of bis-(4-chlorophenyl)-sulfone. Calculations using coherent neutron scattering results show that the soft-mode contribution cannot explain the rapid increase of the spin-lattice relaxation rates close to the transition temperature. Calculations of the spectral densities taking into account the existence of a central-peak phenomenon describe both 35Cl NQR and 2H NMR spin-lattice relaxation rates on approaching the phase transition. In this way, the width of the central peak can be estimated to be in the range of several gigahert