C. S. Jacobsen

Infrared properties of the ambient pressure organic superconductor (BEDT-TTF)2I3

Abstract The infrared principal axes of superconducting β-(BEDT-TTF)2I3 are found to be directed along the chain axis and perpendicular to the axis in the a-b plane. At low temperatures well-defined plasma edges appear in both directions. Adopting a simplified 2-D orthorhombic band structure model, effective transfer integrals are found to be  = 0.10 eV and t perp; = 0.13 eV . A cross-over from semiconductor-like optical properties at 300 K to metallic-like optical properties at 40 K is observed in both directions. This is the first such observation in any organic conductor.

Spectroscopic signatures of spin-charge separation in the quasi-one-dimensional organic conductor TTF-TCNQ.

The electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ is studied by angle-resolved photoelectron spectroscopy (ARPES). The experimental spectra reveal significant discrepancies to band theory. We demonstrate that the measured dispersions can be consistently mapped onto the one-dimensional Hubbard model at finite doping. This interpretation is further supported by a remarkable transfer of spectral weight as a function of temperature. The ARPES data thus show spectroscopic signatures of spin-charge separation on an energy scale of the conduction bandwidth.

Crystal structure, magnetic susceptibility and thermopower of superconducting and non-superconducting Nd1.85Ce0.15CuO4+y

Abstract An experimental study of superconducting and non-superconducting Nd1.85Ce0.15CuO4+y, including structure determination by neutron powder diffraction, recording of oxygen changes by gas volumetry, and susceptibility and thermoelectric measurements, is reported. Difference neutron diffraction patterns from samples prepared on-line at the spectrometer show that the structures of superconducting and non-superconducting samples are identical within the limits set by the statistical errors of our data. Simultaneous gas volumetric measurements reveal that Δy

Spin injection between epitaxial Co2.4Mn1.6Ga and an InGaAs quantum well

Electrical spin injection in a narrow [100] In0.2Ga0.8As quantum well in a GaAs p‐i‐n optical device is reported. The quantum well is located 300nm from an AlGaAs Schottky barrier and this system is used to compare the efficiencies and temperature dependences of spin injection from Fe and the Heusler alloy Co2.4Mn1.6Ga grown by molecular-beam epitaxy. At 5K, the injected electron spin polarizations for Fe and Co2.4Mn1.6Ga injectors are 31% and 13%, respectively. Optical detection is carried out in the oblique Hanle geometry. A dynamic nuclear polarization effect below 10K enhances the magnetic field seen by the injected spins in both devices. The Co2.4Mn1.6Ga thin films are found to have a transport spin polarization of ∼50% by point contact Andreev reflection conductivity measurements.

Trends in the infrared and near infrared properties of organic conductors

It is demonstrated that the infrared spectrum of a typical organic linear chain conductor can be understood as follows. The near infrared shows typical plasmon behaviour, and it appears that the plasma frequency is related to the crystal structure and one-electron parameters as in ordinary metals. Thus band structure parameters can be derived and the infrared oscillator strength for the corresponding non-interacting electron gas can be predicted. In all cases the actual measured oscillator strength is significantly smaller. The information contained in this deviation can be quantified by means of a sum rule, which relates oscillators strength and electron kinetic energy. The latter is a measure of the Coulomb driven electron localisation.

A new organic low temperature conductor: HMTSF-TNAP

Abstract Conductivity and optical data on a new organic, conducting charge transfer salt Δ2, 2′-Bi-(4,5-trimethylene-1,3-diselenole) 11,11′,12,12′-tetracyano-2, 6-napthoquinodimethane (HMTSF-TNAP) are given. σ(300 K )= 2400 ± 600 Ω -1 cm -1 . A maximum in σ(T) is found at TM = 47 K with σ(TM)/σ(300 K) = 6.0 ± 10%, and σ(1.5 K ) > 250 Ω -1 . σ(T) is well defined in the high temperature region, but is sample dependent at low temperatures. The optical data indicate a bandwidth and carrier density comparable to that of HMTSF-TCNQ.

The geometry and structural properties of the 4,8,12-trioxa-4,8,12,12c-tetrahydrodibenzo[cd,­mn]pyrene system in the cationic state. Structures of a planar organic cation with various monovalent and divalent anions

The geometry of the 4,8,12-trioxa-4,8,12,12c-tetrahydrodibenzo[cd,mn]pyrene system in the cationic state was established by X-ray structural resolution of the salts formed between the cation and various anions. The geometry was found to be planar for the 4,8,12-trioxa-4,8,12,12c-tetrahydrodibenzo[cd,mn]pyrenylium and 2,6,10-tri(tert-butyl)-4,8,12-trioxa-4,8,12,12c-tetrahydrodibenzo[cd,mn]pyrenylium cations with the monovalent anions I(-), BF(4)(-), PF(6)(-), AsF(6)(-), HNO(3).NO(3)(-) and CF(3)SO(3)(-), and the divalent anions S(2)O(6)(2-) and Mo(6)Cl(14)(2-). The salts were found to crystallize in distinct space groups following a characteristic pattern. Mixed cation-anion stacking resulted in space groups with high symmetry: Pbca in three cases and R3;c in one; a temperature study of the latter was made at ten different temperatures. The formation of dimers of anions and cations resulted in lower-symmetry space groups, mainly monoclinic (P2(1)/n, P2(1)/c and C2/c), but also P1;.

Characterisation of the transport properties of conducting polypyrrole films

Abstract The conducting polypyrrole film, prepared electrochemically in aqueous solution has a dense, homogenous non-crystalline structure. Temperature dependent studies of thermopower and of DC and 35GHz conductivities have been performed on a group of such films. In order to interpret the results, a model is suggested by extending the Motts Variable Range Hopping (VRH) model to a system of mobile localized states. In this model a parameter l is introduced, which stands for the mean distance that a mobile localized state carrying an electron can be displaced before the electron hops away. In the case of conducting polypyrrole these states are referred to those localized on polarons and bipolarons. This extended VRH model reproduces the measured DC-conductivity and thermopower, and the fitted value for l is of the order of several pyrrole units. The finite value of l indicates the that the polaron and bipolaron defects in the polypyrrole films are mobile and they can move along the polymer chain as charge carriers.

Size-dependent effects in exchange-biased planar Hall effect sensor crosses

Exchange-biased planar Hall effect magnetic field sensor crosses with arm width w have been studied as function of w. For large values of w, the magnetic behavior is hysteresis-free and follows the single domain Stoner–Wohlfarth model. When w is decreased, hysteresis is observed in the sensor response. For intermediate values of w, the magnetization reversal takes place in two steps, and for small values of w, the magnetization reversal takes place in a single step. Based on electrical measurements, magnetic force microscopy, and micromagnetic simulations, the observations are explained by an increasing magnetic shape anisotropy of the arms of the cross. We propose a simple analytical model that captures the essential physics of the observations and parameterizes the effects of the cross-shape on the central part of the cross.

Magnetic properties of Fe1−xMnx/Fe nanocomposites

We have prepared nanocomposites of mixtures of ferromagnetic α-Fe and antiferromagnetic γ-Fe50Mn50 nanoparticles, and studied their magnetic and structural properties by magnetization measurements, Mossbauer spectroscopy, and x-ray diffraction. A sample consisting of a 1:1 mixture of the two materials showed enhanced coercivity, but almost negligible exchange bias at room temperature after field cooling from 520 K. However, samples with higher content of γ-Fe50Mn50 showed significant exchange bias. The mechanisms for exchange bias and enhanced coercivity in the system are discussed.