J. Wosnitza

kappa-(BEDT-TTF)2Cu[N(CN)(2)]Br: a fully gapped strong-coupling superconductor.

High-resolution specific-heat measurements of the organic superconductor kappa-(BEDT-TTF)(2)-Cu[N(CN)(2)]Br in the superconducting ( B = 0) and normal ( B = 14 T) states show a clearly resolvable anomaly at T(c) = 11.5 K and an electronic contribution, C(es), which can be reasonably well described by strong-coupling BCS theory. Most importantly, C(es) vanishes exponentially in the superconducting state which gives evidence for a fully gapped order parameter.

Magnetic-field- and temperature-dependent Fermi surface of CeBiPt

The half-Heusler compounds CeBiPt and LaBiPt are semimetals with very low charge-carrier concentrations as evidenced by Shubnikov–de Haas (SdH) and Hall-effect measurements. Neutron-scattering results reveal a simple antiferromagnetic structure in CeBiPt below TN = 1.15 K. The band structure of CeBiPt sensitively depends on temperature, magnetic field and stoichiometry. Above a certain, sample-dependent, threshold field (B>25 T), the SdH signal disappears and the Hall coefficient reduces significantly. These effects are absent in the non-4f compound LaBiPt. Electronic-band-structure calculations can well explain the observed behaviour by a 4f-polarization-induced Fermi-surface modification.

Superconductivity of a Crystalline Ga84-Cluster Compound

We report on magnetization and resistivity measurements of a cluster compound in which negatively charged Ga84 entities are strongly bonded to organic ligands and confined in an ionic crystal structure. The macroscopic single crystals have a resistivity in the range of 100 Ωcm at room temperature with a semiconducting-like temperature dependence. They reveal a superconducting transition at Tc≈7.2 K and an upper critical field of Bc2=13.8 T. The presence of the superconducting state in the regularly arranged Ga84 clusters implies an electronic coupling between the individual clusters.

Specific heat and critical fields of the organic superconductor β″–(BEDT-TTF)2SF5CH2CF2SO3

We report on specific-heat, magnetization, and ac-susceptibility measurements of {beta}{sup {double_prime}}-(BEDT-TTF){sub 2}SF{sub 5}CH{sub 2}CF{sub 2}SO{sub 3}, an organic superconductor with T{sub c}=4.5 K, where BEDT-TTF stands for bis(ethylenedithio)-tetrathiafulvalene. The jump of the specific heat C at T{sub c} and the exact form of the specific-heat difference between C in the superconducting (B=0 T) and in the normal (B=3.5 T) state can be well described by BCS theory with strong coupling. We estimate an electron-phonon coupling parameter {lambda}{approx}1.1. From measurements of C in magnetic fields we extract the upper critical field B{sub c2}(T). Low-field dc-magnetization measurements were used to determine the temperature dependence of the lower critical field B{sub c1}. From T{sub c} down to {approximately}3 K the characteristic suppression of B{sub c1}{sup {perpendicular}}(T) below the sensitivity limit is observed. {copyright} {ital 1998} {ital The American Physical Society}

The first observation of dynamical chirality by means of polarized neutron scattering in the triangular-lattice antiferromagnet

The polarization-dependent part of the neutron scattering in the triangular-lattice antiferromagnet is studied. This scattering appears in an external magnetic field and is determined by the projection of a chiral fluctuation on the sample magnetization (the dynamical chirality, DC). It is shown that the DC cross section is an odd function of the energy transfer, , and is proportional to the magnetic field up to 10 kOe above . A Be filter is used instead of the analyser for measuring the DC scattering integrated over . It is shown that below the field dependence of this intensity reveals two features related to the acoustic spin waves and to a new type of low-energy magnetic excitation, presumably of topological nature. The temperature dependence of the acoustic spin waves near is in qualitative agreement with dynamical scaling.

Specific heat of EuxSr1-xO near the ferromagnetic phase transition

High-resolution measurements of the specific heat,C, near the ferromagnetic phase transition of the diluted ferromagnetic system EuxSr1−xO are reported. Samples with four different concentrations (x=1,x=0.9,x=0.7 andx=0.5) have been studied and the expected phase diagram, i.e. the linear decrease ofTc with decreasingx, is confirmed. Our specific-heat data of the pure EuO samples yield a critical exponent α=−0.12±0.02 in contrast to literature results, but in agreement with the value expected theoretically for this three-dimensional Heisenberg system. The origin of the discrepancies is traced down to differences in data analysis.

Fermiology of the organic superconductor β″-(ET)2SF5CH2CF2SO3

We present a detailed Fermi-surface (FS) investigation of the quasi two-dimensional (2D) organic superconductor (T c = 4.5 K) β-(ET) 2 SF 5 CH 2 CF 2 SO 3 . In line with previous investigations, de Haas-van Alphen measurements in pulsed fields up to 60 T show a single oscillation frequency, F 0 = 200 T, which corresponds to a FS size of about 5% of the first Brillouin zone. Angular dependent magnetoresistance oscillations (AMROs) are utilized for the exact determination of the in-plane FS, which is found to be a strongly elongated ellipsoid with an axes ratio of about 1:9. Transport measurements in static fields up to 33 T show an unusual temperature dependence of the Shubnikov-de Haas (SdH) signal, i.e., a decrease of the SdH amplitude with decreasing temperature.

Vortex dynamics of the organic superconductor κ-(ET)2Cu[N(CN)2]Br

Abstract We report on ac-susceptibility and dc-magnetization measurements of the quasi two-dimensional (2D) organic superconductor κ-(ET) 2 Cu[N(CN) 2 ]Br. We determined the temperature, frequency and angular dependence of the irreversibility field B irr for two single-crystalline samples with different superconducting transition temperatures ( T c =11.5 K and T c =11.0 K) and Meissner fractions (50% and 20%, respectively). We find for both samples at T / T c ≈0.75 a crossover from an exponential temperature dependence at low T , B irr ∝exp(− AT / T c ), to a power law close to T c , B irr ∝(1− T / T c ) α with 1.3 α B irr changes from 2D to 3D behavior, respectively. The exponential B irr ( T ) dependence is interpreted as evidence for the breakdown field of the proximity-coupled superconducting ET layers. In the irreversible field–temperature range a logarithmic time dependence of the magnetization is found, and analyzed with a thermally activated flux creep model in order to extract more reliable values for the lower critical field B c1 .

Specific heat of the spin-Peierls compound CuGeO3

The specific heat of the quasi-onedimensional magnetic compound CuGeO3 shows a sharp anomaly at the spin-Peierls transition temperatureTsp. The experimental decrease of the magnetic specific heat belowTsp indicates the presence of a spin gap as observed previously with inelastic neutron scattering. A magnetic field of 6T suppressesTsp only slightly but reduces the spin gap by a factor of two.

Specific Heat near the Ferromagnetic Phase Transition of EuxSr1-xS

The specific heat near the phase transition of the diluted ferromagnetic system EuxSr1-xS has been studied for 0.59 ≤ x ≤ 1. Our data yield that the critical exponent α describing the specific-heat anomaly decreases from the nearly 3d Heisenberg value α = 0.14 ± 0.02 for EuS to α ≈ -0.9 ± 0.2 for samples near the limiting concentration of ferromagnetic ordering xc = 0.51. These results are in every respect consistent with recent theoretical predictions of concentration-dependent effective exponents of disordered magnetic systems.