S. Wanka

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.

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}

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 .

Organic conductors: Fermi-surface and specific-heat studies

Abstract We briefly review some of the recent high-field de Haas–van Alphen (dHvA) results we obtained for the quasi-two-dimensional organic metals and superconductors of the type α-(ET) 2 X , where ET (=BEDT–TTF) is bis(ethylenedithio)-tetrathiafulvalene and X is a monovalent anion. We further report on new dHvA, Shubnikov–de Haas, and specific-heat results for the recently synthesized organic superconductor β″− (ET) 2 SF 5 CH 2 CF 2 SO 3 (T c =4.5 K ) . The Fermi-surface topology, the possible influence of the low dimensionality on the quantum oscillations, and the influence of many-body interactions are discussed. The results indicate the importance of electron–phonon interaction and point towards strong coupling.

Thermodynamic properties of the organic superconductor κL-(ET)2Ag(CF3)4·TCE

Abstract We present specific-heat, magnetization, and ac-susceptibility measurements of the recently discovered organic superconductor κL-(ET)2Ag(CF3)4·TCE, where ET (= BEDT-TTF) stands for bisethylenedithio-tetrathiafulvalene and TCE is the neutral solvent 1,1,2-trichloroethane. We observed a very large anisotropy of the critical field B ∗ extracted from ac-susceptibility data in accord with the enhanced ET-layer separation due to the large anions. The strong frequency dependence of B ∗ reveals the weak pinning forces in the material. Magnetization and specific-heat, C, measurements prove the bulk effect of superconductivity. The linear coefficient of C is γ ≈ 50 mJ/molK2 which is a factor 9 larger than expected from a free-electron picture. Indications for strong coupling are found.

Two-dimensional magnetic quantum oscillations observed in an organic metal

The de Haas-van Alphen (dHvA) signal of the organic superconductor {beta}{double_prime}-(BEDT-TTF){sub 2}SF{sub 5}CH{sub 2}CF{sub 2}SO{sub 3} shows an inverse-sawtooth wave form which proves the existence of an ideal two-dimensional (2D) Fermi surface. The dHvA wave shape can almost perfectly be described by a 2D theory assuming a constant chemical potential. This either implies the existence of the predicted quasi-one-dimensional band with an exceptionally large density of states or the chemical potential may be pinned due to localized states near the Fermi energy.

The specific heat of the two-dimensional organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br

We report on high-resolution specific-heat measurements of the organic superconductor κ-(BEDT-TTF)2Cu[N(CN) 2 ]Br in different magnetic fields up to 14 T where the material is in the normal state. The data can reasonably well be described by strong-coupling BCS theory. In contrast to previous results suggesting nodes of the order parameter, the electronic contribution of the specific heat for B = 0 vanishes exponentially towards low temperatures giving clear evidence for a fully-gapped order parameter.

Interlayer dissipation in magnetic fields for H∥J in κ-(ET)2I3

In this paper, we report transport measurements of the interlayer magnetoresistance with field, H, parallel to the current, J, in the organic superconductor κ-(bis(ethylenedithio)-tetrathiafulvalene (ET))2I3. The isothermal magnetoresistance R(H) displays a peak as a function of field for 1>T/Tc≥0.4. At lower temperatures T/Tc<0.4, R(H) increases monotonically with field. Comparison with several other organic superconductors suggests the interlayer magnetoresistance peak is intrinsic to layered systems. Quantitative analysis shows that the magnetoresistance at small field can be fitted to the stacked Josephson-junction model with the normalized coupling energy proportional to (1−T/Tc)3/2/H.

Dimensional crossover of the vortex dynamics in κ-(ET)2X salts

Abstract We investigated the temperature and angular dependence of the irreversibility field, Birr, of the quasi-two-dimensional organic superconductors κ-(ET)2X with X = I3, Cu(NCS)2, and Cu[N(CN)2]Br by means of ac-susceptibility measurements. We find a crossover from an exponential temperature dependence at low temperatures and high fields, Birr∝ exp(−AT/Tc), to a power law close to Tc, Birr∝ (1−T/Tc)α, with α in the range of 1.3 to 2. Concomitantly, the angular dependence of Birr changes from two-dimensional (2D) to three-dimensional (3D) behavior, respectively. The exponential Birr(T) dependence is interpreted as the influence of the breakdown field of proximitycoupled superconducting ET layers on the pinning mechanism. The power law near Tc is explained by the usual 3D vortex pinning.