DYNAMIC DISORDER AT INTERMEDIATE TEMPERATURE AND ITS EFFECT ON THE MAGNETIC PROPERTIES OF THE ORGANIC SUPERCONDUCTOR κ-(BEDTTTF)2Cu[N(CN)2]Br

The k-(BEDT-TTF)X superconducting salts, [where BEDT-TTF is bis(ethylenedithio)-tetrathiafulvalene, abbreviated as ET, and X is a monovalent anion like Cu[N(CN) ]Br 2 , and for either Cu[N(CN) ]Cl 2 or ( )2 Cu NCN and so on], exhibit interestinmagnetic and superconducting phase transitions [1]. They are quasi-two-dimensional and the interplane coupling is very weak. The basic structural unit is a dimer consisting of two BEDTTTF molecules stacked on top of one another. This layered structure leads to highly anisotropic electronic properties. These organic superconductors have similar characteristic superconducting properties including the intrinsic Josephson Effect and the mixed-state properties. This similarity suggests the existence of the vortex phase transition in the organic layered superconductors as observed in HTSC. Because the temperature scale is much lower in organic materials, the thermal fluctuation is expected to be small compared to HTSC. Thus, the comparison between the high-Tc and organic superconductors can give important clues as to the nature of vortex phase transitions. Besides these anomalies around 50 K, unusual time dependencies in magnetic and transport properties have been reported for both deuterated and hydrogenated k-Br near 80 K. For - H - Br 8 k , the superconducting properties have been found to depend on the thermal history, in particular on how fast the sample had been cooled through 80 K. As mentioned above, the ground state of - D - Br 8 k is strongly sample-dependent: both superconducting as well as non-superconducting crystals are found. Furthermore, superconducting as well as insulating (possibly antiferromagnetic) phases in separated volume parts of the same sample have been reported. Their relative volume fraction was found to depend on the cooling rate Vc employed at around 80 K[2-3-4]: in fast cooled samples, a strong decrease of the diamagnetic signal has been observed, which has been interpreted as indicating a suppression of the superconducting in favour of the magnetic phase

Vortex phase transition and superconducting properties of organic quasi-two-dimensional k-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br

International audienceWe report investigations of the low temperature dc susceptibility and the magnetization on the layered organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br near 80K and the effect of disorder on the superconducting transition temperature Tc. The shielding effect (S) and the critical current density Jc were studied (with H parallel to the c axis of the crystal). Jc can be estimated by analysis of magnetic hysteresis measurement using the Bean model. For each temperature value, we observed two regimes in the critical current density Jc(H). This result implies that there exists a first-order phase transition in the vortex system in this organic superconductor. Our results show that the magnetic properties of these compounds depend strongly on the cooling rate. The structural transformation which occurs at the vicinity of 80K very strongly influences the physics of vortex lattice and the associated magnetic behavior

Disorder effect and the vortex phase transition in layered organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br

We report systematic detailed measurement of the superconducting properties on the layered organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br near 80 K and the effect of disorder on the superconducting transition temperature Tc. The reversible part of the magnetization (reversible magnetization) and the magnetic entropy change (ΔS) were studied. Using the London model, we determine the temperature dependence of the London penetration depth (λ) from the slope of the linear M(lnH) dependence of the isothermal magnetization. Our results show the strong dependence of ΔS and λ on the temperature. The structural transformation which occurs at the vicinity of 80 K very strongly influences the physics of vortex lattice and the associated magnetic behavior