Vortex phase transition with decoupling of the adjacent layers in the organic superconductorκ−(BEDT−TT

Abstract

The temperature dependence of the local magnetic induction measured by using a miniature Hall probe shows a tiny step at a field-dependent temperature in the titled compound. This result implies that there exists a first-order phase transition in the vortex system in this organic superconductor as reported in high-T c cuprates. The height of the step is of the order of 10 mG, from which the entropy change at the transition is estimated to be about 0.1 ͑k B /vortex͒/layer. The measurements of the Josephson plasma resonance reveal that the interlayer phase coherence abruptly changes across the transition line, indicating the decoupling of the adjacent layers. ͓S0163-1829͑98͒51310-0͔Owing to large thermal fluctuations and the effect of layered structures, the H-T phase diagram of the mixed state of high-T c superconductors ͑HTSC͒ is drastically different from that of the conventional superconductors. In particular, it is widely accepted that the vortex lattice in HTSC melts into the vortex liquid at a first-order phase transition well below the mean-field upper critical field. 1-5 The equilibrium magnetization measurements in Bi 2 Sr 2 CaCu 2 O 8ϩy ͑Refs. 1 and 2͒ and YBa 2 Cu 3 O 7Ϫ␦ ͑Refs. 3 and 4͒ have revealed that the density of vortices discontinuously decreases upon cooling at the thermodynamic phase transition like freezing water. The calorimetric measurement in untwinned YBa 2 Cu 3 O 7Ϫ␦ ͑Ref. 5͒ clearly indicates that the transition is first order. The nature of the transition, however, is not yet fully understood. One of the unresolved questions is whether or not the decoupling-the loss of coherence between the layers-takes place at the melting transition. To date experiments on this issue are controversial. 6-9 For example, two papers reporting flux transformer-type measurements in Bi 2 Sr 2 CaCu 2 O 8ϩy have drawn the contradictory conclusions. 8,9 It should be noted, however, that in such experiments only the coherence of vortices between top and bottom faces of the entire crystal can be evaluated instead of the coherence between the adjacent layers.Microscopic information on the phase coherence between the adjacent layers can be obtained experimentally by using the Josephson plasma resonance ͑JPR͒ as a probe. 10-13 The resonance occurs when the plasma frequency p of the superconducting carriers across the Josephson-coupled layers coincides with the measurement frequency. In the mixed state, p 2 is proportional to the interlayer phase coherence ͗cos n,nϩ1 ͘, where n,nϩ1 is the gauge-invariant phase difference between the layers n and nϩ1, and ͗¯͘ denotes thermal and disorder averaging. 14 Quite recently, Matsuda et al. 12 have found that ͗cos n,nϩ1 ͘ shows different temperature dependence between well above and below the melting transition line in Bi 2 Sr 2 CaCu 2 O 8ϩy . However, whether or not the loss of the interlayer coherence occurs at the transition line is still unclear. The -type BEDT-TTF superconducting salts, where BEDT-TTF is bisethylenedithio-tetrathiafulvalene, have the layered structure ...

Interlayer Phase Coherence in the Vortex Matter Phases ofBi2Sr2CaCuShibauchi1, Nakano2, Sato3 et al. 1999Phys. Rev. Lett.Self Cite10311751View full textAdd to dashboardCite

“…From these results we conclude that the decoupling occurs at the FOT line, not at the T x line. Moreover, we note that the first-order decoupling transition was also found in a layered organic superconductor k-͑BEDT-TTF͒ 2 Cu͓N͑CN͒ 2 ͔Br [19].…”

Ginzburg-Landau theory of type II superconductors in magnetic field

“…An examples of organic superconductor (Shibauchi et al, 1998) was given in (Li et al, 2006b). The data of BSCCO (Beidenkopf et al, 2005) are compared to the theoretical results on Fig.…”

Josephson plasma resonance inκ−(BEDT−TTF)2Cu(NCSMola1, King2, McRaven3 et al. 2000Phys. Rev. B225320View full textAdd to dashboardCite

“…[1][2][3][4][5] Magneto-optical experiments have been carried out by many groups, and for a wide range of materials, e.g., Bi 2 Sr 2 CaCu 2 O 8ϩ␦ ͑BSCCO͒, 1,3 YBa 2 Cu 3 O 6Ϫy ͑YBCO͒, 3 as well as several OS's. 4,5 In each case, these experiments have demonstrated that a Josephson plasma resonance ͑JPR͒ is observed in the interlayer conductivity, and that this resonance may be used as an extremely sensitive tool for probing the vortex structure/dynamics.…”