Millimeter-Wave Magneto-optical Determination of the Anisotropy of the Superconducting Order Parameter in the Molecular Superconductorκ-(BEDT−TTF)2

Abstract

We have used a millimeter-wave magneto-optical technique to study the angle dependence of the high-frequency conductivity of the molecular superconductor k-͑BEDT-TTF͒ 2 Cu͑NCS͒ 2 . The data strongly suggest that the superconducting gap has nodes directed along the b and c directions of the crystal, in agreement with recent theoretical predictions. This supports the idea that the superconductivity in k-͑BEDT-TTF͒ 2 Cu͑NCS͒ 2 is d wave in nature, and is mediated by spin fluctuations. PACS numbers: 74.25.Gz, 74.20.Mn, 74.25.Nf, 74.70.Kn The family of superconducting charge-transfer salts k-͑BEDT-TTF͒ 2 X, where X can, for example, be Cu͑NCS͒ 2 , Cu͓N͑CN͒ 2 ͔Br, or I 3 , has attracted considerable recent attention. A variety of experiments have suggested that the superconducting gap function may contain nodes at certain points on the Fermi surface; e.g., the 13 C NMR spin-lattice relaxation rate [1] varies as T 3 and the thermal conductivity [2] is proportional to T below the superconducting critical temperature T c . In addition, microwave penetration-depth studies [3] show a non-BCS-like behavior of the penetration depth as a function of T and the electronic component of the specific heat [4] has an unconventional field dependence below T c . However, Shubnikov-de Haas [5,6], magnetic breakdown [7], and angle-dependent magnetoresistance oscillation [8] experiments demonstrate that these salts have well-defined quasi-two-dimensional (Q2D) Fermi surfaces, indicating that the quasiparticles can be described by Fermi-liquid theory at low temperatures (the Fermi surface is shown schematically as an inset of Fig. 1). Furthermore, it has been possible to fit experimental Fermi-surface-topology data to a simplified model of the tight-binding band structure (the so-called effective dimer model) to a good degree of accuracy [5,[9][10][11].The combination of unconventional superconductivity and a tractable analytical representation of the band structure makes the k-͑BEDT-TTF͒ 2 X superconductors attractive for theoretical studies, and a number of authors [9,10,12,13] have explored the possibility of d-wave superconductivity mediated by spin fluctuations. Such approaches predict that the superconducting order parameter will have four, roughly perpendicular, gap nodes, the exact orientation of the nodes depending on the underlying details of the Fermi surface [10,12]. Experiments performed thus far have merely detected the probable presence of the gap nodes, without giving information about their relative orientation. In this paper, we use a millimeter-wave magneto-optical technique to determine the orientation of the gap nodes in k-͑BEDT-TTF͒ 2 Cu͑NCS͒ 2 . The data appear to be in good qualitative agreement with the predictions of Schmalian [10].The experiments were carried out on a number of crystals of k-͑BEDT-TTF͒ 2 Cu͑NCS͒ 2 , grown by FIG.

Lattice dynamics and electron-phonon coupling in theβ−(BEDT−TTF)2I3Girlando1, Masino2, Visentini3 et al. 2000Phys. Rev. B351380View full textAdd to dashboardCite

“…36 On the other hand, evidences are also accumulating towards non-conventional coupling mechanisms in organic superconductors, such as spin-fluctuation mediated superconductivity. 49 Similar apparently contrasting experimental evidences are also found for cuprates, 50 pointing to a superconductivity mechanism where both electron-phonon coupling and antiferromagnetic spin correlations are taken into account. (%) lattice internal lattice internal 216 98 173 90 151 26 68 133 148 86 10 130 136 8 88 124 123 48 48 114 8 86 95 112 24 72 91 90 56 40 84 8 13 34 44 74 56 26 12 71 69 22 78 67 8 40 18 36 62 22 44 10 20 57 23 44 28 48 43 55 11 26 33 44 22 30 17 41 22 …”

A Spin Fluctuation Model for d-Wave Superconductivity

“…Several groups [52][53][54][55] have used spinfluctuation theory to predict the position of nodes of the superconducting order parameter of these materials. An unconventional order-parameter with nodes of the gap is indeed supported by NMR [56][57][58] , thermal conductivity 59 , millimeter transmission 61 and STM 62 experiments. However, the last two experiments seem to come to different conclusions as far as the position of the nodes is concerned.…”

Coulomb enhancement of superconducting pair-pair correlations in a34-filled model forκ−(BEDT-TTF)2X

“…1). Magneto-optical 99 and specific heat measurements in a magnetic field find the nodes to coincide with the crystal axes 100 . Thermal conductivity 101 and STM measurements 94,97 however find the nodes between the crystal axes, although STM measurements on a partially deuterated κ-Br suggest a mixture of two order parameters 98 .…”