Persistence to High Temperatures of Interlayer Coherence in an Organic Superconductor

Abstract

The interlayer magnetoresistance ρzz of the organic metal κ-(BEDT-TTF)2Cu(NCS)2 is studied in fields of up to 45 T and at temperatures T from 0.5 K to 30 K. The peak in ρzz seen in inplane fields, a definitive signature of interlayer coherence, remains to T s exceeding the Anderson criterion for incoherent transport by a factor ∼ 30. Angle-dependent magnetoresistance oscillations are modeled using an approach based on field-induced quasiparticle paths on a 3D Fermi surface, to yield the T dependence of the scattering rate τ −1 . The results suggest that τ −1 does not vary strongly over the Fermi surface, and that it has a T 2 dependence due to electron-electron scattering.PACS numbers: 74.70. Kn, 71.20.Rv, 78.20.Ls The past two decades have seen a tremendous blossoming of interest in compounds that possess quasi-twodimensional (Q2D) electronic bandstructure; examples include crystalline organic metals [1,2,3,4], cuprates [5] and layered ruthenates [6]. These materials may be described by a tight-binding Hamiltonian in which the ratio of the interlayer transfer integral t ⊥ to the average intralayer transfer integral t || is ≪ 1 [1,2,3,4]. The question arises as to whether the interlayer charge transfer is coherent or incoherent in these materials, i.e. whether or not the Fermi surface (FS) is three dimensional (3D), extending in the interlayer direction. Various criteria for interlayer incoherence have been proposed, including [7] where T is the temperature. In this picture, thermal fluctuations "wipe out" details of the interlayer periodicity [7]. The consequent interlayer incoherence is used as a justification for a number of theories which are thought to be pivotal in the understanding of Q2D materials (see e.g. [7,8,9,10]). It is therefore important to test assertions such as Eq. 1. To this end, we have measured the magnetic-field-orientation dependence of the resistance of the organic metal κ-(BEDT-TTF) 2 Cu(NCS) 2 [9] using fields of up to 45 T and T s of up to 30 K (Fig. 1). This material was chosen because its FS is well known [9, 11] (see Fig. 2(c)), previous low-T experiments have shown that the interlayer transfer integral is t a = 0.065 ± 0.007 meV [11,12] and modest T s allow the inequality in Eq. 1 to be exceeded by orders of magnitude (t a /k B ≈ 0.5 K). Our data show that interlayer coherence survives to at least k B T > ∼ 30t a (c.f. Eq. 1). In addition, the results in this paper determine the low-T scattering rate τ −1 for the quasiparticles. Recent works stress that an understanding of the effects of the scattering mechanism in organic metals gives vital information about the mechanism for superconductivity [13,14]. Electrical contacts were made to single crystals (∼ 0.7 × 0.5 × 0.1 mm 3 ) of κ-(BEDT-TTF) 2 Cu(NCS) 2 using 12.5 µm Pt wire bonded by graphite paste; contact resistances were ∼ 10 Ω. Current and voltage terminals were arranged such that the measured resistance R zz is proportional to the interlayer component of the resistivity tensor, ρ zz [11]. The measurements in Figs. 1 and ...

Angle-dependent magnetoresistance oscillations due to magnetic breakdown orbits

“…3(b)), revealing the underlying BAMROs. The fact the BAMRO features survive at scattering times and T s at which the SdH cannot be observed is further evidence that their mechanism is related to semiclassical quasiparticle trajectories across the Fermi surface, similar to those invoked to explain Yamaji oscillations [6,26].…”

Quantum interference and weak localization effects in the interlayer magnetoresistance of layered metals

“…We suggest that weak localization may be the origin of the feature near = 2 that Singleton et al 13 assigned to the coherence peak. The peak width appears to broaden with increasing temperature ͓as would be expected for decreasing , and hence increasing B ͑T͔͒.…”

“…-(BEDT-TTF) 2 Cu(NCS) 2 The magnitude and temperature dependence of the inelastic-scattering rate in -͑BEDT-TTF͒ 2 Cu͑NCS͒ 2 was recently determined using AMRO. 13 The data was fit to…”

“…[6][7][8] More recent developments have extended this to allow one to not only determine the anisotropies in the Fermi surface, but also the anisotropy of scattering on the Fermi surface. [7][8][9][10][11][12][13] In addition to the shape of the Fermi surface, the coherence of interlayer transport in layered metals has been a complicated and controversial issue. 14 In order for AMRO to be observable, only coherence between neighboring layers is required, i.e., weakly incoherent transport, and for almost all angles, this leads to AMRO that is equivalent to fully coherent transport ͑i.e., there is a three-dimensional Fermi surface͒ perpendicular to the planes.…”

Anisotropic scattering in angular-dependent magnetoresistance oscillations of quasi-two-dimensional and quasi-one-dimensional metals: Beyond the relaxation-time approximation

“…Notably, the precise meaning of lifetimes extracted from quantum oscillations in magnetotransport and from cyclotron resonance experiments is still a matter of discussion in the literature. 29,35,36,[39][40][41][42] If a lifetime determined from AMRO data using a relaxationtime approximation is to be usefully compared with those obtained elsewhere then its physical significance must be clarified. 11,27,39 Addressing this issue is the main purpose of this paper.…”