In-plane superfluid density and microwave conductivity of the organic superconductorκ-(BEDT-TTF)2Cu[N(CN)2]Br: Evidence for

Abstract

We report the in-plane microwave surface impedance of a high-quality single crystal of κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br. In the superconducting state, we find three independent signatures of d-wave pairing: (i) a strong, linear temperature dependence of superfluid density; (ii) deep in the superconducting state the quasiparticle scattering rate ∼ T 3 ; and (iii) no BCS coherence peak is observed in the quasiparticle conductivity. Above T c , the Kadowaki-Woods ratio and the temperature dependence of the in-plane conductivity show that the normal state is a Fermi liquid below 23 K, yet resilient quasiparticles dominate the transport up to 50 K. It has been widely argued that the doped Mott insulator describes the essential physics of the cuprates.1 Similarly, the physics of the κ-(ET) 2 X salts (ET is an abbreviation of BEDT-TTF) appears to be connected to the bandwidth-controlled Mott transition.2 Thus, it is essential to identify and understand the important similarities and differences between these two classes of quasi-two-dimensional superconductor. In both the cuprates and the κ-(ET) 2 X salts, the superconducting critical temperature is only two orders of magnitude smaller than the Fermi temperature; in this sense, both are high-temperature superconductors.A broad consensus that the cuprates are d-wave superconductors was quickly reached.3-6 However, the nature of the pairing state of the κ-(ET) 2 X salts has taken longer to understand due to the lack of a "smoking gun" experiment. 7,8 Early on there was clear evidence for singlet pairing. [9][10][11][12] This, and the low symmetry of the organics, limits the pairing symmetry to be either s-wave (A 1g representation of the D 2h point group) or d-wave (B 2g ).13 Early heat capacity experiments suggested s-wave pairing, 14,15 but more recent low-temperature data point to d-wave pairing. 16 Measurements of the NMR relaxation rate support unconventional pairing. [9][10][11][12] Disorder studies show a reduction in T c with increasing scattering 17,18 but, for larger scattering rates, the suppression of T c is less than expected for non-s-wave superconductors.Attempts to locate the nodes expected in a d-wave superconductor have not yet yielded a simple picture. The in-plane thermal conductivity shows a fourfold angular variation with minima at 45• to the crystal axes, 19 whereas when a magnetic field is rotated in the plane both the heat capacity 20 and the millimeter wave absorption 21 have minima when the field is aligned with the crystal axes. At first sight these results seem contradictory, but both experiments are extremely difficult to interpret 22 and a complicated phase diagram could occur as a function of field strength and temperature. 20,22 Nevertheless, as this has not yet been observed, these measurements have not yet settled the pairing symmetry. There have also been attempts to directly image the gap via scanning tunneling microscopy (STM).23 Some care is needed with the interpretation of these experiments as the coherence peaks, the key feature of ...

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

“…In contrast to early specific heat measurements that supported swave pairing 88,89 , more recent measurements are consistent with a nodal order parameter 90 . Microwave penetration depth measurements 91,92 as well as STM tunneling experiments [93][94][95][96][97][98] are also consistent with nodes.…”

Superfluid density and microwave conductivity of FeSe superconductor: ultra-long-lived quasiparticles and extendeds-wave energy gap

“…From the surface impedance we obtain the high frequency conductivity s s s =i 1 2 . The real part of the conductivity is plotted in figure 1(b [46,60], and the organic superconductor κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br [61], although only in Ortho-I YBa 2 Cu 3 O 6.993 is the enhancement as dramatic. In all cases the physics is the same-on passing through T c there is a sudden collapse in quasiparticle scattering that vastly outpaces the steadier condensation of quasiparticles into the superfluid condensate.…”

Near-degeneracy of extendeds+dx2−y2anddxy

“…Although superconducting κ-type charge transfer salts have been investigated, for instance, in studies of specific heat 3,[8][9][10][11][12] , surface impedance 13 , thermal conductivity 14 , millimeter-wave transmission 15 , scanning tunneling spectroscopy (STS) [16][17][18][19] and elastic constants 20 , no consensus about the symmetry of the superconducting pairing has been reached so far. Some of the experiments are in favor of s-wave symmetry 3,8,9 , while other studies have proposed d-wave states with contradictory positions of the nodes in the superconducting order parameter [10][11][12][13][14][15][16][17][18] . Evidence for a mixed-symmetry order parameter was recently provided in Refs.…”