Investigation of the superconducting gap structure in κ-(BEDT-TTF)₂Cu(NCS)₂ and κ-(BEDT-TTF)₂Cu[N(CN)₂]Br by means of thermal-conductivity measurements

Abstract

We report temperature-dependent thermal-conductivity, κ, measurements on the layered quasi-two-dimensional organic superconductors κ-(BEDT-TTF)Cu(NCS) and κ-(BEDT-TTF)Cu[N(CN)]Br down to 160 mK. The results for κ-(BEDT-TTF)Cu(NCS) may be consistent with a nodal superconducting (SC) gap structure as indicated by a non-negligible remnant linear contribution when [Formula: see text] is extrapolated to [Formula: see text]. For κ-(BEDT-TTF)Cu[N(CN)]Br, contrary to expectations, higher κ values are observed in the superconducting regime as compared to the normal, high-field state evidencing a dominant phonon contribution to κ in the superconducting state. The strong increase of κ in the normal state below T for both samples indicates strong electron-phonon scattering. Our results highlight the need for thermal-conductivity measurements performed down to significantly lower temperatures to determine the symmetry of the SC gap.

Investigation of Many‐Body Effects in the Quasi‐Two‐Dimensional Electronic System of Organic Charge‐Transfer Salts

“…More recent results obtained for temperature‐dependent thermal‐conductivity indicate a nodal superconducting gap function for κ ‐(BEDT‐TTF) 2 Cu(NCS) 2 . In contrast κ ‐(BEDT‐TTF) 2 Cu(NCS) 2 those results indicate a significant phonon contribution to the gap in the superconducting state, which would be inconsistent with the model discussed above.…”

Umklapp scattering in unconventional superconductors: Microwave conductivity shows that κ−(BEDT−TTF)2Cu[N(CN)2Cavanagh1, Powell2 2019Phys. Rev. B7020View full textAdd to dashboardCite

“…Despite being one of the most studied organic superconductors, the pairing symmetry of κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br (κ-Br) remains contested [7,[16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34].…”

“…Three different gap symmetries have been proposed in κ-Br: a full gapped s-wave order parameter [18]; d xy pairing [ Fig. 2(a)], where both the presence and the placement of nodes are required by symmetry [24,25,29,30,50]; and a d x 2 −y 2 + s order parameter [22,[31][32][33]51] [Fig. 2(b)].…”

New insights into experiments on unconventional superconductors: The influence of symmetry and the superconducting gap of κ-(BEDT-TTF)2Cu[N(CN)2]Br

“…The focus of this thesis is an organic superconductor, κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br (κ-Br), in which the identification of the gap function has been particularly contentious. The exact form of the superconducting gap in this material has been a matter of considerable disagreement for several decades [42,62,63,[74][75][76][77][78][79][80][81][82]. This material has the highest critical temperature (at ambient pressure) of the BEDT-TTF based superconductors [65] (see Fig.…”

“…Similarly, penetration depth measurements were inconclusive [90][91][92] until recently, with more precise measurements showing a power law temperature dependence suggestive of a nodal superconducting state [82]. Recent measurements of the low temperature thermal conductivity have been taken as an indication of a fully gapped superconducting state [81], while the nuclear magnetic relaxation, which will be discussed in more detail in Chapter 2, has consistently indicated nodal superconductivity [84,85]. The density of states from the surface tunneling spectroscopy has been found to show some indication of multiple coherence peaks, which has been linked to a complicated mixed order parameter [63], though similar signals have been observed in other multi-band materials with a superconducting gap magnitude that varies between bands [72].…”