Low Temperature Penetration depth of \kappa-(ET)_2Cu[N(CN)_2]Br and \kappa-(ET)_2Cu(NCS)_2

TL;DR

This study measures the low-temperature penetration depth in two organic superconductors, revealing a fractional power law behavior that suggests unconventional superconductivity possibly influenced by impurities.

Contribution

It provides high-precision measurements of the penetration depth down to 0.4 K, showing fractional power law behavior and discussing implications for the pairing symmetry and impurity effects.

Findings

In-plane penetration depth varies as T^1.5 at low temperatures.

Data are consistent with a d-wave superconductor with impurities.

Interplane penetration depth measured as approximately 100 μm.

Abstract

We present high precision measurements of the penetration depth \lambda of single crystals of \kappa-(ET)_2Cu[N(CN)2]Br and \kappa-(ET)_2Cu(NCS)_2 at temperature down to 0.4 K. We find that, at low temperatures, the in-plane penetration depth varies as a fractional power law, \lambda\sim T^1.5. W hilst this may be taken as evidence for novel bose excitation processes, we show that the data are also consistent with a quasi-linear variation of the superfluid density, as is expected for a d-wave superconductor with impurities. Our data for the interplane penetration depth show similar features and give a direct measurement of the absolute value, \lambda(0)=100\pm 20 \mu m.