On the superconducting state of the organic conductor $(TMTSF)_2ClO_4$

TL;DR

This paper investigates the superconducting properties of the organic conductor $(TMTSF)_2ClO_4$, revealing complex magnetic behavior and phase transitions that challenge conventional understanding of its pairing symmetry and critical fields.

Contribution

It provides new insights into the spin susceptibility and critical field behavior of $(TMTSF)_2ClO_4$, highlighting a crossover in magnetic response at high fields.

Findings

Spin susceptibility decreases at low fields indicating singlet pairing.

A sharp crossover at ~15 kOe in spin-lattice relaxation rate.

Upper critical field exceeds paramagnetic limit, suggesting unconventional superconductivity.

Abstract

$(TMTSF)_2ClO_4$ is a quasi-one dimensional organic conductor and superconductor with $T_c=1.4$K, and one of at least two Bechgaard salts observed to have upper critical fields far exceeding the paramagnetic limit. Nevertheless, the $^{77}$Se NMR Knight shift at low fields reveals a decrease in spin susceptibility $\chi_s$ consistent with singlet spin pairing. The field dependence of the spin-lattice relaxation rate at 100mK exhibits a sharp crossover (or phase transition) at a field $H_s\sim15$kOe, to a regime where $\chi_s$ is close to the normal state value, even though \hc2 $\gg H_s$.