Upper critical field divergence induced by mesoscopic phase separation in the organic superconductor (TMTSF)2ReO4

TL;DR

This study investigates how mesoscopic phase separation in (TMTSF)2ReO4 leads to a divergence in the upper critical magnetic field, revealing a transition from 3D to 2D superconducting behavior linked to filamentary structures.

Contribution

It demonstrates the link between phase coexistence, filamentary superconductivity, and the divergence of the upper critical field in an organic superconductor, highlighting the role of mesoscopic phase separation.

Findings

Observation of a linear increase in critical field at low fields.

Identification of a divergence in the critical field above a crossover point.

Evidence of ordered filamentary structures with similar sizes.

Abstract

Due to the competition of two anion orders, (TMTSF)2ReO4, presents a phase coexistence between semiconducting and metallic (superconducting) regions (filaments or droplets) in a wide range of pressure. In this regime, the superconducting upper critical field for H parallel to both c* and b' axes present a linear part at low fields followed by a divergence above a cross-over field. This cross-over corresponds to the 3D-2D decoupling transition expected in filamentary or granular superconductors. The sharpness of the transition also demonstrates that all filaments are of similar sizes and self organize in a very ordered way. The distance between the filaments and their cross-section are estimated.