Superconducting and spin-density wave phases probed by scanning tunneling spectroscopy in the organic conductor $\mathrm{(TMTSF)_{2}ClO_{4}}$

TL;DR

This study uses scanning tunneling microscopy to investigate the local electronic states of the organic superconductor (TMTSF)2ClO4, revealing inhomogeneous surface states and evidence of unconventional superconductivity linked to quantum criticality.

Contribution

First direct STM investigation of surface electronic states in (TMTSF)2ClO4, showing inhomogeneous superconducting and spin-density wave phases influenced by disorder and quantum criticality.

Findings

Homogeneous superconducting state in bulk, inhomogeneous at surface

Linear V-shape DOS indicates d-wave like order parameter

Non-linear U-shape DOS in disordered spin-density wave areas

Abstract

By scanning tunneling microscopy (STM) we have probed the local quasi-particle density of states (DOS) of the Bechgaard salt organic superconductor $\mathrm{(TMTSF)_{2}ClO_{4}}$ in slowly cooled single crystals cleaved under ultrahigh vacuum conditions. In well STM imaged crystallographic surface planes, the local DOS has been probed for different surface areas at temperatures above and below the critical temperature of superconducting or insulating spin-density wave states. While a rather homogeneous superconducting state is expected in the bulk from previous studies, depending on the degree of disorder introduced by cleavage in the anion lattice, an inhomogeneous granular state is predominantly observed at the surface. A pronounced linear V-shape profile of the local DOS is observed from intermediate to the lowest energy scale in the less disordered superconducting surface areas. This supports the existence of an unconventional d-wave like order parameter with nodes at low energy, which is preceded by more energetic fluctuations attributed to quantum criticality of the material. At higher energy disorder combined to correlations deplete further the DOS. By contrast a non-linear U-shape characterizes the local low energy DOS profile for the more disordered and insulating surface areas of the spin-density wave state. The experimental results are compared quantitatively with those predicted by the renormalization group theory of the quasi-one dimensional electron gas model and its description of the superconducting and spin-density wave states that are interlinked by quantum criticality in the Bechgaard salts.