STM spectroscopy of an organic superconductor

✍ Scribed by K. Ichimura; T. Arai; K. Nomura; S. Takasaki; J. Yamada; S. Nakatsuji; H. Anzai

• Publisher: Elsevier Science
• Year: 1997
• Tongue: English
• Weight: 187 KB
• Volume: 21
• Category: Article
• ISSN: 0749-6036
• DOI: 10.1006/spmi.1996.0386

✦ Synopsis

Electron tunneling spectroscopy of the organic superconductor κ-(BEDT-TTF) 2 Cu(NCS) 2 using low temperature scanning tunneling microscope (STM) is reported. The tunneling differential conductance in the superconducting phase was obtained in the b-c plane of a single crystal, by varying the tip position on the sample surface. The differential conductance is reduced near zero bias voltage and enhanced at the gap edge, associated with the superconducting gap structure below T c 10 K. The gap width differs slightly from sample to sample, while the overall functional shape of the conductance is sample-independent. The tunneling conductance is reduced to almost zero near zero bias voltage, while it is finite inside the gap edge. The curve obtained cannot be fit to the BCS density of states with s-wave pairing symmetry, even if the life-time broadening of one-electron levels is taken into account. Finite conductance inside the gap edge suggests anisotropy of the gap. However, the conductance curve obtained is not explained by a simple d-wave symmetry for (k k k). The reduced conductance near zero bias voltage suggests a finite gap. An anisotropic model with a finite gap, in which (k k k) varies depending on the direction in k k k-space, is examined. The tunneling conductance in the low-energy region is almost fit by the model with min = 2 meV and max = 6 meV. The finite conductance is explained by introducing a small effect of life time broadening. We conclude that the gap is anisotropic and is finite (at least min = 2 meV) on the entire Fermi surface.