Anisotropy in Conductance of a Quasi-One-Dimensional Metallic Surface State Measured by a Square Micro-Four-Point Probe Method

We have devised a ``square micro-four-point probe method'' using an independently driven ultrahigh-vacuum four-tip scanning tunneling microscope, and succeeded for the first time to directly measure anisotropic electrical conductance of a single-atomic layer on a solid surface. A quasi-one-dimensional metal of a single-domain Si(111)4×1-In had a surface-state conductance along the metallic atom chains (σ_∥) to be 7.2(±0.6)×10^-4 S/ at room temperature, which was larger than that in the perpendicular direction (σ_⊥) by ~60 times. The σ_∥ was consistently interpreted by a Boltzmann equation with the anisotropic surface-state band dispersion, while the σ_⊥ was dominated by a surface-space-charge-layer conductance.