"Checkerboard stripe" electronic state on cleaved surface of NdO_0.7F_0.3BiS₂ probed by scanning tunneling microscopy

We present scanning tunneling microscopy measurements on a cleaved surface of the recently discovered superconductor NdO_0.7F_0.3BiS₂ with a transition temperature (T_c) of 5.1 K. Tunneling spectra at 4.2K (below T_c) and 22K (above T_c) show a large spectroscopic gap (∼40 mV), which is inconsistent with the metallic nature demonstrated in bulk measurements. Moreover, we find two interesting real-space electronic features. The first feature is a "checkerboard stripe" electronic state characterized by an alternating arrangement of two types of nanocluster. In one cluster, one-dimensional electronic stripes run along one Bi-Bi direction, whereas, in the other cluster, the stripes run along the other Bi Bi direction. The second feature is a nanoscale electronic inhomogeneity whose microscopic source seems to be atomic defects on the cleaved surface or dopant F atoms.