A scanning tunneling microscopy study of a new superstructure around defects created by tip-sample interaction on 2H-NbSe2

A low temperature scanning tunneling microscope (LT-STM) was used to investigate a new superstructure on the cleaved surface of a 2H-NbSe2 single crystal after introduction of structural defects through bias voltage pulses during tunneling at 4.2 K. A charge density wave (CDW) with a root 13 x root 13 reconstruction was observed in the vicinity of the defects and the well-known 3 x 3 CDW was observed far from these defects. Multiple layers inside the defects were also exposed and showed the new modulation of the CDW on all of the Se layers. This indicates a local 2H to 1T phase transition for the NbSe2 crystal structure. Two other interesting observations are also included: a disordered CDW-like phase of the root 13 x root 13 structure near the atomic steps and an anomalous distortion in the underlying atomic lattice revealed by STM images. A local heating mechanism is proposed to explain the creation of these novel structures.