Scanning Tunneling Microscopy of Atoms and Charge Density Waves in 1T-TaS sub 2 , 1T-TaSe sub 2 and 1T-VSe sub 2.

摘要

The layer structure dichalcogenide materials TaS sub 2 and TaSe sub 2 grow in several different phases depending on the coordination between the Ta and chalcogenide atoms and the number of three layer sandwiches per unit cell. The 1T phase has octahedral coordination between the Ta and chalcogenide atoms and has one three layer sandwich per unit cell. The high temperature Fermi surfaces (FSs) of the 1T phase Ta based materials exhibit a favorable nesting condition and undergo a charge-density-wave (CDW) transition at temperatures well above room temperature. At low temperatures the CDWs form a sqrt 13 /ovr string/a /times/ sqrt 13 a commensurate superlattice. STM scans on the 1T phases confirm the presence of an extremely strong CDW modulation inducing z-deflections in the constant current mode of anomalously large values. 1T-VSe sub 2 is also a member of the VB layer structure dichalcogenide group and band structure calculations show the high temperatures FS to be similar to that of 1T-TaSe sub 2 . However, sufficient differences exist such that the CDW formation is quite different. The CDW superlattice is observed to form only below room temperature and locks into a 4/ovr string/a /times/ 4/ovr string/a superlattice below /approximately/80K rather than the sqrt 13 /ovr string/a /times/ sqrt 13 /ovr string/a one observed in 1T-TaSe sub 2 . Based on electron and neutron diffraction results on stoichiometric 1T-VSe sub 2 two phase transitions are detected, a second order transition at 110K and a first order transition at /approximately/80K. 20 figs. (ERA citation 14:005581)