SCANNING-PROBE-MICROSCOPY STUDIES OF SUPERLATTICE STRUCTURES AND DENSITY-WAVE STRUCTURES IN 2H-NBSE2, 2H-TASE2, AND 2H-TAS2 INDUCED BY FE DOPING

The intercalation of Fe into the van der Waals gap in the 2H phase transition-metal dichalcogenides NbSe2, TaSe2, and TaS2 produces many interesting electronic, magnetic, and structural effects. The scanning tunneling microscope (STM) and atomic force microscope (AFM) prove to be very sensitive to these changes and we report a wide range of results as a function of Fe concentration. All three materials support similar 3a0 X 3a0 charge-density-wave (CDW) structures in the pure state at low temperatures. At low concentrations of Fe the CDW superlattice is still strong at 4.2 K and persists to high concentrations of Fe. At high concentrations, the Fe becomes ordered in the octahedral holes in the van der Waals gaps, and superlattices of the form 2a0 X 2a0 and square-root 3a0 X square-root 3a0 are observed. These can be detected at both 300 and 4.2 K. STM spectroscopy at 4.2 K shows that in 2H-FexNbSe2 and 2H-FexTaSe2 the energy gap in the electronic spectrum is initially reduced, but stabilizes at higher Fe concentrations and remains well defined for the ordered 2a0 X 2a0 phase. A transition from a CDW to a mixed CDW and spin-density-wave state is indicated, since these high Fe concentration phases are antiferromagnetic. In 2H-FexTaS2 both 2a0 X 2a0 and square-root 3a0 X square-root 3a0 superlattices are observed. The 2a0 X 2a0 regions show a large energy gap, while the square-root 3a0 X square-root 3a0 do not. The latter phase is ferromagnetic and would not be expected to exhibit a gap. The development of the electronic structures over the entire range of Fe concentrations has been followed by STM and AFM and can be tracked in detail.