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A scanning tunneling microscope {STM) has been used to measure energy gaps in the chargedensity-wave (CDW) phases of the layer-structure dichalcogenides and in the high-temperature superconductor Bi&Sr,CaCu&08. Measured values of Ac& at 4.2 K for 2H-TaSe"2H-TaS"and 2H-NbSe2 are 80, 50, and 34 meV giving values of 26C&/k& T, equal to 15.2, 15.4, and 23.9, indicating strong coupling in these CDW systems. Measured values of 6&. & at 4.2 K in 1T-TaSe. and 1T-TaS, are -150 meV for both materials giving 2hcp~/k~r, . =5.8. STM scans of Bi,Sr,CaCu. O, at 4.2 K resolve atoms on the BiO"layer and show possible variations in electronic structure. The energy gap determined from I versus V and dI/d V versus V curves is in the range 30 -35 meV giving values of 2A/kBT, =8. Spectroscopy measurements with the STM can exhibit large zero-bias anomalies which complicate the analysis of the energy-gap structure, but adequate separation has been accomplished.

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The intercalation of Fe into the van der Waals gap in the 2H phase transition-metal dichalcogenides NbSe&, TaSe"and TaS~produces many interesting electronic, magnetic, and structural eA'ects. 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 3aoX3ao 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 2ao X2ao and &3ao X &3ao are observed. These can be detected at both 300 and 4.2 K. STM spectroscopy at 4.2 K shows that in 2H-Fe"NbSe2 and 2H-Fe"TaSe,the energy gap in the electronic spectrum is initially reduced, but stabilizes at higher Fe concentrations and remains well defined for the ordered 2aoX2ao 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-Fe"TaS2 both 2aoX28o and &3aoX&3ao superlattices are observed. The 2aoX2ao regions show a large energy gap, while the &3ao X &3ao 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.