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.

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Dilute concentrations of impurities in NbSe& crystals change the charge-density-wave (CDW) energy gaps very rapidly while the onset temperatures show very small changes. Fe concentrations of & 1% reduce the two CDW energy gaps by approximately 30% while Co concentrations of & 3% increase the low-temperature CDW energy gap by approximately 37Vo and the high-temperature CDW gap by approximately 28%. The real-space scanning-tunneling-microscope scans show changes in the relative CDW modulation amplitudes consistent with the observed changes in the energy gaps.We have used a scanning tunneling microscope (STM) operating at 4.2 K to image the surface structure of charge-density waves (CDW's) in pure NbSe3 and in NbSeq doped with Fe and Co impurities. In addition, we have measured the corresponding energy-gap structure appearing in the dI/dV vs V curves at 4.2 K. The addition of dilute impurities to NbSe3 does not disorder the CDW structure, but does produce very large shifts in the energy gaps associated with both the high-temperature (144 K) and the low-temperature (59 K) CDW's. The energy gaps can be shifted by 28-37% and the magnitude of the structure associated with the density-of-states (DOS) change at the gap edge can be substantially changed. Changes in CDW amplitude modulation are also observed in the real-space STM images and show a direct correlation with the energy-gap measurements.As previously shown' for pure NbSe3, the STM, which measures the local density of states (LDOS) at the Fermi level at the position of tip, detects a substantial CDW modulation on all three chains of the unit cell in the b-c plane. This result is also confirmed for the NbSeq crystals doped with Fe and Co, although the relative CDW amplitudes on the separate chains show significant changes. The three pairs of chains in NbSe3 are electronically and structurally inequivalent and are denoted by I, I', II, II', and III, III' in the band-structure calculation by Shima and Kamimura. ' The CDW with onset at 144 K is associated with the pair of chains denoted by III, III' and has wavelength components of (0.0ao, 4. 115bo, 0.0eo).The CDW with its onset at 59 K has been previously assigned' to the chain pair I, I' and has wavelength cornponents of (2.00ap, 3.802bp, 2.00co). NbSeq has a monoclinic unit cell. A cross section perpendicular to the chain axis (b axis) with the chain identifications is shown in Fig. 1. Chains II, II' were conjectured by Wilson to have little or no conduction electron density and were thought not to be involved in the CDW transition.The previous STM results' on pure NbSe3, and the STM results reported in this paper, detect a substantial conduction electron density over chain II as well as a strong CDW modulation at the wavelength of the lowtemperature CDW. Recent NMR data also show a substantial conduction electron density on chains II, II', although the NMR results are not conclusive with the involvement of chains II, II' in the formation of the lowtemperature CDW.

the various linear chain materials when they are in the CDW phase.The crystals of TaaNiSey grow as needles up to 20 mm in length and have monoclinic symmetry with lattice parameters of ao = 13.827 A, bo = 3AS2 A, co = 18.577 A, and j8 = 108.8°. The crystal structure has been analyzed in detail by Sunshine and Ibers [7]. The chains are oriented along the b axis and there are four formula units Scanning tunneling microscope images of Ta2NiSe7 taken at 77 and 4.2 K show a complex electronic rearrangement occurring below the charge-density-wave (CDW) onset temperature of 52.5 K. A large energy gap of 41.0 ±3.0 meV is observed and a CDW amplitude modulation of wavelength ~2^o is detected. The electronic transfer between chains appears to drive the CDW formation.PACS numbers: 61.16. Di, 71.45.Lr, 73.20.Dx, 73.40.Gk A scanning tunneling microscope (STM) study of the compound Ta2NiSe7 shows that electronic modifications characteristic of a charge-density wave (CDW) are observed at 4.2 K. The electronic spectrum measured at 4.2 K shows a large energy gap of ACDW =41.0 ± 3.0 meV and the local density of states (LDOS) shows a modulation with a wavelength of --2bo, consistent with the wave vector of q = (0,0.483,0) measured by Fleming et al.[1] using x-ray and electron diffraction. The STM scans at 4.2 K also show a significant rearrangement of electron density on the chains compared to the uniform density observed at 77 K.The CDW transition occurs at 52.5 K and the resulting electronic rearrangement produces STM scans which show a very complex LDOS within the unit cell characterized by one chain with very strong intensity, and one much weaker chain. However, the CDW modulation along the chains as detected by the STM at 4.2 K is very weak, in contrast to the strong electronic rearrangement between chains. This suggests that the CDW may in fact be driven by the initial electron transfer between chains rather than a simple nesting of the high-temperature Fermi surface, a significantly different sequence than observed in other linear chain CDW materials such as NbSe3.Ta2NiSe7 is a linear chain compound similar in some respects to FeNbaSeio, although the latter is driven into a metal-insulator transition [2] by the formation of a CDW while TaiNiSev remains a semimetal below the CDW transition. The tunneling parameters used for Ta2NiSe7 are very similar to those used [3-6] for the semimetal NbSea and electronic structural changes appear to dominate the STM scans of both materials below the CDW transitions. However, in contrast to NbSes, the CDW amplitude modulation along the chain is extremely weak in Ta2NiSe7 and the STM image is dominated by a very strong charge transfer between the two chains in the unit cell. This gives rise to a strong difference in STM response for the two chains in the unit cell, even though the chains are equivalent with respect to height and atomic structure. Above the CDW transition at 52.5 K they appear equivalent in both STM and AFM images (see Fig. 2 for STM image). In this respect the STM results on...

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