Applications of scanning tunneling microscopy to the study of charge density waves

Abstract

Scanning tunneling microscopy (STM) studies of the surfaces of transition metal di- and tri-chalcogenides have been used to detect a variety of charge-density-wave (CDW) contributions to the surface charge modulation at 77 and 4.2K. In the 1T phases of TaSe₂ and TaS₂ strong charge maxima are observed which correspond to the √13 a₀ × √13 a₀ superlattice generated by the CDWs formed as standing waves from the conduction electrons. The charge-density contours located between the charge maxima show major contributions from the detailed arrangement of surface Se or S atom. The z-deflection observed from the total surface charge-density modulation in the 1T phases is extremely large in the range 1.0 to 2.5Å. The STM scans show the same general structure for the 1T phases at both 77 and 4.2K, but variations in the z-deflection suggest some temperature dependence of the CDW amplitude. The observations are consistent with band structure considerations and the large electron transfer associated with the CDWs. In 2H-TaSe₂ at 77K and in 2H-NbSe₂ at 4.2K the CDWs are much weaker than in the 1T phases and contribute only small deflections to the STM scans which are mainly dominated by the atomic modulation of the surface charge-density. The linear chain compound NbSe₃ has two CDW transitions, one at 144K and one at 59K. The STM scans at 77K with only one CDW present and only 20% of the Fermi surface gapped show no detectable contribution to the surface charge modulation at the CDW wavelength. The z-deflection shows a large surface modulation and resolves the three chains per surface unit cell, but the STM pattern can be matched to the expected height and charge variations of the surface Se atoms. At 4.2K the two CDWs present in NbSe₃ gap approximately 80% of the Fermi surface and a modulation at the CDW wavelength of ~ 4b₀ along the chain axis can be analyzed in terms of the band structure and CDW formation. Initial STM scans have also been performed at 77K on the linear chain compound TaS₃. The CDW onset occurs at 216K with gapping of the entire Fermi surface and formation of a semiconducting phase at low temperatures. A clear CDW modulation has not yet been observed, but reasonable STM scans can be obtained and improved resolution should make it possible to detect the CDW structure. Technical problems associated with operation of the STMs at both 77 and 4.2K will also be discussed.