Jixia Dai, Eduardo Calleja, Jacob Alldredge, Xiangde Zhu, Lijun Li, Wenjian Lu, Yuping Sun, Thomas Wolf, Helmuth Berger, Kyle McElroy
In condensed matter physics only a handful of models have both elegance in their construction and are thought to describe the behavior of numerous materials. The Peierls transition [1], for electrons crystallizing into charge-density waves (CDW), is one such model that has been successfully applied to many 1D systems [2]. However, in the quasi-2D electron systems of the layered transition metal dichalcogenides (TMD) there is still a controversy about the nature of the transitions to CDW phases, i.e. whether they are described by a Peierls-type mechanism or by a lattice driven model. By performing scanning tunneling microscopy (STM) experiments on the canonical TMD-CDW systems, we have successfully imaged the electronic modulation and the lattice distortion separately for the first time. Across three materials (2H-TaS2, TaSe2, and NbSe2), we found dominant lattice contributions instead of the electronic modulation expected from Peierls transitions, in contrast to the CDW states that show the hallmark of contrast inversion, in 2D [3-5] and 1D [6] systems. Our results imply that the periodic lattice distortion (PLD) plays a vital role in the formation of CDW phases in the TMDs, which is in agreement with the theoretical studies [7-9], especially recent work by Johannes et al. [8,9]. Our results illustrate the importance of taking into account the more complicated lattice degree of freedom, and they will prove useful in the studies of other correlated electron systems.
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http://arxiv.org/abs/1303.0956
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