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Scanning Tunneling Microscope studies on 2D charge density wave systems:

Event Details

Event Dates: 

Thursday, August 30, 2012 - 6:00am

Speaker Name(s): 

Jixia Dai

Speaker Affiliation(s): 

CU Boulder
Seminar Type/Subject

Event Details & Abstract: 

Although being discovered more than 30 years ago, the exact driving mechanism of 2D charge density wave in transitional metal dichalcogenide systems (2H-TaSe2, TaS2, NbSe2, etc.) still remains mysterious. The basic question concerns whether a 2D electronic system can undergo such a CDW transition all by itself or some kind of lattice transition happens in the first place. Performing scanning tunneling microscopy experiment on cleaved surface of 2H-NbSe2, 2H-TaSe2 and 2H-TaS2, we were able to for image the electronic modulation and lattice distortion separately for the first time. In all three of the samples, we found very strong lattice distortion comparing to the electronic modulation. This new result contradicts the well accepted CDW behavior of pure electronic modulation and suggests that periodic lattice distortion plays an important role in the CDW transitions of TMDs. Our results are in agreement with some other recent experimental and theoretical results. In order to study the electronic modulation in details, we also did local density of states (LDOS) measurements within a CDW unit cell. Our results indicate that high energy electrons instead of the electrons around the fermi energy are involved in the electron modulation. It shows that 35meVĀ  is not the correct energy scale of CDW in TaSe2, and the entire conduction band might be involved (at least from -200 to 200meV). As the last part, I will present some work on Cu-doped TaS2 (Cu0.04TaS2) superconductor sample. Our attention is about the effect of dopant atoms on local CDW formation. The small contribution (0.1 e-/Cu) of electrons to the conduction band by the copper dopants is inconsistent with the large decrease of TCDW. Our data indicates that there is a large inhomogeneity on the z-position of ionic location, which is a natural result of copper intercalation. Therefore, we suggest that the local strain created by copper dopant is possibly the killer of CDW. This result is also consistent with the lattice-driven CDW picture.