A high-resolution study of the electronic structure of niobium triselenide in normal and charge density wave states with angle-resolved photoemission spectroscopy.

摘要

Angle resolved photoemission spectroscopy (ARPES) has become the dominant technique for experimentally measuring valence band structures of crystalline surfaces and other low-dimensional systems. It is the only available tool that has the ability to directly measure energy band dispersions as a function of quantum number k, the Bloch wave vector of occupied levels in the crystal. This application stems from the inherent surface sensitivity of the photoelectric effect, and the conservation of surface parallel electron wave vector during photoemission.;In this study, ARPES has been applied to measure the electronic structure of the quasi-one-dimensional (quasi-1D) metal NbSe3. NbSe 3 is a linear-chains material, with a large anisotropy in its charge-transport properties. At low temperatures, NbSe3 carries two independent charge density wave (CDW) states attributed to Peierls transitions. A phonon mode of q = 2kF softens (o( q) → 0), resulting in modified crystal and reciprocal-space structures. Metallic energy bands are back-folded at +/-k F, opening insulating gaps and removing pieces of the Fermi surface. The resulting electronic configuration is characterized by a periodic charge density with wavelength lambda = pi/kF. ARPES measurements, which are sensitive to k-dependent structure, directly image these transformations.;The valence band structure of NbSe3 has been mapped along the reciprocal chain direction, both above and below T1 = 145 K. Structural changes in the metallic bands are striking, and appear only at certain locations in the Brillouin zone, confirming the quasi-1D nature. The CDW1 wave vector and energy gap are determined directly from these spectra. Fermi contours are measured, and little change is noted between the two temperature regimes. This supports a model of the CDW1 transition where the Peierls distortion only affects two of six chains in the crystal unit cell. The persistence of weak CDW1 structure well above T 1 agrees with a previous study into fluctuation enhancements. Perpendicular wave vector is calibrated from a band calculation. Analysis of the data provides average band width and curvature of the valence metallic band in both phases. Broadening of spectral features is observed at all temperatures, and some reasons for this are discussed.