‘‘Extraordinary’’ Surface Phase Transition at (100) Surface of NbSe3

Abstract

We have investigated both q1 and q2 charge-density wave transitions occuring in NbSe3 by means of low temperature scanning tunneling microscopy under ultra-high vacuum on in-situ cleaved (100) surface. High-resolution topographical images with molecular resolution were obtained in the temperature range between 5K and 140K. We studied the temperature dependency of the order parameters of both transitions using Fourier transform of the STM images. Our results show that at the free (100) NbSe3 surface, the low temperature q2 CDW transition occurs above 70K, i.e. more than 10K above the bulk transition temperature reported by x-ray diffraction experiment or by tunneling junction measurements involving an unfree NbSe3 surface. We propose that as q2 phonon modes in NbSe3possess a component transverse to the surface, they might be softer in the top layer than in the bulk, leading to an extraordinary phase transition for the q2 CDW at surface. Additionnally, at variance with previously reported STM measurements, our images show a bias voltage polarity dependency that could be interpreted in a lower density of states on Se atoms of chain II for the occupied states than for the empty states.