Surface Charge Density Wave Transition in NbSe$_3$

TL;DR

This study investigates surface charge-density wave transitions in NbSe₃ using STM, revealing a higher surface transition temperature than the bulk and suggesting a Berezinskii-Kosterlitz-Thouless type transition due to 2D fluctuations.

Contribution

The paper provides the first direct STM measurements of surface CDW transition temperatures in NbSe₃, highlighting a surface enhancement and analyzing the transition as a 2D BKT-type phase transition.

Findings

Surface CDW transition occurs at 70-75 K, above the bulk T₍₂b₎=59 K.

Surface transition exhibits characteristics of a 2D Berezinskii-Kosterlitz-Thouless transition.

Surface critical temperature is more than 15 K higher than the bulk transition.

Abstract

The two charge-density wave (CDW) transitions in NbSe$_3$ %at wave numbers at $\bm{q_1}$ and $\bm{q_2}$, occurring at the surface were investigated by scanning tunneling microscopy (STM) on \emph{in situ} cleaved $(\bm{b},\bm{c})$ plane. The temperature dependence of first-order CDW satellite spots, obtained from the Fourier transform of the STM images, was measured between 5-140 K to extract the surface critical temperatures (T$_s$). The low T CDW transition occurs at T$_{2s}$=70-75 K, more than 15 K above the bulk T$_{2b}=59$K while at exactly the same wave number. %determined by x-ray diffraction experiments. Plausible mechanism for such an unusually high surface enhancement is a softening of transverse phonon modes involved in the CDW formation.% The large interval of the 2D regime allows to speculate on % %the special Berezinskii-Kosterlitz-Thouless type of the surface transition expected for this incommensurate CDW. This scenario is checked by extracting the temperature dependence of the order % %parameter correlation functions. The regime of 2D fluctuations is analyzed according to a Berezinskii-Kosterlitz-Thouless type of surface transition, expected for this incommensurate 2D CDW, by extracting the temperature dependence of the order parameter correlation functions.