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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 byscanning tunneling microscopy STM on \emph{in situ} cleaved $\bm{b},\bm{c}$plane. The temperature dependence of first-order CDW satellite spots, obtainedfrom the Fourier transform of the STM images, was measured between 5-140 K toextract the surface critical temperatures T$ s$. The low T CDW transitionoccurs at T$ {2s}$=70-75 K, more than 15 K above the bulk T$ {2b}=59$K while atexactly the same wave number. %determined by x-ray diffraction experiments.Plausible mechanism for such an unusually high surface enhancement is asoftening of transverse phonon modes involved in the CDW formation.% The largeinterval of the 2D regime allows to speculate on % %the specialBerezinskii-Kosterlitz-Thouless type of the surface transition expected forthis incommensurate CDW. This scenario is checked by extracting the temperaturedependence of the order % %parameter correlation functions. The regime of 2Dfluctuations is analyzed according to a Berezinskii-Kosterlitz-Thouless type ofsurface transition, expected for this incommensurate 2D CDW, by extracting thetemperature dependence of the order parameter correlation functions.



Autor: Christophe Brun, Zhao-Zhong Wang, Pierre Monceau, Serguei Brazovskii

Fuente: https://arxiv.org/







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