Weak dimensionality dependence of charge density wave transition in 2H-NbSe2

Abstract

The effect of dimensionality on charge density wave (CDW) transition temperature (TCDW) in 2H-NbSe2 is still under debate. Raman measurements uncovered highly enhanced TCDW for few-layer samples, while scanning tunneling microscopy results suggest comparable value of bulk crystals. Here, we obtained high-quality crystals of 2H-NbSe2 with residual resistivity ratio up to 120 and processed thin flakes by mechanical exfoliation. Electrical resistance measurements were carried out on crystals with different thickness to monitor the dimensionality dependence of TCDW, superconducting Tc, and upper critical field Hc2. It is revealed that when the bulk crystal evolves into few layers, the TCDW only increases slightly, though the variations of Tc and upper critical field Hc2 are consistent with previous results. The observed weak dependence of long-range CDW order on dimensionality agrees well with the recent theoretical calculations on anharmonic spectra. These results reconcile experiment and theory, and thus shed light on the mechanism of CDW for thin flakes of 2H-NbSe2.