Effect of nonmagnetic impurities on the Raman spectra of the superconductor niobium diselenide

Abstract

We recently published the results of observation of the superconducting energy gap in $2H-\mathrm{N}\mathrm{b}{\mathrm{Se}}_2$ by Raman scattering. The gap is observed by the appearance of two new Raman-active modes of different symmetries $A$ (18 ${\mathrm{cm}}^{-1\mathrm{}}$) and $E$ (15 ${\mathrm{cm}}^{-1\mathrm{}}$) when the sample is immersed in superfluid helium. Theoretical calculations by Balseiro and Falicov show that these gap excitations gain their Raman activity by coupling to low-lying phonons. Crystal growth techniques, which produce nonmagnetic impurities, are known to have a small effect on superconductivity while tending to inhibit the formation of charge-density waves (CDW) in $2H-\mathrm{N}\mathrm{b}{\mathrm{Se}}_2$. By studying such diverse samples, we present data to show the necessity of the CDW state to observe the superconducting gap excitation. The role of impurities in suppressing CDW formation is also revealed from the data.