Raman Scattering From the Superconductor Niobium-Diselenide

Abstract

At 33 K 2H-NbSe(,2) distorts into an incommensurate charge density wave (CDW) state. The Raman spectrum shows amplitude modes of A and E symmetry near 40 cm('-1) induced by the CDW. At 7 K this material becomes superconducting, and at 2 K we observe additional Raman peaks near 20 cm('-1), one each of A and E symmetry. These are close in energy to the energy gap 2(DELTA), as measured by far infrared transmission. The Raman activity of the gap excitations is believed to be caused by coupling to the Raman active CDW phonon modes. Such coupling is shown directly by the changes in the Raman spectra at 2 K due to an applied magnetic field. The zero field results can be explained by a model (by C. A. Balseiro and L. M. Falicov) in which the optical CDW phonon modes couple to superconducting electrons, and the resulting phonon self energy is calculated using the BCS theory for the electrons.

Extensions of the zero field theory to include lifetime broadening effects and consequences of changing the electron-phonon coupling are presented. The results in an external magnetic field are explained in terms of a complex gap parameter.

These results are confirmed by experiments on samples of 2H-NbSe(,2) that have their CDW transition suppressed by non-magnetic impurities but still retain their superconducting properties.