Infrared Optical Properties of the Quasi-One-Dimensional Conductors NbSe sub 3 and (TMTSF) Sub 2 ClO sub 4

摘要

The technique of far-infrared Fourier-transform spectroscopy has been used to study the properties of two materials, NbSe sub 3 and (TMTSF) sub 2 ClO sub 4 . NbSe sub 3 undergoes incommensurate charge-density-wave (CDW) transitions and exhibits electrical transport by sliding CDW's. (TMTSF) sub 2 ClO sub 4 undergoes order-disorder, spin-density-wave, and superconducting transitions at ambient pressure. It is a member of the first class of organic materials to be synthesized which exhibit superconductivity. We have done a Kramers-Kronig analysis of the far-infrared reflectance of NbSe sub 3 to obtain an estimate of the optical constants. We have used models of the frequency-dependent conductivity to fit the data. General arguments show that at 2K a CDW energy gap exists between 120 and 190 cm exp -1 , the relaxation time(s) of the free carriers and CDW pinned mode is >3 x 10 exp -12 s, and the ratio of the free-carrier concentration to band mass is <2 x 10 exp 20 cm exp -3 /m sub 0 . The strength of the pinned mode obtained from the detailed fit is inconsistent with the classical model of CDW transport. We have also measured the far-infrared reflectance of (TMTSF) sub 2 ClO sub 4 and performed a Kramers-Kronig analysis to obtain the optical constants. A large peak in the conductivity occurs above 100 cm exp -1 , and a smaller peak occurs at approx. 25 cm exp -1 . We have no explanation at present for the large peak, but we believe that the small peak is a phase phonon. It occurs when the orientations of the ClO sub 4 anions order and thereby induce a charge-density wave in the lattice. Our data, however, are inconsistent with an interpretation based upon a theory of quasi-one-dimensional fluctuational superconductivity. (ERA citation 09:000783)