Investigation of Superconducting State of Sr_2RuO_4 by Far Infrared Spectroscopy

G.V. Sudhakar Rao, and M. Reedyk
Department of Physics, Brock University, St. Catharines, ON, L2S 3A1, Canada

N. Kikugawa, and Y. Maeno
Department of Physics, Kyoto University, Kyoto 606-8502, Japan and
CREST, Japan Science and Technology Corporation, Kawaguchi, Saitama
332-0012, Japan

The ratio of the far infrared reflectance of Sr_2RuO_4 in the
superconducting state to that in the normal state has been measured along
the a- and c-axes using an ac-plane oriented crystal and within the
ab-plane using an ab-plane oriented crystal. The measurements were taken
above and below the 1.42 K superconducting transition temperature of both
the samples using a helium-3 cryostat and step-and- integrate
Martin-Puplett type polarizing interferometer with 1 cm^(-1) resolution.
For light polarized within the ab-plane, the superconducting state
reflectance is enhanced over that of the normal state below ~10 cm^(-1).
This energy interval coincides with the superconducting energy gap
established via tunneling measurements [1]. The enhancement is
significantly larger than that expected for a BCS s-wave superconductor.
Along the c-axis no such enhancement is observed within the energy range
investigated.

Work at Brock University was supported by the Natural Sciences and
Engineering Research Council of Canada.

[1] M.D. Upward, L.P. Kouwenhoven, A.F. Morpurgo, N.Kikugawa, Z.Q. Mao,
and Y. Maeno, Phys. Rev. B 65, 220512 (2002)