Abstract:
Polarized reflectance measurements of the quasi I-D charge-transfer salt (TMTSFh CI04
were carried out using a Martin-Puplett-type polarizing interferometer and a 3He refrigerator
cryostat, at several temperatures between 0.45 K and 26 K, in the far infrared, in
the 10 to 70 cm- 1 frequency range.
Bis-tetramethyl-tetraselena-fulvalene perchlorate crystals, grown electrochemically and
supplied by K. Behnia, of dimensions 2 to 4 by 0.4 by 0.2 mm, were assembled on a flat
surface to form a mosaic of 1.5 by 3 mm. The needle shaped crystals were positioned
parallel to each other along their long axis, which is the stacking direction of the planar
TMTSF cations, exposing the ab plane face (parallel to which the sheets of CI04 anions
are positioned). Reflectance measurements were performed with radiation polarized
along the stacking direction in the sample.
Measurements were carried out following either a fast (15-20 K per minute) or slow
(0.1 K per minute) cooling of the sample. Slow cooling permits the anions to order near
24 K, and the sample is expected to be superconducting below 1.2 K, while fast cooling
yields an insulating state at low temperatures.
Upon the slow cooling the reflectance shows dependence with temperature and exhibits
the 28 cm- 1 feature reported previously [1].
Thermoreflectance for both the 'slow' and 'fast' cooling of the sample calculated relative
to the 26 K reflectance data indicates that the reflectance is temperature dependent,
for the slow cooling case only.
A low frequency edge in the absolute reflectance is assigned an electronic origin given its strong temperature dependence in the relaxed state. We attribute the peak in the
absolute reflectance near 30 cm-1 to a phonon coupled to the electronic background.
Both the low frequency edge and the 30 cm-1 feature are noted te shift towards higher
frequcncy, upon cntering the superconducting state, by an amount of the order of the
expected superconducting energy gap.
Kramers-Kronig analysis was carried out to determine the optical conductivity for the
slowly cooled sample from the measured reflectance. In order to do so the low frequency
data was extrapolated to zero frequency using a Hagen-Ru bens behaviour, and the high
frequency data was extended with the data of Cao et al. [2], and Kikuchi et al. [3].
The real part of the optical conductivity exhibits an asymmetric peak at 35 cm-1, and
its background at lower frequencies seems to be losing spectral weight with lowering of
the temperature, leading us to presume that a narrow peak is forming at even lower
frequencies.
