The Superconducting Energy Gap of Rb 3 C 60 as Measured by IR Transmission in Thin Films.
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Abstract The superconducting energy gap of Rb 3 C6 0 has been measured by infrared transmission through a thin film. Resistivity measurements on the same samples in situ indicate a transition to superconductivity at the expected temperature, -30K, with distinct gap features appearing in the transmission below this temperature. The results are interpreted in terms of the BCS theory.
Introduction At temperatures below the critical temperature, T 0 , most of the known superconductors develop a well defined gap in the electronic density of states. In the weak-coupling BCS theory, the zero temperature gap is expected to have a value 2A = 3.5kBTC. For strong electron-phonon coupling, a gap value exceeding 3.5 kBTC is expected. Accurate measurements of the temperature dependence of the gap can answer fundamental questions about the nature of a superconductor. Measurements of the gap in Rb 3 C 60 have already been carried out by other groups, each reporting a different value for 2A ranging from 2 to 5.2kBTc [1, 2, 3]. The great disparity in reported values further stresses the need for an accurate and reliable measurement of the energy gap. The most recent and most accurate measurements to date are those of Degiorgi et al., carried out in a reflecting geometry on both polycrystalline pellets [4, 5] and single crystals [6]. It is to their results that the work presented here is best compared.
Measurement of the Gap The Rb 3 C 60 sample measured in this experiment was prepared in a special glass sample chamber designed for infrared (IR) transmission measurements on thin films [7]. The chamber is made of Pyrex glass with two silicon windows sealed to opposite ends of a 1cm high cylindrical sample area. One of the two windows serves as a C 60 substrate and four leads patterned on it allow in-situ resistivity measurements. C 60 is evaporated under high vacuum onto the substrate and an alkali-metal film, in this case Rb, is deposited in an appendage to the sample space. The chamber is then sealed off, preventing further contamination from the vacuum system and allowing the alkali metal to getter any remaining contaminants.
295 Mat. Res. Soc. Symp. Proc. Vol. 359 01995 Materials Research Society
Rb 3C 6o Spectra ratioed to 35K
Rb 3C60 Thin Film Spectra (below Tc) 6~
100 and 700 cm" data combined
Sample 73, BNL April..94
25 K .30 K
~
14
Order of traces
o0
6.4,10 K
15 K
E
6.4, 10 KM 15 K 20 K 25, 30K
612 (D
(a
25 K
)
E 6.4 K...0
cc
0.
1-2
c
0,
Tdoet_
0
Sample ton6
0,tepedgwamdnnoib
100 200
300
400
500
20
600
100
73, BNL April '94m
200
300
400
Wave number (cm')
Wave number (cm-) Figure 1. The raw data for the 40 -700cm- 1 region, below TC. A distinct gap feature appears below 30 K.
Figure 2. Ratios of the superconducting spectra to the normal-state spectrum for Rb3 C6 0. COrnpare to the fits shown in Figure 3.
To dope the C6 0 , the appendage is warmed in an oil bath to increase the alkali metal's vapor pressure and force the metal to migrate towards the C 60 film. Doping is monitored in
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