Infrared and Raman Spectroscopy of Phonons in n-Type High T c Superconductors
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INFRARED AND RAMAN SPECTROSCOPY OF PHONONS IN n-TYPE HIGH Tc SUPERCONDUCTORS M. K. CRAWFORD 1 , G. BURNS 2 G. V. CHANDRASHEKHAR 2 , F. H. DACOL2 , W. E. FARNETH1 , E. M. MC CARRON, I111 and R. J. SMALLEY1 1E. I. du Pont de Nemours & Co., Inc., CR&D Department, Experimental Station, Wilmington, DE 19880-0356 2 IBM Research Division, T. J. Watson Research Center, Yorktown Heights, NY 10598-0218 ABSTRACT We have measured the infrared (IR) reflectance and Raman spectra of electron doped and undoped Nd2 CuO 4 and Pr 2 CuO 4. Utilizing the results from doped ceramics and undoped ab plane oriented single crystals, we observe and assign all the IR active phonons (3A2u + 4Eu) predicted by group theory. We have also observed the Raman active modes involving atom motion along the c axis. We compare our results to those found in the related hole superconductors based upon La2 CuO 4 . INTRODUCTION The recently discovered 1 electron superconductors crystallize in the T' structure which is closely related to the T structure of La2 CuO 4 (Fig. 1). From the point of view of lattice vibrations there is one important difference between these two structures. The oxygen atoms which lie off the Cu-O sheet, O(4d) in the T' structure and O(4e) in the T structure, are located at different sites (Wyckoff 4d and 4e, respectively). This difference leads to a change in the group theoretical classification of the Raman active phonons in the two structures 2 . In particular, the symmetry species of the Raman modes are (Alg + Big + 2Eg) in the T' structure. Only the Nd atom moves in the A1 mode and only O(4d) moves in the B1 mode. This is different than in La2 CuO4, where there are (2A1 + 2Eg) Raman modes, with both O(4e) and La able to move in the Alg vibrations. Both structures have (3 A2u + 4 Eu) IR allowed modes.
Nd (4e)
0-
S(4d)
La (4e)Cu (2a) o(4c)
))
Nd (4e) La (4e) -
Figure 1. The T (left) and T (right) structures.
Mat. Res. Soc. Symp. Proc. Vol. 169.
1990 Materials Research Society
O-(
1014
EXPERIMENTAL Sample preparation is described elsewhere 2 ,3 . The IR reflectance and Raman measurements have also been described previously4 . 5 . RESULTS In Fig. 2a we show the IR reflectance from the ab face of a single crystal platelet of Nd2 CuO 4 . We also show the results of an oscillator fit to the observed reflectance. The transverse optic (TO) phonon energies used in the fit were determined from the maxima in Im (e)4 obtained by a Kramers-Kronig analysis of the reflectance (Fig. 2b). We observe all Eu modes.
E-rgy (e--')
Figure 2. IRdata for single crystal Nd2CuO4.
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_
ICnEr(Ce--) Figure 3. IRdata for ceramic (Nd2.xCex)Cu04.
In order to measure the 3A2u (c axis) modes, we utilize the fact that, in a doped ceramic, the high conductivity in the ab plane screens the 4Eu phonon response while leaving the 3A2u modes virtually unaffected. In Fig. 3a we show the IR reflectance of an undoped ceramic Nd2CuO 4 sample, in Fig. 3b we show the doped ceramic reflectance and oscillator fit to the 3A2u modes and in Figure 3c we show Im(e)
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