Optical spectra and electronic structure of high Tc oxide superconductors

Physica C 162-164 (1989) 1117-1118 North-Holland

OPTICAL SPECTRAAND ELECTRONIC STRUCTURE OF HIGH Tc OXIDE SUPERCONDUCTORS Jiro TANAKA, Koji KAMIYA*, Masaaki SHIMIZU, Chizuko TANAKA, Hiroyuki OZEKI and Satoru MIYAMOTO Department of Chemistry, Faculty of Science, Nagoya University, Chikusa, Nagoya, 464-01, Japan * I n s t i t u t e for Molecular Science, Okazaki, 444, Japan Reflection spectra of single crystals of La1.85Sro.1sCu04, YBapCuRO~q, Bl~(Sr, Ca)~CupOB and Tl.Ca^Ba^Cu~O are measured from far IR to uv reglon. We'asEi~B-exci~on ban~s ~n IR region to L i~te~ba~dXtransitions by the calculation of energy band. Dielectric functions are determined from the spectra, which indicate importance of electron exciton interaction for pair formation mechanism. I. Introduction

A single crystal of Lal.85Sro.15Cu04 is grown by Kojima and Tanaka of Yamanashi University (1)

Dielectric functions and exclton bands of cuprate superconductor are of particular importance to find the mechanism of pair

and that of TlzCa2Ba2Cu30x is grown at Sumitomo Electric Industries.(2) Other crystals are

formation.

made in our laboratory.

We investigate them by measuring

reflection spectra of single crystals. 3. Reflection Spectra Reflection spectra of the (001) faces of

2. Experimental Method and Materials Normal incident reflection spectra are measured with three spectrophotometers, each of which consists of a microscope and a Fourier interferometer or a double menochromator. 1.0

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these crystals are shown in Figs.l-3 together with dielectric functions determined from these spectra.

The optical conductivity spectra show

contributions of free carrier and exciton bands.

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W ~ E NUMBER / cn"T1 FIGURE 1 Reflection spectra and dielectric functions of Lal.85Sro.15Cu04

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WAVE NUMBER / cm-I FIGURE 2 Reflection spectra and dielectric functions of Bi2(Sr, Ca)3Cu208

I 118

J. Tanaka et al. / Optical spectra and electronic structure

The peak positions of exciton bands are characterized by layered s t r u c t u r e of CuO2 plane;

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x(Y)

420 and 2200 cm-3 in LaSrCuO (one layer), 400,

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950 and 3500 cm-1 in BiSrCaCuO (two layers) and 520, 1230 and 5000 cm- I in TlCaBaCuO (three layers).

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FIGURE 3 Reflection spectra and dielectric functions of T12Ca2Ba2Cu30X (Tc-II6K) 4. Energy Band Structure of CuO2 Plane Energy band structure of the CuO2 plane is calculated with a novel approach (3) in order to give assignment of the exciton of CuO2

FIGURE 4 Calculated energy band and photoemission spectra of BiSrCaCuO (4) frequency coupled with exciton mpe. The real part of I / c i s negative for a l l of these crystals in the range below~pe originated from exciton-plasmon oscillation. The attractive interaction bewtween electrons is mediated by excitons existing in the above range. Ordinary plasma frequency ~ P appears at lower energy region. ACKNOWLEDGEMENTS We thank to Prof.H. Kojima and Dr.K. Tada

plane. The calculated energy bands are shown in

(Sumitomo Electric Ind.) for g i f t of crystals.

Fig, 4 to compare with photoemission spectra of BiSrCaCuO,(4) The exciton bands at 420 and

REFERENCES

2100 cm-1 in LaSrCuO are explained as interband t r a n s i t i o n s to the unoccupied part of ltOMO(A) band from the degenerate next HOMO(B) bands, both of which are composed of O(Zpo) o r b i t a l s . 5. D i e l e c t r i c Functions The maximum of -Im(1/£) corresponds to plasma

I. I.Tanaka and H. Kojima, Nature 337 (1989) 21. 2. T.Kotani, T. Kaneko, H.Takei and K.Tada, Jap.J.Appl.Phys. to be published. 3. J.Tanaka, K. Kamiya and C.Tanaka, unpublished work. 4. T.Takahashi et al. Phys.Rev.B 39 (198g) 6636