Optical absorption of CuGeO3

Abstract

The absorption coefficient of CuGeO 3 was measured in the energy range of 1-4 eV, in a temperature interval between 300 and 20 K, for light polarized along the b and c crystal axes. A weak absorption band at 1.7 eV is attributed to copper d-d transitions partially allowed by electron-phonon interactions; this is followed by a region of transparency leading to a strong exponential edge above 3 eV, which is identified with the onset of charge-transfer transitions. ͓S0163-1829͑96͒53040-7͔

Scaling of theH-TPhase Diagram ofCuGeO3

“…The phonon involved in this process has been determined in Ref. [18] and corresponds very likely to the IR active phonon athv p 28.5 meV [21]. With that analysis one gets S values of about 6 which correspond to a very strong electron-phonon interaction.…”

“…Therefore one observes around 1.47 eV the so-called zerophonon line (ZPL) for which l 0 and the energy is E ZPL . Of course, due to the coupling of the different Cu 21 ions, one expects the corresponding transition to be broadened and this is indeed observed since the width G of the spectra is of the order of 10 meV and does not vary much with T and H. Because this width remains much smaller than that of the main absorption band (ഠ0.5 eV) [18] one may still apply, in a first approximation to our case, the results known for isolated centers. In this context the probability to absorb a photon in the lth vibronic state, neglecting the constant electronic part of the matrix element, is given by P͑l͒ exp͑2S͒S l ͞l!.…”

“…Furthermore, since R is not expected to vary significantly neither with T nor H one arrives at T ͞T 0 exp͓2͑a 2 a 0 ͒d͔ exp͑2Dad͒, where all optical functions are dependent on the photon energyhv. The absorption strength is strongly polarized [16,18], but all discontinuities related to the phase transitions are not dependent on this polarization [16]. Since it is not easy to perform polarized measurements in a high pressure cell, we report here on nonpolarized measurements only.…”

“…This absorption band, centered at E ഠ 1.63 eV, has been analyzed carefully [18,19]. It results in a broad band with a maximum absorption coefficient a͑E͒ ranging from 500 to 800 cm 21 (depending on the polarization), and shows that in the range of 1.47 eV the remaining absorption coefficient is expected to be less than 1 cm 21 .…”

“…For low absorbing media R reduces to ͑n 2 1͒ 2 ͑͞n 1 1͒ 2 , where n is the refractive index (ϳ2.5 for CuGeO 3 [18]), implying R ഠ 0.13. In such conditions, when measuring the ratio of the transmission coefficient with respect to that of a reference point (T 0 ), the denominator of Eq.…”