ELSEVIER Physica C 341-348 (2000) 615-616
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A C o m p a r i s o n b e t w e e n the F e h r e n b a c h e r - R i c e a n d the L i e c h t e n s t e i n - M a z i n m o d e l s
S. J. Liu ~, J. -Y. Lin b, J. M. Chen ¢, P. Nachimuthu ~, C. W. Luo ~, K. H. Wu', J. Y. Juang', T. M. Uen', and Y. S. Gou ~
'Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan blnstitute of Physics, National Chiao Tung University, Hsinchu, Taiwan
~Synchrotron Radiation Research Center, Hsinchu, Taiwan
Unoccupied electronic states of Yi.xPrx.Ba2Cu307.y ( x = 0 - 1) thin films have been investigated using high- resolution polarization-dependent O K-edge x-ray absorption spectroscopy (XANES). After determining the hole distribution among different oxygen sites of the Pr-doped thin films, we compare the experimental data with the Fehrenbacher and Rice (FR) model and the Liechtenstein and Mazin (LM) model, both of which are based on the hybridization of Pr 4f._t~2.y2) and O 2p, orbitals in PrBa2Cu3OT.y Our experimental results are more consistent with the FR model.
Among the rare earth element-substituted
isomorphic YBa2Cu3OT.y superconductors,
PrBa2Cu307.y is an exception that fails to exhibit
superconductivity. The suppression of
superconductivity in the Yt.~Pr~Ba2Cu3OT.y system is strongly related to the Pr concentration. A large number of theoretical models have been proposed to explain the absence of superconductivity in PrBa2Cu3OT.y. However, Fehrenbacher and Rice (FR) proposed a localized Pr 4f~.~2)-O 2p, hybridized state which binds doped holes to Pr sites and causes a hole depletion in the CuO2 planes [1]. The FR model also assumes that the O 2p. orbitals rotate by an angle of about 45 ° and point toward the central Pr ion. Based on the idea of Pr 4f~t~2.y2)-O 2p, hybridization (FR band), Liechtenstein and Mazin (LM) proposed another model in which the direct hopping between oxygen orbitals is taken into account [2]. They found that, in PrBa2Cu3OT.y , there forms an additional hole-depleting band which crosses the Fermi level and consequently grabs holes from the CuO2 band. However, in their model the O 2p. holes are treated as planar (P~y) character. It means that the rotation angle of O 2p. orbitals is 0 °. Since FR band may be closely related to the novel superconductivity in PrBa~Cu3OT.y which has been reported recently [3,4], it is of interest to put more stringent tests on these two competing models. To the rotation angle, denoted as y, of O 2p, orbitals to the CuO 2 plane, the predictions of the FR model and the LM model are quite different. In terms of the LM
0921-4534/00/$ - see front matter ~) 2000 Elsevier Science B.E PII S0921-4534(00)00615-8
model, the rotation angle increases with Pr doping, while it decreases with Pr doping according to the FR model [4]. We derive the rotation angles for each Pr doping level from the O ls XANES of Y ,
xPrxBa,2Cu306.9 t h i n f i l m s .
The well c-axis-oriented Yl.~Pr~Ba2Cu306.9 thin films with thickness of 400 nm were deposited on SrTiO3 substrates by pulse laser deposition. The O K-edge x-ray absorption spectra were carded out using linearly polarized synchrotron radiation fi'om 6-m high-energy spherical grating monochromator (HSGM) beamline located at SRRC, Taiwan. The in- plane spectra (E//ab, 0=0 °, 0 is the angle between the incoming beam and the sample surface normal) were obtained in a normal-incidence alignment. The samples were then rotated, with 0=60 ° and 75 ° , to obtain the polarized x-ray absorption spectra I(0). According to l(O)=Iw/,b COS2(0) + Iw/csin2(O), we can obtain the out-of-plane spectra (E//c, 0=-90 °) spectra. The hole numbers can be obtained by integrating the cross sections of the spectra. The rotation angle y for each sample with different Pr doping can be calculated by the following equations which are modified from those in Ref. [5],
n~=nF(2 - n/), nFR=nF(ny - 1), 2F/E//ab~'F/FRCOS2y 'q" F/ZR q- F/chain, nw/¢=nrRsin2 y + n~ex, F/Pr "F nFR -F nz~ + napcx + rich,in=0.9 (1)
6 1 6 S.,~ Liu et al./Physica C 341-348 (2000) 615-616 OJ 0 e- E t~ ,.Q U 6 4 ¸ 2 ¸ 0 ¸ 8 6 ¸ O ¢.. --s ,-- 4 J~ 2 526 528 530 532 534 P h o t o n e n e r g y ( e V )
apical oxygen sites (n,pe~). For Pr-doped samples, the peak also contains contributions from the FR state. In Fig. 2, the rotation angles o f O 2p~ orbitals, y, calculated by using equations (1) in which we take n/=1.66, are plotted as a function of Pr concentration, x. It shows that ydecreases with increasing x.
Based on our measurements regarding the rotation angle o f O 2p. orbitals to the CuO2 planes, our preliminary results are more consistent with the FR model than the LM model.
This work was supported by National Science Council of Taiwan ROC under Contract Nos. NSC 89-2112-M-009-030 and NSC 89-2112-M-009-007, Ca ¢p 45 'ID 40
~35
O 3o "6 lip 25 Ca e- 20 t~ t= 15 O~
~o 5 e- I ' - 0 0 . 0 • yl_xPrxBa2CuaOe.9 FR model • • 0.2 0.4 0.6 0.8 1.0 Pr concentration, x Fig. 1. O ls absorption spectra of YBazCu3069 andPrBa2Cu306. 9 for polarization (A) E//ab and (B) E//c.
Where nr~ denotes the number of holes transferred to Pr, nn~ the number o f holes situated on the planar O 2p. orbitals, nv the density of Pr 4+ ions, nza the number o f holes situated on the planar O 2p. orbitals, n~,~, the number of holes on the O sites in the CuO chains, n,pex the number o f holes on the apical O sites, nf the Pr 4+ 4 f electrons. The O K-edge XANES o f YBa2Cu306.9 and PrBa~Cu306. 9 for E//ab and E//c are plotted in Fig. 1. No spectrum contribution from the SrTiO3 substrate was observed as reported in Ref. [6]. In the E//ab spectra, for YBa~Cu306.9, the peak at about 528.5 eV is ascribed to the Zhang-Rice (ZR) band [4] and hole state in the CuO chains. However, for Pr-doped samples the peak also contains contributions from the FR state. In the E//c spectra, the peak at about 527.5 eV of YBa2Cu306.9 is wholly attributed to hole state in the
Fig. 2. The relation o f the rotation angle o f O 2p,, y, and the Pr concentration, x. The possible value of y for x--0.8 is from Ref. [5]. The two solid curves are predictions of the FR (sin2?=(2/3)(1-x)/(2-x)) [4] and LM [7] models. The dash line indicates the argument that, because of dispersion of the FR band at large x, the FR model should give the same value o f y as the LM model at x=l [4].
References
[ 1] R. Fehrenbacher and T. M. Rice, Phys. Rev. Lett. 70 (1993) 3471.
[2] A. I. Liechtenstein and I. I. Mazin, Phys. Rev. Lett. 74 (1995) 1000.
[3] Z. Zou et al, Phys. Rev. Lett. 8ll (1998) 1074. [4] I. I. Mazin, Phys. Rev. B 60 (1999) 92. [5] M. Merz et al., Phys. Rev. B 55 (1997) 9160. [6] S. Gerhold et al., Phys. Stat. Sol. 215 (1999) 579. [7] I. I. Mazin and A. I. Liechtenstein, Phys. Rev. B
