Local structural and electronic changes accompanying photodoping in YBA2Cu3O6+x

PHYSICA ELSEVIER

Physica C 292 (1997) 163-170

Local structural and electronic changes accompanying photodoping in Y B a z C u 3 0 6 + x T.A. Tyson

a,* J.F. Federici a, D. Chew a, A.R. Bishop b, L. Furenlid c, W. Savin a, W. Wilber d a Department of Physics, New Jersey Institute of Technology, Newark, NJ 07102, USA b Los Alamos National Laboratory, Los Alamo& NM 87545, USA ¢ Brookhaven National Laborato~, Upton, Long Island, NY 11973, USA a Physical Sciences Directorate, Army Research Laboratory. Fort Monmouth, NJ 07703, USA

Received 21 August 1997; revised 26 September 1997; accepted 29 September 1997

Abstract Polarized X-ray absorption measurements reveal that photodoping of oxygen deficient YBa2Cu306+ . (YBCO) produces a local axial distortion of the CuO~ chains. The broad normal state axial Cu(1)-O(4) bond distribution becomes two well-defined peaks in the photodoped state and suggest local ordering. XANES measurements reveal a transfer of hole density into the CuO 2 planes. These results support a common structure-related transport enhancement mechanism in chemically doped and photodoped high temperature superconductor systems. © 1997 Elsevier Science B.V. Keywords." XAFS; Structure of photodoping; Cu-O chains; Cu-O planes

1. Introduction Transition metal oxides are a class of compounds with highly anisotropic charge distributions. Their crystal structures are the result of a very delicate balance of short and long range forces [1]. The transfers of charge on a microscopic scale is expected to have significant effects on the bulk properties such as the conductivity. In the layered high temperature superconductor (HTSC) systems (YBa2Cu306+ x (YBCO), for example), pressure induced transfers of charge (holes) from the CuO x

* Corresponding author. Tel.: + 1 201 642 4681; Fax: + 1 201 596 5794.

chains to the CuO 2 planes have been shown to increase the critical temperature (T~) [2-6]. For a fixed values of x, it has been found that a local ordering of the ab-plane oxygen atoms in oxygen deficient YBCO enhances the magnitude of T~ [ 7 10]. Both the a- and c-axes were observed to contract during ordering by ~ 0.04%. In addition, it has been observed that He + irradiation induced atomic disorder reduces the magnitude of Tc [11,12]. These measurements suggest a connection between the local atomic arrangement and the transport properties of YBCO. Indirect structural analysis of Y B a z C u 3 0 7 was obtained from inelastically scattered neutrons [13]. The structure factor, S(Q,E), exhibited an anomalous behavior at T~ similar to critical phenomena and a local bucking with a <

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T.A. Tyson et al. / Physka C 292 (1997) 163-170

110 > structure was found. Ion channelling measurements have found changes in the Cu(1), Cu(2) and 0(4) positions in the ab-plane at T~ [14]. In addition, X-ray absorption fine structure (XAFS) measurements on the fully doped system revealed increases in the c-axis Cu-O(4) pair correlations near T~ [15] and found evidence for the existence of an anharmonic double-well axial oxygen potential [16-20] or, more generally, a two-site axial C u - O distribution [21,22]. Kudinov and co-workers [23-28] reported changes in the normal state conductivity of oxygen deficient YBCO (in the semiconducting phase) on exposure to optical light (photodoping). The resistance was found to decrease by as much as 50% with a maximum near x = 0.38. Unlike the transient photoconductivity found in YBCO which had a decay time of 10 - 9 s [29-32] or the photoconductivity in insulating La2CuO 4 with a decay time of 10 s at room temperature [33], the photoinduced state was found to persist for days if the sample was maintained at liquid nitrogen temperatures. When induced at room temperature the decay time was found to be on the order of hours [34-36]. In the metallic phase, photodoping has been found to increase the critical temperature of YBCO [37-39] and Sr doped YBCO [40]. Interestingly, X-ray diffraction measurements of the c-axis lattice constant revealed a contraction by ~ 0.04% when in the persistent photoinduced conductivity (PPC) state [34,35,41]. Due to the long lifetime of the photoinduced state, the YBCO system provides an ideal model for the study of mechanism for photoinduced changes in transport, in particular, and the origin of superconductivity in transition metal oxides, in general. The small contraction of the c-axis observed by X-ray diffraction (a global or long-range structural probe) suggest that significant changes in the local structure accompany the PPC state. However, no local structural studies have been performed on the photodoped system. In analogy with a studies of local structural changes accompanying the metal insulator transition in the doped perovskite [42-46], LaMnO 3, and temperature dependent studies of HTSC systems discussed above, we have examined the local structure about Cu in the normal and photodoped phases by c-axis polarized XAFS analysis. The c-axis orientation was chosen because of the changes seen by

X-ray diffraction tbr the c-axis planes and because of the ability to distinguish the chain (ribbon) Cu(1)-O(1) distance ( ~ 1.9 i ) from the plane Cu(2)-O(4) distance ( ~ 2.3 A) (see Ref. [47] for structural information on YBCO). The X-ray absorption near edge structure (XANES) revealed a transfer of electrons out of the CuO 2 planes. Analysis of the high energy or XAFS signal revealed that the broad normal state c-axis Cu(l)-O(4) bond distribution becomes a two component-peak in the photodoped state suggesting ordering. The long Cu(2)-O(4) bond remained unchanged with photodoping apart from changes in the bond correlation.

2. Experimental methods Thin films (1300 A) of YBa2Cu306+.~ were prepared by pulsed laser deposition as described in Ref. [48,49]. X-ray diffraction was used to characterize the films. The oxygen stoichiometry was determined by comparison of the X-ray diffraction determined c-axis length with bulk derived values [50]. Spectra were measured at Brookhaven National Laboratory's National Synchrotron Light Source (NSLS) beam line X18B under dedicated synchrotron radiation production conditions. S i ( l l l ) crystals were used to monochromatize the X-ray beam. The spectra were taken in fluorescence mode using an Ar-filled Lytle ion chamber and a 3-absorption length Ni filter. Out-of-plane polarized measurements of the c-axis oriented films were performed on a films with x = 0.4 (with an orientation error of ± 15°). The detector position was optimized to eliminate diffraction peaks. A Dysplex closed cycle He system was used to maintain the sample temperate at 95 K. Absorption spectra were collected in the undoped state. Subsequently, photodoping was accomplished using a Hg vapor lamp to expose a region of film 3 mm in diameter (1 W / c m 2) for 10 h, which is adequate to produce a fully doped state [34,35,38]. An ORIEL IR-blocking filter was used to reduce sample heating. X-ray photographs were taken to ensure that only the optically excited region of the film was probed by the X-ray beam. Use of a 1300 thick film ensured that optical penetration length was greater than the film thickness [51,52], thus ensuring

T.A. Tyson et al. / Physica C 292 (1997) 163-170

that a significant fraction of the measured film volume was excited by optical photons. The cryostat window was made of 3-ml thick Kapton with a cutoff near 450 nm. Hence the combination of the window and the IR-blocking filter produced an optical window with range ~ 4 5 0 to ~ 800 nm (or 1.5-2.8 eV). The reduction of the XAFS data was performed using standard procedures [53,54]. Calibration was accomplished by defining the first inflection point in the spectrum of a Cu foil as 8979 eV. The ionization threshold, E 0, was set at this value. The parameter A E 0 12.8 eV was obtained from a fit to CuO. The data were normalized by the value of a first order polynomial fit over the pre-edge region to zero and the value of a second order polynomial fit over the region above the edge to unity at E 0. Nine to thirteen scans were taken in both the normal and photodoped state. After comparison, 10-12 individual scans were averaged for the both doped and =

165

Table 1 c-axis C u - O structural parameters of YBa2Cu3064 Bond Normal

R

(A)

N

Cu(l)-O(4) 1.87+0.01 Cu(2)-O(4) 2.25+0.02

o-

(A)

2.0(fixed) 0.13-t-0.01 2.0(fixed) 0.15_+0.01

Photodoped Cu(I)-O(4) 1.81 _+0.01 1.1 ±0.2 0.07+_0.01 Cu(l)-O(4) 2.00-t-0.01 0.8_+0.1 0.04_+0.01 Cu(2)-O(4) 2.25(fixed) 2.0(fixed) 0.13+0.01 Errors on XAFS derived parameters are based on deviations of fit from data. All measurements were performed at 95 K. Parameters held fixed are labeled as such. E 0 was fixed at the value obtained from a fit CuO. SO was held fixed at 0.87 for all fits. SO is an atomic matrix element which should vary significantly only if the Cu content is varied. Two shell fits to the normal state Cu(1)-O(4) distribution collapsed to a single shell.

undoped sample. The XAFS were extracted from the spectra as the difference between the normalized spectra and an adjustable spline function fit through

1.2 o

=(D

1

O

0.8

U_

0.6

* E

i

. . . .

i

. . . .

i

. . . .

i

. . . .

i

. . . .

,

. . . .

I

. . . .

t

. . . .

i

. . . .

i

. . . .

0.6

0

Z

N

E O

z

¢

0.4

0.4

¢.-

0.2

"8

0.2

41.2 t-

8970

8980

8990

9000

9010

9020

0 8970

8980

8990 9000 Energy (eV)

9010

9020

Fig. 1. The XANES spectra of YBa2Cu306. 4 for the normal (solid line) and photodoped (dotted line) states reveal that photodoping induces a transfer of holes (4pz) to Cu(2). The inset displays the difference between the two spectra. All measurements reported in this work were performed at 95 K. (Additional spectral weight has also been transferred from outside of the range shown. However, this weight is a minor component. The quantitative structural information has been extracted from a detailed analysis of the high-energy region (Figs. 2-4) of the spectra.)

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T.A. T~'son et a l . / Physica C 292 (1997) 163-170

the post-edge region, the parameters of which were adjusted to minimize low frequency residuals in the Fourier transform. Information about the c-axis C u - O bond distributions was obtained by transforming the k 3 weighted raw data over the range 2.8 < k < 11.7 A - i (see Fig. 2), and then filtering the first shell over the range 0.8-2.0 A. The filtered shell was then fit to theoretical model signals (over the range 3 . 8 < k < 1 0 . 7 ,~-=) based on the photoelectron scattering factors of FEFF6 [55]. Average bond lengths, R, average coordination numbers, N, and Debye-Waller factors, ~r, were extracted from these fits (Table 1). The number of independent parameters that could be extracted for the fit is N'= 2 AkAR/oZC+ 2 = 7 [52], where Ak = 6.9 A-~ and A R = 1.2 A.

3. Results and discussion

Fig.

1 displays the XANES spectra of in the normal (solid line) and doped states (dotted line). A clear difference is seen between the two spectra. Since the dipole Cu K-edge (1 s) absorption samples the p density of vacant states (holes) on the Cu sites the difference of the two spectra shown in the inset shows a transfer of holes from the main line final states down to the shoulder (at the peak position in difference spectrum; inset). This feature has been assigned in previous XANES studies of YBCO to be a Cu(2) ls ~ 4pz transition [56-60] (called B' in Ref. [59] Fig. 2). Consequently, we conclude that electrons are being transferred out of the Cu(2) planes. YBa2Cu306. 4

30 23 = m

t.-.-

2o

25 10

,,p, o < x

20

E o

-io -20

15

2

4

6k(A.1)8

lo

C ,.-. I--

10 ." i

tl,..

"r-

\

~J

5

O

u_

0 0

1

2

.3

R(A)

4

5

Fig. 2. Magnitude of the Fourier transform of k 3. XAFS for k = 2.8-11.7 A ~ for normal (solid line) and photodoped (dotted line) oYBa2Cu306.4 system. The first peak (1-2 A) corresponds to the c-axis C u - O bond distribution. The second and third peaks centered near 3 A and 4 A contain the C u ( 2 ) - Y / C u ( I , 2 ) - B a and Cu(2)-Cu(l) contributions, respectively. Changes occur both in the nearest neighbor C u - O distribution and at the Cu(2)-Cu(l) position. Note that the peak positions in the raw Fourier transforms are at a shorter distance than the crystallographic bonds due to corrections from the central atom phase shift and the scattering atom phase function. The inset displays the corresponding raw data in k-space.

167

T.A. 7~vsonet al. / Physica C 292 (1997) 163-170

Detailed structural information can be obtained by analyzing the high energy spectrum or XAFS (Fig. 2, inset). In Fig. 2 we display the Fourier transform of the XAFS spectrum of YBaeCu306. 4 in the normal and photodoped states. Note the structural changes in the first shell Cu(1,2)-O(4) distribution (Cu(l,2) is defined to be Cu(1) or Cu(2)). Changes are also seen in the region of the Cu(2)-Cu(1) contribution. However these latter changes may be the result of effects on the linear Cu(2)-O(4)-Cu(1) multiple scattering contributions due to changes in the positions of the 0(4) atoms. We focus on the changes in the c-axis oxygen shell about Cu(1,2) by filtering the first shell (Fig. 3) and fitting a model structure to the spectrum in order to extract structural parameters: R (bond distance) and o- (bond correlation or Debye-Waller factor), holding the coordination number, N, fixed. The structural model found with the lowest leastsquare error spectra (Fig. 4 and Table 1), was one in which the long Cu(2)-O(4) bond is unchanged (except for o-) while the short Cu(1)-O(4) bond is transformed from a broad distribution to two more well-defined peaks in the photodoped state (the origin of the 'beat' effect in Fig. 3 and Table 1). A two-shell fit was found to satisfactorily model the fully doped system (Table 1). Attempts to fit the normal spectrum Cu(1)-O(4) distribution to twopeaks collapsed to one-peak fits. In the literature, the enhanced photoconductivity in oxygen deficient YBCO has been explained by two primary mechanisms, oxygen ordering and Fcenter formation or photoinduced charge transfer [23-32,41,39,48,49,61,62]. For values of x near 0.4, it becomes energetically favorable to transfer electrons from the CuO 2 planes to the CuO x chains resulting in oxygen defect ordering or equivalently chain ordering along the b-axis thus producing a tetragonal to orthorhombic phase transition. In the photoinduced charge transfer model, the photoproduced electrons (of the hole pairs) are trapped in defects or in unoccupied p levels of O in the chains leading to an enhancement of hole concentration in the CuO 2 planes. The measurements presented here are most consistent with oxygen ordering involving regions of the ordered orthorhombic phase and the disordered tetragonal phase. A two-site axial Cu(1)-O(4) in-chain distribution is expected for Cu(1)-O(4) bonds within

4 2 co u. < x

0 -2 -4

.. .

4

.

.

i

.

4.8

.

.

I

i

5.6

i

i

i

i

6.4

i

i

i

,

,

7.2

i

i

8

,

,

L

L

,

8.8

,

,

i

~

9.6

,

,

i

10.4

k (A-1)

Fig. 3. The Cu-O signals obtained by filtering the transformed data (Fig. 2) over the range 0.8-2.0 A. The solid line and dotted lines correspond to the normal and doped systems, respectively. Note that a near cancellation of the doped signal occurs near k ~ 9.6 ~-i. This 'beat' is due to the interference of signals and indicates the presence of a pair of Cu-O peaks with close separation Ar~Tr/2~k.=O.16 A (see Table 1). (A previous study showed that the contribution from higher shells is negligible [17-22].)

and external to b-axis chain fragments. It is quite possible that the photoinduced conductivity change is facilitated both by in-chain ordering and by electron trapping as some experiments suggest [48,49,61,62]. In terms of the relationship of the photodoping effect to high temperature superconductivity, we note that the split axial oxygen position observed in this work is quite similar to that the double-well axial oxygen potential observed in [16-20] or, more generally, a two-site axial C u - O distribution found by Booth et al. [21] and Stern et al. [22] in the fully doped bulk YBCO. Hence, chemical doping and photodoping, which both increase the in-plane hole density, produce structural states with similar transport enhancing local distortions (see Ref. [63,64] for a discussion of local distortions accompanying chemical doping). More recently, Bianconi et al. [65-67] have also found multiple axial oxygen positions in polarized XAFS measurements on single crystals of Bi2Sr2CaCu2Os+y and La1.85Sr0JsCuO 4. The bimodal apical oxygen distances were determined to be associated with a alternating low temperature orthorhombic (LTO) and low temperature tetragonal (LTT) striped structure in the CuO 2 plane producing two distinct orientations of the CuO 4 planes, about

168

T.A. Tyson et al. / Phvsica C 292 (1997) 163-170

10

~-

4

6

/3

10

U~

.<

Z0~I

X

15

(bl

[

'

'

'

'

I

'

'

'

'

[

'q

10 5 0 -,5 4

6

8

10

k (i Fig. 4. Model lit to the filtered signal for YBa2Cu3064 in the normal (a) and photodoped (b) states over the range 3.80-11.7 A i. In panel (a), the total signal is given as the lower curve (solid line) with the full fit given by the dashed line. The individual components (shells) are also given with the theoretical model signal (see text) represented by the dashed lines. Shell 1 is at 1,87 ,~ and shell 2 is at 2.25 A, (Table 1), The solid line shown for each shell is the residual obtain by subtracting off all other shells from the total signal. In the case of the normal system only two c-axis Cu-O bonds (short Cu(1)-O(4) due to chains and long Cu(2)-O(4) due to planes) were found. Panel (b) displays the corresponding fits for the photodoped state with shell 1 is at 1.8l >,, shell 2 is at 2.00 A and shell 3 is at 2.25 A (Table 1).

the conducting plane Cu atoms. It is argued that the confinement of charge carriers to the LTT region, in the in-plane superlattice of quantum stripes separated by large energy barriers, increases the density of states at the Fermi level and enhances T~ by a factor of ~ 10 [68,69]. The similarity in distortions found in the photodoped system (coupled with the c-axis contraction observed with this effect) and the chemically doped systems suggest a common mechanism for transport enhancement. Higher resolution c-axis XAFS measurements as well as measurements on a-

and b-axis oriented films should assist in defining the mechanism. In addition, a search for superlattice peaks which track photodoping will complement these measurements. Finally, since a Gaussian model may introduce errors in extracted parameters for systems with large disorder [70,71], the detailed nature of the distributions will be further refined with more accurate fits to anharmonic potentials as was carried out in Ref. [16-20]. In summary, we have found evidence of hole transfer into the CuO2 planes accompanying

T.A, Tyson et al. / Physica C 292 (1997) 163-170

photodoping

in o x y g e n

deficient YBCO.

A local

d i s t o r t i o n o f the C u O x c h a i n s a n d a t r a n s f e r o f h o l e d e n s i t y into the C u O 2 p l a n e s are f o u n d to o c c u r w i t h p h o t o d o p i n g . T h e s e results s u g g e s t a c o m m o n struct u r e - r e l a t e d t r a n s p o r t e n h a n c e m e n t m e c h a n i s m in the chemically doped and photodoped high temperature superconductor systems.

Acknowledgements D a t a a c q u i s i t i o n w a s p e r f o r m e d at the N a t i o n a l S y n c h r o t r o n L i g h t S o u r c e ( N S L S ) at B r o o k h a v e n N a t i o n a l L a b o r a t o r y w h i c h is f u n d e d by the U S D e p a r t m e n t o f E n e r g y . T A T a c k n o w l e d g e s the support in part f r o m N e w J e r s e y Institute o f T e c h n o l o g y (NJIT) Grants #421600 and #421930. We thank Prof. J. G r o w o f N J I T for a s s i s t a n c e w i t h X - r a y d i f f r a c t i o n c h a r a c t e r i z a t i o n a n d Dr. K. S y e d o f N S L S for t e c h n i c a l assistance. W e are i n d e b t e d to Prof. F. B r i d g e s o f the U n i v e r s i t y o f C a l i f o r n i a at S a n t a C r u z for useful a n d s t i m u l a t i n g d i s c u s s i o n s .

References [1] R.E. Cohen, Nature 358 (1992) 136. [2] B. Butcher, E. Kaldis, Ch. Kruger, P. Watcher, Europhys. Lett. 34 (I 996) 391. [3] C.C. Almasan, S.H. Han, B.W. Lee, LM. Paulius, M.B. Maple, B.W. Veal, J.W. Downey, A.P. Paulikas, Z. Fisk, J.E. Schirber, Phys. Rev. Lett. 69 (1992) 680. [4] J.J. Neumeier, T, Bjornholm, M.B. Maple, I,K. Schuller, Phys. Rev. Lett. 63 (1989) 2516. [5] K. Murata, Y. Hoda, H. Oyanagi, Y. Nishihara, H. lhara, N. Terada, R. Sugise, M. Hirabayashi, M. Tokumoto, Y. Kimura, Proceedings of the MRS International Meeting on Advanced Materials: Superconductivity, vol. 6, 1989, p. 681 [6] R. Griessen, Phys. Rev. B 36 (1987) 5284, and references therein. [7] J.D. Jorgensen, S. Pei, P. Lightfoot, H. Shi, A.P. Paulikas, B.W. Veal, Physica C 167 (1990) 571. [8] B.W. Veal, H. You, A. Paulikas, H. Shi, Y. Fang, J.W. Downey, Phys. Rev. B 42 (1990) 4770. [9] T. Zeisk, R. Sonntag, D. Hohlwein, N.H. Andersen, T. Wolf, Nature 353 (1991) 542. [10] R.J. Cava, Nature 346 (1990) 110. [11] A.P. Menushenkov, A.Y. lgnatov, A.A. Ivanov, D.I. Kochubey, V.A. Chernov, S.G. Nikitenko, Nucl. Instrum. Meth. Phys. Res. A 359 (1995) 236. [12] A.Yu. Ignatov, A.P. Menushenkov, A.A. Ivanov, D.I. Kochubey, Physica C 234 (1994) 68.

169

[13] M. Arai, K. Yamada, Y. Hidaka, S. Itoh, Z.A. Bowden, A.D, Taylor, Y. Endo, Phys. Rev. Lett. 69 (1992) 359. [14] R.P. Sharma, F.J. Rotella, J.D. Jorgensen, L.E. Rehn, Physica C 174 (1991) 409. [15] H. Oyanagi, H. Kimura, T. Terashima, Y. Bando, J. Phys. Soc. Jpn. 64 (1995) 2563. [16] S.D. Conradson, I.D. Raistrick, A.R. Bishop, Science 248 (1990) 1394. [17] J. Muste de Leon, I. Batistic, A.R. Bishop, S.D. Conradson, S.A. Trugman, Phys. Rev. Lett. 68 (1992) 3236. [18] J. Muste de Leon, S.D. Conradson, I. Batistic, A.R. Bishop, I.D. Raistrick, M.C. Aronson, F.H. Garzon, Phys. Rev. B 45 (1992) 2447. [19] J. Muste de Leon, S.D. Conradson, I. Batistic, A.R. Bishop, Phys. Rev. B 44 (1991) 2442. [20] J. Muste de Leon, S.D. Conradson, I. Batistic, A.R. Bishop, Phys. Rev. Lett. 65 (1990) 1675. [21] C.H. Booth, F. Bridges, J.B. Boyce, T. Ciaeson, B.M. Lairson, R. Liang, D.A. Bonn, Phys. Rev. B 54 (1996) 9542. [22] E.A. Stern, M. Quian, Y. Jacoby, S.M. Heald, H. Maeda, Physica C 209 (1994) 331. [23] V.I. Kudinov, Physica B 194 (1994) 1187. [24] V.I. Kudinov, 1.L. Chaplygin, A.I. Kirilyuk, N.M. Kreines, R. Laiho, E. Lahderanta, C. Ayache, Phys. Rev. B 47 (1993) 9017. [25] V.I. Kudinov, I.L. Chaplygin, A.I. Kirilyuk, N.M. Kreines, R. Laiho, E. Lahderanta, Phys. Lett. A 157 (1991) 290. [26] V.I. Kudinov, I.L. Chaplygin, A.I. Kirilyuk, N.M. Kreines, Physica C 185 (1991) 1241. [27] V.I. Kudinov, A.I. Kirilyuk, N.M. Kreines, Phys. Lett. A 151 (1990) 358. [28] A.I. Kirilyuk, N,M. Kreines, V.I. Kudinov, JETP Lett. 52 (1990) 49. [29] G. Yu, A.J. Heeger, Int. J. Mod. Phys. B 7 (1993) 3751. [30] G. Yu, C.H. Lee, A.J. Heeger, N. Herron, E.M. McCarron, Phys. Rev. Lett. 67 (1991) 2581. [31] G. Yu, C.H. Lee, A.J. Heeger, N. Herrom E.M. McCarron, C. Lin, G.A. Spalding, C.A. Nordman, A.M. Goldman, Phys. Rev. B. 45 (1991) 4964. [32] G. Yu, A.J. Heeger, G. Stuck, N. Herron, E.M. McCarron, S.S. Comm. 72 (1992) 345. [33] T. Thio, R.J. Birgeneau, A. Cassanho, M.A. Kastner, Phys. Rev. B 42 (1990) 10800. [34] D. Lederman, J. Hansen, I.K. Schuller, E. Osquiguli, Y. Bruynseraede, Appl. Phys. Lett. 64 (1994)653. [35] E. Osquiguil, G. Nieva, J. Guimpel, J. Hasen, Y. Bruynseraede, I.K. Schuller, J. Supercond. 7 (1994) 127. [36] G. Nieva, E. Osquiguil, J. Guimpel, M. Maenhoudt, B. Wuyts, Y. Bruynseraede, M.B. Maple, I.K. Shuller, Phys. Rev. B 46 (1992) 14249. [37] G. Nieva, E. Osquiguil, J, Guimpel, M. Maenhoudt, B. Wuyts, Y. Bruynseraede, M.B. Maple, I.K. Schuller, Appl. Phys. Lett. 60 (1992) 2159. [38] K. Tanabe, S. Kuobo, F. Hosseini, H. Teherani, Asano, M. Suzuki, Phys. Rev. Lett. 72 (1994) 1537. [39] W. Gob, W. Lang, W. Markowitsch, V. Schlosser, W. Kula, R. Sobolewski, Solid State Commun. 96 (1995) 431.

170

T.A. Tyson et al. / Physica C 292 (1997) 163-170

[40] T.R. Tsai, C.L. Lin, J.L. Lin, S.S. Pal, D.H. Chen. C.C. Chi, M.K. Wu, Chin. J. Phys. 34 (1996) 661. [41] K. Kawamoto, I. Hirabayashi, Phys. Rev. B 49 (1995) 3655. [42] T.A. Tyson, J. Mustre de Leon, S.D. Conradson, A.R. Bishop, J.J. Neumeier, H. Roder, J. Zang, Phys. Rev. B 53 (1996) 13985. [43] T.A. Tyson et al., Phys. Rev. B, in press. [44] C.H. Booth, F. Bridges, G.J. Snyder, T.H. Geballe, Phys. Rev. B 54 (1996) R15606. [45] S. Billinge, R.G. DiFran~esco, G.H. Kwei, J.J. Neumeier, J.D. Thompson, Phys. Rev. Lett. 77 (1996) 715. [46] D. Louca et al., unpublished. [47] R.M. Hazen, in: D.M. Ginsberg (Ed.), Physical Properties of High Temperature Semiconductors II. World Scientific, Singapore, 1990. [48] D. Chew, J.F. Federici, J. Gutierrez-Solana, G. Molina, W. Savin, W. Wilber, Appl. Phys. Lett. 69 (1996) 3260. [49] J.F. Federici, D. Chew, B. Welker, W. Savin, J. GutierrezSolana, T. Fink, W. Wilber, Phys. Rev. B 52 (1995) 15592. [50] R.J. Cava, B. Batlogg, K.M. Rabe, E.A. Rietman, P.K. Gallager, L.W. Rupp Jr., Physica C !56 (1988) 523. [51] H. Yasuoka, H. Maxaki, T. Terashima, Y. Bando, Physica C 175 (1991) 192. [52] R. Boyn, K. L5be, H.U. Habermeier, N. Prub, Physica C 181 (1991) 75. [53] P.A. Lee, P.H. Citrin, P. Eisenberger, B.M. Kincaid, Rev. Mod. Phys. 53 (1981 ) 679. [54] E.A. Stern, Phys. Rev. B 48 (1993) 9825. [55] J.J. Rehr, S.I. Zabinsky, R.C. Albers, Phys. Rev. Lett. 69 (1992) 3397.

[56] J. Rohler, A. Larisch, R. Schafer, Pbysica C 191 (1992) 57, and references therein. [57] K.B. Garg, A. Bianconi, S. Della, A. Longa, C.M. De Santis, A. Marcelli, Phys. Rev. B 38 (1998) 244. [58] N.V. Bausk, L.N. Mazalov, A.I. Rykov, V.F. Vratskikh, M.R. Predtechenskii, Y.D. Varlamov, Struct. Chem. 33 (1992) 408. [59] S.M. Heald, J.M. Tranquada, A.R. Modenbaugh, Y. Xu, Phys. Rev. B 38 (1988) 761. [60] T.A. Tyson et al., unpublished. [61] J. Hansen et al., Phys. Rev. B 51 (1995) 1342. [62] A. Hoffman et al., unpublished. [63] T. Egami, S. Billinge, Prog. Mater. Sci. 38 (1994) 359, and references therein. [64] Y. Bar-Yam, T. Egami, J. Mustre-de Leon, A.R. Bishop (Eds.), Lattice Effects in High Tc Superconductors, World Scientific, Singapore, 1992, and references therein. [65] A. Bianconi, N.L. Saini, T. Rossetti, A. Lanzara, A. Parali, M. Missori. Y. Oyanagi, H. Yamaguchi, Y. Nishihara, D.H. Ha, Phys. Rev. B 54 (1996) 12018. [66] A. Bianconi, N.L. Saini, A. Lanzara, M. Missori, T. Rossetti, Y, Oyanagi, H. Yamaguchi, K. Oka, T. Ito, Phys. Rev. Lett. 76 (1996) 3412. [67] A, Bianconi, M. Lusignoli, N.L. Saini, P. Border, A. Kvick, P.G. Radaelli, Phys. Rev. B 54 (1996) 4310. [68] A. Bianconi, Solid State Commun. 89 (1994) 993. [69] A. Bianconi, M. Missori, N.L. Saini, Y. Oyanagi, H. Yamaguchi, DH. Ha, Y. Nishihara, J. Supercond. 8 (1995) 545. [70] E.D. Crozier. A.J. Seary, Can. J. Phys. 58 (1980) 1388. [71] P. Eisenberger, G.S. Brown, S.S. Comm. 29 (1980)481.