PHYSICA®
ELSEVIER
Physica C 277 (1997) 145-151
Hole states in oxycarbonate high-Tc superconductor
(T108Cr02)SrnCu2(CO3)O 7 probed by soft X-ray absorption
spectroscopy
J.M. Chen a, R.S. Liu h, C. Martin c, F. Letouze c, B. Raveau c
a Synchrotron Radiation Research Center (SRRC), Hsinchu, Taiwan
b Department of Chemistry, National Taiwan University, Taipei, Taiwan
c Laboratoire CRISMAT-ISMRA, Boulevard du Marechal, Caen Cedex, France
Received 5 December 1996; revised manuscript received 10 January 1997
Abstract
We report O K-edge and Cu L23-edge X-ray-absorption near-edge-structure (XANES) spectra of oxycarbonate
(Tlo.sCro.2)Sr4Cu2(CO3)O7 obtained using a bulk-sensitive X-ray fluorescence yield technique. The prominent features of
the O ls absorption edge in the (Tlo.sCro.2)Sr4Cu2(CO3)O 7 compound are three distinct pre-edge peaks at 528.2, 529.3 and
530.5 eV, respectively. We ascribed these pre-edge peaks to excitations of O ls electrons to predominant O 2p holes located
in the CuO 2 planes, in the apical oxygen sites, and in the (TI,Cr)-O planes, respectively. The average number of holes within
the in-plane oxygen sites per CuO 2 sheet is approximately the same for both the (Tlo.sCro.2)Sr4Cu2(CO3)O 7 and
T13(CrO4)SrsCu4Oi6 systems. Conversely, the average hole content in the apical oxygen sites decreases significantly in
(Tlo.sCro.2)Sr4fu2(CO3)O 7 as compared to Tl3(CrO4)SrsCtl4016. The reduction of the average hole concentration in the
apical oxygen sites is accompanied by the enhanced superconductivity in (Tl0.gCro.2)Sr4Cu2(CO3)OT, indicating that the O
2p holes on the apical oxygen sites play an important role to control the Tc. The behavior of the high-energy shoulders in the
Cu L23-edge absorption spectra coincides with that of the pre-edge peak at 528.2 eV in the O K-edge absorption spectra.
1. Introduction
The insulating oxycarbonate compound Sr2CuO 2-
(CO3) was first reported by von Schnering et al. [1].
This compound consists of a block layer [Sr2CO 3]
and a CuO 2 sheet. Thereafter, the successful substitution of carbonate groups for copper in the perovskite structure has produced a large number of
new oxycarbonate superconductors, such as, (Hg0. 3Pbo.7)Sr4Cu2(CO3)O7 [2], (Tlo.5Pbo.5)Sr4Cu2(CO3)0 7 [3], (Bio.sHgo.s)Sr4Cu2(CO3)O 7 [4], TI(Sr2Ba2)Cu2CO307 [5], Tl(Sr4_xBax)CU2(CO3)O 7 [6],
(Hg0.sPb0.5)(Sr4_xBaa)Cu2(CO3)O 7 [7], (Tll_xBi.,)Sr4fu2(CO3)O 7 [8], etc. The crystal structure of
those oxycarbonate cuprates has already been reported. As an example, the structure of the oxycarbonate (TI,M)Sr4Cu2(CO3)O7 ( M = P b or Bi) is
built up from the intergrowth of double rock-salt-type
layers {[(T1,M)O](SrO)} and single [SrCuO 3] perovskite, linked through single carbonate [SrCO3]
layer [3,8]. Thus, its structure can be described as the
intergrowth of 1201-type layers [(TI,M)Sr2CuO 5]
with the classical [Sr2CuO2CO3] (S2CC) units, and
is called normal " $ 2 C C - 1 2 0 1 " structure. It is re-
0921-4534/97/$17.00 Copyright © 1997 Elsevier Science B.V. All fights reserved
Pll S0921-4534(97)00092-0
146
J.M. Chen et a l . / Physica C 277 (1997) 145-151
markable that the thallium oxycarbonate (T10.sPb0.s)Sr4Cu2(CO3)O7 exhibits the highest Tc of 70 K,
although it derives from two nonsuperconductors,
(Tio.sPbo.5)Sr2CuO 5 [9] and Sr2CuO2(CO 3) [10]. A
similar phenomenon has been recently experimentally proven for a superconducting oxycarbonate Bi~
Srafu2CO308 with a T~ of 30 K [11], which is
composed of an intergrowth of the 2201-type superconductor Bi2SrRCuO 6 with a Tc of 22 K [12] and of
the nonsuperconducting oxycarbonate Sr2CuO 2(CO3). There are also the case of the mercury and
the thallium-based oxycarbonate compounds (Hg0. 3Pb0.7)Sr4Cu2CO307 [2], Hg(Sr2Ba2)Cu2CO307 [5],
TI(Sr2Ba2)Cu2CO307 [6], and (Tlo.sBio.5)Sr4Cu 2(CO3)O 7 [8], which are the intergrowth of the 1201type Hg or TI cuprate and the Sr2CuO2(CO 3) structure. The superconducting transition temperature of
these compounds is systematically higher than that
of the parent 1201-type cuprates. This shows that the
introduction of carbonate layers into the 1201 or
2201-type parent structure can play a crucial role to
enhance superconductivity. The physical properties
of the oxycarbonate cuprates have been widely studied [2-8], however, no investigations of electronic
structure on these compounds have been performed.
It is therefore of great interest to understand the
variation of electronic structure of the oxycarbonate
cuprates related to the hole concentration and the
superconductivity.
It has been experimentally demonstrated that holes
in the p-type cuprate superconductors are responsible
for superconductivity from the correlation between
the superconducting transition temperature and the
hole concentration [13]. The investigation of the
unoccupied electronic states near the Fermi level of
these compounds is therefore an important first step
toward comprehensive understanding of the origin of
superconductivity. For years, soft X-ray-absorption
spectroscopy using synchrotron radiation has been
widely applied to obtain valuable information regarding the local density of unoccupied states at both the
oxygen and copper sites in the cuprate superconductors [ 14].
In this paper we report O K-edge and Cu L23-edge
soft-X-ray absorption measurements of the oxycarbonate (T10.sCr0.2)Sr4Cu2(CO3)O 7 using a bulk-sensitive X-ray fluorescence yield detection method.
Based on the electron diffraction (ED) and high-reso-
lution electron microscopy (HREM) studies of
(Tlo.8Cro.2)Sr4Cu2(CO3)O 7, its structure results from
a shearing phenomenon of the normal "$2CC-1201"
structure, along (110) plane of the "$2CC-1201"
structure. It is therefore called the (ll0)-collapsed
"$2CC-1201" structure [15]. In addition, in order to
understand the variation of electronic structure due
to the introduction of the carbonate layer into the
single-thallium-layer compounds, O K-edge X-ray
absorption spectrum of 1201-type parent compound
with a nominal composition of (Tlo.TsCro.25)Sr2CuO5
has also been carried out in this study. The insertion
of carbonate layer in (Tlo.8Cro.2)Sr4Cu2(CO3)O 7 is
located in the apical position. It is therefore expected
that the hole density in the apical directions may be
changed in (Tlo.sCro.2)Sr4Cu2(CO3)O 7 as compared
to the parent compound (Tlo.75Cro.25)Sr2CuOs. However, it should be pointed out that the substitution of
Cr for T1 in the 1201-type structure results in the
formation of an ordered copper oxychromate
T13(CrO4)SrsCu4016 with the A2mm space grou~o
and lattice constants a = 3.7803 A, b = 15.2573 A
and c = 17.6737 ,~ [16]. It exhibits a Tc of 25 K.
The study of electronic states near the Fermi level of
the oxycarbonate cuprates will give rise to open new
vistas to understand the microscopic mechanism of
high-Tc superconductivity in the layered cuprates.
o
2. Experimental
Details on the preparation of samples were reported elsewhere [15,16]. In brief, high purity powders of TI203, Cr203, SrO 2, CuO, Sr2CuO 3, and
carbonate SrCO 3 were weighed in the appropriate
proportions to form a nominal composition of
(T108Cr02)SraCu2(CO3)O 7. The powders were
mixed in an agate mortar, pressed into bars, put in an
alumina crucible, and then sealed in an evacuated
quartz ampoule. The tubes were introduced in a hot
furnace at 850°C, held for 12 hours, and then
quenched to room temperature. The Tl3(CrOg)Sr8Cu4Oz6 compounds were prepared by similar procedures but with high post-annealing temperature at
920°C for 6 hours [16]. The purity and crystallinity
were checked out by X-ray diffraction (XRD) and
electron diffraction (ED) techniques. The superconducting properties were studied as a function of
147
J.M. Chen et a l . / Physica C 277 (1997) 145-151
temperature using an AC Lakeshore susceptometer
with an applied field of 5 G; no demagnetizations
were made.
The X-ray absorption measurements were carried
out using the 6 m high-energy spherical grating
monochromator (HSGM) beamline of the Synchrotron Radiation Research Center (SRRC) in Taiwan. Bulk-sensitive X-ray fluorescence yield spectra
with a probing depth of thousands of angstroms were
recorded using a microchannel plate (MCP) detector.
This MCP detector is composed of a dual set of
MCPs with an electrically isolated grid mounted in
front of them. For X-ray fluorescence detection, the
grid was set to a voltage of 100 V while the front of
the MCPs was set to - 2000 V and the rear to - 200
V. The grid bias insured that no positive ions were
detected while the MCP bias insured that electrons
would not be detected. The MCP detector was located ~ 2 cm from the sample and oriented parallel
to the sample surface. Photons were incident at an
angle of 45 ° with respect to the sample normal. The
incident photon intensity (10) was measured simultaneously by an 80% transmission Ni mesh located
after the exit slit of the monochromator. All the
measurements were normalized to I 0. The photon
energies were calibrated using the known O K-edge
and Cu L-edge absorption peaks of the CuO compound. The energy resolution of the monochromator
was set to approximately 0.22 and 0.45 eV for the O
K-edge and Cu L-edge absorption measurements,
respectively.
0.0
/CO.<>OO.O
~. -0.2
/
"~ -0.4
~< -0.6
-0.8
.
20
30
40
,
,
,
50
.
60
,
70
Temperature (K)
Fig. 1. Temperature dependence of AC susceptibility of the
oxycarbonate (Tlo.sCro.2)Sr4Cu~(CO3)O 7 sample.
529.3 and 530.5 eV, respectively, and a broad band
at ~ 537 eV. The O I s X-ray absorption spectra in
Fig. 2 can be separated into two regions: below and
above energy ~ 532 eV. It has been proven by
inverse photoemission experiments that the empty d
states of Sr are located at about 5 - 1 0 eV above the
.d
3. Results and discussion
In Fig. 1 we show the temperature dependence of
the AC susceptibility of (Tl0.sCr0.2)Sr4Cu2(CO3)O 7.
As deduced from Fig. 1, its superconducting transition temperature is 68 K which is one of the highest
observed in the oxycarbonate cuprates.
In Figs. 2a and 2b O K-edge X-ray absorption
near edge structure (XANES) spectra of (Tl0.8Cr0.2)S r 4 C u 2 ( C O 3 ) O 7 and TI3(CrO4)Sr8Cu4Oj6 , respectively, are displayed in the energy range of 525-555
eV obtained by a non-surface-sensitive X-ray fluorescence yield detection technique. As noted from
Fig. 2, the prominent features in the O 1s absorption
spectrum are three discrete pre-edge peaks at 528.2,
Z
i
525
I
530
i
I
535
~
Photon
I
540
,
I
545
,
I
550
,'
555
Energy (eV)
Fig. 2. O K-edge X-ray-absorption near-edge-structure spectra of
(a) (Tlo.sCro.2)Sr4Cu2(CO3)O 7 and (b) T ] 3 ( C r O 4 ) S r 8 C u 4 O i 6
obt a i n e d using the X-ray-fluorescence-yield detection method. These
spectra have been normalized to one CuO 2 sheet per formula.
148
J.M. Chen et a l . / Physica C 277 (1997) 145-151
Fermi level [17]. Thus, excitations of the O ls
electrons to the Sr 4d and T1 6p empty states hybridized with O 2p states are probably responsible
for the high-energy peaks above 535 eV. The preedge peaks below 532 eV are attributed to transitions
from O ls electrons to holes with the predominant
2p symmetry on the oxygen sites. The observed
multiple pre-edge peaks in Fig. 2 may be related to
different binding energies of O ls levels of
nonequivalent oxygen sites. In order to understand
the variations in the density of doped O 2p holes per
CuO 2 sheet, the O K-edge absorption spectra in Fig.
2 have been normalized in the range 535-555 eV
according to their ideal oxygen stoichiometry and
then divided by the number of CuO 2 layers per
formula. This provides the absolute intensities of the
relevant pre-edge peaks originating from different
oxygen sites for the (Ti0.8Cr0.2)SraCu2(CO3)O7 and
T13(CrO4)SrsCu4Oi6 compounds.
Based on the crystal structure of (T10.8Cr0.2)Sr4Cu2(CO3)O7 [15], there exist several different oxygen sites distributed in the in-plane CuO 2 planes, in
the apical oxygen sites, and in the (TI,Cr)-O planes,
respectively. The distinct pre-edge features in the O
ls X-ray absorption spectrum originating from different oxygen environments have been found in the
related high-To superconductor compounds, such as
Tl2Ba2Ca2Cu3Oi0, and T12Ba2CaCu20 s. Near the
O ls edge, three pre-edge peaks with maxima at
528.3, 529.4 and 530.6 eV are revealed in the TI 2Ca2Ba2Cu3 O~0 high-T~ superconductor reported by
Krol et al. [18]. These peaks are ascribed to core-level
excitations of oxygen ls electrons to empty states
with the mainly O 2p character located in the CuO 2,
BaO and TIO planes, respectively. This view is
supported by local density approximation (LDA)
band-structure calculations [19] and by X-ray photoemission measurements of the O 1s levels [20]. In
addition, it has been shown by polarized X-ray absorption spectroscopy that, in the T12Ba2CaCu208
compound, the low-energy pre-edge peak centered at
528.3 eV has mainly O 2p,y symmetry from the
two-dimensional CuO2 planes, and the higher-energy pre-edge peak at 529.4 has predominantly O
2pz symmetry from the apical oxygen sites [21].
Moreover, the first pre-edge peak at ~ 528.3 eV in
the related high-Tc superconductors, such as,
(T10.sPbo.5)Sr2(Cal_~Yx)Cu207 [22] and HgBa 2-
Ca~_ ICUnO2n+2+¢~ (n = 1-3) [23], was assigned to
O 2p hole states within the CuO 2 planes.
Therefore, in accord with results from other p-type
cuprate superconductors [14,18,22-24], the first
pre-edge peak at 528.2 eV in the XANES spectrum
of (Tl0.sCr0.2)Sr4Cu2(CO3)O 7 in Fig. 2a was attributable to excitations of O 1s electrons to O 2p
holes located in the CuO 2 planes. The other pre-edge
peaks at 529.3 eV and 530.5 eV in Fig. 2a were
ascribed to transitions into O 2p holes in the apical
oxygen sites and the (TI,Cr)-O planes, respectively.
However, another possible final state associated with
the transition at 530.5 eV is the upper Hubbard band
of the Cu 3d states highly hybridized with the O 2p
states. Due to strong on-site correlation on the copper sites in the cuprate compounds, such a band has
always been assumed to exist [25]. Therefore, the
peak at 530.5 eV may be due to a superposition of
unoccupied O 2p states originating from the (T1,Cr)-O
layers and the upper Hubbard band related to the
CuO 2 planes. The peak assignment of the O K-edge
XANES spectrum of T! 3(CrOg)Sr8Cu 40 16 is similar
to that of (Tl0.sCr0.2)SraCu2(CO3)O 7.
The peaks at ~ 532.2 and ~ 533.6 eV may be
due to surface contamination since those peaks exhibit a greater intensity in the surface-sensitive
total-electron yield spectra. Existence of surface contamination has been reported by many researchers. It
is suggested that these peaks are arisen from absorption of hydrides, water, and CO 2 on the surface [26].
Figs. 3a and 3b show the pre-edge region of the O
K-edge X-ray absorption spectra of (Tl0.sCr0.2)Sr 4Cu 2(CO3)O7 and T13(CrOa)Sr8Cu4Oi6 , respectively.
As shown, the spectral weight of the low-energy
pre-edge peak at 528.2 eV is similar from
(Tl0.8Cr0.2)Sr4Cu2(CO3)O 7 to TI3(CrO4)SrsCu4016.
This indicates that the average number of holes
within the in-plane oxygen sites per CuO 2 plane is
approximately the same for both systems. Conversely, high-energy pre-edge peak at 529.3 eV originating from the apical oxygen sites decreases significantly in intensity in (T10.sCr0.2)SraCu2(CO3)O 7 as
compared to Tl3(CrO4)SrsCu4Oi6.
It has been demonstrated that the concentration of
O 2p holes in the CuO 2 planes is strongly correlated
with Tc [18]. Also several theories for high-To superconductors suggest that the apical O 2p= orbital
plays a crucial role on superconductivity [27-30].
149
J.M. Chen et al./Physica C 277 (1997) 145-151
For example, the presence of apical oxygen atoms
makes the CuO2 plane easier t o dope with holes
[30]. In addition, superconductors with pyramidal
copper coordination show a very large pressure enhancement of Tc, whereas, in superconductors with
no apical oxygen atoms, the pressure effect on T~ is
very small [28]. Recently, a clear correlation has
been found between the Tc and the Cu bond valence
sum due to variation in bonding of the copper ion to
the apical oxygen [31]. Similarly, Ohta et al. found a
correlation between T~ and the energy difference
between apical O 2pz states and planar O 2Pxy states
[32]. Grant et al. have recently pointed out the
importance of the apical O 2pz states in determining
the nature and dispersion of quasiparticle states of
p-type doped cuprates [33]. Also Di Castro et al.
have explained the suppression of Tc above a certain
dopant concentration by the occupancy of holes on
Cu 3d3z2_r~ and apical O 2pz hybrids [34]. It should
be pointed out that the oxycarbonate (Tl0.sCr0.2)Sr 4Cu2(CO3)O 7 is a superconductor with a T~ of 68 K,
while T13(CrO4)SrsCu4OI6 exhibits a Tc of 25 K
(b)
O
z
i
526
I
527
i
I
528
I
i
529
i
I
530
I
I
531
I
532
Photon Energy (eV)
Fig. 3. Pre-edge region of the O K-edge X-ray-absorption nearedge-structure spectra of (a) (Tlo.sCro.2)Sr4Cu2(CO3)O 7 (solid
circles) and (b) T13(CrO4)SrsCu 4Oi6 (solid triangles). These spectra have been normalized to one CuO 2 plane per formula.
.6
._.9.
>0
0
U
0
2
.s
0
z
I
920
*
I
930
*
I
940
i
I
950
i
I
960
Photon Energy (eV)
Fig. 4. Copper L23-edge X-my fluorescence yield spectra of the
(a) (TlosCroz)Sr,Cu2(CO3)O 7 (solid circles) and (b)
"l13(CrO4)SrsCu4Oi6 samples (solid triangles). These spectra have
been normalized to one CuO 2 sheet per formula.
[16]. As noted from Fig. 3, the significant decrease
in the average O 2p hole concentration in the apical
oxygen sites in (Tl0.sCr0.2)Sr4Cu2(CO3)O 7 is accompanied by a substantial increase in T~, Our data
give evidence in support of the hypothesis that holes
in apical oxygen sites may have a negative influence
on the superconductivity in the layered cuprates.
In Figs. 4a and 4b are shown the Cu L23-edge
X-ray-absorption near-edge-structure X-ray-fluorescence-yield spectra of (T10.sCr0.2)Sr4Cu2(CO3)O 7
and TI 3(CrO4)SrsCu 4° 16, respectively, in the energy
range of 920 to 960 eV. The data have been normalized to the number of CuO 2 sheets per formula. For
both systems, the strong excitonic peaks at 931.5 eV
and 951.3 eV are very close to the L23 peaks observed in the Cu L23-edge absorption spectrum of
CuO and are attributed to transitions from the
Cu(2P3/ej/2)3d9-O2p 6 ground states (formally
Cu ÷2) to the Cu(2p3/zz/2)-]3dl°-O2p 6 excited
states, where (2P3/2)-j denotes a 2p3/2 hole [35].
Two shoulders at the high-energy side of the main
peaks, as shown in Figs. 4a and 4b, are assigned to
150
J.M. Chen et aL / Physica C 277 (1997) 145-151
excitations of the Cu(2p3/2 l/2)3d9L ground states
(formally Cu +3) to the Cu(2p3/2.1/z)-t3d~°L excited states, where L denotes the O 2p ligand hole
[36]. According to the curve-fitting analyses, the new
features are found to center at ~ 933.1 and ~ 952.9
eV, respectively. In addition, the integrated intensity
of the high-energy shoulder at ~ 933.1 eV remains
approximately unchanged from the (T]0.8Cr0.2)Sr4Cu2(CO3)0 7 to the Yl3(CrOa)Sr8CU4Ol6 samples.
Because there is only one type of Cu site in the unit
cell of (Tlo.sCro.2)Sr4Cu2(CO3)O 7 and Yl3(CrO4)SrsCu4Ol6 compounds (i.e., no Cu-O chains as in
YBa2Cu30 7_ ~) these high-energy features in the Cu
L23-edge absorption spectra can be obviously characterized as a result of the O 2p hole in the CuO 2
planes. It is noted that the behavior of these high-energy shoulders coincides with that for the pre-edge
peak at 528.2 eV in the O K-edge absorption spectra
shown in Fig. 2. This gives an evidence to verify the
assignment that the pre-edge peak at 528.2 eV originates from the CuO 2 planes.
and ~ 952.9 eV are assigned to the transitions from
the Cu(2p3/2 I/2)3d9L ground states to the
Cu(2p3/2j/z)-i3dl°L excited states, where L denotes the O 2p ligand hole. The behavior of these
shoulders correlates with that of the pre-edge peak at
528.2 eV in the O K-edge absorption spectra. Based
on the present X-ray absorption studies, the O 2p
holes in the apical oxygen sites play a crucial role in
controlling the T~ up to 68 K in (T10.sCr0.2)Sr4Cu2(C03)07.
Acknowledgements
We would like to thank all the members of SRRC
for their technical support. This research is financially supported by SRRC and National Science
Council of the Taiwan under grant numbers NSC
86-2613-M-213-010 and NSC 86-2113-M-002-020.
References
4. Conclusion
In this study, O K-edge and Cu L23-edge X-rayabsorption near-edge-structure measurements of the
(T10.8C r0.2)S r4Cu 2(C O 3)O 7 and TI 3(CrO4)SrsCu4OI6 compounds were performed to search
for the hole distributions among different oxygen
sites and their role in superconductivity. Near the O
ls edge, three distinct pre-edge peaks at 528.2, 529.3
and 530.5 eV, respectively, are clearly revealed.
These pre-edge peaks were ascribed to excitations of
O 1s electrons to predominant O 2p holes located in
the CuO 2 planes, in the apical oxygen sites, and in
the (T1,Cr)-O planes, respectively. The average number of holes within the in-plane oxygen sites per
CuO 2 plane is approximately the same for both
(T10.sCr0.2)Sr4Cu2(CO3)O 7 and TI3(CrO4)SrsCu4Oi6 systems. Conversely, average hole content in the apical oxygen sites decreases significantly
in (Tl0.sCr0,2)SrnCu2(CO3)O 7 as compared to
TI 3 ( C r O a ) S r 8Cu 4OI6. The reduction of average O 2p
hole concentration in the apical oxygen sites is accompanied by the enhanced superconductivity in
(TIo.sCr0.2)Sr4Cu2(CO3)O7. In the Cu Lz3-edge absorption spectra, high-energy shoulders at ~ 933.1
[1] H.G. Von Schnering, M. Hartweg, L. Waltz, T. Popp, T.
Beeker and M. Schwarz, Jahresbericht des MPI fiir
Festk6rperforschung (Stuttgart, 1988) p. 94.
[2] C. Martin, A. Maignan, M. Hervieu, M. Huve, C. Michel, A.
Maigann, G. Van Tendeloo and B. Ravean, Physica C 222
(1994) 19.
[3] M. Huve, C. Michel, A. Maignan, M. Hervieu, C. Martin and
B. Raveau, Physica C 205 (1993) 219.
[4] D. Pelloquin, M. Hervieu, C. Michel, A. Maignan and B.
Raveau, Physica C 227 (1994) 215.
[5] M. Uehara, S. Sahoda, H. Nataka, J. Akimitsu and Y.
Matsui, Physica C 222 (1994) 27.
[6] F. Goutenoire, M. Hervieu, A. Maignan, C. Michel, C.
Martin and B. Raveau, Physica C 210 (1993) 359.
[7] M. Huve, C. Michel, A. Maignan, M. Hervieu, C. Martin and
B. Raveau, Physica C 205 (1993) 219.
[8] A. Maignan, M. Huve, C. Michel, M. Hervieu, C. Martin and
B. Raveau, Physica C 208 (1993) 149.
[9] G.H. Kwei, J.B. Shi and H.C. Ku, Physica C 174(1991) 180.
[10] Y. Miyazaki, H. Yamane, T. Kajitani, T. Oku, K. Hiraga, Y.
Morii, K. Fuchisaki, S. Funahashi and T. Hirai, Physica C
191 (1992) 434.
[11] D. Pelloquin, M. Caldes, A. Maignan, C. Michel, M. Hervieu
and B. Raveau, Physica C 208 (1993) 121.
[12] C. Michel, H. Hervieu, M.M. Borel, A. Grandin, F. Deslandes, J. Provost and B. Raveau, Z. Phys. B: Condensed Matter
68 (1987) 421.
[13] J.B. Torrance, Y. Tokura, A.I. Nazzal, A. Bezinge, T.C.
Huang and S.S.P. Parkin, Phys. Rev. Lett. 61 (1988) 1127.
[14] J. Fink, N. Niicker, E. Pellegrin, H. Romberg, M. Alexander
J.M. Chen et aL /Physica C277 (1997) 145-151
and M. Knupfer, J. Electron. Spectrosc. Relat. Phenom. 66
(1994) 395.
[15] A. Maignan, D. Pelloquin, S. Malo, C. Michel, M. Hervieu
and B. Raveau, Physica C 249 (1995) 220.
[16] C. Michel, F. LetouzE, C. Martin, M. Hervieu and B. Raveau,
Physica C 262 (1996) 159.
[17] H.M. Meyer I!I, T.J. Wagnener, J.H. Weaver and D.S.
Ginley, Phys. Rev. B 39 (1989) 7243.
[18] A. Krol, C.S. Lin, Y.L. Soo, Z.H. Ming, Y.H. Kao, Jui H.
Wang, Min Qi and G.C. Smith, Phys. Rev. B 45 (1992)
10051.
[19] P. Marksteiner, J. Yu, S. Massidda, A.J. Freeman, J. Reinger
and P. Weinberger, Phys. Rev. B 39 (1989) 2894.
[20] S.M. Butorin, J.H. Guo, N. Wassdahl, P. Skytt, J. Nordgren,
Y. Ma, C. StriSm, L.G. Johansson and M. Qvarford, Phys.
Rev. B 51 (1995) 11915.
[21] H. Romberg, N. Niicker, M. Alexander, and J. Fink, D.
Hahn, T. Zetterer, H.H. Otto and K.F. Renk, Phys. Rev. B.
41 (1990) 2609.
[22] J.M. Chen, R.S. Liu, and W.Y. Liang, Phys. Rev. B 54
(1996) 12587.
[23] E. Pellegrin, J. Fink, C.T. Chen, Q. Xiong, Q.M. Lin and
C.W. Chu, Phys. Rev. B 53 (1996) 2767.
[24] G. Mante, Th. Schmalz, R. Manzke, M. Skibowski, M.
Alexander and J. Fink, Surf. Sci. 269/270 (1992) 1071.
[25] D. Vaknin, S.K. Shiha, D.E. Moneton, D.C. Johnston, J.M.
Newsam, C.R. Safiva and H.E. King, Jr., Phys. Rev. Lett. 58
(1987) 2802.
151
[26] Z. lqbal, E. Leone, R. Chin, A.J. Signorelli, A. Bose and H.
Eckhardt, J. Mater. Res. 2 (1987) 768.
[27] E. Kaldis, P. Fischer, A.W. Hewat, E.A. Hewat, J. Karpinski
and S. Rusiecki, Physica C 159 (1989) 668.
[28] C. Murayama, N. M6ri, S. Yomo, H. Takagi, S. Uchida and
Y. Tokura, Nature 339 (1989) 293.
[29] H. Matsukawa and H. Fukuyama, J. Phys. Soc. Jpn. 59, 1723
(1990).
[30] J.B. Torrance, A. Bezinge, A.I. Nazzal, A. Bezinge, T.C.
Huang, S.S.P. Parkin, D.T. Keane, S.J. LaPlaca, P.M. Horn
and G.A. Held, Phys. Rev. B 40 (1989) 8872.
[31] D.M. De Leeuw, W.A. Groon, L.F. Feiner and E.E. Havinga,
Physica C 166 (1990) 133.
[32] Y. Ohta, T. Tohyama and S. Maekawa, Phys. Rev. B 43
2968 (1991).
[33] J.B. Grant and A.K. McMahan, Phys. Rev. B 46 (1990)
8840.
[34] C. Di Castra, L.F. Feiner and M. Gilli, Phys. Rev. Lett. 66
(1991) 3209.
[35] M. Grioni, J.B. Goedkoop, R. Schoorl, F.M.F. de Groot, J.C.
Fuggle, F. Schafers, E.E. Koch, G. Rossi, J.M. Esteva and
R.C. Kamatak, Phys. Rev. B 39 (1989) 1541.
[36] A. Bianconi, M. DeSantis, A. Di Ciccio, A.M. Flank, A.
Fronk, A. Fontaine, P. Legarde, H.K. Yoshida, A. Kotani and
A. Marcelli, Phys. Rev. B 38 (1988) 7196; Physica C
153-155 (1988) 1760.