11 September 1998
Chemical Physics Letters 294 Ž1998. 209–216
Soft X-ray absorption study of ž Nd 1.05yx Pr x / Ba 1.95 Cu 3 O 7 using
synchrotron radiation
J.M. Chen
a,)
, R.S. Liu b, M.J. Kramer c , K.W. Dennis c , R.W. McCallum
c
a
b
Synchrotron Radiation Research Center (SRRC), Hsinchu, Taiwan
Department of Chemistry, National Taiwan UniÕersity, Taipei, Taiwan
c
Ames Laboratory, Iowa State UniÕersity, Ames, IA 50011, USA
Received 15 May 1998; in final form 13 July 1998
Abstract
O K-edge and Cu L 23-edge X-ray-absorption spectra for the series of ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 compounds with
x s 0.0–0.5 were measured to investigate how the variation of hole states related to the superconductivity. Near the O 1s
edge, the prepeaks at ; 527.6 and ; 528.3 eV are ascribed to transitions into O 2p hole states within the CuO 3 ribbons and
the CuO 2 planes, respectively. As deduced from O 1s X-ray absorption spectra, the O 2p hole concentrations in the CuO 2
planes and the CuO 3 ribbons decrease monotonically with increasing the Pr doping. The present results clearly demonstrate
that the suppression of superconductivity upon substituting Pr into the ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 system results predominantly from the hole depletion effect. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction
Chemical substitution of Pr for Y in the
ŽY1y x Pr x .Ba 2 Cu 3 O 7 system reduces the Tc value
with superconductivity disappearing for x ) 0.55,
although PrBa 2 Cu 3 O 7 forms the same orthorhombic
structure as YBa 2 Cu 3 O 7 w1,2x. This is in contrast to
other earth-earth ŽR. substitutions ŽNd, Sm, Eu, Gd,
Dy, Ho, Er, Tm, Yb and Lu. which remain superconducting with Tc near 90 K w3x. In addition, it has
been demonstrated that the depression of Tc by the
Pr substitution in ŽR 1y x Pr x .Ba 2 Cu 3 O 7 strongly depends on the rare-earth elements w4,5x. The smaller
atomic number of rare-earth elements in the host
ŽR,Pr.Ba 2 Cu 3 O 7 compounds leads to the greater
decrease in Tc when the Pr ions are substituted into
)
Corresponding author. E-mail: [email protected]
the R sites. The suppression of superconductivity in
ŽR,Pr.Ba 2 Cu 3 O 7 has been a remarkable puzzle in
the cuprate superconductors.
An enormous amount of experimental investigations have been carried out on the ŽY1y x Pr x .Ba 2 Cu 3 O 7 system in an attempt to understand the
origin of the quenching of superconductivity in
ŽR,Pr.Ba 2 Cu 3 O 7 w6–8x. The models for explaining
the depression of superconductivity in ŽR,Pr.Ba 2Cu 3 O 7 include hole-filling w9x, hole localization w10x,
percolation w11x and magnetic pair-breaking w9,12x.
However, no model allows for a consistent interpretation of all the experimental data. As an example,
the high-temperature susceptibility measurements on
PrBa 2 Cu 3 O 7 show an effective Pr moment ; 2.7
u B , comparable to that of the free Pr 4q ions Ž2.5 u B .
w13x. As a result, when Y is substituted by Pr in
ŽY1y x Pr x .Ba 2 Cu 3 O 7 , some of the holes will be filled
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 8 6 3 - X
210
J.M. Chen et al.r Chemical Physics Letters 294 (1998) 209–216
by the extra electrons donated by the tetravalent Pr
ions, consequently reducing the hole concentration in
the CuO 2 planes. However, this proposal was criticized in view of experimental results based on structure investigation w14x, X-ray absorption spectroscopy w7x and Raman spectroscopy w15x which
were interpreted in terms of the simple Pr 3q ion.
It has been proposed by Fehrenbacher and Rice
ŽFR. that the crystal structure of PrBa 2 Cu 3 O 7 favors
a strong hybridization between Pr 4f states and conduction holes in the CuO 2 planes with a mixture of
4f 2 L ŽPr 3q . and 4f 1 ŽPr 4q . configuration, where L
denotes a ligand hole in the O 2p orbitals w16x. This
hybridized state is competitive in energy to the hole
states in the CuO 2 planes. The FR model assumed
hole depletion in the CuO 2 planes, not because of
higher Pr valence but because of transfer of the holes
from the CuO 2 planes into the FR state which binds
doped holes to the Pr sites. However, the FR model
still can not explain the R-dependence on the suppression of Tc in ŽR 1yx Pr x .Ba 2 Cu 3 O 7 .
Recently, Liechtenstein and Mazin ŽLM. calculated the electronic structure of ŽR 1y x Pr x .Ba 2 Cu 3 O 7
using ab initio local density approximation plus Hubbard parameter including Coulomb correlation in the
f shell w17x. They found that, in PrBa 2 Cu 3 O 7 , there
forms an additional hole-depleting band which
crosses the Fermi level and consequently grabs holes
from the CuO 2 band. On doping different rare-earth
elements in ŽR 1y x Pr x .Ba 2 Cu 3 O 7 , the position of
this hole-depleting band shifts with the atomic number of the rare earth, through the energy level of 4f
orbitals, leading to the R-dependence of the suppression of Tc w17x.
On the basis of band-structure calculations w18x
and X-ray absorption measurements w19,20x, it has
been shown that the holes near the Fermi level in
YBa 2 Cu 3 O 7 are distributed between the CuO 2 planes
and CuO 3 ribbons. If the LM model is valid, it is
expected that, in the ŽY1y x Pr x .Ba 2 Cu 3 O 7 system,
the hole concentrations originating from the CuO 2
planes and the CuO 3 ribbons should decrease with
increasing the Pr doping.
It is well known that hole states play a pivotal
role for superconductivity in the p-type cuprate superconductors. Therefore, a knowledge of the electronic structure near the Fermi level of these compounds is an important step toward unveiling the
mechanism of superconductivity. The X-ray absorption spectra are determined by electronic transitions
from a selected atomic core level to the unoccupied
electronic states near the Fermi level. X-ray absorption near edge structure ŽXANES. is therefore a
direct probe of the character and local density of
hole states responsible for high-Tc superconductivity.
To improve the understanding on the mechanism
of Tc-suppression, a detailed study on other
ŽR 1y x Pr x .Ba 2 Cu 3 O 7 systems would be helpful. We
therefore measure systematically the variation of
electronic structure near the Fermi level for the
series of ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 compounds. The
chemical substitution of the Pr ion for the Nd ion in
ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 leads to a monotonic decrease of the superconducting temperature from Tc
s; 92 K for x s 0.0 to Tc s; 20 K for x s 0.3.
The compound becomes an semiconductor at x 0 0.4
w21x. There are two reasons for using slightly overdoped rare earth in this system. First, unlike
YBa 2 Cu 3 O 7y d , stoichiometric NdBa 2 Cu 3 O 7 do not
form in 100% O 2 w22x. Secondly, it has been shown
that optimal doping occurs for 0.05 Nd on the Ba
sites in the Nd 1q x Ba 2yx Cu 3 O 7q d compounds w23x.
For these reason, all samples in this study were
synthesized using 1.05 Nd atoms per formula units.
Neutron diffraction results show that the excess Nd
occupies the Ba site and that approximately xr2
formula units of oxygen are added for each additional Nd above 1.0 in fully oxygenated samples
w24x. In this study, O 1s-edge and Cu 2p-edge X-ray
absorption measurements in ŽNd 1.05yx Pr x .Ba 1.95Cu 3 O 7 for x s 0.0–0.5 were performed in order to
investigate how the variation of hole states near the
Fermi level related to the superconductivity.
2. Experimental
Details on the preparation of samples were reported elsewhere w21x. In brief, Nd 2 O 3 and BaCO 3
were dried at 10008C and 7508C for 24 h, respectively. Pr6 O 11 was dried at 9008C for 24 h and at
4008C for 24 h to insure the formation of stoichiometric Pr6 O 11. In order to fully oxidize CuO, it was
dried at 5508C for 5 days. Samples were made using
conventional solid-state reactions. ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 for x s 0.0–0.5 by 0.1 was prepared
J.M. Chen et al.r Chemical Physics Letters 294 (1998) 209–216
by mixing and heating appropriate amounts of
Nd 2 O 3 , Pr6 O 11 , BaCO 3 and CuO powders at ;
8808C in air for 24 h. This was repeated, with
grinding the sample between calcinations Žup to 4
times.. Samples were then annealed at 9508C for 48
h in 100% O 2 and ground again. For the X-ray
absorption measurements, 400 mg aliquots from the
larger 10 g batches were pressed into 6 mm pellets
and annealed as described above but with an additional oxygenation step of 4508C for 48 h. X-ray
diffraction ŽXRD. showed that all samples were
single phase with an orthorhombic structure. The a
axis showed no change with x, while c lattice
decreased and the b lattice increased. DC SQUID
results showed that Tc decreases linearly with x,
samples becoming normal at x s 0.4 w21x.
Using the 6-m high-energy spherical grating
monochromator ŽHSGM. beamline, the X-ray absorption measurements were performed at the Synchrotron Radiation Research Center ŽSRRC. with an
electron beam energy of 1.5 GeV and a maximum
stored current of 240 mA. X-ray-fluorescence-yield
spectra were recorded using a microchannel plate
ŽMCP. detector w25x. This MCP detector is composed
of a dual set of MCPs with an electrically isolated
grid mounted in front of them. X-ray fluorescence
yield measurement is strictly bulk sensitive with a
probing depth of thousands of angstroms. During the
X-ray fluorescence yield measurements, the grid was
set to a voltage of 100 V while the front of the
MCPs was set to y2000 V and the rear to y200 V.
The negative MCP bias was applied to expel electrons before they entered the detector, while the grid
bias insured that no positive ions were detected. The
MCP detector was located at ; 2 cm from the
sample and oriented parallel to the sample surface.
Photons were incident at an angle of 458 with respect
to the sample normal. The incident photon intensity
Ž I0 . was measured simultaneously by a Ni mesh
located after the exit slit of the monochromator. All
the absorption spectra were normalized to Io. The
photon energies were calibrated using the known
O K-edge and Cu L 3-edge absorption peaks of CuO.
The energy resolution of the monochromator was set
to ; 0.22 and ; 0.45 eV in the O K-edge and
Cu L-edge X-ray absorption measurements, respectively. All the measurements were performed at room
temperature.
211
3. Results and discussion
On the basis of the strong similarity of O 1s
absorption spectra and resonant high-energy inverse
photoemission spectra on Bi 2 Sr2 CaCu 2 O 8 , it is suggested that the core–hole effect in O K-edge X-ray
absorption near edge structure ŽXANES. spectrum of
the cuprate superconductors can be ignored w26x. In
addition, based on the dipole selection rules, only the
local unoccupied states with the O 2p character are
probed in the oxygen K-edge X-ray absorption spectra. Therefore, if the hole states near the Fermi level
in the p-type cuprates are of primarily O 2p character, a prepeak should be visible in the O 1s XANES
spectrum with an intensity proportional to the O 2p
hole concentration.
In Fig. 1, the O K-edge X-ray absorption spectra
for the series of ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 samples
with x s 0.0–0.5 are shown in the energy range of
525–550 eV obtained using a bulk-sensitive total
X-ray fluorescence yield method. The major features
Fig. 1. O K-edge X-ray absorption spectra for the series of
ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 samples with x s 0.0–0.5. The absorption spectra for various compounds with different x values
were normalized to the same height at the main peak of ; 537
eV.
212
J.M. Chen et al.r Chemical Physics Letters 294 (1998) 209–216
in the O 1s X-ray absorption spectrum for sample
with x s 0.1 are two distinct prepeaks at ; 528.3
and ; 529.2 eV with a shoulder at ; 527.6 eV and
a broad peak at ; 537 eV. The low-energy prepeaks
with energy below 532 eV are ascribed to photoexcitation of the O 1s core electrons to holes with predominantly 2p character on the oxygen sites. Unoccupied states related to the Ba 4d, Cu 4s or Cu 4p
states hybridized with O 2p states could be the origin
for the enhanced peaks above 532 eV in
ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 w8,10x. The O K-edge Xray absorption spectra for various compounds with
different x values in Fig. 1 were normalized to the
same height at the main peak of ; 537 eV.
The crystal structure of NdBa 2 Cu 3 O 7 is composed of two CuŽ2.OŽ2.OŽ3. layers separated by a
Nd plane. The unit of CuO 2 and Nd planes are
separated by a CuO 3 ribbon consisting of a BaOŽ4.
plane, a CuŽ1.OŽ1. chain along the b axis and
another BaOŽ4. plane. Therefore, there exists four
nonequivalent oxygen sites in NdBa 2 Cu 3 O 7 , OŽ2.
and OŽ3. within the CuŽ2.O 2 layers, OŽ4. in the BaO
planes and OŽ1. in the CuŽ1.O chains. With respect
to the O 1s edge, the observed different O 1s thresholds in Fig. 1 may be a result of chemical shifts due
to the influence of charges on the oxygen sites and
the site-specific neighborhood. Band-structure calculations based on the local-density approximation
ŽLDA. have been successful to calculate the electronic structure of the cuprate superconductors w27x.
According to the LDA band-structure calculations in
YBa 2 Cu 3 O 7 by Krakauer et al. w28x, the OŽ2,3.
atoms in the CuO 2 planes and the OŽ1. atom in the
CuO chain are predicted to have the largest and the
lowest binding energy of the O 1s level, respectively.
Based on the polarization-dependent X-ray absorption measurements on single-crystal YBa 2 Cu 3 O 7y d
w19x, it was found that OŽ2. 1s level is about 0.8 eV
lower in energy than the OŽ4. 1s level. The OŽ1. 1s
binding energy is in between those of OŽ4. and
OŽ2,3.. These results are in consistent with the theoretical predictions. In O K-edge X-ray absorption
spectra of YBa 2 Cu 3 O 7y d , the prepeaks at ; 527.8
eV are attributed to transitions into O 2p holes in the
CuO 3 ribbons Žapical oxygen sites and CuO chains..
The high-energy prepeak at ; 528.5 eV is ascribed
to transitions into O 2p hole states within the CuO 2
planes w19,29x.
The orthorhombic NdBa 2 Cu 3 O 7y d compound is
isomorphic with YBa 2 Cu 3 O 7y d Žspace group:
Pmmm .. With increasing the Nd content in
Nd 1q x Ba 2yx Cu 3 O 7q d , the compound becomes tetragonal at x ; 0.2 Žspace group: P4 r mmm. for
fully oxygenated samples. The tetragonal structure is
isomorphic with YBa 2 Cu 3 O6 w24x. When oxygen is
removed from the NdBa 2 Cu 3 O 7y d compounds, the
system behaves analogously to YBa 2 Cu 3 O 7y d with
a reduction in total hole concentrations and Tc w23x.
In addition, when oxygen content is reduced in
PrBa 2 Cu 3 O 7y d , a strong reduction of the prepeak
structure in O 1s X-ray absorption spectrum is observed w10x, as seen for the YBa 2 Cu 3 O 7y d compounds w19x. The low-energy loss function of
PrBa 2 Cu 3 O 7y d Ž d ; 0.3. is very similar to that
measured for the YBa 2 Cu 3 O 7y d Ž d ; 0.2. compound. As shown in Fig. 1, the O 1s X-ray absorption spectrum of ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 for x s
0.0 exhibits similar features as observed in
YBaCu 3 O 7y d with d s 0 w19x. We therefore adopt
the same assignment scheme for the O 1s X-ray
absorption spectra of ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 . The
high-energy prepeak at ; 528.3 eV is attributed to
the excitation of O 1s electrons to O 2p holes in the
CuO 2 planes. The low-energy prepeaks at ; 527.6
eV in Fig. 1 are due to the superposition of O 2p hole
states in the apical oxygen sites and the CuO chains.
Based on X-ray absorption studies in La 2y xSr x CuO4 , the absorption peak at ; 529.2 eV in Fig.
1 can be ascribed to transitions between O 1s core
states and the empty upper Hubbard Cu 3d conduction band w30,31x. This kind of pre-edge structure is a
result of hybridization in the ground state of the
Cu3d9 and Cu3d10 L states, where L is ligand hole
from the O 2p band w32x. Due to the strong on-site
correlation effects on the copper sites in the cuprate
superconductors, an upper Hubbard band ŽUHB. has
always been assumed to exist w32x. As noted from
Fig. 1, the prepeak at ; 528.3 eV originating from
the O 2p hole states in the CuO 2 layers shows a
decrease in spectral weight with increasing the dopant
concentration of Pr. This result clearly reveals that
the chemical substitution of Pr for Nd in
ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 leads to a reduction in
hole concentration within the CuO 2 planes.
In order to investigate the variation of the hole
distribution among different oxygen sites as a func-
J.M. Chen et al.r Chemical Physics Letters 294 (1998) 209–216
213
tion of the Pr doping, these prepeaks shown in Fig. 1
were analyzed by fitting each spectrum with Gaussian functions. In Fig. 2 the integrated intensity of
each prepeak, normalized against that of main peak
at ; 537 eV, is plotted as a function of Pr content x
in ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 . As noted from Fig.
2a,b, the hole content from the CuO 2 planes decreases monotonically with increasing the Pr doping,
while that from apical oxygen sites and CuO chains
reduces slowly. According to the LM model, the
hole reduction upon substituting R by Pr in
ŽR 1y x Pr x .Ba 2 Cu 3 O 7 originates from the hole transfer from the CuO 2 planes and CuO 3 ribbons into the
LM band. As shown, the depletion in hole concentration with Pr doping in the ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7
system provides an evidence in support of LM model.
It has been experimentally shown that the concentration of O 2p holes in the CuO 2 planes is strongly
correlated with Tc in the p-type cuprtaes w33x. Accordingly, the Tc value in the ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 system should decrease with increasing
Fig. 3. Pr M 45 -edge and Cu L 23-edge X-ray absorption spectra of
ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 for x s 0.0–0.5. The absorption spectra for various compounds with different x values were normalized to same height at the Cu L 3 peak of 931.3 eV.
Fig. 2. X-dependence of the relative intensity of hole states in
ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 on oxygen sites originating from the
Ža. CuO 2 planes, Žb. CuO 3 ribbons and Žc. upper Hubbard band
ŽUHB.. The solid curves are drawn as a guide for the eye.
the dopant concentration of Pr. Our experimental
results clearly reveal that the quenching of superconductivity with Pr-doping in ŽNd 1.05yx Pr x .Ba 1.95Cu 3 O 7 results predominantly from the hole depletion.
In addition, as shown in Fig. 2c, the peak at
; 529.2 eV originating from the UHB shows a
monotonic increase in spectral weight as the Pr
doping increases. This change is related to a spectral
weight transfer of states from the UHB to doping-induced hole states near the Fermi level w34x. The
similar behavior has also been observed in O 1s
absorption spectra of other p-type cuprate superconductors w29,30,35x.
Fig. 3 presents the Pr M 45 -edge and Cu L 23-edge
X-ray-fluorescence-yield spectra for the series of
ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 compounds with x s
0.0–0.5 in the photon energy range of 925–960 eV.
As marked on the top of Fig. 3, the peak at ; 930.0
eV is related to the transition from Pr 3d 5r2 electrons
into 4f states. As shown, the Cu L 3-edge absorption
spectra are asymmetric and a shoulder exhibits at the
214
J.M. Chen et al.r Chemical Physics Letters 294 (1998) 209–216
in the O 2p orbital w36x. Because two different Cu
sites Ži.e. CuŽ2.O 2 plane and CuŽ1.O chain. exist in
the unit cell of the ŽNd 1.05yx Pr x .Ba 2 Cu 3 O 7 compounds, the high-energy shoulder can be recognized
as the holes in the CuO 2 layers and the CuO 3
ribbons. Therefore, the intensity of this shoulder can
be regarded as the total concentrations of hole states
in the cuprate superconductors, as the prepeaks in the
O 1s X-ray absorption spectra.
In Fig. 4a the integrated intensity of the high-energy shoulder at ; 932.6 eV, normalized against
that of the Cu L 3 peak at 931.3 eV, is plotted as a
function of the Pr content x in ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 . The integrated intensity was estimated
by fitting the Cu L 3 peak and the shoulders centered
at ; 930.0 and ; 932.6 eV with Gaussian functions. As noted from Fig. 4a, the high-energy shoulder originating from the Cu3d9 L defect states shows
a linear decrease in spectral weight with increasing
the Pr content. This indicates that the total hole
concentrations in the CuO 2 planes and the CuO 3
Fig. 4. Ža. Relative intensity of defect state at ;932.6 eV on the
Cu sites and Žb. relative intensity of Pr M 5 peak at ;930.0 eV as
a function of the com positional param eter x in
ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 . The solid lines are drawn as a guide
for the eye.
high-energy side of the Cu L 3 peak. Based on the
curve-fitting analyses, the Cu L 3 absorption spectra
can be decomposed into two peaks at 931.3 and
; 932.6 eV, respectively. Due to strong Coulomb
interaction of the Cu 3d states with the core hole in
the 2p level, the pronounced excitonic peak at 931.3
eV shown in Fig. 3 is ascribed to the excitations of
the CuŽ2p 3r2 .3d9 ground states Žformal Cu2q state.
to the CuŽ2p 3r2 .y1 3d10 excited states, where
CuŽ2p 3r2 .y1 denotes a Cu 2p 3r2 hole. The Pr M 45edge and Cu L 23 -edge X-ray absorption spectra for
various compounds with different x values in Fig. 3
were normalized to the same height at the Cu L 3
peak of 931.3 eV. In addition, based on the photoemission studies and cluster calculation on divalent
Cu compounds, the high-energy shoulder at ; 932.6
eV is attributed to the transitions from the
CuŽ2p 3r2 .3d9 L defected states into the CuŽ2p 3r2 .y1 3d10 L excited states, where L denotes a ligand hole
Fig. 5. Nd 3d-edge X-ray absorption spectra of ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 for x s 0.1, 0.2, 0.3, 0.4 and 0.5. Insert is the plot
of the intensity of Nd M 5 peak at ;979.0 eV as a function of the
Pr doping. The curve is drawn as a guide for the eye.
J.M. Chen et al.r Chemical Physics Letters 294 (1998) 209–216
ribbons in ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 decreases as
the dopant concentration of Pr increases. This result
is in consistent with the conclusion deduced from
O 1s X-ray absorption spectra presented above. In
addition, as shown in Fig. 4b, the spectral weight of
Pr M 5 peak at ; 930.0 eV increases linearly with
increasing the Pr concentration in the compounds.
This supports that the Nd ions in the
ŽNd 1.05yx Pr x .Ba 2 Cu 3 O 7 system are partially substituted by the Pr ions.
Fig. 5 presents the Nd 3d-edge X-ray absorption
spectra of ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 for x s 0.1–
0.5. As noted from Fig. 5, the spectra show two
multiplet structures separated by the Nd 3d 5r2 –
Nd 3d 3r2 spin–orbital splitting. The absorption peak
at ; 979.0 eV is attributed to the excitations of the
Nd 3d 5r2 core states to the empty 4f states. As
shown in the insert of Fig. 5, the intensity of Nd M 5
peak at ; 979.0 eV decreases linearly with increasing the Pr doping in the compounds. This also
confirms the Nd ions in the ŽNd 1.05yx Pr x .Ba 1.95Cu 3 O 7 system partially substituted by the Pr ion.
4. Conclusion
In this study, we report the O K-edge and Cu L 23 edge X-ray absorption spectra for the series of
ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 compounds with x s
0.0–0.5. Near the O 1s edge, the multiple prepeaks
are related to the different O 1s binding energies of
the nonequivalent oxygen sites in the crystal structure. As deduced from the O 1s X-ray absorption
spectra, the hole concentrations originating from the
CuO 2 planes and the CuO 3 ribbons decrease monotonically with increasing of the Pr doping in
ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 . This result is in consistent with the predictions by the LM model. In the Cu
L 23 -edge X-ray absorption spectra, the high-energy
shoulders are attributed to the transitions from
Cu3d9 L defected states into Cu2p – 1 3d10 L excited
states, where L denotes the ligand O 2p hole in the
CuO 3 ribbons and the CuO 2 planes. The intensity of
high-energy shoulder shows a monotonic decrease as
the Pr content increases, corresponding to the reduction in the total hole concentrations in
ŽNd 1.05yx Pr x .Ba 1.95 Cu 3 O 7 . This is also evidenced
by the O K-edge absorption spectra. The present
215
XANES results clearly reveal that the superconductivity suppression with the Pr substitution in
ŽNd 1.05y x Pr x .Ba 1.95 Cu 3 O 7 arises predominantly
from the hole depletion effect.
Acknowledgements
We thank the SRRC staff for their technical support. This research is financially supported by SRRC
and National Science Council of the Republic of
China under Grant Number NSC 86-2613-M-213010. Work at Ames Laboratory was supported by the
Director of Energy Research, US Department of
Energy under Contract No. W-7405-ENG-82.
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