Pr-induced superconductivity suppression
in Y
x
Pr
1x
Ba
2
Cu
4
O
8
probed by soft-X-ray
absorption spectroscopy: a comparative studies
of Y
x
Pr
1x
Ba
2
Cu
4
O
8
and Y
x
Pr
1x
Ba
2
Cu
3
O
7
J.M. Chen
a,*, S.J. Liu
b, J.M. Lee
a, I.P. Hong
c, J.-Y. Lin
d,
Y.S. Gou
b, H.D. Yang
caSynchrotron Radiation Research Center (SRRC), Hsinchu, Taiwan, ROC bDepartment of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan, ROC
cDepartment of Physics, National Sun Yat-Sen University, Kaohsiung, Taiwan, ROC dInstitute of Physics, National Chiao Tung University, Hsinchu, Taiwan, ROC
Received 23 September 2002; in final form 17 December 2002
Abstract
The variation of hole states with Pr doping for YxPr1xBa2Cu3O7and YxPr1xBa2Cu4O8has been investigated by O K-edge X-ray absorption spectroscopy. Upon Pr substitution, Tcsuppression rate in YxPr1xBa2Cu4O8 is slower than that in YxPr1xBa2Cu3O7. For both systems, hole carriers in the CuO2planes and CuO chains decrease monotonically
with increasing Pr doping level. The depletion rate of hole carriers in the CuO2 planes with Pr doping in
YxPr1xBa2Cu4O8is considerably slower than that in YxPr1xBa2Cu3O7. The oxygen content affects the depletion rate of hole carriers in cuprates with Pr doping.
Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction
After the discovery of high-Tc cuprate
super-conductors, Pr substitution in YBa2Cu3O7 has
stimulated much research interest [1]. The absence of superconductivity and the anomalously high-Neel temperature 17 K of PrBa2Cu3O7 have long
remained controversial puzzles [2,3]. In contrast, it
has been recently reported that PrBa2Cu3O7single
crystals grown by the oxygen-annealed traveling-solvent floating-zone method show superconduc-tivity at85 K and Tc enhancement to more than
100 K under pressure [4,5]. These results contradict other reports on the insulating PrBa2Cu3O7single
crystals. However, these reports are still contro-versial due to the lack of reproducibility of data. The absence or occurrence of superconductivity in PrBa2Cu3O7has reopened the field for discussion.
The substitution of Y by Pr in YxPr1x
Ba2Cu3O7leads to a significant decrease in Tc with
www.elsevier.com/locate/cplett
*
Corresponding author.
E-mail address:[email protected](J.M. Chen).
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00032-0
superconductivity disappearing for Pr content greater than 0.55 [6]. For YxPr1xBa2Cu4O8; Tc
decreases monotonically with increasing Pr doping level from Tc¼ 80 K for x ¼ 1 to Tc¼ 6 K for
x¼ 0.3 [7]. The critical Pr concentration required to quench superconductivity in YxPr1xBa2Cu4O8
is estimated to be0.72, which is larger than that of0.55 in YxPr1xBa2Cu3O7. PrBa2Cu3O7 is an
insulator, while the PrBa2Cu4O8 compound
ex-hibits the metallic behavior in the CuO2 planes at
low temperature [8]. The crystal structure of YBa2Cu4O8 is composed of two Cu(2)O(2)O(3)
layers separated by a Y plane. The unit of CuO2
and Y planes is separated by a Cu2O4 ribbon
consisting of a BaO(4) plane, two Cu(1)O(1) chains along the b-axis and another BaO(4) plane. Upon Pr substitution, the YxPr1xBa2Cu4O8
compounds remain the orthorhombic structure isomorphic with YBa2Cu4O8. The structure of
YxPr1xBa2Cu4O8 is thus very similar to that of
YxPr1xBa2Cu3O7 except the double CuO chains
instead of a single CuO chain. Accordingly, the comparative studies of YxPr1xBa2Cu4O8 and
YxPr1xBa2Cu3O7 would be helpful to improve
the understanding on the mechanism of Pr-in-duced superconductivity suppression in PrBa2
Cu3O7 and Pr-doped cuprates.
It has been well established that a key param-eter controlling the Tc value of the hole-doped
superconducting cuprates is the carrier concen-tration in the CuO2 planes [9]. Tc as a function of
hole concentration in the CuO2planes is found to
follow a parabolic curve for many p-type high-Tc
cuprate superconductors [10]. Several methods have been applied to obtain the hole concentration for cuprate superconductors, including the Hall coefficient measurement [11], bond-valence-sum calculation [12], idometric titration technique [13], X-ray absorption spectroscopy [14,15], thermo-electric power measurement [16], etc. Among them, X-ray absorption spectroscopy using syn-chrotron radiation has some advantages over other measurements. X-ray absorption near-edge structure (XANES) measurements provide a direct probe on the local density of hole carriers at the distinct oxygen and copper sites in superconduc-tive copper-oxide phases [9]. In particular, polari-zation-dependent X-ray absorption measurements
are able to provide detailed information on the orbital character of hole states and the distribution of hole carriers in the p-type cuprates [14].
While the enormous majority of research has been conducted on the RxPr1xBa2Cu3O7 systems
(R¼ rare earth elements), no investigations on electronic structure of the YxPr1xBa2Cu4O8
compounds have been performed. Utilizing high-resolution O K-edge X-ray absorption spectra, we report a comparative study on the variation of hole carriers over the different oxygen sites with Pr doping for the YxPr1xBa2Cu3O7 (x¼ 0–1) thin
films and polycrystalline YxPr1xBa2Cu4O8
(x¼ 0–1) samples.
2. Experimental
The detailed procedures for the preparation of samples were reported elsewhere. In brief, the polycrystalline YxPr1xBa2Cu4O8 (x¼ 0–1)
sam-ples were prepared by the nitric pyrolysis method and oxalate coprecipitation method [17,18]. The annealing temperature is 800°C for 0 6 x 6 0:4 and 870 °C for 0:6 6 x 6 1. As confirmed by X-ray diffraction (XRD), all samples under study are single phase. The YxPr1xBa2Cu3O7(x¼ 0–1) thin
films with thickness 4000 AA were deposited on the SrTiO3 substrates by pulsed laser deposition.
These thin films are highly preferentially oriented with the c-axis perpendicular to the surface of thin films.
X-ray absorption measurements were per-formed at the 6-m high-energy spherical grating monochromator (HSGM) beamline of the Syn-chrotron Radiation Research Center (SRRC) in Taiwan. The X-ray absorption spectra were re-corded by the X-ray fluorescence yield mode using a microchannel plate detector. X-ray fluorescence yield measurement is a strictly bulk-sensitive technique with a probing depth of thousands of angstrom. The absorption spectra were corrected for the energy-dependent incident photon intensity as well as for the self-absorption effects and nor-malized to tabulated standard absorption cross-section in the energy range of 600–620 eV. The polarization-dependent X-ray absorption spectra with several incidence angles h were measured by
rotating the sample around a vertical axis. h is the incident angle between the incoming synchrotron beam and surface normal. The absorption spec-trum for h¼ 0° corresponds to the normal inci-dence geometry where the electric field vector E of the linearly polarized synchrotron light is parallel to the ab plane of thin films. In order to obtain E//c spectra, the thin films were rotated to achieve different incidence angles of h¼ 0°, 45°, 60° and 75°. According to IðhÞ ¼ IE==absin2ðhÞ þ IE==c
cos2ðhÞ, the E//c absorption spectrum can be
de-duced. The photon energy was calibrated using the known O K-edge absorption peaks of CuO. The energy resolution of the monochromator was set to0.2 eV for the O K-edge energy range.
3. Results and discussion
In Fig. 1, polarized O K-edge X-ray absorption spectra of the YxPr1xBa2Cu3O7 (x¼ 0–1) thin
films for E//ab polarization are reproduced. The O 1s X-ray absorption spectra for the YxPr1xBa2
Cu3O7 thin films can be divided into two regions.
The low-energy pre-edge peaks with energy below 530 eV are attributed to transitions from the O 1s core electrons into hole states with mainly O 2p character on the oxygen sites. The strong rise in spectral weight of the absorption spectra above 535 eV may be due to continuum absorption of the Ba 4d, Cu 4s or Cu 4p states hybridized with O 2p states [9,14]. As noted, the X-ray absorption spectra with energy beyond 540 eV for all the thin films under study exhibit very similar features and are independent of Pr doping. According to dipole selection rules, for E//ab, only the unoccupied electronic states with O 2pxy symmetry are acces-sible for the O 1s transition and in the E//c case, the empty O 2pzstates are probed. It has been well established that, for YBa2Cu3O7, the absorption
feature at 528.4 eV is ascribed to hole states in the CuO2planes (i.e., the Zhang–Rice (ZR) states),
while the shoulder at 527.8 eV corresponds to unoccupied states in the CuO chains [9,14]. The absorption peak at529.5 eV is attributed to the upper Hubbard band (UHB). Feature B shows an increase in intensity with increasing Pr doping level and is assigned to the Pr 5d states [19].
As noted from Fig. 1, upon Pr substitution, spectral weight of the ZR states is strongly re-duced, but the UHB intensity is significantly en-hanced at the same time. This change is attributed to the well-known transfer of spectral weight from the ZR states to the UHB due to strong correla-tion effects in the CuO2 planes. Furthermore, the
peak positions of the UHB and feature B are shifted to lower energies with increasing Pr con-tent. This implies that the O 1s core level and Fermi level are shifted to higher energies with in-creasing dopant concentration of Pr [14].
Many models were proposed to explain the superconductivity suppression in PrBa2Cu3O7 and
related Pr-doped cuprates. These models include hole-filling [20], hole localization [21], percolation [22], magnetic pair-breaking [20,23], hybridization [24], disorder on Ba site [25], hole transfer from
Fig. 1. Polarized O K-edge X-ray absorption spectra of the YxPr1xBa2Cu3O7(x¼ 0–1) thin films for E//ab polarization.
planes to chains [26], etc. Among them, the exis-tence of Pr 4fzðx2y2Þ–O 2pp hybridized states
pro-posed by Fehrenbacher and Rice (FR) is considered to the most promising model [24]. Based on this model, the Pr 4fzðx2y2Þ O 2pp
hy-bridized state is competitive in energy to hole states in the CuO2planes. The FR model proposed
that hole depletion in the CuO2 planes with Pr
doping is due to transfer of hole carriers from the CuO2 planes into the FR states which bind doped
holes to the Pr sites. However, there is no direct evidence in support of the existence of the FR states.
Fig. 2 presents the O K-edge X-ray absorption spectra of the YxPr1xBa2Cu3O7 (x¼ 0–1) thin
films for E//c polarization. For YBa2Cu3O7, the
absorption peak at527.8 eV is assigned to the O 2pzhole states in the apical oxygen sites. Upon Pr
substitution, an additional small band around 527.8–529.5 eV is observed. If this additional band is attributed to hole transfer from planes and chains to the apical oxygen sites, it is expected that the reduction of hole carriers within the CuO2
planes and CuO chains in Fig. 1 should be ap-proximately equal to the increase of hole numbers in the apical oxygen sites in Fig. 2. In contrast, the increase in the absorption peak at 527.8 eV in Fig. 2 is significantly smaller than the reduction of spectral weight in the ZR states and CuO chains in Fig. 1 for a fixed Pr content. It is thus ascribed the additional spectral weight in the E//c spectra of YxPr1xBa2Cu3O7 to the Pr 4f-O 2p hybridized
states or FR states [14,27]. As noted, this band lies in the almost same energy range as the ZR states observed for the E//ab spectra below the UHB. Accordingly, the substitution of Y by Pr in YxPr1xBa2Cu3O7 induces a transfer of hole
car-riers from the ZR states to the energetically fa-vored FR states, as evidenced in Fig. 1. The experimental results provide direct spectroscopic evidence in support of the FR model.
In Fig. 3, the O K-edge X-ray absorption spectra for the series of YxPr1xBa2Cu4O8 samples with
x¼ 0–1 are presented in the energy range of 525–560 eV. The major features in the O 1s X-ray absorption spectra for those samples are two dis-tinct pre-edge peaks at 528.5 eV and 529.6 eV with a shoulder at527.8 eV, and a broad peak at 537 eV. According to the local-density approxi-mation (LDA) band-structure calculations in YBa2Cu3O7, the O(2, 3) atoms in the CuO2planes
and the O(1) atom in the CuO chain are predicted to have the largest and the lowest binding energy of the O 1s level, respectively [28]. In contrast, in YBa2Cu4O8, the O(1) atom in the CuO chain is
predicted to have the largest binding energies of the O 1s level [29]. This is due to the fact that in YBa2Cu4O8, the O(1) atom is coordinated with
three Cu atoms while in YBa2Cu3O7there exist only
two next-nearest Cu atoms. These predications are consistent with the experimental results [29].
Based on polarization-dependent X-ray ab-sorption measurements on the YBa2Cu4O8 single
Fig. 2. Polarized O K-edge X-ray absorption spectra of the YxPr1xBa2Cu3O7 (x¼ 0–1) thin films for E//c polarization.
The absorption spectrum of PrBa2Cu3O7is plotted as a dashed
crystals, the pre-edge peaks at 528.7 eV are as-cribed to transitions into the O 2p hole states within the CuO2planes and CuO chains [29]. The
orthorhombic YxPr1xBa2Cu4O8 compounds are
isomorphic with YBa2Cu4O8. We therefore adopt
the same assignment scheme for the O 1s X-ray absorption spectra of YxPr1xBa2Cu4O8. The
low-energy pre-edge peak at 527.8 eV in Fig. 3 cor-responds to excitations of the O 1s electrons to the O 2p hole states in the apical oxygen sites. The high-energy pre-edge peaks at 528.5 eV are due to the superposition of the O 2p hole states in the CuO2 planes and CuO chains. The absorption
peak at 529.6 eV is ascribed to transitions be-tween the O 1s core electrons and the upper Hubbard Cu 3d conduction band. This kind of pre-edge structure is a result of hybridization in the ground state of the Cu3d9and Cu3d10L states,
where L is ligand hole from the O 2p band. Due to the strong on-site correlation effects on the copper sites in the cuprate superconductors, an UHB has always been assumed to exist. As noted from Fig. 3, the pre-edge peaks at528.5 eV originating from the O 2p hole states in the CuO2 layers and
CuO chains show a slight decrease in spectral weight with increasing the dopant concentration of Pr. This result clearly reveals that chemical sub-stitution of Pr for Y in YxPr1xBa2Cu4O8causes a
reduction in hole concentration within the CuO2
planes and CuO chains.
To quantify the variation of hole carriers over the different oxygen sites as a function of Pr con-tent, the pre-edge peaks shown in Figs. 1 and 3 were analyzed by fitting each spectrum with Gaussian functions. In order to define peak width for those three peaks, the results from the single crystals were referenced [14,15,29]. The energy shift of the UHB upon Pr substitution was also taken into account. The hole distribution and spectral weight of the UHB as a function of Pr content for YxPr1xBa2Cu3O7 are presented in
Figs. 4a and b. It can be seen from Fig. 4a that hole carriers within the CuO2 planes and CuO
chains in YxPr1xBa2Cu3O7 decrease significantly
with increasing Pr doping level. Accordingly, the UHB shows a monotonic increase in intensity as Pr content increases, as presented in Fig. 4b. In Figs. 4c–e, the integrated intensity of each pre-edge peak is plotted as a function of Pr content in YxPr1xBa2Cu4O8. As noted from Figs. 4c and d,
the hole content in the CuO2 planes and CuO
chains reduces slowly with increasing Pr doping level, while that in the apical oxygen sites remains nearly constant. In contrast, spectral weight of the UHB increases moderately as Pr doping level in-creases, as shown in Fig. 4e.
It is intriguing to compare the depletion rate of hole carriers with Pr doping for YxPr1xBa2Cu4O8
and YxPr1xBa2Cu3O7. Fig. 5 presents the change
rates of hole carriers in the ZR states and CuO chains and spectral weight of the UHB for YxPr1xBa2Cu4O8and YxPr1xBa2Cu4O8 relative
to the undoped parent cuprates for x¼ 1. As noted from Fig. 5a, upon Pr substitution, the reduction rate of hole carriers within the ZR states and CuO chains in YxPr1xBa2Cu4O8is considerably slower
Fig. 3. O K-edge X-ray absorption spectra of the polycrystal-line YxPr1xBa2Cu4O8(x¼ 0–1) samples.
than that in YxPr1xBa2Cu3O7. Moreover, the
increased rate of spectral weight in the UHB with Pr doping in YxPr1xBa2Cu4O8 is significantly
higher than that in YxPr1xBa2Cu3O7, as shown in
Fig. 5b. It is known that the UHB is strongly correlated with the ZR states and presumed to have no correlation with hole states in the CuO chains [9]. Thus, the results in Fig. 5b confirm that the hole depletion rate in the ZR states with Pr substitution in YxPr1xBa2Cu4O8 is slower than
that in YxPr1xBa2Cu3O7. As well established, the
hole concentration in the ZR states is strongly correlated with Tc. Thus, the Tc suppression rate
with Pr doping in YxPr1xBa2Cu4O8 should be in
principle slower than that in YxPr1xBa2Cu3O7,
which is indeed found to be the case.
It has been shown that the extent of the Pr 4f–O 2p hybridization in Pr-doped cuprates is determined predominantly to the Pr–O(2, 3) bond length as well as the O(2, 3)–Pr–O(2,3) bond an-gle and the energy level difference between the Fermi level EF and the Pr 4f level [30,31]. As
compared to YxPr1xBa2Cu3O7, the
correspond-ing YxPr1xBa2 Cu4O8 compounds exhibit a
longer c-axis parameter due to the double CuO chains. Based on X-ray diffraction studies, the Pr–O(2, 3) bond length in PrBa2Cu4O8 is slightly
shorter than that in PrBa2Cu3O7[32,33]. It is thus
expected that the hybridization between the Pr 4f states and O 2p orbitals in YxPr1xBa2Cu4O8 is
less pronounced than that in corresponding YxPr1xBa2Cu3O7. It has been proposed that the
energy of the hole-depletion band (or FR states) may depend on the extent of the Pr 4f–O 2p hybridization [34]. In other words, the enlarged
Fig. 5. Variation rates of (a) hole carriers in the CuO2
pla-ne + CuO chain and (b) spectral weight of the UHB for YxPr1xBa2Cu3O7and YxPr1xBa2Cu4O8relative to the
und-oped parent cuprates for x¼ 1. Fig. 4. Integrated cross-sections of (a) hole states in the CuO2
planes + CuO chains and (b) the upper Hubbard band (UHB) as a function of Pr concentration in YxPr1xBa2Cu3O7.
Inte-grated cross-sections of (c) hole states in the apical oxygen sites, (d) hole states in the CuO2 planes + CuO chains, and (e) the
upper Hubbard band (UHB) as a function of Pr content in YxPr1xBa2Cu4O8. The solid curves are drawn as a guide for
lattice in YxPr1xBa2Cu4O8 reduces the extent of
the Pr 4f–O 2p hybridization and in turn leads to the decrease in position of the hole-depletion band (or FR states) relative to the ZR states. Accordingly, as compared to YxPr1xBa2Cu3O7,
the resultant hole depletion rate in corresponding YxPr1xBa2Cu4O8 is mitigated and the remaining
hole content in the ZR states is higher, as evi-denced in Fig. 5.
It has been demonstrated that hole depletion in cuprates upon Pr doping arises from not only the ZR states but also hole states in the CuO chains [14]. For RxPr1xBa2Cu3O7, the hole reduction
with Pr substitution originates predominantly from the ZR states [14,35]. However, due to dou-ble CuO chains in YxPr1xBa2Cu3O7, the energy
level of hole states in the CuO chains might be more overlapped with the ZR states [29]. Thus, another possibility for slow reduction rate in the ZR states with Pr doping in YxPr1xBa2Cu4O8
compared to YxPr1xBa2Cu3O7is that the transfer
of hole carriers in the CuO chains to the FR states in the former system may make a significant con-tribution. Here, we assumed that the total reduc-tion of hole carriers from the ZR sates and CuO chains in YxPr1xBa2Cu3O7is the same as that in
YxPr1xBa2Cu4O8 for a fixed Pr content.
Ac-cordingly, the hole depletion in the ZR states with Pr doping in YxPr1xBa2Cu4O8is less than that in
YxPr1xBa2Cu3O7. The high content of hole
car-riers in the ZR states might be the possible reason for metallic properties of the CuO2 planes in
YxPr1xBa2Cu4O8for Pr content greater than 0.72
at low temperature [7,8]. In other words, due to the significant decrease of hole carriers in the ZR states with Pr doping in YxPr1xBa2Cu3O7, it is
expected that PrBa2Cu3O7 should be an insulator.
The monotonic depletion of hole carriers in the CuO2 planes and CuO chains with increasing Pr
content in YxPr1xBa2Cu4O8and YxPr1xBa2Cu3
O7provides evidence in support of hole-depletion
effect based on the Pr 4f–O 2p hybridization [35].
4. Conclusion
We have studied the variation of hole states with Pr doping related to the Tc suppression for
YxPr1xBa2Cu3O7 and YxPr1xBa2Cu4O8 by O
K-edge X-ray absorption spectroscopy. We dem-onstrate spectroscopic evidence of the existence of FR states. For both systems, hole carriers in the CuO2 planes and CuO chains decrease
monotoni-cally with increasing Pr doping level, which are caused by the transfer of hole carriers from the ZR band and CuO chains to the localized FR band. The depletion rate of hole carriers in the ZR states with Pr substitution in YxPr1xBa2Cu4O8is slower
than that in YxPr1xBa2Cu3O7. The experimental
results provide clear evidence in support of hole-depletion effect based on the Pr 4f–O 2p hybrid-ization. Based on present XANES studies, the oxygen content affects the hole depletion rate of cuprates with Pr doping.
Acknowledgements
We gratefully acknowledge the SRRC staff for their technical support. This work is supported by the SRRC and National Science Council of the Republic of China (Grant No. NSC 89-2113-M-213-013).
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