The question of Pr in HTSC
Introduction
The discovery of high-temperature superconductivity (HTSC) in 1986 by Bednorz and Mueller in LaBaCuO [1] has triggered a cascade of experimental and theoretical research concerning the underlying mechanism of this phenomenon, but there exists yet no clear answer to the origin of HTSC and its high Tc. A reasonable approach to tackle the above shortcoming is to remove oxygen or substitute different elements in HTSC compounds. Among different well-known families of HTSC compounds, i.e., LaBaCuO (La214), YBCO (Y123), BSCCO (Bi2201, Bi2212, Bi2223), substitution of Pr for Y in Y123 or, in general, in R123 (R=rare earth element) has been the subject of a tremendous amount of research and controversy.
Soon after the discovery of Y123 with Tc above 90 K [2], great interest was shown in substituting different elements for atoms on different sites in Y123. Substitution of Y with R (except Ce, Pr, Pm, Tb) shows no important effect on Tc, crystal structure, or normal properties of the system [3]. Hence, in essence, Y or R has no crucial role in HTSC. More important is the fact that the presence of magnetic R ions preserves HTSC. I turn to the above four exceptions. Pm is radioactively unstable, and Pm123 has not been studied. On the basis of ion size, the 123 phase should exist for Ce, Pr and Tb. It does exist for Pr [4], but not for Ce and Tb, probably due to a failure of Ce and Tb to appear in their trivalent state.
With standard synthesis conditions for bulk ceramics, the formation of the perovskite MBO3 (M: Ce, Pr or Tb) [5], competes with the formation of the 123 phase. However, Pr has different valence states: Pr2O3 (3+), Pr7O12 (3.4+), Pr9O16 (3.56+), Pr6O11 (3.67+), and PrO2 (4+). The partial presence of the Pr ions in their trivalent state helps stabilize the 123 structure, and under the usual conditions of synthesis, the 123 phase structure dominates for Pr. The valence of Pr in R1−xPrxBa2Cu3O7−δ (RPr123) has been the subject of detailed research and great controversy. (Hereafter, x denotes the Pr concentration, and δ denotes the oxygen deficiency).
Partial substitution of Pr for elements of different families of HTSC has been studied, but most puzzling effects arise out of Pr substitution for R in R123 superconducting (SC) systems. Substitution of Pr for R in R123, quenches SC [6] and Tc goes to zero for certain xc, which depends on the element R. The suppression of Tc with x does not follow the Abrikosov–Gor’kov (AG) magnetic pair-breaking theory [7]. An explanation for the lack of SC in Pr123 might provide insight into the origin of the SC in HTSC. The quenching of SC in Pr123 may also help our understanding of the interplay between magnetism and SC. Furthermore, Pr123 is interesting for applications, as insulating layers between Y123 layers in multilayer films.
Another striking feature of Pr is its magnetic ordering temperature TN (Pr123)=17 K [8] which is one or two orders of magnitude larger than expected relative to other elements (e.g., TN (Gd123)=2 K, the highest besides Pr123). The novel Pr–Cu magnetic ordering phase at low temperature [9] is another anomalous feature of the Pr123 system.
It is impossible to review all of the massive literature on Pr in HTSC. For related reviews see Refs. [10], [11]. The aim of this review is to survey the experimental and theoretical research, discuss the important issues and recent developments about the role of Pr in HTSC, and focus upon controversies. The author apologizes in advance for omissions from an incomplete reference list.
This review is organized as follows: Structure and related properties are discussed in Section 2. The valence of Pr, being controversial, is discussed in Section 3. Transport properties, such as normal state resistivity, magnetoresistance, Hall effect, transition temperature, and metal insulator transition (MIT) are covered in Section 4. Section 5 collects effects due to the ion size of the host R elements. Section 6 covers magnetic ordering, Section 7 pressure effects, and Section 8 complimentary compounds. Section 9 discusses effects of mis-substitution, and finally different models are reviewed in Section 10.
Section snippets
Structural properties
Extensive research has been done on the crystal structure of the insulating Pr-based compound to determine the differences between its crystal structure and other SC R123 compounds. Soderholm et al. [6] pioneered this subject. Tretyakov and Goodilin recently thoroughly reviewed the chemical principles of preparing HTSC, including 292 references [12]. The samples used for most of the studies are polycrystalline, prepared by the solid-state reaction between R2O3, Pr6O11, BaCO3 or BaO, and CuO.
Valence of Pr
The central issue in Pr-based or doped compounds is the question of Pr valence. Is it 3+, 4+, or mixed? If Pr is trivalent like the other R elements, why is SC quenched in Pr123? and if Pr is tetravalent, why cannot the spectroscopy measurements confirm that? Despite enormous research, this question remains open and highly controversial.
Techniques employed to determine the valence of Pr in HTSC include: XANES, EELS, ellipsometry, photoemission, and χac. I focus first on spectroscopy. Lytle et
Transport properties
In this section, I discuss transport measurements, such as normal state resistivity, magnetoresistance, Hall effect, SC transition temperature Tc, and metal–insulator transition (MIT), which are widely used to analyze HTSC features, including effects arising from the presence of Pr.
Ion-size effect
Pr is a rare earth element (La to Lu). Thus, detailed studies of effect of Pr doping in R123 compounds should aid in better understanding the role of Pr in HTSC. Experimental studies of RPr123 reveal a special dependence of many characteristic parameters of Pr on the size of the R host ion. Moreover, in RPr123 systems the continuous composition, x, provides a unique possibility to study the SC and magnetic subsystem interference.
Studies of ion-size effects by Guan and coworkers have revealed
Magnetic ordering
An anomalously high-antiferromagnetic (AFM) ordering temperature of Pr in Pr123 has been reported from magnetic susceptibility, heat capacity, and neutron diffraction measurements. This temperature is about two orders of magnitude higher than expected from either dipolar direct exchange or the RKKY interaction alone. In addition, TN is found to have low field dependence [86], [90], and a low size of the ordered Pr moment [91].
A neutron diffraction study on Pr123 revealed a novel Pr–Cu
Pressure effects
An effective avenue toward understanding the mechanism of conduction in HTSC is to search for correlation between the normal and SC state properties under variation of basic physical parameters, such as charge concentration variation through cation substitution or the change of atomic positions through application of pressure [95]. However, there are distinct differences between physical pressure and chemical pressure. While the former has equal effects on each atom, chemical pressure may have
Complimentary compounds
Many investigations have been carried out on other Pr-doped compounds to examine effects of crystal chemistry and/or chemical pressure on Pr SC suppression, or to compare Pr-doping with proton irradiation effects in creating effective pinning centers [100]. Structural, electrical, and magnetic properties have been investigated upon Ca substitution for R or Ba ions [101]. The critical current density decreased with Pr-substitution and increased with Ca-substitution in the GdPrCa123 system [102].
Mis-substitution effects
Blackstead et al. [14] reported observing an inhomogeneous granular SC in films of Pr123. Zou et al. [111] later found that some part of an oxygen annealed travelling solvent floating zone (TSFZ) Pr123 crystal grown in a deoxidized atmosphere exhibits SC about 85 K. The substitution ratio x of Pr atoms to Ba sites for their SC single crystals is indefinite because of the unavoidable nonuniform distribution of the substitution ratio over a TSFZ-Pr123 single crystal. The c-axis lattice parameter
Models
Many proposals and models, mostly qualitative, have been put forth to explain SC suppression in RPr123 and insulating Pr123. The most important and generally considered models are:
- (1)
Strong exchange coupling of the Pr 4f moments with the spins of the holes in the CuO2 plane, resulting in magnetic pair breaking on the basis of the AG theory [7]. This model is inconsistent with the nonlinearity of Tc(x) [38], the MIT at xc [124], and the recovery of SC by partial Ca substitution for Pr in RPr123
Acknowledgements
In preparing this manuscript, I am especially indebted to the works of my students: Z. Yamani, V. Daadmehr, H. Khosroabadi, M.R. Mohammadizadeh, H. Naghshara, H. Shakeripour, and G. Zolfagharkhani in the Magnet Research Laboratory.
This work was supported in part by the Office of Vice President for Research at Sharif University of Technology.
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