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Regular Articles

Vol. 12, No. 8, pp. 30–36, Aug. 2014. https://doi.org/10.53829/ntr201408ra1

High-temperature Superconductivity without Doping—Synthesis of Conceptually New Superconductors

Yoshiharu Krockenberger and Hideki Yamamoto

Abstract

High-temperature superconductivity is commonly associated with cuprate superconductors as their superconducting transition temperatures are still highest among all of the superconducting materials. It is these cuprate superconductors that drive scientists to reveal the very mechanism of its superconductivity and to derive models capable of predicting new superconducting material systems. Cuprate superconductors are built up of copper-oxygen planes, sandwiched by rare-earth or alkaline-earth oxide layers. The copper-oxygen layers form infinitely long planes, and it is those planes where superconductivity takes place. In particular, the copper ions in those planes may have three distinct coordinations, i.e., octahedral, pyramidal, and square-planar. Quite generally, electronic states are a function of crystal structure, crystal symmetry, and the elements building up the crystal structures. Nevertheless, it has been widely assumed that the copper-oxygen planes are inherently insulating and superconductivity is induced by doping carriers into those copper-oxygen planes. Indeed, such an approach is suitable for the copper-oxygen layers with octahedral and pyramidal coordinated copper, where superconductivity is induced by hole doping. For cuprates with square-planar coordinated copper, however, we demonstrate here that elimination of defects by annealing is the key to inducing superconductivity and doping is not a prerequisite for the emergence of superconductivity.

Keywords: undoped superconductor, high-T_c cuprates, molecular beam epitaxy

1. Introduction

High-temperature superconductivity is a key technology for many present and future milestones ranging from transportation [1], over medical applications [2] towards elementary particle physics [3]. Among all superconducting materials known so far, the discovery of cuprate superconductors was not only awarded by the Nobel prize [4] but still hold record high superconducting transition temperatures as high as 150 K [5]. Despite all efforts and progress made to understand the underlying mechanism driving superconductivity in those cuprates, conclusive theoretical models remain elusive. Solving the enigma of cuprate superconductivity, however, holds the key for materials with even higher superconducting transition temperatures. One of the crucial factors that drives superconductivity in cuprates are the copper-oxygen planes and the local environment of the Cu ions in those planes. Generally, three different geometries are stabilized in cuprate superconductors, i.e., octahedral, pyramidal, and square-planar. In many cases, those coordination cages do not appear exclusive but as combinations, e.g., pyramidal and square-planar in Bi₂Sr₂Ca₂Cu₃O_10+δ. The local coordination of the copper ions has far reaching implications on the electronic structure. Revealing electronic structures subject to a specific coordination is therefore of importance. La_2-xSr_xCuO₄ is a cuprate with solely octahedrally coordinated copper and it is one of the most widely studied systems among cuprate superconductors and commonly termed “T-phase.” Cuprates with solely square-planar coordinated copper are much less studied owing to the difficult and cumbersome process in synthesis as well as inducing superconductivity to them. Hence, data reported for these systems are significantly dressed and dominated by electronic correlations stemming from impurity and defect related influences. In contrast to those earlier reports, we can synthesize cuprate superconductors by molecular beam epitaxy under oxidizing conditions. Molecular beam epitaxy is a well-known and widely used technique for synthesizing semiconductors and semiconductor-related devices. We further refined and optimized this technique with the scope of ultra-high quality cuprate materials [6]. Crystalline materials with the utmost degree of perfection are mandatory prerequisites in revealing electronic and magnetic correlations in cuprates with respect to the various coordination geometries of copper. Two cuprates, where copper is square-planar coordinated, are known, i.e., Nd₂CuO₄–structure [7, 8] and infinite-layer structure [9]. While the Nd₂CuO₄–structure (T’-structure) can be synthesized under ambient conditions, the formation of bulk infinite-layer cuprates occurs only under high pressure. The T’-structure belongs to the Ruddelsden-Popper series of transition metal oxides [10–12], which is also the case for the T-structure. In contrast to other transition metal oxides, where the transition metal ion is octahedrally coordinated, the copper ion is forced to be square-planar coordinated in the T’-structure and the force is brought by the tolerance factor [13]. The defects appear at the apical position of copper, forming locally pyramidal coordinated copper. Electronically, this situation represents an insulating and antiferromagnetic ground state. Upon elimination of those defects (by elaborate annealing), the entire electronic structure changes from an antiferromagnetic insulator to a superconducting metal. In the following we focus on our efforts to elucidate the electronic structure of ultra-high quality T’-cuprate superconductors where such defects have been eliminated by annealing.

2. Experimental

High quality thin films of Pr₂CuO₄ are grown by molecular beam epitaxy onto (001) SrTiO₃ substrates [14]. Pr and Cu are evaporated by e-beam evaporation from metal-sources and the evaporation rate is controlled by electron impact emission spectroscopy [15–18]. Ozone or radio frequency (RF) activated oxygen is used as an oxidizing agent. The growth is monitored by reflection high energy electron diffraction (RHEED) which allows not only fine tuning of growth conditions but a real-time feedback of film growth. In Fig. 1, the crystal structure of Pr₂CuO₄, high-resolution reciprocal space map (HRRSM) of the epitaxial relation, and high-resolution transmission electron microscopy (HRTEM) image of Pr₂CuO₄ films on (001) SrTiO₃ substrates are shown, respectively. Here, growth conditions are tuned in order to allow for a relaxed but epitaxial growth of single phase and c-axis oriented Pr₂CuO₄ on SrTiO₃. Throughout the following experiments and discussions, the film thickness was kept constant at 100 nm. In Fig. 2, X-ray diffraction of a Pr₂CuO₄ film on (001) SrTiO₃ substrate is shown. Using a Nelson-Riley [19] function allows for a precise determination of c-axis lengths as this is an important control parameter.

Fig. 1. (a) Crystal structure of Pr₂CuO₄, (b) high-resolution reciprocal space map of Pr₂CuO₄ grown on (001) SrTiO₃ substrate, and (c) high-resolution transmission electron microscopy image of Pr₂CuO₄ grown on (001) SrTiO₃ substrate taken at the interface.

Fig. 2. X-ray diffraction pattern taken of Pr₂CuO₄ grown on (001) SrTiO₃ substrate.

As discussed in the introduction, the as-grown Pr₂CuO₄ is insulating due to the presence of over-stoichiometric oxygen occupying the apical sites of copper [20–24]. As it is well known from Ce doped Pr_2-xCe_xCuO₄, annealing is required for the induction of superconductivity—otherwise, the system remains insulating and antiferromagnetic. During the annealing process, the apical oxygen leaves the Pr₂CuO₄ crystal. Such a process is a diffusion process though in case of Pr₂CuO₄ a detailed understanding remains elusive owing to the three different oxygen positions in the T’-crystal structure [25–29]. Here, we applied a two-step annealing process [15], which allows for an effective evacuation of apical sites, while regular oxygen positions are not affected. The annealing procedure requires that thermodynamic constraints of Pr₂CuO₄ itself are not violated (this leads to decomposition). At the same time, an appropriate annealing is achieved only near or on thermodynamic boundary on a delicate balance of kinetically driven competition.

3. Results and discussions

3.1 Standard annealing

The term “standard annealing” may seem confusing though it simply describes annealing conditions used for the induction of superconductivity at doping levels of x = 0.15; i.e., Pr_1.85Ce_0.15CuO₄ [30, 31]. As annealing of Ce doped Pr₂CuO₄ is carried out under ultra-high vacuum, only two annealing parameters, time and temperature, are to be considered. Applying the standard annealing approach leads to phase diagram of Pr_2-xCe_xCuO₄ as shown in Fig. 3. At low Ce doping levels (x < 0.11) the system is insulating and superconductivity sets in for x > 0.12. This phase diagram (Fig. 3) inevitably is identical to what has been reported for single crystals or powder samples.

Fig. 3. (a) Thermodynamic phase diagram in the vicinity of the growth conditions of Pr_2-xCe_xCuO₄ (red area) and annealing conditions (blue area) and (b) superconducting phase diagram of Pr_2-xCe_xCuO₄ grown on (001) SrTiO₃ substrates as a result of the annealing procedure shown on the left.

3.2 2-step annealing

Applying standard annealing conditions to Pr_2-xCe_xCuO₄ is effective for the induction of superconductivity for x > 0.11. In Pr_2-xCe_xCuO₄, the addition of Ce not only acts as a dopant but as a defect [32]. If less Ce is incorporated into the Pr_2-xCe_xCuO₄ crystal, the crystallite dimensions rapidly increase. The path taken by oxygen leaving Pr_2-xCe_xCuO₄ crystals is predominantly along the CuO₂ planes until reaching a grain boundary. The thermodynamic environment under standard annealing conditions is unsuitable for establishing such a condition over long periods of time or at elevated temperatures due to the simultaneously occuring defects at regular oxygen sites. This diffusion problem of identical species at competing sites is not limited to thin film but to a much larger extent to single crystals owing to their reduced surface/volume ratio. However, this problem can be overcome by introducing an intermediate annealing step. This situation is visualized in the thermodynamic phase diagram plotted in Fig. 4. In contrast to the standard annealing procedure, Pr₂CuO₄ is kept at a temperature comparable or even higher than the synthesis temperature used during its growth [33]. Moreover, an environment with small but finite partial oxygen pressure is stabilized for 60 min before the sample temperature is cooled to the second annealing step. In Fig. 5, the temperature dependence of resistivity is plotted for all three positions of the thermodynamic phase diagram. The as-grown Pr₂CuO₄ is insulating and its resistivity further increases after the 1^st annealing step. This behavior indicates that additional defects, tapering the electron mobility, are created. Schematically, this is shown in Fig. 6. While the as-grown Pr₂CuO₄ posseses defects primarily related to occupied apical sites, the first annealing step is about to increase the defects within the CuO₂ planes. Consequently, the absolute resistivity value increases. Simultaneously, the overall crystallographic dimensions remain nearly unaffected. This does change, however, dramatically after the second annealing step and the c-axis value shrinks noticably. Most dramatic is the influence on the resistivity behavior as it not only changes from an insulating to a metallic state but to superconductivity. This behavior is again common to what is known to happen during the conventional annealing process for Ce-doped Pr_2-xCe_xCuO₄ (x > 0.11). Accordingly, our study has revealed an intrinsic electronic phase diagram for Pr_2-xCe_xCuO₄, which was hampered by enigmatic and cumbersome processes in optimizing annealing conditions.

Fig. 4. (a) Thermodynamic phase diagram in the vicinity of the growth conditions of Pr_2-xCe_xCuO₄ (red area) and annealing conditions (green and blue area) and (b) superconducting phase diagram of Pr_2-xCe_xCuO₄ grown on (001) SrTiO₃ substrates as a result of the annealing procedure shown on the left. Data denoted by triangles are taken from Ref. [34].

Fig. 5. (a) Temperature dependence of resistivity of Pr₂CuO₄ thin films in as-grown state (green dashed line), after the 1^st annealing step (step I), and after the 2^nd annealing step (step II) and (b) trace of the crystallographic lattice parameters of Pr₂CuO₄ thin films.

Fig. 6. Microscopic view of oxygen vacancies and defects at each synthesis or annealing step.

4. Conclusion

It has been assumed that the high-T_c cuprates universally have an insulating ground state in their undoped state due to electronic correlations, and doping is necessary to change the correlations and induce superconductivity. However, a series of our studies have revealed that the cuprates with the square-planar coordinated Cu have a metallic ground state and show superconducting transition after removal of defects from the playground of high-T_csuperconductivity, i.e., the copper-oxygen planes. This observation is against the main paradigm in theories of high-T_c superconductivity. Although, at the moment, this conclusion is still under intense debate, the very result of our research will be a turning point in the quest for high-T_c superconductivity once further investigations support it.

References

[1]	K. Kitazawa, “Superconductivity: 100th Anniversary of Its Discovery and Its Future,” Japanese Journal of Applied Physics, Vol. 51, 010001, 2012.
[2]	E. M. Haacke, R. W. Brown, M. R. Thompson, and R. Venkatesan, “Magnetic Resonance Imaging: Physical Principles and Sequence Design,” Wiley, 1999.
[3]	M. Riordan, “The Demise of the Superconducting Super Collider,” Phys. Perspect, Vol. 2, No. 4, pp. 411–425, 2000.
[4]	J. G. Bednorz and K. A. Müller, “Possible high T_c superconductivity in the Ba−La−Cu−O system,” Z. Physik B - Condensed Matter, Vol. 64, No. 2, pp. 189–193, 1986.
[5]	N. Takeshita, A. Yamamoto, A. Iyo, and H. Eisaki, “Zero Resistivity above 150 K in HgBa₂Ca₂Cu₃O_8+δ at High Pressure,” Journal of the Physical Society of Japan, Vol. 82, 023711, 2013.
[6]	H. Yamamoto, Y. Krockenberger, and M. Naito, “Multi-source MBE with High-precision Rate Control System as a Synthesis Method Sui Generis for Multi-cation Metal Oxides,” Journal of Crystal Growth, Vol. 378, pp. 184–188, 2013.
[7]	Y. Tokura, H. Takagi, and S. Uchida, “A Superconducting Copper Oxide Compound with Electrons as the Charge Carriers,” Nature Vol. 337, pp. 345–347, 1989.
[8]	H. Müller-Buschbaum and M. W. Wollschläger, “Über ternäre Oxocuprate. VII. Zur Kristallstruktur von Nd₂CuO₄,” Zeitschrift für anorganische und allgemeine Chemie, Vol. 414, No. 1, pp.76–80, 1975.
[9]	Y. Krockenberger, K. Sakuma, and H. Yamamoto, “Molecular Beam Epitaxy and Transport Properties of Infinite-layer Sr_0.90La_0.10CuO₂ Thin Films,” Applied Physics Express, Vol. 5, No.4, 043101, 2012.
[10]	S. N. Ruddlesden and P. Popper, “New Compounds of the K₂NiF₄Type,” Acta Crystallographica, Vol. 10, No. 8, pp. 538–539, 1957.
[11]	S. N. Ruddlesden and P. Popper, “The Compound Sr₃Ti₂O₇ and Its Structure,” Acta Crystallographica, Vol. 11, No. 1, pp. 54–55, 1958.
[12]	R. E. Schaak and T. E. Mallouk, “Perovskites by Design: A Toolbox of Solid-state Reactions,” Chemistry of Materials, Vol. 14, No. 4, pp. 1455–1471, 2002.
[13]	V. M. Goldschmidt, “Die Gesetze der Krystallochemie,” Die Naturwissenschaften, Vol. 14, No. 21, pp. 477–485, 1926.
[14]	Y. Krockenberger, H. Yamamoto, A. Tsukada, M. Mitsuhashi, and M. Naito, “Unconventional Transport and Superconducting Properties in Electron-doped Cuprates,” Physical Review B, Vol. 85, 184502, 2012.
[15]	H. Yamamoto, O. Matsumoto, Y. Krockenberger, K. Yamagami, and M. Naito, “Molecular Beam Epitaxy of Superconducting Pr₂CuO₄ Films,” Solid State Communications, Vol. 151, No. 10, pp. 771–774, 2011.
[16]	H. Yamamoto, Y. Krockenberger, and M. Naito, “Augmented Methods for Growth and Development of Novel Multi-cation Oxides,” Proc. of SPIE 8987, Oxide-based Materials and Devices V, 89870V, 2014.
[17]	C. Lu, C. D. Blissett, and G. Diehl, “An Electron Impact Emission Spectroscopy Flux Sensor for Monitoring Deposition Rate at High Background Gas Pressure with Improved Accuracy,” Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, Vol. 26, No. 4, pp. 956–960, 2008.
[18]	C. Lu, M. J. Lightner, and C. A. Gogol, “Rate Controlling and Composition Analysis of Alloy Deposition Processes by Electron Impact Emission Spectroscopy (EIES),” Journal of Vacuum Science & Technology, Vol. 14, No.1, pp. 103–107, 1977.
[19]	J. B. Nelson and D. P. Riley, “An Experimental Investigation of Extrapolation Methods in the Derivation of Accurate Unit-cell Dimensions of Crystals,” Proceedings of the Physical Society, Vol. 57, No. 3, pp. 160–177, 1945.
[20]	A. J. Schultz, J. D. Jorgensen, J. L. Peng, and R. L. Greene, “Single-crystal Neutron-diffraction Structures of Reduced and Oxygenated Nd_2−xCe_xCuO_y,” Physical Review B, Vol. 53, 5157, 1996.
[21]	P. G. Radaelli, J. D. Jorgensen, A. J. Schultz, J. L. Peng, and R. L. Greene, “Evidence of Apical Oxygen in Nd₂CuO_y Determined by Single-crystal Neutron Diffraction,” Physical Review B, Vol. 49, 15322, 1994.
[22]	P. W. Klamut, “Evidence for Interstitial Oxygen and Possible Role of Reduction in Nd_2−xCe_xCuO_{4 + y},” Journal of Alloys and Compounds, Vol. 194, No. 1, pp. L5–L7, 1993.
[23]	C. Sułowski, A. Sikora, P. W. Klamut, and R. Horyñ, “Transport Properties and Oxygen Sublattice Time Relaxation of Nd₂CuO_4−δ,” Solid State Communications, Vol. 112, No. 11, pp. 601–604, 1999.
[24]	T. Klimczuk, W. Sadowski, J. Olchowik, and E. Walker, “Crystal Growth and the Influence of Oxygen Stoichiometry on Electrical Resistivity of Nd_2−xCe_xCuO_4-y Single Crystals,” Superconductor Science and Technology, Vol. 12, No. 4, pp. 199–202, 1999.
[25]	N. L. Allan and W. C. Mackrodt, “Defect Chemistry of La₂CuO₄, Nd₂CuO₄ and Pr₂CuO₄ Doped by Tetravalent Cations: Relevance to High-T_c Behaviour,” Philosophical Magazine Letters, Vol. 60, No. 5, pp. 183–186, 1989.
[26]	N. L. Allan and W. C. Mackrodt, “Oxygen Interstitial Defects in High-T_c Oxides,” Molecular Simulation, Vol. 12, No. 2, pp. 89–100, 1994.
[27]	P. Ghigna, G. Spinolo, A. Filipponi, A.V. Chadwick, and P. Hanmer, “EXAFS Evidence of Interstitial Oxygen Defects in Nd₂CuO_4+δ,” Physica C: Superconductivity, Vol. 246, pp. 345–350, 1994.
[28]	Y. Idemoto, K. Fueki, and M. Sugiyama, “Diffusion Coefficients of Oxygen in Nd₂CuO_4−δ,” Journal of Solid State Chemistry, Vol. 92, No. 2, pp. 489–495, 1991.
[29]	Y. Idemoto, K. Uchida, and K. Fueki, “Anisotropic Diffusion of Oxygen in Nd₂CuO_4−δ Single Crystal,” Physica C: Superconductivity, Vol. 222, No. 3–4, pp. 333–340, 1994.
[30]	Y. Krockenberger, H. Yamamoto, M. Mitsuhashi, and M. Naito, “Universal Superconducting Ground State in Nd_1.85Ce_0.15CuO₄ and Nd₂CuO₄,” Japanese Journal of Applied Physics, Vol. 51, No. 1, 010106, 2012.
[31]	Y. Krockenberger, J. Kurian, A. Winkler, A. Tsukada, M. Naito, and L. Alff, “Superconductivity Phase Diagrams for the Electron-doped Cuprates R_2−xCe_xCuO₄ (R=La, Pr, Nd, Sm, and Eu),” Physical Review B, Vol. 77, 060505, 2008.
[32]	K. Byrappa and T. Ohachi, “Crystal Growth Technology,” Springer, 2003.
[33]	Y. Krockenberger, H. Irie, O. Matsumoto, K. Yamagami, M. Mitsuhashi, A. Tsukada, M. Naito, and H. Yamamoto, “Emerging Superconductivity Hidden Beneath Charge-transfer Insulators,” Sci. Rep. Vol. 3, 2235, 2013.
[34]	M. Brinkmann, T. Rex, H. Bach, and K. Westerholt, “Extended Superconducting Concentration Range Observed in Pr_2−xCe_xCuO_4−δ,” Phys. Rev. Lett., Vol. 74, No. 24, pp. 4927–4930, 1995.

Yoshiharu Krockenberger: Distinguished Researcher, Thin-Film Materials Research Group, NTT Basic Research Laboratories.
He received his diploma in physics from the Technical University of Munich in 2002 and his Ph.D. in physics from the Darmstadt University of Technology in 2006. His principal research areas are superconductivity, strongly correlated electron systems and thin film growth. He has been working at NTT since 2010. In the meantime, he was a research scientist at RIKEN, Advanced Scientific Institutes, in the cross-correlated materials research group of Prof. Y. Tokura (2006–2010).

Hideki Yamamoto: Group Leader of Thin-Film Materials Research Group, Executive Manager, Research Planning Section, NTT Basic Research Laboratories.
He received his B.S., M.S., and Ph.D. in chemistry from the University of Tokyo in 1990, 1992, and 1995, respectively. His principal research areas are thin film growth, surface science, and superconductivity. He has been working at NTT since 1995. He was a visiting scholar of Geballe Laboratory for Advanced Materials, Stanford University (2004–2005). He is a member of JSAP (The Japan Society of Applied Physics), JPS (The Physical Society of Japan), SSSJ (The Surface Science Society of Japan), APS (The American Physical Society), and MRS (The Materials Research Society).

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