High-Temperature Superconductivity

I. Introduction

Due to the high-temperature superconductivity, scientists were able to solve many problems associated with the use of superconducting materials in energy, electronics, and high-current engineering, computer science, physics of elementary particles (superconducting accelerators), the mining industry (magnetic separators) and medicine (superconducting tomography). One of the most successful applications is their use in creation of SQUIDs and their use in various industries, such as the medicine, the oil industry, and the scientific laboratories. In addition, the technology of high-temperature superconducting SQUID rapidly develops, and its range of use is significantly expanded by simplifying the operational issues. Thus, superconductivity is a relevant issue and requires further and more detailed studies.

II. YBCO as The High-Temperature Superconductor

A. Superconductors of the 2^nd Type

The superconductors of the 2^nd type are usually characterized by the existence of two critical fields [1]. When the field is less than the lower critical one, the magnetic flux does not penetrate into the superconductor. If someone increases the field to the upper critical one, then going through it, the sample returns to the normal state, and the field completely penetrates into it, i. e. the sample becomes an ordinary conductor. At the average values of the field, there is its partial penetration into the sample.

The critical field defines the difference between specific free energies of superconducting and normal phases [1]. Therefore, the existence of the critical field of superconductor leads to the inability of current flow, which is greater than a certain critical value. If this threshold is overcome, the superconductivity breaks down. Niobium (Nb) is a pure metal, which belongs to the superconductors of the 2nd type. The impurities can transform the materials, which are the superconductors of the 1st type in their pure state, into the superconductors of the 2nd type. The superconducting high-temperature oxides also belong to the superconductors of the 2nd type [1].

B. The Perovskite Structure of the Superconducting High-Temperature Oxides

A common feature of the superconducting oxides is their belonging to the perovskite compounds with oxygen defect structure. So, the crystal structure of YBa₂Cu₃O₇ is classified as perovskite. In the unit cell of the ideal perovskite (ABO₃), ions B occupy the vertices, ions of oxygen are positioned at the midpoints of the ribs, and the ion A is placed inside the cell [2]. Although, there is an ion inside the cell, the grade is not body-centered, but primitive, there is no translation within the volume and the ion A entirely belongs to the cell. The main structural elements of the perovskite structure are the octahedron BX₆ and cuboctahedron AX₂ [2]. The octahedron BX₆ is connected by the common vertices, form an infinite three-dimensional frame (isotropic structure), in the cavities of which the atoms A are located. To predict the possible formation of the compounds with the perovskite structure, people usually use Goldschmidt geometric stability criterion, which determines the allowable size of the cations A and B (or the distance A-X and B-X). It is noteworthy that the formation of the perovskite structure with a perfect stoichiometry (1: 1: 3) is possible in the case of filling the A, B, and X-position with a large number of atoms. In this case, the atoms that create the compounds (phases), should have a similar crystal chemistry for placing them on the same type locations.

In the case of ideal oxide perovskites’ composition, besides the geometric conditions, the condition of electrical neutrality should be performed [3]. I. e. the summary positive charge of the cations should be equal to +6 to compensate the negative one. However, for the most of the cations, this condition is not realized, but, nevertheless, the anion-deficient structures with the compound of ABO_3.5 are being formed. In such instance, the formation of the perovskite structure occurs due to the possibility of demonstration, primarily in the B-type cations, of the lower coordination numbers [3]. For the oxides with the perovskite structure on the basis of Cu, the formation of the anion-deficient perovskite structures is caused by the high stability of the coordination environment of copper atoms with coordination number of less than 6. The Jahn-Teller effect in the anion-deficient perovskite structures leads to a large distortion of the coordination polyhedrons of Cu. The frame of the perovskite structure becomes layered: the ordering of oxygen vacancies leads to the formation of layers of CuO₂.

C. History of the Superconductivity Research

After H. Kamerlingh-Onnes discovered the superconductivity in 1911, there was the tendency to search for the new superconductors among the common metals (Hg, Pb, Nb), and then among the double (Nb₃Sn, Nb₃Ga) and triple (Nb₃(Al,Ge)) intermetallic compounds [4]. The search among these compounds was hindered due to their belonging to the insulators. The first oxide superconducting compound with the perovskite structure SrTiO₃ with the critical temperature of T_c = 0.3-0.5 K was found in 1964, in the USA. In 1974-75, the superconductivity was observed in LiTi₂O₄ (T_c = 11 K) and BaPb_1-xBi_xO₃, where the critical temperature varied depending on the composition, and reached the maximum value of T_c = 13 K for x = 0.25.

The development of high-temperature superconductivity started in 1986 with the discovering of superconductivity in the La_2-xBaCuO₄ compound at the temperature of 30 – 35 K [4]. The possibility of the replacement of Ba by Sr allowed to reach the critical temperature value of T_c = 36 K. The ceramic Y-Ba-Cu-O with T_c = 92 K was obtained in 1987 and in 1988 the researchers synthesized the bismuth and thallium compounds with T_c = 110-120 K. Recently, the researchers have learned to make the superconducting films on the basis of YBCO. They obtained these thin films, called the second generation, by spraying a superconducting material on a substrate, which has a special texture, setting the direction of the crystal growth [5].

D. Yttrium-Barium-Copper-Oxide. Its Characteristics and Synthesis

YBa₂Cu₃O₇ is the most studied and frequently used compound, in particular, as the material for the creation of superconducting films. It happens due to its cost, as well, since yttrium is one of the most widely available rare earth elements. The superconductors based on yttrium-barium-copper-oxide compound maintain the superconductivity at the temperature up to 92 K, therefore, one can use the liquid nitrogen for their cooling.

The compound YBa₂Cu₃O_7-δ with the reduced oxygen content (δ = 0.6 – 1.0) is an antiferromagnetic insulator [6]. With the decrease of the oxygen defect, the Néel temperature quickly decreases from 400 K (δ = 0.85) to zero (δ = 0.6), the compounds with δ < 0.6 become superconductors (critical temperature of the transition to the superconducting state T_c = 92 K at δ = 0 – 0.1). The region of the existence of high-temperature superconductivity in the phase diagram (temperature vs. composition) is directly adjacent to the line which corresponds to the transition from the insulator to the metal [6]. Close to the same line, there is a transition from antiferromagnet material to the nonmagnetic metal. In the compound of YBa₂Cu₃O_7-δ oxygen octahedrons, creating oxygen ordered vacancies transforms into pyramids and squares [6]. As a result, there are copper-oxygen planes and chains.

There are several methods of producing the YBCO, however, the most popular one is Carbon Combustion Synthesis of Oxides (CCSO) [7]. This method involves heating a mixture of the initial components to form the stable rhombic phase of YBCO at a room temperature. The samples for the synthesis are prepared in the form of stoichiometric mixture of oxides Y₂O₃, CuO and BaO. Barium oxide is usually prepared by the preliminary firing of BaCO₃. Earlier YBCO synthesis was performed using BaCO₃, but the combined reaction of barium carbonate decomposition with the synthesis of YBCO is extremely difficult to uniquely interpret [7].

III.  Conclusions

Thus, the history of the superconductivity research started in 1911 and led to the discovery of YBa₂Cu₃O_7-δ in 1987, which transforms to the superconducting state at a temperature of 92 K. The compound of YBa₂Cu₃O_7-δ is classified as perovskite with the oxygen defect structure and belongs to the superconductors of the 2^nd type. It is the most studied and frequently used compound, in particular, as the material for the creation the superconducting films, due to its lower cost.

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