Handbook of High-Temperature Superconductor

Neeraj Khare

• Publisher: CRC Press
• Year: 2003
• eBook ISBN: 9781135531669
• Topic: Physical Sciences
• Subtopic: Electrical Engineering & Telecommunications

Devoted to the preparation, characterization and evaluation of HTS electronic devices, the Handbook of High-Temperature Superconductor Electronics provides information on using high-Tc thin films and junctions to increase speed, lessen noise, lower power consumption and enhance upper frequency limits in superconductor electronics. Compiled by a gro

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Introduction to High-Temperature Superconductors

Neeraj Khare

National Physical Laboratory, New Delhi, India

1.1 INTRODUCTION

The discovery of superconductivity in copper oxide perovskite (1) has opened a new era of research in superconducting materials. This class of materials not only show hightemperature superconductivity but also show properties that are different from classical superconductors. This offers a great challenge to understanding the basic phenomenon that causes superconductivity in these materials and to developing the appropriate preparation methods so that these can be exploited for a wide range of applications. During the last one and half decades after the discovery of high-T_c materials, several high-T_c superconductors have been discovered which show superconductivity at temperatures higher than liquid-nitrogen temperature (77K). There has also been great progress in understanding the properties of these materials, developing different methods of preparation, and realizing superconducting devices which use these superconductors.

This chapter will give a brief description of the historical developments in raising the transition temperature (T_c) of the superconductors, preparation, and structure of the material. Different properties of the high-T_c materials such as critical magnetic field, penetration depth, coherence length, critical current density, weak link, and so forth are also discussed.

1.2 RAISING THE TRANSITION TEMPERATURE

Superconductivity is the phenomenon in which a material loses its resistance on cooling below the transition temperature (T_c). Superconductivity was first discovered in mercury by Onnes (2) in 1911. The temperature at which mercury becomes superconducting was found to be close to the boiling point of liquid helium (4.2 K). Subsequently, many metals, alloys, and intermetallic compounds were found to exhibit superconductivity. The highest T_c known was limited to 23.2K (3) in the Nb₃Ge alloy; however, in September 1986, Bednorz and Muller (1) discovered superconductivity at 30K in La-Ba-Cu-O. The phase responsible for superconductivity was identified to have nominal composition of La_2−xBa_xCuO_4−y (x= 0.2). The discovery of high-temperature superconductivity in ceramic cuprate oxides by Bednorz and Muller led to unprecedented effort to explore new superconducting oxide material with higher transition temperatures. The value of T_c in La_2−xBa_xCuO₄ was found to increase up to 57K with the application of pressure (4). This observation in La_2−xBa_xCuO₄ material raised the hope of attaining even higher transition temperatures in cuprate oxides. This, indeed, turned out to be true when Chu and co-workers (5) reported a remarkably high superconductivity transition temperature (T_c) of 92K on replacing La by Y in nominal composition Y_1.2Ba_0.8CuO_4−y. Later, different groups identified (6–8) that the superconducting phase responsible for 90K has the composition YBa₂Cu₃O_7−y.

The discovery of superconductivity above the boiling point of liquid nitrogen led to extensive search for new superconducting materials. Superconductivity at transition temperatures of 105K in the multiphase sample of the Bi-Sr-Ca-Cu-O compound was reported by Maeda et al. (9) in 1988. The highest T_c of 110K was obtained in the Bi-Sr- Ca-Cu-O compound having a composition Bi₂Sr₂Ca₂Cu₃Oio (10, 11). Sheng and Hermann (12) substituted the nonmagnetic trivalent Tl for R in R-123, where R is a rareearth element. By reducing the reaction time to a few minutes for overcoming the highvolatility problem associated with T1₂O₃, they detected superconductivity above 90K in TlBa₂Cu₃O_x samples in November 1987. By partially substituting Ca for Ba, they (13) discovered a T_c~120K in the multiphase sample of Tl-Ba-Ca-Cu-O in February 1988. In September 1992, Putillin et al. (14) found that the H_gBa₂CuO_x (Hg-1201) compound with only one CuO2 layer showed a T_c of up to 94K. It was, therefore, rather natural to speculate that T_c can increase if more CuO₂ layers are added in the per unit formula to the compound. In April 1993, Schilling et al. (15) reported the detection of superconductivity at temperatures up to 133K in HgBa₂ Ca₂Cu₃O_x. The transition temperature of HgBa₂Ca₂Cu₂O_x was found to increase to 153K with the application of pressure (16).

Figure 1.1 depicts the evolution in the transition temperature of superconductors starting from the discovery of superconductivity in mercury. The slow but steady progress to search for new superconductors with higher transition temperatures continued for decades until superconductivity at 30K in La-Ba-Cu-O oxide was discovered in 1986. Soon after this, other cuprate oxides such as Y-Ba-Cu-O, Bi-Sr-Ca- Cu-O, Tl-Ba-Ca-Cu-O with superconductivity above the liquid-nitrogen temperature were discovered.

Table 1.1 gives a list of some of high-T_c superconductors with their respective transition temperature, crystal structure, number of Cu-O layers present in unit cell, and lattice constants. Transition temperature has been found to increase as the number of Cu- O layer increases to three in Bi-Sr-Ca-Cu-O, Tl-Ba-Ca-Cu-O, and Hg-Ba-Ca-Cu-O compounds. In all of the cuprate superconductors described so far, the superconductivity is due to hole-charge carriers, except for Nd_2−xCe_xCuO₄ (T_c~20K), which is an n-type superconductor (17). The superconductor Ba_0.6K_0.4BiO₃, which does not include Cu, was reported by Cava et al. (18) in 1988 exhibiting T_c~30K. A homologous series of compounds (Cu, Cr)Sr₂Ca_n−1Cu_xO_y [Cr12(n−1)n] has been synthesized under high pressure.

FIGURE 1.1 The evolution of the transition temperature (T_c) subsequent to the discovery of superconductivity.

TABLE 1.1 Transition Temperature (T_c), Crystal Structure and Lattice Constants of Some High-T_c Superconductors

In the Cr series, the value of n can be changed from 1 to 9, with a maximum T_c of 107K at n=3. The Pr(Ca)Ba₂Cu₃O_y compound has also been synthesized under high pressure, showing a transition temperature of 97K (19).

1.3 CRYSTAL STRUCTURE OF HIGH-T_c SUPERCONDUCTORS

The structure of a high-T_c superconductor is closely related to perovskite structure. The unit cell of perovskite consists of two metal (A, B) atoms and three oxygen atoms, with the general formula given as ABO₃. The ideal perovskite structure is shown in Fig. 1.2a. Atom A, sitting at the body-centered site, is coordinated by 12 oxygen atoms. Atom B occupies the corner site and the oxygen atom occupies the edge-centered position. Figure 1.2b shows the unit cell of La_2−xBa_xCuO₄, which has a tetragonal symmetry and consists of perovskite layers separated by rock-salt-like layers made of La (or Ba) and O atoms. This compound is often termed 214 because it has two La, one Cu, and four O atoms. The 214 compound has only one CuO₂ plane. Looking at the exact center of Fig. 1.2b, the CuO₂ plane appears as one copper atoms surrounded by four oxygen atoms, with one LaO plane above the CuO₂ plane and one below it. The entire structure is layered. The LaO planes are said to be intercalated. The CuO₂ plane is termed the conduction plane, which is responsible for superconductivity. The intercalated LaO planes are called “charge-reservoir layers.” When the intercalated plane contains mixed valence atoms, electrons are drawn away from the copper oxide planes, leaving holes to form pairs needed for superconductivity.

The structure of YBa₂Cu₃O₇ is shown in Fig. 1.2c. The unit cell of YBa₂Cu₃O₇ consists of three pseudocubic elementary perovskite unit cells (8). Each perovskite unit cell contains a Y or Ba atom at the center: Ba in the bottom unit cell, Y in the middle one, and Ba in the top unit cell. Thus, Y and Ba are stacked in the sequence [Ba-Y-Ba] along the c-axis. All corner sites of the unit eell are occupied by Cu, which has two different coordinations, Cu(1) and Cu(2), with respect to oxygen. There are four possible crystallographic sites for oxygen: O(1), O(2), O(3), and O(4). The coordination polyhedra of Y and Ba with respect to oxygen are different. The tripling of the perovskite unit cell (ABO₃) leads to nine oxygen atoms, whereas YBa₂Cu₃O₇ has seven oxygen atoms accommodating the deficiency of two oxygen atom...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Preface
• Contributors
• 1 Introduction to High-Temperature Superconductors
• 2 Epitaxial Growth of Superconducting Cuprate Thin Films
• 3 High-Temperature Superconducting Multilayer Ramp-Edge Junctions
• 4 Step-Edge Josephson Junctions
• 5 Conductance Noise in High-Temperature Superconductors
• 6 Noise in High-Temperature Superconductor Josephson Junctions
• 7 High-Temperature RF SQUIDs
• 8 High-Temperature SQUID Magnetometer
• 9 High-Temperature Superconducting Digital Circuits
• 10 High-Temperature Superconductor Microwave Devices
• 11 High-Temperature Superconducting IR Detectors
• 12 Cryocoolers and High-Tc Devices
• 13 High-Temperature Superconductor Electronics: Status and Perspectives