1 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 high-temperature superconductivity but also show properties that are differ- ent from classical superconductors. This offers a great challenge to understanding the basic phenomenon that causes superconductivity in these materials and to de- veloping 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 discov- ery of high-T c materials, several high-T c superconductors have been discovered which show superconductivity at temperatures higher than liquid-nitrogen tem- perature (77 K). There has also been great progress in understanding the proper- ties 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 crit- ical magnetic field, penetration depth, coherence length, critical current density, weak link, and so forth are also discussed. Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. 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 discov- ered 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 23.2 K (3) in the Nb 3 Ge alloy; however, in September 1986, Bednorz and Muller (1) discovered superconductivity at 30 K in La–Ba–Cu–O. The phase responsible for supercon- ductivity was identified to have nominal composition of La 2Ϫx Ba x CuO 4Ϫy (x ϭ 0.2). The discovery of high-temperature superconductivity in ceramic cuprate ox- ides by Bednorz and Muller led to unprecedented effort to explore new supercon- ducting oxide material with higher transition temperatures. The value of T c in La 2Ϫx Ba x CuO 4 was found to increase up to 57 K with the application of pressure (4). This observation in La 2Ϫx Ba x CuO 4 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 tran- sition temperature (T c ) of 92 K on replacing La by Y in nominal composition Y 1.2 Ba 0.8 CuO 4Ϫy . Later, different groups identified (6–8) that the superconduct- ing phase responsible for 90 K has the composition YBa 2 Cu 3 O 7Ϫy . The discovery of superconductivity above the boiling point of liquid nitro- gen led to extensive search for new superconducting materials. Superconductivity at transition temperatures of 105 K in the multiphase sample of the Bi–Sr–Ca–Cu–O compound was reported by Maeda et al. (9) in 1988. The high- est T c of 110 K was obtained in the Bi–Sr–Ca–Cu–O compound having a compo- sition Bi 2 Sr 2 Ca 2 Cu 3 O 10 (10,11). Sheng and Hermann (12) substituted the non- magnetic trivalent Tl for R in R-123, where R is a rare-earth element. By reducing the reaction time to a few minutes for overcoming the high-volatility problem as- sociated with Tl 2 O 3 , they detected superconductivity above 90 K in TlBa 2 Cu 3 O x samples in November 1987. By partially substituting Ca for Ba, they (13) discov- ered a T c ϳ 120 K in the multiphase sample of Tl–Ba–Ca–Cu–O in February 1988. In September 1992, Putillin et al. (14) found that the HgBa 2 CuO x (Hg-1201) compound with only one CuO 2 layer showed a T c of up to 94 K. It was, therefore, rather natural to speculate that T c can increase if more CuO 2 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 133 K in HgBa 2 Ca 2 Cu 3 O x . The transition temperature of HgBa 2 Ca 2 Cu 3 O x was found to increase to 153 K with the application of pressure (16). Figure 1.1 depicts the evolution in the transition temperature of supercon- ductors starting from the discovery of superconductivity in mercury. The slow but steady progress to search for new superconductors with higher transition temper- 2 Khare Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. atures continued for decades until superconductivity at 30 K in La–Ba–Cu–O ox- ide 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 respec- tive 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 super- conductors described so far, the superconductivity is due to hole-charge carriers, except for Nd 2Ϫx Ce x CuO 4 (T c ϳ 20 K), which is an n-type superconductor (17). The superconductor Ba 0.6 K 0.4 BiO 3 , which does not include Cu, was reported by Cava et al. (18) in 1988 exhibiting T c ϳ 30 K. A homologous series of compounds (Cu,Cr)Sr 2 Ca nϪ1 Cu n O y [Cr12(nϪ1)n] has been synthesized under high pressure. Introduction to High-Temperature Superconductors 3 FIGURE 1.1 The evolution of the transition temperature (T c ) subsequent to the discovery of superconductivity. Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. In the Cr series, the value of n can be changed from 1 to 9, with a maximum T c of 107 K at n ϭ 3. The Pr(Ca)Ba 2 Cu 3 O y compound has also been synthesized under high pressure, showing a transition temperature of 97 K (19). 1.3 CRYSTAL STRUCTURE OF HIGH-T c SUPERCONDUCTORS The structure of a high-T c superconductor is closely related to perovskite struc- ture. The unit cell of perovskite consists of two metal (A, B) atoms and three oxy- gen atoms, with the general formula given as ABO 3 . The ideal perovskite struc- ture 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 occu- pies the edge-centered position. 4 Khare TABLE 1.1 Transition Temperature (T c ), Crystal Structure and Lattice Constants of Some High-T c Superconductors High-T c superconductors T c Crystal Formula Notation (K) n a structure Lattice constants (Å) La 1.6 Ba 0.4 CuO 4 214 30 1 Tetragonal a ϭ 3.79, c ϭ 13.21 La 2Ϫx Sr x CuO 4 214 38 1 Tetragonal a ϭ 3.78, c ϭ 13.23 YBa 2 Cu 3 O 7 123 92 2 Orthorhombic a ϭ 3.82, b ϭ 3.89, c ϭ 11.68 YBa 2 Cu 4 O 8 124 80 2 Orthorhombic a ϭ 3.84, b ϭ 3.87, c ϭ 27.23 Y 2 Ba 4 Cu 7 O 14 247 40 2 Orthorhombic a ϭ 3.85, b ϭ 3.87, c ϭ 50.2 Bi 2 Sr 2 CuO 6 Bi-2201 20 1 Tetragonal a ϭ 5.39, c ϭ 24.6 Bi 2 Sr 2 CaCu 2 O 8 Bi-2212 85 2 Tetragonal a ϭ 5.39, c ϭ 30.6 Bi 2 Sr 2 Ca 2 Cu 3 O 10 Bi-2223 110 3 Tetragonal a ϭ 5.39, c ϭ 37.1 TlBa 2 CuO 5 Tl-1201 25 1 Tetragonal a ϭ 3.74, c ϭ 9.00 TlBa 2 CaCu 2 O 7 Tl-1212 90 2 Tetragonal a ϭ 3.85, c ϭ 12.74 TlBa 2 Ca 2 Cu 3 O 9 Tl-1223 110 3 Tetragonal a ϭ 3.85, c ϭ 15.87 TlBa 2 Ca 3 Cu 4 O 11 Tl-1234 122 4 Tetragonal a ϭ 3.86, c ϭ 19.01 Tl 2 Ba 2 CuO 6 Tl-2201 80 1 Tetragonal a ϭ 3.86, c ϭ 23.22 Tl 2 Ba 2 CaCu 2 O 8 Tl-2212 108 2 Tetragonal a ϭ 3.86, c ϭ 29.39 Tl 2 Ba 2 Ca 2 Cu 3 O 10 Tl-2223 125 3 Tetragonal a ϭ 3.85, c ϭ 35.9 HgBa 2 CuO 4 Hg-1201 94 1 Tetragonal a ϭ 3.87, c ϭ 9.51 HgBa 2 CaCu 2 O 6 Hg-1212 128 2 Tetragonal a ϭ 3.85, c ϭ 12.66 HgBa 2 Ca 2 Cu 3 O 8 Hg-1223 134 3 Tetragonal a ϭ 3.85, c ϭ 15.78 (Nd 2Ϫx Ce x ) CuO 4 T 30 1 Tetragonal a ϭ 3.94, c ϭ 12.07 (Nd, CeSr) CuO 4 T* 30 1 Tetragonal a ϭ 3.85, c ϭ 12.48 a n represents the number of Cu-O planes in the unit cell. Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. Figure 1.2b shows the unit cell of La 2Ϫx Ba x CuO 4 , 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 CuO 2 plane. Look- ing at the exact center of Fig. 1.2b, the CuO 2 plane appears as one copper atoms sur- rounded by four oxygen atoms, with one LaO plane above the CuO 2 plane and one below it. The entire structure is layered. The LaO planes are said to be intercalated. The CuO 2 plane is termed the conduction plane, which is responsible for supercon- ductivity. 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 YBa 2 Cu 3 O 7 is shown in Fig. 1.2c. The unit cell of YBa 2 Cu 3 O 7 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 cell 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 (ABO 3 ) leads to nine oxygen atoms, whereas YBa 2 Cu 3 O 7 has seven oxygen atoms accommodat- Introduction to High-Temperature Superconductors 5 F IGURE 1.2 Structure of (a) perovskite ABO 3 , (b) (La,Ba) 2 CuO 4 , and (c) YBa 2 Cu 3 O 7 . Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. ing the deficiency of two oxygen atoms. Thus, the structure of the 90 K phase de- viates from the ideal perovskite structure and, therefore, is referred to as an oxy- gen-deficient perovskite structure. Oxygen atoms are missing from the Y plane (i.e., z ϭ 1/2 site); thus, Y is surrounded by 8 oxygen atoms instead of the 12 if it had been in ideal perovskite structure. Oxygen atoms at the top and bottom planes of the YBa 2 Cu 3 O 7 unit cell are missing in the [100] direction, thus giving (Cu–O) chains in the [010] direction. The Ba atom has a coordination number of 10 oxy- gen atoms instead of 12 because of the absence of oxygen at the (1/2 0 z) site. The structure has a stacking of different layers: (CuO)(BaO)(CuO 2 )(Y)(CuO 2 )(BaO) (CuO). One of the key feature of the unit cell of YBa 2 Cu 3 O 7Ϫ␦ (YBCO) is the presence of two layers of CuO 2 . The role of the Y plane is to serve as a spacer be- tween two CuO 2 planes. In YBCO, the Cu–O chains are known to play an impor- tant role for superconductivity. T c maximizes near 92 K when ␦ Ϸ 0.15 and the structure is orthorhombic. Superconductivity disappears at ␦ Ϸ 0.6, where the structural transformation of YBCO occurs from orthorhombic to tetragonal. The crystal structure of Bi-, Tl-, and Hg-based high-T c superconductors are very similar to each other. Like YBCO, the perovskite-type feature and the pres- ence of CuO 2 layers also exist in these superconductors. However, unlike YBCO, Cu–O chains are not present in these superconductors. The YBCO superconduc- tor has an orthorhombic structure, whereas the other high-T c superconductors have a tetragonal structure (see Table 1.1). The Bi–Sr–Ca–Cu–O system has three superconducting phases forming a homologous series as Bi 2 Sr 2 Ca nϪ1 Cu n O 4ϩ2nϩy (n ϭ 1, 2, and 3). These three phases are Bi-2201, Bi-2212, and Bi-2223, having transition temperatures of 20, 85, and 110 K, respectively (10,11). The structure of Bi-2201 together with Bi- 2212 and Bi-2223 is shown in Fig. 1.3. All three phases have a tetragonal struc- ture which consists of two sheared crystallographic unit cells. The unit cell of these phases has double Bi–O planes which are stacked with a shift of (1/2 1/2 z) with respect to the origin. The stacking is such that the Bi atom of one plane sits below the oxygen atom of the next consecutive plane. The Ca atom forms a layer within the interior of the CuO 2 layers in both Bi-2212 and Bi-2223; there is no Ca layer in the Bi-2201 phase. The three phases differ with each other in the number of CuO 2 planes; Bi-2201, Bi-2212, and Bi-2223 phases have one, two, and three CuO 2 planes, respectively. The c axis of these phases increases with the number of CuO 2 planes. The lengths of the c axis are 24.6 Å, 30.6 Å, and 37.1 Å respec- tively for the Bi-2201, Bi-2212, and Bi-2223 phases. The coordination of the Cu atom is different in the three phases. The Cu atom forms an octahedral coordina- tion with respect to oxygen atoms in the 2201 phase, whereas in 2212, the Cu atom is surrounded by five oxygen atoms in a pyramidal arrangement. In the 2223 struc- ture, Cu has two coordinations with respect to oxygen: one Cu atom is bonded with four oxygen atoms in square planar configuration and another Cu atom is co- ordinated with five oxygen atoms in a pyramidal arrangement. 6 Khare Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. Figure 1.4 shows the unit cells of two series of the Tl–Ba–Ca–Cu–O super- conductor (20). The first series of the Tl-based superconductor containing one Tl–O layer has the general formula TlBa 2 Ca nϪ1 Cu n O 2nϩ3 , whereas the second se- ries containing two Tl–O layers has a formula of Tl 2 Ba 2 Ca nϪ1 Cu n O 2nϩ4 with n ϭ 1, 2, and 3. In the structure of Tl 2 Ba 2 CuO 6 , there is one CuO 2 layer with the stack- ing sequence (Tl–O) (Tl–O) (Ba–O) (Cu–O) (Ba–O) (Tl–O) (Tl–O). In Tl 2 Ba 2 CaCu 2 O 8 , there are two Cu–O layers with a Ca layer in between. Similar to the Tl 2 Ba 2 CuO 6 structure, Tl–O layers are present outside the Ba–O layers. In Tl 2 Ba 2 Ca 2 Cu 3 O 10 , there are three CuO 2 layers enclosing Ca layers between each of these. In Tl-based superconductors, T c is found to increase with the increase in CuO 2 layers. However, the value of T c decreases after four CuO 2 layers in TlBa 2 Ca nϪ1 Cu n O 2nϩ3 , and in the Tl 2 Ba 2 Ca nϪ1 Cu n O 2nϩ4 compound, it decreases after three CuO 2 layers. The crystal structure of HgBa 2 CuO 4 (Hg-1201), HgBa 2 CaCu 2 O 6 (Hg- 1212), and HgBa 2 Ca 2 Cu 3 O 8 (Hg-1223) is similar to that of Tl-1201, Tl-1212, and Tl-1223 (Fig. 1.4) with Hg in place of Tl (21). It is noteworthy that the T c of the Hg compound (Hg-1201) containing one CuO 2 layer is much larger as compared to the one-CuO 2 -layer compound of thallium (Tl-1201). In the Hg-based super- conductor, T c is also found to increase as the CuO 2 layer increases. For Hg-1201, Hg-1212, and Hg-1223, the values of T c are 94, 128, and 134 K respectively, as shown in Table 1.1. The observation that the T c of Hg-1223 increases to 153 K un- der high pressure (16) indicates that the T c of this compound is very sensitive to the structure of the compound. Introduction to High-Temperature Superconductors 7 FIGURE 1.3 Unit cells of the Bi 2 Sr 2 Ca nϪ1 Cu n O x compound with n ϭ 1, 2, and 3. (Adapted from Ref. 11.) Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. 1.4 PREPARATION OF HIGH-T c SUPERCONDUCTORS High-T c superconductors are prepared in the form of bulk, thick films, thin films, single crystals, wires, and tapes. Fabrication in the form of wires and tapes are re- quired for high-current applications. On the other hand, thick and thin films are needed for electronic application. Strict control of the stoichiometry of the com- position is very much required for preparing high-T c superconductors with desir- able characteristics. Even a small change in oxygen content or a small change in cation doping level can transform the material from a superconductor to a low-car- rier-density metal or even to an insulator. The following paragraphs give a brief 8 Khare FIGURE 1.4 Unit cells of the Tl 1 Ba 2 Ca nϪ1 Cu n O 2nϩ3 compound containing one Tl–O layer and the Tl 2 Ba 2 Ca nϪ1 Cu n O 2nϩ4 compound containing two Tl–O lay- ers for n ϭ 1, 2, and 3. (Adapted from Ref. 20.) Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. description of high-T c superconductors in the form of bulk and thick films. The preparation of high-T c thin films is given in more detail in the other chapters of this book. The simplest method for preparing high-T c superconductors is a solid-state thermochemical reaction involving mixing, calcination, and sintering. The appro- priate amounts of precursor powders, usually oxides and carbonates, are mixed thoroughly using a ball mill. Solution chemistry processes such as coprecipitation, freeze-drying, and sol–gel methods are alternative ways for preparing a homoge- nous mixture. These powders are calcined in the temperature range from 800°C to 950°C for several hours. The powders are cooled, reground, and calcined again. This process is repeated several times to get homogenous material. The powders are subsequently compacted to pellets and sintered. The sintering environment such as temperature, annealing time, atmosphere, and cooling rate play a very important role in getting good high-T c superconducting materials. The (La 1Ϫx Ba x ) 2 CuO 4Ϫ␦ high-T c superconductor is prepared by heating a mixture of La 2 O 3 , BaCO 3 , and CuO in a reduced oxygen atmosphere at 900°C. After re- grinding and reheating the mixtures, the pellet is prepared and sintered at 925°C for 24 h. The YBa 2 Cu 3 O 7Ϫ␦ compound is prepared by calcination and sintering of a homogenous mixture of Y 2 O 3 , BaCO 3 , and CuO in the appropriate atomic ratio. Calcination is done at 900–950°C, whereas sintering is done at 950°C in an oxy- gen atmosphere. The oxygen stoichiometry in this material is very crucial for ob- taining a superconducting YBa 2 Cu 3 O 7Ϫ␦ compound. At the time of sintering, the semiconducting tetragonal YBa 2 Cu 3 O 6 compound is formed, which, on slow cooling in oxygen atmosphere, turns into superconducting YBa 2 Cu 3 O 7Ϫ␦ . The up- take and loss of oxygen are reversible in YBa 2 Cu 3 O 7Ϫ␦ . A fully oxidized or- thorhombic YBa 2 Cu 3 O 7Ϫ␦ sample can be transformed into tetragonal YBa 2 Cu 3 O 6 by heating in a vacuum at temperature above 700°C. The preparation of Bi-, Tl-, and Hg-based high-T c superconductors is difficult compared to YBCO. Problems in these superconductors arise because of the existence of three or more phases having a similar layered structure. Thus, syntactic intergrowth and defects such as stacking faults occur during synthesis and it becomes difficult to isolate a single superconducting phase. For Bi–Sr–Ca–Cu–O, it is relatively simple to prepare the Bi-2212 (T c ϳ 85 K) phase, whereas it is very difficult to prepare a single phase of Bi-2223 (T c ϳ 110 K). The Bi-2212 phase appears only after few hours of sintering at 860–870°C, but the larger fraction of the Bi-2223 phase is formed after a long reaction time of more than a week at 870°C (11). Although the substitution of Pb in the Bi–Sr–Ca–Cu–O compound has been found to promote the growth of the high-T c phase (22), a long sintering time is still required. Toxicity and low vapor pressure of Hg–O and Tl–O make fabrication of Hg- and Tl-based high-T c superconductors much more difficult and one has to follow special precautions and stringent control on the preparation atmosphere. The Tl- based superconductor is prepared by thorough mixing of Tl 2 O 3 , BaO, CaO, and Introduction to High-Temperature Superconductors 9 Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. CuO in appropriate proportions and pressing the powders into a pellet. The pellet is wrapped in a gold foil and fired at 880°C for 3h in a sealed quartz tube con- taining 1 atm oxygen to reach superconductivity (20). For the preparation of a Hg-based high-T c superconductor (15), first a pre- cursor material with the nominal composition Ba 2 CaCu 2 O 5 is obtained from a ho- mogenous mixture of the respective metal nitrates by sintering at 900°C in oxy- gen. Dry boxes are used for grinding and mixing of the powders. After regrinding and mixing with HgO powder, the pressed pellet is sealed in an evacuated quartz tube. This tube is placed horizontally in a tight steel container and sintered at 800°C for a few hours. Several techniques such as screen printing (23–27), spin-coating (28) and spray pyrolysis (29–33) are used in preparing high-T c thick films. For the screen printing or spin-coating method, the first step is to prepare homogenous powders of high-T c materials; this is accomplished by solid-state reaction or by a chemical route involving mixing, calcination, and sintering of appropriate powders in the form of oxides or carbonates. After sintering the powders are sieved through a screen woven from stainless steel or nylon wire. The diameter of the screen wire and the size of the opening can vary depending on the process requirement. The opening size is usually given in terms of a standard mesh number that varies from 100 to 400. The fine sieved powders are converted into thick paste by mixing with an organic solvent such as propylene glycol, octyl alcohol, heptyl alcohol, tri- ethanolamine, or cyclohexagonal. In the screen-printing technique, thick paste is used for printing the substrate through the mesh screen and dried at an appropri- ate temperature. In the spin-coating method, one drop of the paste is put on the substrate and the substrate is spun to get a uniform coating of the material. The re- sultant films are fired at a suitable annealing temperature. In general, single-crys- tal and polycrystalline substrates of magnesium oxide (MgO), strontium titnate (SrTiO 3 ), lanthanum aluminate (LaAlO 3 ), yattria-stabilized zirconia (YSZ), and aluminum oxide (Al 2 O 3 ) are used for the high-T c thick-film preparation. For YBCO thick films, the sintering temperature is kept between 940°C and 970°C followed by slow cooling in an oxygen atmosphere (23). In order to achieve YBCO films with a larger grain size and higher current density, the firing temper- ature is increased to 1000°C (24). Bi-2212 high-T c films are prepared by firing the films at 880–885°C. It has been found that partial melting and quenching of the Bi- 2212 films from 885°C to room temperature leads to a T c as high as 96 K (25). For high-T c films with a Bi-2223 phase, the films are fired at ϳ880°C for a few min- utes and then annealed at 864°C for a duration of 70–80 h (26). The preparation of Tl–Ba–Ca–Cu–O thick films requires a two-step process (27). In the first step, a film of Ba–Ca–Cu–O is prepared, and in the second step, this precursor film is heated in Tl 2 O 3 vapor followed by slow cooling to room temperature. Spray pyrolysis is another simple and inexpensive technique for preparing high-T c films (30–33). For YBCO film, an aqueous solution for the spray is pre- 10 Khare Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. [...]... High- Tc superconductors Tc (K) ⌬ (mV) 2 /kBTc Ref YBa2Cu3O7Ϫ␦ Bi2Sr2CaCu2O8ϩ␦ Bi1,7Pb0,3Sr2CaCu2Ox Tl2Ba2CuO6 Tl2Ba2Ca2Cu3Ox HgBa2Ca2Cu3O8ϩ␦ (Nd2Ϫx Cex)CuO4Ϫy Ba1Ϫx Kx BiO3 85 62 96 86 114 1 32 22 24.5 20 20 26 25 30 48 3.7 4.6 6 7.5 6.3 6.7 6.1 8.5 3.9 3.9 51 53 54 55 56 57 58 58 For high- Tc superconductors, a higher value of the energy gap-to-Tc ratio is observed as compared to the weakly coupled BCS superconductor. .. Copyright © 20 03 by Marcel Dekker, Inc All Rights Reserved Introduction to High- Temperature Superconductors 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 23 RJ Cava, B Batlogg, JJ Krajewski, R Farrow, LW Rupp, AE White Jr, K Short, T Kometani Superconductivity near 30 K without copper Ba0.6K0.4BiO3 Perovskite Nature 3 32: 814–816, 1988 YS Yao, YF Xiong, D Jin, JW Li, JL Luo, ZX Zhao The high- pressure... and superconductivity of Pr0.5Ca0.5Ba2Cu3Oz Physica C 28 2 28 7:49– 52, 1997 SSP Parkin, VY Lee, AI Nazzal, R Savoy, R Beyers, SJ LaPlaca Tl1CanϪ1Ba2CunO2nϩ3 (nϭ1 ,2, 3): A new class of crystal structures exhibiting volume superconductivity at up to Ӎ 110 K Phys Rev Lett 61:750–753, 1988 SN Putillin, EV Antipov, M Marezio Superconductivity above 120 K in HgBa2CaCu2O6ϩ␦ Physica C 21 2 :26 6 27 0, 1993 D Shi, MS... Quasiparticle and Josephson tunneling of overdoped Bi2Sr2CaCu2O8ϩ␦ single crystals Phys Rev B 61:3 629 –3640, 20 00 Q Huang, JF Zasadzinski, KE Gray, JZ Liu, H Claus Electron tunneling study of the normal and superconducting states of Bi1.7Pb0.3 Sr2CaCu2Ox Phys Rev B 40:9366– 9369, 1989 L Ozyuzer, Z Yusof, JF Zasadzinski, L Wei-Ting, DG Hinks, KE Gray Tunneling spectroscopy of Tl2Ba2CuO6 Physica C 320 :9 20 ,... 70:493–496, 1993 N Khare Symmetry of Order Parameter of High- Tc Superconductors In: A Narlikar, ed Studies of High Temperature Superconductor vol 20 New York: Nova Science Publishers, 1996, pp 187 21 5 Copyright © 20 03 by Marcel Dekker, Inc All Rights Reserved 26 Khare 70 RC Dynes The order parameter of high Tc superconductors; experimental probes Solid State Commun 92: 53– 62, 1994 W Braunisch, N Knauf,... Observation of Josephson pair tunneling between a high- Tc cuprate (YBa2Cu3O7Ϫ␦) and a conventional superconductor (Pb) Phys Rev Lett 72: 226 7 22 70, 1994 P Chaudhari, SY Lin Symmetry of the superconducting order parameter in YBa2Cu3O7Ϫ␦ epitaxial films Phys Rev Lett 72: 1084–1087, 1994 DA Wollman, DJ Van Harlingen, J Giapintzakis, DM Ginsberg Evidence for dx2Ϫy2 pairing from the magnetic field modulation of YBa2Cu3O7–Pb... 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 Copyright © 20 03 by Marcel Dekker, Inc All Rights Reserved Introduction to High- Temperature Superconductors 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 25 M Boekholt, M Hoffmann, G Guntherodt Detection of an anisotropy of the superconducting gap in Bi2Sr2CaCu2O8ϩ␦ single crystals by Raman and tunneling spectroscopy Physica C 175: 127 –134, 1991 L... the order parameter in the high- Tc superconductor YBa2Cu3O7Ϫ␦ Nature 373 :22 5 22 8, 1995 CC Tsuei, JR Kirtley, M Rupp, JZ Sun, A Gupta, MB Ketchen, CA Wang, ZF Ren, JH Wang, M Bhushan Pairing symmetry in single-layer tetragonal Tl2Ba2CuO6ϩ␦ superconductors Science 27 1: 329 –3 32, 1996 JR Kirtley, P Chaudhari, MB Ketchen, N Khare, SY Lin, T Shaw Distribution of magnetic flux in high- Tc grain boundary junctions... of this ratio (ϳ3.9) (58) Table 1 .2 shows values of the energy gap-to-Tc ratio [2 (0)/kBTc] for some of high- Tc superconductors Angle-resolved photoelectron spectroscopy has been used to investigate the energy gap in different k directions in Bi -22 12, and an anisotropy of the gap value in the a-b plane was noted (59), which indicates the possibility of the existence of nodes in the energy gap Low -temperature. .. anisotropy in Tl2Ba2CaCu2Ox Phys Rev B 42: 6758–6761, 1990 DH Wu, S Sridhar Pinning forces and lower critical fields in YBa2Cu3Oy crystals: Temperature dependence and anisotropy Phys Rev Lett 65 :20 74 20 77, 1990 U Welp, M Grimsditch, H You, WK Kwok, MM Fang, GW Crabtree, JZ Liu The upper critical field of untwinned YBa2Cu3O7Ϫ␦ crystals Physica C 161:1–5, 1989 G Blatter Vortex matter Physica C 28 2 28 7:19 26 , 1997 . of the CuO 2 layers in both Bi -22 12 and Bi -22 23; there is no Ca layer in the Bi -22 01 phase. The three phases differ with each other in the number of CuO 2 planes; Bi -22 01, Bi -22 12, and Bi -22 23. ϭ 3.87, c ϭ 27 .23 Y 2 Ba 4 Cu 7 O 14 24 7 40 2 Orthorhombic a ϭ 3.85, b ϭ 3.87, c ϭ 50 .2 Bi 2 Sr 2 CuO 6 Bi -22 01 20 1 Tetragonal a ϭ 5.39, c ϭ 24 .6 Bi 2 Sr 2 CaCu 2 O 8 Bi -22 12 85 2 Tetragonal. structure of HgBa 2 CuO 4 (Hg- 120 1), HgBa 2 CaCu 2 O 6 (Hg- 121 2), and HgBa 2 Ca 2 Cu 3 O 8 (Hg- 122 3) is similar to that of Tl- 120 1, Tl- 121 2, and Tl- 122 3 (Fig. 1.4) with Hg in place of Tl (21 ). It is