High Temperature Superconductors: Materials ...

ASM I n a u g u r a l L e c t u r e 2009

High Temperature Superconductors: Materials, Mechanisms and Applications R. Abd-Shukor, F.A.Sc.

Academy of Sciences Malaysia

00Prelims_(i-iv)3.indd 1

10/23/09 8:08:11 PM

© Akademi Sains Malaysia 2009. All rights reserved. No part of this publication may be produced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior permission of the Copyright owner. The views and opinions expressed or implied in this publication are those of the authors and do not necessarily reflect the views of the Academy of Sciences Malaysia.

Perpustakaan Negara Malaysia

Cataloguing-in-Publication Data

R. Abd-Syukor, 1961High temperature superconductors: materials, mechanisms and applications/ R. Abd-Shukor. Bibliography: p. ISBN 978-983-9445-37-4 1. High temperature superconductors. 2. Superconductors, I. Title. 537.623

Printed by Percetakan J. R. No. 4, Jalan 15/48A, Sentul Raya Boulevard, 51000 Sentul, Kuala Lumpur. 2009


10/23/09 8:08:11 PM

High Temperature Superconductors: Materials, Mechanisms and Applications


10/23/09 8:08:11 PM

R. Abd-Shukor, F.A.SC. Roslan Abd-Shukor is currently a Professor at the School of Applied Physics, Universiti Kebangsaan Malaysia. He obtained his BSc in 1982 and MSc in 1985 in physics from Northern Illinois University, USA and PhD in Solid State Physics from the University of Arkansas, USA in 1991. He has been actively involved in high-temperature superconductor research since 1989. Superconductors are materials that can conduct electricity without any resistance which have many electrical and electronic applications. He has studied the propagation of ultrasonic waves in a wide range of hightemperature superconductors and showed that the two dimensional character of these materials naturally leads to high-temperature superconductivity. He has published more than 100 papers in international refereed journals and has written and co-edited several books. He has served as Editorial/Advisory Board of many leading international journals including Superconductor Science and Technology from 2004 to 2008 and Editor-in-Chief for Sains Malaysiana since 2006. He was a Visiting Fellow at Princeton University in 2003 and Kyoto University in 1996, and a Visiting Scholar at Texas A & M University in 2009. On the education front, he co-trained and led the national team to the International Physics Olympiad since 2002. Prof Roslan has received several awards inter alia the Malaysia Toray Science and Technology Award in 2006, National Young Scientist Award in 1999, Anugerah Khas Maal Hijrah Negeri Sembilan in 2008 and the Excellent Malay in Education Award in 2006 in conjunction with the 60th anniversary of UMNO Malaysia. He is a member of the American Physical Society and a fellow of the Malaysian Solid State Science and Technology Society, The Institute of Physics, United Kingdom and the Academy of Sciences, Malaysia.


10/23/09 8:08:11 PM

Contents

Abstract

1

I.

Introduction

2

II.

High Temperature Superconductor Research in Malaysia

7

III.

Superconducting Materials

9

IV.

Acoustic Properties and Electron-phonon Coupling in Cuprate Superconductors

16

V.

Nanomagnet-superconductor Hybrid for Energy Transport

24

VI.

Superconductor in Space

31

VII.

Conclusion

34

Acknowledgement

35

References

36


10/23/09 8:08:11 PM


10/23/09 8:08:11 PM

High Temperature Superconductors: Materials, Mechanisms and Applications ABSTRACT A surprising variety of new superconducting materials has been discovered in recent years. Many compounds with light elements such as fullerenes, oxides, borides, nitrides, some organic materials and also heavy fermions have been found to superconduct at various temperatures. Hitherto, superconductors have proven to be highly varied in composition but elusive and mysterious. The juxtaposition of superconductivity and magnetism at the nanoscale in some of these new materials has paved the way to a rich and exciting new field in condensed matter and materials research. An overview of superconductor research in Malaysian institutions is presented in this paper. Some of the new superconducting materials and their possible mechanisms, conventional and exotic, are presented. The possible role of lattice vibrations in the mechanisms of high temperature superconductivity and the study of this via acoustic methods are discussed. Frozen flux superconductors in a nanomagnet-superconductor hybrid system are also discussed. Key words: Cuprates; mechanisms; phonons; superconductor; space experiment; high temperature; historial development; acoustic; elastic; properties; materials; nanomagnet; Malaysia

ABSTRACT IN BAHASA MALAYSIA Berbagai jenis superkonduktor baru yang tidak diduga telah ditemui sejak beberapa tahun yang lalu. Banyak sebatian dengan unsur ringan seperti fuleren, oksida, borida, nitrida, bahan organik dan fermion berat telah ditemui mensuperkonduksi dalam berbagai suhu. Superkonduktor terbukti bahan yang terdiri daripada komposisi yang berbagai serta elusif dan penuh misteri. Persandingan kesuperkonduksian dan kemagnetan pada skala nano dalam sebahahgian bahan baru ini telah membuka bidang baru yang kaya dan menarik dalam fizik bahan dan fizik jirim terkondensasi. Penyelidikan superkonduktor yang dijalankan di institusi-institusi di Malaysia dibentangkan dalam kertas ini. Sebahagian daripada bahan superkonduktor baru dan mekanisme kesuperkonduksian, konvensional dan eksotik juga dibincangkan. Peranan getaran kekisi dalam mekanisme superkonduktor suhu tinggi yang dikaji melalui kaedah akustik dibincangkan. Sistem fluks beku dalam sistem nanomagnet-superkonduktor juga dibincangkan.

01Intro_(1-42)4.indd 1

10/23/09 8:22:47 PM

2

ASM Inaugural Lecture 2009

I. INTRODUCTION The mythical Greek monster Hydra, which was known to grow more heads when one was severed, would be even more powerful if each head was powered by perpetual energy transporters. In our modern days, such energy transporters could be made from what are known as superconductors. Such a system should be able to withstand power spikes and maintain stability under extreme conditions. Like a perpetual motion machine, electric current can flow freely in a superconductor without any loss of energy. Since time immemorial, advances in technology have depended greatly on humanity’s ability to manipulate materials. The transition from one historically distinct period to another is usually correlated to the materials used. The Stone Age stretches back at least two-and-a-half million years. As metalworking progressed, humanity proceeded through epochs of technological advancement now known as the Bronze Age and the Iron Age. Today, we have invented many new materials to replace conventional metals in common applications. In general, materials can be classified into four categories according to their electrical resistivity: insulator, semiconductor, conductor and superconductor. Insulators, such as quartz and diamond, are materials with the highest resistivity. Semiconductors, such as silicon and germanium, exhibit a lower resistivity compared to insulators but higher than that of conductors. One of the most interesting materials used and studied extensively today is the superconductor — a material that allows direct current to flow without any measurable resistance. It also expels external magnetic field, an effect normally called the Meissner effect (Photo 1). Superconductors are materials that are opening the gateway of the twenty first century and promising the realization of a long-time dream. The field of superconductivity underwent a major boost in 1986 following the discovery of high temperature superconductivity in the copper oxide based materials, which were later found to superconduct up to temperatures above the boiling point of liquid nitrogen.

Photo 1. A magnet levitating over a superconductor. This phenomenon is also known as the Meissner effect.


10/23/09 8:22:47 PM


3

Superconductors and perfect conductors are both resistanceless materials. However, they are distinctly different concepts. Perfect conductors are hypothetical materials that are 100% pure, implying that normal electrons can flow without any scattering or other processes that could result in resistivity. Electrical conduction in superconductors, however, arises from the pairing of charge carriers into Cooper pairs. The Cooper pairs propagate by sidestepping any processes that can give rise to a loss of energy. This article is divided into seven sections. In Section I, we give an introduction to the basic concepts and historical development of superconductors. In Section II, we give an overview of superconductor research in Malaysia. This is followed by a discussion on the various superconducting materials that have been discovered up to the present day in Section III. Section IV concerns the acoustic and elastic properties of the high temperature superconductors. The nanomagnet-superconductor hybrid for current conductors is discussed in section V. Results from a space-based superconductor experiment are discussed in the Section VI. Concluding remarks are given in Section VII. Sections I and II are written for the general reader. Historical Development Superconductivity was discovered by H. K. Onnes in 1911 following his success in liquefying helium three years earlier. The first superconducting element discovered was mercury, which exhibited a dramatic drop in resistivity at 4.2 K from 0.03 Ω to -6 3 × 10 Ω within a temperature range of 0.01 K (Figure 1). Following this discovery, a number of superconducting elements were discovered, including lead, aluminium and several alloys that superconduct at very low temperatures. These are known as conventional superconductors. Among the conventional superconductors, MgB2, discovered in 2001, has the highest transition temperature, (Tc) 39 K. Until recently, due to the low functional temperature and high cost of cooling, the applications of these materials was restricted to large laboratories with liquid helium production facilities and hospitals where Magnetic Resonance Imaging has proven to be an indispensable tool. The discovery of high temperature superconductivity in the copper oxide based materials by J. G. Bednorz and K. A. Müller in 1986 resulted in worldwide interest in these materials. Intense research efforts into superconductivity were undertaken during the last two decades. Hitherto, more than 100 000 papers on superconductivity have been published worldwide. Many of the copper oxide based materials, such as YBa2Cu3O7-d, were found to have a Tc higher than the boiling point of liquid nitrogen (~ 77 K). The high transition temperature translates into reduced operational and maintenance costs and thus a wider range of application. Figure 2 shows the evolution of the highest


10/23/09 8:22:47 PM

4


known transition temperature of a superconductor with the year of discovery. Since the field of superconductivity started in 1911, a total of eleven physicists have been awarded the Noble Prize for their contributions to the field (Table 1).

Figure 1. Electrical resistance of Hg at low temperature (Onnes 1913) which showed a transition temperature at 4.2 K. TABLE 1. NOBEL PRIZE IN PHYSICS FOR CONTRIBUTION TO SUPERCONDUCTIVITY

Year


Scientists

Contribution

1913

H. K. Onnes

Properties of materials at low temperature

1972

J. Bardeen, L. Cooper, and R. Schrieffer

Microscopic (BCS) theory of conventional superconductors

1973

I. Giaever and B. Josephson

Tunnelling effects in superconductor

1986

J. G. Bednorz and K.A. Müller

Discovery of the copper oxide-based high temperature superconductor

1991

P. de Gennes

Studies on complex systems including superconductivity

2003

V.L. Ginzburg and A.A. Abrikosov

For pioneering contributions to the theory of superconductors

10/23/09 8:22:47 PM


5

Figure 2. The evolution of Tc according to the year of discovery.

Although many electronic devices operate at room temperature, better performance can be achieved by going to lower temperatures. The two properties of superconductors, zero DC resistivity and perfect diamagnetism, can be use to enhance the performance of many devices. In general, applications of superconductor can be divided into two categories: large-scale and small-scale. Large-scale applications include high-speed trains, magnetic energy storage, magnetic resonance imaging (MRI) for medical applications, and high energy physics experiments. Small-scale applications include Josephson devices, Superconducting Quantum Interference Devices (SQUID), microwave devices and resonators. The practical applications of conventional superconductors are limited due to the very low operating temperature. The discovery of higher Tc materials extends the feasible applications of superconductors. Small-scale applications are expected to be commercialised earlier than large-scale applications due to the complexity in fabricating these materials in a bulk form suitable for commercial applications. A highly elaborate design is needed to cool a large-scale system.


10/23/09 8:22:47 PM

6


The critical current density, Jc, is the maximum current that can flow across a cross-sectional area before a superconductor reverts to normal. One way to understand this is to recall that a current through any wire generates a magnetic field with its maximum value at the surface of the conductor. The self-generated field at the wire’s surface cannot exceed the critical field, and the current must be correspondingly small if the superconducting state is to persist. The Jcs in the conventional superconductors are sufficiently large to carry current suitable for applications. The Jc of the high temperature superconducting materials however, needs to be improved before they can be used extensively in commercial applications. After years of intense research by scientists and engineers, superconductor technology is set to provide solutions in all major technological sectors in this century: information technology, satellite communications, mass transport, power transmission and generation, medical instruments and microelectronics. Currently, the annual world market for superconductor-based products is approximately RM34 billion (USD 10 billion). This annual figure is expected to increase to RM510 billion (USD 150 billion) by year 2020. In Malaysia the use of conventional cables for power transmission results in energy loss which amounts to RM3 billion per year. Furthermore Malaysia needs to invest in superconductor R&D in order to compete with other industrialized nations of the world in developing this advanced technology. Targeted and niche areas of superconductor applications need to be identified with serious research efforts, and funding must be put forward.


10/23/09 8:22:47 PM


7

II. HIGH TEMPERATURE SUPERCONDUCTOR RESEARCH IN MALAYSIA Since the discovery of high temperature superconductivity in the cuprates in 1987, a number of researchers in Malaysian universities have conducted research on these materials (Yahaya 1997). Today, about 20 researchers are actively involved in superconductor research in Malaysia. The superconductor research programme in Malaysia aims to promote and develop superconductivity related industries through fundamental and applied research, thereby contributing to the development of the nation’s economy. At the School of Applied Physics, Universiti Kebangsaan Malaysia (UKM), research in superconductivity encompasses three topics: the study of the basic mechanism of high temperature superconductor (HTSC) using ultrasonic methods, the use magnetic nanoparticles as pinning centres for monofilament and multifilament superconductor tapes and the synthesis of novel materials. The theory behind high temperature superconductivity remains a mystery despite many years of research worldwide. At UKM, ultrasonic methods have been employed to study the elastic properties and the possible role of phonons in the mechanism of the copper oxide based superconductors. Of particular interest is the strength of the electron-phonon coupling in these two dimensional systems. Our results show the importance of the interplay between the Debye frequency and electron-phonon coupling in a two dimensional system, and their variations have a combined effect in governing the transition temperature. The electron-phonon coupling is very small in these materials, which indicates that superconductivity is due to the non-diagonal electron-phonon interaction of the in-plane oxygen breathing mode. The successful application of superconductors will partly depend on the success of fabricating conductors, such as wires and tapes, with high current carrying capacity. One of the methods used to improve the current capacity is to use magnetic nanoparticles in HTSC tapes. In principle, this can result in a frozen flux superconductor where improvement in current carrying capacity is expected. Magnetic nanoparticles of about 50 nm size have been employed to enhance the properties of the Bi-based and Tl-based tapes. Magnetic impurities of the nano size are important because these particles can interact with the flux, which is about the same size. Our results indicate a significant enhancement in the current density, and research effort in this direction is ongoing (Abd-Shukor & Kong 2009; Ismail et al. 2004; Lau et al. 2006).


10/23/09 8:22:47 PM

8


Perhaps the most interesting aspect of superconductivity research at UKM is the search for superconductivity in novel materials. This includes the synthesis of the derivatives of existing families of superconductors as well as novel compounds with lower dimensions. Facilities at UKM include pulsed echo ultrasonic equipment, an AC susceptometer, low temperature cryostats, wire and tape drawing facilities and numerous high temperature furnaces. At Universiti Putra Malaysia, the research effort is on thin film fabrication and synthesis of the Bi- and Y-based superconductors by novel chemical techniques. Important aspects of high Tc superconductivity research, such as the interplay of superconductivity and magnetism and the effect of magnetic and non-magnetic nano particles on the transport critical current density of wires and tapes are studied (e.g. Halim et al. 1999; 2000). Facilities include pulsed laser ablation, low temperature cryostats, numerous furnaces and a Vibrating Sample Magnetometer. At Universiti Teknologi MARA, research efforts are on ultrasonic studies and the fabrication of Tl-based tapes using dip-coating method and the fabrication and critical current density studies of other dip-coated multicore high-temperature superconductors and tapes. In addition, development and testing for a novel design for high-temperature superconductors, the effect of chemical substitution on sound velocity and ultrasonic attenuation in colossal magnetoresistive materials and elemental substitutions on formation and superconductivity of TlSr2CaCu2O7 ceramics were also part of the research (e.g. Yahya et al. 2007). Facilities include low temperature cryostats, a dip-coating facility and pulsed-echo ultrasonic equipment. The crystal structure of various superconducting materials and the effect of magnetic ions were actively studied at Universiti Sains Malaysia (USM) in the early period (Fun et al. 1994). The mechanism of high temperature superconductivity in the cuprates was also studied extensively at USM in the early days of the field (e.g. Lee 1989). At Universiti Tenaga Nasional (UNiTEN), research on applications of superconductors as devices, such as a fault current limiter, is actively pursued. Each group carried out their own research, but in collaboration with each other. Meetings are held several times a year and attended by a representative from each group. Due to the similarity in the characterization technique, magnetic materials such as colossal magneto-resistive (CMR) manganites are also studied together with superconductors. The total funding for superconductor research in Malaysia from 1989 to 2008 is estimated to be not more than RM4 million. This estimate is based from IRPA, Science Fund, Scientific Advancement Fund Allocation (SAGA), Malaysia universities research grant, Malaysia Toray Science Foundation and international agencies. Despite the low priority given by funding agencies in Malaysia for superconductor research, Malaysian researchers have published about 150 papers on superconductivity in internationally refereed journals and several hundred papers in conferences from 1989 to early 2009.


10/23/09 8:22:47 PM

9


III. SUPERCONDUCTING MATERIALS At the forefront of high temperature superconductivity research, synthesis of new materials and the understanding of the basic mechanisms of these novel materials have generated worldwide interest. Superconductors are materials that allow electricity to flow without any resistance. This unique feature is important in many applications in electronics, communications, transportation and medical equipment. Superconductors can be divided into several classes according to their Tc, structure and the nature of their superconducting properties: conventional superconductors, Chevrel phase superconductors, organic superconductors, heavy fermions, copper oxide-based high Tc superconductors, non-copper oxide-based superconductors and quaternary carbide compound. In recent years, a number of new superconducting materials have been discovered. The most recent discovery is the existence of iron arsenide based materials with Tcs near 26 K (Kamihara et al. 2008). The classes of some superconducting materials known today are shown in Table 2.

TABLE 2. CLASSES OF SUPERCONDUCTING MATERIALS Class of materials


Example

Tc (K)

Metal and alloys

Al, Pb, Nb3Sn, NbTi, MgB2

≤ 39

Organic Superconductor

K3C60, [BEDT-TTF]2X

< 42

Heavy Fermions

CeCu2Si2

99.9%) metal oxides and carbonates. Samples 12.5 mm in diameter and of 2 mm thickness were sintered in air at various temperatures, followed by annealing in oxygen and various gases. The sound velocity was measured using a Matec 7700 system that utilizes the pulse-echo-overlap technique. The samples were bonded to a transducer using Nonaq stopcock grease [Figure 7(a)]. The ultrasonic waves were propagated along the direction of pressing using X-cut (longitudinal) or Y-cut (shear) quartz transducers at 5 – 10 MHz. The velocity and measurement was performed in an Oxford Instruments liquid nitrogen cryostat model DN 1711. The absolute longitudinal and shear velocities were evaluated at 80 K. A typical ultrasonic echo train is shown in Figure 7(b). (a)

(b) Rf signal Transducer Nonaq grease

Superconductor Figure 7 (a). Schematic of a single transducer setup; (b) A typical echo pattern of an ultrasonic pulse travelling through a copper oxide based ceramic superconductor.

01Intro_(1-42)4.indd 18

10/23/09 8:22:48 PM

19


The temperature-dependent electrical resistance measurements were carried out using the four-point probe technique with silver-paint contacts. Powder XRD analyses with CuKα radiation using a Siemens D5000 diffractometer were used to confirm that the samples are single phased. The sound velocity in an ideal, void-free material can be estimated from the measured velocity using the relation, v I = v M

ρ th , where vI is the void free ρ

velocity, th is the theoretical density and vM is the measured velocity. The Debye

h 3N temperature, θD, can be estimated using the standard formula, θ D = k 4 V where

3

v

3 m

=

1

v

3 l

+

2

v3

1

3

v m,

, h is the Planck constant, k is the Boltzmann constant, N is

the number of mass points, V is the atomic volume and vm is the mean velocity. The Debye temperature and electron-phonon coupling constant were estimated in the weak coupling limit of the BCS theory as well as in the van Hove scenario and are shown in Table 3. The Tc and θD in the table are from our previous reports on ultrasonic studies of high temperature superconductors (for example, Nik-Jaafar, et al. 2005; Jaafar & Abd-Shukor 2001; Yahya & Abd-Shukor 1999). The uncertainty in θD is about 2.5 % of the absolute value. Tc and θD do not show a linear relationship when compared between samples of different composition. For example the Tl2Ba2Ca2Cu3O10 with the highest Tc showed the lowest θD. However, for samples of similar composition save for the oxygen contents, for example ErBa2Cu3O6.9 and ErBa2Cu3O6.3, the superconducting sample (O6.9) showed a higher θD. This is also true for the EuBa2Cu3O6.98 and EuBa2Cu3O6.9 samples. In such cases, higher oxygen content (higher Tc) shows a higher θD. In the Zn doped GdBaSrCu3O7- samples, the disruption of the Cu spins by non-magnetic Zn resulted in a decrease in Tc, although an increase in θD was observed. Zn doped samples also showed a reduction of the electron-phonon coupling constant in both the BCS and the van Hove scenario. The electron-phonon coupling constant calculated using the weak coupling limit, λBCS, is between 0.28 and 1.11 as shown in Table 3 and plotted against Tc in Figure 8. The position of the van Hove singularity with respect to the Fermi level depends on the doping level. In addition, a linear resistance versus temperature relation is evidence that the Fermi surface is near the van Hove singularity (Lee & Read 1987). All the samples listed in Table 3 showed the metallic normal state

−1

behaviour where the equation Tc = 2.72TF e λ was employed to calculate the van Hove electron-phonon coupling constant. The van Hove scenario electron-phonon coupling constant, λVH, is between 0.025 and 0.060, which are proportional to the transition temperature of the samples studied (Figure 9) and much closer to the

01Intro_(1-42)4.indd 19

10/23/09 8:22:48 PM

20


values of various electron-phonon coupling constants found in the Bi based and RBa2Cu3O7- superconductors (λ is centred at around 0.04) from Raman scattering data (Devereaux et al. 1998; Friedl et al. 1990; Pattnaik & Newns 1989). The λVH seems to agree well with other experimental data and should be relevant to superconductivity in the cuprates. If electron-phonon interaction is playing a role in the Cooper pair formation in the cuprate superconductors, then this small momentum transfer is important for the pair formation. Figure 9 shows a slightly better correlation between Tc and electron-phonon coupling constant in the van Hove scenario compared to the standard BCS theory as shown in Figure 8. According to the conventional theory, there are two dependencies that govern the electron–phonon coupling. Usually, the coupling constant is determined by the density of states given as λ = N(0)V, with V reasonably constant. In the doped HTSC, this can be interpreted as the variation in the density of states near the Fermi level. The van Hove singularity moves closer or further away from the Fermi level depending on doping. The coupling constant in the BCS-McMillan scheme can also C be related to the characteristic phonon frequency , with λ ~ , where C is a M ω constant and M the ionic mass. In this case, an elastically softer material will have a stronger electron-phonon coupling. The two expressions for λ, however, are related because materials with a high density of states are elastically softer (McMillan 1968). However, in the cuprates, a decrease in the electron-phonon coupling constant does not necessarily result in a lower Tc. The corresponding changes in the characteristic phonon frequency may instigate suppression of the transition temperature and vice-versa. The variation in the characteristic phonon (Debye) frequency and the electron-phonon coupling constant has a combined effect on Tc, as in the case of other types of two-dimensional superconductors (Ghosh et al. 2003). Detailed studies of the phonon and oxygen vibration modes in the CuO2 planes have been widely reported (Devereaux et al. 2004; Piekarz et al. 1999). It is interesting to note that the electron-phonon coupling constant for the breathing mode is λ = 0.02 (Devereaux et al. 2004), the same order of magnitude as the λVH values determined in this work. Although the electron-phonon coupling constant in the van Hove scenario is small, i.e., λVH 1, a magnetic rod generates a flux line which is tightly bound within the rod, resulting in a frozen flux superconductor. In a magnetic system with characteristic length L, where < L < λ, strong interaction between the flux line network and a magnetic subsystem can be expected. In principle, this can result in a frozen flux superconductor, where improvement in current carrying capacity is expected. Magnetic impurities of the nanometer scale is vital because these particles can interact with the flux, which is about the same size.

01Intro_(1-42)4.indd 25

10/23/09 8:22:49 PM

26


␥-Fe2O3 Magnetic Nanorod as a Flux Pinning Centre In this section, we discuss briefly the effect of the addition of -Fe2O3magnetic nanorods on the pinning strength of Ag-sheathed Bi(Pb)-Sr-Ca-Cu-O tapes. Our results indicate a significant enhancement in the current density and research efforts in this direction are ongoing. Table 4 shows the details of the thermo-mechanical treatment process for all samples and the critical current density (Jc) at 77 K in selffield and in applied field B = 0.10 T. Figure 12(a) shows the micrograph of the nanorod-like structure of -Fe2O3 and Figure 12(b) shows the micrograph of Bi-SrCa-Cu-O tapes with a distribution of -Fe2O3 as white dots. Details of thermomechanical treatment process for all samples are shown in Table 4. In the self-field environment, addition of -Fe2O3 increases the Jc of Ag-sheathed Bi(Pb)-Sr-Ca-Cu-O tapes from 1560 A/cm2 for sample S to 6460 A/cm2 for sample SN. The Jc improves further to 9560 A/cm2 in sample SN-R after thermo-mechanical treatments. It is important to note that the improved Jc in sample SN is due to the enhancement of the pinning strength of the tapes, bolstered by the addition of -Fe2O3 powder. However, only a slight increase in the pinning strength of sample SN-R is observed after the thermo-mechanical treatments.

TABLE 4. DETAILS OF THERMO-MECHANICAL TREATMENT PROCESS FOR ALL SAMPLES. CRITICAL CURRENT DENSITY (Jc) AT 77 K IN SELF-FIELD AND IN APPLIED FIELD B = 0.10 T

Sample

Heat treatment

S

845 oC 50 h

S-R

No. of intermediate rolling

Jc (B = 0 T) (A/cm2)

Jc (B = 0.1 T) (A/cm2)

0

1560

128

845 oC 150 h

2

5400

562

S-T

845 oC 150 h

0

1370

116

SN

845 oC 50 h

0

6460

801

SN-R

845 oC 150 h

2

9560

1400

Source: Lau et al. 2006

In order to confirm these results, two conventional methods of comparing the pinning strength of the samples are used. The first method compares the pinning strength of the tape through the ability of the samples to maintain their Jc in increasing magnetic field B. Graph of Jc versus B shows the decrease rate of Jc for

01Intro_(1-42)4.indd 26

10/23/09 8:22:49 PM


27

the samples from the highest to the lowest can be arranged as followed: S, S-T, S-R, SN-R and SN. Consequently, the pinning strength for the samples, arranged from the highest to the lowest, is SN, SN-R, S-R, S-T and S. The second method compares the pinning strength by comparing the samples’ field (Bpmax) at the peak of the normalized pinning force density Fp/Fmax versus applied field B curve. Larger Bpmax indicates stronger pinning strength. The Bpmax value for the samples from the highest to the lowest can be arranged as followed: SN, SN-R, S-R, S-T and S. In general, although the pinning strengths of SN are shown to be higher than those of SN-R, both of the results agree with the conclusion that the pinning strength of the samples with the addition of -Fe2O3 nano powder are higher than samples without the addition of -Fe2O3.

(a)

(b)

Figure 12. (a) Micrograph of nanorod-like structure of -Fe2O3 and (b) micrograph of Bi-Sr-Ca-Cu-O tapes with -Fe2O3 shown as white dots (Abd-Shukor et al. 2006 b, © IOP Publishing).

This work shows experimentally that magnetic nanorods can enhance the transport capacity of high temperature superconductor tapes (Lau et al. 2006). We believe that -Fe2O3 can further enhance the Jc of the Bi-based tapes if prepared under the optimum conditions. Magnetic impurities generally suppress superconductivity. However, our result shows that magnetism at the nanoscale can be employed to enhance the flux pinning capability of Ag sheathed Bi(Pb)-Sr-CaCu-O superconductor tapes, in line with previous calculation on frozen flux superconductors with magnetic nanorod as pinning centres.

01Intro_(1-42)4.indd 27

10/23/09 8:22:49 PM

28


Fe3O4 Magnetic Nanoparticles as Flux Pinning Centres Figure 13 shows the TEM micrograph of the Fe3O4 nanoparticles used in this work. The average size is approximately 40 nm. Figure 14 shows the zero-resistance transition temperature, Tc and critical current density versus Cr2O3 content in bulk Bi1.6Pb0.4Sr2Ca2Cu3O10. The maximum Jc and Tc coincide and are exhibited by the sample with 0.1 weight % of nano Cr2O3. Figure 15 shows the transition temperature and critical current density versus Fe3O4 content in bulk Bi1.6Pb0.4Sr2Ca2Cu3O10. The maximum Jc and Tc also coincide and are exhibited by the sample with 0.01 weight % of nano Fe3O4. It is interesting to note that the weight % of nanoparticles that showed the optimum superconducting properties is an order of magnitude less in the Fe as compared to the Cr containing nanoparticles. XRD patterns show that the Bi2223 is the majority phase while the (Bi,Pb)2Sr2CaCu2O8 (Bi2212) is the minority phase in all samples. Table 5 shows the Jc of the Bi1.6Pb0.4Sr2Ca2Cu3O10 tapes with and without the addition of nanoparticles at 30 K and 77 K in zero fields. The Jc of the Cr2O3 added samples at 77 K (3850 A/cm2) is not much different than that of the nonadded samples. However, nanosized Cr2O3 enhanced the critical current density tremendously at a lower temperature of 30 K (20 720 A/cm2). Nevertheless, Jc of the Fe3O4 added tapes is much higher than the Cr2O3 added tapes at 30 K and 77 K.

Figure 13. TEM micrograph of Fe3O4 nanoparticles (Abd-Shukor & Kong 2009, © Maney Publishing).

01Intro_(1-42)4.indd 28

10/23/09 8:22:49 PM

29


TABLE 5. JC OF Cr2O3 AND Fe3O4 NANOPARTICLES ADDED Bi1.6Pb0.4Sr2Ca2Cu3O10 TAPES IN ZERO FIELDS Jc at 30 K (A/cm2)

Jc at 77 K (A/cm2 )

Bi1.6 Pb0.4 Sr2Ca2Cu3O10

13 180

3730

Bi1.6Pb0.4Sr2Ca2Cu3O10-(Cr2O3)0.1

20 720

3850

Bi1.6Pb0.4Sr2Ca2Cu3O10-(Fe3O4)0.01

24 550

6090

Composition

Nanoparticles can be used to enhance the critical current density in high temperature superconductors. When magnetic nanoparticles are employed, a lesser quantity is required to optimize Jc and Tc. The actual mechanism that lead to the optimization of Jc at different composition for the Fe and non-Fe added sample is an interesting problem for further study. Instead of relying on the condensation energy associated with their core, the full vortex magnetic energy, through the addition of magnetic nanoparticles in Bi1.6Pb0.4Sr2Ca2Cu3O10, has the ability to enhance Jc in the nano Fe3O4 added materials (Abd-Shukor & Kong 2009).

Figure 14. Tc and Jc versus Cr2O3 content (wt %) in bulk Bi1.6Pb0.4Sr2Ca2Cu3O10 (Abd-Shukor & Kong 2009, © Maney Publishing).

01Intro_(1-42)4.indd 29

10/23/09 8:22:49 PM

30


Figure 15. Tc and Jc versus Fe3O4 content in bulk Bi1.6Pb0.4Sr2Ca2Cu3O10 (Abd-Shukor & Kong 2009, © Maney Publishing).

01Intro_(1-42)4.indd 30

10/23/09 8:22:49 PM


31

VI. SUPERCONDUCTOR IN SPACE On a frigid, wintery Tuesday, 23 January 1999, a Delta 2 rocket blasted off from the Vandenberg Air Force Base in California, USA (Photo 2). One of the payloads onboard this NASA rocket is a South African satellite SUNSAT. It is a low earth orbit satellite intended for an altitude of 600–840 km. SUNSAT is a microsatellite built by the Faculty of Engineering, University of Stellenbosch, South Africa. The weight of the satellite is 64 kg with dimensions 450 mm × 450 mm × 600 mm. Apart from carrying an experiment module from Malaysia, this satellite also carried other payloads including a remote sensing camera and sun sensors. The Malaysian experiment consisted of two modules weighing 28 g (Photo 3). The material for testing was placed on the exterior, while the electronic module was placed in the interior. Both modules were fabricated at UKM.

Photo 2. The Delta 2 rocket blasts off the Vandenberg Air Force Base in California, USA. One of the payloads onboard this NASA rocket is a South African satellite SUNSAT carrying several modules, including a superconductor experiment from Malaysia.

The mission of the experiment is to study the effects of radiation and extreme space conditions on a sample of the high temperature superconductor YBCO. The superconductor was mounted on the exterior, while the electronic module was placed in the interior of the satellite. The data that were obtained over the course of

01Intro_(1-42)4.indd 31

10/23/09 8:22:49 PM

32


24 months demonstrated that the superconductor is very stable under extreme space conditions. Figure 16 shows a typical variation of resistance of YBCO with temperature for three-earth orbit cycles. The time interval between successive peaks in the figure is about 100 min, which is equivalent to one earth orbit cycle. The satellite also rotates about its axis five times for every earth-orbit cycle, and the several smaller temperature peaks in between the larger peaks is due to this rotation.

Photo 3. Flight model of the superconductor experiment aboard SUNSAT. The electronic module is on the left, and the materials module containing a superconductor and a glassy carbon (smaller) disc is on the right.

The normal state resistance of YBa2Cu3O7-d is very sensitive to the oxygen content. For example, the resistivity of normal state YBa2Cu3O6.3 (nonsuperconducting state) is several orders of magnitude higher than normal state YBa2Cu3O7 (superconducting). A rise in resistance of the YBCO sample indicates the amount of oxygen that left the sample. There is no observable change in the resistance throughout the two-year period. This indicates that the material is stable in an exposed space environment. This also indirectly indicates that the oxygen is stable in the YBCO crystal lattice (Abd-Shukor et al. 2006c). This result indicates the potential use of superconductors as a perfect satellite communication component in the future. The stability of 2N3819 FET as a constant current source for space applications was also tested on SUNSAT. Our data indicate that the FET performed reasonably well in space for at least 24 months. (Alwi et al. 2001).

01Intro_(1-42)4.indd 32

10/23/09 8:22:49 PM


33

Figure 16. Resistance of YBCO superconductor over three earth-orbit cycle.

01Intro_(1-42)4.indd 33

10/23/09 8:22:49 PM

34


VII. CONCLUSION The development of superconductivity, especially in the cuprates, has been discussed. The major topics include an historical perspective, superconductor research in Malaysia, novel materials, acoustics and the role of phonons in the cuprates and nanomagnet-superconductor hybrid system. Most of these discussions stem from superconductor research performed for almost two decades at Universiti Kebangsaan Malaysia. The mechanism for high temperature superconductivity has been discussed by combining various models and is supported by ultrasonic measurements. By employing the theory for conventional superconductivity (also known as BCS theory) and taking into account the anisotropic (two-dimensional) nature of the crystal structure of these superconductors, where the van Hove scenario is applicable, the mechanism can be explained. This understanding can be useful in advancing the applications of high temperature superconductors. The use of liquid nitrogen instead of the more expensive liquid helium has renewed the possibility of moving superconductor technology into power applications. In order to realize these applications, significant improvements has to be made in the current carrying capacity of HTSC wires, notably in high magnetic field environment. Improvements in length and performances are expected by optimizing the fabrication process until it is suitable for creating cables and windings for prototype superconducting devices. Our research indicates that a frozen flux superconductor can be generated in HTSC tapes with the incorporation of magnetic nanorods. These results provide a new approach to fabricating HTSC tapes for power transmission.

01Intro_(1-42)4.indd 34

10/23/09 8:22:50 PM


35

ACKNOWLEDGEMENT This research has been made possible by the co-operation and assistance of many institutions and individuals. I wish to thank Universiti Kebangsaan Malaysia, especially the Faculty of Science and Technology, for providing much support for this research. I am grateful to Prof Emeritus Dato’ Dr Muhammad Yahya, F.A.Sc. (UKM), Prof Dr Abdul Halim Shaari (UPM) and Associate Prof Ahmad Kamal Yahaya (UiTM) for their co-operation and assistance. Also, my appreciation to the graduate students who have conducted countless experiments in the laboratory. I also wish to thank the Ministry of Science, Technology and Innovation (MOSTI) for the IRPA and Science Fund grant; the Academy of Science and MOSTI for the SAGA grant; the Ministry of Higher Education for the Fundamental Research Grant; Malaysia Toray Science Foundation; Third World Academy of Science, Italy for the research grants; and the Brain Gain Malaysia Programme (BGM) for the International Fellowship.

01Intro_(1-42)4.indd 35

10/23/09 8:22:50 PM

36


REFERENCES Abd-Shukor, R 2002, ‘Acoustic Debye temperature and the role of phonons in cuprates and related superconductors’, Superconductor Science & Technology, vol. 15, no. 3, pp. 435–438. Abd-Shukor, R 1992, ‘Elastic anomaly of YBa2Cu3O7-d at the superconducting transition’, Japanese Journal of Applied Physics Part 2-Letters, vol. 31, no. 8A, pp. L1034–L1036. Abd-Shukor, R 2007, ‘Electron-phonon coupling constant of cuprate based high temperature superconductors’, Solid State Communications, vol. 142, no. 10, pp. 587–590. Abd-Shukor, R 2003, ‘Ketertiban dalam superkonduktor suhu tinggi’, Syarahan Perdana, Bangi: Penerbit Universiti Kebangsaan Malaysia, 64 p. Abd-Shukor, R & Kong, W 2009, ‘Magnetic field dependent critical current density of Bi-Sr-Ca-Cu-O superconductor in bulk and tape form with addition of Fe3O4 magnetic nanoparticles’, Journal of Applied Physics vol. 50, vol. 7, art. no. 07E311, pp. 07E311.1–07E311.2. Abd-Shukor, R & Arulsamy, AD 2000, ‘Superconductivity and transport critical current density of Nd substituted Tl-1212 phase Tl(Sr1-xNdx)2CaCu2O7’, Journal of Physics D-Applied Physics, vol. 33, no.7, p. 836–839. Abd-Shukor, R & Jaafar, NAN 1999, ‘Formation and superconductivity of Pr and Nd-substituted Tl-1212 phase (Tl0.85Cr0.15)Sr2CaCu2O7’, Journal of Materials Science-Materials in Electronics, vol. 10, no. 9, pp. 677–681. Abd-Shukor, R, Kek, CA & Yeoh, WK 2006a, ‘Superconducting properties of Ce-substituted TaSr2(Gd,Ce)2Cu2Oy’, Journal of Materials Science, vol. 41, no. 12, pp. 3799–3803. Abd-Shukor, R., Yahya, S.Y., Jumali, M.H., Hamadneh, I., Halim, S.A. 2006b. “Transport Critical Current Density of Ag-Sheathed Bi-Sr-Ca-Cu-O Multifilament Superconductor Tapes with Magnetic Nanopowders y-Fe2O3”, Journal of Physics: Conf. Ser. vol. 43, pp. 71–74. Abd-Shukor, R, Othman, M, Alwi, HA, Deraman, M & Aziz, AZA 2006c, ‘Fabrication and results of materials experiment in space on SUNSAT’, Journal of Science & Technology in the Tropics, vol. 2, no. 1, pp. 45–49. Abd-Shukor, R & Tee, KS 1998, ‘Effectiveness of Bi versus Pb for superconductivity in the TlSr2CaCu2O7 (1212) phase’, Journal of Materials Science Letters, vol. 7, no. 2, pp. 103–106. Abd-Shukor, R & Kong, W 2009, ‘Nanoparticles as flux pinning center in bulk and Ag-sheathed Bi1.6Pb0.4Sr2Ca2Cu3O10 high temperature superconductor Tapes’, Materials Research Innovation, accepted for publication. Abd-Shukor, R & Quah, CT 1994. ‘Superconductivity of 1212-type phase TlSr2(Ca,Cr)Cu2O7 and TlSr2(Sr,Tm)Cu2O7’, Materials Science and Engineering B-Solid State Materials for Advanced Technology, vol. 23, no. 1, pp. 54–57. Abrecht M, Ariosa, D, Cloetta, D, Mitrovic, S, Onellion, M, Xi, XX, Margaritondo, G & Pavuna, D 2003, ‘Strain and high temperature superconductivity: Unexpected results from direct electronic structure measurements in thin films’, Physical Review Letters, vol. 91, no. 5, pp. 057002.1– 057002.4.

01Intro_(1-42)4.indd 36

10/23/09 8:22:50 PM


37

Abrikosov, AA, Campuzano, JC & Gofron, K 1993, ‘Experimentally observed extended saddle-point singularity in the energy-spectrum of YBa2Cu3O6.9 and YBa2Cu4O8 and some of the consequences’, Physica C, vol. 214, no. 1–2, pp. 73–79. Alwi, HAB, Abd-Shukor, R, Othman, M 2001, ‘Stability of 2N3819 FET as a constant current source for space applications’, Singapore Journal of Physics, vol. 17, no. 1, pp. 47–52. Bhattacharya, S, Higgins, MJ, Johnston, DC, Jacobson, AJ, Stokes, JP, Goshorn, DP & Lewandowski, JT 1988, ‘Elastic anomalies and phase-transitions in high-Tc superconductors’, Physical Review Letters, vol. 60, no. 12, pp. 1181–1184. Bulaevskii, LN & Chudnovsky, EM 2001, ‘Ferromagnetic film on a superconducting substrate’, Physical Review B, vol. 63, no. 1, art. no. 012502, 3 p. Dessau, DS, Shen, ZX, King, DM, Marshall, DS, Lombardo, LW, Dickinson, PH, Loeser, AG, Dicarlo, J, Park, CH, Kapitulnik, A & Spicer, WE 1993, ‘Key features in the measured bandstructure of Bi2Sr2CaCu2O8+delta – flat bands at e(f) and fermi-surface nesting’, Physical Review Letters, vol. 71, no. 17, pp. 2781–2784. Devereaux, TP, Cuk, T, Shen, ZX & Nagaosa, N 2004, ‘Anisotropic electron-phonon interaction in the cuprates’, Physical Review Letters, vol. 93, no. 11, p. 4. Devereaux, TP, Virosztek, A, Zawadowski, A, Opel, M, Muller, PF, Hoffmann, C, Philipp, R, Nemetschek, R, Hackl, R, Erb, A, Walker, E, Berger, H & Forro, L 1998, ‘Enhanced electronphonon coupling and its irrelevance to high T-c superconductivity’, Solid State Communications, vol. 108, no. 7, pp. 407–411. Erdin, S, Kayali, AF, Lyuksyutov, IF & Pokrovsky, VL 2002, ‘Interaction of mesoscopic magnetic textures with superconductors’, Physical Review B, vol. 66, no. 1, pp. 144 141–144 147. Friedl, B, Thomsen, C, Schonherr, E & Cardona, M 1990, ‘Electron-phonon coupling of apex oxygen in RBa2Cu3O7-delta’, Solid State Communications, vol. 76, no. 9, pp. 1107–1110. Fun, H, Yang, KP, Lai, CC, Ku, HC & Lee TJ 1994, ‘Structures and antiferromagnetic Pr ordering mechanism of Tl(Ba2-xSrx)PrCu2O7-y compounds by rietveld analysis’, Physica C, vol. 223, no. 3–4, pp. 267–275. Getino, JM, Rubio, H & Dellano, M 1992, ‘Cooper pairing in the van Hove singularity scenario of high-temperature superconductivity’, Solid State Communications, vol. 83, no. 11, pp. 891–893. Ghosh, AK, Nakamura, H, Tokunaga, M & Tamegai, T 2003, ‘Superconductivity in Y2PdGe3-xSix: interplay between Debye temperature and coupling constant’, Physica C-Superconductivity and its Applications, vol. 388, pp. 567–568 . Gofron, K, Campuzano, JC, Abrikosov, AA, Lindroos, M, Bansil, A, Ding, H, Koelling, D & Dabrowski, B 1994, ‘Observation of an extended van-hove singularity in YBa2Cu4O8 by ultrahighenergy resolution angle-resolved photoemission’, Physical Review Letters, vol. 73, no. 24, pp. 3302–3305.

01Intro_(1-42)4.indd 37

10/23/09 8:22:50 PM

38


Haldar, P, Roigjanicki, A, Sridhar, S & Giessen, BC 1988, ‘A new intermediate Tc oxide superconductor with a double perovskite structure – the 1-2-1 phase (Tl,Bi)(Sr,Ca)2CuO4.5+delta’, Materials Letters, vol. 7, no. 1–2, pp. 1–4. Halim, SA, Khawaldeh, SA, Mohamed, SB & Azhan, H 1999. ‘Superconducting properties of Bi2-xPbxSr2Ca2Cu3Oy system derived via sol-gel and solid state routes’, Materials Chemistry and Physics, vol. 61, no. 3, pp. 251–259. Halim, SA, Saleh, AK, Azhan, H, Mohamed, SB, Khalid, K & Suradi, J 2000, Synthesis of Bi1.5Pb0.5Sr2Ca2Cu3Oy via sol-gel method using different acetate-derived precursors’, Journal of Materials Science, vol. 35, no. 12, pp. 3043–3046. Hamadneh, I, Kuan, YW, Hui, LT & Abd-Shukor, R 2006, ‘Formation of Tl0.85Cr0.15Sr2CaCu2O7-delta superconductor from ultrafine co-precipitated powders’, Materials Letters, vol. 60, no. 6, pp. 734–736. Hoen, S, Bourne, LC, Kim, CM & Zettl, A 1988, ‘Elastic response of polycrystalline and single-crystal YBa2Cu3O7’, Physical Review B, vol. 38, no. 16, pp. 11 949–11 951. Ismail, M., R. Abd-Shukor, I. Hamadneh & S. A. Halim. 2004, Transport current density of Ag-sheathed superconductor tapes using Bi-Sr-Ca-Cu-O powders prepared by the co-precipitation method, Journal of Materials Science, vol. 39, no. 10, pp. 3517–3519. Jaafar, NAN & Abd-Shukor, R 2001, ‘Elastic properties of superconducting and non-superconducting DyBaSrCu3O7-delta’, Physics Letters A, vol. 288, no. 2, pp. 105–110. Kamihara, Y, Watanabe, T, Hirano, M & Hosono, H 2008, ‘Iron-based layered superconductor La[O1-xFx]FeAs (x=0.05-0.12) with T-c=26 K’, Journal of the American Chemical Society, vol. 130, no. 11, pp. 3296-+. Kayali, MA & Pokrovsky, VL 2004, ‘Anisotropic transport properties of ferromagnetic-superconducting bilayers’, Physical Review B, vol. 69, no. 13, art. no. 132 501. Krunavakarn, B, Udomsamuthirun, P, Yoksan, S, Grosu, I & Crisan, M 1998, ‘The gap-to-T-c ratio of a van Hove superconductor’, Journal of Superconductivity, vol. 11, no. 2, pp. 271–273. Labbe, J & Bok, J 1987, ‘Superconductivity in alkaline-earth-substituted La2CuO4 – a theoreticalmodel’, Europhysics Letters, vol. 3, no. 11, pp. 1225–1230. Lanzara, A, Bogdanov, PV, Zhou, XJ, Kellar, SA, Feng, DL, Lu, ED, Yoshida, T, Eisaki, H, Fujimori, A, Kishio, K, Shimoyama, JI, Noda, T, Uchida, S, Hussain, Z & Shen, ZX 2001, ‘Evidence for ubiquitous strong electron-phonon coupling in high-temperature superconductors’, Nature, vol. 412, no. 6846, pp. 510–514. Lau, KT, Yahya, SY & Abd-Shukor, R 2006, ‘Enhanced flux pinning in Ag-sheathed Bi(Pb)-Sr-Ca-Cu-O superconductors tapes with addition of magnetic nanorod gamma-Fe2O3’, Journal of Applied Physics, vol. 99, no. 12, pp. 123904–123907. Lee, BS 1989, ‘Anharmonicities and cooperative Jahn-Teller phase transitions in the high-Tc superconductors La2-xMxCuO4 (M = Sr, Ba)’, Physica C, vol. 57, no. 1, pp. 72–82.

01Intro_(1-42)4.indd 38

10/23/09 8:22:50 PM


39

Lee, CS, Safa-Sefat, A, Greedan, JE & Kleinke, H 2003, ‘Synthesis, structure, and physical properties of mixed valent Mo2SbS2, the first superconducting antimonide-sulfide’, Chemistry of Materials, vol. 15, no. 3, pp. 780–786. Lee, PA & Read, N 1987, ‘Why is Tc of the oxide superconductors so low’, Physical Review Letters, vol. 58, no. 25, pp. 2691–2694. Lyuksyutov, IF & Naugle, DG 1999, ‘Frozen flux superconductors’, Modern Physics Letters B, vol. 13, no. 15, pp. 491–497. Lyuksyutov, IF & Naugle, DG 2000, ‘Frozen flux in a superconducting film’, Physica C-Superconductivity and Its Applications, vol. 341, pp. 1267–1268. Lyuksyutov, IF & Naugle, DG 2003, ‘Magnetic nanorods/superconductor hybrids’, International Journal of Modern Physics B, vol. 17, no. 18–20, pp. 3713–3716. Maeda, H, Tanaka, Y, Fukutomi, M & Asano, T 1988, ‘A new high-Tc oxide superconductor without a rare-earth element’, Japanese Journal of Applied Physics Part 2-Letters, vol. 27, no. 2, pp. L209–L210. Marmorkos, IK, Matulis, A & Peeters, FM 1996, ‘Vortex structure around a magnetic dot in planar superconductors’, Physical Review B, vol. 53, no. 5, pp. 2677–2685. Martin, C, Provost, J, Bourgault, D, Domenges, B, Michel, C, Hervieu, M & Raveau, B 1989, ‘Structural peculiarities of the 1212 superconductor Tl0.5Pb0.5Sr2CaCu2O7’, Physica C, vol. 157, no. 3, pp. 460–468. McMillan, WL 1968, ‘Transition temperature of strong-coupled superconductors’, Physical Review, vol. 167, no. 2, pp. 331. Nagamatsu, J, Nakagawa, N, Muranaka, T, Zenitani, Y & Akimitsu, J 2001, ‘Superconductivity at 39 K in magnesium diboride’, Nature, vol. 410, no. 6824, pp. 63–64. Newns, DM, Tsuei, CC & Pattnaik, PC 1995, ‘Vanhove scenario for d-wave superconductivity in cuprates’, Physical Review B, vol. 52, no. 18, pp. 13 611–13 618. Nik-Jaafar, NA, Halim, SA & Abd-Shukor, R 2005, ‘Sound velocity and ultrasonic attenuation in RuSr2GdCu2O8’, Superconductor Science & Technology, vol. 18, no. 10, pp. 1365–1369. Novikov, DL & Freeman, AJ 1993, ‘Vanhove singularities and the role of doping in the stabilization, synthesis and superconductivity of HgBa2Can-1CunO2n+2+delta’, Physica C, vol. 216, no. 3–4, pp. 273–283. Parkin, SSP, Lee, VY, Nazzal, AI, Savoy, R, Beyers, R & Laplaca, SJ 1988, ‘Tl1Can-1Ba2CunO2n+3 (n=1,2,3) — a new class of crystal-structures exhibiting volume superconductivity at up to congruent-to 110-K’, Physical Review Letters, vol. 61, no. 6, pp. 750–753. Pattnaik, PC & Newns, DM 1989, ‘Estimate of electron phonon coupling-constant in high-Tc superconductors’, Physica C, vol. 157, no. 1, pp. 13–15.

01Intro_(1-42)4.indd 39

10/23/09 8:22:50 PM

40


Piekarz, P, Konior, J & Jefferson, JH 1999, ‘Electron-phonon interaction in the cuprates: Breathing versus buckling mode’, Physical Review B, vol. 59, no. 22, pp. 14 697–14 701. Putilin, SN, Antipov, EV, Chmaissem, O & Marezio, M 1993, ‘Superconductivity at 94-K in HgBa2CuO4+delta’, Nature, vol. 362, no. 6417, pp. 226–228. Reznik, D, Pintschovius, L, Ito, M, Iikubo, S, Sato, M, Goka, H, Fujita, M, Yamada, K, Gu, GD & Tranquada, JM 2006, ‘Electron-phonon coupling reflecting dynamic charge inhomogeneity in copper oxide superconductors’, Nature, vol. 440, no. 7088, pp. 1170–1173. Schilling, A, Cantoni, M, Guo, JD & Ott, HR 1993, ‘Superconductivity above 130-K in the Hg-Ba-Ca-Cu-O system’, Nature, vol. 363, no. 6424, pp. 56–58. Sheng, ZZ & Hermann, AM 1988a, ‘Bulk superconductivity at 120-K in the Tl-Ca Ba-Cu-O System’, Nature, vol. 332, no. 6160, pp. 138–139. Sheng, ZZ & Hermann, AM 1988b, ‘Superconductivity in the rare-earth-free Tl-Ba-Cu-O system above liquid-nitrogen temperature’, Nature, vol. 332, no. 6159, pp. 55–58. Sheng, ZZ, Hermann, AM, Vier, DC, Schultz, S, Oseroff, SB, George, DJ & Hazen, RM 1988, ‘Superconductivity in the Tl-Sr-Ca-Cu-O system’, Physical Review B, vol. 38, no. 10, pp. 7074– 7076. Shimada, D & Tsuda, N 2002, ‘On the isotope effect in T-c for cuprate superconductors’, Physica C-Superconductivity and Its Applications, vol. 371, no. 1, pp. 52–56. Skinta, JA, Kim, MS, Lemberger, TR, Greibe, T & Naito, M 2003, ‘Superconducting density of states from the magnetic penetration depth of electron-doped cuprates La2-xCexCuO4-y and Pr2-xCexCuO4-y’, Journal of Low Temperature Physics, vol. 131, no. 3–4, pp. 359–368. Sonin, EB 2002, ‘Comment on “Ferromagnetic film on a superconducting substrate’’, Physical Review B, vol. 66, no. 13, pp. 36 501. Szczesniak, R, Mierzejewski, M, Zielinski, J & Entel, P 2001, ‘Modification of the isotope effect by the van Hove singularity of electrons on a two-dimensional lattice’, Solid State Communications, vol. 117, no. 6, pp. 369–371. Tang, YQ, Sheng, ZZ, Chen, ZY, Li, YF & Pederson, DO 1992, ‘New 100 K TlSr2(Ca,Cr)Cu2O7 superconducting films’, Applied Physics Letters, vol. 60, no. 24, pp. 3057–3059. Vozmediano, MAH, Gonzalez, J, Guinea, F, Alvarez, JV & Valenzuela, B 2002, ‘Properties of electrons near a Van Hove singularity’, Journal of Physics and Chemistry of Solids, vol. 63, no. 12, pp. 2295–2297. Wu, MK, Ashburn, JR, Torng, CJ, Hor, PH, Meng, RL, Gao, L, Huang, ZJ, Wang, YQ & Chu, CW 1987, ‘Superconductivity at 93-K in a new mixed-phase Y-Ba-Cu-O compound system at ambient pressure’, Physical Review Letters, vol. 58, no. 9, pp. 908–910. Yahya, AK & Abd-Shukor, R 1999, ‘Ultrasonic velocity measurements on superconducting and nonsuperconducting TlSr2(Ca1-xPrx)Cu2O7-d’, Physica C-Superconductivity and its Applications, vol. 314, no. 1–2, pp. 117–124.

01Intro_(1-42)4.indd 40

10/23/09 8:22:50 PM


41

Yahya, AK, Abdullah, WF, Imad, H, Jumali, MH 2007, ‘Changes in doping state of (Tl, Pb)Sr1212 superconductors with Yb substitution at Sr site’, Physica C: Superconductivity and its Applications, vol. 463–465 (Suppl.), pp. 474–477. Yahaya, M 1997. ‘Teknologi filem nipis: penggunaan dan cabaran pada industri menjelang abad ke-21’, di Syarahan Perdana, Bangi: Penerbit Universiti Kebangsaan Malaysia. Zhang, QM, Shao, HM, Huang, YN, Shen, HM & Wang, YN 1997, ‘Internal friction and lattice anomalies of single-phase Hg-1223’, Physica Status Solidi a-Applied Research, vol. 160, no. 1, pp. 61–65.

01Intro_(1-42)4.indd 41

10/23/09 8:22:50 PM

01Intro_(1-42)4.indd 42

10/23/09 8:22:50 PM