Bulg. J. Phys.
41 (2014) 305–314
M.A. Malik, B.A. Malik
Department of Physics, University of Kashmir, Hazratbal 190 006, Srinagar,
India
Received 29 December 2014
Abstract.
The immense potential of high temperature superconductivity for technological use has kept the subject alive ever since its discovery. Though the mechanism of high temperature superconductivity still remains elusive, a lot of progress has been made resulting in a major reduction in the number of proposed mechanisms under consideration. At present, electron phonon interaction or spin fluctuations are considered to be central to the mechanism of superconductivity. While the transition temperature has not been improved over the last few years and room temperature superconductivity may not be achieved in near future, many technological applications are in use and being constantly improved. New materials with a better promise are also being discovered. This paper attempts to look back at the progress made so far on all fronts and elucidate the challenges of understanding the mechanism and achieving room temperature superconductivity.
PACS codes: 74.72.-h, 74.72.-b
1 Introduction
Superconductivity is a phenomenon exhibited by certain materials by virtue of which they lose all their electric resistance below certain sufficiently low temperature accompanied by a total expulsion of magnetic field from within. These two properties (zero resistance and perfect diamagnetism) straight away imply revolutionary applications of these materials ranging from loss-less transmission lines, more efficient energy production, computers with reduced size and power consumption, very strong magnets, levitating trains to medical diagnostics etc.
However, for a very long time after the discovery [1], the practical applications could not be realized mainly because superconductivity appeared to be a very low temperature phenomenon and the materials had to be cooled to below liquid
Helium temperatures which is expensive as well as difficult to handle.
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M.A. Malik, B.A. Malik
This changed with the remarkable breakthrough of the 1986 discovery of superconductivity by Bednorz and Muller in La-Ba-Cu-O system showing superconductivity up to a transition temperature ( T c
) of around 35K [2]. Soon the transition temperature, well above liquid Nitrogen were achieved [3] that ushered an era of high temperature superconductivity. At some stage it appeared that room temperature superconductivity may not be a distant dream though at present the record high transition temperature is 164K [4] well above the liquid Nitrogen temperature. Liquid Nitrogen being cheap, plentiful as a coolant opened up the possibilities of practical applications of superconductors. Currently, superconductors are used in a variety of electronic applications, medical diagnostics, construction of superconducting magnets etc.
Though eleven physicists have received 6 Nobel Prizes for their contributions to superconductivity over the past century, experimental techniques have improved dramatically and nearly 200,000 [5] papers have been published in this field, physicists do not have complete theoretical explanation for high temperature superconductivity. The mechanism of high temperature superconductivity continues to be one of the most fascinating and challenging topics in condensed-matter physics.
2 Materials
Since the discovery of superconductivity in the cuprate family La-Ba-Cu-O ( T c upto 35 K), hundreds of cuprate oxide superconductors have been synthesized with varying advantages. An exhaustive description of almost all High T c superconducting cuprate families is given in [6, 7]. We shall here mention only some of these cuprate families in order of increasing transition temperatures. Soon after the discovery of Bednorz and Muller, Wu et al. [3] discovered superconductivity in Y-Ba-Cu-O system with transition temperature well above the liquid nitrogen temperature ( T c
= 93 K in YBa
2
Cu
3
O
7 − x
, x = 0 .
07 ). Later, superconductivity was discovered in the Bi-Sr-Ca-Cu-O system at a transition temperature of above 105 K [8]. In the same year, Thalium based cuprates were found to show superconductivity at 120 K [9]. Further, it followed that T
C enhanced by increasing the number of conducting CuO
2 could be layers. Another class of the Carbon based families-the oxycarbonates could show superconductivity upto a T c of 117 K [10]. The critical temperature reached a value of 133 K in Mercury based cuprates [11]. The transition temperature could further be increased by applying high pressure. At present the record high transition temperature is 164 K in Mercury based cuprates under high pressure which is within a factor of two of room temperature. The unprecedentedly high transition temperatures enabled the use of a cheap and easily available coolant – the liquid Nitrogen. Classification and reported optimized T c in [6].
values of cuprate HTS compounds may be found
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High Temperature Superconductivity: Materials, Mechanism and Applications
One of the most important characteristics of these High T c cuprates is the presence of CuO
2 layers which determine most of their properties. They exhibit metallic behavior which is highly anisotropic showing conductivity mainly in the CuO
2 planes. Perpendicular to the planes, the conductivity is much smaller.
Further, the phase diagram of the high-Tc cuprate superconductors is highly complex, exhibiting regions of antiferromagnetism, semiconduction, superconductivity, strongly correlated electrons and metallic properties when plotted as a function of doping. Understanding the phase diagram, crystal chemistry and stability of these materials is still in progress.
Though there has been no report of an increase in T c in high temperature cuprate oxides for the last decade, significant progress has been reported for non-cuprate oxide superconductors. Alkali ion doped C
60
52 K [12]. MgB
2 was found to superconduct at showed superconductivity at 39 K [13]. Superconductivity is well explained by the BCS theory in both these materials. The latest addition being the discovery of superconductivity in iron-based materials LaFePo with T c
∼ 4 K started in 2006 [14] during a systematic study on electromagnetic functionalities in transition-metal oxypnictides with a ZrCuSiAs type structure and related pnictides [15, 16]. Because of very small critical temperature the discovery did not attract too much attention. However, the real breakthrough occurred in 2008, when Hosono and co-workers reported the discovery of 26 K superconductivity in fluorine-doped LaFeAsO [17] marking the beginning of worldwide efforts to investigate this new family of superconductors. Immediately after Takahashi et al. [18] reported an increase in T c
LaFeAsO
0 .
89
F
0 .
11
(to 43 K) for under high pressure, two groups in China [19–21] reported a high est T c
T c
(55 K) for SmFeAsO
1 − x
F x under an ambient pressure. The high-
(55–56 K) in non-cuprate superconductors are observed for some FeSCs such as SmFeAsO
0 .
9
F
0 .
1
[21], SmFeAsO
0 .
85
[22], SmFeAsO
0 .
8
H
0 .
2
[23] and
Gd
0 .
8
Th
0 .
2
SmFeAsO [24]. Wang et al. [25] have used interface engineering method in single unit cell FeSe films on SrTiO
3 to push the transition temperature to above 50 K. There are various reasons that make FeSC so special. First, they promise interesting physics that stems from the coexistence of superconductivity and magnetism. Second, they provide a much wider variety of compounds for research and, with their multi-band electronic structure, they offer the hope of finally discovering the mechanism of high-temperature superconductivity and finding the way to increase T c
. Lastly, the FeSC are very promising materials for technological applications.
Because elemental iron is strongly magnetic, the discovery of Fe-based superconductors (FeSCs) with highT c was completely unexpected. Owing to the unique properties of FeSc and distinct differences from cuprates and conventional superconductors, today research on F eSC is in the mainstream of condensed matter physics [33, 39, 40]. Many families of FeSCs with different structures and compositions but all sharing a common iron-pnictogen (P, As) or ironchalcogen plane (Se, Te) are already known [26–29, 31–33]. These materials
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M.A. Malik, B.A. Malik have more complex structure than cuprates and superconduct at much lower temperatures but provide valuable information about the underlying mechanism of high temperature superconductivity.
One must bear in mind that superconductors have been discovered by serendipity, not by prediction from fundamental principles. We need to find new superconducting materials with higher transition temperatures and with lower intrinsic anisotropies in order to raise their current-carrying ability. The ultimate materials challenge is to find a room-temperature superconductor. Possibility of room temperature superconductivity has been debated even before the discovery of high temperature superconductivity. As early as in 1964 Ginzburg [34] and Little [35] theortically predicted the possibility of superconductivity well above the room temperature by invoking a non-phonon mechanism. In a recent paper, Pines [36] has indicated that spin fluctuations may lead to room temperature superconductivity as there appears to be no magnetic ceiling that would prevent it. A team of researchers at MIT have reported the existence of charge density waves that would explain the phenomenon of high T c superconductivity and hence lead to room temperature superconductivity. Kawashima [37] suggest that room temperature superconductor may be obtained by bringing alkanes into contact with a graphite surface. Room temperature superconductivity in substitutionally doped graphene via a combined mechanism involving phononic and electronic processes has been reported by Sinha and Jindal [38]. However, as of now, there is no confirmation of a reproducible room temperature superconductivity and the only materials with T c above the liquid Nitrogen temperature are still the copper oxide superconductors.
3 Mechanism
The properties exhibited by conventional superconductors were very well explained by the BCS theory [41]. The BCS theory envisages an attractive interaction between electrons mediated by phonons resulting in the formation of the so called Cooper pairs. The electrons in these pair states are no longer required to obey the Fermi-Dirac statistics. The theory was very successful, making many predictions that were quickly confirmed by experiment. For most of the conventional superconductors this theory satisfactorily explains the properties like Isotope Effect, Specific heat Jump, Penetration Depth, the Energy Gap,
Coherence Length and others as summarized in [42, 43]. The BCS theory was further validated by the flux quantization measurement [44] and Josephson effect [45] both of which suggested that supercurrents involve pair of electrons.
However, the theory also implied that the forces binding the Cooper pairs were very feeble, so they would be ripped apart by thermal vibrations at anything other than extremely low temperatures and therefore superconductivity might not occur above 30 K. The discovery of superconductivity in the cuprate oxides, especially above the liquid Nitrogen temperature, openly challenged the BCS
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High Temperature Superconductivity: Materials, Mechanism and Applications theory on many fronts. Many of the properties of these materials like small or no isotope shift, short coherence length, unprecedentedly high transition temperatures, electric and magnetic anisotropies etc., were markedly different from conventional superconductors. These observations were difficult to reconcile with the conventional phonon mediated pairing mechanism and hence BCS theory appeared to be inadequate for high Tc superconductors. Ironically, Bednorz and Muller’s choice of compounds was influenced by BCS theory. The noncuprate high temperature superconductors, however, seem to be more BCS-like.
As an alternative to the BCS theory, almost all elementary excitations of solids have been tried with little success. Among them, Spin fluctuation mechanism
(where pairs are bound because of magnetic interactions between the electrons’ spins) seems promising. Qualitatively, the spin fluctuation mechanism can be put in the following manner: When an electron moves in a high T c superconductor, its spin creates a spin-density wave around it. This spin-density wave in turn causes a nearby electron to fall into the spin depression created by the first electron. Hence a cooper pair is formed in the same manner as in the phonon mechanism except that now the mediators are the spin density waves. The initial hint in support of the spin fluctuation mechanism came from the fact that all of the ferromagnetic superconductors are very close to the border between a ferromagnetic and a nonmagnetic ground state, close to a ferromagnetic quantum critical point at which magnetic fluctuations are maximized. By establishing the link between high energy spin excitations and kinks in the fermionic band dispersions along the nodal directions, and estimation of the spin-fermionic coupling constant, Dahm et al. [46] demonstrated that spin fluctuations have sufficient strength to mediate high T c superconductivity generating d-wave superconducting state with transition temperature exceeding 150 K. NMR measurements by
Hattori et al. [47] have provided strong evidence that ferromagnetic fluctuations provide the pairing force that leads to superconductivity in U CoGe in which ferromagnetism and superconductivity seem to coexist. Recently, Suchitra et al. [48], based on the angle-resolved quantum oscillation measurements in the underdoped copper oxide YBa
2
Cu
3
O
6+ x
, have found that ripples of electrons, known as charge density waves or charge order, create twisted pockets of electrons in these materials from which superconductivity emerges.
Like in cuprates, phonon’s alone appeared to be unlikely candidates as the pairing mediators in FeSCs.
A calculation of electron-phonon coupling in
LaFeAsOF revealed it to be at most a very poor electron-phonon superconductor [49]. However, a recalculation of this quantity results in a potential increase of the coupling constant by up to 50% when the effects of magnetism are included [50].
Liu et al. [51] found an absence of any O isotope effect in SmFeAsO
1 − x
F x
, but did find a significant Fe isotope effect ( α exponent of ∼ 0 .
35 ) on both the magnetic and superconducting transitions in both
SmFeAsO
1 − x
F x and Ba
1 − x
K x
Fe
2
As
2 suggesting that phonons are at least intermediate players. However, there are many indications that combined phonon-
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M.A. Malik, B.A. Malik exciton mechanism could explain the T c values very well as it is believed that the coupling is strong in iron based superconductors. Singh et al. also proposed that bi-exciton may be involved in pairing mechanism [52]. Many theoretical approaches found that strong antiferromagnetic correlation (spin fluctuation) triggers s ± pairing next to the antiferromagnetism. The match between spin fluctuation structure and fermiology might drive superconductivity in at least some of these materials. Therefore, antiferromagnetic spin fluctuations could possibly be the major mediating glue in the iron-based superconductors [53–56].
Experiments that probe the symmetry of the SC phase provide important information about the energy and momentum dependence of Cooper pairing in this new class of high-Tc superconductors. Even though the iron-pnictide family is vast, many experiments performed on different systems or different chemical compositions of the same crystalline system have shown remarkable consistency. For instance, NMR experiments were quick to determine from Knight shift measurements that the SC state spin symmetry is likely singlet, implying an even order parameter symmetry (i.e. s-wave, d-wave, etc). This experiment has been done on the main members of the FeSC family, including samples of the 1111 [57, 58], 122 [59] and 11 [60] systems, so it is reasonable to assume that SC in the iron-pnictides is universally spin-singlet. Lu et al. [61] used inelastic neutron scattering to show that low-energy spin excitations in iron pnictides (BaFe
2 − x
Te x
As
2
) change from fourfold symmetric to twofold symmetric at temperatures corresponding to the onset of the in-plane resistivity anisotropy.
Their observation that the resistivity and spin excitation anisotropies both vanish near optimal superconductivity implies a close connection between the two.
Similarity and differences between copper oxide and iron based superconductors have been discussed in a recent paper on the basis of theoretical and experimental understanding [62]. While the pairing mechanism and non-Fermi liquid behaviors in transport properties may have a common origin between the two systems, the strengths of electron correlation are different: Cuprate is a doped Mott insulator, while iron pnictide is an itinerant system with a weak correlation. Recent studies by Luca et al. [63], based on theory and simulations, reveal that the theoretical explanation for copper and iron superconductors could be the same, and could even apply to other materials, i.e. there could be a unified theory for these superconductors.
While a lot has been learnt in the last 27 years, a theory that completely describes
High Tc superconductivity continues to remain elusive. One reason for this is that the materials in question are generally very complex, multi-layered crystals, making theoretical modelling extremely difficult.
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High Temperature Superconductivity: Materials, Mechanism and Applications
4 Applications
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. These applications include high-speed trains, magnetic energy storage, magnetic resonance imaging (MRI) for medical applications, Josephson devices, Superconducting Quantum Interference Devices (SQUID), Magnetoencephalography, microwave devices and resonators to high energy physics experiments.
SQUIDs deserve a special mention because of their versatility finding applications in magnetic materials characterization, medicine, NMR, MRI, geophysics, quantum computing etc. SQUID magnetometers may be the most sensitive measurement device known. The threshold for SQUID is of the order of 1 fT, making it capable of measuring extremely feeble magnetic fields. In comparison, magnetic field of heart is around 50,000 fT and that of brain is a few fT. Because of their extreme sensitivity, SQUIDs have established themselves as very accurate devices for both Magnetocardiography and Magnetoencephalography. SQUIDs are being considered for a search for the Dark Matter. The idea is that when axions of a given mass/energy enter a microwave cavity sited from a liquidhelium-cooled superconducting solenoid, they will interact with the field and decay into photons. These photons can then be amplified and detected using extremely sensitive SQUIDs. Such experiments are underway in the university of Washington.
The other prospective application of superconductivity is lossless Power Transmission. In India, 22% of electric power is lost in transmission alone. A superconducting wire can transmit a dc current without losses or an ac current with an extremely small loss. In the west, people are contemplating to utilize the benefit of superconductivity in transporting energy on a scale of many gigawatts to terawatts from remote generation facilities (the Sahara desert), which otherwise would be futile owing to huge transmission losses. The fact that HTSC are extreme type II, characterized by very high upper critical field values, opens up the possibility of superconducting Maglev Trains. Superconducting Motors and generators could be made with a weight of about one tenth that of conventional devices for the same output. The basic elements of the world’s first “quantum“ computer are superconducting quantum bits. In the Fermilab, superconducting magnets were successfully used to increase the ultimate particle energy and the operating cost was greatly reduced. Recently, experts from the CERN Superconductors team obtained a world record current of 20 kA at 24 K in an electrical transmission line consisting of two 20-metre long cables made of M gB
2 superconductor. This result makes the technology a viable solution for long-distance power transportation.
One limitation with the technological applications of high temperature cuprate
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M.A. Malik, B.A. Malik oxide superconductors is their inherent low critical current density J c
. There are ways to improve J c by introducing artificial pinning centers.
J c has been improved by a factor of 2-3 by flux pinning [64] but the challenge is to increase it by a factor of around 10. Recently, Kim et al. [65] have been successful in raising the critical current to 1500 A/cm width in SmBCO superconductor. This is the highest value ever reported for a high temperature superconductor.
The relatively recent discovery of superconductivity in iron based superconductors has opened up a new dimension. Owing to their much higher H c than cuprates and high isotropic critical currents [66–68], they are attractive for electrical power and magnetic applications, while the coexistence of magnetism and superconductivity makes them interesting for spintronics [69].
In order to improve the values of superconducting parameters ( J c
, H c and T c
), for better technological applications, many non-superconducting materials are added in the superconducting state. Among these, the addition of BaZrO
3 and
Ag in cuprate superconductors have received prominance recently [70–72]. Our group is currently working on improving J c and H c by adding Ag and BaZrO
3 in cuprate superconductors (YBa
2
Cu
3
O
7
) in both bulk and film form. We are also studying effect of irradiation on these materials for enhancing superconducting parameters due to flux pinning. This tunning of the superconducting parameters for advanced applications is currently dominating the research on technological front. Recently, Nazarova et al. [73] have shown that the addition of Ag in FeSe enhances T c and sharpens the transition. They have also shown that Ag doping improves the magnetoresistance and enhances the upper critical field values as well. In another recent work, Nazarova et al. [74] have reported a manifold increase in the intergranular critical current in Ag doped F eSe besides an improvement in pinning, screening ability and activation energy. This makes
Fe-based superconductors very important for technological applications.
While the technological applications of HTSC are immense, a fringe benefit is that because of their ability to reduce size and weight and in the process save energy, HTSC promises to address the global environmental issues as well.
Acknowledgement
The authors would like to acknowlege the IUAC (UGC) project grant No. UFR-
52307.
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