Normal electronic conductors have electrical resistance to the motion of electrons whenever a current flows through the material. A voltage must be applied in order to replace this energy lost as heat. A superconductor, however, has no resistance at all. Many metals, but not all, show electrical resistance at ordinary room temperatures but turn superconductive when refrigerated near to absolute zero.
This behaviour of superconductors is exciting today for a variety of commercial applications and in research because the limits of superconductors are a long way from being reached.
. In 1911 superconductivity was first observed in mercury by Dutch physicist Heike Kamerlingh Onnes of Leiden University (shown above). When he cooled it to the temperature of liquid helium, 4 degrees Kelvin (-452F, -269C), its resistance suddenly disappeared. The Kelvin scale represents an "absolute" scale of temperature. Thus, it was necessary for Onnes to come within 4 degrees of the coldest temperature that is theoretically attainable to witness the phenomenon of superconductivity. Later, in 1913, he won a Nobel Prize in physics for his research in this area.
Resistance is classically due to collisions of free electrons with thermally displaced ions with impurities and defects in the metal. This approach can not explain superconductivity as electrons always suffer some collisions so resistance can never be zero. This is put to good use in light bulbs.
The best normal conductors have weak interactions between the electrons and the lattice which is why they are good conductors, but this prevents them from becoming superconductors.
The only way to describe superconductors is to use quantum mechanics. The model used is the BSC theory (named after the 3 men who derived it, Bardeen, Cooper and Schrieffer), which was first suggested in 1957[4]. It states that lattice vibrations play an imp
Type 1 and Type 2 Superconductors
The first superconductors were of little use in a practical sense, because they could not carry a significant amount of current. These are known as type 1 or “soft” superconductors[10]. They require the coldest temperatures (to slow down molecular vibrations sufficiently to allow unimpeded electron flow in accordance with BCS Theory) to become superconductive and exhibit a very sharp transition to a superconducting state and “perfect” diamagnetism.
Diagram 6: Type 1 superconductivity showing a sharp transition
For a type 1 superconductor the critical current is a consequence of the critical magnetic field, Hc[11]. Hc is low in type 1 superconductors along with their critical current densities (important in wire manufacturing) and therefore they have been of little interest to magnet builders or the electric utilities[12].
Type 2 or “hard” superconductors are comprised of metallic compounds and alloys such as “perovskites” (metal oxide ceramics). They achieve higher Tc than type 1 by a mechanism that is still unclear[13]. They differ from type 1 in that their transition from a normal to a superconducting state is gradual across a region of “mixed state” or vortex behaviour. They admit the magnetic field into their interiors while still remaining superconducting. It has been these type 2 superconductors that contemporary scientific and commercial superconducting magnets are wound.
States of Superconductors
In both types of superconductors the electrons combine in pairs under the critical temperature to form macroscopic material waves.
In type 1, the conventional metals and metalloids, the electrons interact with the lattice vibration, whereby both electrons in the Cooper Pair have S= 0 and L=0 (quantum numbers) and can be described by an s-wave function. The wave-function, therefore has the same characteristics along every axis of the lattice.
In type 2 superconductors, the unconventional ceramic compounds, the electron pair processes for an S=0 state; L= 0, 2h, 4h etc. due to quantum mechanic restrictions. The compound always takes on the lowest possible energy and therefore, in most superconductors the (S=0, L=0) state occurs.
7 Paul Brown, Heidelberg University, 2004
High Temperature Superconductors (HTS)
For over 75 years superconductivity remained a low temperature phenomenon, and it was theoretically shown and widely believed that high temperature Superconductivity was impossible, that the highest transition temperature, or critical temperature (Tc), could not go above 30K (according to BCS theory). This changed in 1986, when J.G.Bednorz and K.A.Müller discovered the barium-doped structures of LaCuO4[14], which broke the 30K limit.
With the 30K barrier broken the race was on to find still higher transition temperatures. The first was via strontium substitution: La2-xSrxCuO4 giving a Tc of 38K[15]. It was also found that under extreme pressure the critical temperature could be increased to 50K[16].
The next step was to simulate pressure via chemical substitution. This was done by adding yttrium to the perovskite structure of BaCuO3. Surprisingly the compound (YBa2Cu3O7) went superconducting at 92K[17].
This was important as it put superconductivity in the range of liquid nitrogen and so hundreds of labs joined the race.
It was found that nearly any of the rare earth metals could be substituted for yttrium without any significant change to the transition temperature[18].
The structure of the compound is that of a sandwich with planes of copper oxide in the centre, where the superconducting current flows. The other elements act only as spacers.
The record Tc today is owned by HgBa2Ca2Cu3O8, which by room pressure has a Tc of 135K and under pressure can reach 164K[19]. One of today’s theories predicts an upper limit of 200K for superconductivity, while others predict no limit.
All of these HTSs were brittle ceramic compounds. This is surprising as ceramics are normally insulators. The theory behind this is still not fully understood[20]. This brittleness causes drawbacks in practical applications, such as drawing out wires. Another drawback is the magnetic properties of these materials.
Most HTSs are produced form metastable materials; this means that the thermodynamically stabile reactants are forced into forming the compound either by high pressure and temperature or by doping. This method of synthesis, however, does not represent in any way an absolute criterion for HTS synthesis.
Type II superconductors are, for the most part, comprised of metallic compounds and alloys. This class of superconductors generally has a much higher critical temperature than those in Type I. They achieve a higher critical temperature than Type 1 superconductors by a mechanism that is still not completely understood. It is believed that it relates to the planar layering within the crystalline structure. The highest critical temperature reached is currently 138 K. Debates still arise as to whether or not an upper limit exists for a critical temperature to be found.
REVIEW OF LITERATURE:
1)TITLE:Hard superconductivity of a soft metal in the
quantum regime
MUSTAFA M. ÖZER1, JAMES R. THOMPSON1,2 AND HANNO H. WEITERING
Submitted on:27 january 2006
Superconductivity is inevitably suppressed in reduced dimensionality. The thin superconducting wires or films can be before they lose their
superconducting properties have important technological ramifications and go to the
heart of understanding coherence and robustness of the superconducting state in
quantum-confined geometries. Here, we exploit quantum confinement of itinerant
electrons in a soft metal to stabilize superconductors with lateral dimensions of the
order of a few millimeters and vertical dimensions of only a few atomic layers10.These extremely thin superconductors show no indication of defect- or fluctuationdriven suppression of superconductivity and sustain supercurrents of up to 10% of
the depairing current density. The extreme hardness of the critical state is attributed
to quantum trapping of vortices. This study paints a conceptually appealing, elegant
picture of a model nanoscale superconductor with calculable critical state properties.
It indicates the intriguing possibility of exploiting robust superconductivity at the
nanoscale.
| 2)Title: | | Nonlinear diffusion in hard and soft superconductors |
| Authors: | | |
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| Publication Date: | | 09/1994 |
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| Bibliographic Code: | | 1994PhyC..231..147G |
We discuss the diffusion of magnetic flux in a field-cooled (``hard'') superconducting slab in a creep regime in which E ~ |J|σ J. Bryksin and Dorogovtsev recently discussed flux diffusion in a pinningless (``soft'') superconductor in which E ~ |B|J. This problem is closely related to the flux-creep one with σ=1, and provides additional insight into the possible types of behaviour. We list a series of possible long-term asymptotic solutions of a scaling form, which are either analytically exact or accurately calculated. We check numerically that the relevant long-term solution is approached after various initial conditions. Amongst other conclusions we find S=d(In|M|)/d(Int)-->-1/σ or -1/2σ, after application and removal of an additional field, aJump to main content
3)Limited flux jumps in hard superconductors
R G Mints and A L Rakhmanov
Inst. of High Temperatures, Moscow, USSR
Limited flux jumps in superconductors are investigated under the conditions when the heating of the sample is not too high. The surface temperature rise, electric field and magnetic flux change associated with the instability development are calculated. The theory is compared with experiments, and a satisfactory agreement is found.
Print publication: Issue 12 (14 December 1983)
4)Magnetic instabilities in hard superconductors
R G Mints and Aleksandr L Rakhmanov
The magnetic instabilities in hard and combined Type II superconductors in detail give the criteria for stability of the critical state with respect to magnetic-flux jump.Then the total effect of magnetic and thermal diffusion, as well as that of the structure of a combined superconductor, on the magnetic-field value for a flux jump. The theoretical results will be compared with the existing experimental data.
Print publication: Issue 3 (1977)
Superposition of currents in hard superconductors placed into crossed AC and DC magnetic fields
FISHER L. M. (1) ; KALINOV A. V. (1) ; VOLOSHIN I. F. (1) ; BALTAGA I. V. ; IL'ENKO K. V. ; YAMPOL'SKII V. A. ;
Publishing year:1996
The superposition of currents in YBCO melt-textured samples placed into crossed ac and dc magnetic fields is predicted and observed. This superposition is a direct consequence of the critical state model. The dc magnetic field distribution is shown to become uniform wherever the ac field has penetrated. Owing to this nonlinear process, the area of the dc magnetization loop diminishes and eventually disappears completely with an increase of the ac field magnitude. This means that under the action of the external ac field, the static magnetic properties of hard superconductors change and tend to the well known properties of soft ones.
SUMMARY:
Superconductors conduct electricity with little or no resistance. Organic superconductors contain carbon and are less dense than their ceramic or metallic counterparts; they also offer unusual potential for fine-tuning of electrical properties. Argonne National Laboratory long has carried out the major U.S. effort to synthesize and identify organic superconductors. Nearly 100 new superconductors of this type have been produced, with critical temperatures (at which a superconductor loses all electrical resistance) as high as -260 degrees C, or -434 degrees F. Recently, the first superconductor composed entirely of organic components (with no metal atoms) was synthesized, with a transition temperature in this range. Although this remains far lower than the highest known transition temperature for ceramics, scientists still expect that a high-temperature organic superconductor may be possible, such that liquid nitrogen (at -196 degrees C, or -321 degrees F) could be used as the coolant instead of the more costly liquid helium, thus making practical applications more feasible. The new compound has a two-dimensional, layered structure, which may provide significant insight into the nature of superconductivity.
These advances will help scientists develop a theory of how organic superconductors work and contribute to the design of new materials with higher transition temperatures. The all-organic material is ideal for studies of magnetic and charge transport properties because there is no possibility of contamination from metallic impurities.
APPLICATIONS:
Superconductivity already has important applications, such as medical diagnostic equipment, and many more uses are possible if transition temperatures are high enough. The availability of purely organic superconductors greatly expands the possibilities, especially for applications in which weight is a factor
Superconducting high speed train system comprising a rail including at least one elongated hard superconducting member disposed horizontally along the running direction of the train and having a hollow or gap portion extending in the elongated direction, and a train body including a superconducting magnet for generating a magnetic field perpendicular to the hard superconducting member, thereby floating the body from the rail by the magnetic force acting between the superconducting magnet and the hard superconducting member.
LIMITATIONS:
Limitations on performance of Superconductor oversampling ADCs
For the development and optimization of superconductor oversampling modulators, We highlight the importance of specially engineered and parasitic components of the feedback loop. In particular, LR circuits operating as low-pass filters are capable of providing a noticeable SNR improvement and dramatically reducing the dynamic range requirements for used SFQ comparators. On the other hand, the feedback loop delay and time-jitter in timing circuits are able to spoil the potentially extremely high performance of superconductor oversampling ADCs. We also developed a simple formula describing time-jitter in superconductor
BIBLOGRAPHY:
| 1): |
2)www.iop.org/EJ/abstract/0022-3727/16/12/026
3)http://www.freepatentsonline.com
4)www.sciencedirect.com/science
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