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ABSTRACT
This paper describes about high temperature superconductors and their applications in
the elite electronic world. It also explains the reasons behind this theory along with recent
breakthroughs in high temperature superconductors.
The discovery of high-temperature superconducting materials in 1986 sparked a dream
of an amazing new electrical & electronics world , a world of loss- free power
transmission from coast to coast, of enormously powerful computers, and of levitated
trains passing in a blur of speed.
High-temperature superconductors are generally considered to be those that demonstrate
superconductivity at or above the temperature of liquid hydrogen, or - 196 °C (77 K),
since this is the most easily attainable cryogenic
temperature. Conventional
superconductors, by contrast, require temperatures no higher than a few degrees above
absolute zero (- 273.15 °C or - 459.67 °F). Though it is extremely cold by everyday
standards, in the field of superconductivity, 77 K is considered high temperature.
Recently, other unconventional superconductors have been discovered. Some of them
also have unusually high values of the critical temperature Tc, and hence they are
sometimes also called high-temperature superconductors, although the record is still held
by a cuprate perovskite material (Tc=138 K, that is - 135 °C) (although slightly higher
transition temperatures have been achieved under pressure).
High-Tc superconductivity is believed to originate from strongly interacting or "paired"
electrons moving through copper oxide layers. A single atom of zinc, a strong scatterer of
electrons, substituted for an atom of copper, which would be the source of any paired
electrons, proved to be an ideal probe for studying the underlying physics of high-Tc
superconductivity.
Superconductors have also been used to make digital circuits (e.g. based on the Rapid
Single Flux Quantum technology) and microwave filters for mobile phone base stations.
Promising future applications include high-performance transformers, power storage
devices, electric power transmission, electric motors (e.g. for vehicle propulsion, as in
Victorians or maglev trains), magnetic levitation devices, and Fault Current Limiters.
However superconductivity is sensitive to moving magnetic fields so applications that
use alternating current (e.g. transformers) will be more difficult to develop than those that
rely upon direct current
Scientist believe that an entire periodic table will have to be put together to make a room
temperature superconductor .Nevertheless it is believed by some researchers that if room
temperature superconductivity is ever achieved it will be in a different family of
materials.
With the devlopment of new oxide superconductors having Tc of 125k or above, there
has been a tremendous excitement in scientific world. This opens a new age of high
temperatures superconducting dervices,which have widespread commercial applications.
INTRODUCTION
One of the most interesting and unusual properties of solids is that certain metals and
alloys exhibit almost zero resistivity when they are cooled to sufficiently low
temperatures .This phenomenon is called superconductivity.
Superconductivity occurs in a wide variety of materials, including simple elements like
tin and aluminium, various metallic alloys, some heavily-doped semiconductors and a
family of cuprate-perovskite ceramic materials known as high- temperature
superconductors. Superconductivity does not occur in noble metals like gold and silver,
nor in most ferromagnetic metals.
In 1986 the discovery of high- temperature superconductors, with critical temperatures in
excess of 90 kelvin, spurred renewed interest and research in superconductivity for
several reasons. As a topic of pure research, these materials represented a new
phenomenon not explained by the current theory. And, because the superconducting state
persists up to more manageable temperatures, more commercial applications are feasible,
especially if materials with even higher critical temperatures could be discovered.
In a class of superconductors known as type II superconductors (including all known
high-temperature superconductors), an extremely small amount of resistivity appears at
temperatures not too far below the nominal superconducting transition when an electrical
current is applied in conjunction with a strong magnetic field (which may be caused by
the electrical current). This is due to the motion of vortices in the electronic superfluid,
which dissipates some of the energy carried by the current.
If the current is sufficiently small, the vortices are stationary, and the resistivity vanishes.
The resistance due to this effect is tiny compared with that of non-superconducting
materials, but must be taken into account in sensitive experiments. However, as the
temperature decreases far enough below the nominal superconducting transition.
Most prominent materials in the high- Tc range are the so-called cuprates, such as
La1.85Ba0.15CuO4, YBCO (Yttrium-Barium-Copper-Oxide) and related substances.All
known high-Tc(critical temperature) superconductors are so-called Type-II
superconductors. A Type-II superconductor allows magnetic field to penerate its interior
in the units of flux quanta, creating 'holes' (or tubes) of normal metallic regions in the
superconducting bulk. This property makes high-Tc superconductors capable of
sustaining much higher magnetic fields.
According to BCS theory, super electrons are responsible for superconductivity. They
exist as copper pairs. They form a bound single system. Their motions are correlated.
Electron-electron interaction via lattice deformation, copper pair formation, flux
quantisation ..etc are underlining principles for high temperature superconductors.
ANALYSIS AND DISCUSSION
The radical idea that high temperature superconductivity and related phenomena occur in
certain materials is because of quantum- mechanical fluctuations in these materials
increase as temperature decreases. Usually such fluctuations, which determine the
properties of all matter in the universe, decrease as temperature decreases.
Varma's theory did not explain the nature of the fluctuations; he accomplished this in a
theory he proposed in 1996, while still at Bell Labs, in which he noted that in copper
oxide materials, also known as cuprates, superconductivity is associated with the
formation of a new state of matter in which electric current loops form spontaneously,
going from copper to oxygen atoms and back to copper. His theory concluded that the
quantum- mechanical fluctuations are the fluctuations of these current loops. Physicists
consider these fluctuations in the current loops to be fluctuations of time.
In a normal conductor, an electrical current may be visualized as a fluid of electrons
moving across a heavy ionic lattice. The electrons are constantly colliding with the ions
in the lattice, and during each collision some of the energy carried by the current is
absorbed by the lattice and converted into heat (which is essentially the vibrational
kinetic energy of the lattice ions.) As a result, the energy carried by the current is
constantly being dissipated. This is the phenomenon of electrical resistance.
The situation is different in a superconductor. In a conventional superconductor, the
electronic fluid cannot be resolved into individual electrons. Instead, it consists of bound
pairs of electrons known as Cooper pairs. This pairing is caused by an attractive force
between electrons from the exchange of phonons. Due to quantum mechanics, the energy
spectrum of this Cooper pair fluid possesses an energy gap, meaning there is a minimum
amount of energy E that must be supplied in order to excite the fluid. Therefore, if E is
larger than the thermal energy of the lattice (given by kT, where k is Boltzmann's constant
and T is the temperature), the fluid will not be scattered by the lattice. The Cooper pair
fluid is thus a superfluid, meaning it can flow without energy dissipation.
(Behavior of heat capacity (cv) and resistivity () at the superconducting phase
transition)
The onset of superconductivity is accompanied by abrupt changes in various physical
properties, which is the hallmark of a phase transition. For example, the electronic heat
capacity is proportional to the temperature in the normal (non-superconducting) regime.
At the superconducting transition, it suffers a discontinuous jump and thereafter ceases to
be linear. At low temperatures, it varies instead as e- a/T for some constant a. (This
exponential behavior is one of the pieces of evidence for the existence of the energy gap.)
The order of the superconducting phase transition was long a matter of debate.
Experiments indicate that the transition is second-order, meaning there is no latent heat.
In the seventies calculations suggested that it may actually be weakly first-order due to
the effect of long-range fluctuations in the electromagnetic field. Only recently it was
shown theoretically with the help of a disorder field theory, in which the vortex lines of
the superconductor play a major role, that the transition is of second order in the type II
and of first order (i.e., latent heat) with in the type I regime, and the two regions are
separated by a tricritical point.
The Meissner effect was explained by London and London, who showed that the
electromagnetic free energy in a superconductor is minimized provided
where H is the magnetic field and is the penetration depth.
This equation, which is known as the London equation, predicts that the magnetic field in
a superconductor decays exponentially from whatever value it possesses at the surface.
The Meissner effect breaks down when the applied magnetic field is too large.
Superconductors can be divided into two classes according to how this breakdown
occurs. In high temperature superconductors, raising the applied field past a critical value
Hc1 leads to a mixed state in which an increasing amount of magnetic flux penetrates the
material, but there remains no resistance to the flow of electrical current as long as the
current is not too large. At a second critical field strength Hc2, superconductivity is
destroyed. The mixed state is actually caused by vortices in the electronic superfluid,
sometimes called fluxons. Most pure elemental superconductors (except niobium,
technetium, vanadium and carbon nanotubes) are Type I, while almost all impure and
compound superconductors are Type II(High temperature superconductors).
BREAKTHROUGHS IN HIGH TEMPERATURE
SUPERCONDUCTORS
Between 1986 and 1994 most advances in the field of superconductivity related to the
discovery of new superconductor "systems" and compounds. In recent years, except for
the discovery of additional elements that will superconduct under extreme high pressure,
superconductor news has been mainly about novel ways to employ the new ceramic
superconductors, innovative fabricating techniques and atypical superconductors
.
NEW INNOVATIONS
Silicon Becomes A Superconductor
Silicon -- the archetypal semiconductor -- has at long last been shown to demonstrate
superconductivity. By substituting 9% of the silicon atoms with boron atoms
Superconducting Qubits May Enable Quantum Computing
Boson-Mediated Electron Pairing Observed in HT Superconductors
Superconductors to Facilitate World's First Artificial Sun
Multi-walled Carbon Nanotubes Superconduct at 12K
DNA Nanowires Exhibit SQUID- like Behavior
Superconductor Makes Nano-Refrigerator Possible
High Tc, Low Toxicity: 115K Superconductivity in Sn-3212-Tm
Larger, non-spherical pure carbon fullerenes that superconduct have recently been
discovered .superconductivity is found in single-walled carbon nanotubes at around 15
Kelvin. Silicon-based fullerides like Na2Ba6Si46 will also superconduct. However, they
are structured as infinite networks, rather than discrete molecules. Fullerenes are
technically part of a larger family of organic conductors,
APPLICATIONS
Superconductors are used to make some of the most powerful electromagnets
known to man, including those used in MRI machines and the beam-steering
magnets used in particle accelerators
Superconductors have also been used to make digital circuits (e.g. based on the
Rapid Single Flux Quantum technology) and microwave filters for mobile phone
base stations.
Superconductors are used to build Josephson junctions which are the building
blocks of SQUIDs (superconducting quantum interference devices), the most
sensitive magnetometers known.
Series of Josephson devices are used to define the SI volt. Depending on the
particular mode of operation, a Josephson junction can be used as photon detector
or as mixer
The large resistance change at the transition from the normal- to the
superconducting state is used to build thermometers in cryogenic micro-
calorimeter photon detectors.
DISADVANTAGES
There are some potential problems when using these higher temperature superconductors.
The material will come out of the furnace much harder and more difficult to re-
grind.
Higher temperatures induce a risk of melting the materials, especially if the
temperature indicator is inaccurate.
Moisture may slowly destroy the superconductivity of the material.
ARE HIGH TEMPERATURE SUPERCONDUCTORS THE
FUTURE
Supercomputers, SQUIDS, electric power transmission, motors, and magnetically
levitated trains are just some of the things superconductors can do; without wasting any
energy.
Are superconductors the future After understanding what superconductivity is and what
it is doing today I am convinced they are the future of us or the generations to come.
Some day superconductors will replace the conductors of electricity we use today.
Superconductors will save billions of dollars for countries of the world, and make life
easier for us all. I believe superconductors are the future.
CONCLUSION
With the devlopment of new oxide superconductors having Tc of 125k or above, there
has been a tremendous excitement in scientific world. This opens a new age of high
temperatures superconducting dervices,which have widespread commercial
applications.Many scientist believe that an entire periodic table will have to be put
together to make a room temperature superconductor.
REFERENCES
Tinkham, Michael (2004). Introduction to Superconductivity, 2nd ed.,
Tipler, Paul; Llewellyn, Ralph (2002). Modern Physics, 4th ed.,
P.K Paliniswamy ,Solid state physics
S.O Pillai , solid state physics
www.superconductorlinks.com