Superconductor material. Supervision: dr.issam rashid . Done by : hend zaid . What is a Superconductor?? Superconductor :An element inter-metallic alloy, or compound that will conduct electricity without resistance below a certain temperature called critical temperature (T c ).
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Done by :hendzaid.
What is a Superconductor??
Superconductor:An element inter-metallic alloy, or compound that will conduct electricity without resistance below a certain temperature called critical temperature (Tc).
Resistance is undesirable because it produces losses in the energy flowing through the material.
3) In 1935, Fritz and Heinz London showed that the Meissner effect was a consequence of the minimization of the electromagnetic free energy carried by superconducting current.
4) In 1957 ,John Bardeen, Leon Cooper, and John Schrieffer put a theory ( became know as the BCS theory ) and they own a Nobel Prize in 1972. The mathematically-complex BCS theory explained superconductivity at temperatures close to absolute zero for elements and simple alloys.
5) In 1962 Brian D. Josephson predicted that electrical current would flow between 2 superconducting materials - even when they are separated by a non-superconductor or insulator. Nobel Prize in 1973.
Vortices (for type 2)
Under a small magnetic field (with a weak magnet or a magnet far from the sample), type2 react like type I superconductors and completely expel the magnetic field. But when the magnetic field is stronger, they prefer to adopt a compromise situation and allow some of the magnetic field to penetrate along “vortices”.
These vortices are like tubes that go through the sample from side to side and in which the material is no longer superconducting since the magnetic field can penetrate it. The material then becomes a sieve. In order to enable this magnetic field to go through the vortex, the material develops superconducting currents circulating around this pillar in a spiral motion justifying the name, “vortex”.
Resistance In a superconductor
below a temperature called the “critical temperature”, the electric resistance very suddenly falls to zero,the material conducts current perfectly. This is incomprehensible because the flaws and vibrations of the atoms should cause resistance in the material when the electrons flow through it. However, in a superconductor, the electric resistance is equal to zero although the flaws and vibrations still exist.!!!!
Explanation : the electrons form a quantum collective state that is not sensitive to collisions anymore. The electrons are not slowed,
and the electric
a) The black arrows represent the external magnetic field applied to the superconducting sample.b) Currents appear on the surface of the superconductor (in red). These currents create a magnetic field (the external magnetic field is not represented).c) The total magnetic field is the sum of the applied magnetic field and the magnetic field that appears as a result of the reaction. In the superconductor, as a whole, the magnetic field is equal to zero.
Occurrence of Superconductivity
critical current density (Jc): It corresponds to the maximum current that can go through a superconductor. If the current exceeds this value, the material becomes a normal metal again and starts resisting and heating like any other metal. More often, physicists use the term “current density” (jc), that is the current divided by the cross section of the electric wire. For instance, in a cuprate, the current density is more than 10 000 amperes per cm2.
4) The penetration length λ:
When a magnetic field is applied to a superconductor, it is
expelled and can only penetrate a short depth of the surface of
the sample, measured as the penetration length. It is usually very
small, from 10 to 100 nanometres.
5) The gap:
The superconductor is characterized by its wave function formed from the Cooper pairs. This wave function has an energy called “gap”: it corresponds to the minimum energy needed to break one of the Cooper pairs.
“Pauli exclusion principle”, only allows the existence of such a condensate if the waves which compose it are carried by particles called bosons. Unfortunately, electrons. are fermions, not bosons. !!!!!
The solution: in order to form a condensate, electrons must form pairs first. Indeed, a pair of electrons can form a boson. In conventional superconductors, the creation of electron pairs and the formation of the condensate happen instantaneously.
The Cooper pairs will move at the same speed,
the electrons of the pairs
still moving at
This proposed equations are consistent with the Meissner effect and can be used with Maxwell's equations to predict how the magnetic field and surface current vary with distance from the surface of a superconductor.In order to account for the Meissner effect, the London brothers proposed that in a superconductor, Maxwell's equations is replaced by equ. 1 & 2 :
V=RI = - = = 0 B = constant.
= - B ….. equ. 1
Mercury is the metal in which superconductivity was first observed. It had to be cooled to -269°C, really close to absolute zero. The best superconductor we have yet discovered becomes superconducting below -135°C. The physics of superconductors is hence closely linked to the study of very low temperatures accessible with specific cooling techniques, using cryogenic liquids such as liquid helium or nitrogen, and even more sophisticated tools.
In order to reach very low temperatures – a branch of physics called “cryogenics” – we use machines that pump the heat out of an object in order to cool it.
liquid oxygen (90.2 K)
liquid nitrogen (77.4K)
liquid hydrogen (20.3 K)
liquid helium (4.2 K)
Nanophysics corresponds to physics at the scale of the nanometer, a billionth of a meter, or a hundred thousandth of the width of a hair. It corresponds to the size of about ten atoms.
Observing and manipulating superconductivity at the scale of the nanometer is one of the issues of modern research, not only to understand superconductivity better, but also to make non-superconducting materials superconducting, or to invent new electronic components with very strange properties.
Applications of superconductivity
Power cable made of superconductors;
Maglev, the fastest train in the world, uses superconductors.
A SQUID circuit made of superconductors
Any question please??