Superconductivity
Superconductivity
Superconductivity
properties of superconductors.
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3. 4.
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Types of superconductors
DC & AC Josephson effect Applications
L9-10
12-28
APPLIED PHYSICS
CODE : 07A1BS05 I B.TECH CSE, IT, ECE & EEE UNIT-5: CHAPTER-2 NO. OF SLIDES :37
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Lecture-7
SUPERCONDUCTIVITY.
Superconductivity
is a phenomenon occurring in certain materials at extremely low temperatures, characterized by almost zero electrical resistance and the exclusion of the interior magnetic field (the Meissner effect).
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Superconductivity occurs in a wide variety of materials, including simple elements like tin and aluminium, various metallic alloys and some heavily-doped semiconductors. Superconductivity does not occur in noble metals like gold and silver, nor in most ferromagnetic metals.
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Resistivity
O
Tc
TEMP(K)
Critical temperature
The temperature at which the transition from normal state to superconducting state takes place on cooling in the absence of magnetic field is called the critical temperature or the transition temperature
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A magnet levitating above a high-temperature superconductor, cooled with liquid nitrogen. Persistent electric current flows on the surface of the superconductor, acting to exclude the magnetic field of the magnet (the Meissner effect). This current effectively forms an electromagnet that repels the magnet.
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The minimum magnetic field strength required to destroy superconductivity of substance,below Tc is called critical magnetic
field (Hc) at that temperature. Hc = Ho [1-(T/Tc)2].
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Meissner effect.
NORMAL CONDUCTER. SUPERCONDUCTER B
T<Tc
T > Tc
SUPERCONDUCTER EXPELS MAGNETIC LINES OF FORCE.
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Levitation Experiments
Meissner Effect
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Types of Superconductors.
Depending
on the way of transition from superconducting state to normal state by the application of magnetic field, superconductors are classified into Type-I superconductors and Type-II superconductors.
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L Lecture-9 e c t u r e 1
TYPE-I SUPERCONDUCTORS
Superconductors exhibiting complete Meissner effect (perfect diamagnetism) are called Type-I Superconductors. They are also known as soft Superconductors.
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Transition
between normal and superconducting states is sharp and well defined. There is only one value of critical magnetic field Hc.
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Critical
temperatures are low. Hence these are not commercially useful but are useful to understand the exciting phenomenon of superconductivity. Type-I Superconductors are mostly of pure specimens. Examples: Pure specimens of Al, Zn, Hg and Sn.
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TYPE-I SUPERCONDUCTERS
SUPER CONDUCTING STATE
NORMAL STATE.
Hc
RELATION BETWEEN MAGNETIZATION AND APPLIED MAGNETIC FIELD FOR TYPE-I SUPER CONDUCTERS.
TYPE-II SUPERCONDUCTERS
They are developed from alloys, compounds, ceramics, transition metals etc. For any Type2 material, two critical values of applied magnetic field Hc1 and Hc2 can be identified. In between, there is a thermodynamic critical magnetic field Hc corresponding to that of type1 materials.
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The
material behaves as a perfect superconductor in the range 0<H<Hc1. When H>Hc2 the material returns to normal state. Nb and Zr are some examples of this type.
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TYPE-II SUPERCONDUCTERS.
SUPERCONDUCTIONG STATE.
NORMAL STATE.
Hc1
Hc2
Super electrons
According
to London brothers, a superconductor is composed of two distinct type of electrons, i.e., normal electrons and super electrons. super electrons experience no scattering.
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Lecture-10
Penetration depth
According to London equations, the magnetic flux does not drop to zero suddenly at the surface of Type-I superconductors, but decreases exponentially. The depth from the surface at which the magnetic flux density falls to 1/e of its initial value at the surface is called penetration depth.
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BCS theory
According
to BCS theory, superelectrons are responsible for the superconductivity. They exist as Cooper pairs. They form a bound single system. Their motions are correlated.
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Quantum Tunneling
Metal Metal
L e c t u r e 1
Insulater
Quantum Tunneling
Super conducter Metal
Insulater
Vc
L e c t u r e 1
Available States.
QUANTUM TUNNELING
E Ef
E1
Ef
Super Conducter
Insulater
METAL
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Cooper Pairs
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Flux quantization
The
magnetic flux enclosed by a ring is quantized. This concept is known as flux quantization.
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Josephson effect
When
Lecture-11
a thin insulating layer is sandwiched between a metal and a superconductor or two superconductors, electrons can tunnel through the junction. Their wave functions on both sides are highly correlated. This is known as Josephson effect.
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d.c. current flows across the junction of two superconductors separated by a thin insulating layer in the absence of any external electric or magnetic field.
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a.c.Josephson effect
When
d.c. voltage applied across the junction of the two superconductors separated by a thin insulating layer then microwaves are emitted.
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Lecture-12
effect is used to generate microwaves with frequency W = 2eVo/ A.C. Josephson effect is used to define standard volt
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Applications of Superconductors
Lecture-13
1.It is a basis of new generation of energy saving power system. Superconducting generators are smaller in size and less in weight compare with conventional generators. These generators consume very low energy, hence more energy will be saved. 2.All electric power companies are looking forward to the superconducting transmission system that would save most of the energy now being last
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from conventional power lines in the form of useless radiation and heat.
.3.In japan, Superconducting magnets have been used to levitate an experimental train above its track and can drive it at a great speed of 500 Km/h with minimum expenditure of energy. A similar magnetic propulsion system may be used to launch satellites into orbits directly from the earth without the use of rockets.
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4.High efficiency ore-separating machines are built using Super-conducting magnets, which are also used to separate tumour cells from healthy cell by High Gradient Magnetic separation method.
5.Superconducting materials can be used as a memory or storage device in computers, since the current in it can flow without any change in its value with time.
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6.Using Superconducting elements one can build up an extremely fast and large-scale computer in a compact size. The power consumed by this computer will be less than 0.5 watt. 7. The Josephon devices are used to produce microwaves, which are made up of superconductors.
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