superconductors and nanotechnology
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1/23/2017 1Dr A K Mishra, Academic Coordinator, JIT Jahangirabad
Engineering Physics II Unit V
Jahangirabad Institute of Technology, Barabanki
Superconductors In 1911 superconductivity was first observed in mercury by
Dutch physicist Heike Kamerlingh Onnes of Leiden University . When he cooled it to the temperature of liquid helium, 4
degrees Kelvin (-452F, -269C), its resistance suddenly disappeared.
Later, in 1913, he won a Nobel Prize in physics for his research in this area.
Superconductors, materials that have no resistance to the flow of electricity, are one of the last great frontiers of scientific discovery
At the critical temprature the resistance falls to zero
1/23/2017 Dr A K Mishra, Academic Coordinator, JIT Jahangirabad 2
These can be used to make low loss power line and very good electromagnets.
1/23/2017 Dr A K Mishra, Academic Coordinator, JIT Jahangirabad 3
4.0 4.1 4.2 4.3 4.40.0
Temperature dependence of resistivity in superconducting materials
Superconductivity in the metal are depend on the intensity of the field and temperature.
to maintain the Superconductivity both the parameter should be less than their critical values.
1/23/2017Dr A K Mishra, Academic Coordinator,
JIT Jahangirabad 4
The dependence of the temperature on magnetic field can be represented by following equation,
Where Hc is the critical field strength, Hc(0) is the maximum field strength at absolute zero and Tc is the critical temperature. it is clear from the fig the temperature below Tc the materials remains in the superconducting state till a corresponding critical magnetic field is applied when the field is higher than the Hc the superconducting state is destroyed and the materials comes to its normal state.
1/23/2017 Dr A K Mishra, Academic Coordinator, JIT Jahangirabad 5
-Hc(0)[1 = HcTT
Meissner Effect (Effect of magnetic field)
When a material makes transition from normal to superconducting state, it excludes magnetic field from its interior.
This phenomenon is called the meissner effect. Meissner and Ochsenfield (1953) concluded that when
superconductor is cooled in longitudinal magnetic field,thenabove its transition temperature magnetic lines pass through specimen but below transition temperature the magnetic flux is pushed out of the specimen,this indicates that below Tc the metal become perfectely diamagnetic.
Thus phenomenon of exclusion of magnetic flux from the interior of a superconductor below Tc is the Meissner effect.
1/23/2017 Dr A K Mishra, Academic Coordinator, JIT Jahangirabad 6
1/23/2017 Dr A K Mishra, Academic Coordinator, JIT Jahangirabad 7
Below Tc a persistence current generates on the surface and cancel the flux density inside the superconductor.Thus for a superconducting state,if B is zero inside the specimen then we get,
1/23/2017 Dr A K Mishra, Academic Coordinator, JIT Jahangirabad 8
tyconductivi is WhereE = J
law Ohms from Nowc.diamagnetiperfect a is state ctingsupercondu hence
metal, cdiamagneti theofcondition theis This
1 - =HM = lity,susceptibi Magnetic
H - = M
M) + (H = 0
M) + (H = B
1/23/2017 Dr A K Mishra, Academic Coordinator, JIT Jahangirabad 9
0 = E i.e., (J)density current infinitefor zero bemust field electric zero, becomes
)(y resistivitequation above fromclear isIt J = E
E1 = J
may write y weresistivit of in termsAgain
1/23/2017 Dr A K Mishra, Academic Coordinator, JIT Jahangirabad 10
ctor.supercondu a of properties essential andt independen two theare smdiamagneti
perfect andy resistivit theHence effect.Meissner the contradict thisTc oft independen isctor supercondu
theinsideflux magnetic theshows This
constant = B 0 = tB
0 = E puttingBy tB - = E X
have weequation, thirdMaxwell fromBut
when an electric current is set in a superconductor it can persist for long time without any e.m.f.
An induced current can flow in a ring of superconducting material by cooling it in the presence of magnetic field below Tc and then remove the field (fig A),the flux outside the ring disappears but it remains inside the ring (fig B).
1/23/2017 Dr A K Mishra, Academic Coordinator, JIT Jahangirabad 11
TYPE OF SUPERCONDUCTORS Superconductors are divided in to two parts depending on the
mechanism from superconducting state to normal state due to exceed of critical field.TYPE I SUPERCONDUCTORS OR SOFT SUPERCONDUCTOR
Type 1 superconductors - characterized as the "soft" superconductors - were discovered first and require the coldest temperatures to become superconductive.
They exhibit a very sharp transition to a superconducting state and "perfect" diamagnetism - the ability to repel a magnetic field completely.
Type 1 superconductors along with the critical transition temperature (known as Tc) below which each superconducts.
1/23/2017 Dr A K Mishra, Academic Coordinator, JIT Jahangirabad 12
Continued.. Surprisingly, copper, silver and gold, three of the best metallic
conductors, do not rank among the superconductive elements.
For type I superconductormagnetic flux is expelledproducing magnetization
with increasing field untilis reached, at which it
falls to zero with a normal conductor.
1/23/2017 Dr A K Mishra, Academic Coordinator, JIT Jahangirabad 13
Applied Magnetic field (H)
TYPE II SUPERCONDUCTORS OR Hard SUPERCONDUCTOR
Except for the elements vanadium, technetium and niobium, the Type 2 category of superconductors is comprised of metallic compounds and alloys.
Type II have two criticalfield ( & )below behave astype I and above behave as normal conductor.
1/23/2017 Dr A K Mishra, Academic Coordinator, JIT Jahangirabad 14
Continued.. This new category of superconductors was identified by L.V.
Shubnikov at the Kharkov Institute of Science and Technology in the Ukraine in 1936(1) when he found two distinct critical magnetic fields (known as Hc1 and Hc2) in PbTl2.
The first of the oxide superconductors was created in 1973 by DuPont researcher Art Sleight when Ba(Pb,Bi)O3 was found to have a Tc of 13K.
Type 2 superconductors - also known as the "hard" superconductors - differ from Type 1 in that their transition from a normal to a superconducting state is gradual across a region of "mixed state" behavior. Since a Type 2 will allow some penetration by an external magnetic field into its surface.
1/23/2017 Dr A K Mishra, Academic Coordinator, JIT Jahangirabad 15
London Penetration depth According to F.London and H.London,the magnetic field at the
surface of a superconductor does not vanish suddenly but decays exponentially to zero according to following equation,
Where H0 is the field at the surface, x is the distance from the surface and is the characteristics length known as London penetration depth.
1/23/2017 Dr A K Mishra, Academic Coordinator, JIT Jahangirabad 16
H0 = H
Continued. The magnetic field at the surface of the superconductor
decays to at a distance in the interior of superconductor.
1/23/2017 Dr A K Mishra, Academic Coordinator, JIT Jahangirabad 17
0 (x) B
BCS THEORY The first widely accepted quantum mechanical explanation of
superconductivity was presented by J.Barden, L N Copper and John Schriffer (BCS) in 1957.
This theory based on the electron-electron interaction via phonon as a mediator.
Under certain restricted conditions, the electron couple together electron-lattice electron interactions.
Therefore, the electron and phonon interaction is the basic mechanism responsible for superconductivity.
1/23/2017 Dr A K Mishra, Academic Coordinator, JIT Jahangirabad 18
Atoms in a crystal lattice are constantly vibrating. Because they are all connected, these vibrating atoms create waves throughout the metal.
These waves are called phonons. The more the atoms are vibrating (ie. the hotter the material),
the larger the phonons. In superconductors (at low temperatures) the phonons are
small, and any distortion caused by the electrons is reflected in phonons.
These phonons can attract electrons to form cooper pairs.
1/23/2017 Dr A K Mishra, Academic Coordinator, JIT Jahangirabad 19
Continued. As shown below the moving electron causes the lattice to
distort and an increa