superconductors and nanotechnology

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  • 1/23/2017 1Dr A K Mishra, Academic Coordinator, JIT Jahangirabad

    Engineering Physics II Unit V

    Presentation By

    Dr.A.K.Mishra

    Associate Professor

    Jahangirabad Institute of Technology, Barabanki

    Email: akmishra.phy@gmail.com

    Arun.Kumar@jit.edu.in

  • 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

  • Continued..

    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

    Temperature (K)

    Resis

    tanc

    e

    T C

    O

    Temperature (K)

    Resis

    tanc

    e

    4.0 4.1 4.2 4.3 4.40.0

    1.0

    1.5

  • 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

    Normal MetalSuperconductor

    Tc00

  • Continued..

    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

    ]c

    -Hc(0)[1 = HcTT

    2

  • 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

  • Continued

    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,

  • Continued

    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

    0

    0

  • Continued

    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

  • Continued

    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

  • Persistent current

    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)

    Mag

    netiz

    atio

    n (M

    )

    H C

    I Type

    HCM

    agne

    tizat

    ion

    (M)

    H C

    I Type

  • 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

    Mag

    netiz

    atio

    n (M

    )

    H C

    II Type

    HC1 HC2

    HC1 HC2HC1

    HC2

  • 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

    ex

    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

    eB 0

    x

    0 x

    x

    B0

    B

    eB-x

    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

  • Continued.

    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

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