Superconductors

An exciting field of Physics!

General Objective

To understand the nature of superconductivity

Specific Objectives:You will be able to

1. Define Superconductivity

2. State the history of Superconductors

3. List the Properties, Types and Applications of

Superconductors

Prerequisite knowledgeElectrical Conductivity

Thermal conductivity

Conducting materials

1. Low resistive materials (Ex: Al, Cu, Ag)

2. high resistive materials (Ex: Tungsten, Platinum)

3. Zero resistive materials (Ex: Nichrome, Mercury, Tungsten, Platinum and Alloys)

Introduction Before the discovery of superconductivity, it was

thought that the electrical resistance of a conductor

becomes zero only at absolute zero.

But it is found that, in some materials the electrical

resistance becomes zero, when they are cooled to very

low temperature.

Ex: Electrical resistance of pure mercury suddenly drops

to zero when it is cooled below 4.2 Kelvin and becomes

a superconductor.

Superconductivity?

The phenomenon of losing the resistivityabsolutely, when cooled to sufficiently lowtemperature is called superconductivity.

Superconductors are materials that conductelectricity with no resistance.

This means that, unlike the more familiarconductors such as copper or steel, asuperconductor can carry a current indefinitelywithout losing any energy at temperatures nearabsolute zero.

SO - 1

Comparisons of TemperaturesTemperatures F C K

water boils 212.0 100.0 373.2

body temp 98.6 37.0 310.2

room temp 77.0 25.0 298.2

water freezes 32.0 0.0 273.2

mercury freezes -37.8 -38.8 234.4

dry ice -108.4 -78.0 195.2

liquid Oxygen -297.4 -183.0 90.2

liquid Nitrogen -320.8 -196.0 77.2

liquid Helium -452.1 -269.0 4.2

absolute zero -459.7 -273.2 0.0

HISTORY OF SUPERCONDUCTIVITY

1911- Kamerlingh Onnes (Nobel prize in 1913) while studying the

resistance of solid mercury at cryogenic temperatures using the recently-

discovered liquid helium as a refrigerant. At 4.2 K, the resistance abruptly

disappeared.

1913- lead (7 K), 1941 niobium nitride(16 K)

1895 – Helium - William Ramsay in England (Isolation of Helium).

1908 – Liquid Helium (−269 °C) (about 4 K) - H. Kamerlingh Onnes

Res

ista

nce

(Ω

)

4.0 4.1 4.2 4.3 4.4

Temperature (K)

0.15

0.10

0.0Tc

Transition Temperature (or) Critical Temperature (Tc)

The temperature at which a normal conductor loses itsresistivity and becomes a super conductor is known astransition temperature or critical temperature.

Low temperature superconductors ( > Tc ) i.e., lowtransition temperature superconductors.

High temperature superconductors ( < Tc ) i.e., hightransition temperature superconductors.

Superconduction transition is reversible. i.e., abovecritical temperature (Tc ), the superconductor againbecomes normal conductor.

Critical Temperature (Tc)

TEMPERATURE

RESISTIVITY

11

Superconducting elements

•Ferromagnetic elements are not superconducting•The best conductors (Ag, Cu, Au..) are not superconducting •Nb has the highest TC = 9.2K from all the elements

• Electrical resistance,

• Magnetic property,

• Meissner effect,

• Effect of electric current and pressure,

• Isotopic effect

Properties of Superconductors

Properties of Superconductors

1. Electrical Resistance

The electrical resistance of a superconducting material is very less.

It is of the order of 10-5 cm

SO -3

When super conducting materials are subjected to very largevalue of magnetic field, the super conducting property isdestroyed.

Critical magnetic field (Hc ): The minimum magnetic fieldsrequired to destroy the superconducting state is called the

critical magnetic field (Hc

)

H0 – Critical field at 0K

T - Temperature below TC

TC - Transition Temperature

Hc

= Ho[1- (T/Tc)

2]

Superconducting

Normal

T (K) TC

H0

HC

2. Magnetic Property

3. The Meissner Effect (1933 )

Superconductors push out magnetic fields

Act as perfect diamagnets

Magnetic fields does not penetrate the sample

Meissner Effect is reversible

A complete expulsion of all magnetic field by a

superconducting material is called “Meissner effect”

H ≤ HCCurrents i appear, to cancel B. i x B on the

superconductor produces repulsion.

4. Effect of electric current

When a large value of A.C. current is applied to

a super conducting material it induces some

magnetic field in the material and because of

this magnetic field, the super conducting

property of the material is destroyed.

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What destroys superconductivity?

High temperatures:strong thermal vibration of the lattice predominate over the electron-phonon coupling.

Magnetic field: the spins of the C-P will be directed parallel.

(should be antiparallel in C-P)

A current: produces magnetic field which in destroys superconductivity.

Current density

Temperature

Magnetic field

Types of Super Conductors

• There are two types of super conductors based on their variation in magnetisation, due to external magnetic field applied.

1. Type I super conductor (or) Soft super conductor

2. Type II super conductor (or) Hard super conductor

Magnetisation

• The term magnetisation or the intensity ofmagnetisation is the process of converting anon magnetic material into a magneticmaterial.

• The magnetic moment (M=IA) per unit volume(I = M/V).

Type I (Soft) Super Conductor• When the super conductor is

kept in the magnetic field andif the field is increased thesuper conductor becomes anormal conductor abruptly atcritical magnetic field. Thistype of materials are termedas Type I superconductors.

• Below Hc, the specimenexcludes all the magnetic linesof force and exhibits perfectMeissner effect.

• Type I superconductors areperfect diamagnets.

Normal state

Super conductor

Type II (Hard) Super Conductor• When the super conductor is kept in

the magnetic field and if the field isincreased, below the lower critical fieldHc1, the materials exhibits perfectdiamagnetism (super conductor) andabove Hc1, the magnetisation decreasesand hence magnetic flux startspenetrating through the material.

• The material is said to be in a mixedstate between Hc1 and Hc2.

• Above Hc2, it becomes normalconductor.

• The materials which loses its superconducting property gradually due tothe increase in magnetic field are calledtype II super conductors.

SC State

Normal state

TYPE I SUPERCONDUCTORS TYPE I SUPERCONDUCTORS

Sudden loss of magnetisation Gradual loss of magnetisation

Exhibit Meissner Effect Does not exhibit complete Meissner Effect

No mixed state Mixed state present - Gradual transition from Superconducting

state to normal state

One HC = 0.1 tesla , and the value is low Two HCs – HC1 & HC2 (≈30 tesla) value is high

Only one critical field Tc Perfect Diamagnetic and completely follows Meissner effct

below Hc1 Electrically superconductor between Hc1 and Hc2

Super current flows on material surface Super current can flow over the bulk of the material

Soft superconductor, Eg.s – Pb, Sn, Hg Hard superconductor, Eg.s – Nb-Sn, Nb-Ti

Cannot carry large currents Can carry large currents when field is in between Hc1 and Hc2;

Used to generate very high magnetic fields

Can tolerate impurities without affecting

the superconducting properties.

Cannot tolerate impurities, i.e., the impurity affects the

superconducting property

Superconducting

-M

Normal

Mixed

HC1 HCHC2

H

-M

HHC

Superconducting

Normal

Nobel Prizes for superconductivity

Kamerlingh Onnes (1913),

Bardeen, Leon N. Cooper, and J. Robert Schrieffer (1972),

Brian D. Josephson (1973),

Georg Bednorz and Alex K. Muller (1987)

Alexei A. Abrikosov, Vitaly L. Ginzburg, and Anthony J.

Leggett (2003), "for pioneering contributions to the theory of

superconductors and superfluids"

Important Factors to define a Superconducting State

• 1. critical temperature (Tc)

2. critical field (Hc)

3. critical current density (Jc).

BCS theory (1957)

The Origin of Superconductivity

The theory describes superconductivity as a microscopic effect caused

by a condensation of pairs of electrons into a boson-like state.

(Bosons are one of the two fundamental classes of subatomic particles,)

John Bardeen, Leon Cooper and Bob Schrieffer

“ B. C. S.”Nobel Prize in 1972 for their microscopic theory in 1957 nearly 50 years after their discovery by Kamerlingh Onnes!

Describes why

materials are

superconducting

Cooper pair• The two electron interacting attractively in the

phonon field are called cooper pair.

Electrons pairs, called Cooper pairs,

which propagate throughout the lattice

HIGH Tc SUPERCONDUCTORS

Low (Tc) Superconductors High (Tc) Superconductors

Superconductors that require

liquid helium coolant are

called low temperature

superconductors. Liquid

helium temperature is 4.2K

above absolute zero

Superconductors having their

Tc values above the

temperature of liquid nitrogen

(77K) are called the high

temperature superconductors.

Cost Saving and Cost Increase

Applications of Superconductivity

Zero resistance

No energy lost,

Novel uses…

Need refrigeration,

fabrication costs….

• Magnetic levitation,

• SQUIDS

• Cryotron,

Applications of Superconductivity

1. MAGLEV or MAGNETIC LEVITATION

Potential to exceed 6,400 km/h if

deployed in an evacuated tunnel.

Highest recorded speed,

603 km / h, Japan, April 2015

The levitation coils are installed on thesidewalls of the guide way.

When the on board superconductingmagnets pass through the coils, an electriccurrent is induced (electromagnetstemporarily).

As a result, the forces push thesuperconducting magnet upwards and oneswhich pull them upwards simultaneously,thereby levitating the Maglev vehicle.

2. SQUID (Superconductor Quantum Interference Device)

The most sensitive type of detector known to science to measure

very small magnetic fields.

Superconducting Quantum Interference Devices can measure tiny fields – such as those due to currents flowing in your heart muscle

Invented in 1964 by Robert Jaklevic, John Lambe, Arnold

Silver, and James Mercereau of Ford Research Labs

Principle :

Small change in magnetic field, produces variation in the flux quantum.

Construction:

The superconducting quantum interference device (SQUID) consists of two

superconductors separated by thin insulating layers to form two parallel

Josephson junctions.

Types

Two main types of SQUID:

1) RF SQUIDs – has only one Josephson junction

2)DC SQUIDs have two or more junctions.

Thereby,

• more difficult and expensive to produce.

• much more sensitive.

How it worksPhase change due to

external magnetic fieldCurrent flow

Voltage change

Due to B field Due to junctions Must be quantized

3. CRYOTRON

• It is a magnetically operated current switch.

Principle:

• The super conducting property disappears when the magnetic field is grater than critical field (Hc)

PROBLEMS

The transition temperature of Pb is 7.2K. But at 5 K it loses the superconductingproperty If subjected to a magnetic field of 3.3 x104 A/m. Find the maximum value of H which will allow the metal to retain its superconductivity at 0K

Solution: Hc

= Ho[1-(T/Tc)

2]

Ho

= Hc

/ [1-(T/Tc)2] = 3.3 x104 A/m /1-(25/51.28)

Ans: 6.37x104 A/m

The transition temperature of lead is 7.26K. The maximum critical field for the material is 8 x105A/m. Lead has to be used as a superconductor subjected to a magnetic field of 4 x104A/m. What precaution will have to be taken?

T = Tc [1- Hc(T)/ H

c(o)]1/2 = 7.08K

The temp of the metal should be held below 7.08K

The dream - “Tomorrow’s Superconducting World”

350 mph levitated Intercity trains

Underground rapid transit: Heathrow to Gatwick in 10

minutes

Computing: 1000 times fastersupercomputers

Cargo-carrying

submarines,all-electric US Navy

Energy Saving:power lines

electric motorstransformers

Medical Diagnostics:Magnetic Resonance Imaging SQUID:

Brain activity Heart function

Information Technology: much faster, wider band

communications

magnetically launched space shuttle

Some of these dreams are already reality…

Japanese levitating train has superconducting magnets onboard

Superconducting power cable installed in Denmark

SQUID measure-ment of neuro-

magnetic signals

(nuclear) magnetic resonance imaging of the brain, in the field from a superconducting magneth

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Formative Assessment

1. In superconductivity, the electrical resistance of material becomes

• Zero

• Infinite

• Finite

• All of the above

• Zero

2. The superconducting state is perfectly _____ in nature.

• Diamagnetic

• Paramagnetic

• Ferromagnetic

• Ferromagnetic

• Diamagnetic

3. Which of the following are the properties of superconductors?

• They are diamagnetic in nature

• They have zero resistivity

• They have infinite conductivity

• All of the above

• All of the above

4. Superconductivity was first observed by

• 1 : Ohm

• 2 : Ampere

• 3 : H.K. Onnes

• 4 : Schrieffer

• 3 : H.K. Onnes

5. The first successful theory on superconductivity was due to

• 1 : Schrieffer

• 2 : Onnes

• 3 : Ampere and Schrieffer

• 4 : Bardeen Cooper and Schrieffer

• 4 : Bardeen Cooper and Schrieffer

Stimulating question

1. Even though Nb3Sn has produced highmagnetic field than Nb-Ti, why it is not used inthe MRI?

Stimulating question

2. How does a magnetic levitation train stops?

A linear motor (propulsion coils) mounted inthe track. This linear motor operates to propelthe train forward, and when it is necessary tostop the train, the linear motor acts in reverse.

Thank You