Brian D. Josephson

The Discovery of Tunnelling Supercurrents

The Nobel Prize in Physics 1973

Josephson Effect (see also hand-out)

In 1962 Josephson predicted Cooper-pairs can tunnel through a weak link at zero voltage difference. Current in junction (called Josephson junction – Jj) is then equal to:

21sin cJJ

Electrical current flows between two SC materials - even when they are separated by a non-SC or insulator. Electrons "tunnel" through this non-SC region, and SC current flows.

The SQUID may be configured as a magnetometer to detect incredibly small magnetic fields - small enough to measure the magnetic fields in living organisms.

Threshold for SQUID: 10-14 TMagnetic field of heart: 10-10 TMagnetic field of brain: 10-13 T

• Many uses in everyday life

•Making measurements using SQUIDs

(magnetic susceptibility, static nuclear susceptibility, Nuclear Magnetic resonance...)

• Biomagnetism

(magnetoencephalography [MEG], magnetocardiogram)

• Scanning SQUID microscopy

• Geophysical applications of SQUID

(oil prospecting, earthquake prediction, geothermal energy surveying)

• Higher Temperature SQUIDs

(nondestructive testing of materials...)

JJ’s essential in Superconducting Interference Devices

Georg Bednorz and Alex Muller

received theNobel Prize

1987 for discovery of the

first of the copper-oxide

superconductors

this is how it was announced

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Possible High Tc superconductivity in the Ba – La – Cu – O

system

resis

tivit

y (

c

m)

Temperature ( K )

35 K

10 K

Their sample at first became more resistive as it cooled!

At 35 K, when the sample was5000 x more resistive than copper,

the resistance began to fall …

Only by 10 K had the resistance fallen to (possibly)

zero !

High-Tc Superconductivity

Alex Müller and Georg Bednorz

Paul Chu

164 K

93 K !

77 K liquid

nitrogen

1910 1930 1950 1970 1975 1980 19901985 20001995 2005

HgPb Nb

NbO

NbNV3Sn

NbAlSi

Nb3Sn

Ba(Pb,Bi)O3

organic materials

NbGe3Ba(K,Bi)O3

Doped buckyballsA3C60

MgB2 39K

Liquid N2

Liquid He0 -

10 -

70 -

30 -20 -

120 -

60 -50 -40 -

110 -

90 -100 -

80 -

130 -

140 -

150 - Temperature (K)

conventional superconductors

(La,Ba)CuOBednorz and Muller

TlBaCaCuO

Bi2Sr2Ca2Cu3Ox

HgBaCaCuO

pressure

~ 155K

“High temperature” “cuprate”

superconductorsYBa2Cu3O7-d

pressure

The crystal structures of High-Tc superconducting materials all have

copper-oxide CuO2 layers

Is a room-temperature superconductor out there

waiting to be discovered ??

Have we reached the maximum

possible Tc in this class of materials?

We are still not sure exactly why this is important!

General features of cuprate superconductorsCu-O sheets (with square pyramidal or

octahedral coordination)Charge reservoirs in the form of Cu-O

chains or TlO(BiO) layersSuperconducting cuprates have AFM parent

members (La2CuO4, YBa2Cu3O6, Bi2CaSr2LnO8)

Anisotropic properties (e.g. ab<c)Hole superconductors (residing on

oxygen):Tc is maximum at a critical hole concentration

Typical values of important parameters (HTSC)Hc1 & Hc2 (parallel to c-axis): 1 T & 120T: ~1400 Å: 10-30 Å in ab plane

~ 3 Å in perpendicular planeJc: ~104 amp./cm2 (bulk)

106-107 amp./cm2 (thin films)

The dream - “Tomorrow’s Superconducting World”

350 mph levitated Intercity

trains

Underground rapid transit: Heathrow to

Gatwick in 10 minutes

Computing: 1000 times faster supercomputers

Cargo-carrying

submarines,

all-electric US Navy

Energy Saving:

power lineselectric motors

transformers

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 magnet

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Transmission Lines

• 15% of generated electricity is dissipated in transmission lines

• Potential 100-fold increase in capacity

• BNL Prototype: 1000 MW transported in a diameter of 40 cm

Pirelli Cables & Systems

Superconducting magnetsAn electrical current in a wire creates a magnetic field around a wire. The

strength of the magnetic field increases as the current in a wire increases.

Because SCs are able to carry large currents without loss of energy, they are

well suited for making strong magnets. When a SC is cooled below its Tc and a

magnetic field is increased around it, the magnetic field remains around the SC.

If the magnetic field is increased to a critical value Hc the SC will turn normal.

• Support a very high current density with a very small resistance• A magnet can be operated for days or even months at nearly constant field

A typical Nb3Sn SC magnet.It produces 10.8T with a current of 146A. Bore diameter is 3.8 cm.

Cross-section of multifilamentNb-Ti of 1mm overall diameter,consisting from 13255 5-mfilaments

Other Uses of Superconductivity

• Fault current limiters

• Electric motors

• Electric generators

• Petaflop computers (thousand trillion floating point operations

per second)

Applications of Superconductivity

Trade off between:

Cost Saving and Cost Increase

Zero resistance, no energy lost, novel

uses…

Need refrigeration, fabrication costs….

John Bardeen, Leon Cooper and Bob Schrieffer

“ B. C. S.”Nobel Prize 1972 for their theory of 1957 which explained conventional superconductors: nearly 50 years after their

discovery by Kamerlingh Onnes!

Who knows tomorrowB may stand for Bhavya, Bikramjit, Bhaskar,.....

C may stand for (unfortunately we don’t have anybody in our class!)

S may stand for Samaya, Swagnik, Surabhi, Sherya, Shamik, Sourav

(there are many contenders!!)

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