Josephson effectJosephson effect (see also hand-out)(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.

Brian D. Josephson

The Discovery of Tunnelling Supercurrents

The Nobel Prize in Physics 1973

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

Fig.1 Neuromag Ltd.122 MEG systemArrays of gradiometer dc SQUID detectors are contained within a helmet surrounded by a liquid helium reservoir for cooling

Fig.2 Neuromag Ltd.122 sensor array

Fig. MRI scan of a human scull

Uses of SC magnetsUses of SC magnets

Transmission LinesTransmission 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

TelecommunicationsTelecommunications

• Superconductors are used as efficient filters in cellular telephone towers (now 700 worldwide)

• Separate signals of individual phone calls.

• Because of electrical resistance, conventional interference filters eat away part of the signal.

Conductus Clearsite system

Superconducting magnetsSuperconducting 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 SuperconductivityOther 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….

High-THigh-Tc c SuperconductivitySuperconductivity

Alex Müller and Georg Bednorz

Paul Chu

164 K

295 K,room temp

373 K,BP of water

273 K,FP of water

77 K,Liquid Nitrogen

4.2 K,Liquid Helium

138 K,Highest Tc for HgBaCaCuO

K.A. Muller J. G. Bednorz

The Discovery of superconductivity in ceramic materials

The Nobel Prize in Physics 1987