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: 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: T Magnetic field of heart: T Magnetic field of brain: 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 system Arrays 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 magnets

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

Telecommunications 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 magnets An 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 multifilament Nb-Ti of 1mm overall diameter, consisting from  m filaments

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….

High-T c Superconductivity 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