Transcript SUPERCONDUCTIVITY - University of Crete

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:
J  J c sin1  2 
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
JJ’s essential in Superconducting Interference Devices
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 T
Magnetic field of heart: 10-10 T
Magnetic 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...)
Fig.2 Neuromag Ltd.122
sensor array
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. 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.
Conductus Clearsite system
• Because of electrical resistance,
conventional interference filters eat
away part of the signal.
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.
A typical Nb3Sn SC magnet.
It produces 10.8T with a current
of 146A. Bore diameter is 3.8 cm.
• 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
Cross-section of multifilament
Nb-Ti of 1mm overall diameter,
consisting from 13255 5-mm
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-Tc Superconductivity
164 K
Paul Chu
Alex Müller and Georg Bednorz
373 K,BP of water
295 K,room temp
273 K,FP of water
138 K,Highest Tc for HgBaCaCuO
77 K,Liquid Nitrogen
4.2 K,Liquid Helium
K.A. Muller
J. G. Bednorz
The Discovery of superconductivity in ceramic materials
The Nobel Prize in Physics 1987