Transcript SeminarPresentations\Superconductors Josephson Junctions and

SUPERCONDUCTIVITY, JOSEPHSON
JUNCTIONS, AND SQUIDS
NICK HILL
EE 4611
4-20-16
OUTLINE
• Superconductivity:
• Definition
• Properties
• Josephson Junctions:
• Definition
• Properties
• SQUIDs (Superconducting Quantum
Interference Devices)
• Architecture
• Principle of operation
• Uses
SUPERCONDUCTIVITY
Superconductivity is a phenomenon that occurs
when certain materials are cooled below a
critical temperature. This phenomenon is
characterized by a material having zero
electrical resistance and the expulsion of
magnetic fields within the material (Meissner
effect).
Photo from www.wou.edu
MEISSNER EFFECT
• The Meissner effect is a property of
superconductors that when they are placed in a
magnetic field and cooled below their critical
temperature (Tc) that the magnetic field is ejected
from the bulk of the superconductor.
Photo from www.conceptualphysicstoday.com
LONDON PENETRATION DEPTH
• Inside the Superconductor there is
still some magnetic that will rapidly
be attenuated as the depth
increases.
Photo from hyperphysics.phy-astr.gsu.edu
LONDON MOMENT
• The London moment is the property
of a superconductor that as it spins, it
creates a magnetic field around it’s
self, the axis of the magnetic field
also lines up with the axis of rotation
of the sphere.
Photo from einstein.stanford.edu
COOPER PAIRS
• At Low temperatures there is a
state where pairs of electrons
can have a lower energy than
the Fermi-energy. This means
that the pair is bound together.
This allows the electrons to
behave like a superfluid and
creates some of the properties
of superconductors.
Photo from webs.mn.catholic.edu.au
JOSEPHSON JUNCTIONS
•
Josephson junctions are created by having
two superconductors that are separated by
a weak link that can either be created by
an insulator (S-I-S), or a non
superconducting conductor (S-N-S), or a
constriction in the superconducting material
(S-s-S). At this junction the cooper pairs will
tunnel through the junction.
Photo from www.nature.com
DC JOSEPHSON EFFECT
• The DC Josephson effect is when a
DC current is created across the
junction in the absence of an
external magnetic field. It is
proportional to the sine of the phase
shift across the junction and the
frequency of the voltage applied
across the junction.
Photo from www.nsa.gov
AC JOSEPHSON EFFECT
• The AC Josephson effect is when a
DC voltage is applied across the
junction and an AC current will be
developed with a frequency of Ic
and a frequency of
. This
means that a Josephson junction can
act as a voltage to frequency
converter.
Photo from www.advancedlab.org
SUPERCONDUCTING QUANTUM INTERFERENCE
DEVICES (SQUIDS)
• SQUIDs are a type of extremely
sensitive magnetometer that contains
a Josephson junction. They are so
sensitive that they can detect a field
change of 5*10^(-14) gauss
(1/10,000,000,000,000 of the
earths magnetic field) within a few
days.
Photo from www.learner.org
PRINCIPLE OF OPERATION
• DC SQUIDs are a current loop with two Josephson junctions in it that will carry
a current of I/2 in each branch in the absence of an external magnetic field.
In the presence of a field, there is a secondary current produced that
increases the current of one branch and decreases the current of another
branch. This will continue until there is a critical current reached that will then
cause the magnetic field to switch and thus the current to change.
APPLICATIONS FOR SQUIDS
• MRI and MEG (Magnetoencephalography) to take high resolution images of
neurons in the brain.
• Gravity detectors
• Quantum computers
SOURCES
•
Ran, Shannon K’doah (2004). Gravity Probe B:
Exploring Einstein's Universe with
Gyroscopes (PDF).NASA. p. 27.
•
Bhadeshia123. "Superconductivity: SQUIDS."
YouTube. YouTube, 09 Nov. 2014. Web. 18 Apr.
2016.
•
Josephson, Brian. “The Discovery of Tunneling
Supercurrents” Nobel lecture 12 Dec. 1973
nobelprize.org. Web. 18 Apr. 2016.
•
John C. Gallop (1990). SQUIDS, the Josephson
Effects and Superconducting Electronics. CRC
Press. pp. 3, 20. ISBN 0-7503-0051-5.
KEY POINTS
•
Superconductivity is a phenomenon where a material has zero electrical resistance
and will expel any magnetic fields within the material.
•
Josephson junctions are junctions that allow the quantum tunneling of Cooper pairs
•
Josephson Junctions are formed with a type of barrier that could either be an
insulator, normal conductor, or a restriction in a superconductor.
•
SQUIDS are a type of Magnetometer
•
SQUIDS are used in an array of things from MRI to super sensitive gyroscopes.