Superconductivity, Josephson Junctions, and squids Nick Hill EE 4611 4-20-16

Outline Superconductivity: Josephson Junctions: Definition Properties Josephson Junctions: 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.