Super Conductivity Josephsen Junctions And SQUIDS By: Keegan Warner

Outline Superconductors Josephson junctions SQUIDS History New technologies Application Josephson junctions Josephson Effect Josephson Voltage Standard SQUIDS How Squids work DC Squids AC Squids Superconductors are Supercool

Superconductivity Phenomenon of zero resistance in a superconductor when cooled below a certain temperature depending on the material Only some materials can become a superconductors; materials like copper and silver retain some resistance even at absolute zero because of impurities or defects Different than physics idea of perfect conductivity because it includes complete removal magnetic field lines from the superconductor Supercurrent: current flowing infinitely across a superconductor without any applied voltage * field exists on the edge of material for about 100nm Something i thought was cool: Experiments have demonstrated that supercurrents in superconducting coils can persist for years without any measurable degradation. Experimental evidence points to a current lifetime of at least 100,000 years. But Theoretically it could last forever

History Superconductivity was discovered in 1911 by Heike Kamerlingh Onnes Using recently created liquid helium he was able to lower mercury to 4 K, at this point the resistance completely disappeared 1913 Lead was discovered to be a superconductor at 7 K 1933 Meissner discovered superconductors expel their internal magnetic fields; named the Meissner effect 1941 Niobium nitride was discovered to be a superconductor at 16 K 1987 Peroskite (Ca Ti oxide) superconducts at 35 K This marked an important step because it allows liquid nitrogen to be the coolant instead of liquid helium, which is much easier to use and produce Niobium plus nitrogen. Only used as a superconductor and anti-reflective lenses Peroskite Calcium Titanium oxide

Recent superconductors Scientists are continuously developing and experimenting for new higher temperature superconductors 1993 superconducting ceramics were discovered, up to 133 K Highest ever recorded superconductor Problems with using ceramics extremely fragile and hard to shape 2008 iron family of superconductors discovered Iron Oxygen Flouride superconducts up to 55 K New predictions of superconductors working at higher temperature if you keep them under high pressures Hydrogen sulfide works at 80 K at 160 Gigapascals

Applications for superconductivity Superconductors make the strongest electromagnets yet to known to man These magnets are used in MRI machines and potentially in digital circuits in the future Potential applications for power transfer with zero loss to resistance Smart grid, power lines, maglev trains, magnetic levitation Also Josephson Junctions

Josephson Junctions A device made from two superconductors separated by a weak thin insulator Created when you separate two superconductors by a thin insulator a continuous current appears through the insulator Predicted and published by Brian David Josephson in 1962 at his senior year at Cambridge when he was just 22 It was heavily disputed by highly recognized scientists at the time The next year experiments in Bell labs proved his hypothesis Brian Josephson was awarded the Nobel prize for physics in 1973

Josephson Effect On either side of the insulator the electrons in the superconductors get together into pairs, and those pairs get together to form a quantum wave If the insulator is thin enough the quantum waves will spill through the insulator by a quantum tunneling effect, thus creating a current through the insulator When you apply a fixed current across the junction the current across the insulator begins to alternate in frequency directly related to the voltage This relationship within the superconductors isn’t affected by material properties This is important because a direct relation between frequency and voltage is not easily obtained

Josephson Voltage Standard Josephson junctions create a direct relationship between frequency and voltage Since frequency can be easily measured using standard atomic clocks throughout the world Now scientists all over the world can use the Josephson junctions to create a voltage standard to calibrate their machines, this is called the Josephson voltage standard However the official SI unit has not been changed

SQUID’s Super sensitive measuring device used to measure magnetic fields; a magnetometer Uses two Josephson junctions in parallel in a superconducting loop Can measure fields as low as 5 aT, 10 ^-18 So far Squids are the most sensitive magnetometers ever created Can be used to measure magnetic fields in the brain in real time Used whenever scientists need a strong magnetometers Physics, archaeology, geology, biology Superconducting quantum interference device aT = atto Tesla

Creation of DC SQUIDS First creation of SQUID was 1964 by Ford Research labs Without any magnetic field the current is split equally across the two branches When a magnetic field is applied a circular current forms around the loop creating a magnetic field to cancel out the introduced field When this induced current exceed the material's critical current, a voltage across the SQUID is induced This voltage can be measured and used to measure the magnetic field

Creation of AC SQUIDS The first SQUID based on the AC Josephson effect was made in 1965 by Robert Jeklevic Only uses one Josephson junction because the magnetic field will fluctuate with the AC voltage AC SQUIDS have to be connected to resonant tanks where the changing magnetic field can be measured AC SQUIDS are less sensitive but are easier to manufacture and maintain than DC SQUIDS

Superconductors are Supercool Most Squids are made from traditional superconductors that have to be cooled by liquid helium This limits Squids to only be used in a lab where the scientists can control the safety precautions of using liquid helium Helium can freeze the air in the pipelines that causes dangerous pressure buildups Recently Squids have been made using the high temperature superconductors possibly allowing Squids to be used in the field with liquid nitrogen Helium solid frozen air that blocks pipe lines and causes dangerous pressure buildup

Conclusion Superconductors are materials that when cooled to a certain temperature lose all their resistance and expel magnetic field Josephson Junctions are two superconductors separated by a small insulator where there is a current that tunnels through the insulator Josephson junctions directly relate voltage and frequency, this can be used to create a universal standard for the Volt Squids are created from Josephson junctions and are super powerful magnetometers Superconductors are Supercool

References Ame Ee Science Studio (4/10/2017) Josephson Effect http://www.supraconductivite.fr/en/index.php?p=applications-squid-josephson Christopher L Henley (2011) Josephson Junctions http://www.lassp.cornell.edu/clh/BT-GL/6.5AD.pdf Fagaly (2006) Superconducting Quatum Interference Device Instruments and Applications. Review of Scientific Instruments R Nave (4/10/2017) Superconductivity http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/scond.html Josephson Effect. 27 march 2017. In Wikipedia. Retrieved 4/10/2017 Superconductivity. 5 April 2017. In Wikipedia. Retrieved 4/10/2017 Brain David Josephson. 1 April 2017. In Wikipedia. Retrieved 4/10/2017

Key concepts Superconductors lose all resistance when cooled past certain temperature and expel magnetic fields within the material Josephson junctions create a direct relationship between voltage and frequency With the direct voltage frequency relationship there is a worldwide reproducible voltage standard called the Josephson voltage standard Squids are super powerful magnetometers using josephson junctions to detect magnetic fields Traditional superconductors must be kept cold with liquid helium but new superconductors are being discovered that can use liquid nitrogen instead A list of five concepts, key points, or discussions from your presentation that could be used as a basis for questions on our Final Exam! Your colleagues in our class will use these lists and your posted presentations as Final Exam study material!!!