Superconductivity

In the year of 1890 Heike Kemmerlingh Onnes in Leiden pioneered low temperature physics with his invention on the technique to liquefy Hydrogen and in 1906 to liquefy Helium. Initiated by this works, in 1911 he performed a study to observe the resistance of pure metals i.e. mercury at very low temperature (cryogenic). At that time, many believed that electrons flowing through a conductor would be completely halt at absolute zero temperature, or in other words the resistivity become infinitely large. The Result of Kammerlingh Onnes study stated that at the temperature of 4.2 K, the resistance of mercury is abruptly disappeared. He realized that there was a phase transition occurred and then he referred this phenomenon as “supraconductivity”, later adopted as “superconductivity.” We can safely describe superconductivity as a phenomenon where resistivity of several metals or alloys is disappeared at certain Temperature.

Numerous effort has been done to explain the phenomenon of superconductivity after its first discovery in 1911. From phenomenological macroscopic explanation to the microscopic explanation of BCS theory. Superconductor has several unique properties, that is; Magnetic field effect, Meissner effect, isotope effect. The Magnetic field effect can be described as disappearance of superconductivity under influence of magnetic field, there is critical value of magnetic field Hc in which above certain value superconductivity disappear. Superconductivity can be classified into two types based on its critical magnetic field; type I where there is only one critical field and type II where there are two critical field, the upper critical field Hc2 which separates normal phase from superconducting phase and the lower critical field Hc1 which separates superconducting mixed phase from the meissner phase, which is the same as the superconducting phase of type I. Meissner effect described as perfect diamagnetism that appears inside superconductor material. This effect appears because there is surface current in seperconductor that generate additional magnetic field that cancel the external magnetic field. The isotope effect is described as proportionality of transition temperature Tc to the isotope nucleus mass and this dependence gave clue that superconductivity was related to lattice vibration.

Microscopic quantum mechanical explanation of superconductivity is successfully explained by the BCS theory in 1957. BCS Stands for Bardeen Cooper Schrieffer, the last name of its founders John Bardeen, Leon Cooper and John Schrieffer. BCS theory stated that superconductivity appears from pairing of electrons with opposite spin and wave number under influence of lattice vibration or phonons. Electron moving through conductor attract positive charge nucleus in lattice, resulting lattice displacement which also attract another electron with different spin and wave number so that these two electrons are correlated. In superconductor a lot of electron pair appear, they are strongly overlapped and formed bose-einstein condensate and lose electrical resistance.

Superconductor found its application in many fields. In medical imaging, superconductor is used in MRI (Magnetic Resonance Imaging) apparatus which is used to visualize internal structure of human body. SQUID (Superconducting Quantum Interference Device) is a very sensitive magnetometer used to measure extremely weak magnetic fields, based on superconducting loops containing Josephson junctions, SQUID is useful in wide area of research from biology to experimental physics. There are many other application of superconductivity in our daily life such as linear motor car in magnetic levitation train.

• This article is part of course report submitted for “Frontier in Science II” course at the Graduate School of Science, Tohoku University, Japan. Part of this writing is inspired by the lecture on superconductivity by Professor Sumio Ishihara of Tohoku University.