Bridges 3—6 are in the center of the film. Figure 3 a Resistance vs temperature in films of different dimensions and different distances between contacts. The resistivity is geometry independent.
The inset in b shows the experimental setup and the set of current images used to generate the correction factor calculated in Eq. Figure 4 The geometrical factor C , calculated in Eq. The inset shows an AFM image of the film topography near an etched step. Figure 6 Resistivity vs temperature for four different optimally doped cuprates. The average derivative is used in Fig. Sign up to receive regular email alerts from Physical Review B. Journal: Phys. X Rev.
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Dynamic Hubbard model: Effect of finite boson frequency , F. Kuzmenko, Phys. Webb, F. Inversion of optical reflectance in the fullerenes , F. Asymptotic limit for H c2 in Eliashberg Theory , F. This article has been cited by other articles in PMC. Tanaka, F.
Applied Phys. Beams Phys. Kamerlingh Onnes, who found that the electrical resistance of mercury vanished suddenly when the metal was cooled to a temperature of about 4 kelvin K , which is 4 degrees Celsius above absolute zero Table If an electrical current is established in a ring of frozen mercury that is maintained at that.
By contrast, such a current dies away quickly in an ordinary conducting material such as copper. In the s, another characteristic of superconducting materials was described.
If an ordinary metal is placed in a magnetic field, the magnetic field permeates the material. Superconducting materials act differently. Some of them expel the magnetic field completely; others allow only partial penetration. This Meissner effect, as it is called, is responsible for the ability of permanent magnets to be levitated above a superconductor.
In expelling the field of the magnet, the superconductor generates its own magnetic field, which pushes the magnet away and allows the magnet to float over the superconducting sample. A full microscopic theory of superconductivity was achieved in the s by three physicists, John Bardeen, Leon N. Cooper, and J. Robert Schrieffer. The Bardeen-Cooper-Schrieffer BCS theory starts with the picture of a normally conducting metal whose atoms are arranged in a three-dimensional crystal structure.
Some of the loose-. The flow of these electrons is the electrical current that runs through the metal. These electrons do not have limitless freedom, however. Some of them interact with impurities in the metal and with the vibrations of the atoms that form the crystal lattice. The interaction of the electrons with the atomic vibrations causes the electrical resistance found in metals at ordinary temperatures. Electrons, which have a negative charge, ordinarily repel each other. The essence of the BCS theory is that under some circumstances they can have a net attractive interaction and thus form pairs.
The BCS theory pictures an electron traveling through a lattice of metal ions, which are positively charged because they have lost some of their electrons. The negatively charged electron may be attracted to another negatively charged electron by way of the oscillations of the metal ions. The two electrons form a pair—a Cooper pair, in the language of the BCS theory.
All the Cooper pairs act together as a unified quantum system. Circulating currents made up of Cooper pairs do not decay in the ordinary manner of currents composed of single electrons. One useful way to picture the interaction is to say that one negatively charged electron attracts the positively charged ions around it, causing a slight distortion in the lattice. The distortion produces a polarization, an area of increased positive charge. A second electron is attracted by this pocket of positive charge; it thus becomes coupled to the first electron, following it through the lattice.
Another description of the same phenomenon is that the electrons are coupled by the interchange of a virtual particle, the phonon. Phonons represent the vibrations of the lattice.
In either picture, the electron-phonon coupling allows the electrons to pair and then to flow unhindered through the lattice. Anything that destroys the Cooper pairs—for example, heat that increases the lattice vibrations above a certain limit—destroys superconductivity. Among other things, the BCS theory explains why superconductivity occurs in metals only at very low temperatures. At higher temperatures, thermal motion begins to break Cooper pairs apart.
Above a given transition temperature, all the pairs are broken and superconductivity vanishes. BCS theory also explains why metals that are good conductors at room temperature may not be superconductors at low temperatures: they do not have a large enough phonon-electron interaction to allow Cooper pairs to form.
In the BCS theory, a material's transition temperature, the temperature at which it becomes superconducting, depends only on three. Another feature of superconducting materials is the isotope effect. If an atom of one element in a superconducting material is replaced by an isotope of greater mass, the transition temperature of the material generally goes down.
This so-called isotope effect occurs because transition temperature is approximately proportional to the frequency of the lattice vibrations, and isotopes of greater mass have lower vibration frequencies. The isotope effect, first demonstrated in mercury in , was fundamental to the development of the BCS theory, because it strongly implied that phonons were the glue that held Cooper pairs together. The success of the BCS theory came against a background of slow, dogged advances in superconducting materials research.
Starting from the 4.
Theory of Superconductivity: From Weak to Strong Coupling (Condensed Matter Physics) [A.S Alexandrov] on goystofetel.ga *FREE* shipping on qualifying. Editorial Reviews. Review. "The book presents both a useful description of the fundamentals of Theory of Superconductivity: From Weak to Strong Coupling ( Condensed Matter Physics) - Kindle edition by A.S Alexandrov. Download it once and read it on your Kindle device, PC, phones or tablets. Use features like.
By the mids, the record transition temperature was 23 K, in a niobium-germanium compound. For practical purposes, characteristics other than transition temperature are also important.
Superconductivity can be destroyed by a high current density or a strong magnetic field. Technologically, the most desirable superconducting material has a high transition temperature and a high critical current and remains superconducting in a strong magnetic field. One such material that combines these qualities is a niobium-titanium compound that is used in superconducting magnets such as those found in medical magnetic resonance imaging devices and those being built for the Superconducting Super Collider, the world's largest particle accelerator.
Another property of superconductors, described by Brian Josephson in the early s, is the Josephson effect. If two samples of superconducting material are separated by a thin barrier, some of the Cooper-paired electrons will tunnel through. This tunneling is explained by quantum theory, in which subatomic particles are described equally well as wave packets. It is the wave aspect of electrons that allows tunneling to occur. The electron as a particle cannot pass through a barrier; as a wave, it can.