had a great success in explaining the macroscopic properties of superconductors.
2. found that the critical temperature of a superconductor depends on the of the constituent .
3. of superconductivity was finally proposed by John Bardeen (1908-1991), Leon Cooper, and John Schrieffer.
4. the first commercial superconducting wire, a - alloy, was developed at Westinghouse Electric Corporation.
5. had believed that the forbade superconductivity at temperatures above about 30 K.
LESSON 4
Active vocabulary
Superconductors – Сверхпроводники
resistivity удельное – сопротивление
magnetic field – магнитное поле
heat capacity – теплоемкость
critical temperature – критическая температура
properties – свойства
particle accelerator – ускоритель частиц
voltage – напряжение
electric current – сверхпроводящее состояние
superconducting state – электрический ток
current – ток
coils – катушки
universe – вселенная
conductor – проводник
ionic lattice – ионная решетка
vibrational kinetic energy- вибрационная кинетическая энергия
phenomenon – явление
critical temperature – критическая температура
fluid – жидкость
Superconductors possess both common and individual properties according to each kind. An example of a common property of superconductors is that they all have exactly zero resistivity to low applied currents when there is no magnetic field present. Individual properties include the heat capacity and the critical temperature at which superconductivity is destroyed.
Most of the physical properties of superconductors vary from material to material, such as the heat capacity and the critical temperature above which superconductivity disappears. On the other hand, there is a class of properties that are independent of the underlying material. For instance, all superconductors have exactly zero resistivity to low applied currents when there is no magnetic field present. The existence of these "universal" properties implies that superconductivity is a thermodynamic phase and that these distinguishing properties are largely independent of microscopic details.
Electric cables used by the European Organization for Nuclear Research (CERN). Regular cables (background) for 12,500 amps of electric current used at a particle accelerator called the Large Electron-Positron Collider (LEP); superconductive cable (foreground) for the same amount of electric current used at the Large Hadron Collider (LHC).
The simplest method to measure the electrical resistance of a sample of some material is to place it in an electrical circuit in series with a current source "I" and measure the resulting voltage "U" across the sample. The resistance of the sample is given by Ohm's law:
If the voltage is zero, then the resistance is zero, which means that the electric current is flowing freely through the sample and the sample is in its superconducting state.
Superconductors are also able to maintain a current with no applied voltage whatsoever, a property exploited in superconducting electromagnets such as those found in MRI machines. Experiments have demonstrated that currents in superconducting coils can persist for years without any measurable degradation. Experimental evidence points to a current lifetime of at least 100,000 years, and theoretical estimates for the lifetime of persistent current exceed the lifetime of the universe.
In a normal conductor, an electrical current may be visualized as a fluid of electrons moving across a heavy lattice (the conducting material), consisting of atoms that are electrically neutral. The electrons are constantly colliding with the ions (electrically neutral atoms) in the lattice, and during each collision some of the energy carried by the current is absorbed by the lattice and converted into (which is essentially the vibrational , energy due to motion of the lattice ions). As a result, the energy carried by the current is constantly dissipated. This is the phenomenon of electrical resistance.
In superconductors, on the other hand, the electronic fluid is not made up of individual electrons, but rather pairs of electrons called Cooper pairs, held together by an attractive force arising from the microscopic vibrations in the lattice. According to quantum mechanics, this Cooper pair fluid requires a minimum amount of energy, ∆E, for it to conduct an electrical current. Specifically, the energy supplied to the fluid needs to be greater than the thermal energy (temperature) of the lattice in order for superconductivity to appear. This is why superconductivity is achieved at extremely low temperatures.
Superconducting phase transition
In superconducting materials, the characteristics of superconductivity appear when the temperature T is lowered below a critical temperature Tc. The value of this critical temperature varies from material to material. Conventional superconductors usually have critical temperatures ranging from less than 1 K to around 20 K. Solid , for example, has a critical temperature of 4.2 K. As of 2001, the highest critical temperature found for a conventional superconductor is 39 K for magnesium diboride (MgB2), although this material displays rather exotic properties that there is doubt about classifying it as a "conventional" superconductor. Cuprate superconductors can have much higher critical temperatures: YBCO (YBa2Cu3O7), one of the first cuprate (copper based) superconductors to be discovered, has a critical temperature of 92 K, and mercury-based cuprates have been found with critical temperatures in excess of 130 K. The explanation for these high critical temperatures remains unknown.
The onset of superconductivity is accompanied
