changes in various physical properties, which is the hallmark of a phase transition (when a material changes state, such as from solid to liquid). One such change, as seen above with the Cooper pairing, is that the electronic fluid in a normal conductor becomes a Cooper pair fluid in the superconducting state and this fluid also becomes a superfluid.
Meissner effect
When a superconductor is placed in a weak external , the field penetrates the superconductor for only a short distance, called the penetration depth, after which it decays rapidly to zero. This is called the Meissner effect, and is a defining characteristic of superconductivity. For most superconductors, the penetration depth is on the order of 100 nanometers.
The Meissner effect states that a superconductor expels all magnetic fields. Suppose we have a material in its normal state, containing a constant internal magnetic field. When the material is cooled below the critical temperature, we would observe the abrupt expulsion of the internal magnetic field. An equation (known as the London equation) predicts that the magnetic field in a superconductor decays exponentially from whatever value it possesses at the surface.
The Meissner effect breaks down when the applied magnetic field is too large. Superconductors can be divided into two classes according to how this breakdown occurs.
In Type I superconductors, superconductivity is abruptly lost when the strength of the applied field rises above a critical value. Depending on the geometry of the sample, one may obtain an intermediate state consisting of regions of normal material carrying a magnetic field mixed with regions of superconducting material containing no field.
In Type II superconductors, raising the applied field past a critical value leads to a mixed state in which an increasing amount of magnetic flux (an amount of something that flows through a unit area in a unit time) penetrates the material, but there remains no resistance to the flow of electrical current as long as the current is not too large.
At a second critical field strength, superconductivity is destroyed. Most pure superconductors (except , , and ) are Type I, while almost all impure and compound superconductors are Type II.
Applications
Superconductors are used to make some of the most powerful electromagnets known to man, including those used in MRI machines and the beam-steering magnets used in . They can also be used for magnetic separation, where weak magnetic particles are extracted from a background of less or non-magnetic particles, as in the industries.
Superconductors have also been used to make digital circuits and filters for mobile phone base stations.
Superconductors are used to build Josephson junctions, which are the building blocks of SQUIDs (superconducting quantum interference devices) – the most sensitive magnetometers known. Series of Josephson devices are used to define the SI volt. Depending on the particular mode of operation, a Josephson junction can be used as a photon detector or as mixer. The large resistance change at the transition from the normal- to the superconducting state is used to build thermometers in cryogenic photon detectors.
Other early markets are arising where the relative efficiency, size, and weight advantages of devices based on high-temperature superconductors outweigh the additional costs involved.
Promising future applications include high-performance , power storage devices, electric power transmission, (such as for propulsion of vactrains or ), magnetic levitation devices, and fault current limiters. However, superconductivity is sensitive to moving magnetic fields, so applications that use alternating current (such as transformers) will be more difficult to develop than those that rely upon direct current.
Superconductors in popular culture
Superconductivity is a popular device in science fiction, due to the simplicity of the underlying concept – zero electrical resistance – and the rich technological possibilities. One of the first mentions of the phenomenon occurred in 's novel Beyond This Horizon (1942). Notably, the use of a fictional room temperature superconductor was a major plot point in the Ringworld novels by Larry Niven, first published in 1970. Organic superconductors were featured in a science fiction novel by physicist Robert L. Forward. Also, superconducting magnets may be invoked to generate the powerful needed by Bussard ramjets, a type of spacecraft commonly encountered in science fiction.
The most troublesome property of real superconductors, the need for cryogenic cooling, is often circumvented by postulating the existence of room temperature superconductors. Many stories attribute additional properties to their fictional superconductors, ranging from infinite heat (thermal) conductivity in Niven's novels to providing power to an interstellar travel device in the Stargate movie and TV series (real superconductors conduct heat poorly, though superfluid has immense but finite heat conductivity).
Answer the questions.
1. What does the Large Electron-Positron Collider (LEP) mean?
2. What is the simplest method to measure the electrical resistance?
3. What are superconductors also able to maintain?
4. In what type is superconductivity abruptly lost when the strength of the applied field rises above a critical value?
5. Are superconductors used to make some of the most powerful electromagnets?
Put in a preposition where necessary.
1. Most ___ the physical properties ___ superconductors vary ___ material ___material, such as the heat capacity and the critical temperature ____ which superconductivity disappears. 2. Electric cables use ___ the European Organization ___ Nuclear Research (CERN). 3. Regular cables (background) ___ 12,500 amps ___ electric current used ___ a particle accelerator called the Large Electron-Positron Collider (LEP); superconductive cable (foreground) ___ the same amount ___ electric current used ___the Large Hadron Collider (LHC). 4. The energy supplied __ the fluid needs to be greater than the thermal energy (temperature) __the lattice in order for superconductivity __appear.
Complete the sentences with the relative clause: which, when.
1. An example of a common property of superconductors is that they all have exactly zero resistivity to low applied currents ___there is no magnetic field present. 2.Individual properties include the heat capacity and the critical temperature at ___ superconductivity is destroyed. 3.Most of the physical properties of superconductors vary from material to material, such as the heat capacity and the critical temperature above ___ superconductivity disappears. 4. If the voltage is zero, then the resistance is zero, ___means that the electric current is flowing freely through the sample and the sample is in its superconducting state. 5. In superconducting materials,
