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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

Text 4

Elementary properties of superconductivity

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 ionic 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 heat (which is essentially the vibrational kinetic energy, 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 T_c. 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 mercury, 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 by abrupt 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 magnetic field, 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 elemental superconductors (except niobium, technetium, vanadium and carbon nanotubes) 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 particle accelerators. 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 pigment industries.

Superconductors have also been used to make digital circuits and microwave 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 micro-calorimeter 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 transformers, power storage devices, electric power transmission, electric motors (such as for propulsion of vactrains or maglev trains), 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 Robert A. Heinlein'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 magnetic fields 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 helium has immense but finite heat conductivity).

Task №1

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?

Task №2

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.

Task №3

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, the characteristics of superconductivity appear ___the temperature T is lowered below a critical temperature T_c.

Task №4

Put in a/an or the where necessary.

1. ___simplest method to measure ___ electrical resistance of ___sample of some material is to place it in ___electrical circuit in series with ___current source "I" and measure the resulting voltage "U" across ___sample. 2. In __normal conductor, ___electrical current may be visualized as ___fluid of electrons moving across __heavy ionic lattice (the conducting material), consisting of atoms that are electrically neutral. 3. When __ superconductor is placed in __ weak external magnetic field, ___ field penetrates ___ superconductor for only __ short distance, called ___penetration depth. 4. This is called ___ Meissner effect, and is ___defining characteristic of superconductivity.

Task №5

Put the verb into the correct form.

1. Experiments ___ that currents in superconducting coils can persist for years without any measurable degradation. (demonstrate) 2. Experimental evidence ___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. (point) 3.Conventional superconductors usually___ critical temperatures ranging from less than 1 K to around 20 K. (have) 4. The Meissner effect ___down when the applied magnetic field is too large. (break) 5. ___ Meissner effect states that ____superconductor expels all magnetic fields.

Task №6

Complete the sentence. Use the comparative or superlative form.

1. ____(simple) 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. 2. ___ (high) critical temperature found for a conventional superconductor is 39 K for magnesium diboride (MgB₂). 3. ___ (troublesome) property of real superconductors, the need for cryogenic cooling, is often circumvented by postulating the existence of room temperature superconductors.4. Superconductors are used to build Josephson junctions, which are the building blocks of SQUIDs – ____ (sensitive) magnetometers known

Task №7

Put questions to the underlined words

1. Individual properties include the heat capacity and the critical temperature at which superconductivity is destroyed.

2. All superconductors have exactly zero resistivity to low applied currents when there is no magnetic field present.

3. In superconducting materials, the characteristics of superconductivity appear when the temperature T is lowered below a critical temperature T_c.

4. When a superconductor is placed in a weak external magnetic field, the field penetrates the superconductor for only a short distance, called the penetration depth, after which it decays rapidly to zero.

5. At a second critical field strength, superconductivity is destroyed.