Superconductivity

If mercury is cooled below 4.1 K, it loses all electric resistance. This discovery of superconductivity by H. Kammerlingh Onnes in 1911 was followed by the observation of other metals which exhibit zero resistivity below a certain critical temperature. The fact that the resistance is zero has been demonstrated by sustaining currents in superconducting lead rings for many years with no measurable reduction. An induced current in an ordinary metal ring would decay rapidly from the dissipation of ordinary resistance, but superconducting rings had exhibited a decay constant of over a billion years!

One of the properties of a superconductor is that it will exclude magnetic fields, a phenomenon called the Meissner effect.

The disappearance of electrical resistivity was modeled in terms of electron pairing in the crystal lattice by John Bardeen, Leon Cooper, and Robert Schrieffer in what is commonly called the BCS theory.

A new era in the study of superconductivity began in 1986 with the discovery of high critical temperature superconductors.

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Critical Temperature for Superconductors

The critical temperature for superconductors is the temperature at which the electrical resistivity of a metal drops to zero. The transition is so sudden and complete that it appears to be a transition to a different phase of matter; this superconducting phase is described by the BCS theory. Several materials exhibit superconducting phase transitions at low temperatures. The highest critical temperature was about 23 K until the discovery in 1986 of some high temperature superconductors.

Materials with critical temperatures in the range 120 K have received a great deal of attention because they can be maintained in the superconducting state with liquid nitrogen (77 K).

Material
Tc
Gallium
1.1K
Aluminum
1.2 K
Indium
3.4 K
Tin
3.7 K
Mercury
4.2
Lead
7.2 K
Niobium
9.3 K
La-Ba-Cu-oxide
17.9 K
Y-Ba-Cu-oxide
92 K
Tl-Ba-Cu-oxide
125 K


Type I superconductorsType II superconductors
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Superconductivity concepts

Reference Rohlf,Ch 15
 
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Types I and II Superconductors

There are thirty pure metals which exhibit zero resistivity at low temperatures and have the property of excluding magnetic fields from the interior of the superconductor (Meissner effect). They are called Type I superconductors. The superconductivity exists only below their critical temperatures and below a critical magnetic field strength. Type I superconductors are well described by the BCS theory.

Starting in 1930 with lead-bismuth alloys, a number of alloys were found which exhibited superconductivity; they are called Type II superconductors. They were found to have much higher critical fields and therefore could carry much higher current densities while remaining in the superconducting state.

The variations on barium-copper-oxide ceramics which achieved the superconducting state at much higher temperatures are often just referred to as high temperature superconductors and form a class of their own.

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Type I Superconductors

The 27 pure metals listed in the table below are called Type I superconductors. The identifying characteristics are zero electrical resistivity below a critical temperature, zero internal magnetic field (Meissner effect), and a critical magnetic field above which superconductivity ceases.

The superconductivity in Type I superconductors is modeled well by the BCS theory which relies upon electron pairs coupled by lattice vibration interactions. Remarkably, the best conductors at room temperature (gold, silver, and copper) do not become superconducting at all. They have the smallest lattice vibrations, so their behavior correlates well with the BCS Theory.

While instructive for understanding superconductivity, the Type I superconductors have been of limited practical usefulness because the critical magnetic fields are so small and the superconducting state disappears suddenly at that temperature. Type I superconductors are sometimes called "soft" superconductors while the Type II are "hard", maintaining the superconducting state to higher temperatures and magnetic fields.
Mat. Tc
Be 0
Rh 0
W 0.015
Ir 0.1
Lu 0.1
Hf 0.1
Ru 0.5
Os 0.7
Mo 0.92
Zr 0.546
Cd 0.56
U 0.2
Ti 0.39
Zn 0.85
Ga 1.083
Mat. Tc
Gd* 1.1
Al 1.2
Pa 1.4
Th 1.4
Re 1.4
Tl 2.39
In 3.408
Sn 3.722
Hg 4.153
Ta 4.47
La 6.00
Pb 7.193

*Gd at Tc=1.1 is questionable. Source is Rohlf, Ch 15, but this may be a misprint. Ga has Tc about 1.1, so Ga value may have been attributed to Gd.

Note also the three metals at right which were formerly included as Type I superconductors in the above table, but have been shown to exhibit Type II properties.
Mat. Tc
V 5.38
Tc 7.77
Nb 9.46
Type I superconductors on periodic table

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Superconductivity concepts

Reference
Rohlf
Ch 15
 
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Type II Superconductors

Superconductors made from alloys are called Type II superconductors. Besides being mechanically harder than Type I superconductors, they exhibit much higher critical magnetic fields. Type II superconductors such as niobium-titanium (NbTi) are used in the construction of high field superconducting magnets.

Type-II superconductors usually exist in a mixed state of normal and superconducting regions. This is sometimes called a vortex state, because vortices of superconducting currents surround filaments or cores of normal material.


New superconductor: magnesium diboride
Superconducting plutonium compound
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