Magnetic Materials

Superconductors

  Introduction
Magnetic Properties of Materials
Superconductors
A History of Superconductivity
Molecule-Based Magnets
Spintronics
References

Superconductors

Superconductors are materials which exhibit no electrical resistance below a certain temperature defined as the critical temperature (TC). Prior to 1986, the highest TC reported was 20 K for Nb3Ge and Nb3Sn.5 In 1986-87, a group lead by Johannes Bednorz and Karl Müller reported the ceramic oxides La2-xBaxCuO4-x and YBa2Cu3O7 (Aldrich product 32,862-6) superconduct above the boiling point of nitrogen (77 K).6,7 Materials whose TC is greater than the boiling point of nitrogen (a common, readily available, cryogenic coolant) are referred to as high-temperature superconductors (HTS). For their work Bednorz and Müller were awarded the Nobel Prize in Physics in 1987.8

Other more exotic compounds such as fullerides have also exhibited superconducting properties. Fullerides of the formula Ax@C60 (A = K, Rb, Cs) are reported to have superconducting character.9 Although superconductive compounds have been known for nearly a century, the relatively mundane compound magnesium boride has only recently been demonstrated to exhibit superconductivities. Magnesium boride, MgB2 (Aldrich product 55,391-3) is not only superconductive but its critical temperature is surprisingly high for a simple ceramic material (Tc = 39 K).10 Figure 2 shows an image of a MgB2 wire segment with a tungsten boride core. The wire is formed by reaction of magnesium vapor with a boron filament. The grain structure seen in this image is visible under polarized light.11 See Table 4 for a comparison of some critical temperatures.

MgB2 wire segment

Figure 2. The cross section of a MgB2 wire segment. (Image courtesy of D.K. Finnemore, S.L. Bud¢ko, P.C. Canfield, Ames Laboratory, Iowa State University.)
 

Table 4. Critical temperatures of some superconductors.

Compound or Element TC (K) Compound or Element TC (K)
Mercury 4 Nb3Sn 18
Vanadium 5.4 Nb3Ge 23
Lead 7.2 Ba0.6K0.4BiO3 30
Technetium 7.8 Cs2Rb@C60 33
Niobium 9.5 MgB2 39
Sulfur (at 93 Gpa) 10 La1.85Sr0.15CuO4 40
(CH3CH2)2Cu(NCS)2 11.4 Tl2Ba2CuO6 80
LiTi2O4 12 YBa2Cu3O7 93
BaPb0.75Bi0.25O3 13 Tl2Ba2CaCu2O8 105
YNi2B2C 15.5 BiScCO (BiSr2Ca3Cu3O10) 110
NbN 16 Tl2Ba2Ca3Cu4O12 115
V3Ga 16.5 Tl2Ba2Ca2Cu3O10 125
Sulfur (at 160 Gpa)* 17 HgBa2Ca2Cu3O10 134
V3Si 17 HgBa2Ca2Cu3O10 (at 30 Gpa)** 164
Nb3Al 17.5    

*Highest reported Tc for an element **Highest reported TC to date

Superconductivity is governed not only by a critical temperature but also by a critical magnetic field (HC) and a critical current density (JC). The critical magnetic field refers to an applied magnetic field, such that, if an applied field becomes too large (greater than HC) superconductivity will be lost. Critical temperature and critical field are inversely proportional such that just below TC, the superconducting state can only be maintained in a very weak applied field, whereas, near 0 K, a larger applied field can be tolerated (see Figure 2). Similarly, JC is the maximum current that can be passed through a superconducting material before it reverts back to a non-superconducting state. This is a critical factor for power applications such as practical superconductor-based electronics would have a JC greater than 106 amp·cm-1.


Figure 3. Effects of temperature and magnetic field on the superconducting state.

Aside from traditional metals based superconductors and HTS cuprate-based ceramics, more recent work has focused upon molecular and fullerene based superconductors. See below for highlights in the timeline of superconductor research.

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