Magnetic Materials: Superconductors

Superconductors

Superconductors are materials which exhibit no electrical resistance below a certain temperature defined as the critical temperature (T_C). Prior to 1986, the highest T_C 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 La_2-xBa_xCuO_4-x and YBa₂Cu₃O₇ (product 328626) superconduct above the boiling point of nitrogen (77 K).^6,7 Materials whose T_C 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 A_x@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 (product 553913) is not only superconductive but its critical temperature is surprisingly high for a simple ceramic material (T_c = 39 K).¹⁰ Figure 1 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 Table 1 for a comparison of some critical temperatures.

Figure 1. 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 1Critical 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 T_C to date

Superconductivity is governed not only by a critical temperature but also by a critical magnetic field (H_C) and a critical current density (J_C). The critical magnetic field refers to an applied magnetic field, such that, if an applied field becomes too large (greater than H_C) superconductivity will be lost. Critical temperature and critical field are inversely proportional such that just below T_C, the superconducting state can only be maintained in a very weak applied field, whereas, near 0 K, a larger applied field can be tolerated (Figure 2). Similarly, J_C 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 J_C greater than 106 amp·cm^-1.

Figure 2. 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.