Section: 12 | Properties of Superconductors |

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PROPERTIES OF SUPERCONDUCTORS

Jens Hänisch, Stuart C. Wimbush, Lev I. Berger, and B. W. Roberts

The following tables list superconducting properties of elements, compounds, and alloys (“low-temperature superconductors”). The extensive series of cuprates and iron-based compounds (“high-temperature superconductors”) are listed separately in this section. Individual tables are also given here for thin films, elements under high pressures, and for the critical magnetic fields of selected compounds and alloys.

The historically first observed and most distinctive property of a superconductor is its total loss of electrical resistance at some critical temperature T_c, which is characteristic of each material. Figure 1(a) schematically illustrates two possible superconducting transitions. The temperature interval ΔT_c over which the transition between the normal and superconducting states takes place may be of the order of as little as 10^–5 K or as much as several kelvin, depending on the material. The sharp vertical discontinuity in resistance is indicative of that observed in a single crystal of a pure element. The broad transition suggests the shape seen for materials that are strained or inhomogeneous. Careful testing of the upper resistivity limit for the fully superconducting state has shown it to be less than 4 × 10^–23 Ω cm. This is at least 10¹⁷ times less resistive than copper at room temperature — comparable to the difference in resistivity between copper and glass.

The defining property of a superconductor, however, that distinguishes it from a notional perfect conductor, is its perfect diamagnetism: a superconducting body expels a magnetic field from its interior. A Type I superconductor expels a magnetic field up to some critical field B_c, whereupon it reverts to the normal state as shown in the B-T phase diagram of Figure 1(b). A Type II superconductor, in contrast, passes from the perfect diamagnetic state to a mixed state at a lower critical field B_c1 before attaining the normal resistive state at an upper critical field B_c2 as shown on the magnetization curve of Figure 1(c). In the mixed state between B_c1 and B_c2, regions of normal material exist within the superconducting matrix, allowing the penetration of quantized magnetic flux through the body. The number of flux lines present increases in proportion to the applied field B until at B_c2 the flux line density has become so large as to drive the entire volume of the superconducting body normal. This gradual penetration of magnetic flux enables the superconductivity to persist to much higher fields than it otherwise would. This property of Type II superconductors has enabled the development, half a century after their discovery, of technologically relevant superconducting materials. Tables 5 and 6 list some of the available critical field data of Type II materials.

The great majority of superconducting materials are Type II. The elements in pure form — with the exception of vanadium, niobium, technetium, and protactinium — as well as a very few precisely stoichiometric and well-annealed compounds, such as TaSi₂, Bi₂Pt, boron-doped SiC, and SnAs, are Type I.

Metallurgical Aspects

Superconducting properties are very sensitive to the material state, and this sensitivity has been used in a reverse sense to study and specify the detailed state of alloys. The mechanical state, the homogeneity, and the presence of impurity atoms and other electron-scattering centers are all capable of influencing the critical temperature and the current-carrying capability in magnetic fields. This sensitivity underlines a general problem in the tabulation of properties for superconducting materials. The occasional divergent values of the critical temperature or critical fields quoted for a Type II superconductor may lie in the variation in sample preparation. Critical temperatures of materials studied early in the history of superconductivity must be evaluated in light of the probable metallurgical state of the material, as well as the availability of less pure starting elements.

These tables are based on the original work by L. I. Berger and B. W. Roberts and are compiled from a large number of sources spanning the primary literature. Individual references for each datum are too numerous to list. Secondary sources, as listed under the references, have been used to ensure the accuracy and completeness of the work.

FIGURE 1. Physical properties of superconductors. (a) Resistivity vs. temperature for a pure and perfect lattice (solid line); impure and/or imperfect lattice (broken line). (b) Magnetic-field temperature dependence for Type I superconductors. (c) Schematic magnetization curve for Types I and II superconductors.

Symbols in tables: T_c: Critical temperature; H_o: Critical magnetic field in the T = 0 limit; θ_D: Debye temperature; and γ: Electronic-specific heat.

References

Lawson, A. C., J. Less-Common Metals 23, 103, 1971. [https://doi.org/10.1016/0022-5088(71)90016-6]
Savitskii, E. M., et al., Superconducting Materials, Plenum Press, New York, 1973. [https://doi.org/10.1007/978-1-4615-8672-2]
Douglass, D. H., Ed., Superconductivity in d- and f-Band Metals, Plenum Press, New York, 1976. [https://doi.org/10.1007/978-1-4615-8795-8]
Roberts, B. W., J. Phys. Chem. Ref. Data 5, 581, 1976. [https://doi.org/10.1063/1.555540]
Roberts, B. W., Properties of Selected Superconductive Materials 1978 Supplement, Nat. Bur. Stand. Tech. Note 983, 1978. [https://doi.org/10.6028/NBS.TN.983]
Fischer, Ø., Appl. Phys. 16, 1, 1978. [https://doi.org/10.1007/BF00931416]
Fischer, Ø., Maple, M. B., Eds., Topics in Current Physics, Vols. 32&34: Superconductivity in Ternary Compounds I&II, Springer-Verlag, Berlin, 1982. [https://doi.org/10.1007/978-3-642-81868-4]
Fischer, K., et al., Supraleitende Materialien, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, Germany, 1982.
Chapnik, I. M., J. Mat. Sci. Lett. 4, 370, 1985. [https://doi.org/10.1007/BF00719818]
Buzea, C., Robbie, K., Supercond. Sci. Technol. 18, R1, 2004. [https://doi.org/10.1088/0953-2048/18/1/R01]
Griveau, J.-C., and Colineau, É, C. R. Phys. 15, 599, 2014. [https://doi.org/10.1016/j.crhy.2014.07.001]
Hamlin, J. J., Physica C 514, 59, 2015. [https://doi.org/10.1016/j.physc.2015.02.032]

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TABLE 1. Selective Properties of Superconductive Elements

Element	T_c/K	B_o/mT	θ_D/K	γ/mJ mol^–1K^–1
Continued on next page...
Al	1.175 ± 0.002	10.49 ± 0.03	420	1.35
Am (α)	0.79
Am (β)	1.1
Be (α)	0.026		1390	0.21
Cd	0.517 ± 0.002	2.8 ± 0.1	209	0.69
Ga (α)	1.083 ± 0.001	5.83 ± 0.02	325	0.60
Ga (β)	5.9, 6.2	56
Ga (γ)^*	7	95
Ga (Δ)^*	7.85	81.5
Hf (α)	0.128	1.27	256	2.21
Hf (β)	0.506
Hg (α)	4.154 ± 0.001	41.1 ± 0.2	87, 71.9	1.81
Hg (β)	3.949	33.93	93	1.37
In	3.4087 ± 0.001	28.15 ± 0.2	109	1.672
Ir	0.1125 ± 0.001	1.600 ± 0.005	425	3.19
La (α)	4.88 ± 0.02	80 ± 1	151	9.8
La (β)	6.00 ± 0.1	109.6, 160	139	11.3
Li	0.0004
Lu (α)	0.1 ± 0.03	35 ± 5	210	10.2

^*Type II superconductor.

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