Superconductivity diamagnetism

The transition from the normal diamagnetic state to superconductive diamagnetism is characterised by one or more transition temperatures Tc identified by the inflection points on the x versus T curve. A very good identification of the transition temperature is provided by the maximum of the imaginary (out of phase) susceptibility x" measured in an alternating magnetic field (Fig. 7.2). 

Figure 6.29. Superconducting diamagnetic shielding observed for various A3C60 compounds in polycrystalline powder form. values can be accurately measured from the onset temperatures. (Reproduced by permission from ref 12.)...

Properties. Pure thorium metal is a dense, bright silvery metal having a very high melting point. The metal exists in two allotropic modifications. Thorium is a reactive, soft, and ductile metal which tarnishes slowly on exposure to air (12). Having poor mechanical properties, the metal has no direct stmctural appHcations. A survey of the physical properties of thorium is summarized in Table 1. Thorium metal is diamagnetic at room temperature, but becomes superconducting below 1.3—1.4 K. 

The Meissner Effect and Levitation. Besides the absence of electrical resistance, a superconducting material is characterized by perfect diamagnetism. The exclusion of magnetic field lines from a material when it passes from a normal state to a superconducting state is shown schematically in Figure 3. 

Silver-white lustrous soft metal highly malleable and ductile face-centered tetragonal crystalline structure (a=4.583A, c=4.93GA) diamagnetic metal density 7.31 g/cm at 20°C melts at 156.6°C vaporizes at 2,072°C electrical resistivity 8.4 x Kh ohm-cm superconducting at 3.38°K (—269.8°C) hardness 0.9 (Brinnel) tensile strength 26.19 atm modulus of elasticity 10.8 GPa thermal neutron absorption cross-section 190 10 bams soluble in acids. 

The reported Meissner fraction was an important property to follow during the early days of superconductivity research. The magnitude of this diamagnetic effect, when properly measured, gives a good indication of the homogeneous nature of the materials being prepared in different laboratories. The first reported values were low, 2 to 15%, but as the composition, structure, and synthetic conditions became better known, the values increased to 70-80%— indicative of bulk superconductivity in an essentially pure product. 

This is consistent with the result that Horowitz et al. have found on submicron particles obtained by a hyponitrite method they are too small to give any diamagnetic behavior. However, the heat capacity measurement shows a break at 90 K that corresponds to about 90% of superconducting phase in the powder (16). 

In what follows we should bear in mind that the generation of a diamagnetic metallic state (irrespective of whether it is a superconductor or not) will not be favored by a half-filled band of electrons. Either a Peierls distortion or the generation of an antiferromagnetic insulating state will result, with a ferromagnet being less likely for the reasons discussed. Superconductivity in these materials is in fact only observed if electrons are removed, or (less commonly to date) added to the half-filled band. Considerable effort is underway to theoreti-... 

Diamagnetism Negative Small" X = constant Organic materials, superconducting metals, and other metals (e.g., Bi)... 

As discussed earlier, the original BCS theory predicted that superconductors were perfect diamagnets. It was therefore with surprise that solids were recently discovered that are both ferromagnetic and superconducting. 

In addition to the zero resistivity, superconducting materials are perfectly diamagnetic in other words, magnetic fields (up to a limiting strength that decreases as the temperature rises toward Tc) cannot penetrate them (the Meissner effect). This is a consequence of the mobile, paired state of the electrons. Indeed, it is the demonstration of the Meissner effect, rather than lack of electrical resistivity, that is usually demanded as evidence of superconductive behavior. One entertaining consequence of the Meissner effect is that small but powerful magnets will float (levitate) above the surface of a flat, level superconductor.30... 

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