Nuclear Magnetic Resonance Studies of Type II Superconductors
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NUCLEAR MAGNETIC RESONANCE
STUDIES OF TYPE II
31
SUPERCONDUCTORS*
F. Y. FRADIN Materials Science Division, Argonne National Laboratory,
Argonne,
IL
60439
ABSTRACT The results of nuclear magnetic resonance (nmr) experiments on simple type-I superconductors were among the first and most important verifications of the BCS theory of superconductivity. In this paper, the application of nmr techniques to the study of superconducting properties in the more complex type-Il superconductors will be reviewed. The discussion will include the effect of material parameters (e.g., degree of long range crystalline order, density of states at the Fermi level, effects of magnetic dopants) on the superconducting properties, including size of the superconducting gap, vortex structure, upper-critical field Hc2, and variations in Tc. Emphasis will be placed on high Tc, high Hc2 materials, i.e., A15 compounds and the ternary Chevrel phases.
INTRODUCTION This paper will focus on the materials properties important for the understanding and the development of useful type-Il superconductors. Nuclear magnetic resonance (nmr) techniques have a historical coupling to the phenomena of superconductivity starting with the elegant experiments of Hebel and Slichter
[1]
that
verified
a crucial aspect
of the microscopic
theory of
superconductivity proposed by Bardeen et al. [2]. This article will deal only peripherally with the important topic of nmr experiments in the superconducting state; an excellent review article dealing with nmr studies of the physical phenomena associated with superconductivity has been written by MacLaughlin [3]. The emphasis in this article will be on the relationship between nmr and normal state parameters, such as the degree of crystalline order, the density of states at the Fermi level, and the coupling of the conduction electrons to magnetic dopants that determine the strength of the Cooper pairing and thus the critical temperature Tc and critical field Hc2. Although much of the physics of superconductivity was first established on the simple type-I, nontransition metal superconductors, in recent years a great deal of research has dealt with the more complex type-Il superconductors (see for example, the two volume review edited by Parks 14]). These transition metal alloys and intermetallic compounds are of technological importance due to their high values of Tc, Hc2, and critical currents JC" Because of the complexity of the metallurgical behavior, the electronic structure, and the physical properties of these transition metal systems, a large number of problems must be solved by the materials scientist in order to optimize their superconducting properties. Since this field is already very broad, this review will not attempt to be comprehensive but will try to illustrate the utility of rmr by selective examples mostly taken from the authors own work. *Work supported by the U.S. Department of Energy.
32 MIXED STATE The principal feature that makes type-Il superconductivity technologically important is th
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