TY - JOUR
T1 - Nmr in superconductors
AU - Weger, Meir W.
PY - 1972/1/1
Y1 - 1972/1/1
N2 - Superconductivity is a striking physical phenomenon; it is a manifestation of quantum effects on a macroscopic scale; consequently, the most striking physical phenomena associated with it, are macroscopic ones—the total disappearance of resistivity, exclusion of flux, flux quantization in macroscopic cylinders, interference effects between junctions a macroscopic distance apart. NMR, being a microscopic tool, does not display such striking effects, but is quite suitable to investigate the microscopic nature of the superconducting state. We shall discuss various microscopic features of superconductivity, and nmr experiments relating to them. Some of the properties of superconductors are: (1) The pairing of electrons with opposite spins, which is the basic feature of the BCS theory, and which manifests itself in a reduction of the Knight shift. (2) The creation of a gap, which causes the number of excitations at low temperatures to be proportional to exp(— 2Δ/kT), and the relaxation rate to be proportional to this quantity. (3) The coherent nature of the superconducting state, which causes 1/T1just below Tc to exceed 1/T1in the normal state. (4) The property of gapless superconductivity, observed in some systems (mainly with magnetic impurities, or in a magnetic field), which affects 1/T1. (5) In a magnetic field, some ('type II’) superconductors possess an array of magnetic flux lines. Some information about the geometry of this array can be obtained from the nmr linewidth and lineshape. (6) Inside the flux lines, superconductivity is effectively destroyed and consequently the relaxation rate there, is characteristic of the normal metal. (7) The magnetic flux lines are not rigid. Therefore, they fluctuate thermally yielding an additional relaxation mechanism. (8) Very near Tc, the gap parameter A is not constant (in time and space), but fluctuates thermally. This yields an additional relaxation mechanism, which should be relatively strong in superconductors with a quasi one-dimensional band structure. This effect has apparently been observed.
AB - Superconductivity is a striking physical phenomenon; it is a manifestation of quantum effects on a macroscopic scale; consequently, the most striking physical phenomena associated with it, are macroscopic ones—the total disappearance of resistivity, exclusion of flux, flux quantization in macroscopic cylinders, interference effects between junctions a macroscopic distance apart. NMR, being a microscopic tool, does not display such striking effects, but is quite suitable to investigate the microscopic nature of the superconducting state. We shall discuss various microscopic features of superconductivity, and nmr experiments relating to them. Some of the properties of superconductors are: (1) The pairing of electrons with opposite spins, which is the basic feature of the BCS theory, and which manifests itself in a reduction of the Knight shift. (2) The creation of a gap, which causes the number of excitations at low temperatures to be proportional to exp(— 2Δ/kT), and the relaxation rate to be proportional to this quantity. (3) The coherent nature of the superconducting state, which causes 1/T1just below Tc to exceed 1/T1in the normal state. (4) The property of gapless superconductivity, observed in some systems (mainly with magnetic impurities, or in a magnetic field), which affects 1/T1. (5) In a magnetic field, some ('type II’) superconductors possess an array of magnetic flux lines. Some information about the geometry of this array can be obtained from the nmr linewidth and lineshape. (6) Inside the flux lines, superconductivity is effectively destroyed and consequently the relaxation rate there, is characteristic of the normal metal. (7) The magnetic flux lines are not rigid. Therefore, they fluctuate thermally yielding an additional relaxation mechanism. (8) Very near Tc, the gap parameter A is not constant (in time and space), but fluctuates thermally. This yields an additional relaxation mechanism, which should be relatively strong in superconductors with a quasi one-dimensional band structure. This effect has apparently been observed.
UR - http://www.scopus.com/inward/record.url?scp=67649394039&partnerID=8YFLogxK
U2 - 10.1351/pac197232010325
DO - 10.1351/pac197232010325
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AN - SCOPUS:67649394039
SN - 0033-4545
VL - 32
SP - 325
EP - 338
JO - Pure and Applied Chemistry
JF - Pure and Applied Chemistry
IS - 1-4
ER -