Negative Surface Free-Energy Effects in Superconducting Niobium

Onset of flux-bundle migration into superconducting niobium strips

“…The experimental value (H c1 ϭ1250Ϯ250 G) presents a large error but is similar to the value (H c1 ϭ1413 G) given in Ref. 10. The pinning current was determined following the critical state model of C. P. Bean.…”

“…The lower critical field H c1 (4.2 K͒ϭ1410 G has been taken from Ref. 10. The ratio H m /H c1 decreases systematically as the sample width increases but the experimental points are well below all the theoretically predictions.…”

“…The H c2 value ͑ϭ 2590 G͒ was taken from Ref. 10. The number of quanta per flux bundle is then given by…”

Superconductivity of Pb--Bi alloys under high pressure

“…For type 11, the per fect diamagnetism ceases at the lower critical field, but M does not become zero until a higher critical field, is reached and zero elec trical resistance persists up to By analogy, a thermodynamic criti cal field can be defined by VH^/Stc ® -J* MdH. An important experimental observation is that for a suitable sample either type of magnetization curve is reversible (12,13,14). Many superconductors are not reversible and the superconducting properties of the Q!-p phase of Pb-Bi are extremely irreversible (15).…”

The superconducting properties of high purity niobium

“…Reversible type-II behavior is not confined to alloys, but has been found Variation of magnetic field and order parameter in the interface reg i on also in the pure elements niobium and vanadium (13,14).…”