Superconducting properties of ultrapure niobium

TL;DR

This paper investigates the superconducting properties of ultrapure niobium, examining critical magnetic fields, magnetization, and critical currents, revealing type transitions and surface effects relevant for applications.

Contribution

It provides a detailed analysis of how purity influences niobium's superconducting behavior and its transition between type I and II superconductivity with temperature.

Findings

Ultrapure niobium is a type I superconductor near Tc.

It transitions to type II at lower temperatures.

Critical current dependence aligns with force-free current models.

Abstract

The results of determination of critical magnetic fields, magnetization curves and critical currents of niobium samples of different purity are presented. It is shown that ultrapure niobium near Tc is a superconductor of the first type and becomes a superconductor of the second type with decreasing temperature due to the temperature dependence of the Ginzburg-Landau parameters. The dependence of the hysteresis of the magnetization curves of massive samples on the surface state is investigated. A comparison of the superconducting parameters with the parameters of the electron spectrum of niobium is carried out. The dependence of the critical current of niobium wires on the longitudinal magnetic field agrees with the assumption of a force-free current distribution.