Equilibrium properties of the mixed state in superconducting niobium in a transverse magnetic field: Experiment and theoretical model

TL;DR

This study combines experimental measurements and a new theoretical model to analyze the equilibrium magnetic properties of the mixed state in high-purity niobium superconductors, especially in perpendicular magnetic fields, revealing limitations of existing theories.

Contribution

A novel theoretical model for the mixed state in type-II superconductors is developed and experimentally validated, addressing gaps in existing theories and providing new insights into vortex properties.

Findings

First-time equilibrium magnetization data in perpendicular fields.

Existing theories do not match new experimental data.

A new model accurately describes the mixed state in perpendicular magnetic fields.

Abstract

Equilibrium magnetic properties of the mixed state in type-II superconductors were measured with high purity bulk and film niobium samples in parallel and perpendicular magnetic fields using dc magnetometry and scanning Hall-probe microscopy. Equilibrium magnetization data for the perpendicular geometry were obtained for the first time. It was found that none of the existing theories is consistent with these new data. To address this problem, a theoretical model is developed and experimentally validated. The new model describes the mixed state in an averaged limit, i.e. %without detailing the samples' magnetic structure and therefore ignoring interactions between vortices. It is quantitatively consistent with the data obtained in a perpendicular field and provides new insights on properties of vortices. % and the entire mixed state. At low values of the Ginzburg-Landau parameter, the model converts to that of Peierls and London for the intermediate state in type-I superconductors. It is shown that description of the vortex matter in superconductors in terms of a 2D gas is more appropriate than the frequently used crystal- and glass-like scenarios.