Superconducting vortex lattices beyond SANS: A study of the intermediate mixed state of niobium using neutron interferometry, SANS and USANS

Superconducting vortex lattices can be regarded as a macroscopic lattice, formed by topological entities. Analogous to condensed matter, a large variety of phases is observed for vortex matter, resembling the particle like character and reflecting the underlying physical properties.

We present a comprehensive experimental study on the type-II/1 superconductor Nb which focuses on the transformation of its homogeneous vortex lattice into an inhomogeneous domain structure at the onset of vortex attraction. By means of SANS, USANS, and neutron grating interferometry, the vortex lattice and the micrometer-scale vortex domain structure as well as its distribution was investigated.

We focus on the transformation of the vortex lattice at the transition to the intermediate mixed state, which is characterized by vortex attraction. We find that the phase separation of the vortex lattice into an irregular domain structure takes place via a process showing strong similarity to spinodal decomposition. While pinning disorders the domain morphology, the characteristic length scale of the domain structure is governed by interplay of field distortion energy and domain surface tension. Finally, geometric barriers in the disk-shaped samples provoke an inhomogeneous distribution of domains on the macroscopic scale.

Visualizing the morphology of vortex lattice domains in a bulk type-II superconductor
T. Reimann, S. Mühlbauer, M. Schulz, B. Betz, A. Kaestner, V. Pipich, P. Böni and C. Grünzweig.
Nature communications 6, article number 8813, (2015)

Neutron dark-field imaging of the domain distribution in the intermediate state of lead
T. Reimann, M. Schulz, C. Grünzweig, A. Kaestner, A. Bauer, P. Böni and S. Mühlbauer.
Journal of Low Temperature Physics, issue 0022-2291,1-10 (2015)

Domain formation in the type-II/1 superconductor niobium: Interplay of pinning, geometry, and attractive vortex-vortex interaction
T. Reimann, M. Schulz, D.R.F. Mildner, M. Bleuel, A. Brulet, R. Harti, G. Benka, A. Bauer, P. Böni and S. Mühlbauer
Accepted, Phys Rev. B. (2017)