Absence of hexagonal to square structural transition in LiFeAs vortex matter

TL;DR

This study uses scanning tunneling microscopy to examine vortex structures in LiFeAs, finding no transition from hexagonal to square lattice up to 12 T, contrary to previous reports, and highlighting factors influencing vortex lattice formation.

Contribution

It provides direct experimental evidence that the hexagonal to square vortex lattice transition does not occur in LiFeAs up to 12 T, challenging prior findings.

Findings

No hexagonal to square vortex lattice transition observed up to 12 T.

Hexagonal vortex lattice remains highly ordered up to 8 T.

Factors like cooling procedure and sample stoichiometry influence vortex lattice structure.

Abstract

We investigated magnetic vortices in two stoichiometric LiFeAs samples by means of scanning tunneling microscopy and spectroscopy. The vortices were revealed by measuring the local electronic density of states (LDOS) at zero bias conductance of samples in magnetic fields between 0.5 and 12 T. From single vortex spectroscopy we extract the Ginzburg-Landau coherence length of both samples as $4.4\pm0.5$ nm and $4.1\pm0.5$ nm, in accordance with previous findings. However, in contrast to previous reports, our study reveals that the reported hexagonal to square-like vortex lattice transition is absent up to 12 T both in field-cooling and zero-field-cooling processes. Remarkably, a highly ordered zero field cooled hexagonal vortex lattice is observed up to 8 T. We argue that several factors are likely to determine the structure of the vortex lattice in LiFeAs such as (i) details of the cooling procedure (ii) sample stoichiometry that alters the formation of nematic fluctuations, (iii) details of the order parameter and (iv) magnetoelastic coupling.