Flux creep regimes and vortex phase diagram in β-FeSe single crystals

Abstract

We analyze the relationship between critical current densities (J_C) and flux creep rates (S) in β-FeSe single crystals. This analysis was based on magnetization measurements. Additionally, we establish correlations with the recently reported magnetic field-induced geometrical deformation of the vortex lattice, transitioning from hexagonal to square shape due to a rhombic distortion [A. V. Putilov et al. Phys. Rev 99 (2019) 144514]. The results show that the magnetic field dependence of J_c displays distinct regimes, which is reflected by changes in S. The vortex dynamics is analyzed within the framework of the collective creep theory. S is characterized by low pinning energies and glassy exponents according to the expectation for small-bundles at low temperatures and magnetic fields where a hexagonal vortex lattice was reported. Conversely, we observe a systematic increase in S, resembling a shift from small to large vortex bundles, at magnetic fields corresponding to the rhombic distortion. Last, the relaxation rates exhibit significant values for magnetic fields where a square vortex lattice is expected, suggesting a potential crossover from elastic to plastic creep. Our findings highlight a direct relationship between vortex lattice deformations and a decrease in vortex pinning related to vortex-defect interactions.