Evidence of unconventional vortex states in the Chevrel phase superconductor PbMo6Se8

We present a comprehensive investigation of superconducting properties in the Chevrel phase compound PbMo₆Se₈ by x-ray diffraction, dc magnetization, resistivity, heat capacity, and magnetic relaxation experiments. We determine a bulk superconducting transition temperature Tc of 3.8 K. Key superconducting parameters such as lower [μ₀Hc1(0)] and upper [μ₀Hc2(0)] critical fields, coherence length [ξGL(0)], penetration depth [λGL(0)], Ginzburg-Landau parameter (κ), Pauli paramagnetic field (μ₀HPˢᵗʳ), and Maki parameter (αM) are obtained. The BCS model provides a reasonable description of the heat-capacity data. An unconventional vortex state is evident from the investigation of the unusual fishtail effect and nonmonotonous magnetic field dependency of the vortex pinning energy obtained from the magnetic relaxation experiments, indicating a deviation from the ideal type II superconducting vortex, in PbMo₆Se₈. A detailed μ₀H-T phase diagram has been drawn illustrating a multiphase vortex crossover.