Evidence of unconventional vortex states in the Chevrel phase superconductor PbMo6Se8
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 (𝜇0𝐻str𝑃), 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.
(a) Volume susceptibility after demagnetization correction measured at 1 mT. (b) Isothermal magnetization at 1.9 K. Inset shows the same in expanded scale. (c) Evolution of magnetic isotherms from 1.9 to 3.75 K. Observation of two magnetic anomalies called 𝐻𝑚 and 𝐻SP and their shift with temperature. (d) Fitting of lower critical field (𝐻𝑐1) from isothermal magnetization. (e) Temperature-dependent resistivity and associated parallel resistor model fit. Magnetic field dependence of superconducting transition in resistivity data in the inset. (f) Fitting of upper critical field (𝐻𝑐2) from Ginzburg-Landau theory from 50% drop of resistivity data.
