Fluctuations in the conductivity of Ba0.72K0.28Fe2As2single crystal are studied systematically by resistance measurements as a function of temperature and magnetic field. A clear Maki−Thompson and Aslamakov−Larkin (MT–AL) two- to three-dimensional (2D–3D) crossover is found on the excess conductivity (σ) curves as the temperature approaches the superconducting critical temperature,Tc. 3D fluctuations in superconductivity are realized near Tc that are well fitted to experimental data by the 3D Aslamazov–Larkin theory. The Maki–Thompson model shows a 2D conductivity fluctuation above the 2D-3D temperature transition,T0, which depends on magnetic field. Results show that the 2D-3D dimensional crossover moves to lower temperature with increasing magnetic field. The values of the transition temperature and the crossover in the reduced temperature, ln(ε0), as functions of magnetic field were used to determine the coherence length and the lifetime,τϕ, of the fluctuational pairs at the temperature of 35 K. Analysis of the Ba0.72K0.28Fe2As2single crystal gives a value of 3.76×10 −12 sfortheτϕin the absence of magnetic field and it decreases to 2.4×10−12 sinmagnetic fieldof13T.

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