Magnetic-field-driven quantum metallic state and scaling relationship in polycrystalline vanadium films

Abstract Two-dimensional (2D) and three-dimensional (3D) polycrystalline vanadium films grown on SiO 2 substrates have been prepared by magnetron sputtering method. The superconductor-insulator transition (SIT) has been observed by adjusting the film thickness, average crystallite size and magnetic field respectively. The complete superconducting 3D vanadium film deposited at 523 K has an obvious two-step development of superconductivity when a magnetic field is applied. This behavior is attributed to non-uniform coupling due to variations in grain sizes and the intergranular distances across the sample. The metallic state induced by the magnetic field can be explained by a model that incorporates both thermally activated phase slippage and quantum phase slippage. Both the 2D vanadium film with thickness of 12 nm and the 3D vanadium film with average crystallite size of 11.9 nm exhibit a trend towards superconducting transition where the resistance decreases to a nonzero value. The SIT, tuned by adjusting the perpendicular magnetic field, has been observed in the above two films. The scaling behavior near the critical magnetic field <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>B</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">c</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> distinguishes a superconducting phase from an insulating phase, indicating a quantum phase transition induced by a finite-size effect. The upper critical magnetic field <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>H</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">c</mml:mi> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> (0) increases as the grain size decreases, likely due to a reduction in the coherence length resulting from a decrease in the average free path with decreasing size.