Scaling behaviour of magneto-resistance in clean YBa 2Cu 3O 7− δ single crystals in the vortex liquid state

Abstract

We present temperature and angular studies of the ohmic magneto-resistivity of clean twinned and detwinned YBa 2Cu 3O 7− δ single crystals. The scaling behaviour of thermodynamic and dynamic properties of the vortex liquid phase, in the region H c1≪ H< H c2, was analysed. Using the melting field B m, as en example, we demonstrate that when the applied field is tilted an angle θ away from the crystalline c-axis, the thermodynamic properties of the vortex system are controlled by the effective field B eff= Bε θ , where ε θ =(cos 2 θ+ ε 2sin 2 θ) 0.5 is the angular dependent anisotropy parameter (with ε=0.134) and B= μ 0 H. We observe a suppression of the `melting kink' (which is a fingerprint of the first order melting transition) when the applied field is larger than the multicritical field B mc. However, this transition recovers when the sample is tilted at an angle θ such that Bε θ < B mc. We find that this `effective field' model can also describe phenomena associated with vortex dynamics, but only for sufficiently low angles ( θ<60°). From ρ( T)/ B scaling of resistivity for different magnetic fields (at θ=0°), we show that the free flux-flow model applies for a considerable part of the resistive transition.