Normal-Superconducting Phase Transition Obscured by Current Noise

There is a large volume of experimental research on the normal- superconducting phase transition, both in zero field and the so- called ``vortex-glass'' transition in a field. For these phase transitions, resistive behavior at low currents is expected for $T > T_c$, and non-linear current versus voltage isotherms are expected below $T_c$. We show theoretically and experimentally that the addition of current noise to nonlinear voltage versus current curves will create ohmic behavior. Thus, current noise will create ohmic behavior at low currents even for temperatures below $T_c$, and isotherms that are actually \textit{below} $T_c$ will appear to be \textit{above} $T_c$. This obscures the phase transition and leads to incorrect values for $T_c$ and the critical exponents $\nu$ and $z$. Most reports in the literature do not explicitly mention filtering, yet the transition temperature and the critical exponents extracted from the conventional analysis are inaccurate if current noise is not filtered out. Thus, current noise is a possible explanation for the wide range of critical exponents found in the literature.