Negative magnetoresistance in YBCO single crystals: Spin fluctuations?

The authors present results for the magnetoresistance in a number of single crystals of YBCO. They all have Tc in the vicinity of 93K with transitions widths of around 0.5K. Between 93K and 100K the positive magnetoresistance due to superconducting fluctuations is present and the authors are able to fit the data to theory yielding values of the coherence length in the {open_quote}c{close_quote} axis of around 0.2nm. However, above 100K a negative contribution to the resistivity appears. Above about 115K the magnetoresistance is dominated by this negative contribution. The negative contribution is proportional to the field and its magnitude is consistent with a spin fluctuation contribution to the magnetoresistance. The magnitude of the effect increases as the temperature is reduced until it peaks at around 100K.