COEXISTENCE OF 2 LENGTH SCALES IN X-RAY AND NEUTRON CRITICAL SCATTERING - A THEORETICAL INTERPRETATION

The theory of critical phenomena in the presence of quenched disorder is applied to x-ray and neutron experiments which suggest two different length scales for critical fluctuations. Long-range random strains associated with defects generated near the sample surface can induce crossover to a "disordered" fixed point with different critical exponents, while the bulk retains the ordinary critical behavior. For Ho and Tb, it is proposed that the dominant defects are dislocation dipoles, resulting in a critical exponent for the second correlation length νs=1, in reasonable agreement with experiments. © 1995 The American Physical Society.