Pre-yield non-affine fluctuations and a hidden critical point in strained crystals

A crystalline solid exhibits thermally induced localised {\em non-affine} droplets in the absence of external stress. Here we show that upon an imposed shear, the size of these droplets grow until they percolate at a critical strain, well {\em below} the value at which the solid begins to yield. This critical point does not manifest in bulk thermodynamic or mechanical properties, but is {\em hidden} and reveals itself in the onset of inhomogeneities in elastic moduli, marked changes in the appearance and local properties of non-affine droplets and a sudden enhancement in defect pair concentration. Slow relaxation of stress and an-elasticity appear as observable dynamical consequences of this hidden criticality. Our results may be directly verified in colloidal crystals with video microscopy techniques but are expected to have more general validity.