Sketch of a DNA hairpin structure and its free energy landscape. (a) A DNA hairpin construct is linked between a superparamagnetic bead and a coverslip surface. The DNA hairpin construct will be unzipped to the ssDNA state at a big stretching force, indicated by an extension jump . (b) The free energy landscape of the DNA hairpin at a force close to the critical force shows two energy minima representing the hairpin state and ssDNA state, respectively. The distance between the hairpin state and the transition state defines unfolding distance , while the distance between the ssDNA and the transition state defines folding distance .