Separating pairing from quantum phase coherence dynamics above the   superconducting transition by femtosecond spectroscopy
  In classical superconductors an energy gap and phase coherence appear simultaneously with pairing at the transition to the superconducting state. In high-temperature superconductors, the possibility that pairing and phase coherence are distinct and independent processes has led to intense experimental search of their separate manifestations, but so far without success. Using femtosecond spectroscopy methods we now show that it is possible to clearly separate fluctuation dynamics of the superconducting pairing amplitude from the phase relaxation above the critical transition temperature. Empirically establishing a close correspondence between the superfluid density measured by THz spectroscopy and superconducting optical pump-probe response over a wide region of temperature, we find that in differently doped Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} crystals the pairing gap amplitude monotonically extends well beyond T_{c}, while the phase coherence shows a pronounced power-law divergence as T\rightarrow T_{c}, thus showing for the first time that phase coherence and gap formation are distinct processes which occur on different timescales. 
Ivan Madan, Toru Kurosawa, Yasunori Toda, Migaku Oda, Tomaz Mertelj,   Primoz Kusar, Dragan Mihailovic
cond-mat.supr-con cond-mat.mtrl-sci cond-mat.str-el