Exploring Superconductivity and Pseudogap using High Resolution Scanning Tunneling Microscopy.

摘要

A correct theory of the high temperature cuprate superconductors must describe not only superconductivity, but also the parent phase from which it develops. To this end, it is important that experiments be done both below and above the superconducting transition temperature. For mainly experimental reasons, however, the majority of measurements on cuprates using scanning tunneling microscopy are done below the transition temperature. Here I will present measurements which overcome these difficulties and have been taken across the entire doping-temperature phase diagram. In particular, we find that for overdoped Bi2Sr2CaCu2O8+delta , the spectral gap and its temperature evolution are well described in a d-wave BCS context, with local nanoscale variation. However, the temperature scale, on average, exceeds the bulk T c of the material. Second, we find that the underdoped spectrum is characterized by two distinct energy scales, and can be best understood as arising from separate contributions from superconductivity, at low energies, and pseudogap, at high energies. The high energy portion of the spectrum continues to couple to the disorder, while the low energy portion develops a universal d-wave behavior, suggesting that the energy scale for superconductivity has saturated below optimal doping. Finally, we find periodic modulations of the local density of states at all dopings, which exist all the way up to the pseudogap temperature T*. These modulations cannot be accounted for by impurity scattering, and are most likely associated with a short range stripe order as they show strong enhancement near a hole concentration of 1/8. Importantly, this means that this order cannot be the cause of the psuedogap, which increases with underdoping even below 1/8 doping. Our measurements suggest an alternative scenario in which pseudogap is a prerequisite for a number of possible orderings, including the Mott insulator, fluctuating stripe order, and superconductivity. The competition between these phases over different portions of the Fermi surface and different patches in real space leads to the complex phase diagram of the hole doped cuprates.