Condensed-Matter Physics & Materials Science Seminar

The ground state of a superconductor is commonly described by a complex order parameter where the amplitude is a measure of the pair density (& energy gap), and the phase describes the long range order & supercurrent response to an applied field. Back in the early 70s at Bell Labs,
Testardi showed that laser light could be used to produce
non-equilibrium conditions in a superconductor. Pair-breaking by the laser resulted in an excess quasiparticle population and a decreased order parameter amplitude. A large number of studies followed to explore the ultra-fast dynamics of this non-equilibrium state, including the high-Tc cuprates. The NSLS has had an active program to study
non-equilibrium superconductivity by laser-pump, THz probe spectroscopy using pulsed synchrotron radiation.
With the advent of linac-based sources of intense coherent THz pulses, a new type of non-equilibrium state for a superconductor can now be realized. The spectral content for such a pulse can be limited to photon energies below the threshold for pair-breaking, and the field strengths can be sufficiently large to drive large currents in the pair condensate, exceeding the critical current density on an ultra-fast time scale. In one sense, this corresponds to a state where the order parameter phase has been driven far from equilibrium - rather than the order parameter amplitude - resulting in the loss of long range order.
The nature of such a state and how it recovers is presently unknown.
This presentation will describe the NSLS Source Development Lab linac as a source of intense THz pulses and presents initial results for destroying superconductivity in a NbN film using a single sub-THz coherent pulse.