Andrea Cavalleri

Abstract:

We report possible light-induced superconductivity in the organic molecular solid K3C60, a superconductor at equilibrium below Tc=20 K. In our experiment we excited this alkali-doped fulleride with strong femtosecond pulses, tuned to be resonant with local molecular vibrational modes. By means of THz time-domain spectroscopy, we detected the pump-induced changes in the conductivity spectrum as a function of pump-probe time delay. Strikingly, at temperatures up to 100 K, we measured a light-induced response with the same optical properties of the equilibrium superconductor. An interpretation in terms of non-linear coupling between different vibrational modes may give hints to explain this emergent physics away of equilibrium.

Abstract:

Motivated by recent pump-probe experiments indicating enhanced coherent c-axis transport in underdoped YBCO, we study Josephson junctions periodically driven by optical pulses. We propose a mechanism for this observation by demonstrating that a parametrically driven Josephson junction shows an enhanced imaginary part of the low-frequency conductivity when the driving frequency is above the plasma frequency, implying an effectively enhanced Josephson coupling. We generalize this analysis to a bilayer system of Josephson junctions modeling YBCO. Again, the Josephson coupling is enhanced when the pump frequency is blue detuned to either of the two plasma frequencies of the material. We show that the emergent driven state is a genuine, nonequilibrium superconducting state, in which equilibrium relations between the Josephson coupling, current fluctuations, and the critical current no longer hold.

Abstract:

Light fields at terahertz and mid-infrared frequencies allow for the direct excitation of collective modes in condensed matter, which can be driven to large amplitudes. For example, excitation of the crystal lattice has been shown to stimulate insulator-metal transitions, melt magnetic order or enhance superconductivity. Here, we generalize these ideas and explore the simultaneous excitation of more than one lattice mode, which are driven with controlled relative phases. This nonlinear mode mixing drives rotations as well as displacements of the crystal-field atoms, mimicking the application of a magnetic field and resulting in the excitation of spin precession in the rare-earth orthoferrite ErFeO 3. Coherent control of lattice rotations may become applicable to other interesting problems in materials research-for example, as a way to affect the topology of electronic phases.