Time- & angle-resolved photoemission spectroscopy investigation of strongly correlated electron systems & graphene

10.04.2013, 14:00

Dr Isabella Gierz, Max Planck Institute for the Structure and Dynamics of Matter, Hamburg

We use time- and angle-resolved photoemission spectroscopy to directly map the momentum- and energy-dependent dynamics of strongly correlated electrons in charge density wave (CDW) compounds and massless Dirac carriers in graphene after strong optical excitation Strong electronic correlations in low-dimensional metals often result in a periodic lattice distortion and a redistribution of charges that opens up band gaps at the Fermi level. The resulting CDW ground state exhibits novel collective excitations where either the amplitude or the phase of the complex order parameter oscillate in time. We investigated the electronic response of the prototypical one-dimensional CDW compound blue bronze (K0.3MoO3). Prompt depletion of the CDW condensate by photo-doping launches coherent oscillations of the amplitude mode at 1.7 THz. Furthermore, we observe oscillations at about half this frequency, which we attribute to coherent excitation of phasons via parametric amplification of phase fluctuations through anharmonic coupling to the amplitude mode.

The optical properties of graphene are made unique by the linear band structure and the vanishing density of states at the Dirac point. It has been proposed that even in the absence of a semiconducting bandgap, a relaxation bottleneck at the Dirac point may allow for population inversion and lasing at arbitrarily long wavelengths. Furthermore, efficient carrier multiplication by impact ionization has been discussed in the context of light harvesting applications. In lightly hole-doped epitaxial graphene samples, we explore excitation in the mid- and near-infrared, both below and above the minimum photon energy for direct interband transitions. While excitation in the mid-infrared results only in heating of the equilibrium carrier distribution, interband excitations give rise to population inversion, suggesting that terahertz lasing may be possible. However, in neither excitation regime do we find indication for carrier multiplication, questioning the applicability of graphene for light harvesting.