Andrea Cavalleri

Abstract:

Competition between electron localization and delocalization in Mott insulators underpins the physics of strongly correlated electron systems. Photoexcitation, which redistributes charge, can control this many-body process on the ultrafast ^1,2 timescale. So far, time-resolved studies have been carried out in solids in which other degrees of freedom, such as lattice, spin or orbital excitations ^3-5 , dominate. However, the underlying quantum dynamics of bareg electronic excitations has remained out of reach. Quantum many-body dynamics are observed only in the controlled environment of optical lattices ^6,7 where the dynamics are slower and lattice excitations are absent. By using nearly single-cycle near-infrared pulses, we have measured coherent electronic excitations in the organic salt ET-F 2 TCNQ, a prototypical one-dimensional Mott insulator. After photoexcitation, a new resonance appears, which oscillates at 25THz. Time-dependent simulations of the Mottg Hubbard Hamiltonian reproduce the oscillations, showing that electronic delocalization occurs through quantum interference between bound and ionized holong doublon pairs. © 2011 Macmillan Publishers Limited. All rights reserved.

Abstract:

Photoinduced phase transitions are of special interest in condensed matter physics because they can be used to change complex macroscopic material properties on the ultrafast timescale. Cooperative interactions between microscopic degrees of freedom greatly enhance the number and nature of accessible states, making it possible to switch electronic, magnetic or structural properties in new ways. Photons with high energies, of the order of electron volts, in particular are able to access electronic states that may differ greatly from states produced with stimuli close to equilibrium. In this study we report the photoinduced change in the lattice structure of a charge and orbitally ordered Nd 0.5 Sr 0.5 MnO 3 thin film using picosecond time-resolved X-ray diffraction. The photoinduced state is structurally ordered, homogeneous, metastable and has crystallographic parameters different from any thermodynamically accessible state. A femtosecond time-resolved spectroscopic study shows the formation of an electronic gap in this state. In addition, the threshold-like behaviour and high efficiency in photo-generation yield of this gapped state highlight the important role of cooperative interactions in the formation process. These combined observations point towards a ĝ€̃ hidden insulating phaseĝ€™ distinct from that found in the hitherto known phase diagram. © 2011 Macmillan Publishers Limited. All rights reserved.

Abstract:

Polarons, mixed modes of solids comprising electronic and lattice excitations, underpin the electronic properties of strongly-correlated oxides. We demonstrate their coherent formation in ferromagnetic La0.7Sr0.3MnO3, enabled via direct lattice excitation in the midinfrared. © OSA / UP 2010.

Abstract:

Control of onsite electronic wavefunctions is achieved in the organic conductor (BEDT-TTF)-F2TCNQ by resonant excitation of localized molecular vibrational modes of the BEDT molecule. In this way, the onsite two-particle Coulomb repulsion can be modulated. © OSA/UP 2010.