Light-induced insulator-metal transitions and magnetic switching in manganites
Manganites are a prototype of correlated electron materials exhibiting many exotic phenomena, such as charge ordered phases, orbital and magnetic ordering, phase separation, and colossal magnetoresistence. Most of these phenomena arise from the strong interactions between charge, orbital, spin, and lattice degrees of freedom. They also display a variety of insulator to metal phase transitions, which are initiated by temperature, magnetic field, pressure, and light. Their control, e.g., in the case of colossal magnetoresistence, has important ramifications in both fundamental science and technology.
Our group has demonstrated such an insulator-metal phase transitions in manganites by selective excitation of a phonon mode with intense femtosecond mid-IR radiation. We carried out experiments in insulating Pr0.7Ca0.3MnO3, using light fields resonant with the Mn-O stretching mode to modulate the crystal structure without generating hot carriers, thus driving the system into a new phase whilst remaining in the electronic ground state . Combining time-resolved optical spectroscopy and resistivity measurement, we were able to show that this resonant excitation induces a nanosecond-lived increase in sample conductivity by six orders of magnitude. Only recently, density functional theory calculations by A. Subedi and A. George disclosed a microscopic mechanism, based on the framework of nonlinear phononics, for this vibrationally induced phase transition .
The same type of optical stimulation was applied to the charge, orbitally and antiferromagnetically ordered manganite La0.5Sr1.5MnO4. Femtosecond measurements of the transient optical birefringence revealed that the direct lattice excitation non-thermally melts this charge and orbital order . We found strong indications that the lattice distortions are the true underlying cause for ultrafast phase transitions in the manganites. We further concluded that also photo-doping of these materials couples to the electronic system indirectly through the concomitant excitation of lattice distortions. Indeed, most recent experiments using 4-fs laser pulses revealed a temporal bottleneck for the photo-induced melting of this charge and orbital order, being limited by the atomic motions associated with Jahn-Teller distortions .
These light-driven phase transitions in the manganites are accompanied by a melting of antiferromagnetic order that can be monitored via time-resolved resonant soft x-ray diffraction at the 640 eV Mn L-edge . We were able to exploit the potential of this technique with femtosecond time resolution at the LCLS Free Electron Laser to demonstrate the lattice driven melting of magnetic order in a manganite after vibrational excitation .
 Control of the electronic phase of a manganite by mode-selective vibrational excitation
M. Rini, R. Tobey, N. Dean, J. Itatani, Y. Tomioka, Y. Tokura, R. W. Schoenlein & A. Cavalleri
 Photoinduced Melting of Antiferromagnetic Order in La0.5Sr1.5MnO4 Measured Using Ultrafast Resonant Soft X-Ray Diffraction
H. Ehrke, R. I. Tobey, S. Wall, S. A. Cavill, M. Först, V. Khanna, Th. Garl, N. Stojanovic, D. Prabhakaran, A. T. Boothroyd, M. Gensch, A. Mirone, P. Reutler, A. Revcolevschi, S. S. Dhesi and A. Cavalleri
 Driving magnetic order in a manganite by ultrafast lattice excitation
M. Först, R. I. Tobey, S. Wall, H. Bromberger, V. Khanna, A. L. Cavalieri, Y.-D. Chuang, W. S. Lee, R. Moore, W. F. Schlotter, J. J. Turner, O. Krupin, M. Trigo, H. Zheng, J. F. Mitchell, S. S. Dhesi, J. P. Hill, and A. Cavalleri