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Coherent orbital waves in the photo-induced insulator–metal dynamics of a magnetoresistive manganite


Photo-excitation can drive strongly correlated electron insulators into competing conducting phases1,2, resulting in giant and ultrafast changes of their electronic and magnetic properties. The underlying non-equilibrium dynamics involve many degrees of freedom at once, whereby sufficiently short optical pulses can trigger the corresponding collective modes of the solid along temporally coherent pathways. The characteristic frequencies of these modes range between the few GHz of acoustic vibrations3 to the tens or even hundreds of THz for purely electronic excitations. Virtually all experiments so far have used 100 fs or longer pulses, detecting only comparatively slow lattice dynamics4,5. Here, we use sub-10-fs optical pulses to study the photo-induced insulator–metal transition in the magnetoresistive manganite Pr0.7Ca0.3MnO3. At room temperature, we find that the time-dependent pathway towards the metallic phase is accompanied by coherent 31 THz oscillations of the optical reflectivity, significantly faster than all lattice vibrations. These high-frequency oscillations are suggestive of coherent orbital waves6,7, crystal-field excitations triggered here by impulsive stimulated Raman scattering. Orbital waves are likely to be initially localized to the small polarons of this room-temperature manganite, coupling to other degrees of freedom at longer times, as photo-domains coalesce into a metallic phase.

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Figure 1: Time-resolved measurement of the nanosecond conductivity transients induced by photo-excitation in single-crystal Pr0.7Ca0.3MnO3 at 77 K.
Figure 2: Femtosecond optical reflectivity measurements of Pr0.7Ca0.3MnO3 at 77 K.
Figure 3: Excitation process in Pr0.7Ca0.3MnO3 in the 2.5 eV photon-energy range.
Figure 4: Femtosecond optical reflectivity measurements of Pr0.7Ca0.3MnO3 at 300 K.


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The authors are grateful to the following colleagues for discussions: P. B. Allen, E. Dagotto, M. Grueninger, A. T. Boothroyd. Work at the University of Oxford was supported by the European Science Foundation through a European Young Investigator Award, and by Oxford University Press through a John Fell Award. S.W. and A.C. acknowledge support from the European Community Access to research infrastructure action of the Improving Human Potential Programme (LASERLAB Europe). Work at Lawrence Berkeley National Laboratory was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the US Department of Energy.

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Correspondence to D. Polli, M. Rini, S. Wall or A. Cavalleri.

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Polli, D., Rini, M., Wall, S. et al. Coherent orbital waves in the photo-induced insulator–metal dynamics of a magnetoresistive manganite. Nature Mater 6, 643–647 (2007).

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