Ultrafast Electronic Phase Transition in La1/2Sr3/2MnO4 by Coherent Vibrational Excitation: Evidence for Nonthermal Melting of Orbital Order

An ultrafast electronic phase transition, associated with melting of orbital order, is driven in La(1/2)Sr(3/2)MnO(4) by selectively exciting the Mn-O stretching mode with femtosecond pulses at 16 mu m wavelength. The energy coupled into this vibration is less than 1% of that necessary to induce the transition thermally. Nonthermal melting of this electronic phase originates from coherent lattice displacements comparable to the static Jahn-Teller distortion.