We report on ultrafast optical experiments in which femtosecond midinfrared radiation is used to excite the lattice of complex oxide heterostructures. By tuning the excitation energy to a vibrational mode of the substrate, a long-lived five-order-of-magnitude increase of the electrical conductivity of NdNiO(3) epitaxial thin films is observed as a structural distortion propagates across the interface. Vibrational excitation, extended here to a wide class of heterostructures and interfaces, may be conducive to new strategies for electronic phase control at THz repetition rates.

Many advanced applications of X-ray free-electron lasers require pulse durations and time resolutions of only a few femtoseconds. To generate these pulses and to apply them in time-resolved experiments, synchronization techniques that can simultaneously lock all independent components, including all accelerator modules and all external optical lasers, to better than the delivered free-electron laser pulse duration, are needed. Here we achieve all-optical synchronization at the soft X-ray free-electron laser FLASH and demonstrate facility-wide timing to better than 30 fs r.m.s. for 90 fs X-ray photon pulses. Crucially, our analysis indicates that the performance of this optical synchronization is limited primarily by the free-electron laser pulse duration, and should naturally scale to the sub-10 femtosecond level with shorter X-ray pulses.

At low-temperatures (T < T N =110 K < T CO/OO =220 K), single-layer La 0.5 Sr 1.5 MnO 4 exhibits CE-type charge, spin and orbital order, characterized by in-plane "zig-zag" ferromagnetic chains. These chains are antiferromagnetically coupled with one another, in and out of plane [11,12,13]. Resonant soft Xray diffraction is directly sensitive to this spin and orbital order, when the incident photon energy is tuned to the 2p→3d transitions (Mn L 2,3 edges), and provides both momentum-dependent and spectroscopic information [14,15]. Figure 1 The temporal evolution of the integrated diffraction spot intensity at the magnetic (¼ ¼ ½) wave vector, obtained from the fits as described above, is reported in Figure 2(a). Diffraction was reduced by 8%, with a single time constant of 12.2 ps. For comparison, we display the significantly faster response response measured after excitation with 5-mJ/cm 2 pulses at 800-nm wavelength [23,24,25], which reveals a prompt collapse of magnetic order on the 250 fs time resolution of the experiment.This observation of different timescales is evidence that lattice driven magnetic disordering must follow a different physical path than for electronic excitation in the near infrared.In Figure 2 timescale and amplitude, with the orbital order only reduced by only 3% with a single-exponential decay time of 6.3 ps. We note that this lattice-driven orbital disordering is slower than was observed previously by time-dependent optical birefringence [19]. However, time dependent optical birefringence, proportional to the orbital order parameter squared in equilibrium [18], is a less direct method than the resonant x-ray diffraction used here.Throughout these dynamics, we see no transient change in the position and width of the scattered diffraction spots for either order and conclude that the correlation lengths are not perturbed. This is shown in Figure 2(c) where we exemplarily plot the transient width of the magnetic diffraction spot together with its peak position. The latter is constant within < 1×10 -5 (calculated standard deviation). , among which we find the Raman-active Jahn-Teller mode depicted in Fig. 3(c). Thus, according to the IRS model, rectification of the mid-infrared mode is able to relax the cooperative Jahn-Teller distortion, which has no infrared activity and thus cannot be driven directly by mid-infrared excitation. Importantly, the Jahn-Teller mode shown in Fig. 3(c) relaxes the splitting between crystal field levels and reduces the ordering of the orbitals. In turn, this weakens the exchange interaction that stabilizes the CE-type order and would thus lead to a smaller equilibrium magnetization, or to a lower equivalent Neel temperature.We stress that in contrast to the case of La 0.7 Sr 0.3 MnO 3 [27], in which the envelope of the infraredactive E u mode drives a low-frequency 1.2-THz rotational (E g ) mode impulsively, the Jahn-Teller A g mode has a higher frequency (15 THz) than the inverse 130-fs envelope of the infrared-active mode.Thus, in La 0.5 Sr 1.5 MnO 4 , the A...