The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a photoinduced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, to identify when and how fast these phase transitions occur, methods that can probe the symmetry change in the time domain are required. Here we show that a time-dependent change in the coherent phonon spectrum can probe a change in symmetry of the lattice potential, thus providing an all-optical probe of structural transitions. We examine the photoinduced structural phase transition in Vo 2 and show that, above the phase transition threshold, photoexcitation completely changes the lattice potential on an ultrafast timescale. The loss of the equilibrium-phase phonon modes occurs promptly, indicating a non-thermal pathway for the photoinduced phase transition, where a strong perturbation to the lattice potential changes its symmetry before ionic rearrangement has occurred.

Many ultrafast solid phase transitions are treated as chemical reactions that transform the structures between two different unit cells along a reaction coordinate, but this neglects the role of disorder. Although ultrafast diffraction provides insights into atomic dynamics during such transformations, diffraction alone probes an averaged unit cell and is less sensitive to randomness in the transition pathway. Using total scattering of femtosecond x-ray pulses, we show that atomic disordering in photoexcited vanadium dioxide (VO2) is central to the transition mechanism and that, after photoexcitation, the system explores a large volume of phase space on a time scale comparable to that of a single phonon oscillation. These results overturn the current understanding of an archetypal ultrafast phase transition and provide new microscopic insights into rapid evolution toward equilibrium in photoexcited matter.

The competition between electron localization and de-localization in Mott insulatorsunderpins the physics of strongly-correlated electron systems. Photo-excitation, which redistributes charge between sites, can control this many-body process on the ultrafast In Mott insulators, conductivity at low energies is prevented by repulsion among electrons.This state is fundamentally different from that of conventional band insulators, in which Bragg scattering from the lattice opens gaps in the single particle density of states. The electronic structure of Mott insulators is, therefore, sensitive to doping. Photo-excitation, in analogy to static doping, can trigger large changes in the macroscopic properties viii .However, the coherent physics driving these transitions has not been fully observed because the many-body electronic dynamics are determined by hopping and correlation processes that only persist for a few femtoseconds.We report measurements of coherent many-body dynamics with ultrafast optical spectroscopy in the one-dimensional Mott insulator ET-F2TCNQ. Several factors make this possible: ET-F2TCNQ has a narrow bandwidth (~ 100 meV), which corresponds to hopping times of tens of femtoseconds; the material has a weak electron-lattice interaction; we use a novel optical device producing pulses of 9 fs at the 1.7 m Mott gap; we study this physics in a one-dimensional system, allowing the evolution of the many-body wavefunction to be calculated and compared with experimental data. The characteristics of this new peak are time dependent, as visualized in Fig. 2c, where we have normalized the reflectivity at each time step. Two contours are shown in Fig. 2c. On the blue side, a prompt red-shift and recovery of the resonance is observed, whereas the red side shows a longer-lived component, containing a damped oscillatory response at 25 THz. StaticRaman data on ET-F2TCNQ does not show any equivalent features, strongly suggesting that the oscillation is not due to coherent phonons, but of an electronic origin xiv .To investigate such dynamics, we used a one-dimensional Mott-Hubbard Hamiltonian for a half-filled chain, with N = 10 sites, with electron hopping, t, and onsite and nearest neighbourCoulomb repulsion U and V, where, c †l, and cl, are the creation and annihilation operators for an electron at site l with spin , nl, is the number operatorand nl = nl, + nl,. We described the initial state, where  represents a many-body wavefunction with one electron per site and total spin-vector . This reflects the fact that, at room temperature, charges are localized, but posses no magnetic ordering.We calculate the static optical conductivity (see methods section) to find values of U, V and t that provide the best fit to the experimental results. The best fit, shown in Fig. 3c (t = -200 fs), gave U = 820 meV, V = 100 meV and t = 50 meV. It was not possible to fit the optical conductivity using U and t alone and inter-site correlation energy, V, was needed xv .These static parameters were used to fit to the...

The extreme electro-optical contrast between crystalline and amorphous states in phase-change materials is routinely exploited in optical data storage and future applications include universal memories, flexible displays, reconfigurable optical circuits, and logic devices. Optical contrast is believed to arise owing to a change in crystallinity. Here we show that the connection between optical properties and structure can be broken. Using a combination of single-shot femtosecond electron diffraction and optical spectroscopy, we simultaneously follow the lattice dynamics and dielectric function in the phase-change material Ge2Sb2Te5 during an irreversible state transformation. The dielectric function changes by 30% within 100 fs owing to a rapid depletion of electrons from resonantly bonded states. This occurs without perturbing the crystallinity of the lattice, which heats with a 2-ps time constant. The optical changes are an order of magnitude larger than those achievable with silicon and present new routes to manipulate light on an ultrafast timescale without structural changes.

Measuring how the magnetic correlations evolve in doped Mott insulators has greatly improved our understanding of the pseudogap, non-Fermi liquids and high-temperature superconductivity. Recently, photo-excitation has been used to induce similarly exotic states transiently. However, the lack of available probes of magnetic correlations in the time domain hinders our understanding of these photo-induced states and how they could be controlled. Here, we implement magnetic resonant inelastic X-ray scattering at a free-electron laser to directly determine the magnetic dynamics after photo-doping the Mott insulator Sr2IrO4. We find that the non-equilibrium state, 2 ps after the excitation, exhibits strongly suppressed long-range magnetic order, but hosts photo-carriers that induce strong, non-thermal magnetic correlations. These two-dimensional (2D) in-plane Néel correlations recover within a few picoseconds, whereas the three-dimensional (3D) long-range magnetic order restores on a fluence-dependent timescale of a few hundred picoseconds. The marked difference in these two timescales implies that the dimensionality of magnetic correlations is vital for our understanding of ultrafast magnetic dynamics.

Photo-excitation can drive strongly correlated electron insulators into competing conducting phases, 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 vibrations 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 dynamics. Here, we use sub-10-fs optical pulses to study the photo-induced insulator-metal transition in the magnetoresistive manganite Pr(0.7)Ca(0.3)MnO(3). 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 waves, 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.

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...

Strain engineering is an emerging route for tuning the bandgap, carrier mobility, chemical reactivity and diffusivity of materials. Here we show how strain can be used to control atomic diffusion in van der Waals heterostructures of two-dimensional (2D) crystals. We use strain to increase the diffusivity of Ge and Te atoms that are confined to 5 Å thick 2D planes within an Sb2Te3–GeTe van der Waals superlattice. The number of quintuple Sb2Te3 2D crystal layers dictates the strain in the GeTe layers and consequently its diffusive atomic disordering. By identifying four critical rules for the superlattice configuration we lay the foundation for a generalizable approach to the design of switchable van der Waals heterostructures. As Sb2Te3–GeTe is a topological insulator, we envision these rules enabling methods to control spin and topological properties of materials in reversible and energy efficient ways.