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Understanding how light modifies long-range order is key to improve our ability to control material functionality on an ultrafast timescale. Transient spatial heterogeneity has been proposed in many materials, but isolating the dynamics of different regions experimentally has been challenging. Here we address this issue and measure the dynamics of orbital order melting in the layered manganite, La0.5Sr1.5MnO4, and isolate the surface dynamics from the bulk for the first time. Bulk measurements show orbital order is rapidly suppressed, but the correlation length surprisingly increases. However, the surface dynamics, show a stronger suppression and a significant decrease in correlation length. By isolating the surface changes, we find that light preferentially melts a less ordered surface and the loss of long-range order is likely driven by the formation of local and disordered polarons. Melting the disordered surface effectively increases the average correlation of the bulk probed volume, resolving the contradictory response. These results show that surface scattering methods are necessary to understand both surface and bulk dynamics in heterogeneous materials., Comment: 22 pages, 8 figures

We present a new set up for resonant inelastic hard X-ray scattering at the Bernina beamline of SwissFEL with energy, momentum, and temporal resolution. The compact R=0.5 m Johann-type spectrometer can be equipped with up to 3 crystal analysers and allows efficient collection of RIXS spectra. Optical pumping for time-resolved studies can be realized with a broad span of optical wavelengths. We demonstrate the performance of the set-up at overall ~180 meV resolution in a study of ground-state and photoexcited (at 400 nm) honeycomb 5d iridate $\alpha$-$\mathrm{Li_2IrO_3}$. Steady-state RIXS spectra at the Iridium ${L_3}$-edge (11.214 keV) have been collected and are in very good agreement with data collected at synchrotrons. The time-resolved RIXS transients (pumped minus unpumped spectra) exhibit changes in the energy-loss region <2 eV, whose features mostly result from the hopping nature of 5d electrons in the honeycomb lattice. These changes are ascribed to modulations of the Ir-to-Ir intersite transition scattering efficiency, which we associate to a transient screening of the on-site Coulomb interaction., Comment: 17 + 7 pages, 4 + 6 figures

Interactions between the different degrees of freedom form the basis of many manifestations of intriguing physics in condensed matter. In this respect, quantifying the dynamics of normal modes that themselves arise from these interactions and how they interact with other excitations is of central importance. Of the different types of coupling that are often important, spin-lattice coupling is relevant to several sub-fields of condensed matter physics; examples include spintronics, high-TC superconductivity, and topological materials. While theories of materials where spin-lattice coupling is relevant can sometimes be used to infer the magnitude and character of this interaction, experimental approaches that can directly measure it are rare and incomplete. Here we use time-resolved X-ray diffraction to directly access the spin-lattice coupling by measuring both ultrafast atomic motion and the associated spin dynamics following the excitation of a coherent electromagnon by an intense THz pulse in a multiferroic hexaferrite. Comparing the dynamics of the two different components, one striking outcome is the different phase shifts relative to the driving field. This phase shift provides insight into the excitation process of such a coupled mode. This direct observation of combined lattice and magnetization dynamics paves the way to access the mode-selective spin-lattice coupling strength, which remains a missing fundamental parameter for ultrafast control of magnetism and is relevant to a wide variety of correlated electron physics., Comment: 4 figures

Nonequilibrium states of quantum materials can exhibit exotic properties and enable unprecedented functionality and applications. These transient states are inherently inhomogeneous, characterized by the formation of topologically protected structures, requiring nanometer spatial resolution on femtosecond timescales to resolve their evolution. Using ultrafast total x-ray scattering at a free electron laser and a sophisticated scaling analysis, we gain unique access to the dynamics on the relevant mesoscopic lengthscales. Our results provide direct evidence that ultrafast excitation of LaTe$_3$ leads to formation of topological vortex strings of the charge density wave. These dislocations of the charge density wave exhibit anomalous, subdiffusive dynamics, slowing the equilibration process, providing rare insight into the nonequilibrium mesoscopic response in a quantum material. Our findings establish a general framework to investigate properties of topological defects, which are expected to be ubiquitous in nonequilibrium phase transitions and may arrest equilibration and enhance competing orders.

Magnetic patterns can be controlled globally using fields or spin polarized currents. In contrast, the local control of the magnetization on the nanometer length scale remains challenging. Here, we demonstrate how magnetic domain patterns in a Tm-doped yttrium iron garnet (Tm:YIG) thin film with perpendicular magnetic anisotropy can be permanently and locally imprinted by high intensity photon pulses of a hard x-ray transient grating (XTG). Micromagnetic simulations provide a qualitative understanding of the observed changes in the orientation of magnetic domains in Tm:YIG and XTG-induced changes. The presented results offer a route for the local manipulation of the magnetic state using hard XTG., Comment: 15 pages, 3 figures

NaOsO$_3$ has recently attracted significant attention for the strong coupling between its electronic band structure and magnetic ordering. Here, we used time-resolved magnetic X-ray diffraction to determine the timescale of the photoinduced \afm dynamics in NaOsO$_3$. Our measurements are consistent with a sub-100~fs melting of the \afm long-range order, that occurs significantly faster than the lattice dynamics as monitored by the transient change in intensity of selected Bragg structural reflections, which instead show a decrease of intensity on a timescale of several ps., Comment: 11 pages, 6 figures

Optical-domain Transient Grating (TG) spectroscopy is a versatile background-free four-wave-mixing technique used to probe vibrational, magnetic and electronic degrees of freedom in the time domain. The newly developed coherent X-ray Free Electron Laser sources allow its extension to the X-ray regime. Xrays offer multiple advantages for TG: their large penetration depth allows probing the bulk properties of materials, their element-specificity can address core-excited states, and their short wavelengths create excitation gratings with unprecedented momentum transfer and spatial resolution. We demonstrate for the first time TG excitation in the hard X-ray range at 7.1 keV. In Bismuth Germanate (BGO), the nonresonant TG excitation generates coherent optical phonons detected as a function of time by diffraction of an optical probe pulse. This experiment demonstrates the ability to probe bulk properties of materials and paves the way for ultrafast coherent four-wave-mixing techniques using X-ray probes and involving nanoscale TG spatial periods., Comment: 11 pages, 4 figures

Ultrafast electron delocalization induced by a fs laser pulse is a well-known process and is the initial step for important applications such as fragmentation of molecules or laser ablation in solids. It is well understood that an intense fs laser pulse can remove several electrons from an atom within its pulse duration. [1] However, the speed of electron localization out of an electron gas, the capture of an electron by ion, is unknown. Here, we demonstrate that electronic localization out of the conduction band can occur within only a few hundred femtoseconds. This ultrafast electron localization into 4f states has been directly quantified by transient x-ray absorption spectroscopy following photo-excitation of a Eu based correlated metal with a fs laser pulse. Our x-ray experiments show that the driving force for this process is either an ultrafast reduction of the energy of the 4f states, a change of their bandwidth or an increase of the hybridization between the 4f and the 3d states. The observed ultrafast electron localization process raises further basic questions for our understanding of electron correlations and their coupling to the lattice., Comment: 12 pages, 4 figures

Disentangling the dynamics of electrons and nuclei during nonadiabatic molecular transformations remains a considerable experimental challenge. Here we have investigated photoinduced electron transfer dynamics following a metal-to-ligand charge-transfer (MLCT) excitation of the [Fe(bmip)2]2+ photosensitizer, where bmip = 2,6-bis(3-methyl-imidazole-1- ylidine)-pyridine, with simultaneous femtosecond-resolution Fe K{\alpha} and K\b{eta} X-ray Emission Spectroscopy (XES) and Wide Angle X-ray Scattering (WAXS). This measurement clearly shows temporal oscillations in the XES and WAXS difference signals with the same 278 fs period oscillation. The oscillatory signal originates from an Fe-ligand stretching mode vibrational wavepacket on a triplet metal-centered (3MC) excited state surface. The vibrational wavepacket is created by 40% of the excited population that undergoes electron transfer from the non-equilibrium MLCT excited state to the 3MC excited state with a 110 fs time constant, while the other 60% relaxes to a 3MLCT excited state in parallel. The sensitivity of the K{\alpha} XES spectrum to molecular structure results from core-level vibronic coupling, due to a 0.7% average Fe-ligand bond length difference in the lowest energy geometry of the 1s and 2p core-ionized states. These results highlight the importance of vibronic effects in time-resolved XES experiments and demonstrate the role of metal-centered excited states in the electronic excited state relaxation dynamics of an Fe carbene photosensitizer.

The control of materials properties with light is a promising approach towards the realization of faster and smaller electronic devices. With phases that can be controlled via strain, pressure, chemical composition or dimensionality, nickelates are good candidates for the development of a new generation of high performance and low consumption devices. Here we analyze the photoinduced dynamics in a single crystalline NdNiO$_3$ film upon excitation across the electronic gap. Using time-resolved reflectivity and resonant x-ray diffraction, we show that the pump pulse induces an insulator-to-metal transition, accompanied by the melting of the charge order. Finally we compare our results to similar studies in manganites and show that the same model can be used to describe the dynamics in nickelates, hinting towards a unified description of these photoinduced phase transitions., Comment: 17 pages, 6 figures

The original observation of the Einstein-de Haas effect was a landmark experiment in the early history of modern physics that illustrates the relationship between magnetism and angular momentum. Today the effect is still discussed in elementary physics courses to demonstrate that the angular momentum associated with the aligned electron spins in a ferromagnet can be converted to mechanical angular momentum by reversing the direction of magnetisation using an external magnetic field. In recent times, a related problem in magnetism concerns the time-scale over which this angular momentum transfer can occur. It is known experimentally for several metallic ferromagnets that intense photoexcitation leads to a drop in the magnetisation on a time scale shorter than 100 fs, a phenomenon called ultrafast demagnetisation. The microscopic mechanism for this process has been hotly debated, with one key question still unanswered: where does the angular momentum go on these sub-picosecond time scales? Here we show using femtosecond time-resolved x-ray diffraction that a large fraction of the angular momentum lost from the spin system on the laserinduced demagnetisation of ferromagnetic iron is transferred to the lattice on sub-picosecond timescales, manifesting as a transverse strain wave that propagates from the surface into the bulk. By fitting a simple model of the x-ray data to simulations and optical data, we roughly estimate that the angular momentum occurs on a time scale of 200 fs and corresponds to 80% of the angular momentum lost from the spin system. Our results show that interaction with the lattice plays an essential role in the process of ultrafast demagnetisation in this system., Comment: 39 pages, 2 tables, 7 figures

We study the structural dynamics of photoexcited [Co(terpy)2]2+ in an aqueous solution with ultrafast x-ray diffuse scattering experiments conducted at the Linac Coherent Light Source. Through direct comparisons with density functional theory calculations, our analysis shows that the photoexcitation event leads to elongation of the Co-N bonds, followed by coherent Co-N bond length oscillations arising from the impulsive excitation of a vibrational mode dominated by the symmetrical stretch of all six Co-N bonds. This mode has a period of 0.33 ps and decays on a subpicosecond time scale. We find that the equilibrium bond-elongated structure of the high spin state is established on a single-picosecond time scale and that this state has a lifetime of ~ 7 ps.

A deep understanding of molecular photo-transformations is challenging because of the complex interaction between the configurations of electrons and nuclei. An initial optical excitation dissipates energy into electronic and structural degrees of freedom, often in less than one trillionth (10^-12) of a second. Molecular dynamics induced by photoexcitation have been very successfully studied with femtosecond optical spectroscopies, but electronic and nuclear dynamics are often very difficult to disentangle. X-ray based spectroscopies can reduce the ambiguity between theoretical models and experimental data, but it is only with the recent development of bright ultrafast X-ray sources, that key information during transient molecular processes can be obtained on their intrinsic timescale. In this letter, Free Electron Laser (FEL) radiation is used to measure ultrafast changes in the X-ray Absorption Near Edge Structure (XANES) during the prototypical photoreaction of a spin crossover compound. We reveal its transformation from the ligand-located electronic photoexcitation to the structural trapping of the high spin state. The results require a description beyond a kinetic model and provide a direct observation of a dynamic breathing of the main structural change. The coherent structural oscillations (period of ~265 fs) in the photoproduct potential lose synchrony within ~330 fs, whereas incoherent motions reveal the energy redistribution and vibrational cooling within ~1.6 ps. We foresee that ultrafast X-ray spectroscopies will provide invaluable insight to understand the complex physics of fundamental light induced phenomena, which are of prime interest in a multitude of chemical, physical and biological processes.

We present a reconstruction of the transverse acoustic phonon dispersion of germanium from femtosecond time-resolved x-ray diffuse scattering measurements at the Linac Coherent Light Source. We demonstrate an energy resolution of 0.3 meV with momentum resolution of 0.01 nm^-1 using 10 keV x-rays with a bandwidth of ~ 1 eV. This high resolution was achieved simultaneously for a large section of reciprocal space including regions closely following three of the principle symmetry directions. The phonon dispersion was reconstructed with less than three hours of measurement time, during which neither the x-ray energy, the sample orientation, nor the detector position were scanned. These results demonstrate how time-domain measurements can complement conventional frequency domain inelastic scattering techniques., Comment: 3 figures, 4 pages

A MHz frame rate X-ray area detector (LPD - Large Pixel Detector) is under development by the Rutherford Appleton Laboratory for the European XFEL. The detector will have 1 million pixels and allows analogue storage of 512 images taken at 4.5 MHz in the detector front end. The LPD detector has 500 mm thick silicon sensor tiles that are bump bonded to a readout ASIC. The ASICs preamplifier provides relatively low noise at high speed which results in a high dynamic range of 10^5 photons over an energy range of 5-20 keV. Small scale prototypes of 32x256 pixels (LPD 2-Tile detector) and 256x256 pixels (LPD supermodule detector) are now available for X-ray tests. The performance of prototypes of the detector is reported for first tests under synchrotron radiation (PETRA III at DESY) and Free-Electron-Laser radiation (LCLS at SLAC). The initial performance of the detector in terms of signal range and noise, radiation hardness and spatial and temporal response are reported. The main result is that the 4.5 MHz sampling detection chain is reliably working, including the analogue on-chip memory concept. The detector is at least radiation hard up to 5 MGy at 12 keV. In addition the multiple gain concept has been demonstrated over a dynamic range to 10^4 at 12 keV with a readout noise equivalent to <1 photon rms in its most sensitive mode.