Recent works have suggested that transient suppression of a charge density wave (CDW) by an ultra-short excitation can lead to an inversion of the CDW phase. We experimentally investigate the dynamics of the CDW in K_0.3MoO_3 by time resolved x-ray diffraction after excitation with optical pulses. Our results indicate a transient inversion of the CDW phase close to the surface that evolves into a highly disordered state in less than one picosecond. Numerical simulations solving the Ginzburg-Landau equation including disorder from strong pinning defects reproduce our main observations. Our findings highlight the critical role of disorder in schemes for coherent control in condensed matter systems.

The use of intense terahertz (THz) pulses to manipulate low-energy excitations offers a powerful approach for ultrafast control of electronic and magnetic properties in materials. Theory suggests that circular ionic motions driven by THz fields carry angular momentum, potentially generating internal magnetic fields. Recent experiments in nonmagnetic SrTiO3 (STO) have hinted at such THz-induced fields, but their origin remains debated. Here, we employ ultrafast x-ray diffraction to resolve the time-dependent ionic trajectories in STO following excitation by circularly polarized THz pulses. Our analysis reveals that oxygen ions, despite their lower mass, contribute around 90

The rise of the electronic age sparked a quest for increasingly faster and smaller switches, since this element is ubiquitous and foundational in any electronic circuit to regulate the flow of current. Mott insulators are promising candidates to meet this need as they undergo extremely fast resistive switching under electric field initiated by an avalanche phenomena. However, the nature of the final switched state is still under debate. The spatially resolved micro‐X‐ray Diffraction imaging and micro‐Raman experiments carried out on the prototypal Mott insulator (V 0.95 Cr 0.05 ) 2 O 3 show that the resistive switching is associated with the creation of a conducting filamentary path consisting in an isosymmetric compressed phase without any chemical or symmetry change. This strongly suggests that the avalanche initiated resistive switching mechanism is inherited from the bandwidth‐controlled Mott‐Hubbard transition just like the laser induced insulator to metal transition recently studied in the same system. This discovery may hence ease the development of a new branch of electronics called Mottronics.

X-ray absorption spectroscopy (XAS) of 3d transition metals provides important electronic structure information for many fields. However, X-ray-induced radiation damage under physiological temperature has prevented using this method to study dilute aqueous systems, such as metalloenzymes, as the catalytic reaction proceeds. Here we present a new approach to enable operando XAS of dilute biological samples and demonstrate its feasibility with K-edge XAS spectra from the Mn cluster in photosystem II and the Fe-S centers in photosystem I. This approach combines highly efficient sample delivery strategies and a robust signal normalization method with high-transmission Bragg diffraction-based spectrometers at X-ray free-electron lasers (XFELs) in a damage-free, shot-by-shot mode. These photon-out spectrometers have been optimized for discriminating the metal Mn/Fe Kα fluorescence signals from the overwhelming scattering background present on currently available detectors for XFELs that lack suitable energy discrimination. We quantify the enhanced performance metrics of the spectrometer and discuss its potential applications for acquiring time-resolved XAS spectra of biological samples during their reactions at XFELs.

The rise of the electronic age sparked a quest for increasingly faster and smaller switches, since this element is ubiquitous and foundational in any electronic circuit to regulate the flow of current. Mott insulators are promising candidates to meet this need as they undergo extremely fast resistive switching under electric field initiated by an avalanche phenomena. However, the nature of the final switched state is still under debate. The spatially resolved micro-X-ray Diffraction imaging and micro-Raman experiments carried out on the prototypal Mott insulator (V0.95Cr0.05)(2)O-3 show that the resistive switching is associated with the creation of a conducting filamentary path consisting in an isosymmetric compressed phase without any chemical or symmetry change. This strongly suggests that the avalanche initiated resistive switching mechanism is inherited from the bandwidth-controlled Mott-Hubbard transition just like the laser induced insulator to metal transition recently studied in the same system. This discovery may hence ease the development of a new branch of electronics called Mottronics.

Understanding how light modifies long-range order in quantum materials is key to improving our ability to control functionality. However, this is challenging if the response is heterogeneous. Here we address the most common form of light-induced heterogeneity-surface melting-and measure the dynamics of orbital order in the layered manganite La0.5Sr1.5MnO4. We isolate the surface dynamics from the bulk by measuring the orbital truncation rod and orbital Bragg peak. After photoexcitation, the orbital Bragg peak shows an unusual narrowing, which suggests an increase in correlation length of the probed volume. By contrast, the correlation length at the surface decreases. These differences can be reconciled if the material is heterogeneous, and light melts a less ordered surface. By isolating the surface response, we determine that the loss of long-range order is an incoherent process, which is probably accompanied by the formation of local polarons.

Nowadays, materials science is moving towards the understanding and control of materials in nonequilibrium states by making use of perturbative techniques to investigate their dynamical responses. From this perspective, the use of ultrashort light pulses seems to be a relevant approach as it can selectively address different degrees of freedom in solid-state systems and more particularly electrons. Such a method can help to decipher the physical phenomena arising from electronic correlations and complements a more conventional methodology where the phase diagrams of materials are investigated at thermodynamical equilibrium. Here, we combine femtosecond optical spectroscopy and a high-pressure setup to monitor the ultrafast out-of-equilibrium photo response of a V_{2}O_{3} thin film across the pressure driven insulator-to-metal transition. The experimental results demonstrate the possibility to use the spectroscopy of coherent phonons as a thermodynamical phase marker in V_{2}O_{3} thin films. In addition, the frequency behavior of the ultrafast coherent phonon mode (A_{1g} character) seems to reflect the manifestation of a strong coupling between the lattice and electronic degrees of freedom near first-order transition lines with a pronounced drop in frequency around the critical pressure.

Ultrafast photoexcitation can generate internal compressive stress in Mott insulators that lead to strain waves from free surfaces. These photoinduced elastic waves can trigger phase transitions in materials. However, a comprehensive physical picture of the phase transformation dynamics that includes acoustic-scale propagation has not yet been developed. Here we demonstrate that such a strain-wave mechanism drives the ultrafast insulator-to-metal phase transition in granular thin films of the Mott material V2O3. Our time-resolved optical reflectivity and X-ray diffraction measurements reveal that an inverse ferroelastic shear occurs before the insulator-to-metal transition, which propagates in the wake of a compressive strain wave. These dynamics are governed by the domain size and film thickness, respectively. Our results clarify the morphological conditions for the ultrafast phase transition that is favoured in granular thin films and hindered in single crystals. The resulting physical picture sheds light on the ultrafast phase transitions in quantum materials and future devices based on Mott insulators.

This study describes the fabrication of hybrid two-dimensional (2D)-quantum dot (QD) MoS2-AgInS2 photoconductive devices through the mechanical pressing of a MoS2 flake onto an AgInS2 QD film. The devices exhibit an enhanced photoresponse at both continuous and modulated optical excitations, compared with the bare MoS2 or AgInS2 layer, due to the formation of a built-in electric field near the MoS2/AgInS2 interface. The continuous wave photoresponse is significantly higher due to the effective photoconductive gain when electrons flow freely through the MoS2 flake, whereas holes are effectively trapped in AgInS2 QDs. The study highlights the potential of hybrid 2D-QD MoS2-AgInS2 devices for photovoltaic and optoelectronic applications.

Ultrafast physics opens new avenues for directing materials to different functional macroscopic states on non-thermal dynamical pathways. In any phase transition involving volume and/or ferroelastic deformation, an often overlooked mechanism emerges whereby photoinduced elastic waves drive the transition. However, a comprehensive physical picture of transformation dynamics which includes acoustic scale propagation remained elusive. Here we show that such a strain wave mechanism drives the ultrafast insulator-to-metal phase transition (IMT) in the V2O3 Mott material. We discuss the underlying physics based on time-resolved optical reflectivity and X-ray diffraction probing granular thin films. We evidence the role of strain wave mechanisms in ultrafast changes either with or without symmetry breaking. We reveal inverse ferroelastic shear occurring before the IMT propagating in the wake of compressive strain wave. These dynamics are shown to be governed by the domain size and the film thickness, respectively. A fluence threshold is evidenced for the onset of IMT at macroscopic scale, as well as phase separation at intermediate fluence and complete transformation at saturating fluence. We clarify the morphological conditions for the ultrafast IMT that is favoured in granular thin films, and hindered in single crystals. The resulting physical picture shed new light on ultrafast phase transitions in quantum materials and future devices based ond Mott insulators.

Resonant driving of electronic transitions with coherent laser sources creates quantum coherent superpositions of the involved electronic states. Most time-resolved studies have focused on gases or isolated subsystems embedded in insulating solids, aiming for applications in quantum information. Here, we demonstrate coherent control of orbital wavefunctions in pyrochlore $Tb_{2}Ti_{2}O_{7}$, which forms an interacting spin liquid ground state. We show that resonant excitation with a strong THz pulse creates a coherent superposition of the lowest energy Tb 4f states before the magnetic interactions eventually dephase them. The coherence manifests itself as a macroscopic oscillating magnetic dipole, which is detected by ultrafast resonant x-ray diffraction. The induced quantum coherence demonstrates coherent control of orbital wave functions, a new tool for the ultrafast manipulation and investigation of quantum materials.

We have used temperature-dependent photoconductivity (PC) with different excitation wavelengths and intensities to study the photoexcited charge-carrier transport within GeSn/Ge/Si heterostructures. The evolution of the PC spectra with temperature was analyzed between 10 and 200 K. These strained GeSn films were grown with high enough Sn content such that the band gaps were direct. As such, the relationships between the band gaps and the temperature were determined using photoconductivity spectroscopy. As a result, an anomalous, linear blueshift of the PC spectral edge was found with increasing temperature. This was attributed to the variation of the GeSn band gap within the film, due to variations in strain and the increased contribution to PC from the region of highest Sn content. The change in photocurrent with excitation intensity and temperature demonstrates that conduction occurs predominantly through the GeSn and Ge layers under low optical pumping and through the Si substrate under high optical pumping. A phenomenological model of photoconductivity in GeSn as it depends on strain was proposed. This detailed understanding of the transport of photoexcited carriers along the GeSn layers is critical for developing GeSn/Ge-based optoelectronic devices.

Mott insulators are a class of strongly correlated materials with emergent properties important for modern electronics applications, such as artificial neural networks. Under an electric field, these compounds undergo a resistive switching that may be used to build up artificial neurons. However, the mechanism of this resistive switching is still under debate and may depend on the Mott material involved. Some works suggest an electronic avalanche phenomenon, while others propose an electrothermal scenario. As electric pulses produce both Joule heating and hot carriers, disentangling their respective roles requires the use of another external stimulus. Here, an ultrashort light pulse is used to tune the number of photogenerated carriers and the energy provided to the system. In these pump-pump-probe experiments, a crystal of the Mott insulator GaTa4Se8 is simultaneously excited by electric and laser pulses while an electric probe monitors its conductivity. The study shows that the resistive switching is affected by the number of generated photocarriers rather than by the accumulation of energy deposited by the femtosecond laser. It supports therefore a mechanism driven by generation of hot carriers. Finally, our work opens the possibility to build up an artificial electro-optical "Mott" neuron tuned by a femtosecond laser pulse.

Understanding the photoconductivity mechanisms in two-dimensional materials is essential for future optoelectronic applications. Through the photoconductivity experiments under continuous light excitation or using modulated excitation in the temperature range of 10-300 K, we probe the mechanisms of lateral photoconductivity in monolayer MoS2.

Mott insulators are a class of strongly correlated materials with emergent properties important for modern electronics applications. The key property for application is the Electric Mott insulator to metal Transitions (EMT) whose proposed mechanism is related to the creation of hot electrons by electric field triggering an electronic avalanche after a time delay. This model suggests that the direct creation of hot carriers thanks to a laser pulse should drastically affect the EMT. We have tested this idea by performing pump-pump-probe experiments on single crystals of the Mott insulator GaTa4Se8, whereby electric and laser pulses simultaneously excite the crystal while electric probe monitors its conductivity. Our results show that the concomitant application of femtosecond laser pulse reduced the time delay of EMT by a factor up to. Measurements performed with different laser wavelengths and fluences support moreover that the EMT is driven by a hot carriers generation mechanism.

Mott insulators display puzzling insulator to metal transitions under electric field. We study here the Mott insulator GaV4S8 using time-resolved Photoemission Electron Microcopy and reveal unusually long carrier lifetimes, supporting the mechanism of electronic avalanche.

GaTa 4 Se 8 is a Mott insulator known to exhibit an electric Mott transition, characterized by a drop in electrical resistivity, when an electric field larger than 1 - 10 kV/cm is applied for a few tens of microseconds using electrodes deposited on the sample. Here, we show that a resistivity drop can be induced in this material within less than a picosecond. These dynamics occur after excitation by a high field THz pump pulse and persist for a few picoseconds, well beyond the duration of the pump pulse.

Laser-speckle patterns dynamics caused by transmission irradiation through the ground glass and layer of transparent liquid have been studied. Frame by frame analysis of dynamical speckle video has shown that the decaying of the amplitude of the correlation functions depends on liquids parameters. The results are discussed in terms by percolation model of liquid.

Fluorescent organic dyes solutions are used for non-contact measurement of the millimeter wave absorption in liquids simulating biological tissue. There is still not any certain idea of the physical mechanism describing this process despite the widespread technology of microwave radiation in the food industry, biotechnology and medicine. For creating adequate physical model one requires an accurate command of knowledge concerning to the relation between millimeter waves and irradiated object. There were three H-bonded liquids selected as the samples with different coefficients of absorption in the millimeter range like water (strong absorption), glycerol (medium absorption) and ethylene glycol (light absorption). The measurements showed that the greatest response to the action of microwaves occurs for glycerol solutions: R6G (building-up luminescence) and RC (fading luminescence). For aqueous solutions the signal is lower due to lower quantum efficiency of luminescence, and for ethylene glycol due to the low absorption of microwaves. In the area of exposure a local increase of temperature was estimated. For aqueous solutions of both dyes the maximum temperature increase is about 7 degrees C caused with millimeter waves absorption, which coincides with the direct radio physical measurements and confirmed by theoretical calculations. However, for glycerol solution R6G temperature equivalent for building-up luminescence is around 9 degrees C, and for the solution of ethylene glycol it's about 15 degrees. It is assumed the possibility of non-thermal effect of microwaves on the different processes and substances. The application of this non-contact temperature sensing is a simple and novel method to detect temperature change in small biological objects.