Oleg Yu. Gorobtsov

There is significant technological interest in developing ever faster switching between different electronic and magnetic states of matter. Manipulating properties at terahertz rates requires accessing the intrinsic timescales of both electrons and associated phonons, which is possible with short-pulse photoexcitation. However, in many Mott insulators, the electronic transition is accompanied by the nucleation and growth of percolating domains of the changed lattice structure, leading to empirical timescales dominated by slowly coarsening dynamics. Here we use time-resolved X-ray diffraction and reflectivity measurements to show that the photoinduced insulator-to-metal transition in an epitaxially strained Mott insulating thin film occurs without observable domain formation and coarsening effects, allowing the study of the intrinsic electronic and lattice dynamics. Above a fluence threshold, the initial electronic excitation …

Advancements of high-power laser sources in the THz frequency range have opened up opportunities for coherent intense excitation of infrared (IR)-active phonons in crystalline materials. The nonlinear phononics mechanism utilizes anharmonic coupling between driven IR-active modes and other lattice modes to induce changes in crystal structure and functional properties on picosecond timescales. Recent experimental and theoretical works have noted the potential for phonon-induced strains on these short time scales [npj Quantum Mater. 5, 95 (2020), PRL 129, 167401 (2022)]. A natural way to measure the response of the lattice due to these THz pulses is by employing X-ray diffraction to observe changes to the position and intensities of Bragg peaks.We show, using theory and first-principle calculations, that these lattice anharmonicities are responsible for the expansion of the c-axis in a biaxially strained thin …

We report on the evolution of the charge density wave (CDW) and spin density wave (SDW) order of a chromium film following photoexcitation with an ultrafast optical laser pulse. The CDW is measured by ultrafast time-resolved x-ray diffraction of the CDW satellite that tracks the suppression and recovery of the CDW following photoexcitation. We find that as the temperature of the film approaches a discontinuous phase transition in the CDW and SDW order, the time scales of recovery increase exponentially from the expected thermal time scales. We extend a Landau model for SDW systems to account for this critical slowing with the appropriate boundary conditions imposed by the geometry of the thin film system. This model allows us to assess the energy barrier between available CDW/SDW states with different spatial periodicities.

Mott metal-insulator transitions possess electronic, magnetic, and structural degrees of freedom promising next generation energy-efficient electronics. We report a previously unknown, hierarchically ordered state during a Mott transition and demonstrate correlated switching of functional electronic properties. We elucidate in-situ formation of an intrinsic supercrystal in a Ca2RuO4 thin film. Machine learning-assisted X-ray nanodiffraction together with electron microscopy reveal multi-scale periodic domain formation at and below the film transition temperature (TFilm ~ 200-250 K) and a separate anisotropic spatial structure at and above TFilm. Local resistivity measurements imply an intrinsic coupling of the supercrystal orientation to the material's anisotropic conductivity. Our findings add an additional degree of complexity to the physical understanding of Mott transitions, opening opportunities for designing materials with tunable electronic properties.

New properties and exotic quantum phenomena can form due to periodic nanotextures, including Moire patterns, ferroic domains, and topologically protected magnetization and polarization textures. Despite the availability of powerful tools to characterize the atomic crystal structure, the visualization of nanoscale strain-modulated structural motifs remains challenging. Here, we develop nondestructive real-space imaging of periodic lattice distortions in thin epitaxial films and report an emergent periodic nanotexture in a Mott insulator. Specifically, we combine iterative phase retrieval with unsupervised machine learning to invert the diffuse scattering pattern from conventional X-ray reciprocal-space maps into real-space images of crystalline displacements. Our imaging in PbTiO3/SrTiO3 superlattices exhibiting checkerboard strain modulation substantiates published phase-field model calculations. Furthermore, the …

Non‐equilibrium defects often dictate the macroscopic properties of materials. They largely define the reversibility and kinetics of processes in intercalation hosts in rechargeable batteries. Recently, imaging methods have demonstrated that transient dislocations briefly appear in intercalation hosts during ion diffusion. Despite new discoveries, the understanding of impact, formation and self‐healing mechanisms of transient defects, including and beyond dislocations, is lacking. Here, operando X‐ray Bragg Coherent Diffractive Imaging (BCDI) and diffraction peak analysis capture the stages of formation of a unique metastable domain boundary, defect self‐healing, and resolve the local impact of defects on ionic diffusion in NaxNi1−yMnyO2 intercalation hosts in a charging sodium‐ion battery. Results, applicable to a wide range of layered intercalation materials due to the shared nature of framework layers, elucidate …

Coherent X-ray diffraction imaging (CXDI) is a lensless imaging technique by directly inverting the oversampled diffraction pattern into real-space images. Because of its non-destructive nature and ability to image sub-picometer atomic-lattice displacements with nanometer resolutions, CXDI has been applied to visualize biological cells, strain in nanocrystals, and recently operando changes in energy materials. Yet, it is limited to objects spatially confined in all three dimensions. Here, we will demonstrate the extension of CXDI to mapping periodic lattice distortions in an extended object. By combining the conventional CXDI iterative computation and an unsupervised machine learning clustering algorithm, we imaged the periodic nanotextures in epitaxially strained Ca 2 RuO 4 thin films grown on LaAlO 3 substrate via the inversion of its diffraction pattern. The result reveals the formation of striped periodic …

Driving the lattice structure with resonant phononic excitations presents a novel way for controlling quantum materials. We present the results of an ultrafast diffraction experiment using THz resonant excitation to drive an IR mode in a thin, strained LaAlO 3 epitaxial film. Our results explore predicted and measured ultrafast, IR light induced structural processes in the film. We do not observe the prediction that nonlinear phonon coupling excites a giant ultrafast response in the lowest frequency Raman phonon. The ultrafast structural phase transition associated with this Raman phonon is not observed. However, our results show that the IR pump induces the growth of structural domains on the timescale of several picoseconds. Moreover, we find that a longitudinal acoustic phonon forms in the films, the period of which is several picoseconds and is determined by the film thickness. Finally, we observe that the film …

Hydrogen fuel cells and electrolyzers operating below 600 °C, ideally below 400 °C, are essential components in the clean energy transition. Yttrium‐doped barium zirconate BaZr0.8Y0.2O3‐d (BZY) has attracted a lot of attention as a proton‐conducting solid oxide for electrochemical devices due to its high chemical stability and proton conductivity in the desired temperature range. Grain interfaces and topological defects modulate bulk proton conductivity and hydration, especially at low temperatures. Therefore, understanding the nanoscale crystal structure dynamics in situ is crucial to achieving high proton transport, material stability, and extending the operating range of proton‐conducting solid oxides. Here, Bragg coherent X‐ray diffractive imaging is applied to investigate in situ and in 3D nanoscale dynamics in BZY during hydration over 40 h at 200 °C, in the low‐temperature range. An unexpected activity of …

Electronic instabilities drive ordering transitions in condensed matter. Despite many advances in the microscopic understanding of the ordered states, a more nuanced and profound question often remains unanswered: how do the collective excitations influence the electronic order formation? Here, we experimentally show that a phonon affects the spin density wave (SDW) formation after an SDW-quench by femtosecond laser pulses. In a thin film, the temperature-dependent SDW period is quantized, allowing us to track the out-of-equilibrium formation path of the SDW precisely. By exploiting its persistent coupling to the lattice, we probe the SDW through the transient lattice distortion, measured by femtosecond X-ray diffraction. We find that within 500 femtoseconds after a complete quench, the SDW forms with the low-temperature period, directly bypassing a thermal state with the high-temperature period. We argue …

Bragg coherent X-ray diffractive imaging is a cutting-edge method for recovering three-dimensional crystal structure with nanoscale resolution. Phase retrieval provides an atomic displacement parallel to the Bragg peak reciprocal lattice vector. The derivative of the displacement along the same vector provides the normal strain field, which typically serves as a proxy for any structural changes. In this communication it is found that the other component of the displacement gradient, perpendicular to the reciprocal lattice vector, provides additional information from the experimental data collected from nanocrystals with mobile dislocations. Demonstration on published experimental data show how the perpendicular component of the displacement gradient adds to existing analysis, enabling an estimate for the external stresses, pinpointing the location of surface dislocations, and predicting the dislocation motion in in situ …

In article number 2103521, Jason J. Huang and co-workers use operando single particle X-ray diffraction to probe disorder dynamics in individual cathode particles of Nax [Ni 1/3 Mn 2/3] O 2, an archetypal cathode material for sodium-ion batteries. The high throughput technique reveals the role of order-disorder transitions in the detrimental P2-O2 structural phase transition. These findings serve to guide the design of sodium-ion battery cathodes with a longer cycle life.

Proton-conducting ceramics have attracted interest as moderate-temperature proton conductors for applications such as energy conversion and hydrogen production. Effects of topological defects and interfacial hydrated layers on proton transport and concerns about structural degradation make understanding nanostructure dynamics crucial for improving structural stability and proton transport. However, material polycrystallinity, absorption, and reactive operating conditions have prevented detailed in-situ measurements of the nanostructure dynamics. Here, we find experimentally in-situ in yttrium-doped barium zirconate, an archetypal proton conductor, that contrary to the assumptions of structural stability the nanostructure is unexpectedly dynamic at temperatures as low as 200 C. We achieve this by applying coherent X-ray diffraction to a BaZr0. 8Y0. 2O3-d sintered pellet to image in-situ three-dimensional …

Functional properties of transition-metal oxides strongly depend on crystallographic defects; crystallographic lattice deviations can affect ionic diffusion and adsorbate binding energies. Scanning x-ray nanodiffraction enables imaging of local structural distortions across an extended spatial region of thin samples. Yet, localized lattice distortions remain challenging to detect and localize using nanodiffraction, due to their weak diffuse scattering. Here, we apply an unsupervised machine learning clustering algorithm to isolate the low-intensity diffuse scattering in as-grown and alkaline-treated thin epitaxially strained SrIrO 3 films. We pinpoint the defect locations, find additional strain variation in the morphology of electrochemically cycled SrIrO 3, and interpret the defect type by analyzing the diffraction profile through clustering. Our findings demonstrate the use of a machine learning clustering algorithm for identifying …

Non-equilibrium defects often dictate macroscopic functional properties of materials. In intercalation hosts, widely used in rechargeable batteries, high-dimensional defects largely define reversibility and kinetics1,2,3,4. However, transient defects briefly appearing during ionic transport have been challenging to capture, limiting the understanding of their life cycle and impact. Here, we overcome this challenge and track operando the interaction and impact of metastable defects within NaxNi1-xMnyO2 intercalation hosts in a charging sodium-ion battery. Three-dimensional coherent X-ray imaging3,4,5 reveals transformation and self-healing of a metastable domain boundary, glissile dislocation loop, and stacking fault. A local strain gradient suggests a quantifiable difference in ion diffusion, coincident with the macroscopic change in diffusion coefficient. Analysis of the unexpected4,6 defect anisotropy highlights the importance of mesostructure, suggesting a possible control approach and disputing the rigidity of the framework layers. The shared nature of oxygen framework layers makes our results applicable to a wide range of intercalation materials.

Electronic instabilities drive ordering transitions in condensed matter. Despite many advances in sophisticated microscopic models of the ordered state, a more nuanced, yet profound, question often remains unanswered: How do collective excitations influence the formation of electronic order? Using time-resolved free electron X-ray diffraction technique, we monitor the laser-induced quench and subsequent recovery of a spin density wave (SDW) in a 28 nm thick Cr film. In this talk, we will discuss how the formation of SDW in antiferromagnetic Cr thin film after its quench by a femtosecond (fs) laser is influenced by an acoustic phonon launched by the laser. Our results demonstrate how a coherent ionic lattice vibration markedly modifies the out-of-equilibrium pathway in energy diagram of an electronic phase transformation, highlighting the opportunity for using collective excitations for guiding electronic instabilities.

The spin-phonon interaction in spin density wave (SDW) systems often determines the free energy landscape that drives the evolution of the system. When a passing energy flux, such as photoexcitation, drives a crystalline system far from equilibrium, the resulting lattice displacement generates transient vibrational states. Manipulating intermediate vibrational states in the vicinity of the critical point, where the SDW order parameter changes dramatically, would then allow dynamical control over functional properties. Here we combine double photoexcitation with an X-ray free-electron laser (XFEL) probe to control and detect the lifetime and magnitude of the intermediate vibrational state near the critical point of the SDW in chromium. We apply Landau theory to identify the mechanism of control as a repeated partial quench and sub picosecond recovery of the SDW. Our results showcase the capabilities to influence …

The relationship between charge transport and structural transformations dictates the properties of electrochemical systems. Despite their importance, the reduction–oxidation (redox) reactions within dynamically coexisting structures have so far eluded direct operando investigation. Here, we use resonant X-ray scattering to select X-ray spectra of a crystal structure coexisting with a different structure during a redox-induced phase transformation in P2-Na2/3Ni1/3Mn2/3O2. The spectra of the P2 structure become static midway through the sodium extraction in an operando coin cell, while the overall desodiation proceeds. The coincident emergence of the O2 structure reveals the rigid link between the local redox and the long-range order in this archetypal sodium-ion battery material. Structure-selective X-ray spectroscopy thus opens a powerful avenue for resolving the dynamic chemistry of different structural phases …

The emerging concept of `beam by design' in free-electron laser (FEL) accelerator physics aims for accurate manipulation of the electron beam to tailor spectral and temporal properties of the radiation for specific experimental purposes, such as X-ray pump/X-ray probe and multiple wavelength experiments. `Beam by design' requires fast, efficient, and detailed feedback on the spectral and temporal properties of the generated X-ray radiation. Here a simple and cost-efficient method to extract information on the longitudinal Wigner distribution function of emitted FEL pulses is proposed. The method requires only an ensemble of measured FEL spectra and is rather robust with respect to accelerator fluctuations. The method is applied to both the simulated SASE spectra with known radiation properties as well as to the SASE spectra measured at the European XFEL revealing underlying non-linear chirp of the generated …

Recently, layered transition metal (TM) oxide materials for cathodes have attracted attention in sodium ion batteries (SIBs). Despite appeal of SIBs for large-scale energy storage due to the relative abundance of sodium, its lower toxicity, and ease of production, the SIBs are yet to demonstrate sufficient cycle life. Understanding and imaging defect nucleation and evolution of strain in the layered cathode materials during the battery operation is crucial to overcome the SIB problems and improve material design. We extended operando Bragg coherent diffractive imaging to observe in 3D structural changes within nanoparticles of sodium ion battery cathode. We observe nucleation of defects perpendicular to the layers during the charging of the battery in layered SIB cathode materials. We also observe the differences in interlayer distance within the nanocrystal, implying difference in ionic diffusion. Operando …