Speakers

Title: Single-shot coherent imaging with randomized light

Abstract:

The development of coherent diffractive (lensless) imaging has represented a paradigm shift in X-ray microscopy. This methodology has become widely successful, engendering new computational imaging approaches for the visualization of a variety of materials – nanocrystals, magnetic thin films, catalysts, among many others. Lensless imaging techniques provide access to several materials properties – topography, strain, extended defects, spin textures, and chemical composition – making them a formidable toolset for the characterization of complex materials at the nanoscale.

The development of ultrafast coherent X-ray sources such as the X-ray free electron lasers (XFELs) has created new opportunities for X-ray microscopy studies of the spatiotemporal dynamics of complex matter. A known limitation of some imaging methods, e.g., ptychography, is the requirement of multiple exposures which makes them inherently incompatible with ultrafast time-resolved studies. Other methods (Bragg CDI, holography, etc.) require nanostructured or patterned samples to solve the phase problem from single diffraction patterns.

To overcome these challenges, we developed a new imaging modality – randomized probe imaging – which uses highly structured illumination to enable single-frame and single-shot spatiotemporal imaging of generic, unpatterned samples with nanometric spatial resolution and time resolution limited by the single-shot pulse length of XFEL sources. I will present our recent work demonstrating randomized probe imaging with synchrotron and XFEL sources, which enabled single-frame and single-shot imaging with a flux-limited 140 nm full-pitch spatial resolution and sub-40 fs X-ray pulses, respectively.

Title: Magnetic and ferroelectric heterogeneity and dynamics in oxide electronic materials

Abstract:

Complex oxide electronic materials exhibit complex configurations of ferroelectric and magnetic order that pose important scientific questions and provide the route to novel useful properties. The development of nanoscale order and the dynamics of this order involves mechanisms and manifestations of ordering that are at present not completely understood. The magnetism of complex oxides is particularly challenging because magnetic moments can reside on multiple distinct ions within the crystallographic unit cell and have interactions via mechanisms that depend on the temperature and external field. We have recently applied synchrotron and FEL-based hard x-ray scattering to gain insight into the static magnetic order of magnetic garnets and the dynamics and excitations of this order. Similar questions pertain to the relationships between the structure and polarization of ferroelectric materials. The imaging and scattering methods available at the FLEXON beamline promise to provide new insights into these dynamics, particularly at transition-metal resonances that are not available through hard x-ray techniques.

Title: Imaging a dilute set of domains: how to simplify the extraction of spatial information from coherent scattering experiments

Abstract:

As the ALS and other synchrotrons upgrade to diffraction limited sources, coherent x-ray scattering will become a powerful tool to probe spatial and temporal characteristics of materials. However, as with any new technique, it is important to envision new and reliable ways to perform coherent scattering experiments. Challenges remain in extracting spatial information by scattering from a sample with a large number of random domains which yields a complex speckle pattern. In this talk, we propose a new approach that would yield a simple way to extract temporal correlations with much more spatial information without the need of phase reconstruction. By confining the sampling to a simple spatial structure with only a few elements, a well-defined Fourier transform is easy to track (like a slit interference pattern). This simplification occurs naturally at the onset of a first order phase transition when domains begin to form, which we have observed using resonant coherent x-ray diffraction to study the formation of antiferromagnetic domains in the correlated antiferromagnet PrNiO3. We demonstrate that it is possible to quantitatively extract the arrangements and sizes of the first-formed domains from coherent diffraction patterns. At the onset of the antiferromagnetic transition, the domains are dilute within the beam footprint, thus resulting in relatively simple coherent diffraction patterns.

Based on these insights we propose a way to reverse this approach, and design beam structure on the sample with a well-defined pattern. Then, the speckle pattern is convoluted with a simple function of a few slits, which can be easily interpreted in Fourier space. This will inform us about the directional correlations with well defined spatial length scales, adding a versatile way to study the spatiotemporal correlations in materials.

Title: Coherent x-ray scattering studies of quantum materials

Abstract:

Mesoscale phenomena play an important role in the dynamics of phase transitions in quantum materials. In order to fully understand and tailor nanoscale functionalities of quantum materials, detailed access to the nanoscale regime, correlation length scales and their temporal evolution is required. X-ray photon correlation spectroscopy (XPCS) provides a unique way to characterize nanoscale heterogeneities and their correlations across the phase transition. It allows us to study domain dynamics and fluctuations by capturing high resolution coherent speckle patterns in reciprocal space which can be considered as a fingerprint of the sample in real space. The combination of XPCS with FLEXON capabilities can provide a novel characterization tool for quantum materials. In this talk, I will present our recent XPCS work across the metal-insulator transitions in magnetite and nickelates and discuss the role of nano- and meso-scale heterogeneities in these canonical systems. I will also discuss some of our recent work which utilizes XPCS at ultrafast timescales to study quantum materials as well as new opportunities for XPCS going forward both at slow and fast timescales.

Title: Prospects for coherent x-rays to access intrinsic dynamical heterogeneity in frustrated magnets

Abstract:

In frustrated magnets, competing magnetic interactions prevent the formation of semi-classical magnetically ordered ground states and can give rise to phases of matter exhibiting non-trivial entanglement. However, frustrated magnets are also sensitive to chemical or structural disorder and the physical effects of even minute disorder can mimic experimental signatures of a quantum state. Uncovering the true ground states in these materials requires measurements of the dynamic response functions that has typically the domain of inelastic neutron scattering. Diffraction limited synchrotron upgrades and the development of ultrafast coherent x-ray sources at XFELs now bring about new opportunities to access magnetic fluctuations on ns time scales relevant to the low energy dynamic response functions of frustrated magnets. In particular, XPCS offers the exciting possibility to explore the spontaneous magnetization density fluctuations that are not accessible to the ensemble averaged inelastic neutron scattering cross-section. However, the extreme sample environments required to reach quantum regimes in many frustrated magnets still pose a challenge for coherent x-ray experiments. I will present recent results where resonant x-rays provide new insights into complex magnetic correlations of some model frustrated magnets. Then discuss prospects for future experiments that can use XPCS to access dynamics and address long standing problems in frustrated magnetism.

Title: Heterogeneity in Magnetic Complex Oxide Thin Films

Abstract:

Tunable transitions between crystal phases with distinct physical properties which are triggered by ion migration are potential candidates for emerging technologies such as neuromorphic computing devices. As an example, topotactic transformations involve structural changes between different phases via the loss or gain of material while retaining a crystallographic relationship. Cobaltite oxides La1-xSrxCoO3 serve as a model system for investigating such topotactic transformations due to their high surface reactivity, mobile oxygen vacancies, and the number of related crystal structures including the Grenier (ABO2.7), brownmillerite (BM, ABO2.5), square planar (ABO2), and Ruddlesden-Popper phases (RP, An+1BnO3n+1, n=integer). In addition, La0.67Sr0.33CoO3 (LSCO) displays magnetic heterogeneity in the form of magneto-electronic phase separation consisting of small ferromagnetic (FM)/metallic clusters within an antiferromagnetic (AFM)/insulating matrix. In this talk, I will discuss the evolution of the structural and functional properties of LSCO thin films in which their oxygen vacancy concentration was controlled through the deposition of a strong oxygen getter layer (e.g., Gd)[1,2] or exposure to high temperature anneals under highly reducing environments.[3] With increasing getter layer thickness, our studies showed the coexistence of perovskite and BM phases with a critical oxygen vacancy threshold above which extended BM filaments were observed. In contrast, annealing under highly reducing conditions led to the formation of the rare La1.4Sr0.6Co1+νO4−δ RP phase (where 0 < ν < 1 and 0 < δ < 1) through the loss of both oxygen and cobalt ions. The corresponding magnetic properties were tunable between various FM and AFM phases, and the room temperature resistivity spanned eight orders of magnitude, demonstration the potential of magneto-ionics as the basis for next generation device applications.

[1] D.A. Gilbert, Y. Takamura et al., Phys. Rev. Mater., 2, 104402 (2018)

[2] G. Rippy, Y. Takamura et al., Phys. Rev. Mater., 3, 082001(R) (2019)

[3] I-Ting Chiu, Y. Takamura et al., Phys. Rev. Mater., 5, 064416 (2021)