Many Microscopy techniques have been developed to explore atomic structures of material and biological specimens at a nano-scale. Coherent Diffractive Imaging (CDI) and Atomic Electron Tomography (AET) are the most powerful among these techniques. Mathematics, especially variational optimization, also follows and supports microscopy, helping to solve various image processing problems such as deblurring and denoising. In our latest research, variational methods can help obtain super-resolution ptychography images, marking a substantial improvement in computational microscopy and bypassing the resolution limit. While sub-pixel shifting and structured probes are two crucial keys to the super-resolution problem, total variation and l1 regularization play a significant role in reconstruction. In another research project, we explore the magnetic vector fields created in the vacancy between atoms of a magnetized material. Our analysis shows that the vector tomography needs in-plane rotation and constraint support to guarantee reconstruction. The sparsity inducing l1 minimization is the right tool to generate highly accurate support. Our variational methods have successfully solved a wide range of image processing problems. We will continue to utilize these techniques again in our upcoming research.
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So far Lauren Mason has created 165 blog entries.
Functional materials—like molecular electronics, biomaterials, light-emitting diodes, or new photovoltaic materials—gain their electronic or photonic properties from complex and multifaceted interactions occurring at the elementary scales of their atomic or molecular constituents. In addition, the ability to control the functions of these materials through external stimuli , e.g., in the form of strong optical excitations, enables new properties in the materials, making them appealing for new technological applications. However, a major obstacle to overcome is the combination of the very fast time (billionths of a second) scales and the very small spatial (nanometer) scales which define the many-body interactions of the elementary excitations in the material which define its function. The extremely high time and spatial resolutions needed have been extremely difficult to achieve simultaneously. Many physicists have, therefore, struggled to visualize the interactions within these materials. In a paper recently published in Nature Communications, JILA Fellow Markus Raschke and his team report on a new ultrafast imaging technique that could solve this issue.
Magnetic systems exhibiting chirality have gained significant attention over the past decade. From enabling the efficient movement of magnetic domain walls to stabilizing the topological magnetic quasiparticles known as skyrmions, chirality and chiral interactions may play a central role in the next generation of magnetic memory and computing schemes. The possible responses of chiral magnetic systems to stimuli such as magnetic fields, electrical currents, light, and thermal gradients is at the forefront of thin-film magnetismresearch . I will briefly introduce the underlying interactions that give rise to chiral magnetism, the emergence of topology and discuss some of the surprising systems where chirality and topology are emerging. I will focus on the various techniques that can be combined to image and understand the three-dimensional magnetic structures that emerge.
. Hellman et al., Reviews of Modern Physics 89, 025006 (2017).
Kukreja group at UC Davis is currently hiring for a postdoctoral position focused on ultrafast optical pump
probe studies of quantum materials.
Position Overview: The position will involve research projects focusing on understanding the role of
electronic and magnetic degrees of freedom in quantum materials and their dynamical behavior under
laser excitation. Low temperature optical pump probe setup at Kukreja Laboratory will be utilized to
access fundamental timescales to study the evolution of electronic and magnetic order at femtosecond
to picosecond timescales across phase transitions, while time‐resolved x‐ray diffraction at synchrotron
sources will be utilized to study structural evolution. In order to investigate the ultrafast behavior as a
function of sample ground state, thin films samples will be carefully tuned using parameters such as
epitaxial strain, anion stoichiometry and cation doping.
Candidates interested in optical pump probe or time‐resolved x‐ray studies of complex oxides and
magnetic materials are strongly encouraged to apply. In addition, the position will also involve further
extending and developing the optical pump probe laboratory, and handling large x‐ray scattering datasets
obtained at user facilities such as National Synchrotron Light Source II (NSLS II), Advanced Photon Source
(APS), Advanced Light Source (ALS) and Linac Coherent Light Source (LCLS) and LCLS II,
Project Overview: Quantum materials have emerged as potential candidates to realize energy‐efficient
computing for ever‐increasing technological demands of the internet of things, big data, and cloud
computing. Quantum materials display strong correlations between their spin, charge, orbital, and lattice
degrees of freedom, which results in a rich variety of electronic and magnetic properties. Emergence of
novel quantum states under non‐equilibrium conditions in quantum materials challenges the limits of
understanding at microscopic length scales and ultrafast time scales. However, fundamental
understanding of the role of nanoscale disorder and fluctuations in quantum materials is impeded by the
lack of experimental methods which can access both characteristic lengthscales and timescales. This
project will utilize optical pump‐probe and coherent x‐ray methods to overcome this knowledge gap to
develop spatio‐temporal understanding of complex oxides. These studies will enable mapping of the
domain dynamics and correlations as a function of emergent electronic and magnetic ordering in strongly
correlated systems. These studies will lead to development of a complete overview of electronic,
magnetic, and structural properties of quantum materials with time scales down to the ultrafast regime
and atomic resolution, to unravel nanoscale disorder in quantum materials and its evolution upon optical
Congratulations to Naomi Ginsberg for Receiving the Carol D. Soc Distinguished Graduate Student Mentoring Award for Later-Career Faculty
The Carol D. Soc Distinguished Graduate Student Mentoring Awards are administered by the Graduate Division in collaboration with the Graduate Council of the Academic Senate, funded by a generous bequest from the estate of Carol Soc, a former employee of the Graduate Division. The mentoring awards were a part of the Sarlo Awards Teaching for Excellence program, established in 1997 to honor outstanding faculty at outstanding Northern California colleges and Universities.
The discovery and analysis of X-ray diffraction from crystals by Max von Laue, William Henry Bragg and William Lawrence Bragg in 1912 marked the birth of crystallography. Over the last century, crystallography has been fundamental to the development of many fields of science. However, many samples in physics, chemistry, materials science, nanoscience, geology, and biology are non-crystalline, and thus their 3D structures are not accessible by traditional crystallography. Overcoming this hurdle has required the development of new structure determination methods. In this talk, I will present two methods that can go beyond crystallography: coherent diffractive imaging (CDI) and atomic electron tomography (AET). In CDI, the diffraction pattern of a non-crystalline sample or a nanocrystal is first measured and then directly phased to obtain an image. The well-known phase problem is solved by combining the oversampling method with iterative algorithms. In the first part of the talk, I will briefly discuss the principle of CDI and highlight its capability of direct observation of 3D topological spin textures and their interactions in a ferromagnetic superlattice. In the second part of the talk, I will present a general tomographic method, termed AET, for 3D structure determination of crystal defects and disordered materials at the single atomic level. By combining advanced electron microscopes with powerful computational algorithms, AET has been used to reveal the 3D atomic structure of crystal defects and chemical order/disorder and to precisely localize the 3D coordinates of individual atoms in materials without assuming crystallinity. The experimentally measured coordinates can then be used as direct input for quantum mechanical calculations of material properties such as atomic spin and orbital magnetic moments and local magnetocrystalline anisotropy. As coherent X-ray sources and powerful electron microscopes are under rapid development around the world, we expect that CDI and AET will find broad applications in both the physical and biological sciences.
In a new paper published in the Journal of Physical Chemistry Letters, Jimenez and his team report a new experimental setup to search for the cause of a mysterious fluorescent signal that appears to be from entangled photon excitation. According to Jimenez: “We built a setup where you could use either a classical laser or entangled photons to look for fluorescence. And the reason we built it is to ask: ‘What is it that other people were seeing when they were claiming to see entangled photon-excited fluorescence?’ We saw no signal in our previous work published a year ago, headed by Kristen Parzuchowski. So now, we’re wondering, people are seeing something, what could it possibly be? That was the detective work here.” The results of their new experiments suggested that hot-band absorption (HBA) by the subject molecules, could be the potential culprit for this mysterious fluorescent signal, making it the prime suspect.
Atomically thin or two-dimensional (2D) materials can be assembled into bespoke heterostructures to produce some extraordinary physical phenomena. Likewise, these highly tunable materials are useful platforms for exploring fundamental questions of interfacial chemical/electrochemical reactivity. One exciting and relatively recent example is the formation of moiré superlattices from azimuthally misoriented (twisted) layers. These moiré superlattices result in flat bands that lead to an array of correlated electronic phases. However, in these systems, complex strain relaxation can also strongly influence the fragile electronic states of the material. Precise characterization of these materials and their properties is therefore critical to the understanding of the behavior of these novel moiré materials (and 2D heterostructures in general). In this talk, I will discuss how spontaneous mechanical deformations (atomic reconstruction) and resultant intralayer strain fields at moiré superlattices of twisted bilayer graphene have been quantitatively imaged using 4D-STEM Bragg interferometry. I will also discuss the impact of these mechanical deformations to the electronic band structure of these moiré superlattices and the correlated electronic phases they host. The talk will then explore how various degrees of freedom that are unique to 2D materials may be used to tailor interfacial chemistry at well-defined mesoscopic electrodes and the outlook for new paradigms of functional materials for energy conversion and low-power electronic devices.
Congratulations to Charlie Bevis for Receiving a Marie Skłodowska-Curie Individual Fellowship (MSCA-IF)
The European Commission will support a total of 1156 experienced post-doctoral researchers with €242 million to work at top universities, research centres, private organisations and small and medium-sized enterprises in Europe and the rest of the world. The European Research Executive Agency (REA) received 8356 applications for this call. The Commission will award €206 million to 1025 researchers through European Postdoctoral Fellowships, allowing them to carry out their projects in the EU or countries associated to Horizon Europe. The action provides support to excellent individual researchers to implement an original and personalized research project, while developing their skills through advanced training, international, interdisciplinary and inter-sectoral mobility.
Uncovering structure/function relationships in condensed phase electronic materials is increasingly important for applications from solar energy to quantum optoelectronics. In this talk, we will explore multimodal microscopy to the role of microscopic heterogeneity and defects in emerging semiconductors, focusing on halide perovskites as a system that is not only technologically important, but also scientifically challenging. Halide perovskites are soft, and easily damaged by low doses of electrons. They are also complicated mixed conductors, exhibiting electronic and ionic relaxation dynamics that vary across many orders of magnitude in time and space. To probe these systems, we combine hyperspectral optical spectroscopy, electron microscopy, scanning probe microscopy, and data science tools to understand and control defects for applications in photovoltaics, showing how surface and grain boundary passivation enables the growth of films that approach thermodynamic efficiency limits for performance, while also problem the role of processing additives on microstructure and performance.