Session M38: Tools and Techniques for Exploring Materials Physics at the Frontier of Time and Length Scales

"Ultrafast control of electronic interactions in low-dimensional cuprate superconductors"

Intense ultrashort electromagnetic fields are an increasingly important tool to realize and control novel emergent phases in quantum materials. Among a variety of nonthermal excitation pathways, a particularly intriguing route is represented by the direct light-engineering of effective many-body interactions, such as electron hopping amplitudes and electron-electron repulsion. Achieving a light-induced dynamical renormalization of the screened onsite Coulomb repulsion (“Hubbard U”) has far-reaching implications for the ultrafast manipulation of superconductivity and magnetism in the solid state. In this talk, I will discuss the light-induced renormalization of the Hubbard U in two cuprate superconductors – the quasi-1D compound Sr₂CuO_3+d, and the quasi-2D La_2-xBa_xCuO₄ – and discuss its implications for the control of superconductivity, magnetism, as well as to the realization of other long-range-ordered phases in light-driven quantum materials.  

 

    

Tuning crystal symmetries by out-of-plane shear deformation

Crystal structure plays a critical role in how emergent electronic states form in quantum materials, and this has motivated a growing interest in deforming crystals while measuring their electronic properties. The most straightforward way of accomplishing this is via mechanical strain—by mechanically deforming a crystallite, crystal parameters and symmetries can be tuned and their impact on material properties can be observed. To date, valuable insights have been gained by controlling in-plane strain using piezoelectric elements to stretch or compress a crystal while measuring its electronic transport behavior. On the other hand, out-of-plane strain—accomplished by shearing a crystal—is largely unexplored, but would enable us to study superconductivity, ferroelectricity, topology, and magnetism in new ways. Here I will discuss our efforts to perform out-of-plane shear-based measurements. To this end, we have developed multiple shearing methods based on (1) low temperature scanning probes and (2) a homebuilt "shear cell" that can be inserted inside commercial cryostats/fridges. We will discuss these methods' ability to explore stacking-dependent magnetism in chromium trihalides and phase transitions in iron-based superconductors.   

Time-resolved photoemission with a momentum microscope at LCLS II

The Linac Coherent Light Source (LCLS), the world’s first X-ray free-electron laser (FEL) has been in operation since 2009 and is currently emerging from a major upgrade. The upgraded LCLS II, which will see first light in 2023, employs a superconducting accelerator to increase the X-ray pulse repetition rate from 120 Hz to 1MHz, enabling experiments in a wide range of fields that are currently impossible. One prime example is angle-resolved photoemission spectroscopy (ARPES), which has proven to be a critical tool in advancing our understanding of quantum materials, such as high T_c superconductors and topological states of matter but has been of limited use at FELs due to space-charge effects. The high rep-rate of LCLS II will overcome this limitation, enabling viable time-resolved ARPES, PEEM, and XPD experiments. In this talk I will discuss our plans to deploy a momentum microscope endstation for time-resolved photoemission at LCLS II.

    

Depth-Sensitive Grazing Incidence Crystallography: From Atomic to Mesoscopic Scale In-plane Structures

A high brilliance neutron source, the second target station at the Spallation Neutron Source at ORNL, provides unique conditions for experiments on small thin-film samples with complex structures for wave vector transfer extended from the total reflection region to inverse interatomic distances. The latter scales are accessible with grazing incidence diffractometry with its extreme sensitivity to near surface and interfacial phenomena, as well as to nuclear and magnetic scattering length density distributions across the thickness of thin film heterostructure. Such sensitivity is absent in conventional diffractometry. The solution to access the in-plane atomic scale structures with the depth sensitivity implemented in the M-STAR reflectometer design [1] is a new option specialized for measurements of the grazing incidence lateral diffraction.  This option will combine diffractometry with reflectometry using the same beam-forming and polarization set up, the advantages of ToF mode, as well as the depth-sensitivity.  In view of the remarkable possibilities provided by the GID, it is also important to note that out-of-plane magnetization is not accessible in conventional reflectometry. This property is essential for studying systems with perpendicular anisotropy, as well as with an anti-ferromagnetic structure. Another important property is that the coherence length for GID reduces in any direction to the value of the order l/Dw (where w is azimuthal angle), which is much smaller than the characteristic size of ferromagnetic domains. This means that the GID is kind of local probe for domain magnetization: the diffraction intensity of each individual domain adds up incoherently. A feasibility of GID to study multi-domain state in ferromagnetic thin film heterostructure was demonstrated in the pilot experiments. It has confirmed theoretical expectations and sufficient luminosity of the method.

    

Order-Disorder, Displacive Ferroelectrics, and use of Bragg CDI for characterization of Ferroelectric materials

Nimish Nazirkar¹ and Edwin Fohtung<sup>1

1</sup>Department of Materials Sciences and Engineering, Rensselaer Polytechnic Institute, Troy, NY-12180

Ferroelectricity is observed in materials with non-centrosymmetric crystal structures having built-in Spontaneous Polarization. Ferroelectrics are broadly classified into three types: Displacive, Order-Disorder and electronic¹. Displacive Ferroelectricity relates to the movement of atoms in unit cells leading to spontaneous polarization². Order-Disorder ferroelectricity is related to the changes in phonon modes leading to shifts in bond lengths. Most of the time order-disorder ferroelectricity is followed by the displacement of atoms from equilibrium positions³. Bragg coherent diffraction imaging (BCDI)^4,5 a well-established technique for imaging the internal strain of nanoparticles, can be used to image the displacement of electron densities of the atoms. It can be used to relate displacement fields to the ferroelectric polarization in various ferroelectric materials using the Born-Effective Approximation and Landau Theory. 

References:

[1] http://abinitiomp.org/teaching/mtle6120/slides/insulators.pdf

[2] Wang, C., Wu, J., Zeng, Z. et al. Soft-mode dynamics in the ferroelectric phase transition of GeTe. npj Comput Mater 7, 118 (2021). https://doi.org/10.1038/s41524-021-00588-4

[3] Critical Role of Order-Disorder Behavior in Perovskite Ferroelectric KNbO₃. Zhi Tan, Yuting Peng, Jiao An, Qiming Zhang, and Jianguo Zhu. Inorganic Chemistry 2021 60 (11), 7961-7973.DOI: 10.1021/acs.inorgchem.1c00478

[4] Karpov, D., Liu, Z., Rolo, T. et al. Three-dimensional imaging of vortex structure in a ferroelectric nanoparticle driven by an electric field. Nat Commun 8, 280 (2017). https://doi.org/10.1038/s41467-017-00318-9

[5] Nanoscale topological defects and improper ferroelectric domains in multiferroic barium hexaferrite nanocrystals .D. Karpov, Z. Liu, A. Kumar, B. Kiefer, R. Harder, T. Lookman, and E. Fohtung Phys. Rev. B 100, 054432 – Published 22 August 2019

 

 

    

Deep-ultraviolet transient grating for characterizing nanoscale thermal, elastic, and interfacial properties in high-bandgap materials

The functional properties of complex or nanostructured materials deviate from bulk, due to the increased influence of surfaces and interfaces [1,2]. Characterizing these properties is crucial to designing new, efficient nanoelectronics, energy materials, and quantum technologies. In this work, we demonstrate a deep-ultraviolet (DUV) transient grating to characterize a more general set of materials at smaller length scales than visible-based methods allow. We generate <200nm ultrafast DUV pulses [3] to a excite a sample in a transient grating modality. A transient grating is created by interfering two beams to form a spatially-modulated pattern on the sample surface [4]. The resultant heating causes thermal expansion which launches acoustic waves into the sample, allowing us to study its elastic and thermal properties. The visible transient grating periodicity is limited by the wavelength of the light; however, by using DUV light, we are able to create a smaller period interference pattern down to a few hundred nanometers [5]. Moreover, the DUV pulses are above the bandgap for many visibly-transparent samples, extending our technique to more materials, including diamond and next-generation battery technologies.

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References:

[1] J. Knobloch et al., ACS Appl. Mater. Interfaces 14, 41316 (2022)

[2] A. Beardo et al., ACS Nano 15 (8), 13019 (2021)

[3] J. Ringling, et al., Opt. Lett. 18 (23), 2035 (1993)

[4] J. Rogers et al., Ann. Rev. Mater. Sci. 30, 117 (2000)

[5] A. Maznev et al., Appl. Phys. Lett 113, 221905 (2018)   

Multi-contrast laboratory-based x-ray microscopy with freestanding, high-aspect-ratio gold masks

We are developing a novel x-ray microscope working with laboratory sources, in which phase and scatter images are obtained simultaneously with absorption ones, and image resolution is determined by the apertures in a custom-developed mask placed before the specimen. The ability to produce masks with apertures smaller than a micron enables sub-micron resolution with much larger detector pixel sizes and x-ray focal spots. 

Masks were fabricated based on gold electroplating into a silicon mold coated with various thin films to form a voltage barrier, plating base, and sacrificial layer, followed by the mold removal to obtain the freestanding gold membrane with void slit apertures. The ensuing imaging tests confirmed that an aperture-driven spatial resolution can be attained, and that this does not change when detector pixel size and x-ray focal spot are altered. Early imaging tests show high phase sensitivity, enabling e.g. the detection of individual cells in a cartilage layers. Scatter images were also retrieved. These are sensitive to sample features on length scales below the resolution of the system, which in this case means nanoscale structures. Indeed, features on the 100 nm scale were included in the imaging tests, and were shown to produce a distinctive scattering signal.  

 

    

Designing CENTAUR, the small- and wide-angle neutron scattering instrument with diffraction and spectrometer capability at the Second Target Station of the Spallation Neutron Source

CENTAUR is one of the first eight instruments selected for the Second Target Station of Spallation Neutron Source at Oak Ridge National Laboratory. As a work-horse Small-Angle Neutron Scattering instrument, its capabilities are extended to cover Wide-Angle Neutron Scattering (WANS) and diffraction, therefore provide a simultaneous coverage for structure measurement from atomic scale to hundreds of nanometers. Notably, the WANS and diffraction capability will be unique among SANS instruments in the United States. Centaur will enable in situ/ operando experiments on materials with hierarchical architecture, for kinetic and/or out-of-equilibrium studies of phenomena with time-resolution down to seconds in many areas of materials research including soft matter, polymer science, geology, biology and quantum condensed matter. The spectrometer mode extends the momentum transfer and energy transfer to a lower Q than those typically covered by other spectrometers and provides a unique capability for inelastic SANS experiment. Additionally, beam polarization will enable detailed structural and dynamical investigations of magnetic materials and quantum materials.

 

In this presentation, we will present the progress on the design of the Centaur instrument. And we invite the community to provide feedback and suggestions for the design, and to envision future experiments to take advantage of its full capability.

    

Determination of viscoelastic Poisson’s ratio on a novel combined torsional-axial MultiDrive device

The lateral contraction of a material when stressing the material in axial direction is described by the Poisson’s ratio. In case of viscoelastic materials, like polymers, this parameter is a function of temperature and excitation frequency, when measured in oscillatory mode, and important for e.g. structural mechanics simulations. Methods to determine the viscoelastic Poisson’s ratio are manifold and can be classified in direct methods, which directly measure the change of the specimen dimension, and indirect methods from which the measurement of two moduli like shear modulus and Young’s modulus seems to be the most effective. A new measuring device concept is introduced which allows one to do combined torsional and axial measurements on one single device. In this contribution measurements on both cylindrical and rectangular specimens are presented in order to determine the viscoelastic Poisson’s ratio of different solid polymers. Using a linear and a rotational measuring drive in one instrument enables the determination of complex shear modulus |G*| as well as the complex Young’s modulus |E*| on a single sample in a continuous measurement run. Consecutive frequency sweeps at room temperature in both torsion and tension deformation modes were performed to obtain the viscoelastic Poisson’s ratio. A suite of polymers ranging from amorphous (PMMA, PC), thermoplastic polyurethane (TPU) to thermosets were studied. 

    

Author not Attending

Understanding the structural and electronic correlations such as excitons and phonons in quantum materials is critical for unraveling the fundamental mechanism behind mesoscale phenomena and designing novel device architectures. Normally, the excitons and phonons in different material systems are characterized by optical absorption, photoluminescence, infrared spectroscopy, Raman spectroscopy, and neutron scattering. However, in most of the techniques, the spatial resolution is not enough to comment on the effect of local structural modulations such as defects, moiré patterns, and buried interfaces on the electronic properties of materials. In recent years, correlated electron microscopy and spectroscopy have been used extensively to understand the local structural and electronic properties [1]. In my talk, I will elaborate on how this technique is useful to untangle correlated properties of quantum materials. I will point out two case studies in the field of two-dimensional materials and thin film oxides. In the first case, I would talk about the sensitivity of intralayer excitons to local stacking configurations in twisted transition metal dichalcogenide heterostructures and the effect of excitons on defects in Janus structures [2,3]. In the second case, I would elaborate more on measuring phonons and cyrstal field at the domain walls of ferroelectric oxides [4,5]. At the end of my talk, some of the future challenges involved in electron spectroscopy and some pathways to solve this problem.

    

How many detector pixels do we need for super-resolution ptychography?

To overcome the spatial resolution limit set by aperture-limited diffraction in traditional scanning transmission electron microscopy (STEM), microscopists have developed ptychography enabled by iterative phase retrieval algorithms and high-dynamic-range pixel array detectors. However, the state-of-the-art detectors with thousands of pixels for acquiring full scattering distribution required for ptychography reconstructions suffer from relatively slow acquisition speed compared to differential phase contrast (DPC) imaging with 2x2 pixels, and the slower acquisitions lead to more severe scan noise, drift, and damage. Here, we demonstrated that super-resolution is still possible with only 2x2 detector pixels with increased real space sampling and that the ptychography can significantly outperform the conventional DPC analysis. We optimized experimental and reconstruction parameters through simulated datasets and performed experiments on WSe₂/MoS₂ bilayer Moiré patterns with 2x2 pixels DPC detectors. We successfully achieved super-resolution ptychography reconstruction with rapid acquisition conditions (25pA and only 1µs dwell time) using segmented detectors.

    

Nanoscale ultrafast dynamics in molecular/TMD heterostructure imaged by time-resolved photoemission electron microscopy

Advancements in synthesizing and constructing organic-inorganic 2D van der Waals heterostructures have enabled modular design of novel properties due to the extensive chemical tunability of molecular building blocks. However, these heterostructures and interfaces can have considerable spatial variation, which modifies electronic structure and interfacial charge transfer on the nanoscale and determines functionality. To establish a nanoscale morphology-property relationship as well as understand how Moiré superlattice impacts charge transfer, we investigated an atomically thin perylene diimide (PDI) film deposited on polycrystalline single layer MoS₂ using photoemission electron microscopy (PEEM) with spatial resolution as good as 25 nm. We were able to image molecular orientation of PDI crystals and calculate the relative angles with respect to MoS₂ domains. We mapped out spatially-evolving excited state lifetimes from time- and energy-resolved PEEM. Statistical analysis shows that the nanoscale variation in the excited state dynamics can be attributed to the non-epitaxial PDI growth, which introduces distinct lattice mismatch that structurally modifies interlayer coupling. Our results provide insights on structural variations underpinning the twist angle dependent electronic properties and promote a molecular framework for desired optoelectronic applications.