ERL X-ray Science Workshop 3 Abstracts

ERL Overview and Charge to Workshop

Don Bilderback

Cornell High Energy Synchrotron Source, Cornell University

The principle of operation of an Energy Recover Linac (ERL) is explained. The status and goals of the Cornell ERL project are summarized. Relevant characteristics of a 5 GeV ERL of the type we hope to build are described and some example experiments are given.

Almost-impossible Materials Science by 3D Diffraction Microscopy

Ian McNulty

Advanced Photon Source, Argonne National Laboratory

The capability to image structure in three dimensions at the molecular scale and beyond is essential to solve manifold problems in materials, condensed matter, and biological science. Examples of current materials interest include nanostructure synthesis and self-assembly, magnetic and ferroelectric domain growth and evolution, and defects and strain in low-dimensional structures such as quantum wires. Most microscopes with the necessary resolution can only image surfaces or require many identical copies of the structure to be imaged. Electron and scanning probe methods such as atomic force microscopy are superb tools for studying surfaces and thin films; x-ray crystallography has enjoyed tremendous success in structural biology but depend on highly ordered samples. Coherent x-ray diffraction microscopy offers the tantalizing possibility of molecular-scale 3D imaging of individual non-crystalline structures without the resolution limitations of lenses and multiple scattering of electrons. Paradoxically, the weak interaction of x-rays with matter that makes 3D diffraction microscopy possible also makes it technically daunting. Diffraction tomography, which uses many views projected through the sample to obtain good depth resolution, is one solution. But if multiple copies of the sample are unavailable or many measurements on the sample are impractical due to radiation damage, the only avenue is to obtain all the data in a single measurement. Near-simultaneous acquisition of 3D data could also enable time-resolved studies currently out of reach of today's methods. We examine various schemes to achieve this utilizing the unprecedented coherent flux provided by the proposed ERL in combination with holography to aid phase retrieval from the tremendous quantity of data needed for 3D imaging.

Strain Mapping by Coherent X-Ray Diffraction

Ian Robinson

University College London and Diamond Light Source

Our latest direction in three dimensional imaging by inversion of Coherent X-ray Diffraction (CXD) patterns involves its extension to complex density functions of the interiors of nanocrystals. An ideal crystal, whatever its shape, when illuminated with an ideal plane wave must give rise to a diffraction pattern that is symmetric about the exact Bragg point at the centre of the intensity distribution. This is rarely observed in practice, since imperfect beams can easily contribute to the symmetry breaking. Our most careful measurements on lead nanocrystals grown in-situ in the 34-ID-C beamline at APS, were still found to be weakly non-centrosymmetric. This showed as missing density in the 3D images obtained by phase retrieval and inversion. However when the image function was allowed to become complex, the missing density was recovered, but only because a significant phase structure was introduced.

We interpret the imaginary part of the image as a projection of deformation fields that are attributed to the conditions of growth of the crystal. This talk will provide details of these results and consider some future applications of the CXD method which are highly attractive with the high brilliance of an ERL source.

Quantitative 3D Imaging of Nanostructured Materials by Using Coherent X-Rays

Jianwei Miao

Department of Physics and Astronomy and the Californis Nanosystems Institute, University of California at Los Angeles

Because X-rays have a longer penetration depth than electrons and X-ray wavelengths are on the order of the size of atoms, there exists the prospect for atomic-resolution X-ray microscopes that could visualize arrangement and dynamics of atoms in three dimensions - without the requirement for periodicity (e.g. crystals). X-rays, however, are much more difficult to focus than electrons. By using Fresnel zone plates, the smallest focal spot currently achievable is 30 nm for hard X-rays and 15 nm for soft-X-rays. With 3rd generation synchrotron radiation, we have developed a lensless microscope, which is based upon coherent X-ray scattering in combination with a method of direct phase recovery called oversampling. By using this novel X-ray microscope, we have investigated nanostructured materials in three dimensions. Our work opens a door for non-destructive and quantitative imaging of 3D morphology and 3D internal structures of a wide range of samples including porous materials, semiconductors, quantum dots and wires, inorganic nanostructures and biomaterials at the nanometer level.

Mapping Atomic Structure at Epitaxial Interfaces

Roy Clarke

University of Michigan

Epitaxial heterostructures constitute a wide variety of modern microelectronics devices. In the limit of ever decreasing feature dimensions, now well into the nanoscale in some cases, the interfaces of such devices are crucial to their operation and performance. To date, direct, non-destructive characterization of the atomic-level structure of films and interfaces has not been readily available and this has hampered the design and optimization of heteroepitaxial devices. In this talk we describe recent work using an x-ray interference method which is specific to thin film materials with structurally coherent interfaces. The method, known as Coherent Bragg Rod Analysis (COBRA), is useful for probing such structures with sub-Å spatial resolution while also providing chemical composition information from a three-dimensional map of the electron density (see Figure 1). We illustrate our studies with recent results on GaSb -InAs heterostructures, of interest as infrared sources and detectors. We show that, with detailed knowledge of the interfaces gained from COBRA, it is now feasible to correlate specific molecular beam epitaxy growth conditions with desired electronic characteristics associated with the interface bonding. The proposed Cornell ERL source, with its very high brightness and relatively short pulse duration, offers enticing possibilities to extend such measurements to more complex materials and into the time domain. Our recent COBRA results on ferroelectric PbTiO3 - SrTiO3 heterostructures at the Advanced Photon Source, suggest that this will be a promising area for future studies at ultrabright x-ray sources such as the ERL.

A New Era in Surface Diffraction - In-Situ Pulsed Laser Deposition of Complex Thin Films

P.R. Willmott, C.M. Schlepuetz, R. Herger, and B.D. Patterson

Swiss Light Source

Subtle structural differences in complex metal oxides lead to fundamentally different properties, due to the strong coupling of the valence electrons. On the one hand, this suggests that surface effects (e.g., relaxations and/or reconstructions) can set a lower limit to "downsizing" of thin film devices that exploit bulk effects, while on the other, unexpected new phenomena, such as surface ferroelectricity may occur only in the surface region of such materials. In addition, interpretation of the electronic properties of strongly correlated systems, investigated with ARPES, strongly depends on the reliable determination of the atomic structure down to the electron escape depth, which in many cases is presently missing. There is therefore a need to obtain exact surface structural models, and how these change in thin films with the film thickness, to better understand such phenomena.

In this contribution, I will present initial structural and growth kinetics results obtained from the Surface Diffraction Station at the Swiss Light Source. The 5-circle surface diffractometer has been fit with a customized pulsed laser deposition (PLD) thin film growth chamber. Using this equipment, it has been possible to grow atomically flat layers of materials which have no preferred cleaving plane and are otherwise practically impossible to prepare with sufficient perfection. For example, heteroepitaxial La-Sr-Mn-O thin films were grown on SrTiO3 monolayer by monolayer and the evolution of the surface structure was followed in-situ at each step. The chemical and crystallographic complexity of such systems requires a concomitant increase in independent data points compared to "simpler" semiconductor or elemental systems most commonly studied using surface diffraction to date. This has only been made possible by the use of a novel photon-counting x-ray pixel detector, which allows the acquisition of reliable surface diffraction data sets at unprecedented rates, some 100 times faster than those possible using conventional point detectors.

Other specific examples, highlighting the capabilities of the setup at the Swiss Light Source, will include the growth of textured quasicrystals on sapphire, LaAlO3 grown on SrTiO3, and YBa2Cu3O7 grown on NdGaO3.

Can Coherent Pulses Reveal the Low Energy Charge Modes in Interacting Electron Systems

Peter Abbamonte

University of Illinois at Urbana-Champaign

The canonical property of an interacting electron system is competition among several ground states, i.e. the close proximity of multiple quantum phase boundaries. The signature of a nearby phase boundary is the presence of low energy (~ 10 meV) electronic modes that exhibit symmetry characteristics of the nearby phase, the best known example being the "stripe" excitations in high temperature superconductors. A complete understanding of an interacting system, in a sense, consists of devising a description of these low energy modes.

Charge excitations are manifested in the dynamic structure factor, S(k,w), which can easily be determined from a theory by evaluation of a charge correlation function. One might think, then, that much could be learned about interacting systems by measuring S(k,w) with meV inelastic x-ray scattering. Unfortunately, however, very few of the electrons in a real material participate in these modes; most are tied up in irrelevant, high energy excitations (i.e. plasmons). Further, current analyzer-based meV instruments provide extremely low detection efficiency. So, sadly, the most important piece of information in the science of correlated electron systems still remains hidden.

In this talk I will describe a possible alternative approach to IXS using time-domain, phase-space mapping of scattered, coherent x-ray pulses. This method makes use of the fact that IXS, in contrast to diffraction processes, is incoherent, i.e. violates Liouville's theorem, i.e. broadens phase space area, which can be detected with interferometry techniques. The advantage of this method is that the entire scattered pulse can be used simultaneously, so could be much more efficient than frequency-domain IXS. In addition, the effective energy resolution is equal to the inverse of the repetition rate of the source so can be extremely high. I will discuss some possible experimental implementations based on frequency-resolved, optical gating (FROG) methods, and the relative merit of FEL vs. ERL based sources.

How Can X-ray Intensity Fluctuation Spectroscopy Push the Frontiers of Material Science

Mark Sutton

McGill University

Intensity fluctuation spectroscopy (IFS) is an ideal way to study dynamics and fluctuations in the microstructure of materials. In this talk I will review the current state of IFS measurements and present my thoughts on the important issues in Material Science that can be addressed by XIFS.

X-Ray Detectors on the Horizon

Sol M. Gruner

Cornell High Energy Synchrotron Source (CHESS) and Physics Department, Cornell University

The conduct of a synchrotron radiation experiment is very much constrained by the x-ray detectors that are used. The predominant practice in the past has been to develop general purpose x-ray detectors and to apply these to a variety of experiments. X-ray detectors with CMOS electronics integrated into each pixel are becoming increasingly practical. This allows complex, customized information processing functions to be built into the detector. Examples of relevant present and future detector technologies are reviewed. We close with an interactive discussion to address the question: What functions should be built into detectors to facilitate "almost impossible" materials science experiments?

Inelastic X-Ray Scattering as a Materials Probe Under Extreme Conditions

Esen Ercan Alp

Advanced Photon Source, Argonne National Laboratory

Inelastic X-ray scattering techniques with resolution power exceeding 104-107 are being used to study electronic and collective behavior materials under extreme conditions of heat, pressure and magnetic field. In this presentation, we will review how various inelastic x-ray scattering techniques can shed light to characteristics of materials at macro- and nano-scales. These include study of monolayers or interfaces, impurities, materials under extreme pressure, and liquids at elevated temperatures. Newly developed techniques incorporate highly monochromatic and tunable synchrotron radiation with crystal monochromators and analyzers. This talk will focus on both resonant and non-resonant momentum resolved or integrated inelastic x-ray scattering, as well as nuclear resonant inelastic x-ray scattering, and their applications for materials under high pressure. Future prospects for pushing the limits of these techniques on an Energy Recover Linac type machine will also be presented.

Note: Work performed in collaboration with W. Sturhahn, H. Sinn, T. Toellner, J. Zhao, and A. Alatas, all of Argonne National Laboratory, and their collaborators.

This work is supported by US DOE-BES Materials Science under contract number W-31-109-ENG-38.

Dynamics at the Nanoscale

Eric D. Isaacs

Argonne National Laboratory

It is a well-known paradigm for both inorganic and biological materials that structure determines function. There is also an emerging understanding that structure alone is not sufficient to determine function, but that dynamics plays a critical role. In this talk, we will highlight some frontier problems in dynamics at the nanoscale that can be impacted by the next generation of very bright x-ray sources, such as XFEL and ERL sources, which will provide unprecedented spatial and temporal coherence, as well as sub-picosecond to second temporal resolution.

We discuss two recent examples of early progress in measuring dynamics at the nanoscale using present-day third generation sources. In the first, coherent x-ray scattering measurements of the dynamics of spin and charge density wave domains in chromium reveal, surprisingly, that at low temperatures the domains remain dynamic and temperature independent with a time-scale of seconds. These results are consistent with a picture in which mesoscale domains of spins coherently tunnel between two local minima in the complex energy landscape. We will also describe first steps towards measuring the structural dynamics of small macromolecular model systems in solution as they are deformed by the sudden (sub-picosecond) optical excitation of electron-hole pairs and relax back to their ground state on a time-scale of picoseconds. Such charge-induced distortions are common to a broad range of important nanoscience problems including molecular electronics, photovoltaics, and catalysis.

Nanoscale Ferroelectricity

G. B. Stephenson*

Materials Science Division and Center for Nanoscale Materials, Argonne National Laboratory

One important scientific area for the Cornell ERL will be performing in situ studies of materials synthesis and processing. In this talk I will use examples from our current research at APS to highlight areas where the properties of the proposed ERL x-ray source would have a strong scientific impact.

Ferroelectricity is an example of a cooperative phenomenon that shows strong size effects. The paraelectric-to-ferroelectric phase transition in ultrathin films displays complex behavior driven by a fascinating competition between polarization, strain, electric field, domain wall energy, and surface chemistry. For decades, researchers have found that ferroelectric behavior is typically suppressed in films that are sufficiently thin. Various explanations have been put forward: intrinsic suppression of polarization at surfaces, the effect of depolarizing electric fields, or extrinsic effects of composition or strain. As a result, the factors responsible for the size dependence of the paraelectric-to-ferroelectric phase transition remain unresolved, in particular for the technologically important perovskites. We have been using in situ synchrotron x-ray scattering to investigate the ferroelectric properties of ultra-thin, coherently strained epitaxial films of PbTiO3 as a function of film thickness, temperature, vapor ambient, and electrical boundary conditions. The ability to perform x-ray scattering in the film growth chamber allows us to determine optimum growth conditions, to control the thickness of the films to sub-unit-cell accuracy, and to control surface and film stoichiometry during high temperature study. When films are grown on insulating SrTiO3, we find that the ferroelectric phase forms as nanoscale 180o stripe domains. When films are grown on conducting SrRuO3 layers on SrTiO3, the polar phase forms in a single domain. Although we observe a thickness-dependent TC, in both cases the polar phase is stable at room temperature in films with thicknesses as small as three unit cells.

*Collaborators: R.-V. Wang, F. Jiang, D. D. Fong, S. K. Streiffer, C. Thompson, J. A. Eastman, P. H. Fuoss, A. M. Kolpak, A. M. Rappe, and K. R. Elder.