SIMES Publications
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"Table-top extreme ultraviolet second harmonic generation" — Tobias Helk: Emma Berger, Sasawat Jamnuch, Lars Hoffmann, Adeline Kabacinski, Julien Gautier, Fabien Tissandier, Jean-Philipe Goddet, Hung-Tzu Chang, Juwon Oh, C. Das Pemmaraju, Tod A. Pascal, Stephane Sebban, Christian Spielmann, Michael Zuerch; Science Advances, 05/19/21.
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Emma Berger, Sasawat Jamnuch, Lars Hoffmann, Adeline Kabacinski, Julien Gautier, Fabien Tissandier, Jean-Philipe Goddet, Hung-Tzu Chang, Juwon Oh, C. Das Pemmaraju, Tod A. Pascal, Stephane Sebban, Christian Spielmann, Michael Zuerch
Abstract
The lack of available table-top extreme ultraviolet (XUV) sources with high enough fluxes and coherence properties has limited the availability of nonlinear XUV and x-ray spectroscopies to free-electron lasers (FELs). Here, we demonstrate second harmonic generation (SHG) on a table-top XUV source by observing SHG near the Ti M2,3 edge with a high-harmonic seeded soft x-ray laser. Furthermore, this experiment represents the first SHG experiment in the XUV. First-principles electronic structure calculations suggest the surface specificity and separate the observed signal into its resonant and nonresonant contributions. The realization of XUV-SHG on a table-top source opens up more accessible opportunities for the study of element-specific dynamics in multicomponent systems where surface, interfacial, and bulk-phase asymmetries play a driving role.
"Excited-State Charge Distribution of a Donor−π–Acceptor Zn Porphyrin Probed by N K-Edge Transient Absorption Spectroscopy" — Amy A. Cordones : C. Das Pemmaraju, Robert W. Schoenlein, Jae Hyuk Lee, Ioannis Zegkinoglou, Maria-Eleni Ragoussi, Franz J. Himpsel, Gema de la Torre ; The Journal of Physical Chemistry Letters, 01/22/21.
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C. Das Pemmaraju, Robert W. Schoenlein, Jae Hyuk Lee, Ioannis Zegkinoglou, Maria-Eleni Ragoussi, Franz J. Himpsel, Gema de la Torre
Abstract
Zinc porphyrin solar cell dyes with donor−π–acceptor architectures combine light absorber (π), electron-donor, and electron-acceptor moieties inside a single molecule with atomic precision. The donor−π–acceptor design promotes the separation of charge carriers following optical excitation. Here, we probe the excited-state electronic structure within such molecules by combining time-resolved X-ray absorption spectroscopy at the N K-edge with first-principles time-dependent density functional theory (TD-DFT) calculations. Customized Zn porphyrins with strong-donor triphenylamine groups or weak-donor tri-tert-butylbenzene groups were synthesized. Energetically well-separated N K-edge absorption features simultaneously probe the excited-state electronic structure from the perspectives of the macrocycle and triphenylamine N atoms. New absorption transitions between the macrocycle N atoms and the excited-state HOMO vacancy are observed, and the triphenylamine associated absorption feature blue-shifts, consistent with partial oxidation of the donor groups in the excited state.

"The role of metal substitution in tuning anion redox in sodium metal layered oxides revealed by X-ray spectroscopy and theory" — Iwnetim Abate: Se Young Kim, C. Das Pemmaraju, Michael F. Toney, Wanli Yang, Thomas P. Devereaux, William C. Chueh, and Linda F. Nazar; Angewandte Chemie International Edition, 12/15/20.
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Se Young Kim, C. Das Pemmaraju, Michael F. Toney, Wanli Yang, Thomas P. Devereaux, William C. Chueh, and Linda F. Nazar
Abstract
We investigate high-valent oxygen redox in the positive Na-ion electrode P2-Na0.67−x[Fe0.5Mn0.5]O2 (NMF) where Fe is partially substituted with Cu (P2-Na0.67−x[Mn0.66Fe0.20Cu0.14]O2, NMFC) or Ni (P2-Na0.67−x[Mn0.65Fe0.20Ni0.15]O2, NMFN). From combined analysis of resonant inelastic X-ray scattering and X-ray near-edge structure with electrochemical voltage hysteresis and X-ray pair distribution function profiles, we correlate structural disorder with high-valent oxygen redox and its improvement by Ni or Cu substitution. Density of states calculations elaborate considerable anionic redox in NMF and NMFC without the widely accepted requirement of an A-O-A′ local configuration in the pristine materials (where A=Na and A′=Li, Mg, vacancy, etc.). We also show that the Jahn–Teller nature of Fe4+ and the stabilization mechanism of anionic redox could determine the extent of structural disorder in the materials. These findings shed light on the design principles in TM and anion redox for positive electrodes to improve the performance of Na-ion batteries.
"Site-specific structure at multiple length scales in kagome quantum spin liquid candidates" — Rebecca W. Smaha: Idris Boukahil, Charles J. Titus, Jack Mingde Jiang, John P. Sheckelton, Wei He, Jiajia Wen, John Vinson, Suyin Grass Wang, Yu-Sheng Chen, Simon J. Teat, Thomas P. Devereaux, C. Das Pemmaraju, and Young S. Lee; Physical Review Materials, 12/14/20.
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Idris Boukahil, Charles J. Titus, Jack Mingde Jiang, John P. Sheckelton, Wei He, Jiajia Wen, John Vinson, Suyin Grass Wang, Yu-Sheng Chen, Simon J. Teat, Thomas P. Devereaux, C. Das Pemmaraju, and Young S. Lee
Abstract
Realizing a quantum spin liquid (QSL) ground state in a real material is a leading issue in condensed matter physics research. In this pursuit, it is crucial to fully characterize the structure and influence of defects, as these can significantly affect the fragile QSL physics. Here, we perform a variety of cutting-edge synchrotron x-ray scattering and spectroscopy techniques, and we advance new methodologies for site-specific diffraction and L-edge Zn absorption spectroscopy. The experimental results along with our first-principles calculations address outstanding questions about the local and long-range structures of the two leading kagome QSL candidates, Zn-substituted barlowite Cu3ZnxCu1−x(OH)6FBr and herbertsmithite Cu3Zn(OH)6Cl2. On all length scales probed, there is no evidence that Zn substitutes onto the kagome layers, thereby preserving the QSL physics of the kagome lattice. Our calculations show that antisite disorder is not energetically favorable and is even less favorable in Zn-barlowite compared to herbertsmithite. Site-specific x-ray diffraction measurements of Zn-barlowite reveal that Cu2+ and Zn2+ selectively occupy distinct interlayer sites, in contrast to herbertsmithite. Using the first measured Zn L-edge inelastic x-ray absorption spectra combined with calculations, we discover a systematic correlation between the loss of inversion symmetry from pseudo-octahedral (herbertsmithite) to trigonal prismatic coordination (Zn-barlowite) with the emergence of a new peak. Overall, our measurements suggest that Zn-barlowite has structural advantages over herbertsmithite that make its magnetic properties closer to an ideal QSL candidate: its kagome layers are highly resistant to nonmagnetic defects while the interlayers can accommodate a higher amount of Zn substitution.
"Accuracy in Resolving the First Hydration Layer on a Transition Metal Oxide Surface: Experiment (AP-XPS) & Theory" — Daniel J. Aschaffenburg: Seiji Kawasaki, Chaitanya Das Pemmaraju, and Tanja Cuk; The Journal of Physical Chemistry C, 08/28/20.
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Seiji Kawasaki, Chaitanya Das Pemmaraju, and Tanja Cuk
Abstract
Understanding the equilibrium conditions at the metal oxide/aqueous interface is a key component toward visualizing the structure of water in confined environments and differentiating the catalytic activity of transition-metal oxides. While ambient pressure X-ray photoelectron spectroscopy (AP-XPS) has been the primary technique to investigate the formation of a hydration layer on many surfaces, results over the extended relative humidity (RH) range accessible experimentally have not been compared quantitatively to theoretical predictions. With the use of first-principles theoretical methods and accumulated knowledge of AP-XPS spectral analysis, we do so here for a model surface, TiO2-terminated undoped SrTiO3(100) (STO). The measured distribution of OH and H2O coverages from vacuum up to the first hydration layer is in good agreement with a static density functional theory (DFT) configuration involving partial dissociation of H2O per Ti-atom mediated by H-bonding. Furthermore, ab initio molecular dynamics (AIMD) simulations at 300 K for select coverages (1/4, 1/2, and 1 ML) test the role of fluctuations and entropy in the competition between adsorption and dissociation with coverage. This comparison between theory and experiment for OH and H2O coverages on STO provides a foundation for a more quantitative assessment of the first hydration layer and associated competition between adsorption, dissociation, and H-bonding on transition-metal oxide surfaces.

"Berry curvature memory through electrically driven stacking transitions" — Jun Xiao: Ying Wang, Hua Wang, C. D. Pemmaraju, Siqi Wang, Philipp Muscher, Edbert J. Sie, Clara M. Nyby, Thomas P. Devereaux, Xiaofeng Qian, Xiang Zhang & Aaron M. Lindenberg; Nature Physics, 06/29/20.
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Ying Wang, Hua Wang, C. D. Pemmaraju, Siqi Wang, Philipp Muscher, Edbert J. Sie, Clara M. Nyby, Thomas P. Devereaux, Xiaofeng Qian, Xiang Zhang & Aaron M. Lindenberg
Abstract
In two-dimensional layered quantum materials, the stacking order of the layers determines both the crystalline symmetry and electronic properties such as the Berry curvature, topology and electron correlation1,2,3,4. Electrical stimuli can influence quasiparticle interactions and the free-energy landscape5,6, making it possible to dynamically modify the stacking order and reveal hidden structures that host different quantum properties. Here, we demonstrate electrically driven stacking transitions that can be applied to design non-volatile memory based on Berry curvature in few-layer WTe2. The interplay of out-of-plane electric fields and electrostatic doping controls in-plane interlayer sliding and creates multiple polar and centrosymmetric stacking orders. In situ nonlinear Hall transport reveals that such stacking rearrangements result in a layer-parity-selective Berry curvature memory in momentum space, where the sign reversal of the Berry curvature and its dipole only occurs in odd-layer crystals. Our findings open an avenue towards exploring coupling between topology, electron correlations and ferroelectricity in hidden stacking orders and demonstrate a new low-energy-cost, electrically controlled topological memory in the atomically thin limit.
"Probing ultrafast C–Br bond fission in the UV photochemistry of bromoform with core-to-valence transient absorption spectroscopy" — Benjamin W. Toulson: Mario Borgwardt, Han Wang, Florian Lackner, Adam S. Chatterley, C. D. Pemmaraju, Daniel M. Neumark, Stephen R. Leone, David Prendergast, Oliver Gessner; Structural Dynamics, 10/11/19.
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Mario Borgwardt, Han Wang, Florian Lackner, Adam S. Chatterley, C. D. Pemmaraju, Daniel M. Neumark, Stephen R. Leone, David Prendergast, Oliver Gessner
Abstract
UV pump–extreme UV (XUV) probe femtosecond transient absorption spectroscopy is used to study the 268 nm induced photodissociation dynamics of bromoform (CHBr3). Core-to-valence transitions at the Br(3d) absorption edge (∼70 eV) provide an atomic scale perspective of the reaction, sensitive to changes in the local valence electronic structure, with ultrafast time resolution. The XUV spectra track how the singly occupied molecular orbitals of transient electronic states develop throughout the C–Br bond fission, eventually forming radical Br and CHBr2 products. Complementary ab initio calculations of XUV spectral fingerprints are performed for transient atomic arrangements obtained from sampling excited-state molecular dynamics simulations. C–Br fission along an approximately Cs symmetrical reaction pathway leads to a continuous change of electronic orbital characters and atomic arrangements. Two timescales dominate changes in the transient absorption spectra, reflecting the different characteristic motions of the light C and H atoms and the heavy Br atoms. Within the first 40 fs, distortion from C3v symmetry to form a quasiplanar CHBr2 by the displacement of the (light) CH moiety causes significant changes to the valence electronic structure. Displacement of the (heavy) Br atoms is delayed and requires up to ∼300 fs to form separate Br + CHBr2 products. We demonstrate that transitions between the valence-excited (initial) and valence + core-excited (final) state electronic configurations produced by XUV absorption are sensitive to the localization of valence orbitals during bond fission. The change in valence electron-core hole interaction provides a physical explanation for spectral shifts during the process of bond cleavage.
"Corvus: a framework for interfacing scientific software for spectroscopic and materials science applications" — S. M. Story: F. D. Vila, J. J. Kas, K. B. Raniga, C. D. Pemmaraju and J. J. Rehr; Journal of Synchrotron Radiation, 07/24/19.
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F. D. Vila, J. J. Kas, K. B. Raniga, C. D. Pemmaraju and J. J. Rehr
Abstract
Corvus, a Python-based package designed for managing workflows of physical simulations that utilize multiple scientific software packages, is presented. Corvus can be run as an executable script with an input file and automatically generated or custom workflows, or interactively, in order to build custom workflows with a set of Corvus-specific tools. Several prototypical examples are presented that link density functional, vibrational and X-ray spectroscopy software packages and are of interest to the synchrotron community. These examples highlight the simplification of complex spectroscopy calculations that were previously limited to expert users, and demonstrate the flexibility of the Corvus infrastructure to tackle more general problems in other research areas.
"THz-Pump UED-Probe on a Topological Weyl Semimetal" — Edbert J. Sie: Clara M. Nyby, C. D. Pemmaraju, Su Ji Park, Xiaozhe Shen, Jie Yang, Matthias C. Hoffmann, B. K. Ofori-Okai, Renkai Li, Alexander H. Reid, Stephen Weathersby, Ehren Mannebach, Nathan Finney, Daniel Rhodes, Daniel Chenet, Abhinandan Antony, Luis Balicas, James Hone, Thomas P. Devereaux, Tony F. Heinz, Xijie Wang, and Aaron M. Lindenberg; Optical Society of America, 05/05/19.
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Clara M. Nyby, C. D. Pemmaraju, Su Ji Park, Xiaozhe Shen, Jie Yang, Matthias C. Hoffmann, B. K. Ofori-Okai, Renkai Li, Alexander H. Reid, Stephen Weathersby, Ehren Mannebach, Nathan Finney, Daniel Rhodes, Daniel Chenet, Abhinandan Antony, Luis Balicas, James Hone, Thomas P. Devereaux, Tony F. Heinz, Xijie Wang, and Aaron M. Lindenberg
Abstract
We used THz pulses to drive a topological transition in a Weyl semimetal WTe2 via interlayer shear motion, as crystallographically measured using ultrafast electron diffraction (Sie et al, Nature (2018), in press).
"An ultrafast symmetry switch in a Weyl semimetal" — Edbert J. Sie: Clara M. Nyby, C. D. Pemmaraju, Su Ji Park, Xiaozhe Shen, Jie Yang, Matthias C. Hoffmann, B. K. Ofori-Okai, Renkai Li, Alexander H. Reid, Stephen Weathersby, Ehren Mannebach, Nathan Finney, Daniel Rhodes, Daniel Chenet, Abhinandan Antony, Luis Balicas, James Hone, Thomas P. Devereaux, Tony F. Heinz, Xijie Wang & Aaron M. Lindenberg; Nature, 01/02/19.
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Clara M. Nyby, C. D. Pemmaraju, Su Ji Park, Xiaozhe Shen, Jie Yang, Matthias C. Hoffmann, B. K. Ofori-Okai, Renkai Li, Alexander H. Reid, Stephen Weathersby, Ehren Mannebach, Nathan Finney, Daniel Rhodes, Daniel Chenet, Abhinandan Antony, Luis Balicas, James Hone, Thomas P. Devereaux, Tony F. Heinz, Xijie Wang & Aaron M. Lindenberg
Abstract
Topological quantum materials exhibit fascinating properties1,2,3, with important applications for dissipationless electronics and fault-tolerant quantum computers4,5. Manipulating the topological invariants in these materials would allow the development of topological switching applications analogous to switching of transistors6. Lattice strain provides the most natural means of tuning these topological invariants because it directly modifies the electron–ion interactions and potentially alters the underlying crystalline symmetry on which the topological properties depend7,8,9. However, conventional means of applying strain through heteroepitaxial lattice mismatch10 and dislocations11 are not extendable to controllable time-varying protocols, which are required in transistors. Integration into a functional device requires the ability to go beyond the robust, topologically protected properties of materials and to manipulate the topology at high speeds. Here we use crystallographic measurements by relativistic electron diffraction to demonstrate that terahertz light pulses can be used to induce terahertz-frequency interlayer shear strain with large strain amplitude in the Weyl semimetal WTe2, leading to a topologically distinct metastable phase. Separate nonlinear optical measurements indicate that this transition is associated with a symmetry change to a centrosymmetric, topologically trivial phase. We further show that such shear strain provides an ultrafast, energy-efficient way of inducing robust, well separated Weyl points or of annihilating all Weyl points of opposite chirality. This work demonstrates possibilities for ultrafast manipulation of the topological properties of solids and for the development of a topological switch operating at terahertz frequencies.