Alexander Hume Reid

All Publications

• Comment on "Photoinduced Dynamics and Momentum Distribution of Chiral Charge Density Waves in 1T-TiSe₂" PHYSICAL REVIEW LETTERS Xu, C., McClellan, J., Liu, P., Lin, S., Pan, S., Bell, H. G., Kramer, P. L., Lemons, R., Philip, S. S., Duncan, C. J. R., Kaufman, B., Mo, M., Reid, A. H., Cerullo, G., Zong, A., Zuerch, M. W. 2026; 137 (2)

  View details for DOI 10.1103/p5d8-548h

  View details for Web of Science ID 001824444300004

• Comment on "Photoinduced Dynamics and Momentum Distribution of Chiral Charge Density Waves in 1T-TiSe_{2}". Physical review letters Xu, C., McClellan, J., Liu, P., Lin, S. C., Pan, S., Bell, H. G., Kramer, P. L., Lemons, R., Philip, S. S., Duncan, C. J., Kaufman, B., Mo, M., Reid, A. H., Cerullo, G., Zong, A., Zuerch, M. W. 2026; 137 (2): 029601

  View details for DOI 10.1103/p5d8-548h

  View details for PubMedID 42503136

• The UV Photoinduced Ring-Closing Reaction of Cyclopentadiene Probed with Ultrafast Electron Diffraction. The journal of physical chemistry. A Lederer, J., Bertram, L., Huang, L., Bhattacharyya, S., Boutet, S., Cheng, X., Crane, S. W., England, R. J., Graßl, M., Heald, L., Ji, F., Kramer, P., Lam, H. V., Larsen, K., Le, C., Lin, M. F., Liu, Y., Lopata, K., Minitti, M., Muvva, S. B., Nunes, J. P., Phelps, Z., Santhamma Philip, S., Khakurel, K., Reid, A. H., Rolles, D., Rudenko, A., Saha, S. K., Searles, J., Shen, X., Wang, J., Weathersby, S. P., Weber, P. M., Wolf, T. J., Xiong, Y., Xu, T., Yang, J., Yun, L., Zhao, H., Kirrander, A., Centurion, M. 2025

  Abstract

  Conjugated cyclic organic molecules are common across many fields such as pharmaceuticals, are naturally occurring in biological systems, and are used in synthetic materials. One particular area of interest from a photochemical point of view is the formation of highly strained cyclic organics. We investigate the photoinduced reaction of cyclopentadiene, a five-membered organic ring molecule, which can form strained three and four carbon rings after photoexcitation with UV light, with the gas-phase ultrafast electron diffraction instrument at the SLAC MeV-UED facility. Electron diffraction offers a direct probe sensitive to the nuclear geometry during the reaction, allowing for the determination of the distribution of products formed following photoexcitation. We observe the simultaneous formation of the highly strained ring-closed bicyclo[2.1.0]pentene and vibrationally hot cyclopentadiene within the temporal resolution of the experiment and determine the relative yield of all reaction products. The experimental results are in good agreement with the predictions of trajectory simulations.

  View details for DOI 10.1021/acs.jpca.5c05681

  View details for PubMedID 41259736

• Thermodynamic phase transitions of nematic order in magnetic helices. Science advances Tumbleson, Z., Morley, S. A., Hollingworth, E., Singh, A., Bayaraa, T., Burdet, N. G., Saleheen, A. U., McCarter, M. R., Raftrey, D., Pandolfi, R. J., Esposito, V., Dakovski, G. L., Decker, F. J., Reid, A. H., Assefa, T. A., Fischer, P., Griffin, S. M., Kevan, S. D., Hellman, F., Turner, J. J., Roy, S. 2025; 11 (25): eadt5680

  Abstract

  A nematic phase lacks translation order but has orientational order. Nematic phases have been discovered in a variety of systems, including liquid crystals, correlated materials, and superconductors. Here, we report on a magnetic nematic phase, where the basis components are composed of magnetic helices. We directly probed the order parameters associated with the magnetic helices using resonant soft x-ray scattering and find two distinct nematic phases with complex spatiotemporal signatures. Using x-ray correlation spectroscopy, we find that near the phase boundary between the two nematic phases, fluctuations coexist on multiple disparate timescales. Our micromagnetic simulations and density functional theory calculations show that the fluctuations occur concomitantly with a reorientation of the magnetic helices, indicating spontaneous symmetry breaking and the emergence of additional degrees of freedom. Our results provide a framework for characterizing exotic phases that can be extended to a broad class of physical systems.

  View details for DOI 10.1126/sciadv.adt5680

  View details for PubMedID 40540568

• Deviation from Debye-Waller behavior in single crystalline freestanding NiO membranes studied via ultrafast electron diffraction PHYSICAL REVIEW APPLIED Wisser, J. J., Reid, A., Harbola, V., Luo, D., Shen, X., Kramer, P. L., Lindgren, E. R., Xia, C., Hoffmann, M. C., Lindenberg, A. M., Hwang, H., Suzuki, Y. 2025; 23 (3)

  View details for DOI 10.1103/PhysRevApplied.23.034075

  View details for Web of Science ID 001470096500004

• Electrochemical Control of the Ultrafast Lattice Response of a Layered Semimetal. Advanced science (Weinheim, Baden-Wurttemberg, Germany) de Quesada, F. A., Muscher, P. K., Krakovsky, E. S., Sood, A., Poletayev, A. D., Sie, E. J., Nyby, C. M., Irvine, S. J., Zajac, M. E., Luo, D., Shen, X., Hoffmann, M. C., Kramer, P. L., England, R. J., Reid, A. H., Weathersby, S. P., Dresselhaus-Marais, L. E., Rehn, D. A., Chueh, W. C., Lindenberg, A. M. 2024: e2411344

  Abstract

  The unique layer-stacking in two-dimensional (2D) van der Waals materials facilitates the formation of nearly degenerate phases of matter and opens novel routes for the design of low-power, reconfigurable functional materials. Electrochemical ion intercalation between stacked layers offers a promising approach to stabilize bulk metastable phases and to explore the effects of extreme carrier doping and strain. However, in situ characterization methods to study the structural evolution and dynamical functional properties of these intercalated materials remains limited. Here a novel experimental platform is presented capable of simultaneously performing electrochemical lithium-ion intercalation and multimodal ultrafast characterization of the lattice using both electron diffraction and nonlinear optical techniques. Using the layered semimetal WTe2 as a model system, the interlayer shear phonon mode that modulates stacking between 2Dlayers is probed, showing that small amounts of lithiation enhance the amplitude and lifetime of the phonon, contrary to expectations. This results from the dynamically fluctuating and anharmonic structure between nearly degenerate phases at room temperature, which can be stabilized by electronic carriers accompanying the inserted lithium ions. At high lithiation, the Td' structure emerges and quenches the phonon response. This work defines new approaches for using electrochemistry to engineer the dynamic structure of 2D materials.

  View details for DOI 10.1002/advs.202411344

  View details for PubMedID 39686650

• Ultrafast Symmetry Control in Photoexcited Quantum Dots. Advanced materials (Deerfield Beach, Fla.) Guzelturk, B., Portner, J., Ondry, J., Ghanbarzadeh, S., Tarantola, M., Jeong, A., Field, T., Chandler, A. M., Wieman, E., Hopper, T. R., Watkins, N. E., Yu, J., Cheng, X., Lin, M. F., Luo, D., Kramer, P. L., Shen, X., Reid, A. H., Borkiewicz, O., Ruett, U., Zhang, X., Lindenberg, A. M., Ma, J., Schaller, R. D., Talapin, D. V., Cotts, B. L. 2024: e2414196

  Abstract

  Symmetry control is essential for realizing unconventional properties, such as ferroelectricity, nonlinear optical responses, and complex topological order, thus it holds promise for the design of emerging quantum and photonic systems. Nevertheless, fast and reversible control of symmetry in materials remains a challenge, especially for nanoscale systems. Here, reversible symmetry changes are unveiled in colloidal lead chalcogenide quantum dots on picosecond timescales. Using a combination of ultrafast electron diffraction and total X-ray scattering, in conjunction with atomic-scale structural modeling and first-principles calculations, it is revealed that symmetry-broken lead sulfide quantum dots restore to a centrosymmetric phase upon photoexcitation. The symmetry restoration is driven by photoexcited electronic carriers, which suppress lead off-centering for about 100 ps. Furthermore, the change in symmetry is closely correlated with the electronic properties, and the bandgap transiently red-shifts in the symmetry-restored quantum dots. Overall, this study elucidates reversible symmetry changes in colloidal quantum dots, and more broadly defines a new methodology to optically control symmetry in nanoscale systems on ultrafast timescales.

  View details for DOI 10.1002/adma.202414196

  View details for PubMedID 39584653

• Coupling to octahedral tilts in halide perovskite nanocrystals induces phonon-mediated attractive interactions between excitons. Nature physics Yazdani, N., Bodnarchuk, M. I., Bertolotti, F., Masciocchi, N., Fureraj, I., Guzelturk, B., Cotts, B. L., Zajac, M., Rainò, G., Jansen, M., Boehme, S. C., Yarema, M., Lin, M. F., Kozina, M., Reid, A., Shen, X., Weathersby, S., Wang, X., Vauthey, E., Guagliardi, A., Kovalenko, M. V., Wood, V., Lindenberg, A. M. 2024; 20 (1): 47-53

  Abstract

  Understanding the origin of electron-phonon coupling in lead halide perovskites is key to interpreting and leveraging their optical and electronic properties. Here we show that photoexcitation drives a reduction of the lead-halide-lead bond angles, a result of deformation potential coupling to low-energy optical phonons. We accomplish this by performing femtosecond-resolved, optical-pump-electron-diffraction-probe measurements to quantify the lattice reorganization occurring as a result of photoexcitation in nanocrystals of FAPbBr3. Our results indicate a stronger coupling in FAPbBr3 than CsPbBr3. We attribute the enhanced coupling in FAPbBr3 to its disordered crystal structure, which persists down to cryogenic temperatures. We find the reorganizations induced by each exciton in a multi-excitonic state constructively interfere, giving rise to a coupling strength that scales quadratically with the exciton number. This superlinear scaling induces phonon-mediated attractive interactions between excitations in lead halide perovskites.

  View details for DOI 10.1038/s41567-023-02253-7

  View details for PubMedID 38261834

  View details for PubMedCentralID PMC10791581

• Element-specific ultrafast lattice dynamics in FePt nanoparticles STRUCTURAL DYNAMICS-US Turenne, D., Vaskivskyi, I., Sokolowski-Tinten, K., Wang, X. J., Reid, A. H., Shen, X., Lin, M., Park, S., Weathersby, S., Kozina, M., Hoffmann, M. C., Wang, J., Sebesta, J., Takahashi, Y. K., Granas, O., Oppeneer, P. M., Durr, H. A. 2024; 11 (6)

  View details for DOI 10.1063/4.0000260

  View details for Web of Science ID 001351080800001

• Element-specific ultrafast lattice dynamics in FePt nanoparticles. Structural dynamics (Melville, N.Y.) Turenne, D., Vaskivskyi, I., Sokolowski-Tinten, K., Wang, X. J., Reid, A. H., Shen, X., Lin, M. F., Park, S., Weathersby, S., Kozina, M., Hoffmann, M. C., Wang, J., Sebesta, J., Takahashi, Y. K., Grånäs, O., Oppeneer, P. M., Dürr, H. A. 2024; 11 (6): 064501

  Abstract

  Light-matter interaction at the nanoscale in magnetic alloys and heterostructures is a topic of intense research in view of potential applications in high-density magnetic recording. While the element-specific dynamics of electron spins is directly accessible to resonant x-ray pulses with femtosecond time structure, the possible element-specific atomic motion remains largely unexplored. We use ultrafast electron diffraction (UED) to probe the temporal evolution of lattice Bragg peaks of FePt nanoparticles embedded in a carbon matrix following excitation by an optical femtosecond laser pulse. The diffraction interference between Fe and Pt sublattices enables us to demonstrate that the Fe mean square vibration amplitudes are significantly larger that those of Pt as expected from their different atomic mass. Both are found to increase as energy is transferred from the laser-excited electrons to the lattice. Contrary to this intuitive behavior, we observe a laser-induced lattice expansion that is larger for Pt than for Fe atoms during the first picosecond after laser excitation. This effect points to the strain-wave driven lattice expansion with the longitudinal acoustic Pt motion dominating that of Fe.

  View details for DOI 10.1063/4.0000260

  View details for PubMedID 39582608

  View details for PubMedCentralID PMC11585363

• Response of fs-Laser-Irradiated Diamond by Ultrafast Electron Diffraction JOURNAL OF PHYSICAL CHEMISTRY C Bernal, F., Riffe, E. J., Devlin, S. W., Hamel, S., Lindsey, R. K., Reid, A. H., Mo, M., Luo, D., Kramer, P., Shen, X., Nadarajah, A., Stacey, A., Prawer, S., Whitley, H. D., Schwartz, C. P., Saykally, R. J. 2024

  View details for DOI 10.1021/acs.jpcc.4c05999

  View details for Web of Science ID 001338407700001

• UV-Induced Reaction Pathways in Bromoform Probed with Ultrafast Electron Diffraction. Journal of the American Chemical Society Hoffmann, L., Toulson, B. W., Yang, J., Saladrigas, C. A., Zong, A., Muvva, S. B., Figueira Nunes, J. P., Reid, A. H., Attar, A. R., Luo, D., Ji, F., Lin, M. F., Fan, Q., Weathersby, S. P., Shen, X., Wang, X., Wolf, T. J., Neumark, D. M., Leone, S. R., Zuerch, M. W., Centurion, M., Gessner, O. 2024

  Abstract

  For many chemical reactions, it remains notoriously difficult to predict and experimentally determine the rates and branching ratios between different reaction channels. This is particularly the case for reactions involving short-lived intermediates, whose observation requires ultrafast methods. The UV photochemistry of bromoform (CHBr3) is among the most intensely studied photoreactions. Yet, a detailed understanding of the chemical pathways leading to the production of atomic Br and molecular Br2 fragments has proven challenging. In particular, the role of isomerization and/or roaming and their competition with direct C-Br bond scission has been a matter of continued debate. Here, gas-phase ultrafast megaelectronvolt electron diffraction (MeV-UED) is used to directly study structural dynamics in bromoform after single 267 nm photon excitation with femtosecond temporal resolution. The results show unambiguously that isomerization contributes significantly to the early stages of the UV photochemistry of bromoform. In addition to direct C-Br bond breaking within <200 fs, formation of iso-CHBr3 (Br-CH-Br-Br) is observed on the same time scale and with an isomer lifetime of >1.1 ps. The branching ratio between direct dissociation and isomerization is determined to be 0.4 ± 0.2:0.6 ± 0.2, i.e., approximately 60% of molecules undergo isomerization within the first few hundred femtoseconds after UV excitation. The structure and time of formation of iso-CHBr3 compare favorably with the results of an ab initio molecular dynamics simulation. The lifetime and interatomic distances of the isomer are consistent with the involvement of a roaming reaction mechanism.

  View details for DOI 10.1021/jacs.4c07165

  View details for PubMedID 39374484

• Orbital-selective time-domain signature of nematicity dynamics in the charge-density-wave phase of La1.65Eu0.2Sr0.15CuO4. Proceedings of the National Academy of Sciences of the United States of America Bluschke, M., Gupta, N. K., Jang, H., Husain, A. A., Lee, B., Kim, M., Na, M., Dos Remedios, B., Smit, S., Moen, P., Park, S. Y., Kim, M., Jang, D., Choi, H., Sutarto, R., Reid, A. H., Dakovski, G. L., Coslovich, G., Nguyen, Q. L., Burdet, N. G., Lin, M. F., Revcolevschi, A., Park, J. H., Geck, J., Turner, J. J., Damascelli, A., Hawthorn, D. G. 2024; 121 (23): e2400727121

  Abstract

  Understanding the interplay between charge, nematic, and structural ordering tendencies in cuprate superconductors is critical to unraveling their complex phase diagram. Using pump-probe time-resolved resonant X-ray scattering on the (0 0 1) Bragg peak at the Cu [Formula: see text] and O [Formula: see text] resonances, we investigate nonequilibrium dynamics of [Formula: see text] nematic order and its association with both charge density wave (CDW) order and lattice dynamics in La[Formula: see text]Eu[Formula: see text]Sr[Formula: see text]CuO[Formula: see text]. The orbital selectivity of the resonant X-ray scattering cross-section allows nematicity dynamics associated with the planar O 2[Formula: see text] and Cu 3[Formula: see text] states to be distinguished from the response of anisotropic lattice distortions. A direct time-domain comparison of CDW translational-symmetry breaking and nematic rotational-symmetry breaking reveals that these broken symmetries remain closely linked in the photoexcited state, consistent with the stability of CDW topological defects in the investigated pump fluence regime.

  View details for DOI 10.1073/pnas.2400727121

  View details for PubMedID 38819998

• Multi-objective Bayesian active learning for MeV-ultrafast electron diffraction. Nature communications Ji, F., Edelen, A., Roussel, R., Shen, X., Miskovich, S., Weathersby, S., Luo, D., Mo, M., Kramer, P., Mayes, C., Othman, M. A., Nanni, E., Wang, X., Reid, A., Minitti, M., England, R. J. 2024; 15 (1): 4726

  Abstract

  Ultrafast electron diffraction using MeV energy beams(MeV-UED) has enabled unprecedented scientific opportunities in the study of ultrafast structural dynamics in a variety of gas, liquid and solid state systems. Broad scientific applications usually pose different requirements for electron probe properties. Due to the complex, nonlinear and correlated nature of accelerator systems, electron beam property optimization is a time-taking process and often relies on extensive hand-tuning by experienced human operators. Algorithm based efficient online tuning strategies are highly desired. Here, we demonstrate multi-objective Bayesian active learning for speeding up online beam tuning at the SLAC MeV-UED facility. The multi-objective Bayesian optimization algorithm was used for efficiently searching the parameter space and mapping out the Pareto Fronts which give the trade-offs between key beam properties. Such scheme enables an unprecedented overview of the global behavior of the experimental system and takes a significantly smaller number of measurements compared with traditional methods such as a grid scan. This methodology can be applied in other experimental scenarios that require simultaneously optimizing multiple objectives by explorations in high dimensional, nonlinear and correlated systems.

  View details for DOI 10.1038/s41467-024-48923-9

  View details for PubMedID 38830874

  View details for PubMedCentralID PMC11148007

• Giant Terahertz Birefringence in an Ultrathin Anisotropic Semimetal. Nano letters Sie, E. J., Othman, M. A., Nyby, C. M., Pemmaraju, D., Garcia, C. A., Wang, Y., Guzelturk, B., Xia, C., Xiao, J., Poletayev, A., Ofori-Okai, B. K., Hoffmann, M. C., Park, S., Shen, X., Yang, J., Li, R., Reid, A. H., Weathersby, S., Muscher, P., Finney, N., Rhodes, D., Balicas, L., Nanni, E., Hone, J., Chueh, W., Devereaux, T. P., Narang, P., Heinz, T. F., Wang, X., Lindenberg, A. M. 2024

  Abstract

  Manipulating the polarization of light at the nanoscale is key to the development of next-generation optoelectronic devices. This is typically done via waveplates using optically anisotropic crystals, with thicknesses on the order of the wavelength. Here, using a novel ultrafast electron-beam-based technique sensitive to transient near fields at THz frequencies, we observe a giant anisotropy in the linear optical response in the semimetal WTe2 and demonstrate that one can tune the THz polarization using a 50 nm thick film, acting as a broadband wave plate with thickness 3 orders of magnitude smaller than the wavelength. The observed circular deflections of the electron beam are consistent with simulations tracking the trajectory of the electron beam in the near field of the THz pulse. This finding offers a promising approach to enable atomically thin THz polarization control using anisotropic semimetals and defines new approaches for characterizing THz near-field optical response at far-subwavelength length scales.

  View details for DOI 10.1021/acs.nanolett.4c00758

  View details for PubMedID 38717626

• Direct observation of strong momentum-dependent electron-phonon coupling in a metal. Science advances Mo, M., Tamm, A., Metsanurk, E., Chen, Z., Wang, L., Frost, M., Hartley, N. J., Ji, F., Pandolfi, S., Reid, A. H., Sun, P., Shen, X., Wang, Y., Wang, X., Glenzer, S., Correa, A. A. 2024; 10 (11): eadk9051

  Abstract

  Phonon scattering in metals is one of the most fundamental processes in materials science. However, understanding such processes has remained challenging and requires detailed information on interactions between phonons and electrons. We use an ultrafast electron diffuse scattering technique to resolve the nonequilibrium phonon dynamics in femtosecond-laser-excited tungsten in both time and momentum. We determine transient populations of phonon modes which show strong momentum dependence initiated by electron-phonon coupling. For phonons near Brillouin zone border, we observe a transient rise in their population on a timescale of approximately 1 picosecond driven by the strong electron-phonon coupling, followed by a slow decay on a timescale of approximately 8 picosecond governed by the weaker phonon-phonon relaxation process. We find that the exceptional harmonicity of tungsten is needed for isolating the two processes, resulting in long-lived nonequilibrium phonons in a pure metal. Our finding highlights that electron-phonon scattering can be the determinant factor in the phonon thermal transport of metals.

  View details for DOI 10.1126/sciadv.adk9051

  View details for PubMedID 38478610

• Monitoring the Evolution of Relative Product Populations at Early Times during a Photochemical Reaction. Journal of the American Chemical Society Figueira Nunes, J. P., Ibele, L. M., Pathak, S., Attar, A. R., Bhattacharyya, S., Boll, R., Borne, K., Centurion, M., Erk, B., Lin, M., Forbes, R. J., Goff, N., Hansen, C. S., Hoffmann, M., Holland, D. M., Ingle, R. A., Luo, D., Muvva, S. B., Reid, A. H., Rouzee, A., Rudenko, A., Saha, S. K., Shen, X., Venkatachalam, A. S., Wang, X., Ware, M. R., Weathersby, S. P., Wilkin, K., Wolf, T. J., Xiong, Y., Yang, J., Ashfold, M. N., Rolles, D., Curchod, B. F. 2024

  Abstract

  Identifying multiple rival reaction products and transient species formed during ultrafast photochemical reactions and determining their time-evolving relative populations are key steps toward understanding and predicting photochemical outcomes. Yet, most contemporary ultrafast studies struggle with clearly identifying and quantifying competing molecular structures/species among the emerging reaction products. Here, we show that mega-electronvolt ultrafast electron diffraction in combination with ab initio molecular dynamics calculations offer a powerful route to determining time-resolved populations of the various isomeric products formed after UV (266 nm) excitation of the five-membered heterocyclic molecule 2(5H)-thiophenone. This strategy provides experimental validation of the predicted high (50%) yield of an episulfide isomer containing a strained three-membered ring within 1 ps of photoexcitation and highlights the rapidity of interconversion between the rival highly vibrationally excited photoproducts in their ground electronic state.

  View details for DOI 10.1021/jacs.3c13046

  View details for PubMedID 38317439

• Coupling to octahedral tilts in halide perovskite nanocrystals induces phonon-mediated attractive interactions between excitons NATURE PHYSICS Yazdani, N., Bodnarchuk, M. I., Bertolotti, F., Masciocchi, N., Fureraj, I., Guzelturk, B., Cotts, B. L., Zajac, M., Raino, G., Jansen, M., Boehme, S. C., Yarema, M., Lin, M., Kozina, M., Reid, A., Shen, X., Weathersby, S., Wang, X., Vauthey, E., Guagliardi, A., Kovalenko, M. V., Wood, V., Lindenberg, A. M. 2023

  View details for DOI 10.1038/s41567-023-02253-7

  View details for Web of Science ID 001103093700003

• Spin-mediated shear oscillators in a van der Waals antiferromagnet. Nature Zong, A., Zhang, Q., Zhou, F., Su, Y., Hwangbo, K., Shen, X., Jiang, Q., Liu, H., Gage, T. E., Walko, D. A., Kozina, M. E., Luo, D., Reid, A. H., Yang, J., Park, S., Lapidus, S. H., Chu, J. H., Arslan, I., Wang, X., Xiao, D., Xu, X., Gedik, N., Wen, H. 2023

  Abstract

  Understanding how microscopic spin configuration gives rise to exotic properties at the macroscopic length scale has long been pursued in magnetic materials1-5. One seminal example is the Einstein-de Haas effect in ferromagnets1,6,7, in which angular momentum of spins can be converted into mechanical rotation of an entire object. However, for antiferromagnets without net magnetic moment, how spin ordering couples to macroscopic movement remains elusive. Here we observed a seesaw-like rotation of reciprocal lattice peaks of an antiferromagnetic nanolayer film, whose gigahertz structural resonance exhibits more than an order-of-magnitude amplification after cooling below the Néel temperature. Using a suite of ultrafast diffraction and microscopy techniques, we directly visualize this spin-driven rotation in reciprocal space at the nanoscale. This motion corresponds to interlayer shear in real space, in which individual micro-patches of the film behave as coherent oscillators that are phase-locked and shear along the same in-plane axis. Using time-resolved optical polarimetry, we further show that the enhanced mechanical response strongly correlates with ultrafast demagnetization, which releases elastic energy stored in local strain gradients to drive the oscillators. Our work not only offers the first microscopic view of spin-mediated mechanical motion of an antiferromagnet but it also identifies a new route towards realizing high-frequency resonators8,9 up to the millimetre band, so the capability of controlling magnetic states on the ultrafast timescale10-13 can be readily transferred to engineering the mechanical properties of nanodevices.

  View details for DOI 10.1038/s41586-023-06279-y

  View details for PubMedID 37532936

  View details for PubMedCentralID 10156606

• Understanding and Controlling Photothermal Responses in MXenes. Nano letters Guzelturk, B., Kamysbayev, V., Wang, D., Hu, H., Li, R., King, S. B., Reid, A. H., Lin, M., Wang, X., Walko, D. A., Zhang, X., Lindenberg, A., Talapin, D. V. 2023

  Abstract

  MXenes have the potential for efficient light-to-heat conversion in photothermal applications. To effectively utilize MXenes in such applications, it is important to understand the underlying nonequilibrium processes, including electron-phonon and phonon-phonon couplings. Here, we use transient electron and X-ray diffraction to investigate the heating and cooling of photoexcited MXenes at femtosecond to nanosecond time scales. Our results show extremely strong electron-phonon coupling in Ti3C2-based MXenes, resulting in lattice heating within a few hundred femtoseconds. We also systematically study heat dissipation in MXenes with varying film thicknesses, chemical surface terminations, flake sizes, and annealing conditions. We find that the thermal boundary conductance (TBC) governs the thermal relaxation in films thinner than the optical penetration depth. We achieve a 2-fold enhancement of the TBC, reaching 20 MW m-2 K-1, by controlling the flake size or chemical surface termination, which is promising for engineering heat dissipation in photothermal and thermoelectric applications of the MXenes.

  View details for DOI 10.1021/acs.nanolett.2c05001

  View details for PubMedID 36917456

• Ultrafast Optomechanical Strain in Layered GeS. Nano letters Luo, D., Zhang, B., Sie, E. J., Nyby, C. M., Fan, Q., Shen, X., Reid, A. H., Hoffmann, M. C., Weathersby, S., Wen, J., Qian, X., Wang, X., Lindenberg, A. M. 2023

  Abstract

  Strong coupling between light and mechanical strain forms the foundation for next-generation optical micro- and nano-electromechanical systems. Such optomechanical responses in two-dimensional materials present novel types of functionalities arising from the weak van der Waals bond between atomic layers. Here, by using structure-sensitive megaelectronvolt ultrafast electron diffraction, we report the experimental observation of optically driven ultrafast in-plane strain in the layered group IV monochalcogenide germanium sulfide (GeS). Surprisingly, the photoinduced structural deformation exhibits strain amplitudes of order 0.1% with a 10 ps fast response time and a significant in-plane anisotropy between zigzag and armchair crystallographic directions. Rather than arising due to heating, experimental and theoretical investigations suggest deformation potentials caused by electronic density redistribution and converse piezoelectric effects generated by photoinduced electric fields are the dominant contributors to the observed dynamic anisotropic strains. Our observations define new avenues for ultrafast optomechanical control and strain engineering within functional devices.

  View details for DOI 10.1021/acs.nanolett.2c05048

  View details for PubMedID 36898060

• Symmetry-dependent ultrafast manipulation of nanoscale magnetic domains PHYSICAL REVIEW B Hagstrom, N., Jangid, R., Madhavi, M., Turenne, D., Brock, J. A., Lamb, E. S., Stoychev, B., Schlappa, J., Gerasimova, N., Van Kuiken, B., Gort, R., Mercadier, L., Le Guyader, L., Samartsev, A., Scherz, A., Mercurio, G., Durr, H. A., Reid, A. H., Arora, M., Nembach, H. T., Shaw, J. M., Jal, E., Fullerton, E. E., Keller, M. W., Kukreja, R., Bonetti, S., Silva, T. J., Iacocca, E. 2022; 106 (22)

  View details for DOI 10.1103/PhysRevB.106.224424

  View details for Web of Science ID 000909190700003

• Bidirectional phonon emission in two-dimensional heterostructures triggered by ultrafast charge transfer. Nature nanotechnology Sood, A., Haber, J. B., Carlström, J., Peterson, E. A., Barre, E., Georgaras, J. D., Reid, A. H., Shen, X., Zajac, M. E., Regan, E. C., Yang, J., Taniguchi, T., Watanabe, K., Wang, F., Wang, X., Neaton, J. B., Heinz, T. F., Lindenberg, A. M., da Jornada, F. H., Raja, A. 2022

  Abstract

  Photoinduced charge transfer in van der Waals heterostructures occurs on the 100 fs timescale despite weak interlayer coupling and momentum mismatch. However, little is understood about the microscopic mechanism behind this ultrafast process and the role of the lattice in mediating it. Here, using femtosecond electron diffraction, we directly visualize lattice dynamics in photoexcited heterostructures of WSe2/WS2 monolayers. Following the selective excitation of WSe2, we measure the concurrent heating of both WSe2 and WS2 on a picosecond timescale-an observation that is not explained by phonon transport across the interface. Using first-principles calculations, we identify a fast channel involving an electronic state hybridized across the heterostructure, enabling phonon-assisted interlayer transfer of photoexcited electrons. Phonons are emitted in both layers on the femtosecond timescale via this channel, consistent with the simultaneous lattice heating observed experimentally. Taken together, our work indicates strong electron-phonon coupling via layer-hybridized electronic states-a novel route to control energy transport across atomic junctions.

  View details for DOI 10.1038/s41565-022-01253-7

  View details for PubMedID 36543882

• Light-Driven Ultrafast Polarization Manipulation in a Relaxor Ferroelectric. Nano letters Park, S., Wang, B., Yang, T., Kim, J., Saremi, S., Zhao, W., Guzelturk, B., Sood, A., Nyby, C., Zajac, M., Shen, X., Kozina, M., Reid, A. H., Weathersby, S., Wang, X., Martin, L. W., Chen, L. Q., Lindenberg, A. M. 2022

  Abstract

  Relaxor ferroelectrics have been intensely studied for decades based on their unique electromechanical responses which arise from local structural heterogeneity involving polar nanoregions or domains. Here, we report first studies of the ultrafast dynamics and reconfigurability of the polarization in freestanding films of the prototypical relaxor 0.68PbMg1/3Nb2/3O3-0.32PbTiO3 (PMN-0.32PT) by probing its atomic-scale response via femtosecond-resolution, electron-scattering approaches. By combining these structural measurements with dynamic phase-field simulations, we show that femtosecond light pulses drive a change in both the magnitude and direction of the polarization vector within polar nanodomains on few-picosecond time scales. This study defines new opportunities for dynamic reconfigurable control of the polarization in nanoscale relaxor ferroelectrics.

  View details for DOI 10.1021/acs.nanolett.2c02706

  View details for PubMedID 36450036

• Ultrafast perturbation of magnetic domains by optical pumping in a ferromagnetic multilayer PHYSICAL REVIEW B Zusin, D., Iacocca, E., Le Guyader, L., Reid, A. H., Schlotter, W. F., Liu, T., Higley, D. J., Coslovich, G., Wandel, S. F., Tengdin, P. M., Patel, S. K. K., Shabalin, A., Hua, N., Hrkac, S. B., Nembach, H. T., Shaw, J. M., Montoya, S. A., Blonsky, A., Gentry, C., Hoefer, M. A., Murnane, M. M., Kapteyn, H. C., Fullerton, E. E., Shpyrko, O., Durr, H. A., Silva, T. J. 2022; 106 (14)

  View details for DOI 10.1103/PhysRevB.106.144422

  View details for Web of Science ID 000879551600002

• Large Exchange Coupling Between Localized Spins and Topological Bands in Magnetic Topological Insulator MnBi2 Te4. Advanced materials (Deerfield Beach, Fla.) Padmanabhan, H., Stoica, V. A., Kim, P. K., Poore, M., Yang, T., Shen, X., Reid, A. H., Lin, M., Park, S., Yang, J., Hugo Wang, H., Koocher, N. Z., Puggioni, D., Georgescu, A. B., Min, L., Lee, S. H., Mao, Z., Rondinelli, J. M., Lindenberg, A. M., Chen, L., Wang, X., Averitt, R. D., Freeland, J. W., Gopalan, V. 2022: e2202841

  Abstract

  Magnetism in topological materials creates phases exhibiting quantized transport phenomena with potential technological applications. The emergence of such phases relies on strong interaction between localized spins and the topological bands, and the consequent formation of an exchange gap. However, this remains experimentally unquantified in intrinsic magnetic topological materials. Here, this interaction is quantified in MnBi2 Te4 , an intrinsic antiferromagnetic topological insulator. To achieve this, a multimodal ultrafast approach is employed to interrogate optically induced nonequilibrium spin dynamics. Momentum-resolved ultrafast electron scattering and magneto-optic measurements show that Bi-Te p-like states comprising the bulk topological bands demagnetize via electron-phonon scattering at picosecond timescales. Localized Mn 3d spins, probed by ultrafast resonant X-ray scattering, are found to disorder concurrently with the p-like spins, despite being energetically decoupled from the optical excitation. These results, together with atomistic simulations, reveal that the exchange coupling between localized spins and the bulk topological bands is at least 100 times larger than the primary superexchange interaction, implying an optimal exchange gap of at least 25 meV in the topological surface states. By directly quantifying this exchange coupling, the study validates the materials-by-design strategy of utilizing localized magnetic order to create and manipulate magnetic topological phases, spanning static to ultrafast timescales. This article is protected by copyright. All rights reserved.

  View details for DOI 10.1002/adma.202202841

  View details for PubMedID 36189841

• Interlayer magnetophononic coupling in MnBi2Te4. Nature communications Padmanabhan, H., Poore, M., Kim, P. K., Koocher, N. Z., Stoica, V. A., Puggioni, D., Hugo Wang, H., Shen, X., Reid, A. H., Gu, M., Wetherington, M., Lee, S. H., Schaller, R. D., Mao, Z., Lindenberg, A. M., Wang, X., Rondinelli, J. M., Averitt, R. D., Gopalan, V. 2022; 13 (1): 1929

  Abstract

  The emergence of magnetism in quantum materials creates a platform to realize spin-based applications in spintronics, magnetic memory, and quantum information science. A key to unlocking new functionalities in these materials is the discovery of tunable coupling between spins and other microscopic degrees of freedom. We present evidence for interlayer magnetophononic coupling in the layered magnetic topological insulator MnBi2Te4. Employing magneto-Raman spectroscopy, we observe anomalies in phonon scattering intensities across magnetic field-driven phase transitions, despite the absence of discernible static structural changes. This behavior is a consequence of a magnetophononic wave-mixing process that allows for the excitation of zone-boundary phonons that are otherwise 'forbidden' by momentum conservation. Our microscopic model based on density functional theory calculations reveals that this phenomenon can be attributed to phonons modulating the interlayer exchange coupling. Moreover, signatures of magnetophononic coupling are also observed in the time domain through the ultrafast excitation and detection of coherent phonons across magnetic transitions. In light of the intimate connection between magnetism and topology in MnBi2Te4, the magnetophononic coupling represents an important step towards coherent on-demand manipulation of magnetic topological phases.

  View details for DOI 10.1038/s41467-022-29545-5

  View details for PubMedID 35396393

• Nonequilibrium sub-10 nm spin-wave soliton formation in FePt nanoparticles SCIENCE ADVANCES Turenne, D., Yaroslavtsev, A., Wang, X., Unikandanuni, V., Vaskivskyi, I., Schneider, M., Jal, E., Carley, R., Mercurio, G., Gort, R., Agarwal, N., Van Kuiken, B., Mercadier, L., Schlappa, J., Le Guyader, L., Gerasimova, N., Teichmann, M., Lomidze, D., Castoldi, A., Potorochin, D., Mukkattukavil, D., Brock, J., Hagstrom, N., Reid, A. H., Shen, X., Wang, X. J., Maldonado, P., Kvashnin, Y., Carva, K., Wang, J., Takahashi, Y. K., Fullerton, E. E., Eisebitt, S., Oppeneer, P. M., Molodtsov, S., Scherz, A., Bonetti, S., Iacocca, E., Durr, H. A. 2022; 8 (13): eabn0523

  Abstract

  Magnetic nanoparticles such as FePt in the L10 phase are the bedrock of our current data storage technology. As the grains become smaller to keep up with technological demands, the superparamagnetic limit calls for materials with higher magnetocrystalline anisotropy. This, in turn, reduces the magnetic exchange length to just a few nanometers, enabling magnetic structures to be induced within the nanoparticles. Here, we describe the existence of spin-wave solitons, dynamic localized bound states of spin-wave excitations, in FePt nanoparticles. We show with time-resolved x-ray diffraction and micromagnetic modeling that spin-wave solitons of sub-10 nm sizes form out of the demagnetized state following femtosecond laser excitation. The measured soliton spin precession frequency of 0.1 THz positions this system as a platform to develop novel miniature devices.

  View details for DOI 10.1126/sciadv.abn0523

  View details for Web of Science ID 000778886800029

  View details for PubMedID 35363518

• Ultrafast visualization of incipient plasticity in dynamically compressed matter. Nature communications Mo, M., Tang, M., Chen, Z., Peterson, J. R., Shen, X., Baldwin, J. K., Frost, M., Kozina, M., Reid, A., Wang, Y., E, J., Descamps, A., Ofori-Okai, B. K., Li, R., Luo, S., Wang, X., Glenzer, S. 2022; 13 (1): 1055

  Abstract

  Plasticity is ubiquitous and plays a critical role in material deformation and damage; it inherently involves the atomistic length scale and picosecond time scale. A fundamental understanding of the elastic-plastic deformation transition, in particular, incipient plasticity, has been a grand challenge in high-pressure and high-strain-rate environments, impeded largely by experimental limitations on spatial and temporal resolution. Here, we report femtosecond MeV electron diffraction measurements visualizing the three-dimensional (3D) response of single-crystal aluminum to the ultrafast laser-induced compression. We capture lattice transitioning from a purely elastic to a plastically relaxed state within 5 ps, after reaching an elastic limit of~25 GPa. Our results allow the direct determination of dislocation nucleation and transport that constitute the underlying defect kinetics of incipient plasticity. Large-scale molecular dynamics simulations show good agreement with the experiment and provide an atomic-level description of the dislocation-mediated plasticity.

  View details for DOI 10.1038/s41467-022-28684-z

  View details for PubMedID 35217665

• State-resolved ultrafast charge and spin dynamics in [Co/Pd] multilayers APPLIED PHYSICS LETTERS Le Guyader, L., Higley, D. J., Pancaldi, M., Liu, T., Chen, Z., Chase, T., Granitzka, P. W., Coslovich, G., Lutman, A. A., Dakovski, G. L., Schlotter, W. F., Shafer, P., Arenholz, E., Hellwig, O., Lalieu, M. L. M., Koopmans, B., Reid, A. H., Bonetti, S., Stohr, J., Durr, H. A. 2022; 120 (3)

  View details for DOI 10.1063/5.0076953

  View details for Web of Science ID 000779205300016

• Role of Equilibrium Fluctuations in Light-Induced Order PHYSICAL REVIEW LETTERS Zong, A., Dolgirev, P. E., Kogar, A., Su, Y., Shen, X., Straquadine, J. A. W., Wang, X., Luo, D., Kozina, M. E., Reid, A. H., Li, R., Yang, J., Weathersby, S. P., Park, S., Sie, E. J., Jarillo-Herrero, P., Fisher, I. R., Wang, X., Demler, E., Gedik, N. 2021; 127 (22)

  View details for DOI 10.1103/PhysRevLett.127.227401

  View details for Web of Science ID 000723128100004

• Role of Equilibrium Fluctuations in Light-Induced Order. Physical review letters Zong, A., Dolgirev, P. E., Kogar, A., Su, Y., Shen, X., Straquadine, J. A., Wang, X., Luo, D., Kozina, M. E., Reid, A. H., Li, R., Yang, J., Weathersby, S. P., Park, S., Sie, E. J., Jarillo-Herrero, P., Fisher, I. R., Wang, X., Demler, E., Gedik, N. 2021; 127 (22): 227401

  Abstract

  Engineering novel states of matter with light is at the forefront of materials research. An intensely studied direction is to realize broken-symmetry phases that are "hidden" under equilibrium conditions but can be unleashed by an ultrashort laser pulse. Despite a plethora of experimental discoveries, the nature of these orders and how they transiently appear remain unclear. To this end, we investigate a nonequilibrium charge density wave (CDW) in rare-earth tritellurides, which is suppressed in equilibrium but emerges after photoexcitation. Using a pump-pump-probe protocol implemented in ultrafast electron diffraction, we demonstrate that the light-induced CDW consists solely of order parameter fluctuations, which bear striking similarities to critical fluctuations in equilibrium despite differences in the length scale. By calculating the dynamics of CDW fluctuations in a nonperturbative model, we further show that the strength of the light-induced order is governed by the amplitude of equilibrium fluctuations. These findings highlight photoinduced fluctuations as an important ingredient for the emergence of transient orders out of equilibrium. Our results further suggest that materials with strong fluctuations in equilibrium are promising platforms to host hidden orders after laser excitation.

  View details for DOI 10.1103/PhysRevLett.127.227401

  View details for PubMedID 34889631

• Dynamic lattice distortions driven by surface trapping in semiconductor nanocrystals. Nature communications Guzelturk, B., Cotts, B. L., Jasrasaria, D., Philbin, J. P., Hanifi, D. A., Koscher, B. A., Balan, A. D., Curling, E., Zajac, M., Park, S., Yazdani, N., Nyby, C., Kamysbayev, V., Fischer, S., Nett, Z., Shen, X., Kozina, M. E., Lin, M., Reid, A. H., Weathersby, S. P., Schaller, R. D., Wood, V., Wang, X., Dionne, J. A., Talapin, D. V., Alivisatos, A. P., Salleo, A., Rabani, E., Lindenberg, A. M. 2021; 12 (1): 1860

  Abstract

  Nonradiative processes limit optoelectronic functionality of nanocrystals and curb their device performance. Nevertheless, the dynamic structural origins of nonradiative relaxations in such materials are not understood. Here, femtosecond electron diffraction measurements corroborated by atomistic simulations uncover transient lattice deformations accompanying radiationless electronic processes in colloidal semiconductor nanocrystals. Investigation of the excitation energy dependence in a core/shell system shows that hot carriers created by a photon energy considerably larger than the bandgap induce structural distortions at nanocrystal surfaces on few picosecond timescales associated with the localization of trapped holes. On the other hand, carriers created by a photon energy close to the bandgap of the core in the same system result in transient lattice heating that occurs on a much longer 200 picosecond timescale, dominated by an Auger heating mechanism. Elucidation of the structural deformations associated with the surface trapping of hot holes provides atomic-scale insights into the mechanisms deteriorating optoelectronic performance and a pathway towards minimizing these losses in nanocrystal devices.

  View details for DOI 10.1038/s41467-021-22116-0

  View details for PubMedID 33767138

• High-sensitivity x-ray/optical cross-correlator for next generation free-electron lasers OPTICS EXPRESS Droste, S., Zohar, S., Shen, L., White, V. E., Diaz-Jacobo, E., Coffee, R. N., Reid, A. H., Tavella, F., Minitti, M. P., Turner, J. J., Robinson, J. S., Fry, A. R., Coslovich, G. 2020; 28 (16): 23545–53

  Abstract

  We design and realize an arrival time diagnostic for ultrashort X-ray pulses achieving unprecedented high sensitivity in the soft X-ray regime via cross-correlation with a ≈1550 nm optical laser. An interferometric detection scheme is combined with a multi-layer sample design to greatly improve the sensitivity of the measurement. We achieve up to 275% of relative signal change when exposed to 1.6 mJ/cm2 of soft X-rays at 530 eV, more than a hundred-fold improvement in sensitivity as compared to previously reported techniques. The resolution of the arrival time measurement is estimated to around 2.8 fs (rms). The demonstrated X-ray arrival time monitor paves the way for sub-10 fs-level timing jitter at high repetition rate X-ray facilities.

  View details for DOI 10.1364/OE.398048

  View details for Web of Science ID 000560931200042

  View details for PubMedID 32752349

• Coherent Lattice Wobbling and Out-of-Phase Intensity Oscillations of Friedel Pairs Observed by Ultrafast Electron Diffraction ACS NANO Qian, Q., Shen, X., Luo, D., Jia, L., Kozina, M., Li, R., Lin, M., Reid, A. H., Weathersby, S., Park, S., Yang, J., Zhou, Y., Zhang, K., Wang, X., Huang, S. 2020; 14 (7): 8449-8458

  Abstract

  The inspection of Friedel's law in ultrafast electron diffraction (UED) is important to gain a comprehensive understanding of material atomic structure and its dynamic response. Here, monoclinic gallium telluride (GaTe), as a low-symmetry, layered crystal in contrast to many other 2D materials, is investigated by mega-electronvolt UED. Strong out-of-phase oscillations of Bragg peak intensities are observed for Friedel pairs, which does not obey Friedel's law. As evidenced by the preserved mirror symmetry and supported by both kinematic and dynamic scattering simulations, the intensity oscillations are provoked by the lowest-order longitudinal acoustic breathing phonon. Our results provide a generalized understanding of Friedel's law in UED and demonstrate that by designed misalignment of surface normal and primitive lattice vectors, coherent lattice wobbling and effective shear strain can be generated in crystal films by laser pulse excitation, which is otherwise hard to achieve and can be further utilized to dynamically tune and switch material properties.

  View details for DOI 10.1021/acsnano.0c02643

  View details for Web of Science ID 000557762800067

  View details for PubMedID 32538617

• Orbital dynamics during an ultrafast insulator to metal transition PHYSICAL REVIEW RESEARCH Parchenko, S., Paris, E., McNally, D., Abreu, E., Dantz, M., Bothschafter, E. M., Reid, A. H., Schlotter, W. F., Lin, M., Wandel, S. F., Coslovich, G., Zohar, S., Dakovski, G. L., Turner, J. J., Moeller, S., Tseng, Y., Radovic, M., Saathe, C., Agaaker, M., Nordgren, J. E., Johnson, S. L., Schmitt, T., Staub, U. 2020; 2 (2)

  View details for DOI 10.1103/PhysRevResearch.2.023110

  View details for Web of Science ID 000602780200010

• Nonequilibrium Thermodynamics of Colloidal Gold Nanocrystals Monitored by Ultrafast Electron Diffraction and Optical Scattering Microscopy. ACS nano Guzelturk, B. n., Utterback, J. K., Coropceanu, I. n., Kamysbayev, V. n., Janke, E. M., Zajac, M. n., Yazdani, N. n., Cotts, B. L., Park, S. n., Sood, A. n., Lin, M. F., Reid, A. H., Kozina, M. E., Shen, X. n., Weathersby, S. P., Wood, V. n., Salleo, A. n., Wang, X. n., Talapin, D. V., Ginsberg, N. S., Lindenberg, A. M. 2020

  Abstract

  Metal nanocrystals exhibit important optoelectronic and photocatalytic functionalities in response to light. These dynamic energy conversion processes have been commonly studied by transient optical probes to date, but an understanding of the atomistic response following photoexcitation has remained elusive. Here, we use femtosecond resolution electron diffraction to investigate transient lattice responses in optically excited colloidal gold nanocrystals, revealing the effects of nanocrystal size and surface ligands on the electron-phonon coupling and thermal relaxation dynamics. First, we uncover a strong size effect on the electron-phonon coupling, which arises from reduced dielectric screening at the nanocrystal surfaces and prevails independent of the optical excitation mechanism (i.e., inter- and intraband). Second, we find that surface ligands act as a tuning parameter for hot carrier cooling. Particularly, gold nanocrystals with thiol-based ligands show significantly slower carrier cooling as compared to amine-based ligands under intraband optical excitation due to electronic coupling at the nanocrystal/ligand interfaces. Finally, we spatiotemporally resolve thermal transport and heat dissipation in photoexcited nanocrystal films by combining electron diffraction with stroboscopic elastic scattering microscopy. Taken together, we resolve the distinct thermal relaxation time scales ranging from 1 ps to 100 ns associated with the multiple interfaces through which heat flows at the nanoscale. Our findings provide insights into optimization of gold nanocrystals and their thin films for photocatalysis and thermoelectric applications.

  View details for DOI 10.1021/acsnano.0c00673

  View details for PubMedID 32208676

• Ultrafast time-resolved x-ray scattering reveals diffusive charge order dynamics in La2-x Ba x CuO4. Science advances Mitrano, M., Lee, S., Husain, A. A., Delacretaz, L., Zhu, M., de la Pena Munoz, G., Sun, S. X., Joe, Y. I., Reid, A. H., Wandel, S. F., Coslovich, G., Schlotter, W., van Driel, T., Schneeloch, J., Gu, G. D., Hartnoll, S., Goldenfeld, N., Abbamonte, P. 2019; 5 (8): eaax3346

  Abstract

  Charge order is universal among high-T c cuprates, but its relation to superconductivity is unclear. While static order competes with superconductivity, dynamic order may be favorable and even contribute to Cooper pairing. Using time-resolved resonant soft x-ray scattering at a free-electron laser, we show that the charge order in prototypical La2-x Ba x CuO4 exhibits transverse fluctuations at picosecond time scales. These sub-millielectron volt excitations propagate by Brownian-like diffusion and have an energy scale remarkably close to the superconducting T c. At sub-millielectron volt energy scales, the dynamics are governed by universal scaling laws defined by the propagation of topological defects. Our results show that charge order in La2-x Ba x CuO4 exhibits dynamics favorable to the in-plane superconducting tunneling and establish time-resolved x-rays as a means to study excitations at energy scales inaccessible to conventional scattering techniques.

  View details for DOI 10.1126/sciadv.aax3346

  View details for PubMedID 31453340

• Fluorescence intensity monitors as intensity and beam-position diagnostics for X-ray free-electron lasers JOURNAL OF SYNCHROTRON RADIATION Heimann, P., Reid, A., Feng, Y., Fritz, D. 2019; 26: 358-362

  Abstract

  For LCLS-II, a fluorescence intensity monitor for the non-invasive, pulse-by-pulse normalization of experiments has been developed. A prototype diagnostic was constructed with a microchannel plate assembly and two photodiodes. The diagnostic was then installed in the LCLS SXR instrument Kirkpatrick-Baez mirror chamber with the detectors located above the vertically reflecting mirror. The linearity, noise and position sensitivity of the detectors have been characterized. The photodiode responsivity is suitable for high pulse energies. The microchannel plate detector shows sufficient responsivity over a wide range of pulse energies. The relative signal from the two photodiodes provides a sensitive measure of the X-ray beam position. The fluorescence intensity monitor provides intensity normalization while being compatible with high incident power, a 0.93 MHz repetition rate and ultra-high vacuum.

  View details for DOI 10.1107/S1600577519001802

  View details for Web of Science ID 000460859600008

  View details for PubMedID 30855243

  View details for PubMedCentralID PMC6412172

• An ultrafast symmetry switch in a Weyl semimetal. Nature Sie, E. J., Nyby, C. M., Pemmaraju, C. D., Park, S. J., Shen, X. n., Yang, J. n., Hoffmann, M. C., Ofori-Okai, B. K., Li, R. n., Reid, A. H., Weathersby, S. n., Mannebach, E. n., Finney, N. n., Rhodes, D. n., Chenet, D. n., Antony, A. n., Balicas, L. n., Hone, J. n., Devereaux, T. P., Heinz, T. F., Wang, X. n., Lindenberg, A. M. 2019; 565 (7737): 61–66

  Abstract

  Topological quantum materials exhibit fascinating properties1-3, 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 depend7-9. 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.

  View details for PubMedID 30602749

• Laser Power Meters as Portable X-ray Power Monitors Heimann, P. A., Fritz, D., Krzywinski, J., Moeller, S., Nordlund, D., Reid, A., Stefan, P., Walter, P., Welch, J. edited by Tschentscher, T., Patthey, L., Tiedtke, K., Zangrando, M. SPIE-INT SOC OPTICAL ENGINEERING. 2019

  View details for DOI 10.1117/12.2526119

  View details for Web of Science ID 000502119600005

• THz-Pump UED-Probe on a Topological Weyl Semimetal Sie, E. J., Nyby, C. M., Pemmaraju, C. D., Park, S., Shen, X., Yang, J., Hoffmann, M. C., Ofori-Okai, B. K., Li, R., Reid, A. H., Weathersby, S., Mannebach, E., Finney, N., Rhodes, D., Chenet, D., Antony, A., Balicas, L., Hone, J., Devereaux, T. P., Heinz, T. F., Wang, X., Lindenberg, A. M., IEEE IEEE. 2019

  View details for Web of Science ID 000482226301297

• An Ultrafast Symmetry Switch in a Weyl Semimetal Nature Sie, E. J., et al 2019; 565, 61

  View details for DOI 10.1038/s41586-018-0809-4

• Nonequilibrium electron and lattice dynamics of strongly correlated Bi2Sr2CaCu2O8+delta single crystals SCIENCE ADVANCES Konstantinova, T., Rameau, J. D., Reid, A. H., Abdurazakov, O., Wu, L., Li, R., Shen, X., Gu, G., Huang, Y., Rettig, L., Avigo, I., Ligges, M., Freericks, J. K., Kemper, A. F., Duerr, H. A., Bovensiepen, U., Johnson, P. D., Wang, X., Zhu, Y. 2018; 4 (4): eaap7427

  Abstract

  The interplay between the electronic and lattice degrees of freedom in nonequilibrium states of strongly correlated systems has been debated for decades. Although progress has been made in establishing a hierarchy of electronic interactions with the use of time-resolved techniques, the role of the phonons often remains in dispute, a situation highlighting the need for tools that directly probe the lattice. We present the first combined megaelectron volt ultrafast electron diffraction and time- and angle-resolved photoemission spectroscopy study of optimally doped Bi2Sr2CaCu2O8+δ. Quantitative analysis of the lattice and electron subsystems' dynamics provides a unified picture of nonequilibrium electron-phonon interactions in the cuprates beyond the N-temperature model. The work provides new insights on the specific phonon branches involved in the nonequilibrium heat dissipation from the high-energy Cu-O bond stretching "hot" phonons to the lowest-energy acoustic phonons with correlated atomic motion along the <110> crystal directions and their characteristic time scales. It reveals a highly nonthermal phonon population during the first several picoseconds after the photoexcitation. The approach, taking advantage of the distinct nature of electrons and photons as probes, is applicable for studying energy relaxation in other strongly correlated electron systems.

  View details for DOI 10.1126/sciadv.aap7427

  View details for Web of Science ID 000431374900021

  View details for PubMedID 29719862

  View details for PubMedCentralID PMC5922801

• Magnetic Switching in Granular FePt Layers Promoted by Near-Field Laser Enhancement NANO LETTERS Granitzka, P. W., Jal, E., Le Guyader, L., Savoini, M., Higley, D. J., Liu, T., Chen, Z., Chase, T., Ohldag, H., Dakovski, G. L., Schlotter, W. F., Carron, S., Hoffman, M. C., Gray, A. X., Shafer, P., Arenholz, E., Hellwig, O., Mehta, V., Takahashi, Y. K., Wang, J., Fullerton, E. E., Stoehr, J., Reid, A. H., Duerr, H. A. 2017; 17 (4): 2426-2432

  Abstract

  Light-matter interaction at the nanoscale in magnetic materials is a topic of intense research in view of potential applications in next-generation high-density magnetic recording. Laser-assisted switching provides a pathway for overcoming the material constraints of high-anisotropy and high-packing density media, though much about the dynamics of the switching process remains unexplored. We use ultrafast small-angle X-ray scattering at an X-ray free-electron laser to probe the magnetic switching dynamics of FePt nanoparticles embedded in a carbon matrix following excitation by an optical femtosecond laser pulse. We observe that the combination of laser excitation and applied static magnetic field, 1 order of magnitude smaller than the coercive field, can overcome the magnetic anisotropy barrier between "up" and "down" magnetization, enabling magnetization switching. This magnetic switching is found to be inhomogeneous throughout the material with some individual FePt nanoparticles neither switching nor demagnetizing. The origin of this behavior is identified as the near-field modification of the incident laser radiation around FePt nanoparticles. The fraction of not-switching nanoparticles is influenced by the heat flow between FePt and a heat-sink layer.

  View details for DOI 10.1021/acs.nanolett.7b00052

  View details for Web of Science ID 000399354500047

  View details for PubMedID 28272897

• Diffractive imaging of a rotational wavepacket in nitrogen molecules with femtosecond megaelectronvolt electron pulses NATURE COMMUNICATIONS Yang, J., Guehr, M., Vecchione, T., Robinson, M. S., Li, R., Hartmann, N., Shen, X., Coffee, R., Corbett, J., Fry, A., Gaffney, K., Gorkhover, T., Hast, C., Jobe, K., Makasyuk, I., Reid, A., Robinson, J., Vetter, S., Wang, F., Weathersby, S., Yoneda, C., Centurion, M., Wang, X. 2016; 7

  Abstract

  Imaging changes in molecular geometries on their natural femtosecond timescale with sub-Angström spatial precision is one of the critical challenges in the chemical sciences, as the nuclear geometry changes determine the molecular reactivity. For photoexcited molecules, the nuclear dynamics determine the photoenergy conversion path and efficiency. Here we report a gas-phase electron diffraction experiment using megaelectronvolt (MeV) electrons, where we captured the rotational wavepacket dynamics of nonadiabatically laser-aligned nitrogen molecules. We achieved a combination of 100 fs root-mean-squared temporal resolution and sub-Angstrom (0.76 Å) spatial resolution that makes it possible to resolve the position of the nuclei within the molecule. In addition, the diffraction patterns reveal the angular distribution of the molecules, which changes from prolate (aligned) to oblate (anti-aligned) in 300 fs. Our results demonstrate a significant and promising step towards making atomically resolved movies of molecular reactions.

  View details for DOI 10.1038/ncomms11232

  View details for Web of Science ID 000373622400001

  View details for PubMedID 27046298

  View details for PubMedCentralID PMC4822053

• Indirect excitation of ultrafast demagnetization. Scientific reports Vodungbo, B., Tudu, B., Perron, J., Delaunay, R., Müller, L., Berntsen, M. H., Grübel, G., Malinowski, G., Weier, C., Gautier, J., Lambert, G., Zeitoun, P., Gutt, C., Jal, E., Reid, A. H., Granitzka, P. W., Jaouen, N., Dakovski, G. L., Moeller, S., Minitti, M. P., Mitra, A., Carron, S., Pfau, B., von Korff Schmising, C., Schneider, M., Eisebitt, S., Lüning, J. 2016; 6: 18970-?

  Abstract

  Does the excitation of ultrafast magnetization require direct interaction between the photons of the optical pump pulse and the magnetic layer? Here, we demonstrate unambiguously that this is not the case. For this we have studied the magnetization dynamics of a ferromagnetic cobalt/palladium multilayer capped by an IR-opaque aluminum layer. Upon excitation with an intense femtosecond-short IR laser pulse, the film exhibits the classical ultrafast demagnetization phenomenon although only a negligible number of IR photons penetrate the aluminum layer. In comparison with an uncapped cobalt/palladium reference film, the initial demagnetization of the capped film occurs with a delayed onset and at a slower rate. Both observations are qualitatively in line with energy transport from the aluminum layer into the underlying magnetic film by the excited, hot electrons of the aluminum film. Our data thus confirm recent theoretical predictions.

  View details for DOI 10.1038/srep18970

  View details for PubMedID 26733106

  View details for PubMedCentralID PMC4702181

• Femtosecond gas phase electron diffraction with MeV electrons FARADAY DISCUSSIONS Yang, J., Guehr, M., Vecchione, T., Robinson, M. S., Li, R., Hartmann, N., Shen, X., Coffee, R., Corbett, J., Fry, A., Gaffney, K., Gorkhover, T., Hast, C., Jobe, K., Makasyuk, I., Reid, A., Robinson, J., Vetter, S., Wang, F., Weathersby, S., Yoneda, C., Wang, X., Centurion, M. 2016; 194: 563–81

  Abstract

  We present results on ultrafast gas electron diffraction (UGED) experiments with femtosecond resolution using the MeV electron gun at SLAC National Accelerator Laboratory. UGED is a promising method to investigate molecular dynamics in the gas phase because electron pulses can probe the structure with a high spatial resolution. Until recently, however, it was not possible for UGED to reach the relevant timescale for the motion of the nuclei during a molecular reaction. Using MeV electron pulses has allowed us to overcome the main challenges in reaching femtosecond resolution, namely delivering short electron pulses on a gas target, overcoming the effect of velocity mismatch between pump laser pulses and the probe electron pulses, and maintaining a low timing jitter. At electron kinetic energies above 3 MeV, the velocity mismatch between laser and electron pulses becomes negligible. The relativistic electrons are also less susceptible to temporal broadening due to the Coulomb force. One of the challenges of diffraction with relativistic electrons is that the small de Broglie wavelength results in very small diffraction angles. In this paper we describe the new setup and its characterization, including capturing static diffraction patterns of molecules in the gas phase, finding time-zero with sub-picosecond accuracy and first time-resolved diffraction experiments. The new device can achieve a temporal resolution of 100 fs root-mean-square, and sub-angstrom spatial resolution. The collimation of the beam is sufficient to measure the diffraction pattern, and the transverse coherence is on the order of 2 nm. Currently, the temporal resolution is limited both by the pulse duration of the electron pulse on target and by the timing jitter, while the spatial resolution is limited by the average electron beam current and the signal-to-noise ratio of the detection system. We also discuss plans for improving both the temporal resolution and the spatial resolution.

  View details for DOI 10.1039/c6fd00071a

  View details for Web of Science ID 000392422200026

  View details for PubMedID 27711826

• Nanoscale Confinement of All-Optical Magnetic Switching in TbFeCo - Competition with Nanoscale Heterogeneity NANO LETTERS Liu, T., Wang, T., Reid, A. H., Savoini, M., Wu, X., Koene, B., Granitzka, P., Graves, C. E., Higley, D. J., Chen, Z., Razinskas, G., Hantschmann, M., Scherz, A., Stoehr, J., Tsukarnote, A., Hecht, B., Kimel, A. V., Kirilyuk, A., Rasing, T., Duerr, H. A. 2015; 15 (10): 6862-6868

  Abstract

  Single femtosecond optical laser pulses, of sufficient intensity, are demonstrated to reverse magnetization in a process known as all-optical switching. Gold two-wire antennas are placed on the all-optical switching film TbFeCo. These structures are resonant with the optical field, and they create a field enhancement in the near-field which confines the area where optical switching can occur. The magnetic switching that occurs around and below the antenna is imaged using resonant X-ray holography and magnetic circular dichroism. The results not only show the feasibility of controllable switching with antenna assistance but also demonstrate the highly inhomogeneous nature of the switching process, which is attributed to the process depending on the material's heterogeneity.

  View details for DOI 10.1021/acs.nanolett.5b02743

  View details for Web of Science ID 000363003100087

• Dynamic Structural Response and Deformations of Monolayer MoS2 Visualized by Femtosecond Electron Diffraction NANO LETTERS Mannebach, E. M., Li, R., Duerloo, K., Nyby, C., Zalden, P., Vecchione, T., Ernst, F., Reid, A. H., Chase, T., Shen, X., Weathersby, S., Hast, C., Hettel, R., Coffee, R., Hartmann, N., Fry, A. R., Yu, Y., Cao, L., Heinz, T. F., Reed, E. J., Duerr, H. A., Wang, X., Lindenberg, A. M. 2015; 15 (10): 6889-6895

  Abstract

  Two-dimensional materials are subject to intrinsic and dynamic rippling that modulates their optoelectronic and electromechanical properties. Here, we directly visualize the dynamics of these processes within monolayer transition metal dichalcogenide MoS2 using femtosecond electron scattering techniques as a real-time probe with atomic-scale resolution. We show that optical excitation induces large-amplitude in-plane displacements and ultrafast wrinkling of the monolayer on nanometer length-scales, developing on picosecond time-scales. These deformations are associated with several percent peak strains that are fully reversible over tens of millions of cycles. Direct measurements of electron-phonon coupling times and the subsequent interfacial thermal heat flow between the monolayer and substrate are also obtained. These measurements, coupled with first-principles modeling, provide a new understanding of the dynamic structural processes that underlie the functionality of two-dimensional materials and open up new opportunities for ultrafast strain engineering using all-optical methods.

  View details for DOI 10.1021/acs.nanolett.5b02805

  View details for PubMedID 26322659

• Control of the metal-insulator transition in vanadium dioxide by modifying orbital occupancy NATURE PHYSICS Aetukuri, N. B., Gray, A. X., Drouard, M., Cossale, M., Gao, L., Reid, A. H., Kukreja, R., Ohldag, H., Jenkins, C. A., Arenholz, E., Roche, K. P., Duerr, H. A., Samant, M. G., Parkin, S. S. 2013; 9 (10): 661-666

  View details for DOI 10.1038/NPHYS2733

  View details for Web of Science ID 000325205000018