All Publications


  • Laser-induced transient magnons in Sr3Ir2O7 throughout the Brillouin zone. Proceedings of the National Academy of Sciences of the United States of America Mazzone, D. G., Meyers, D., Cao, Y., Vale, J. G., Dashwood, C. D., Shi, Y., James, A. J., Robinson, N. J., Lin, J., Thampy, V., Tanaka, Y., Johnson, A. S., Miao, H., Wang, R., Assefa, T. A., Kim, J., Casa, D., Mankowsky, R., Zhu, D., Alonso-Mori, R., Song, S., Yavas, H., Katayama, T., Yabashi, M., Kubota, Y., Owada, S., Liu, J., Yang, J., Konik, R. M., Robinson, I. K., Hill, J. P., McMorrow, D. F., Forst, M., Wall, S., Liu, X., Dean, M. P. 2021; 118 (22)

    Abstract

    Although ultrafast manipulation of magnetism holds great promise for new physical phenomena and applications, targeting specific states is held back by our limited understanding of how magnetic correlations evolve on ultrafast timescales. Using ultrafast resonant inelastic X-ray scattering we demonstrate that femtosecond laser pulses can excite transient magnons at large wavevectors in gapped antiferromagnets and that they persist for several picoseconds, which is opposite to what is observed in nearly gapless magnets. Our work suggests that materials with isotropic magnetic interactions are preferred to achieve rapid manipulation of magnetism.

    View details for DOI 10.1073/pnas.2103696118

    View details for PubMedID 34039712

  • Structure of a seeded palladium nanoparticle and its dynamics during the hydride phase transformation COMMUNICATIONS CHEMISTRY Suzana, A. F., Wu, L., Assefa, T. A., Williams, B. P., Harder, R., Cha, W., Kuo, C., Tsung, C., Robinson, I. K. 2021; 4 (1)
  • Charge density waves in cuprate superconductors beyond the critical doping NPJ QUANTUM MATERIALS Miao, H., Fabbris, G., Koch, R. J., Mazzone, D. G., Nelson, C. S., Acevedo-Esteves, R., Gu, G. D., Li, Y., Yilimaz, T., Kaznatcheev, K., Vescovo, E., Oda, M., Kurosawa, T., Momono, N., Assefa, T., Robinson, I. K., Bozin, E. S., Tranquada, J. M., Johnson, P. D., Dean, M. M. 2021; 6 (1)
  • Site-Selective Real-Time Observation of Bimolecuar Electron Transfer in a Photocatalytic System Using L-Edge X-Ray Absorption Spectroscopy** CHEMPHYSCHEM Britz, A., Bokarev, S. I., Assefa, T. A., Bajnoczi, E. G., Nemeth, Z., Vanko, G., Rockstroh, N., Junge, H., Beller, M., Doumy, G., March, A., Southworth, S. H., Lochbrunner, S., Kuehn, O., Bressler, C., Gawelda, W. 2021

    Abstract

    Time-resolved X-ray absorption spectroscopy has been utilized to monitor the bimolecular electron transfer in a photocatalytic water splitting system. This has been possible by uniting the local probe and element specific character of X-ray transitions with insights from high-level ab initio calculations. The specific target has been a heteroleptic [IrIII (ppy)2 (bpy)]+ photosensitizer, in combination with triethylamine as a sacrificial reductant and Fe 3 ( CO ) 12 as a water reduction catalyst. The relevant molecular transitions have been characterized via high-resolution Ir L-edge X-ray absorption spectroscopy on the picosecond time scale and restricted active space self-consistent field calculations. The presented methods and results will enhance our understanding of functionally relevant bimolecular electron transfer reactions and thus will pave the road to rational optimization of photocatalytic performance.

    View details for DOI 10.1002/cphc.202000845

    View details for Web of Science ID 000629277500001

    View details for PubMedID 33410580

  • Imaging the Phase Transformation in Single Particles of the Lithium Titanate Anode for Lithium-Ion Batteries ACS APPLIED ENERGY MATERIALS Assefa, T. A., Suzana, A. F., Wu, L., Koch, R. J., Li, L., Cha, W., Harder, R. J., Bozin, E. S., Wang, F., Robinson, I. K. 2021; 4 (1): 111–18
  • Evolution of ferroelastic domain walls during phase transitions in barium titanate nanoparticles PHYSICAL REVIEW MATERIALS Diao, J., Shi, X., Assefa, T. A., Wu, L., Suzana, A. F., Nunes, D. S., Batey, D., Cipiccia, S., Rau, C., Harder, R. J., Cha, W., Robinson, I. K. 2020; 4 (10)
  • Complete Strain Mapping of Nanosheets of Tantalum Disulfide ACS APPLIED MATERIALS & INTERFACES Cao, Y., Assefa, T., Banerjee, S., Wieteska, A., Wang, D., Pasupathy, A., Tong, X., Liu, Y., Lu, W., Sun, Y., He, Y., Huang, X., Yan, H., Chu, Y. S., Billinge, S. L., Robinson, I. K. 2020; 12 (38): 43173–79

    Abstract

    Quasi-two-dimensional (quasi-2D) materials hold promise for future electronics because of their unique band structures that result in electronic and mechanical properties sensitive to crystal strains in all three dimensions. Quantifying crystal strain is a prerequisite to correlating it with the performance of the device and calls for high resolution but spatially resolved rapid characterization methods. Here, we show that using fly-scan nano X-ray diffraction, we can accomplish a tensile strain sensitivity below 0.001% with a spatial resolution of better than 80 nm over a spatial extent of 100 μm on quasi-2D flakes of 1T-TaS2. Coherent diffraction patterns were collected from a ∼100 nm thick sheet of 1T-TaS2 by scanning a 12 keV focused X-ray beam across and rotating the sample. We demonstrate that the strain distribution around micron- and submicron-sized "bubbles" that are present in the sample may be reconstructed from these images. The experiments use state-of-the-art synchrotron instrumentation and will allow rapid and nonintrusive strain mapping of thin-film samples and electronic devices based on quasi-2D materials.

    View details for DOI 10.1021/acsami.0c06517

    View details for Web of Science ID 000575557800078

    View details for PubMedID 32833418

  • Femtosecond X-Ray Scattering Study of Ultrafast Photoinduced Structural Dynamics in Solvated [Co(terpy)(2)](2+) (vol 117, 013002, 2016) PHYSICAL REVIEW LETTERS Biasin, E., van Driel, T., Kjer, K. S., Dohn, A. O., Christensen, M., Harlang, T., Vester, P., Chabera, P., Liu, Y., Uhlig, J., Papai, M., Nemeth, Z., Hartsock, R., Liang, W., Zhang, J., Alonso-Mori, R., Chollet, M., Glownia, J. M., Nelson, S., Sokaras, D., Assefa, T. A., Britz, A., Galler, A., Gawelda, W., Bressler, C., Gaffney, K. J., Lemke, H. T., Moller, K. B., Nielsen, M. M., Sundstrom, V., Vanko, G., Warnmark, K., Canton, S. E., Haldrup, K. 2020; 124 (19): 199902

    Abstract

    This corrects the article DOI: 10.1103/PhysRevLett.117.013002.

    View details for DOI 10.1103/PhysRevLett.124.199902

    View details for Web of Science ID 000533164500015

    View details for PubMedID 32469585

  • Scaling behavior of low-temperature orthorhombic domains in the prototypical high-temperature superconductor La1.875Ba0.125CuO4 PHYSICAL REVIEW B Assefa, T. A., Cao, Y., Diao, J., Harder, R. J., Cha, W., Kisslinger, K., Gu, G. D., Tranquada, J. M., Dean, M. M., Robinson, I. K. 2020; 101 (5)
  • Ultrafast x-ray diffraction study of melt-front dynamics in polycrystalline thin films SCIENCE ADVANCES Assefa, T. A., Cao, Y., Banerjee, S., Kim, S., Kim, D., Lee, H., Kim, S., Lee, J., Park, S., Eom, I., Park, J., Nam, D., Kim, S., Chun, S., Hyun, H., Kim, K., Juhas, P., Bozin, E. S., Lu, M., Song, C., Kim, H., Billinge, S. L., Robinson, I. K. 2020; 6 (3): eaax2445

    Abstract

    Melting is a fundamental process of matter that is still not fully understood at the microscopic level. Here, we use time-resolved x-ray diffraction to examine the ultrafast melting of polycrystalline gold thin films using an optical laser pump followed by a delayed hard x-ray probe pulse. We observe the formation of an intermediate new diffraction peak, which we attribute to material trapped between the solid and melted states, that forms 50 ps after laser excitation and persists beyond 500 ps. The peak width grows rapidly for 50 ps and then narrows distinctly at longer time scales. We attribute this to a melting band originating from the grain boundaries and propagating into the grains. Our observation of this intermediate state has implications for the use of ultrafast lasers for ablation during pulsed laser deposition.

    View details for DOI 10.1126/sciadv.aax2445

    View details for Web of Science ID 000510488100004

    View details for PubMedID 32010766

    View details for PubMedCentralID PMC6968939

  • Domain Texture of the Orthorhombic Phase of La2-xBaxCuO4 JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM Robinson, I., Assefa, T. A., Cao, Y., Gu, G., Harder, R., Maxey, E., Dean, M. M. 2020; 33 (1): 99–106
  • Elucidation of the photoaquation reaction mechanism in ferrous hexacyanide using synchrotron x-rays with sub-pulse-duration sensitivity JOURNAL OF CHEMICAL PHYSICS March, A., Doumy, G., Andersen, A., Al Haddad, A., Kumagai, Y., Tu, M., Bang, J., Bostedt, C., Uhlig, J., Nascimento, D. R., Assefa, T. A., Nemeth, Z., Vanko, G., Gawelda, W., Govind, N., Young, L. 2019; 151 (14): 144306

    Abstract

    Ligand substitution reactions are common in solvated transition metal complexes, and harnessing them through initiation with light promises interesting practical applications, driving interest in new means of probing their mechanisms. Using a combination of time-resolved x-ray absorption spectroscopy and hybrid quantum mechanics/molecular mechanics (QM/MM) molecular dynamics simulations and x-ray absorption near-edge spectroscopy calculations, we elucidate the mechanism of photoaquation in the model system iron(ii) hexacyanide, where UV excitation results in the exchange of a CN- ligand with a water molecule from the solvent. We take advantage of the high flux and stability of synchrotron x-rays to capture high precision x-ray absorption spectra that allow us to overcome the usual limitation of the relatively long x-ray pulses and extract the spectrum of the short-lived intermediate pentacoordinated species. Additionally, we determine its lifetime to be 19 (±5) ps. The QM/MM simulations support our experimental findings and explain the ∼20 ps time scale for aquation as involving interconversion between the square pyramidal (SP) and trigonal bipyramidal pentacoordinated geometries, with aquation being only active in the SP configuration.

    View details for DOI 10.1063/1.5117318

    View details for Web of Science ID 000500356200029

    View details for PubMedID 31615248

  • Using Ultrafast X-ray Spectroscopy To Address Questions in Ligand-Field Theory: The Excited State Spin and Structure of [Fe(dcpp)(2)](2+) INORGANIC CHEMISTRY Britz, A., Gawelda, W., Assefa, T. A., Jamula, L. L., Yarranton, J. T., Galler, A., Khakhulin, D., Diez, M., Hardee, M., Doumy, G., March, A., Bajnoczi, E., Nemeth, Z., Papai, M., Rozsalyi, E., Szemes, D., Cho, H., Mukherjee, S., Liu, C., Kim, T., Schoenlein, R. W., Southworth, S. H., Young, L., Jakubikova, E., Huse, N., Vanko, G., Bressler, C., McCusker, J. K. 2019; 58 (14): 9341–50

    Abstract

    We have employed a range of ultrafast X-ray spectroscopies in an effort to characterize the lowest energy excited state of [Fe(dcpp)2]2+ (where dcpp is 2,6-(dicarboxypyridyl)pyridine). This compound exhibits an unusually short excited-state lifetime for a low-spin Fe(II) polypyridyl complex of 270 ps in a room-temperature fluid solution, raising questions as to whether the ligand-field strength of dcpp had pushed this system beyond the 5T2/3T1 crossing point and stabilizing the latter as the lowest energy excited state. Kα and Kβ X-ray emission spectroscopies have been used to unambiguously determine the quintet spin multiplicity of the long-lived excited state, thereby establishing the 5T2 state as the lowest energy excited state of this compound. Geometric changes associated with the photoinduced ligand-field state conversion have also been monitored with extended X-ray absorption fine structure. The data show the typical average Fe-ligand bond length elongation of ∼0.18 Å for a 5T2 state and suggest a high anisotropy of the primary coordination sphere around the metal center in the excited 5T2 state, in stark contrast to the nearly perfect octahedral symmetry that characterizes the low-spin 1A1 ground state structure. This study illustrates how the application of time-resolved X-ray techniques can provide insights into the electronic structures of molecules-in particular, transition metal complexes-that are difficult if not impossible to obtain by other means.

    View details for DOI 10.1021/acs.inorgchem.9b01063

    View details for Web of Science ID 000475838000051

    View details for PubMedID 31241335

  • Charge density wave memory in a cuprate superconductor NATURE COMMUNICATIONS Chen, X. M., Mazzoli, C., Cao, Y., Thampy, Barbour, A. M., Hu, W., Lu, M., Assefa, T. A., Miao, H., Fabbris, G., Gu, G. D., Tranquada, J., Dean, M. M., Wilkins, S. B., Robinson, I. K. 2019; 10: 1435

    Abstract

    Although CDW correlations are a ubiquitous feature of the superconducting cuprates, their disparate properties suggest a crucial role for pinning the CDW to the lattice. Here, we report coherent resonant X-ray speckle correlation analysis, which directly determines the reproducibility of CDW domain patterns in La1.875Ba0.125CuO4 (LBCO 1/8) with thermal cycling. While CDW order is only observed below 54 K, where a structural phase transition creates inequivalent Cu-O bonds, we discover remarkably reproducible CDW domain memory upon repeated cycling to far higher temperatures. That memory is only lost on cycling to 240(3) K, which recovers the four-fold symmetry of the CuO2 planes. We infer that the structural features that develop below 240 K determine the CDW pinning landscape below 54 K. This opens a view into the complex coupling between charge and lattice degrees of freedom in superconducting cuprates.

    View details for DOI 10.1038/s41467-019-09433-1

    View details for Web of Science ID 000462721900034

    View details for PubMedID 30926816

    View details for PubMedCentralID PMC6440992

  • Ultrafast X-ray spectroscopy of transition metal compounds relevant for catalysis: a study case of a high-valent Fe complex Gawelda, W., Assefa, T., Britz, A., Diez, M., Khakhulin, D., Galler, A., Torres-Alacan, J., Voehringer, P., Bressler, C. AMER CHEMICAL SOC. 2018
  • Probing Transient Valence Orbital Changes with Picosecond Valence-to-Core X-ray Emission Spectroscopy JOURNAL OF PHYSICAL CHEMISTRY C March, A., Assefa, T. A., Boemer, C., Bressler, C., Britz, A., Diez, M., Doumy, G., Galler, A., Harder, M., Khakhulin, D., Nemeth, Z., Papai, M., Schulz, S., Southworth, S. H., Yavas, H., Young, L., Gawelda, W., Vanko, G. 2017; 121 (5): 2620–26

    Abstract

    We probe the dynamics of valence electrons in photoexcited [Fe(terpy)2]2+ in solution to gain deeper insight into the Fe-ligand bond changes. We use hard X-ray emission spectroscopy (XES), which combines element specificity and high penetration with sensitivity to orbital structure, making it a powerful technique for molecular studies in a wide variety of environments. A picosecond-time-resolved measurement of the complete 1s X-ray emission spectrum captures the transient photoinduced changes and includes the weak valence-to-core (vtc) emission lines that correspond to transitions from occupied valence orbitals to the nascent core-hole. Vtc-XES offers particular insight into the molecular orbitals directly involved in the light-driven dynamics; a change in the metal-ligand orbital overlap results in an intensity reduction and a blue energy shift in agreement with our theoretical calculations and more subtle features at the highest energies reflect changes in the frontier orbital populations.

    View details for DOI 10.1021/acs.jpcc.6b12940

    View details for Web of Science ID 000394080900015

    View details for PubMedID 28580048

    View details for PubMedCentralID PMC5453616

  • A multi-MHz single-shot data acquisition scheme with high dynamic range: pump-probe X-ray experiments at synchrotrons JOURNAL OF SYNCHROTRON RADIATION Britz, A., Assefa, T. A., Galler, A., Gawelda, W., Diez, M., Zalden, P., Khakhulin, D., Fernandes, B., Gessler, P., Namin, H., Beckmann, A., Harder, M., Yavas, H., Bressler, C. 2016; 23: 1409–23

    Abstract

    The technical implementation of a multi-MHz data acquisition scheme for laser-X-ray pump-probe experiments with pulse limited temporal resolution (100 ps) is presented. Such techniques are very attractive to benefit from the high-repetition rates of X-ray pulses delivered from advanced synchrotron radiation sources. Exploiting a synchronized 3.9 MHz laser excitation source, experiments in 60-bunch mode (7.8 MHz) at beamline P01 of the PETRA III storage ring are performed. Hereby molecular systems in liquid solutions are excited by the pulsed laser source and the total X-ray fluorescence yield (TFY) from the sample is recorded using silicon avalanche photodiode detectors (APDs). The subsequent digitizer card samples the APD signal traces in 0.5 ns steps with 12-bit resolution. These traces are then processed to deliver an integrated value for each recorded single X-ray pulse intensity and sorted into bins according to whether the laser excited the sample or not. For each subgroup the recorded single-shot values are averaged over ∼107 pulses to deliver a mean TFY value with its standard error for each data point, e.g. at a given X-ray probe energy. The sensitivity reaches down to the shot-noise limit, and signal-to-noise ratios approaching 1000 are achievable in only a few seconds collection time per data point. The dynamic range covers 100 photons pulse-1 and is only technically limited by the utilized APD.

    View details for DOI 10.1107/S1600577516012625

    View details for Web of Science ID 000386928700015

    View details for PubMedID 27787247

  • Femtosecond X-Ray Scattering Study of Ultrafast Photoinduced Structural Dynamics in Solvated [Co(terpy)(2)](2+) PHYSICAL REVIEW LETTERS Biasin, E., van Driel, T., Kjaer, K. S., Dohn, A. O., Christensen, M., Harlang, T., Chabera, P., Liu, Y., Uhlig, J., Papai, M., Nemeth, Z., Hartsock, R., Liang, W., Zhang, J., Alonso-Mori, R., Chollet, M., Glownia, J. M., Nelson, S., Sokaras, D., Assefa, T. A., Britz, A., Galler, A., Gawelda, W., Bressler, C., Gaffney, K. J., Lemke, H. T., Moller, K. B., Nielsen, M. M., Sundstrom, V., Vanko, G., Warnmark, K., Canton, S. E., Haldrup, K. 2016; 117 (1): 013002

    Abstract

    We study the structural dynamics of photoexcited [Co(terpy)_{2}]^{2+} in an aqueous solution with ultrafast x-ray diffuse scattering experiments conducted at the Linac Coherent Light Source. Through direct comparisons with density functional theory calculations, our analysis shows that the photoexcitation event leads to elongation of the Co-N bonds, followed by coherent Co-N bond length oscillations arising from the impulsive excitation of a vibrational mode dominated by the symmetrical stretch of all six Co-N bonds. This mode has a period of 0.33 ps and decays on a subpicosecond time scale. We find that the equilibrium bond-elongated structure of the high spin state is established on a single-picosecond time scale and that this state has a lifetime of ∼7  ps.

    View details for DOI 10.1103/PhysRevLett.117.013002

    View details for Web of Science ID 000378881300004

    View details for PubMedID 27419566

  • Feasibility of Valence-to-Core X-ray Emission Spectroscopy for Tracking Transient Species JOURNAL OF PHYSICAL CHEMISTRY C March, A., Assefa, T. A., Bressler, C., Doumy, G., Galler, A., Gawelda, W., Kanter, E. P., Nemeth, Z., Papai, M., Southworth, S. H., Young, L., Vanko, G. 2015; 119 (26): 14571–78

    Abstract

    X-ray spectroscopies, when combined in laser-pump, X-ray-probe measurement schemes, can be powerful tools for tracking the electronic and geometric structural changes that occur during the course of a photoinitiated chemical reaction. X-ray absorption spectroscopy (XAS) is considered an established technique for such measurements, and X-ray emission spectroscopy (XES) of the strongest core-to-core emission lines (Kα and Kβ) is now being utilized. Flux demanding valence-to-core XES promises to be an important addition to the time-resolved spectroscopic toolkit. In this paper we present measurements and density functional theory calculations on laser-excited, solution-phase ferrocyanide that demonstrate the feasibility of valence-to-core XES for time-resolved experiments. We discuss technical improvements that will make valence-to-core XES a practical pump-probe technique.

    View details for DOI 10.1021/jp511838q

    View details for Web of Science ID 000357623500013

    View details for PubMedID 26568779

    View details for PubMedCentralID PMC4634714

  • Detailed Characterization of a Nanosecond-Lived Excited State: X-ray and Theoretical Investigation of the Quintet State in Photoexcited [Fe(terpy)(2)](2+) JOURNAL OF PHYSICAL CHEMISTRY C Vanko, G., Bordage, A., Papai, M., Haldrup, K., Glatzel, P., March, A., Doumy, G., Britz, A., Galler, A., Assefa, T., Cabaret, D., Juhin, A., van Driel, T. B., Kjaer, K. S., Dohn, A., Moller, K. B., Lemke, H. T., Gallo, E., Rovezzi, M., Nemeth, Z., Rozsalyi, E., Rozgonyi, T., Uhlig, J., Sundstrom, V., Nielsen, M. M., Young, L., Southworth, S. H., Bressler, C., Gawelda, W. 2015; 119 (11): 5888–5902

    Abstract

    Theoretical predictions show that depending on the populations of the Fe 3d xy , 3d xz , and 3d yz orbitals two possible quintet states can exist for the high-spin state of the photoswitchable model system [Fe(terpy)2]2+. The differences in the structure and molecular properties of these 5B2 and 5E quintets are very small and pose a substantial challenge for experiments to resolve them. Yet for a better understanding of the physics of this system, which can lead to the design of novel molecules with enhanced photoswitching performance, it is vital to determine which high-spin state is reached in the transitions that follow the light excitation. The quintet state can be prepared with a short laser pulse and can be studied with cutting-edge time-resolved X-ray techniques. Here we report on the application of an extended set of X-ray spectroscopy and scattering techniques applied to investigate the quintet state of [Fe(terpy)2]2+ 80 ps after light excitation. High-quality X-ray absorption, nonresonant emission, and resonant emission spectra as well as X-ray diffuse scattering data clearly reflect the formation of the high-spin state of the [Fe(terpy)2]2+ molecule; moreover, extended X-ray absorption fine structure spectroscopy resolves the Fe-ligand bond-length variations with unprecedented bond-length accuracy in time-resolved experiments. With ab initio calculations we determine why, in contrast to most related systems, one configurational mode is insufficient for the description of the low-spin (LS)-high-spin (HS) transition. We identify the electronic structure origin of the differences between the two possible quintet modes, and finally, we unambiguously identify the formed quintet state as 5E, in agreement with our theoretical expectations.

    View details for DOI 10.1021/acs.jpcc.5b00557

    View details for Web of Science ID 000351557800015

    View details for PubMedID 25838847

    View details for PubMedCentralID PMC4368081

  • Disentangling the Electronic and Phononic Glue in a High-T-c Superconductor SCIENCE Dal Conte, S., Giannetti, C., Coslovich, G., Cilento, F., Bossini, D., Abebaw, T., Banfi, F., Ferrini, G., Eisaki, H., Greven, M., Damascelli, A., van der Marel, D., Parmigiani, F. 2012; 335 (6076): 1600–1603

    Abstract

    Unveiling the nature of the bosonic excitations that mediate the formation of Cooper pairs is a key issue for understanding unconventional superconductivity. A fundamental step toward this goal would be to identify the relative weight of the electronic and phononic contributions to the overall frequency (Ω)-dependent bosonic function, Π(Ω). We performed optical spectroscopy on Bi(2)Sr(2)Ca(0.92)Y(0.08)Cu(2)O(8+δ) crystals with simultaneous time and frequency resolution; this technique allowed us to disentangle the electronic and phononic contributions by their different temporal evolution. The spectral distribution of the electronic excitations and the strength of their interaction with fermionic quasiparticles fully account for the high critical temperature of the superconducting phase transition.

    View details for DOI 10.1126/science.1216765

    View details for Web of Science ID 000302062600038

    View details for PubMedID 22461606