Ultrafast

New transient states via photo-excitation

We have been pioneering the application of RIXS to transient states using x-ray free electron lasers.

Figure 1: An illustration of time-resolved RIXS. See M. P. M. Dean et al., Nature Materials 15, 601–605 (2016) for more details.

References

2024

Resolving length-scale-dependent transient disorder through an ultrafast phase transition

Jack Griffiths, Ana F. Suzana, Longlong Wu, Samuel D. Marks, Vincent Esposito, Sébastien Boutet, Paul G. Evans, J. F. Mitchell, Mark P. M. Dean, David A. Keen, Ian Robinson, Simon J. L. Billinge, and Emil S. Bozin

Nature Materials 23, 1041–1047 (2024)

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Material functionality can be strongly determined by structure extending only over nanoscale distances. The pair distribution function presents an opportunity for structural studies beyond idealized crystal models and to investigate structure over varying length scales. Applying this method with ultrafast time resolution has the potential to similarly disrupt the study of structural dynamics and phase transitions. Here we demonstrate such a measurement of CuIr2S4 optically pumped from its low-temperature Ir-dimerized phase. Dimers are optically suppressed without spatial correlation, generating a structure whose level of disorder strongly depends on the length scale. The redevelopment of structural ordering over tens of picoseconds is directly tracked over both space and time as a transient state is approached. This measurement demonstrates the crucial role of local structure and disorder in non-equilibrium processes as well as the feasibility of accessing this information with state-of-the-art XFEL facilities.

2022

Antiferromagnetic excitonic insulator state in Sr₃Ir₂O₇

D. G. Mazzone, Y. Shen, H. Suwa, G. Fabbris, J. Yang, S.-S. Zhang, H. Miao, J. Sears, Ke Jia, Y. G. Shi, M. H. Upton, D. M. Casa, X. Liu, Jian Liu, C. D. Batista, and M. P. M. Dean

Nature Communications 13, 913 (2022)

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Excitonic insulators are usually considered to form via the condensation of a soft charge mode of bound electron-hole pairs. This, however, presumes that the soft exciton is of spin-singlet character. Early theoretical considerations have also predicted a very distinct scenario, in which the condensation of magnetic excitons results in an antiferromagnetic excitonic insulator state. Here we report resonant inelastic x-ray scattering (RIXS) measurements of Sr₃Ir₂O₇. By isolating the longitudinal component of the spectra, we identify a magnetic mode that is well-defined at the magnetic and structural Brillouin zone centers, but which merges with the electronic continuum in between these high symmetry points and which decays upon heating concurrent with a decrease in the material’s resistivity. We show that a bilayer Hubbard model, in which electron-hole pairs are bound by exchange interactions, consistently explains all the electronic and magnetic properties of Sr₃Ir₂O₇ indicating that this material is a realization of the long-predicted antiferromagnetic excitonic insulator phase.

2021

Laser-induced transient magnons in Sr₃Ir₂O₇ throughout the Brillouin zone

Daniel G. Mazzone, Derek Meyers, Yue Cao, James G. Vale, Cameron D. Dashwood, Youguo Shi, Andrew J. A. James, Neil J. Robinson, Jiaqi Lin, Vivek Thampy, Yoshikazu Tanaka, Allan S. Johnson, Hu Miao, Ruitang Wang, Tadesse A. Assefa, Jungho Kim, Diego Casa, Roman Mankowsky, Diling Zhu, Roberto Alonso-Mori, Sanghoon Song, Hasan Yavas, Tetsuo Katayama, Makina Yabashi, Yuya Kubota, Shigeki Owada, Jian Liu, Junji Yang, Robert M. Konik, Ian K. Robinson, John P. Hill, Desmond F. McMorrow, Michael Först, Simon Wall, Xuerong Liu, and Mark P. M. Dean

Proceedings of the National Academy of Sciences 118, e2103696118 (2021)

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Ultrafast manipulation of magnetic states holds great promise for progress in our understanding of new quantum states and technical applications, but our current knowledge of transient magnetism is very limited. Our work elucidates the nature of transient magnetism in gapped antiferromagnets using Sr₃Ir₂O₇ as a model material. We find that transient magnetic fluctuations are trapped throughout the entire Brillouin zone while remaining present beyond the time that is required to restore the original spin network. The results are interpreted in the context of a spin-bottleneck effect, in which the existence of an explicit magnetic decay channel allows for an efficient thermalization of transient spin waves.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.

2019

Ultrafast dynamics of spin and orbital correlations in quantum materials: an energy-and momentum-resolved perspective

Y Cao, DG Mazzone, D Meyers, JP Hill, X Liu, S Wall, and MPM Dean

Philosophical Transactions of the Royal Society A 377, 20170480 (2019)

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Many remarkable properties of quantum materials emerge from states with intricate coupling between the charge, spin and orbital degrees of freedom. Ultrafast photo-excitation of these materials holds great promise for understanding and controlling the properties of these states. Here, we introduce time-resolved resonant inelastic X-ray scattering (tr-RIXS) as a means of measuring the charge, spin and orbital excitations out of equilibrium. These excitations encode the correlations and interactions that determine the detailed properties of the states generated. After outlining the basic principles and instrumentations of tr-RIXS, we review our first observations of transient antiferromagnetic correlations in quasi two dimensions in a photo-excited Mott insulator and present possible future routes of this fast-developing technique. The increasing number of X-ray free electron laser facilities not only enables tackling long-standing fundamental scientific problems, but also promises to unleash novel inelastic X-ray scattering spectroscopies.

2016

Ultrafast energy and momentum resolved dynamics of magnetic correlations in photo-doped Mott insulator Sr₂IrO₄

MPM Dean, Yue Cao, X Liu, S Wall, D Zhu, Roman Mankowsky, V Thampy, XM Chen, JG Vale, D Casa, Jungho Kim, A. H. Said, P. Juhas, R. Alonso-Mori, J. M. Glownia, A. Robert, J. Robinson, M. Sikorski, S. Song, M. Kozina, H. Lemke, L. Patthey, S. Owada, T. Katayama, M. Yabashi, Yoshikazu Tanaka, T. Togashi, J. Liu, C. Rayan Serrao, B. J. Kim, L. Huber, C.-L. Chang, D. F. McMorrow, M. Först, and J. P. Hill

Nature Materials 15, 601–605 (2016)

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Measuring how the magnetic correlations evolve in doped Mott insulators has greatly improved our understanding of the pseudogap, non-Fermi liquids and higherature superconductivity. Recently, photo-excitation has been used to induce similarly exotic states transiently. However, the lack of available probes of magnetic correlations in the time domain hinders our understanding of these photo-induced states and how they could be controlled. Here, we implement magnetic resonant inelastic X-ray scattering at a free-electron laser to directly determine the magnetic dynamics after photo-doping the Mott insulator Sr₂IrO₄. We find that the non-equilibrium state, 2 ps after the excitation, exhibits strongly suppressed long-range magnetic order, but hosts photo-carriers that induce strong, non-thermal magnetic correlations. These two-dimensional (2D) in-plane Neel correlations recover within a few picoseconds, whereas the three-dimensional (3D) long-range magnetic order restores on a fluence-dependent timescale of a few hundred picoseconds. The marked difference in these two timescales implies that the dimensionality of magnetic correlations is vital for our understanding of ultrafast magnetic dynamics.