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Evidence for topological defects in a photoinduced phase transition

Abstract

Upon excitation with an intense laser pulse, a symmetry-broken ground state can undergo a non-equilibrium phase transition through pathways different from those in thermal equilibrium. The mechanism underlying these photoinduced phase transitions has long been researched in the study of condensed matter systems1, but many details in this ultrafast, non-adiabatic regime still remain to be clarified. To this end, we investigate the light-induced melting of a unidirectional charge density wave (CDW) in LaTe3. Using a suite of time-resolved probes, we independently track the amplitude and phase dynamics of the CDW. We find that a fast (approximately 1 picosecond) recovery of the CDW amplitude is followed by a slower re-establishment of phase coherence. This longer timescale is dictated by the presence of topological defects: long-range order is inhibited and is only restored when the defects annihilate. Our results provide a framework for understanding other photoinduced phase transitions by identifying the generation of defects as a governing mechanism.

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Fig. 1: Time evolution of electron diffraction after photoexcitation.
Fig. 2: Dependence of CDW diffraction peak and optical reflectivity on excitation density.
Fig. 3: tr-ARPES spectra showing CDW gap dynamics.
Fig. 4: Summary of CDW recovery timescales and dynamics.

Data availability

The datasets generated and analysed during the current study are available from the corresponding author on reasonable request.

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Acknowledgements

We acknowledge discussions with S. Brazovskii, Z. Ding, T. Xie, P. A. Lee, J. Ruhman, B. Skinner, A. Krikun, W. H. Zurek, S.-Y. Xu and D. Chowdhury. We thank M. Bajaj for assistance on instrumentation. We acknowledge support from the US Department of Energy, BES DMSE (experimental setup and data acquisition), from the Gordon and Betty Moore Foundation’s EPiQS Initiative grant GBMF4540 (data analysis and manuscript writing), the Army Research Office (equipment support for the tr-ARPES), and the Skoltech NGP Program (Skoltech-MIT joint project) (theory). Y.-Q.B. and P.J.-H. acknowledge support from the Center for Excitonics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under award number DESC0001088, as well as the Gordon and Betty Moore Foundation’s EPiQS Initiative through grant GBMF4541 (sample preparation and characterization). Work at Stanford was supported by the US Department of Energy, Office of Basic Energy Sciences, under contract number DE-AC02-76SF00515 (sample growth and characterization). P.W. was supported in part by the Gordon and Betty Moore Foundation’s EPiQS Initiative through grant GBMF4414. E.B. acknowledges support by the Swiss National Science Foundation under fellowship P2ELP2-172290.

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Contributions

A.Z., A.K., T.R., C.L., E.B., E.E., and M.B.Y. performed the time-resolved measurements. J.S. and P.W. synthesized single crystals of LaTe3, supervised by I.R.F. Y.-Q.B. prepared and characterized the samples, supervised by P.J.H. B.F., H.Z., T.R., A.Z. and A.K. built the ultrafast electron diffraction setup. T.R., C.L., E.J.S., E.B. and A.Z. built the 10.75 eV beamline for the tr-ARPES setup. E.E. and M.B.Y. built the optical spectroscopy setup. A.Z. and A.K. performed the data analysis with theoretical input from P.E.D., A.V.R. and B.V.F. A.K. wrote the manuscript with crucial input from A.Z., I.R.F., A.V.R., B.V.F., N.G. and all other authors. This project was supervised by N.G.

Corresponding author

Correspondence to Nuh Gedik.

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Supplementary Information

Supplementary Figure 1–7; References 36–42

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Zong, A., Kogar, A., Bie, YQ. et al. Evidence for topological defects in a photoinduced phase transition. Nature Phys 15, 27–31 (2019). https://doi.org/10.1038/s41567-018-0311-9

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