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Non-thermal separation of electronic and structural orders in a persisting charge density wave

Abstract

The simultaneous ordering of different degrees of freedom in complex materials undergoing spontaneous symmetry-breaking transitions often involves intricate couplings that have remained elusive in phenomena as wide ranging as stripe formation1, unconventional superconductivity1,2,3,4,5,6,7 or colossal magnetoresistance1,8. Ultrafast optical, X-ray and electron pulses can elucidate the microscopic interplay between these orders by probing the electronic and lattice dynamics separately9,10,11,12,13,14,15,16,17, but a simultaneous direct observation of multiple orders on the femtosecond scale has been challenging. Here we show that ultrabroadband terahertz pulses can simultaneously trace the ultrafast evolution of coexisting lattice and electronic orders. For the example of a charge density wave (CDW) in 1T-TiSe2, we demonstrate that two components of the CDW order parameter—excitonic correlations and a periodic lattice distortion (PLD)—respond very differently to 12-fs optical excitation. Even when the excitonic order of the CDW is quenched, the PLD can persist in a coherently excited state. This observation proves that excitonic correlations are not the sole driving force of the CDW transition in 1T-TiSe2, and exemplifies the sort of profound insight that disentangling strongly coupled components of order parameters in the time domain may provide for the understanding of a broad class of phase transitions.

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Figure 1: CDW phase transition in 1T-TiSe2 and its low-energy spectral fingerprint.
Figure 2: Ultrafast photoinduced dynamics of the mid-infrared electronic response.
Figure 3: Terahertz phonon spectrum during ultrafast melting of the electronic order.
Figure 4: CDW amplitude oscillations following a perturbation of the electronic order.

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Acknowledgements

We thank A. Pashkin and R. Bratschitsch for helpful discussions as well as M. Furthmeier, C. Gradl, K. Groh, T. Riedel and C. Sohrt for technical assistance. Support by the European Research Council through ERC grant 305003 (QUANTUMsubCYCLE) is acknowledged. I.E.P. and L.M. were supported by the European Union’s Seventh Framework Programme (FP7-REGPOT-2012-2013-1) under grant agreement No 316165.

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Contributions

M.P., H.D., U.H. and R.H. planned the project; M.P., U.L. and J.-M.M. performed terahertz measurements; M.P., J.D., U.H., J.-M.M., K.R. and R.H. analysed data; K.R. and U.H. provided bulk samples; M.P. prepared thin-film samples; M.P., L.M. and I.E.P. elaborated the theoretical model; M.P., J.D., K.R., I.E.P. and R.H. wrote the paper. All authors contributed to discussions and gave comments on the manuscript.

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Correspondence to M. Porer or R. Huber.

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Porer, M., Leierseder, U., Ménard, JM. et al. Non-thermal separation of electronic and structural orders in a persisting charge density wave. Nature Mater 13, 857–861 (2014). https://doi.org/10.1038/nmat4042

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