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# Ultrafast switching to an insulating-like metastable state by amplitudon excitation of a charge density wave

## Abstract

In correlated electron materials, multiple electronic phases may appear next to each other in their phase diagram, and these can be tuned, for example, by applying static pressure or chemical doping1,2,3. These perturbations modify the subtle balance between the electron transfer energy and Coulomb repulsion between electrons. It is, therefore, tempting to explore whether new states of matter can be accessed through the direct tuning of their order parameters, for example, by driving a collective mode of the emergent phase. Here we demonstrate that the direct excitation of the amplitude mode of a charge density wave (amplitudon) by an intense terahertz pulse in a layered transition metal dichalcogenide compound, namely, 3R-Ta1+xSe2, leads to the appearance of an insulating-like metastable state. The formation dynamics of the metastable phase manifest in the opening of a gap in the optical conductivity spectrum, and we show that they synchronize with an oscillation of the amplitudon. This indicates the intimate interplay between the order parameters of the equilibrium charge density wave and the metastable states.

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## Data availability

Source data are provided with this paper. All other data that support the findings of this paper are available from the corresponding authors upon request.

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## Acknowledgements

This work was supported by JSPS KAKENHI (grant nos. 18H05324, 18H05846, 19H05602, 19H02593 and 19H00653); the A3 Foresight Program; JST CREST grant no. JPMJCR19T3, Japan; JST PRESTO grant no. JPMJPR20AC, Japan; LABEX SEAM grant (ANR-11-IDEX-0005-02); and DIM SIRTEQ grant from Île-de-France region. M.N. was partly supported by the Murata Science Foundation.

## Author information

Authors

### Contributions

N.Y. and H.S. carried out the optical experiments and analyses. H.M., Y.T., M.N. and Y.I. fabricated the thin-film samples. P.H., M.C. and Y.G. performed Raman scattering measurements. N.Y. and R.S. wrote the manuscript. R.S. conceived the project of this study. All the authors contributed to the discussion and interpretation of the results.

### Corresponding authors

Correspondence to Naotaka Yoshikawa or Ryo Shimano.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature Physics thanks the anonymous reviewers for their contribution to the peer review of this work.

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## Extended data

### Extended Data Fig. 1 Polarization dependence of the amplitudon excitation.

Squared amplitude of the CDW amplitude mode oscillation obtained from the power spectra of the optical pump-optical probe dynamics, as a function of the polarization angle of the pump pulse with respect to that of the probe pulse.

### Extended Data Fig. 2 Temperature dependence of the amplitudon excited by a THz pulse.

a, THz pump-induced transmittance change as a function of tpp at various temperatures. Dashed line is a guide to the eyes showing the softening and broadening of the free oscillation of the CDW amplitude mode after the THz pump. The peak disappears above TCDW. b, Amplitude of the Fourier transformation of (a). The CDW amplitude mode is clearly observed centered at 2.3 THz at low temperatures.

### Extended Data Fig. 3 Differential conductivity spectra at long delay.

a, Real and b, imaginary part of the differential optical conductivity spectra at tpp=3, 10, 30 and 150 ps. The sample temperature is 4.3 K and the pulse energy of the THz pump is 0.39 μJ cm−2.

### Extended Data Fig. 4 The pump waveform and pump-probe signal with the broadband THz pulse.

a, The waveform of the pump THz pulse. The inset shows its Fourier spectrum. b, Temporal evolution of the change of the probe electric field $${{\Delta}E}_{{\mathrm{probe}}}$$ normalized by its original value $$E_{{\mathrm{probe}}}$$, as a function of the pump-probe delay time tpp. The vertical dashed line indicates tpp=0.4 ps at which the differential conductivity spectrum shown in Fig. 3f in the main text was measured.

### Extended Data Fig. 5 Experimental setup for the THz pump-THz probe spectroscopy.

Experimental setup for THz pump-THz probe measurements. DS: delay stage, HWP: half waveplate, PM: parabolic mirror, BP: black polyethylene, WGP: wire-grid polarizer, QWP: quarter waveplate, WP: Wollaston prism, BPD: balanced photodiode.

### Extended Data Fig. 6 Experimental setup for the broadband THz pump-THz probe spectroscopy.

Experimental setup for the broadband THz pump-THz probe measurements. DS: delay stage, HWP: half waveplate, PM: parabolic mirror, BP: black polyethylene, WGP: wire-grid polarizer, QWP: quarter waveplate, WP: Wollaston prism, BPD: balanced photodiode.

## Supplementary information

### Supplementary Information

Supplementary Figs. 1–8 and Discussion.

## Source data

### Source Data Fig. 1

Source data for Fig. 1.

### Source Data Fig. 2

Source data for Fig. 2.

### Source Data Fig. 3

Source data for Fig. 3.

### Source Data Fig. 4

Source data for Fig. 4.

## Rights and permissions

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Yoshikawa, N., Suganuma, H., Matsuoka, H. et al. Ultrafast switching to an insulating-like metastable state by amplitudon excitation of a charge density wave. Nat. Phys. 17, 909–914 (2021). https://doi.org/10.1038/s41567-021-01267-3

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