Research ArticleAPPLIED PHYSICS

Hidden CDW states and insulator-to-metal transition after a pulsed femtosecond laser excitation in layered chalcogenide 1T-TaS2−xSex

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Science Advances  20 Jul 2018:
Vol. 4, no. 7, eaas9660
DOI: 10.1126/sciadv.aas9660
  • Fig. 1 Phase diagram of 1T-TaS2−xSex as a function of temperature and Se content.

    Three typical samples used in the present study are the Mott insulators with x = 0 and 0.5 and the superconductor with x = 1.0, as indicated in the diagram.

  • Fig. 2 Alterations of CDW modulation after the pulsed femtosecond-laser excitations.

    Electron diffraction patterns for the x = 0 sample at 10 K along the [001] zone-axis direction demonstrate the structural evolution after femtosecond pulse excitation under (A) dark (that is, CCDW phase; a schematic of the star-of-David cluster is exhibited to illustrate the CCDW superstructure), (B) 3 mJ/cm2, and (C) 5 mJ/cm2 and (D) upon thermal annealing. The space anomaly and orientation anomaly in H-CDW states are evident following laser excitation. The insets show enlarged diffraction spots to illustrate the positional changes of the CDW satellite spots. At relatively high fluence, an additional orientation anomaly appears in the H-CDW state with a rotation (~2°), as indicated in (C). Linear integrated one-dimensional diffraction curves for the first-order satellite spots of (E) CCDW and (F) space-anomaly H-CDW state, indicating remarkable changes in the modulation wave vectors.

  • Fig. 3 Microstructural evolutions for CDW domains driven by different laser fluences.

    Dark-field images for the x = 0 sample obtained by the in situ imaging of the CDW satellite spot along [001] zone-axis direction at 10 K under (A) dark, showing a homogeneous image contrast, (B) 3 mJ/cm2, with an inset for an enlarged local area displaying the visible phase separation, and (C) 5 mJ/cm2, illustrating the variations of PS states and the disappearance of the CCDW domains.

  • Fig. 4 Structural models and theoretical simulations for typical polaron ordered states.

    Schematics of the star-shaped polaron order in the a-b plane and theoretical (or schematic) diffraction pattern in 1T-TaS2−xSex for (A) the CCDW phase. (B) H-CDW domains with δ = 1/9 and schematic diffraction pattern illustrating space anomaly, in agreement with the experimental data of Fig. 2B, in which qH is parallel to qC. (C) H-CDW domain pattern with structural shearing (~2°) (here, the orientation change is exaggerated for clarity), in which the polaron clusters show a local rotation (~2°) relative to the domain walls, yielding a small direction deviation of qC and qH. The diffraction pattern (right column) shows the orientation anomaly for the satellite spots.

  • Fig. 5 Temperature dependence of resistivities for various 1T-TaS2−xSex crystals.

    (A) For the x = 0 sample obtained by thermal cycling. (B) Resistivity drop (indicated by green arrow) for the x = 0 sample at 4 K after a single femtosecond pulse with different fluences. Upon warming, resistivity reverts back at around 100 K. (C) Temperature dependence of resistivity for the x = 0.5 sample obtained by thermal cycling. (D) Photoinduced resistivity drop for the x = 0.5 sample at 4 K after a single femtosecond pulse with different fluences. Upon warming, resistivity reverts back near 120 K. The inset shows the time dependence of resistivity at 4 K at a fixed fluence of 5 mJ/cm2; dynamic saturation is reached for the photoinduced H-CDW state. The arrows indicate a train of femtosecond pulse irradiations.

  • Fig. 6 Resistivity drop under different experiment conditions.

    (A) Drops in resistivity for the x = 0.5 sample at 4 K induced by different numbers of laser pulses within 0.01 s (shutter time); upon warming, resistivities revert back to the thermal equilibrium state at approximately 120 K. The inset shows the rate of resistivity change at 4 K (η = ΔR4K/R4K), obtained under different numbers of laser pulses within 0.01 s. (B) Drops in resistivity induced by a single femtosecond pulse for the x = 0.5 sample at different temperatures.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/7/eaas9660/DC1

    Fig. S1. Resistivity curves for the samples used in this study.

    Fig. S2. TEM bright-field image of 1T-TaS2.

    Fig. S3. In situ observation for the electron diffraction patterns of 1T-TaS1.5Se0.5.

    Fig. S4. CDW free energy schematic.

    Fig. S5. Resistivity curve of 1T-TaSSe with photoexcitation.

    Fig. S6. Schematic diagram of the experimental setup.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Resistivity curves for the samples used in this study.
    • Fig. S2. TEM bright-field image of 1T-TaS2.
    • Fig. S3. In situ observation for the electron diffraction patterns of 1T-TaS1.5Se0.5.
    • Fig. S4. CDW free energy schematic.
    • Fig. S5. Resistivity curve of 1T-TaSSe with photoexcitation.
    • Fig. S6. Schematic diagram of the experimental setup.

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