Research ArticleCONDENSED MATTER PHYSICS

Exploration of metastability and hidden phases in correlated electron crystals visualized by femtosecond optical doping and electron crystallography

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Science Advances  26 Jun 2015:
Vol. 1, no. 5, e1400173
DOI: 10.1126/sciadv.1400173
  • Fig. 1 CDW phase transitions in 1T-TaS2 induced by thermodynamic (temperature, chemical doping, or applying pressure) and femtosecond optical doping.

    (A) Generic phase diagram of 1T-TaS2 under various physical domains (temperature, doping x, or pressure P) reconstructed based on (6, 810). The CDW phase evolution can be characterized by the changes in the hexagonal CDW diffraction peaks at reciprocal vector Q (upper right corner). Upper right shows the SD 13-atom cluster representing the unit cell of C-CDW in real space. The lattice distortion within each star is coupled with a strong charge density redistribution. The angle between the CDW vector bi and lattice vector ai is 13.9°. (B) Scale-up view of the ultrafast electron diffraction pattern, showing the hexagonal diffraction patterns of C-CDW (Q) surrounding the lattice Bragg peaks (G). (C) Photoinduced phase transition from C-CDW (unpumped) all the way to IC-CDW. The number denotes the absorbed photon density nλ in the unit of nm−3. Among the phase transitions, the intensity of CDW reduces approximately by half, and Q rotates 13.9° from C-CDW to IC-CDW.

  • Fig. 2 Phase transitions driven by heating and femtosecond optical pumping using near-infrared (800 nm) and mid-infrared (2500 nm) photons.

    (A) Transitions of 1T-TaS2 upon heating, showing complementary changes in the resistivity and the CDW orientation angle φ extracted on the basis of electron diffraction (35). (B) Comparison between the thermal and optically induced changes of ϕ over absorbed energy density (see the Supplementary Materials for details). The temperature of 1T-TaS2 is at 150 K initially. (C) Optically induced evolution of CDW states characterized by CDW suppression (in ratio, based on unperturbed CDW intensity) and CDW angle Φ at various absorbed photon density for two different pumps: 800 and 2500 nm.

  • Fig. 3 Evolution of CDW phases in 1T-TaS2 induced by mid-infrared pulses at 25 K.

    (A) Optically induced evolution of CDW states characterized by CDW suppression (in ratio, based on unperturbed CDW intensity) and CDW angle Φ at various absorbed photon density at a base temperature TB of 25 K using the 800-nm pump photons. The inset shows the diffraction images of the unpumped (ground state) and excited CDW at the quasi-equilibrium time, +20 ps. The central Bragg peak is masked out to highlight the changes in CDW intensity. (B to D) Angular profiles of the CDW peak at selected pump optical density nλ. At nλ = 2.6 nm−3, the CDW is in a mixed state between NC* and IC. The fit shows the individual components of the IC (dashed gray) and NC* (dashed red) to compose the best fit of the mixed state (solid red), whereas the green curve represents an attempted fit by using the C and IC as the constituent states.

  • Fig. 4 Temperature–optical density phase diagram and phase transition pathways.

    (A) Temperature–photon density phase diagram of 1T-TaS2. (B) Cartoon depiction of the transition pathways between the commensurate state (C) and the incommensurate state (IC) under optically driven and temperature-­driven transformations over the free energy contour, also for the intermediate state triclinic (T) and near-commensurate states (NC and NC*) based on the critical amplitude A2 and the CDW angle change ΔΦ. A is scaled to 0.15 Å at C-CDW state based on (7). The underlying electronic states in different regions of CDW are highlighted in circles.

  • Fig. 5 Femtosecond electron crystallography signatures of nonequilibrium CDW phase transitions induced by selected mid-infrared optical doping.

    (A) Dynamics of CDW state transformations inspected via the rotation of CDW wave vector Q away from C-CDW and the suppression of ICDW(t) [in ratio based on the ICDW (−10 ps)]. The solid lines are drawn on the basis of fitting the staircase rises using a Gauss error function. (B) Diffraction images at selected time scales depicting the transient state of CDW under F = 3.2 mJ/cm2. The central Bragg peak is masked out to highlight the intensity changes of the CDW satellites, which are typically two orders of magnitude weaker. (C) Angular profile of a selected CDW satellite to show the presence of initial (NC*) and final (IC) state population in a mixed state at 4.55 ps. The transient angular profile cannot be properly fitted by using the C as the initial state, suggesting that NC* is a stable doorway state leading to the transition.

  • Table 1 The different CDW phase in 1T-TaS2 and their manifestation in terms of the CDW orientation angle Φ (the angle between the CDW unit cell and that of the unreconstructed atomic lattice) and CDW amplitude.

    The values of transition temperature and angle associated with different CDW phases under the thermodynamic conditions (cooling and warming) are taken from (7, 35). The values of the transition optical dose and the normalized CDW intensity ÎCDW are extracted from the metastable phases obtained at +20 ps in the optically induced phase transitions characterized by femtosecond electron crystallography.

    Thermally driven
    PhasesT on cooling (K)T on warming (K)Φ on cooling (°)Φ on warming (°)
    C-CDW<183<22313.913.9
    T-CDW223 < T < 28013.0 – 12.3
    NC-CDW183 < T < 347280 < T < 35710.9 – 12.312.3 – 11.5
    IC-CDW347 < T < 543357 < T < 54300
    Normal>543>543
    Optical doping–driven (TB = 150 K)
    PhasesOptical density, nλ (nm-3)Φ (°)Normalized intensity, ÎCDW
    C-CDW<0.6513.91
    T-CDW0.65 < nλ < 1.3813.70.91
    NC-CDW1.38 < nλ < 1.7812.90.78
    NC*-CDW1.78 <nλ < 2.3811.70.60
    IC-CDW>2.3800.45

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/1/5/e1400173/DC1

    Text

    Fig. S1. Optical image of a 45-nm-thick exfoliated single-crystal 1T-TaS2 sample supported on a standard 1000-mesh gold TEM grid.

    Fig. S2. Steady-state and dynamical diffraction patterns of 1T-TaS2 in the NC-CDW state taken at room temperature.

    Fig. S3. The selected CDW patterns under different optical excitation fluences for constructing the optical doping–induced phase diagram obtained at TB = 150 K using the 800-nm femtosecond photons for optical doping.

    Fig. S4. The determination of CDW phase boundaries based on the presence of a step or a slope change.

    References (5054)

  • Supplementary Materials

    This PDF file includes:

    • Text
    • Fig. S1. Optical image of a 45-nm-thick exfoliated single-crystal 1T-TaS2 sample supported on a standard 1000-mesh gold TEM grid.
    • Fig. S2. Steady-state and dynamical diffraction patterns of 1T-TaS2 in the NCCDW state taken at room temperature.
    • Fig. S3. The selected CDW patterns under different optical excitation fluences for constructing the optical doping–induced phase diagram obtained at TB = 150 K using the 800-nm femtosecond photons for optical doping.
    • Fig. S4. The determination of CDW phase boundaries based on the presence of a step or a slope change.
    • References (50–54)

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