Research ArticlePHYSICS

Ultrafast dissolution and creation of bonds in IrTe2 induced by photodoping

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Science Advances  27 Jul 2018:
Vol. 4, no. 7, eaar3867
DOI: 10.1126/sciadv.aar3867
  • Fig. 1 Static electron diffraction image of IrTe2 obtained by ultrahigh acceleration voltage 128-kV electron pulse.

    The upper panels are the electron diffraction obtained at 300 K (HT phase) and ~200 K (LT phase). The Miller indices (h k l) are also shown. Magnified view of the diffraction intensity (I) near the bright BP (see the white box in the upper panels) is also shown as an inset. The symmetry breaking, which corresponds to the presence of superlattices, is observed in the LT phase, as shown by arrows, and indicates the existence of a structural phase transition with modulation wave vectors q of (1/5, 0, 1/5) due to Ir-Ir dimerization and Te-Te polymerization (7, 9, 13). The bottom panels are the schematic illustration of IrTe2 layers at the HT and LT phases. The HT phase observed at room temperature is formed by hexagonally arranged iridium (Ir) atoms, sandwiched by two tellurium (Te) layers coordinating the central Ir atom in an octahedral arrangement (7). In the LT phase, the structural change is associated with the phase transition at Ts ~ 260 K in combination with Ir-Ir (red line) and Te-Te (green line) dimerization with cooling from room temperature.

  • Fig. 2 Photoinduced time-dependent structural changes monitored by ultrabright FED.

    (A to D) Differences of the diffraction patterns of the photoinduced and initial LT phases as a function of the delay time between the optical excitation and electron probe pulses, where the index at the HT phase is assigned. Note that red- and blue-colored scale denotes the increase and decrease of diffraction intensity after photoexcitation, respectively. (E) Wide view of the diffraction pattern subtracting of pump-probe and probe images after photoexcitation (+0.6 ps). (F) Relative intensity changes (ΔI/I) for selected BPs and SLs. The intensity of BP from the host lattice is multiplied by two to see more clearly, and the fast dynamical processes are shown in (G). The excitation fluence was ~0.55 mJ/cm2. The sample was photoexcited by 50-fs, 800-nm optical pulses at a repetition rate of 1 kHz. (G) Fast dynamical processes taken by 400- and 800-nm pump pulses, as indicated. Exponential fitting takes into account the electron and pump pulse duration with an error function as a guide to the eye. For ΔISL/ISLIBragg/IBragg), τ1,FED ~ 200 fs (230 fs) and 300 fs (500 fs) for 400- and 800-nm pump, respectively.

  • Fig. 3 DFT calculation using several temperatures.

    (A) Ir-Ir dimerization as a function of electronic temperature. Regions 1 and 2 indicate that the dimerized and nondimerized phases are stable after the electron-electron interaction. The inset shows the potential energy change corresponding to regions 1 and 2. The shape of the energy surfaces is a cartoon, in particular, the height of the barrier at low electronic temperature was not calculated. (B) Calculated forces on Ir ions after optical excitation at 0.61 eV. The direction of the force is indicated by blue arrows. The representative dimerized structure is shown by circles and a red line, and the nondimerized structure is shown by stars. The result presented here is obtained from DFT calculations using the ideal crystal structure. The calculation using a relaxed structure at a certain electron temperature gives the same result. Calculations shown in the inset of (A) were done for electronic temperatures of 0.001 eV (LT) and 0.68 eV (intermediate temperature) and with DFT, as described in the Supplementary Materials. Both positions of the ion and the unit cell parameters were relaxed in the calculation.

  • Fig. 4 Dynamics of the differential reflectivity change.

    (A) Transient reflectivity: relative intensity changes from a bulk sample at the LT phase ~200 K recorded at the excitation wavelength (800 nm, 100 fs) and fluence (1 mJ/cm2). (B) Subtraction from the fitting curve to raw data of (A). (C) FTs of the transient reflectivity of (B). Dots are the result, and a curve as a guide to the eye is the fit of Lorentzians.

  • Fig. 5 Ultrafast photoinduced transient cooperative processes in IrTe2.

    (A) Schematic lattice dynamics obtained from the present study. Component of I, II, and III corresponds to the dynamics of electron-electron, electron-lattice, and thermal relaxation. (B) Schematic DOS of the filled dxy bonding and unoccupied antibonding states with photoexcitation process in the LT phase. The antibonding orbital is shown on the right. (C) Emerging time evolution of the real-space structure of the Ir-Te plane of IrTe2 with the multi-orbital ordered state following photoexcitation with an intense optical pulse. For the characteristic time constant indicated by I, II, and III, the dashed and solid lines indicate the major and minor contributions from the interaction, respectively.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/7/eaar3867/DC1

    Section S1. Experimental methods

    Section S2. Sample preparation

    Section S3. Diffraction pattern of IrTe2

    Section S4. Dynamics on the Bragg and superlattice peaks

    Section S5. Thickness dependence of lattice dynamics for Bragg and superlattice peaks

    Section S6. Power dependence of lattice and electron dynamics

    Section S7. DFT calculations

    Section S8. Lattice temperature increases with the fluence of pump pulse

    Fig. S1. Schematic diagram of the FED setup.

    Fig. S2. Thin films and the TEM image.

    Fig. S3. Delay time dependence of diffraction pattern.

    Fig. S4. Lattice dynamics obtained from several BPs and SLs.

    Fig. S5. Relative diffraction intensity change of IrTe2 with different film thicknesses of 20 to 100 nm.

    Fig. S6. Fluence dependence of FED and optical pump-probe experiments.

    Fig. S7. Difference of electronic free energies of dimerized and HT structures as a function of electronic temperature, as calculated using DFT.

    Fig. S8. Calculated forces on Ir ions after optical excitation.

    Fig. S9. Calculated forces on the Ir ions in the dimerized structure after an instantaneous optical excitation.

    Fig. S10. DOS contributions from the Ir ions.

    References (3032)

  • Supplementary Materials

    This PDF file includes:

    • Section S1. Experimental methods
    • Section S2. Sample preparation
    • Section S3. Diffraction pattern of IrTe2
    • Section S4. Dynamics on the Bragg and superlattice peaks
    • Section S5. Thickness dependence of lattice dynamics for Bragg and superlattice peaks
    • Section S6. Power dependence of lattice and electron dynamics
    • Section S7. DFT calculations
    • Section S8. Lattice temperature increases with the fluence of pump pulse
    • Fig. S1. Schematic diagram of the FED setup.
    • Fig. S2. Thin films and the TEM image.
    • Fig. S3. Delay time dependence of diffraction pattern.
    • Fig. S4. Lattice dynamics obtained from several BPs and SLs.
    • Fig. S5. Relative diffraction intensity change of IrTe2 with different film thicknesses of 20 to 100 nm.
    • Fig. S6. Fluence dependence of FED and optical pump-probe experiments.
    • Fig. S7. Difference of electronic free energies of dimerized and HT structures as a function of electronic temperature, as calculated using DFT.
    • Fig. S8. Calculated forces on Ir ions after optical excitation.
    • Fig. S9. Calculated forces on the Ir ions in the dimerized structure after an instantaneous optical excitation.
    • Fig. S10. DOS contributions from the Ir ions.
    • References (3032)

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