Ultrafast dissolution and creation of bonds in IrTe2 induced by photodoping

Abstract

The observation and control of interweaving spin, charge, orbital, and structural degrees of freedom in materials on ultrafast time scales reveal exotic quantum phenomena and enable new active forms of nanotechnology. Bonding is the prime example of the relation between electronic and nuclear degrees of freedom. We report direct evidence illustrating that photoexcitation can be used for ultrafast control of the breaking and recovery of bonds in solids on unprecedented time scales, near the limit for nuclear motions. We describe experimental and theoretical studies of IrTe₂ using femtosecond electron diffraction and density functional theory to investigate bonding instability. Ir-Ir dimerization shows an unexpected fast dissociation and recovery due to the filling of the antibonding d_xy orbital. Bond length changes of 20% in IrTe₂ are achieved by effectively addressing the bonds directly through this relaxation process. These results could pave the way to ultrafast switching between metastable structures by photoinduced manipulation of the relative degree of bonding in this manner.

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 T_s ~ 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 ΔI_SL/I_SL (ΔI_Bragg/I_Bragg), τ_1,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 d_xy 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.