Ultrafast recovery dynamics of dimer stripes in IrTe2

TL;DR

This study investigates how IrTe2's dimer stripes respond to ultrafast photoexcitation, combining time-resolved XPS with comparisons to TR-ED and TR-ARPES to separate local dimer dynamics from long-range order. Using a 2.4 eV pump, the dimer population is suppressed in about 0.5 ps and recovers over 1–3 ps, with fluence-dependent minimum and relaxation times; long-range stripe order, however, takes tens of picoseconds to recover. The results indicate that lattice heating, driven by energy transfer from the electronic system, governs the initial suppression, while slow reordering of dimers controls the recovery of long-range order. The work highlights the value of site-specific spectroscopy in dissecting multi-scale dynamics in strongly coupled, dimerized phases and suggests directions for resonant excitations to accelerate phase transitions.

Abstract

The transition metal dichalcogenide IrTe2 displays a remarkable series of first-order phase transitions below room temperature, involving lattice displacements as large as 20 percents of the initial bond length. This is nowadays understood as the result of strong electron-phonon coupling leading to the formation of local multicentre dimers that arrange themselves into one-dimensional stripes. In this work, we study the out-of-equilibrium dynamics of these dimers and track the time evolution of their population following an infrared photoexcitation using free-electron lased-based time-resolved X-ray photoemission spectroscopy. First, we observe that the dissolution of dimers is driven by the transfer of energy from the electronic subsystem to the lattice subsystem, in agreement with previous studies. Second, we observe a surprisingly fast relaxation of the dimer population on the timescale of a few picoseconds. By comparing our results to published ultrafast electron diffraction and angle-resolved photoemission spectroscopy data, we reveal that the long-range order needs tens of picoseconds to recover, while the local dimer distortion recovers on a short timescale of a few picoseconds.

Figures (4)

• Figure 1: (a) Representative XPS spectrum of the Ir 4$f_{7/2}$ core-levels, taken at 250 K and 270 eV photon energy. The colored area are Voigt functions used to fit the spectrum and calculate the dimer population ratio $R$. The blue and red components are the Ir$^{m}$ monomer and Ir$^{d}$ dimer satellites, respectively. The grey component is used to fit a metallic asymmetric background. (b) Evolution of the dimer population ratio $R$ across the different first-order phase transitions upon cooling (blue arrow) and warming (red arrow) (data taken with 200 eV photon energy, graph adapted from Ref. Rumo2020). These data were obtained in our previous study using samples from the same growth batch. The colored dashed horizontal lines represent the expected ratios $R$ for ideal surface phases, as depicted by the amount of Ir dimers (red balls) in graph (c) (adapted from Ref. Rumo2020). Blue balls indicate monomer Ir atoms.
• Figure 2: (a) Time-resolved XPS data of the Ir 4$f_{7/2}$ core-levels displayed as a function of pump-probe time delay relative to $t_0$. All data on this figure have been taken at 250 K, with a photon energy of 270 eV and a photoexcitation fluence of 0.79 mJ/cm$^2$. (b) Difference image plot showing data after subtraction of an average of data before $t_0$. This emphasizes 4 different transient effects described in the main text. (c) Transient intensity curves integrated on the data of graph (a) over $\pm 100$ meV around 60.15 eV for screening, 60.70 eV for monomer and 61.15 eV for dimer intensities. (d),(e),(f) XPS spectra at specific time delays (red curves), in comparison with the spectrum averaged before $t_0$ (black curve). The differences between the two spectra are shown on top.
• Figure 3: (a) Transient dimer population ratio $R$ for different photoexcitation fluences, together with their fit (blue). Curves are offset by multiples of 0.15 for clarity. The tick spacings on the vertical axis correspond to 0.2. (b) Same for the lower two photoexcitation fluences. Curves are offset by multiples of 0.04 for clarity. The tick spacings on the vertical axis correspond to 0.02. (c) Minimum ratio $R_\text{min}$ and relaxation time extracted from the fits in graph (a) as a function of fluence.
• Figure 4: Cartoon describing the ultrafast dynamics of the structural phase transition in IrTe$_2$ and the recovery of the long-range order of dimers after a few-eV photoexcitation.