Ultrafast recovery dynamics of dimer stripes in IrTe2

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.

💡 Research Summary

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IrTe₂ is a prototypical transition‑metal dichalcogenide in which strong electron‑phonon coupling drives a series of first‑order structural phase transitions below room temperature. Upon cooling, the material adopts a series of superstructures—most notably the (5 × 1) surface reconstruction—characterized by Ir‑Ir dimers whose bond lengths contract by up to 20 % relative to the high‑temperature phase. These dimers form one‑dimensional stripes and are responsible for distinct satellite features in Ir 4f core‑level X‑ray photoelectron spectroscopy (XPS). While previous time‑resolved angle‑resolved photoemission spectroscopy (TR‑ARPES) and ultrafast electron diffraction (UED) experiments have shown that the long‑range superlattice order melts on a tens‑of‑picoseconds timescale, the intrinsic dynamics of the dimers themselves have not been directly probed.

In this work the authors employ time‑resolved XPS (TR‑XPS) at the FLASH free‑electron laser, using 270 eV X‑ray probe pulses with a temporal resolution of ≈ 250 fs (FWHM). Single crystals of IrTe₂ were cleaved in situ and held at 250 K, where the surface is in the (5 × 1) dimer‑stripe phase. A 515 nm (2.4 eV) infrared pump pulse excites the sample with fluences ranging from 0.30 to 0.84 mJ cm⁻². The Ir 4f₇/₂ spectrum consists of a main monomer peak (≈ 60.7 eV) and a dimer‑related satellite (≈ 61.1 eV). By fitting each spectrum with Voigt functions the authors extract the intensity ratio R = I_d/(I_d + I_m), which serves as a quantitative marker of the dimer population.

The time‑delay scans reveal four distinct phenomena. (i) At time zero a laser‑assisted photoemission (LAPE) artifact shifts part of the photoelectron distribution by the pump photon energy, confirming the temporal overlap of pump and probe. (ii) Within ≈ 0.2 ps an asymmetric broadening of the core‑level line appears, reflecting a transient increase of free carriers above the Fermi level and enhanced screening of the core hole. (iii) Starting around 0.5 ps the dimer satellite intensity drops sharply while the monomer intensity rises, indicating a rapid suppression of the dimer population. (iv) Between 1 and 3 ps the dimer intensity recovers partially, returning the ratio R toward its equilibrium value.

A systematic fluence‑dependence study shows that for fluences ≤ 0.30 mJ cm⁻² the dimer ratio remains unchanged within experimental sensitivity. For higher fluences, R decreases to a minimum value R_min that becomes lower as the fluence increases (down to ≈ 0.15 at 0.84 mJ cm⁻²). The time required to reach R_min also lengthens with fluence, and the subsequent recovery follows a single‑exponential with a characteristic time τ that grows from ≈ 1 ps at low fluence to ≈ 3 ps at the highest fluence. Notably, the onset of dimer suppression is delayed at larger fluences, contrary to a simple linear scaling with absorbed energy. This behavior is consistent with a two‑temperature model in which the electronic subsystem first absorbs the pump energy, then transfers it to the lattice; the lattice temperature rise—and thus the dimer melting—occurs more slowly when a larger amount of energy must be distributed.

The authors therefore conclude that (1) dimer dissolution is not driven directly by the transient electronic population but by the flow of energy from electrons to the lattice, (2) the local dimer distortion recovers on a picosecond timescale, and (3) the restoration of long‑range stripe order, as observed in previous UED studies, requires tens of picoseconds. This establishes a multi‑step recovery pathway: an ultrafast (sub‑picosecond) electronic heating, a picosecond‑scale local structural relaxation, and a slower (tens of picoseconds) re‑establishment of translational symmetry.

The work provides a clear experimental benchmark for theories of strong electron‑phonon coupling in low‑dimensional materials. It demonstrates that time‑resolved core‑level spectroscopy can uniquely access site‑specific structural dynamics that are invisible to techniques probing only momentum‑space electronic structure or diffraction intensities. The observed fluence‑dependent nonlinearity in the dimer melting dynamics offers a stringent test for future time‑dependent density‑functional theory (TD‑DFT) simulations, which have already suggested a key role for antibonding Ir‑Ir orbitals in the photo‑induced dissociation. Moreover, the distinction between rapid local dimer recovery and slower long‑range order reformation may be a generic feature of other dimer‑based charge‑density‑wave or Peierls‑type systems, suggesting broader relevance for controlling phase transitions on ultrafast timescales.

In summary, the study combines femtosecond X‑ray spectroscopy with systematic fluence control to disentangle the hierarchy of timescales governing the non‑equilibrium behavior of IrTe₂. It confirms that strong electron‑phonon coupling can produce an ultrafast collapse of local bonding motifs while preserving a much slower pathway for the re‑establishment of the superlattice, thereby enriching our understanding of how complex structural orders respond to impulsive optical excitation.