[Paper Review] Electronic and structural fingerprints of charge density wave excitations in extreme ultraviolet transient absorption spectroscopy

Tobias Heinrich, Hung-Tzu Chang+4 more|arXiv (Cornell University)|Nov 7, 2022

Molecular Junctions and Nanostructures4 citations

TL;DR

This study introduces table-top extreme ultraviolet (XUV) transient absorption spectroscopy at the Ti M₂,₃ edge to simultaneously probe electronic and lattice dynamics in 1T-TiSe₂, revealing charge density wave (CDW) excitations. By combining femtosecond XUV probing with time-dependent density functional theory (TDDFT) simulations, it identifies coherent A₁g optical phonons (6.0 THz) and A₁g* CDW amplitude modes (3.3 THz), extracting carrier relaxation (360 fs) and phonon heating (1.0 ps) timescales with atomic displacement amplitudes and directions.

ABSTRACT

Femtosecond core-level transient absorption spectroscopy is utilized to investigate photoinduced dynamics of the charge density wave in 1T-TiSe2 at the Ti M2,3 edge (30-50 eV). Photoexcited carriers and phonons are found to primarily induce spectral red-shifts of core-level excitations, and a carrier relaxation time and phonon heating time of approximately 360 fs and 1.0 ps are extracted, respectively. Pronounced oscillations in delay-dependent absorption spectra are assigned to coherent excitations of the optical $A_{1g}$ phonon (6.0 THz) and the $A_{1g}^*$ charge density wave amplitude mode (3.3 THz). By comparing the measured spectra with time-dependent density functional theory simulations, we determine the directions of the momentary atomic displacements of both coherent modes and estimate their amplitudes. This work presents a first look on charge density wave excitations with table-top core-level transient absorption spectroscopy, enabling simultaneous access to electronic and lattice excitation and relaxation.

Motivation & Objective

• To develop a table-top XUV transient absorption technique capable of simultaneously probing electronic and lattice dynamics in quantum materials.
• To investigate photoinduced charge density wave (CDW) excitations in 1T-TiSe₂ using core-level spectroscopy at the Ti M₂,₃ edge (30–50 eV).
• To disentangle contributions from photoexcited carriers and coherent phonons in transient absorption spectra.
• To identify and characterize coherent modes—specifically the A₁g optical phonon and A₁g* CDW amplitude mode—through spectral fingerprints and atomic displacement analysis.
• To extract quantitative relaxation timescales for hot electron cooling and phonon heating using time-resolved data and TDDFT simulations.

Proposed method

• Femtosecond near-infrared laser pulses (40 fs, 2 μm) excite 1T-TiSe₂, followed by time-delayed extreme ultraviolet (XUV) probing via high harmonic generation (HHG) in an Ar-filled gas cell.
• XUV radiation (25–50 eV) is generated using a two-color driving field (800 nm and its second harmonic), producing discrete harmonics spaced by 1.55 eV.
• Transient absorption is measured as ΔOD = OD_pumped − OD_unpumped, with noise suppression via simultaneous reference spectrum acquisition from the HHG source.
• Time-dependent density functional theory (TDDFT) simulations are performed using the FP-LAPW method and PBE functional, including local field effects to model core-level transitions and atomic displacements.
• Spectral fitting uses a multi-component model incorporating electron relaxation (exponential decay), phonon bath heating (step-like rise), and coherent phonon oscillations with damping (sinusoidal decay with phase offset).
• Atomic displacements for the A₁g and A₁g* modes are derived from DFT-optimized structures, with amplitudes and directions assigned based on symmetry and simulated spectral shifts.

Experimental results

Research questions

• RQ1 What are the distinct electronic and lattice contributions to the transient absorption spectrum in 1T-TiSe₂ following photoexcitation at the Ti M₂,₃ edge?
• RQ2 Which coherent vibrational modes—specifically the A₁g optical phonon and A₁g* CDW amplitude mode—are excited, and what are their frequencies and amplitudes?
• RQ3 How do the relaxation dynamics of photoexcited carriers and lattice phonons evolve in time, and what are their characteristic timescales?
• RQ4 Can the direction and magnitude of atomic displacements in coherent modes be determined from the XUV absorption spectra using ab initio simulations?
• RQ5 To what extent can table-top XUV transient absorption spectroscopy resolve coupled electronic and structural dynamics in charge density wave materials?

Key findings

• Photoexcited carriers and phonons primarily induce red-shifts in core-level excitation spectra, indicating changes in electronic structure and lattice distortion.
• Carrier relaxation occurs with a time constant of approximately 360 fs, extracted from the decay of the electronic contribution in the transient absorption signal.
• Phonon heating is observed with a timescale of about 1.0 ps, corresponding to the rise time of the phonon bath component in the fitting model.
• Coherent oscillations at 6.0 THz are assigned to the A₁g optical phonon mode, with a displacement amplitude of 0.0075 Å for Se atoms.
• Oscillations at 3.3 THz are identified as the A₁g* charge density wave amplitude mode, with Se atoms displaced by 0.0084 Å and Ti atoms by 0.0255 Å.
• TDDFT simulations confirm the spectral fingerprints of both modes and allow assignment of the direction and magnitude of atomic displacements, with stored energies of 0.65 meV/unit cell (A₁g) and 0.8 meV/unit cell (A₁g*).