[Paper Review] Optically induced softening of the charge-transfer gap in Sr2CuO2Cl2

J. Steven Dodge, Andreas Schumacher+2 more|ArXiv.org|Oct 27, 2009

Advanced Condensed Matter Physics1 references3 citations

TL;DR

This study uses time- and energy-resolved pump-probe spectroscopy to demonstrate that photoexcitation at 3 eV induces softening of the charge-transfer gap in the insulating cuprate Sr2CuO2Cl2, resulting in the creation of 11–20 low-energy bosons per photon. The observed relaxation dynamics, consistent with anharmonic phonon decay, suggest a significant contribution from both optical phonons and possibly magnetic excitations to carrier energy dissipation.

ABSTRACT

Energy- and time-resolved spectroscopy reveals a photoinduced softening of the charge-transfer gap in the insulating copper oxide Sr2CuO2Cl2 that indicates rapid and efficient photoproduction of optical phonons. By relating the pump-probe signal amplitude to the thermal difference spectrum, we estimate that eleven to twenty optical phonons are created for every one 3 eV photon. Assuming relaxation to the optical absorption edge at 1.5 eV, this corresponds to 70-130 meV per boson. While the lower limit is consistent with relaxation exclusively through optical phonons, the upper limit suggests a significant role for magnetic excitations. We observe a photoinduced bleaching of the gap excitation that we associate with phase space filling, and estimate the excluded area of the photoexcited state to be about nine copper oxide plaquettes. The temporal decay of the pump-probe signal is consistent with anharmonic phonon decay.

Motivation & Objective

• To investigate the relaxation dynamics of photoexcited carriers in the insulating cuprate Sr2CuO2Cl2 following optical excitation.
• To determine the nature and number of low-energy bosonic modes (phonons or magnons) involved in energy relaxation from the 3 eV excitation to the 1.5 eV band edge.
• To quantify the photoinduced spectral changes, including gap softening and bleaching, to infer the spatial extent and character of the photoexcited state.
• To analyze the temporal decay of the pump-probe signal to identify the dominant relaxation mechanisms, particularly anharmonic phonon decay.

Proposed method

• Employed energy- and time-resolved pump-probe spectroscopy with 3 eV pump and variable probe energies (e.g., 2.1 eV) to map the transient optical response.
• Measured the pump-probe signal amplitude as a function of time delay and excitation density to extract decay dynamics and non-exponential relaxation behavior.
• Used a sum of exponentials with logarithmically spaced decay times (Eq. 6) to fit the data, with N = 4 components required for good fit quality.
• Relied on a discretized Laplace transform approach with globally constrained decay times to enable quantitative comparison across excitation densities.
• Calculated the thermal difference spectrum from the pump-probe signal amplitude and related it to the number of bosons created per photon.
• Estimated the excluded area of the photoexcited state via bleaching of the gap excitation, assuming phase space filling.

Experimental results

Research questions

• RQ1 How many low-energy bosons are created per 3 eV photon during carrier relaxation in Sr2CuO2Cl2?
• RQ2 What is the dominant relaxation pathway for photoexcited carriers—optical phonons or magnetic excitations?
• RQ3 How does the temporal decay of the pump-probe signal reflect the underlying anharmonic decay of bosonic modes?
• RQ4 What is the spatial extent of the photoexcited state, as inferred from the bleaching of the charge-transfer excitation?
• RQ5 To what extent does the photoinduced spectral response mimic thermal excitation of bosonic modes?

Key findings

• Photoexcitation at 3 eV induces a softening of the charge-transfer gap, with the pump-probe signal mimicking the effect of raising temperature.
• For every 3 eV photon absorbed, 11 to 20 low-energy bosons are created, corresponding to 70–130 meV per boson upon relaxation to the 1.5 eV absorption edge.
• The lower limit of 70 meV per boson is consistent with optical phonons, but the upper limit suggests a significant contribution from magnetic excitations.
• The pump-probe signal decay is non-exponential and best described by a sum of four exponentials with decay times of 1.1, 3.7, 12.6, and 42.7 ps, indicating complex anharmonic phonon decay.
• The amplitude of the photoinduced signal scales linearly with excitation density up to 2.4×10¹⁹ cm⁻³, indicating a linear response regime.
• The photoinduced bleaching of the gap excitation implies that the photoexcited state excludes an area equivalent to approximately nine CuO₂ plaquettes.