Photoinduced π-π* band gap renormalization in graphite

Area of Science:

• Condensed Matter Physics
• Materials Science
• Physical Chemistry

Background:

• The electronic properties of carbon allotropes are dictated by their π orbitals.
• Understanding the photoinduced, nonequilibrium behavior of these orbitals is crucial.
• The transient behavior of π orbitals in graphite under laser excitation is under investigation.

Purpose of the Study:

• To investigate the photoinduced nonequilibrium behavior of π orbitals in graphite.
• To demonstrate the renormalization of the π-π* band gap in graphite under UV laser excitation.
• To quantify the transient change in the π-π* band gap.

Main Methods:

• Excitation of graphite with a UV laser pulse to achieve high carrier densities.
• Detection of transient reflectivity and decay time using an infrared probe.
• Tuning the UV pump photon energy across the π-π* absorption resonance.

Main Results:

• A transient renormalization of the π-π* band gap was observed when carrier density exceeded 10% of the π* density of states.
• The maximum transient reflectivity and decay time occurred at a photon energy downshifted by 500 meV from the equilibrium absorption maximum.
• This downshift indicates a transient π-π* band gap shrinking near the M point of the Brillouin zone.

Conclusions:

• Photoexcitation can induce a significant, transient shrinking of the π-π* band gap in graphite.
• The observed band gap renormalization is a nonequilibrium phenomenon driven by high carrier densities.
• This finding offers insights into the dynamic electronic behavior of carbon materials under intense light.