Dirac charge dynamics in graphene by infrared spectroscopy (0807.3780v2)

Abstract: A remarkable manifestation of the quantum character of electrons in matter is offered by graphene, a single atomic layer of graphite. Unlike conventional solids where electrons are described with the Schrodinger equation, electronic excitations in graphene are governed by the Dirac Hamiltonian. Some of the intriguing electronic properties of graphene, such as massless Dirac quasiparticles with linear energy-momentum dispersion, have been confirmed by recent observations. Here we report an infrared (IR) spectromicroscopy study of charge dynamics in graphene integrated in gated devices. Our measurements verify the expected characteristics of graphene and, owing to the previously unattainable accuracy of IR experiments, also uncover significant departures of the quasiparticle dynamics from predictions made for Dirac fermions in idealized, free standing graphene. Several observations, including a marked deviation of the energy bands from the simple linear dispersion, point to the relevance of many body interactions to the rather exotic electromagnetic response in graphene.

• The paper demonstrates that graphene’s IR spectroscopy reveals a 15% substrate reflectance suppression, challenging ideal Dirac fermion predictions.
• It quantifies deviations from universal 2D conductivity through detailed reflectance and transmission measurements under varying gate voltages.
• The study finds Fermi velocity renormalization at low biases, underscoring the significant role of many-body interactions in graphene.

Examination of Dirac Charge Dynamics in Graphene via Infrared Spectroscopy

This paper presents a thorough investigation of the charge dynamics within graphene using infrared (IR) spectromicroscopy. The focus lies on understanding how electronic excitations in graphene, which are traditionally characterized by the Dirac Hamiltonian, deviate from the predictions of idealized Dirac fermions when subjected to experimental conditions. The work unearths crucial insights about graphene's electromagnetic response, particularly highlighting the impact of many-body interactions.

The paper centers on IR reflectance and transmission measurements of graphene samples integrated into gated devices at a temperature of 45K. An important aspect of this research is the analysis near the charge neutrality point, characterized by minimum DC conductivity and zero total charge density.

Key Observations and Results

1. Reflectance and Transmission Modifications: The authors report that a monolayer of graphene induces significant shifts in the substrate reflectance, notably a 15% suppression near specific interference structures. This effect, coupled with alterations in the transmission spectra under varying gate voltages, allows an evaluation of graphene's conductivity.
2. 2D Optical Conductivity: The theoretical expectation for undoped graphene's 2D conductivity, given by the "universal" value $\sigma = \pi e^2/2h$, was supported by experimental data within certain spectral ranges. Deviations observed in the IR spectra under gate voltage application highlight the presence of Drude components and interband transitions, offering an enriched view of charge carrier dynamics.
3. Fermi Energy Dynamics: Analysis revealed that Fermi energy variations with gate voltage are symmetric, aligning with the behavior anticipated for Dirac quasiparticles. However, peculiarities such as a voltage-independent broadening of the 2E$_F$ threshold were noted. This broadened feature could not be explained solely by thermal smearing, suggesting more complex underlying mechanisms, potentially including disorder effects or electron-phonon interactions.
4. Velocity Renormalization: A significant finding is the apparent renormalization of the Fermi velocity ($v_F$), which increases at low biases. This suggests an enhancement of many-body interactions in graphene, further complicated by existing theoretical frameworks.

Implications and Future Prospects

The paper challenges the conventional single-particle understanding of graphene's optical and electromagnetic properties. It underscores the necessity of including many-body effects in future theoretical models to accurately depict the nuanced charge dynamics observed. The exploration of disorder and inhomogeneity in graphene, particularly through advanced scattering and spatially resolved techniques, is an evident future direction.

On a practical level, the insights derived here hold implications for the design and optimization of graphene-based optoelectronic devices. Understanding the deviations from idealized behavior under real-world conditions is essential for enhancing device performance and reliability.

In conclusion, this paper expands upon current knowledge of graphene's charge dynamics through a detailed empirical paper, paving the way for refined theoretical models and applications in next-generation electronic devices. Further exploration of inhomogeneities and many-body interactions in graphene remains critical to harness its full potential.