- The paper reveals that phonon-mediated inelastic tunneling generates a gap-like conductance feature centered at the Fermi level in graphene.
- The study employs atomically-resolved scanning tunneling spectroscopy at low temperatures with a tunable back-gate to accurately map electron dynamics.
- Findings highlight the role of out-of-plane acoustic phonons near the K/K’ points, offering insights that could influence the design of future graphene-based devices.
Giant Phonon-induced Conductance in Scanning Tunneling Spectroscopy of Gate-tunable Graphene
The paper presented in this paper investigates the phonon-induced conductance behavior observed in the scanning tunneling spectroscopy (STS) of gate-tunable graphene. Graphene exhibits remarkable electronic properties due to its unique two-dimensional (2D) honeycomb lattice structure, where electron dynamics follow the relativistic Dirac fermion model. This intrinsic attribute makes the graphene an ideal candidate for exploring high-mobility 2D electron systems.
The authors perform atomically-resolved scanning tunneling spectroscopy measurements on mechanically cleaved graphene flake devices with tunable back-gate electrodes. One of the surprising discoveries is a gap-like feature in the tunneling spectrum of graphene that persists across varying electron densities and remains centered at the Fermi level (E_F). This observation is in stark contrast with the naive expectation of a linear density of states from the linear band structure. The paper reveals that the anomaly is linked to a suppression of electronic tunneling near the Fermi level and a simultaneous enhancement of tunneling at higher energies, facilitated by a phonon-mediated inelastic channel.
Key experimental setups include employing an Omicron LT-STM apparatus at low temperatures (T = 4.8 K) and conducting experiments in an ultra-high vacuum (UHV) environment to ensure cleansing and accuracy in the derived spectra. The devices facilitate back-gate voltage modulation, allowing variable carrier density across the graphene samples. In their STS experiments, the researchers have consistently identified the gap feature across numerous graphene flake devices and various STM tip calibrations.
Further characterization of the anomalous energy gap behavior includes examining the gate-voltage dependence of conductance minima and determining the temperature dependence of the tunneling spectra. The phonon-mediated mechanism is corroborated by analyzing measurements at differing temperatures, gate voltages, and tip-sample separations. A significant finding is the identification of out-of-plane acoustic phonon modes near the K/K’ points in reciprocal space as responsible for the inelastic excitations at an energy threshold, hω_0 ≈ 63 meV. The authors confirm the phonon-mediated inelastic excitation mechanism through well-correlated theoretical models and experimental results, estimating the electron-phonon coupling matrix element from extracted electron-phonon coupling data.
This comprehensive approach to studying graphene’s electronic landscape underlines critical tunneling processes involving phonons and inelastic electron dynamics, unearthing new insights into electron-phonon interactions within graphene. The effect of sound-like phonons acting as a ‘floodgate’ significantly enhances tunneling at specific energies, thereby introducing new paradigms for interpreting the electronic structure of graphene and influencing the design and functioning of future graphene-based electronic devices.
These findings have significant implications for understanding and manipulating the electronic properties of graphene under various external perturbations. They prompt further exploration into the effects of electron-phonon coupling in other 2D materials, aiming to harness similar phonon-mediated phenomena for technological applications. Researchers in condensed matter physics and material sciences would find these insights invaluable for expanding the existing knowledge of graphene and optimizing its applications in nanoscale devices.