Observation of Dirac Holes and Electrons in a Topological Insulator

Abstract: We show that in the new topological-insulator compound Bi_{1.5}Sb_{0.5}Te_{1.7}Se_{1.3} one can achieve a surfaced-dominated transport where the surface channel contributes up to 70% of the total conductance. Furthermore, it was found that in this material the transport properties sharply reflect the time dependence of the surface chemical potential, presenting a sign change in the Hall coefficient with time. We demonstrate that such an evolution makes us observe both Dirac holes and electrons on the surface, which allows us to reconstruct the surface band dispersion across the Dirac point.

• The paper demonstrates that BSTS exhibits both Dirac holes and electrons, enabling reconstruction of its surface band structure.
• It employs Shubnikov–de Haas oscillations to measure asymmetric Fermi velocities of 3.5×10^5 m/s for holes and 6.0×10^5 m/s for electrons.
• The paper reveals time-dependent band bending that shifts surface conduction from p-type to n-type with air exposure.

Observation of Dirac Holes and Electrons in a Topological Insulator

The paper presents a detailed investigation into the transport properties of a newly synthesized topological insulator (TI), Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ (BSTS), with a specific focus on understanding the behavior of Dirac quasiparticles. This compound features a surface-dominated conductance where up to 70% of the total conductance is attributed to surface states. Unlike many previously studied TIs, BSTS addresses the significant challenge of mitigating bulk conduction, a common issue in topological insulator studies that obscures surface phenomena.

In this study, BSTS was shown to support a remarkable signature of both Dirac holes and electrons, enabled by time-dependent changes in the surface chemical potential. This variability allowed the researchers to reconstruct the surface band structure across the Dirac point (DP), exploiting Shubnikov-de Haas (SdH) oscillations to gain insight into the Dirac nature of the surface states.

Key numerical findings from the study include effective Fermi velocities ($v_F^*$) of $3.5 \times 10^5$ m/s for Dirac holes and $6.0 \times 10^5$ m/s for Dirac electrons, indicating the asymmetric dispersions of chiral particles above and below the DP. These values diverge from the ideal linear Dirac cone energy-momentum relationship, providing a nuanced perspective on the quasi-linear dispersions intrinsic to Bi-based TIs.

A notable implication of this work is the identification of band bending as a mechanism that modulates carrier type over time. Initially, freshly cleaved surfaces show p-type characteristics transitioning to n-type with air exposure, possibly due to atmospheric doping mechanisms or intrinsic rearrangements. This insight underscores the critical interplay between surface chemical states and Dirac fermion dynamics in TIs.

The research outcomes have both practical and theoretical implications. Practically, optimizing TI compositions to enhance surface conduction is pivotal for developing electronic devices leveraging topological surface states. Theoretically, this work adds to the understanding of quasiparticle dynamics in TIs and paves the way for further exploration into the effects of chemical potential variation on quasiparticle transport properties.

Future research could extend these findings by exploring methods to stabilize the surface states of TIs against atmospheric effects, further refine the bulk insulator properties, and investigate the potential for observing fractional quantum phenomena, as hinted by quantum oscillations in the study. Overall, BSTS presents itself as a valuable platform for topological quantum research, particularly due to its enhanced surface properties and its potential applicability in exploring the physics of Dirac fermions and topological phases.