Observation of topological surface state quantum Hall effect in an intrinsic three-dimensional topological insulator (1409.3778v2)

Abstract: A three-dimensional (3D) topological insulator (TI) is a quantum state of matter with a gapped insulating bulk yet a conducting surface hosting topologically-protected gapless surface states. One of the most distinct electronic transport signatures predicted for such topological surface states (TSS) is a well-defined half-integer quantum Hall effect (QHE) in a magnetic field, where the surface Hall conductivities become quantized in units of (1/2)e2/h (e being the electron charge, h the Planck constant) concomitant with vanishing resistance. Here, we observe well-developed QHE arising from TSS in an intrinsic TI of BiSbTeSe2. Our samples exhibit surface dominated conduction even close to room temperature, while the bulk conduction is negligible. At low temperatures and high magnetic fields perpendicular to the top and bottom surfaces, we observe well-developed integer quantized Hall plateaus, where the two parallel surfaces each contributing a half integer e2/h quantized Hall (QH) conductance, accompanied by vanishing longitudinal resistance. When the bottom surface is gated to match the top surface in carrier density, only odd integer QH plateaus are observed, representing a half-integer QHE of two degenerate Dirac gases. This system provides an excellent platform to pursue a plethora of exotic physics and novel device applications predicted for TIs, ranging from magnetic monopoles and Majorana particles to dissipationless electronics and fault-tolerant quantum computers.

• The paper demonstrates the observation of half-integer quantized QHE from Dirac fermion surface states in BiSbTeSe.
• The study employed backgate tuning and precise transport measurements to suppress bulk conduction and reveal surface-dominated QHE.
• These findings enhance the understanding of topological magnetoelectric effects, advancing prospects for dissipationless electronics and quantum computing.

An Analysis of Topological Surface State Quantum Hall Effect in 3D Topological Insulators

In the paper "Observation of topological surface state quantum Hall effect in an intrinsic three-dimensional topological insulator," the authors present a meticulous paper of the quantum Hall effect (QHE) within the topological surface states (TSS) of an intrinsic three-dimensional topological insulator (TI), specifically BiSbTeSe. This paper is particularly noteworthy due to the unique half-integer quantization of the Hall conductance attributed to the Dirac fermions present within the TSS, differing fundamentally from the integer quanization observed in conventional QHE systems.

Experimental Observations and Implications

The authors report the observation of well-developed QHE arising from TSS in BiSbTeSe, an intrinsic TI where bulk conduction is effectively suppressed, leading to surface-dominated electronic transport even at temperatures near room temperature. Under low temperatures and high magnetic fields, integer-quantized Hall plateaus were observed. These are characterized by each of the two parallel surfaces contributing a half-integer e²/h Hall conductance, leading to a total Hall conductance that sums to integer values. The gating of the samples to align the carrier density on the bottom surface with the top surface revealed odd-integer plateaus, indicative of the interaction between two degenerate Dirac gases.

Theoretical and Practical Implications

The observed well-developed QHE illustrates a platform to explore fundamental physics and potential device applications associated with TIs. Theoretically, the paper strengthens the understanding of axionic electrodynamics and the topological magnetoelectric effect, where the decoupling of the bulk and surface conduction opens avenues for investigating exotic phenomena such as quantized electromagnetic responses. From a practical perspective, the implications for electronics are profound, particularly in the development of dissipationless devices and fault-tolerant quantum computing systems utilizing the robust surface conductance properties of TIs.

Methodological Innovation

The methodology involved the precise tuning of carrier densities via backgate voltages in exfoliated thin flakes of TI. The samples were designed to minimize bulk contributions and ensure uniformity while maintaining adequate thickness for effective modulation using backgates. The research employed multiple spectroscopic and transport measurement techniques to disentangle surface from bulk contributions to the transport properties. This approach allowed the direct measurement of the surface-dominated QHE, characterized by distinctive half-integer shifts in the quantum Hall states as carrier density varied.

Future Directions

Future research directions include the exploration of coupling effects between topological surface states and conventional two-dimensional electron gases, further studies on the implications of the observed half-integer quantum Hall effect for understanding spin-helical Dirac fermions, and the potential application in creating robust quantum computing platforms. Additionally, the role of electron-electron interactions and disorder in broadening Landau levels in such TIs could be further elucidated. The integration of these intrinsic TIs with superconducting or magnetic materials may also provide insights into the realization of novel quantum phenomena such as Majorana fermions, which hold promise for quantum computational applications.

In summary, this work significantly enhances the understanding of topological surface state QHE in three-dimensional topological insulators, providing a compelling case for the unique electronic properties of TSS and their potential applications in next-generation electronic and quantum computing systems.