Coulomb-driven broken-symmetry states in doubly gated suspended bilayer graphene (1010.0989v1)

Abstract: The non-interacting energy spectrum of graphene and its bilayer counterpart consists of multiple degeneracies owing to the inherent spin, valley and layer symmetries. Interactions among charge carriers are expected to spontaneously break these symmetries, leading to gapped ordered states. In the quantum Hall regime these states are predicted to be ferromagnetic in nature whereby the system becomes spin polarized, layer polarized or both. In bilayer graphene, due to its parabolic dispersion, interaction-induced symmetry breaking is already expected at zero magnetic field. In this work, the underlying order of the various broken-symmetry states is investigated in bilayer graphene that is suspended between top and bottom gate electrodes. By controllably breaking the spin and sublattice symmetries we are able to deduce the order parameter of the various quantum Hall ferromagnetic states. At small carrier densities, we identify for the first time three distinct broken symmetry states, one of which is consistent with either spontaneously broken time-reversal symmetry or spontaneously broken rotational symmetry.

• The paper demonstrates that Coulomb interactions drive gapped ordered states and broken symmetry even at zero magnetic fields in bilayer graphene.
• It employs a suspended, doubly gated configuration to precisely control electron density and electric fields, revealing subtle electron-electron interaction effects.
• Experimental observations include complete lifting of the Landau level degeneracy, which is key for advancing high-performance graphitic device technologies.

Overview of "Coulomb-driven broken-symmetry states in doubly gated suspended bilayer graphene"

The paper presents a detailed investigation of bilayer graphene, specifically focusing on the phenomenon of Coulomb-driven broken-symmetry states. Bilayer graphene, exhibiting a parabolic dispersion, is a compelling subject due to its potential for band gap modification through external electric fields and its predicted interaction-induced symmetry-breaking characteristics even at zero magnetic field. The paper elucidates the underlying order of various broken-symmetry states that surface under the influence of both quantum Hall ferromagnetism and at zero magnetic fields. This work is seminal in mapping these phases by systematically varying external magnetic and electric fields.

Experimental Approach

To probe these states, the authors fabricate high-quality bilayer graphene samples that are suspended between top and bottom gate electrodes. The methodological advancement in creating suspended samples is pivotal, allowing perfect control over electron density and perpendicular electric field, which are crucial for discerning the subtle electron-electron interaction effects. The experimental setup includes a doubly gated configuration that enhances the electrostatic tunability due to reduced scattering from the substrate.

Key Observations

1. Gapped Ordered States: The research confirms the presence of gapped ordered states driven by interaction-induced symmetry breaking, observable even at zero magnetic fields. These states are underpinned by the combinations of spin, valley, and layer polarization.
2. Quantum Hall Regime: At large magnetic fields, the paper substantiates the emergence of various quantum Hall states, characterized by an interaction-induced lifting of the lowest Landau level (LL) degeneracy. The experimental observations reveal complete lifting of the 8-fold degeneracy of this level, giving rise to well-defined plateaus at filling factors ν = 0, ±1, ±2, ±3.
3. Zero Magnetic Field Broken-symmetry States: Perhaps most notably, the paper identifies three distinct broken-symmetry states at zero magnetic field, one of which may either be a result of spontaneously broken time-reversal symmetry or spontaneously broken rotational symmetry. These findings could open new frontiers in understanding spontaneous symmetry breaking in two-dimensional electron systems.

Implications and Future Research

This research provides significant insights into the fundamental interactions within bilayer graphene, highlighting the pivotal role of electron-electron interactions in driving broken-symmetry states. These findings have substantial implications for potential electronic and optoelectronic applications, as the ability to manipulate such symmetry-breaking states could lead to advances in low-power and high-performance graphitic devices.

From a theoretical perspective, the paper challenges existing models predicting the nature of order parameters in these systems and prompts further investigations into these complex interactions. The quantitative discrepancies between theoretical predictions and observed transition values highlight the need for models that can better accommodate the nuances of screening effects and LL degeneracies.

In future developments, the exploration of bilayer graphene under even more extreme conditions (higher magnetic fields, lower temperatures, or with more intricate gating configurations) could reveal novel quantum phases. Additionally, similar methodologies can be applied to investigate other two-dimensional materials where electron interactions play a crucial role in the emergence of exotic electronic properties.

In summary, this paper significantly advances our understanding of bilayer graphene, demonstrating the intricate nature of electron-electron interactions and their capacity to foster diverse broken-symmetry states. It paves the way for future theoretical and experimental exploration in the field of two-dimensional materials.