Broken symmetry states and divergent resistance in suspended bilayer graphene

Abstract: Graphene [1] and its bilayer have generated tremendous excitement in the physics community due to their unique electronic properties [2]. The intrinsic physics of these materials, however, is partially masked by disorder, which can arise from various sources such as ripples [3] or charged impurities [4]. Recent improvements in quality have been achieved by suspending graphene flakes [5,6], yielding samples with very high mobilities and little charge inhomogeneity. Here we report the fabrication of suspended bilayer graphene devices with very little disorder. We observe fully developed quantized Hall states at magnetic fields of 0.2 T, as well as broken symmetry states at intermediate filling factors $\nu = 0$, $\pm 1$, $\pm 2$ and $\pm 3$. The devices exhibit extremely high resistance in the $\nu = 0$ state that grows with magnetic field and scales as magnetic field divided by temperature. This resistance is predominantly affected by the perpendicular component of the applied field, indicating that the broken symmetry states arise from many-body interactions.

• The paper presents a breakthrough in fabricating low-disorder, suspended bilayer graphene, enabling clear observation of quantized Hall states at magnetic fields as low as 0.2 T.
• It employs precise electron beam lithography and millikelvin temperature measurements to reveal broken symmetry states at filling factors ν = 0, ±1, ±2, and ±3, highlighting strong many-body interactions.
• The study demonstrates that resistance increases exponentially with the perpendicular magnetic field, underpinning the role of quantum Hall ferromagnetism distinct from monolayer graphene behavior.

Broken Symmetry States and Divergent Resistance in Suspended Bilayer Graphene

The study "Broken Symmetry States and Divergent Resistance in Suspended Bilayer Graphene" by Feldman, Martin, and Yacoby presents an in-depth investigation into the quantum Hall effect (QHE) phenomenon observed in high-quality suspended bilayer graphene devices. This paper engages extensively with the distinctive electronic properties of bilayer graphene, particularly under conditions of low disorder and high carrier mobility.

The primary accomplishment of this paper is the fabrication and analysis of suspended bilayer graphene devices exhibiting notably low disorder, akin to improvements previously achieved in graphene monolayers. The research highlights the observation of fully developed quantized Hall states at relatively low magnetic fields (0.2 T) and identifies broken symmetry states at intermediate filling factors ν = 0, ±1, ±2, and ±3. This phenomenon, coupled with the extremely high resistance observed in states at ν = 0, suggests strong many-body interactions facilitated by quantum Hall ferromagnetism (QHF).

The experimental setup focuses on the behavior of bilayer graphene at zero magnetic fields, as well as under varying magnetic fields and carrier densities. Notably, the resistance of these devices increases exponentially with increasing perpendicular magnetic field components, scaling with the magnetic field divided by temperature. This reinforces the hypothesis that the broken symmetry states are influenced by many-body interactions distinct from those observed in graphene monolayers.

In greater detail, the research demonstrates that the eightfold degeneracy of the zero-energy Landau level (LL) in bilayer graphene is lifted—an occurrence that begins at magnetic fields approximately an order of magnitude lower than similar phenomena in monolayers. The paper attributes the lift in degeneracy to multiple effects, including QHF. The intrinsic interaction energy associated with QHF is presumed to be significantly stronger than that caused by simple Zeeman splitting. This is corroborated by examining the linear dependence of the energy gap on B, differing from traditional QHF models but echoing early observations in GaAs systems.

The fabrication method employed careful mechanical exfoliation and subsequent electron beam lithography to produce samples with minimized disorder. The measurements, performed at millikelvin temperatures, reveal that the suspended bilayer devices achieve conductance plateaus indicative of the rich physics inherent in bilayer graphene, with careful calibration against known filling factors.

This study provides a significant step forward in understanding the interplay between quantum coherence and electronic properties in bilayer systems. The comprehensive analysis of bilayer graphene underlines the importance of many-body interactions in the emergence of novel quantum phases. Future explorations could refine the theoretical models surrounding QHF in bilayer systems and explore potential tunable electronic applications of these materials in quantum computing and nanoelectronics. The findings underscore the critical role of suspended techniques in revealing the intrinsic properties of graphene and its bilayers, potentially informing the synthesis of other two-dimensional van der Waals heterostructures. The implications for graphene-based electronics are substantial, with the potential for innovative device architectures that leverage these quantum properties at attainable magnetic fields and temperatures.