Unconventional fractional quantum Hall states and Wigner crystallization in suspended Corbino graphene

Manohar Kumar,Pertti Hakonen,Antti Laitinen

doi:10.1038/s41467-018-05094-8

Abstract

Competition between liquid and solid states in two-dimensional electron systems is an intriguing problem in condensed matter physics. We have investigated competing Wigner crystal and fractional quantum Hall (FQH) liquid phases in atomically thin suspended graphene devices in Corbino geometry. Low-temperature magnetoconductance and transconductance measurements along with IV characteristics all indicate strong charge density dependent modulation of electron transport. Our results show unconventional FQH phases which do not fit the standard Jain’s series for conventional FQH states, instead they appear to originate from residual interactions of composite fermions in partially filled Landau levels. Also at very low charge density with filling factors nu ,lesssim, 1/5, electrons crystallize into an ordered Wigner solid which eventually transforms into an incompressible Hall liquid at filling factors around ν ≤ 1/7. Building on the unique Corbino sample structure, our experiments pave the way for enhanced understanding of the ordered phases of interacting electrons.

Highlights

Competition between liquid and solid states in two-dimensional electron systems is an intriguing problem in condensed matter physics
In the extreme quantum limit, the interplay between kinetic and potential energy of electrons in the two-dimensional electron gas leads to formation of exotic states like fractional quantum Hall (FQH) state, a many-body state where elementary excitations have fractional electronic charge[1], or even formation of electron solids, i.e., Wigner crystals where electrons freeze into a periodic lattice at very low temperature[2]
Owing to reduced screening in atomically thin graphene, the electrons interact with higher Coulomb interaction energy than in conventional semiconductor heterostructures, providing an extraordinary setting for both unconventional FQH states and Wigner crystals

Summary

Introduction

Competition between liquid and solid states in two-dimensional electron systems is an intriguing problem in condensed matter physics. It was realized that these difficulties are mostly due to either intermixing of ρxx and ρxy[9] or strain-induced pseudomagnetic fields[10] in two-terminal and multi-terminal suspended graphene samples This limits the experimental verification of fractional states to either local compressibility measurements using SET techniques[11] or transconductance measurements[12], both of which techniques have limited access to the complete electrical transport characteristics of fractional states; they probe local properties only. In our magnetoconductance and transconductance measurements, we have resolved a distinctive set of incompressible liquid states with fractional filling factors of {−1/3, 1/5, 2/7, 4/13, 1/3, 4/ 11, 2/5, 3/7, 4/9, 4/7, 3/5, 2/3, 4/5, 4/3} Of these FQH states, ν = 4/13 and 4/11 are unconventional states, formed due to interactions between composite fermions[19]. In addition to our magnetoconductance measurements, IV characteristics and microwave spectroscopy showed evidence of solid phase order of electrons, i.e., Wigner crystallization, at low densities around ν ’ 1=5 À 1=720,21

Methods

Results

Conclusion