Navigation Links
Columbia researchers observe tunable quantum behavior in bilayer graphene

Columbia researchers have observed the fractional quantum Hall effect in bilayer graphene and shown that this exotic state of matter can be tuned by an electric field.

The fractional quantum Hall effect, which can occur when electrons confined to thin sheets are exposed to large magnetic fields, is a striking example of collective behavior where thousands of individual electrons behave as a single system. However, while the basic theory describing this effect is well established, many details of this collective behavior remain not well understood, in part because it is only observable in systems with extremely low disorder.

Graphene, an atomically thin sheet of carbon, is a promising material for study of the fractional quantum Hall effect both because it can it be a nearly defect-free crystal, and because researchers can 'tune' the charge density with an external metal 'gate' electrode and observe how the quantum states evolve in response. Over the past several years, a collaborative effort at Columbia University spanning researchers from Mechanical Engineering, Electrical Engineering and Physics, developed a series of breakthrough fabrication techniques in order to take advantage of this opportunity, allowing them to report the first observation of the fractional quantum Hall effect in graphene in 2009, and the first wide-range tuning of the effect in 2011.

An even more interesting system for study of the fractional quantum Hall effect is so-called bilayer graphene, which consists of two stacked graphene sheets. In this material, use of two metal gate electrodes (above and below) allows independent tuning of the charge density in each layer, which provides a completely new way to manipulate the fractional quantum Hall states. In particular, theory predicts that it should be possible to create exotic 'non-abelian' states that could be used for quantum computation.

While observation of the fractional quantum Hall effect in single layer graphene required simply making cleaner devices, observing this effect in bilayer graphene proved more difficult. "We knew that we could fabricate very clean bilayer graphene structures, but we suffered from our inability to make good electrical contact since bilayer graphene develops an electronic 'band-gap' under the high magnetic fields and low temperatures required for our experiments," says Cory Dean, professor of Physics who recently moved to Columbia University, and lead author on the paper. A critical breakthrough was re-design of the devices so that the charge density in the contact regions could be tuned independently from the rest of the device, which allowed them to maintain good electrical contact even under large magnetic fields. "Once we had this new device structure the results were spectacular."

Reporting in the July 4, 2014 edition of Science, the team demonstrates the existence of the fractional quantum Hall effect in bilayer graphene and shows evidence of a controllable phase transition by application of electric fields. One of the key questions towards understanding the fractional quantum Hall effect in any system is to identify the order associated with the ground state. For example, do all electrons associated within the collective state carry the same spin? In bilayer graphene this question is more complex since there are several degrees of symmetry at play all at once. In addition to spin, electrons can polarize by spontaneously residing entirely on one layer versus the other. This complexity provides an interesting new phase space to explore for new and unusual effects. In particular, several theories have predicted that application of electric fields to bilayer graphene could enable transitions between these ground state orders. "This is a new experimental knob that just is not available in other systems," says James Hone, a professor of Mechanical Engineering and co-author on the paper. The team has confirmed for the first time that varying the applied electric field causes a phase transition, but the exact nature of these different phases remains an open question. "While theory expects that we can tune the ground state order, the complexity of the system makes it difficult to determine exactly which order is actually realized," says Physics Professor and co-author Philip Kim.

"This is where the next phase of our research is headed," says Dean. "The implications for this result could be far reaching," he adds, "While we do not yet see any evidence of non-abelian states, the fact that we are able to modify the nature of the fractional quantum Hall effect by electric fields is a really exciting first step."

While previous efforts have been able to demonstrate different aspects of the sample requirement, no other group has been able to bring this all together into a single device. Dean attributes this success to the unique collaborative environment fostered at Columbia University. "This is truly a remarkable environment," he says, adding, "The open exchange of ideas across several disciplines makes the environment at Columbia a fertile ground for doing great science." Device fabrication and initial testing was done at Columbia University. Measurement under large magnetic fields was then performed by the Columbia team with the aid of the National High Magnetic Field Laboratory user facility in Tallahassee, Florida. "We have established a fantastic relationship with the NHFML over many years," says Dean. "The support provided by the NHMFL personnel at both the technical and scientific level has been invaluable to our efforts."


Contact: Beth Kwon
Columbia University

Related biology technology :

1. Columbia Engineering wins $3 million ARPA-E grant to raise efficiency, lower cost of power grid
2. Columbia engineers make worlds smallest FM radio transmitter
3. Genia Technologies, Columbia University, and Harvard University Awarded $5.25 Million Grant from NIH to Accelerate the Development of NanoTag DNA Sequencing Technology
4. Dr. Szczepan Baran will be Presenting at the International Microsurgical Simulation Society at Columbia University in New York
5. Columbias Nursing School Launches "Keep It Clean for Kids" (KICK)
6. Worlds Largest Group of Medicinal Plant Researchers Endorses ABC-AHP-NCNPR Botanical Adulterants Program
7. Researchers find that computer components can be damaged by key manufacturing processes
8. York U molecular communication researchers send worlds first text message using vodka
9. Researchers split water into hydrogen, oxygen using light, nanoparticles
10. CWRU engineering researchers report nanoscale energy-efficient switching devices at IEDM 2013
11. Berkeley Lab researchers create a nonlinear light-generating zero-index metamaterial
Post Your Comments:
(Date:11/25/2015)... 2015  PharmAthene, Inc. (NYSE MKT: PIP) announced  today ... rights plan (Rights Plan) in an effort to preserve ... under Section 382 of the Internal Revenue Code (Code). ... use of its NOLs could be substantially limited if ... Section 382 of the Code. In general, an ownership ...
(Date:11/25/2015)... Studies reveal the differences in species of bacteria ... for more effective treatment for one of the most commonly ... --> --> Gum disease is one ... relatively little was understood about the bacteria associated with it ... researchers from the WALTHAM Centre for Pet Nutrition together with ...
(Date:11/25/2015)... DIEGO , Nov. 25, 2015  Neurocrine Biosciences, ... Kevin Gorman , President and CEO of Neurocrine Biosciences, ... Healthcare Conference in New York . ... to visit the website approximately 5 minutes prior to ... A replay of the presentation will be available on ...
(Date:11/25/2015)... Muncie, IN (PRWEB) , ... November 25, 2015 ... ... Aeronautics (AMA) and the Organization of Black Aerospace Professionals (OPBAP) has been formalized ... Mathewson and other AMA team leaders met with OPBAP leaders Capt. Karl Minter ...
Breaking Biology Technology:
(Date:11/12/2015)... , Nov. 12, 2015  A golden retriever ... Duchenne muscular dystrophy (DMD) has provided a new lead ... Children,s Hospital, the Broad Institute of MIT and Harvard ... Brazil . Cell, pinpoints ... dogs "escape" the disease,s effects. The Boston Children,s lab ...
(Date:11/10/2015)... Nov. 10, 2015 About ... that helps to identify and verify the identity ... considered as the secure and accurate method of ... a particular individual because each individual,s signature is ... especially when dynamic signature of an individual is ...
(Date:10/29/2015)... Oct. 29, 2015   MedNet Solutions , an ... spectrum of clinical research, is pleased to announce that ... Association (MHTA) as one of only three finalists for ... – Small and Growing" category. The Tekne Awards honor ... shown superior technology innovation and leadership. ...
Breaking Biology News(10 mins):