Graphene-metal interface: an efficient spin and

momentum filter

Jesse Maassen, Wei Ji

and Hong Guo

Department of Physics,

McGill University, Montreal, Canada

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University of Wisconsin-Madison

Motivation(of transport through graphene-metal interface)

Graphene has exceptional properties (i.e. 2D material, zero gap, linear dispersion bands, …)

All graphene-based devices must unavoidably be

electrically contacted to outside world by metal contacts.

Experimental literature looking at the properties at the contact, and how this can largely influence the global response of the device.

University of Wisconsin-Madison

Experimental works:

Motivation(of transport through graphene-metal interface)

Nature Nanotechnology 3, 486 (2008) Phys. Rev. B 79, 245430 (2009)

University of Wisconsin-Madison

Quantitative parameter-free transport calculation of a graphene-metal interface

Our goal

University of Wisconsin-Madison

Theoretical method

• Density functional theory (DFT) combined with nonequilibrium Green’s functions (NEGF)1

• Two-probe geometry under finite bias

NEGF

DFT

HKS

1 Jeremy Taylor, Hong Guo and Jian Wang, PRB 63, 245407 (2001).

SystemLeftlead

Rightlead

- +

Simulation Box

+

-

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Atomic structure

Which metals? What configuration at interface?

Cu, Ni and Co (111) have in-place lattice constants that almost match that of graphene (PRL 101, 26803 (2008))

Found most stable configuration (1stC on metal, 2ndC on hollow site)

After relaxation

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Results (Metal = Copper)

Bandstructure

• Intact Dirac point

• n-doping of graphene

Transport

• Double minima conductance feature

• Gate bias can shift Dirac points relative to each other

deq = 2.95 Å

University of Wisconsin-Madison

Results (Metal = Ni, Co)

Bandstructure Co Ni

• Graphene bands (black + blue) • No more linear dispersion

•Interaction with metal opens a band gap • Band gaps are spin dependent

deq = 2.17 Å deq = 2.13 Å

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Results (Metal = Ni, Co)

Transport

• Spin dependent band gaps small transmission • Large transmission ratios (~30)

• High spin injection efficiency of ~ 80% (spin filter)

University of Wisconsin-Madison

Results (Metal = Cu, Ni & Co)

Momentum filtering

GrapheneMetal +graphene

TOP VIEW

Transportdirection

K

KK

K

K K

Brillouin Zone

EF

K

University of Wisconsin-Madison

Summary

Performed a parameter-free calculation of electronic transport through a graphene-metal interface.

Cu merely n-dopes the graphene resulting in a double Dirac point feature.

Ni & Co opens spin-dependent band gaps in graphene, leading to large values of spin injection efficiencies ~80%.

Filtering of electron direction

University of Wisconsin-Madison

Thank you !

Questions?

• We gratefully acknowledge financial support from NSERC, FQRNT and CIFAR.• We thank RQCHP for access to their supercomputers.