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Graphene-metal interface: an efficient spin and
momentum filter
Jesse Maassen, Wei Ji
and Hong Guo
Department of Physics,
McGill University, Montreal, Canada
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
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
+
-
University of Wisconsin-Madison
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
University of Wisconsin-Madison
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
University of Wisconsin-Madison
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.