1. Graphene-metal interface: an efficient spin and momentum filter
Jesse Maassen, Wei Ji
and Hong Guo
Department of Physics,
McGill University, Montreal, Canada

2. 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

3. Motivation (of transport through graphene-metal interface)
Experimental works:
Nature Nanotechnology 3, 486 (2008)
Phys. Rev. B 79, (2009)
University of Wisconsin-Madison

4. University of Wisconsin-Madison
Our goal
Quantitative parameter-free transport calculation of a graphene-metal interface
University of Wisconsin-Madison

5. University of Wisconsin-Madison
Theoretical method
Density functional theory (DFT) combined with nonequilibrium Green’s functions (NEGF)1
Two-probe geometry under finite bias
System
Left
lead
Right - +
Simulation
Box
NEGF
DFT
HKS 
University of Wisconsin-Madison
1 Jeremy Taylor, Hong Guo and Jian Wang, PRB 63, (2001).

6. 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, (2008))
Found most stable configuration (1stC on metal, 2ndC on hollow site)
2x1:H is physical but also meant to replicate oxide interface (in the sense there is still a large band gap).
After relaxation
University of Wisconsin-Madison

7. Results (Metal = Copper)
Bandstructure
Intact Dirac point
n-doping of graphene
Relaxed structures of both surfaces to compare.
deq = 2.95 Å
Transport
Double minima conductance feature
Gate bias can shift Dirac points relative to each other
University of Wisconsin-Madison

8. University of Wisconsin-Madison
Results (Metal = Ni, Co) Co Ni
Bandstructure
Graphene bands (black + blue)
No more linear dispersion
Interaction with metal opens a band gap
Band gaps are spin dependent
Relaxed structures of both surfaces to compare.
deq = 2.17 Å
deq = 2.13 Å
University of Wisconsin-Madison

9. 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

10. Results (Metal = Cu, Ni & Co)
Momentum filtering
Graphene
Metal +
graphene 
TOP VIEW
Transport
direction K
Brillouin Zone EF K
University of Wisconsin-Madison

11. 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

12. 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.
University of Wisconsin-Madison