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

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

Motivation (of transport through graphene-metal interface) Experimental works: Nature Nanotechnology 3, 486 (2008) Phys. Rev. B 79, 245430 (2009) University of Wisconsin-Madison

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

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, 245407 (2001).

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) 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

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

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

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 Graphene Metal + graphene  TOP VIEW Transport direction K Brillouin Zone EF K University of Wisconsin-Madison

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

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