Tailoring graphene for spintronics

Rodolfo Miranda

Dpt. Física de la Materia Condensada, Universidad Autónoma de Madrid &

Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA-Nano). Spain.

Spintronics with graphene

Spin (“conserver”) transport: Long spin diffusion length (1-100 µm), Spin filter: Spin polarized bands Spin manipulation: Large Spin-Orbit coupling

Topological edge states without dissipation ?

Tunnel barriers for injection

Epitaxial Graphene on Ir(111)

P-doped ED= +100 meV

a) HOMO at -2eV c) LUMO at +1.5 eV

Neutral TCNQ /gr/Ir(111)

Self organization via H bonds Intermolecular bands BUT.. No charge transfer No SOMO No magnetic moment

D. Maccariello et al, Chem. Mat. 26, 2883 (2014)

XPS

b) at +0.7eV

Topographic images

HCP top FCC top atop 4.6 K

Periodically Rippled Graphene/ Ru(0001)

Moiré pattern Periodicity: 29.3 ± 0.8 Å

5 pm corrugation of the C atoms

, PRL 101, 099704 (2008) A.L. Vázquez de Parga et al, PRL 100, 056807 (2008)

Graphene/ Ru(0001) n-doped ED= -900 meV

Graphene/ Ir(111) p-doped ED= +100 meV

F. Calleja et al. Nature Physics 11, 43 (2015)

Charged TCNQ and F4-TCNQ / gr/Ru(0001)

The Kondo Effect Screening of a magnetic moment (below TK) by the formation of a many-body singlet with the conduction band electron bath

Kondo “cloud”

Appearance of a sharp resonance at

the Fermi level

M. Garnica et al, Nature Physics 9, 368 (2013)

FWHM= 10 meV TK=60K

Mapping spin distributions by imaging the Kondo effect

TCNQ on graphene/Ru(0001

TCNQ TK= 58K F4-TCNQ TK=70 K

M. Garnica et al, Nano Lett 14, 4560 (2014)

Mapping spin distributions by imaging the Kondo effect

M. Garnica et al, Surface Sci. (2014)

F4- TCNQ on graphene/Ru(0001

Unpaired electron concentrated in

the C-CN2 terminations

4.2 nm x 5.3 nm STM topograph taken with V=-0.1V , I=30pA;

dI/dV(x,y) at 2 meV: Kondo signal Topography

4.6 K

M. Garnica et al, Nature Physics 9, 368 (2013)

Kondo Effect: Rows of TCNQ on gr/Ru(0001)

The intensity of the Kondo resonance changes along the rows

HCP Blue TCNQ TK= 63K Br. Green TCNQ TK=111 K

a)

b) c) c)

Kondo resonance on 1 ML of TCNQ on graphene/Ru(0001): Only on low sites

D. Maccariello et al, Chem. Mat. 26, 2883 (2014)

7 nm x 7 nm

1 ML TCNQ TK= 140K

14 V = 0 mV V = -50 mV V = 50 mV

dI/dV maps

A Kondo Lattice: F4-TCNQ on gr/Ru(0001) 4.6 K

1 ML F4-TCNQ TK= 81 K

Calculated magnetic moment per unit cell (containing 8 molecules) : 1.3μB

A system with a half filled (nearly) flat band is predicted to have a ferromagnetic ground state.

Spin Density distribution for a TCNQ monolayer on graphene/Ru(0001)

Y. Nagaoka Phys. Rev. 147 392 (1966)

Spin polarized bands for a (charged) flat, free-standing TCNQ ML

M. Garnica et al, Nature Physics 9, 368 (2013)

on gr/Pb/Ir(111)

on gr/Ir(111)

Electronic density at Γ of the first empty band Antisymmetric combination of LUMOs

Andrés Black, Manuel Rodríguez, Daniel Granados

M. Garnica et al, Nature Physics 9, 368 (2013)

Adsorbed molecules induce Long Range Magnetic Order

in epitaxial graphene

Pb-intercalated Graphene/Ir(111) Gr/ Pb/Ir(111) islands Gr/Ir(111) terraces

n-doped ED = -110 meV

p-doped ED= 100 meV

4.6 K

Graphene/Pb/Ir(111): Pseudo- Landau Levels?

ED EF

4.6 K

F. Calleja et al, Nature Physics 11, 43 (2015)

Graphene/Pb/Ir(111): Landau Levels without magnetic field ?

F. Calleja et al, Nature Physics 11, 43 (2015)

Graphene/Pb/Ir(111) vs gr/Ir(111)

Large Spin-Orbit coupling

Small Spin-Orbit coupling

δISO/δx

p n

F. Calleja et al, Nature Physics 11, 43 (2015)

Spin-split Graphene bands by a Giant Spin Orbit interaction induced by Pb atoms

DFT band structure

ΔSO > 50 meV

Tight Binding band structure

Zero longitudinal resistance Quantized transverse resistance

2D Topological Insulator

Graphene + Giant Spin Orbit Coupling: Quantum Spin Hall insulator or 2D Topological Insulator