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O: Fachverband Oberflächenphysik

O 19: Symposium Spin-Orbit Coupling and Spin Relaxation in Graphene and Carbon Nanotubes

O 19.2: Hauptvortrag

Montag, 22. März 2010, 14:35–15:10, H1

Spin-orbit coupling and spin relaxation in carbon nanotube quantum dots — •Ferdinand Kuemmeth — Harvard University, Cambridge, MA, USA

This talk presents transport measurements through single and double quantum dots that elucidate the role of spin-orbit coupling on the energy spectrum and spin-relaxation in carbon nanotubes.

In a one-electron quantum dot the lowest shell consists of four quantum states associated with spin up/down and clockwise/counterclockwise orbital motion around the circumference of the nanotube. These states are resolved by application of a magnetic field parallel to the nanotube. Sub-Kelvin tunneling spectroscopy reveals that spin-orbit coupling breaks the four-fold degeneracy expected at zero magnetic field, by favoring parallel alignment of the electron's orbital and spin magnetic moment. For the one-hole quantum dot antiparallel alignment is observed, demonstrating that spin-orbit coupling also breaks electron-hole symmetry [Nature 452, 449 (2008)].

In double quantum dots Pauli blockade is observed and utilized to study spin-relaxation. In 12C nanotubes the leakage current displays a minimum at B=0 whereas a maximum is observed in 13C devices. We attribute the first effect to spin-relaxation via spin-orbit coupling - a phenomenon which is suppressed near B=0 due to time reversal symmetry. We explain the second effect by hyperfine coupling to the nuclear spins, resulting in fast electron-nuclear flip-flop processes at B=0 [Nature Phys. 5, 321 (2009)].