ABSTRACT
We report ground- and excited-state transport through an electrostatically defined few-hole quantum dot in bilayer graphene in both parallel and perpendicular applied magnetic fields. A remarkably clear level scheme for the two-particle spectra is found by analyzing finite bias spectroscopy data within a two-particle model including spin and valley degrees of freedom. We identify the two-hole ground state to be a spin-triplet and valley-singlet state. This spin alignment can be seen as Hund's rule for a valley-degenerate system, which is fundamentally different from quantum dots in carbon nanotubes, where the two-particle ground state is a spin-singlet state. The spin-singlet excited states are found to be valley-triplet states by tilting the magnetic field with respect to the sample plane. We quantify the exchange energy to be 0.35 meV and measure a valley and spin g factor of 36 and 2, respectively.
ABSTRACT
We report on the temperature dependence of the spin-pumping effect and the Gilbert damping in Co/Pt bilayers grown on Silicon oxide by measuring the change of the linewidth in a ferromagnetic resonance (FMR) experiment. By varying the Co thickness d(Co) between 1.5 nm and 50 nm we find that the damping increases inversely proportional to d(Co) at all temperatures between 300 K and 5 K, showing that the spin pumping effect does not depend on temperature. We also find that the linewidth increases with decreasing temperature for all thicknesses down to about 30 K, before leveling off to a constant, or even decreasing again. This behavior is similar to what is found in bulk ferromagnets, leading to the conclusion that in thin films a conductivity-like damping mechanism is present similar to what is known in crystals.
