Spins, disorder and interactions in GaAs and graphene

This thesis describes experiments on semiconductor spin physics under the influence of diverse disorder and carrier-carrier
interaction. Motivated by recent observations of GaAs spin qubit coherence limited by hyperfine coupling to nuclear-spin en-
semble fluctuations, we started out to find ways to study the electron-spin nuclear-spin coupling or to avoid the nuclear spin
bath altogether. This can be done in several different ways and here we pursued two fairly different approaches. One is the
investigation of the dynamics of nuclear spin polarization in GaAs and the other aims at spin-related effects in graphene na-
nostructures which possibly have negligible nuclear spin contributions due to the natural abundance (about 99 %) of zero-
spin isotopes.
The experiments on GaAs are performed using a non-local spin injection device with Fe ferromagnetic contacts on a degene-
rately n-doped epilayer. At low temperatures, where the injected spin polarization allows dynamic polarization of the nuclear
spins via hyperfine interaction, distinct spin signals are used to study the dynamics of the nuclear spin system both in presen-
ce and absence of net electron spin polarization.
The nuclear spin-lattice relaxation in an unpolarized environment reveals an unexpected breakdown of the Korringa law of
nuclear spin relaxation otherwise valid for metallic systems. This is manifested in the observed deviation from a linear tem-
perature dependence of the nuclear T_1 time and is interpreted as a result of hyperfine coupling to conduction electrons
which are influenced by the interplay of disorder and carrier-carrier interaction. This finding therefore gives important insight
into the strong influence of intimate coupling between the electron and nuclear spin sub-systems.
Transport experiments on lithographically defined graphene quantum dots are performed at low temperatures. Three graphe-
ne quantum dots of different nanometer sizes fabricated on a single graphene flake allow a detailed investigation of the size
dependence of the Coulomb interaction, the energy spectra, and the influence of disorder within the nanostructures. The onset
of Landau quantization in perpendicular magnetic fields reveals signatures of the electron-hole crossover reflecting the
bandstructure symmetry of graphene. Suppression of orbital effects by applying external magnetic fields parallel to the sam-
ple plane allows to address spin effects of the charge transitions in the quantum dots. The observed field dependence of Cou-
lomb blockade peak splittings is not inconsistent with the Zeeman splitting proportional to an expected g-factor of 2. The
transport data evidence strong influence of disorder supposably induced by both charged impurities in the close vicinity of
the quantum dots and by edge disorder as a result of the fabrication process lacking precise control of the edge structures.