[show abstract][hide abstract] ABSTRACT: We demonstrate the growth of graphene nanocrystals by molecular beam methods
that employ a solid carbon source, and that can be used on a diverse class of
large area dielectric substrates. Characterization by Raman and Near Edge X-ray
Absorption Fine Structure spectroscopies reveal a sp2 hybridized hexagonal
carbon lattice in the nanocrystals. Lower growth rates favor the formation of
higher quality, larger size multi-layer graphene crystallites on all
investigated substrates. The surface morphology is determined by the roughness
of the underlying substrate and graphitic monolayer steps are observed by
ambient scanning tunneling microscopy.
[show abstract][hide abstract] ABSTRACT: Spin orbit interaction (SOI) induces a splitting of the conduction bands
in two-dimensional electron systems (2DES) in GaAs. We study the impact
of zero-field spin-splitting on excitations of ultra high mobility 2DESs
by resonant inelastic light scattering experiments. To distinguish
between splitting caused by bulk inversion asymmetry (Dresselhaus) and
structure inversion asymmetry (Rashba), we studied symmetric (two-side
modulation doped) and asymmetric (single-side modulation doped) quantum
wells grown along (001) and (110) crystallographic directions. We probe
the excitation modes as a function of transferred momentum for different
crystallographic directions in the plane of the QW. At large wave
vectors we find a complex splitting of the single-particle intersubband
excitation mode that is strongly dependent on the combination of
Dresselhaus and Rashba SOI. The observed mode splitting is a result of
effective SOI fields in both, ground and first excited subband. Suitable
choices of crystallographic orientations yield Dresselhaus and Rashba
[show abstract][hide abstract] ABSTRACT: We find unexpected low energy excitations of fully spin-polarized composite-fermion ferromagnets in the fractional quantum Hall liquid, resulting from a complex interplay between a topological order manifesting through new energy levels and a magnetic order due to spin polarization. The lowest energy modes, which involve spin reversal, are remarkable in displaying unconventional negative dispersion at small momenta followed by a deep roton minimum at larger momenta. This behavior results from a nontrivial mixing of spin-wave and spin-flip modes creating a spin-flip excitonic state of composite-fermion particle-hole pairs. The striking properties of spin-flip excitons imply highly tunable mode couplings that enable fine control of topological states of itinerant two-dimensional ferromagnets.
[show abstract][hide abstract] ABSTRACT: The FQHE is a result of strongly interacting electrons that can be understood as QHE of composite fermions. We use inelastic light scattering experiments to study the collective excitations of CF with 2 flux quanta focusing on filling factors nu = 4/9, 3/7 and 2/5. For these fillings, the lowest collective excitation modes are spin-waves, which display a distinct spectral weight below the bare Zeeman energy indicating a negative dispersion relation. The determined energies for these "spin-wave roton minima" are in excellent quantitative agreement with numerical calculations. Using the real experimentally transferred momentum the addressed DOS and hence inelastic light scattering spectra can be modeled. We demonstrate that the observed modes are very similar for positive and negative effective magnetic field at the same CF-filling factor.
[show abstract][hide abstract] ABSTRACT: We report on growth of thin large area graphitic layers on dielectric substrate materials by means of molecular beam epitaxy (MBE) under UHV conditions. This solid source MBE technique offers highly controllable conditions without the need of gas precursors or metal surfaces. Our initial experiments on dielectric substrates such as mica, SiO2 and BN clearly demonstrates the potential of this new growth technique. NEXAFS studies show that the binding mechanism in our sheets is dominated by sp^2 bonds and the Raman spectra confirm their graphitic nature. We will also describe STM measurements of the topography and local electronic structure of these films.