299 research outputs found
Transition to Landau Levels in Graphene Quantum Dots
We investigate the electronic eigenstates of graphene quantum dots of
realistic size (i.e., up to 80 nm diameter) in the presence of a perpendicular
magnetic field B. Numerical tight-binding calculations and Coulomb-blockade
measurements performed near the Dirac point exhibit the transition from the
linear density of states at B=0 to the Landau level regime at high fields.
Details of this transition sensitively depend on the underlying graphene
lattice structure, bulk defects, and localization effects at the edges. Key to
the understanding of the parametric evolution of the levels is the strength of
the chiral-symmetry breaking K-K' scattering. We show that the parametric
variation of the level variance provides a quantitative measure for this
scattering mechanism. We perform measurements of the parametric motion of
Coulomb blockade peaks as a function of magnetic field and find good agreement.
We thereby demonstrate that the magnetic-field dependence of graphene energy
levels may serve as a sensitive indicator for the properties of graphene
quantum dots and, in further consequence, for the validity of the
Dirac-picture.Comment: 10 pages, 11 figures, higher quality images available on reques
Transport through graphene nanoribbons: suppression of transverse quantization by symmetry breaking
We investigate transport through nanoribbons in the presence of disorder
scattering. We show that size quantization patterns are only present when SU(2)
pseudospin symmetry is preserved. Symmetry breaking disorder renders transverse
quantization invisible, which may provide an explanation for the necessity of
suspending graphene nanoconstrictions to obtain size quantization signatures in
very recent experiments. Employing a quasi-classical Monte-Carlo simulation, we
are able to reproduce and explain key qualitative features of the full
quantum-mechanical calculations.Comment: 5 figure
Transient localization in the kicked Rydberg atom
We investigate the long-time limit of quantum localization of the kicked
Rydberg atom. The kicked Rydberg atom is shown to possess in addition to the
quantum localization time a second cross-over time where quantum
dynamics diverges from classical dynamics towards increased instability. The
quantum localization is shown to vanish as either the strength of the kicks at
fixed principal quantum number or the quantum number at fixed kick strength
increases. The survival probability as a function of frequency in the transient
localization regime is characterized by highly irregular,
fractal-like fluctuations
Diffractive wave guiding of hot electrons by the Au (111) herringbone reconstruction
The surface potential of the herringbone reconstruction on Au(111) is known
to guide surface-state electrons along the potential channels. Surprisingly, we
find by scanning tunneling spectroscopy that hot electrons with kinetic
energies twenty times larger than the potential amplitude (38 meV) are still
guided. The efficiency even increases with kinetic energy, which is reproduced
by a tight binding calculation taking the known reconstruction potential and
strain into account. The guiding is explained by diffraction at the
inhomogeneous electrostatic potential and strain distribution provided by the
reconstruction.Comment: 10 pages, 9 figure
Angular distribution in two-photon double ionization of helium by intense attosecond soft X-ray pulses
We investigate two-photon double ionization of helium by intense () ultrashort ( as) soft X-ray pulses (E = 91.6 eV). The
time-dependent two-electron Schr\"odinger equation is solved using a coupled
channel method. We show that for ultrashort pulses the angular distribution of
ejected electrons depends on the pulse duration and provides novel insights
into the role of electron correlations in the two-electron photoemission
process. The angular distribution at energies near the ``independent electron''
peaks is close to dipolar while it acquires in the ``valley'' of correlated
emission a significant quadrupolar component within a few hundred attoseconds.Comment: 17 pages, 6 fig
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