2,183 research outputs found
Searches at HERA
Searches for physics beyond the Standard Model have been performed in
high-energy collisions at HERA. No significant deviation from the
Standard Model has been observed while searching for contact interactions,
extra dimensions, leptoquarks, R-parity violating squarks and excited fermions.
Exclusion limits have been inferred which extend or complement bounds from
other colliders. The H1 collaboration has observed a puzzling excess of events
with an high isolated lepton and missing transverse momentum, and
interpretation as flavour changing neutral currents has been explored.Comment: 7 pages, 6 figures, contribution to the Lake Louise Winter Institut
Tight binding description of the electronic response of a molecular device to an applied voltage
We analyze the effect of an external electric field on the electronic
structure of molecules which have been recently studied as molecular wires or
diodes. We use a self-consistent tight binding technique which provides results
in good agreement with ab initio calculations and which may be applied to a
large number of molecules. The voltage dependence of the molecular levels is
mainly linear with slopes intimately related to the electronic structure of the
molecules. We emphasize that the response to the applied voltage is an
important feature which governs the behavior of a molecular device
Assessment of the notions of band offsets, wells and barriers at nanoscale semiconductor heterojunctions
Epitaxially-grown semiconductor heterostructures give the possibility to
tailor the potential landscape for the carriers in a very controlled way. In
planar lattice-matched heterostructures, the potential has indeed a very simple
and easily predictable behavior: it is constant everywhere except at the
interfaces where there is a step (discontinuity) which only depends on the
composition of the semiconductors in contact. In this paper, we show that this
universally accepted picture can be invalid in nanoscale heterostructures
(e.g., quantum dots, rods, nanowires) which can be presently fabricated in a
large variety of forms. Self-consistent tight-binding calculations applied to
systems containing up to 75 000 atoms indeed demonstrate that the potential may
have a more complex behavior in axial hetero-nanostructures: The band edges can
show significant variations far from the interfaces if the nanostructures are
not capped with a homogeneous shell. These results suggest new strategies to
engineer the electronic properties of nanoscale objects, e.g. for sensors and
photovoltaics.Comment: Accepted for publication in Phys. Rev.
Study of Phase Reconstruction Techniques applied to Smith-Purcell Radiation Measurements
Measurements of coherent radiation at accelerators typically give the
absolute value of the beam profile Fourier transform but not its phase. Phase
reconstruction techniques such as Hilbert transform or Kramers Kronig
reconstruction are used to recover such phase. We report a study of the
performances of these methods and how to optimize the reconstructed profiles
Discrete Time Quantum Walk Approach to State Transfer
We show that a quantum state transfer, previously studied as a continuous
time process in networks of interacting spins, can be achieved within the model
of discrete time quantum walks with position dependent coin. We argue that due
to additional degrees of freedom, discrete time quantum walks allow to observe
effects which cannot be observed in the corresponding continuous time case.
First, we study a discrete time version of the engineered coupling protocol due
to Christandl et. al. [Phys. Rev. Lett. 92, 187902 (2004)] and then discuss the
general idea of conversion between continuous time quantum walks and discrete
time quantum walks.Comment: 9 pages, 6 figures, comments welcom
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