11,126 research outputs found
Drastic Reduction of Shot Noise in Semiconductor Superlattices
We have found experimentally that the shot noise of the tunneling current
through an undoped semiconductor superlattice is reduced with respect to the
Poissonian noise value , and that the noise approaches 1/3 of that value
in superlattices whose quantum wells are strongly coupled. On the other hand,
when the coupling is weak or when a strong electric field is applied to the
superlattice the noise becomes Poissonian. Although our results are
qualitatively consistent with existing theories for one-dimensional mulitple
barriers, the theories cannot account for the dependence of the noise on
superlattice parameters that we have observed.Comment: 4 Pages, 3Figure
Electroweak phase transition in the MSSM with four generations
By assuming the existence of the sequential fourth generation to the minimal
supersymmetric standard model (MSSM), we study the possibility of a strongly
first-order electroweak phase transition. We find that there is a parameter
region of the MSSM where the electroweak phase transition is strongly first
order. In that parameter region, the mass of the lighter scalar Higgs boson is
calculated to be above the experimental lower bound, and the scalar quarks of
the third and the fourth generations are heavier than the corresponding quarks.Comment: 12 pages, 2 tables, 2 figure
Higgs bosons of a supersymmetric model at the ILC
We study the scalar Higgs sector of the next-to-minimal supersymmetric
standard model with an extra U(1), which has two Higgs doublets and a Higgs
singlet, in the light leptophobic scenario where the extra neutral gauge
boson does not couple to charged leptons. In this model, we find that the
sum of the squared coupling coefficients of the three neutral scalar Higgs
bosons to , normalized by the corresponding SM coupling coefficient is
noticeably smaller than unity, due to the effect of the extra U(1), for a
reasonable parameter space of the model, whereas it is unity in the
next-to-minimal supersymmetric standard model. Thus, these two models may be
distinguished if the coupling coefficients of neutral scalar Higgs bosons to
are measured at the future International Linear Collider by producing them
via the Higgs-strahlung, fusion, and fusion processes.Comment: 12 pages, 2 figures, 1 table, PR
Origins of anomalous electronic structures of epitaxial graphene on silicon carbide
On the basis of first-principles calculations, we report that a novel
interfacial atomic structure occurs between graphene and the surface of silicon
carbide, destroying the Dirac point of graphene and opening a substantial
energy gap there. In the calculated atomic structures, a quasi-periodic
domain pattern emerges out of a larger commensurate
periodic interfacial reconstruction,
resolving a long standing experimental controversy on the periodicity of the
interfacial superstructures. Our theoretical energy spectrum shows a gap and
midgap states at the Dirac point of graphene, which are in excellent agreement
with the recently-observed anomalous angle-resolved photoemission spectra.
Beyond solving unexplained issues of epitaxial graphene, our atomistic study
may provide a way to engineer the energy gaps of graphene on substrates.Comment: Additional references added; published version; 4 pages, 4 figure
Clustering of vacancy defects in high-purity semi-insulating SiC
Positron lifetime spectroscopy was used to study native vacancy defects in
semi-insulating silicon carbide. The material is shown to contain (i) vacancy
clusters consisting of 4--5 missing atoms and (ii) Si vacancy related
negatively charged defects. The total open volume bound to the clusters
anticorrelates with the electrical resistivity both in as-grown and annealed
material. Our results suggest that Si vacancy related complexes compensate
electrically the as-grown material, but migrate to increase the size of the
clusters during annealing, leading to loss of resistivity.Comment: 8 pages, 5 figure
Uniform current in graphene strip with zigzag edges
Graphene exhibits zero-gap massless-Dirac fermion and zero density of states
at E = 0. These particles form localized states called edge states on finite
width strip with zigzag edges at E = 0. Naively thinking, one may expect that
current is also concentrated at the edge, but Zarbo and Nikolic numerically
obtained a result that the current density shows maximum at the center of the
strip. We derive a rigorous relation for the current density, and clarify the
reason why the current density of edge state has a maximum at the center.Comment: 5 pages, 3 figures; added references and corrected typos, to be
published in J. Phys. Soc. Jpn. Vol.78 No.
Signature of Carrier-Induced Ferromagnetism in Ti_{1-x}Co_{x}O_{2-delta}: Exchange Interaction Between High-Spin Co 2+ and the Ti 3d Conduction Band
X-ray photoemission spectroscopy measurements were performed on thin-film
samples of rutile Ti_{1-x}Co_{x}O_{2-delta} to reveal the electronic structure.
The Co 2p core level spectra indicate that the Co ions take the high-spin Co 2+
configuration, consistent with substitution on the Ti site. The high spin state
and the shift due to the exchange splitting of the conduction band suggest
strong hybridization between carriers in the Ti 3d t2g band and the t2g states
of the high-spin Co 2+. These observations support the argument that room
temperature ferromagnetism in Ti_{1-x}Co_{x}O_{2-delta} is intrinsic.Comment: 4 pages, 5 figures. Accepted for publication in Physical Review
Letter
Electroweak phase transition in a nonminimal supersymmetric model
The Higgs potential of the minimal nonminimal supersymmetric standard model
(MNMSSM) is investigated within the context of electroweak phase transition. We
investigate the allowed parameter space yielding correct electroweak phase
transitoin employing a high temperature approximation. We devote to
phenomenological consequences for the Higgs sector of the MNMSSM for
electron-positron colliders. It is observed that a future linear
collider with GeV will be able to test the model with regard
to electroweak baryogenesis.Comment: 28 pages, 5 tables, 12 figure
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