499 research outputs found
Asymptotic expansions for test statistics and tests for normality based on robust regression
Ankara : Department of Economics of Bilkent University, 1999.Thesis (Ph.D.) -- Bilkent University, 1999.Includes bibliographical references leaves 62-69Önder, A ÖzlemPh.D
Conductance Quantization at zero magnetic field in InSb nanowires
Ballistic electron transport is a key requirement for existence of a
topological phase transition in proximitized InSb nanowires. However,
measurements of quantized conductance as direct evidence of ballistic transport
have so far been obscured due to the increased chance of backscattering in one
dimensional nanowires. We show that by improving the nanowire-metal interface
as well as the dielectric environment we can consistently achieve conductance
quantization at zero magnetic field. Additionally, studying the sub-band
evolution in a rotating magnetic field reveals an orbital degeneracy between
the second and third sub-bands for perpendicular fields above 1T
On Signature Transition and Compactification in Kaluza-Klein Cosmology
We consider an empty (4+1) dimensional Kaluza-Klein universe with a negative
cosmological constant and a Robertson-Walker type metric. It is shown that the
solutions to Einstein field equations have degenerate metric and exhibit
transitioins from a Euclidean to a Lorentzian domain. We then suggest a
mechanism, based on signature transition which leads to compactification of the
internal space in the Lorentzian region as . With the
assumption of a very small value for the cosmological constant we find that the
size of the universe and the internal scale factor would be related
according to in the Lorentzian region. The corresponding
Wheeler-DeWitt equation has exact solution in the mini-superspace giving rise
to a quantum state which peaks in the vicinity of the classical solutions
undergoing signature transition.Comment: 13 pages, 3 figure
Killing-Yano Forms of a Class of Spherically Symmetric Space-Times II: A Unified Generation of Higher Forms
Killing-Yano (KY) two and three forms of a class of spherically symmetric
space-times that includes the well-known Minkowski, Schwarzschild,
Reissner-Nordstrom, Robertson-Walker and six different forms of de Sitter
space-times as special cases are derived in a unified and exhaustive manner. It
is directly proved that while the Schwarzschild and Reissner-Nordstrom
space-times do not accept any KY 3-form and they accept only one 2-form, the
Robertson-Walker space-time admits four KY 2-forms and only one KY 3-form.
Maximal number of KY-forms are obtained for Minkowski and all known forms of de
Sitter space-times. Complete lists comprising explicit expressions of KY-forms
are given.Comment: 28 page
Dynamical Generation of Spacetime Signature by Massive Quantum Fields on a Topologically Non-Trivial Background
The effective potential for a dynamical Wick field (dynamical signature)
induced by the quantum effects of massive fields on a topologically non-trivial
dimensional background is considered. It is shown that when the radius of
the compactified dimension is very small compared with (where
is a proper-time cutoff), a flat metric with Lorentzian signature is
preferred on . When the compactification radius
becomes larger a careful analysis of the 1-loop effective potential indicates
that a Lorentzian signature is preferred in both and and that these
results are relatively stable under metrical perturbations
Diffeomorphism algebra of two dimensional free massless scalar field with signature change
We study a model of free massless scalar fields on a two dimensional cylinder
with metric that admits a change of signature between Lorentzian and Euclidean
type (ET), across the two timelike hypersurfaces (with respect to Lorentzian
region). Considering a long strip-shaped region of the cylinder, denoted by an
angle \theta, as the signature changed region it is shown that the energy
spectrum depends on the angle \theta and in a sense differs from ordinary one
for low energies. Morever diffeomorphism algebra of corresponding infinite
conserved charges is different from '' Virasoro'' algebra and approaches to it
at higher energies. The central term is also modified but does not approach to
the ordinary one at higher energies.Comment: 18 pages, Latex, 2 ps figure
Towards high mobility InSb nanowire devices
We study the low-temperature electron mobility of InSb nanowires. We extract
the mobility at 4.2 Kelvin by means of field effect transport measurements
using a model consisting of a nanowire-transistor with contact resistances.
This model enables an accurate extraction of device parameters, thereby
allowing for a systematic study of the nanowire mobility. We identify factors
affecting the mobility, and after optimization obtain a field effect mobility
of cm/Vs. We further demonstrate the
reproducibility of these mobility values which are among the highest reported
for nanowires. Our investigations indicate that the mobility is currently
limited by adsorption of molecules to the nanowire surface and/or the
substrate.Comment: 13 pages, 5 figures (main text); 7 pages, 2 figures, 2 tables
(supplementary text
On Applications of Campbell's Embedding Theorem
A little known theorem due to Campbell is employed to establish the local
embedding of a wide class of 4-dimensional spacetimes in 5-dimensional
Ricci-flat spaces. An embedding for the class of n-dimensional Einstein spaces
is also found. The local nature of Campbell's theorem is highlighted by
studying the embedding of some lower-dimensional spaces.Comment: 17 pages, standard Latex sourc
- …