882 research outputs found
Size dependent tunneling and optical spectroscopy of CdSe quantum rods
Photoluminescence excitation spectroscopy and scanning tunneling spectroscopy
are used to study the electronic states in CdSe quantum rods that manifest a
transition from a zero dimensional to a one dimensional quantum confined
structure. Both optical and tunneling spectra show that the level structure
depends primarily on the rod diameter and not on length. With increasing
diameter, the band-gap and the excited state level spacings shift to the red.
The level structure was assigned using a multi-band effective-mass model,
showing a similar dependence on rod dimensions.Comment: Accepted to PRL (nearly final version). 4 pages in revtex, 4 figure
Non(anti)commutative SYM theory: Renormalization in superspace
We present a systematic investigation of one-loop renormalizability for
nonanticommutative N=1/2, U(N) SYM theory in superspace. We first discuss
classical gauge invariance of the pure gauge theory and show that in
contradistinction to the ordinary anticommutative case, different
representations of supercovariant derivatives and field strengths do not lead
to equivalent descriptions of the theory. Subsequently we develop background
field methods which allow us to compute a manifestly covariant gauge effective
action. One-loop evaluation of divergent contributions reveals that the theory
simply obtained from the ordinary one by trading products for star products is
not renormalizable. In the case of SYM with no matter we present a N=1/2
improved action which we show to be one-loop renormalizable and which is
perfectly compatible with the algebraic structure of the star product. For this
action we compute the beta functions. A brief discussion on the inclusion of
chiral matter is also presented.Comment: Latex file, 59 pages, 10 figures, One reference adde
A pseudopotential study of electron-hole excitations in colloidal, free-standing InAs quantum dots
Excitonic spectra are calculated for free-standing, surface passivated InAs
quantum dots using atomic pseudopotentials for the single-particle states and
screened Coulomb interactions for the two-body terms. We present an analysis of
the single particle states involved in each excitation in terms of their
angular momenta and Bloch-wave parentage. We find that (i) in agreement with
other pseudopotential studies of CdSe and InP quantum dots, but in contrast to
k.p calculations, dot states wavefunction exhibit strong odd-even angular
momentum envelope function mixing (e.g. with ) and large
valence-conduction coupling. (ii) While the pseudopotential approach produced
very good agreement with experiment for free-standing, colloidal CdSe and InP
dots, and for self-assembled (GaAs-embedded) InAs dots, here the predicted
spectrum does {\em not} agree well with the measured (ensemble average over dot
sizes) spectra. (1) Our calculated excitonic gap is larger than the PL measure
one, and (2) while the spacing between the lowest excitons is reproduced, the
spacings between higher excitons is not fit well. Discrepancy (1) could result
from surface states emission. As for (2), agreement is improved when account is
taken of the finite size distribution in the experimental data. (iii) We find
that the single particle gap scales as (not ), that the
screened (unscreened) electron-hole Coulomb interaction scales as
(), and that the eccitonic gap sclaes as . These scaling
laws are different from those expected from simple models.Comment: 12 postscript figure
Supersymmetric NambuJona-Lasinio Model on four-dimensional Non(anti)commutative Superspace
We construct the Lagrangian of the four-dimensional generalized
supersymmetric NambuJona-Lasinio (SNJL) model, which has
supersymmetry (SUSY) on non(anti)commutative superspace. A special attention is
paid to the examination on the nonperturbative quantum dynamics: The phenomenon
of dynamical-symmetry-breaking/mass-generation on the deformed superspace is
investigated. The model Lagrangian and the method of SUSY auxiliary fields of
composites are examined in terms of component fields. We derive the effective
action, examine it, and solve the gap equation for self-consistent mass
parameters.Comment: 16 pages, TeX mistakes corrected, accepted for publication in JHEP,
25 Jan. 200
The modification of polytetrafluoroethylene surface using hydroxyapatite coating deposited by RF-magnetron method
The ossteointegration rate of the polytetrafluoroethylene (PTFE) dental implants is related to their composition and properties of surface. Osteoconductive calcium phosphate coatings promote bone healing and apposition, leading to the rapid biological fixation of implants. The deposition of the hydroxyapatite (HA) coating on the PTFE surface by the RF magnetron sputtering method and the influence of this modification on the wettability, and physicochemical properties are presented in the present work
New supersymmetric higher-derivative couplings: Full N=2 superspace does not count!
An extended class of N=2 locally supersymmetric invariants with
higher-derivative couplings based on full superspace integrals, is constructed.
These invariants may depend on unrestricted chiral supermultiplets, on vector
supermultiplets and on the Weyl supermultiplet. Supersymmetry is realized
off-shell. A non-renormalization theorem is proven according to which none of
these invariants can contribute to the entropy and electric charges of BPS
black holes. Some of these invariants may be relevant for topological string
deformations.Comment: 24 pages, v2: version published in JHEP, one reference added and
typos corrected, v3: reference adde
Driving current through single organic molecules
We investigate electronic transport through two types of conjugated
molecules. Mechanically controlled break-junctions are used to couple thiol
endgroups of single molecules to two gold electrodes. Current-voltage
characteristics (IVs) of the metal-molecule-metal system are observed. These
IVs reproduce the spatial symmetry of the molecules with respect to the
direction of current flow. We hereby unambigously detect an intrinsic property
of the molecule, and are able to distinguish the influence of both the molecule
and the contact to the metal electrodes on the transport properties of the
compound system.Comment: 4 pages, 5 figure
Non(anti)commutative N=(1,1/2) Supersymmetric U(1) Gauge Theory
We study a reduction of deformation parameters in non(anti)commutative N=2
harmonic superspace to those in non(anti)commutative N=1 superspace. By this
reduction we obtain the exact gauge and supersymmetry transformations in the
Wess-Zumino gauge of non(anti)commutative N=2 supersymmetric U(1) gauge theory
defined in the deformed harmonic superspace. We also find that the action with
the first order correction in the deformation parameter reduces to the one in
the N=1 superspace by some field redefinition. We construct deformed N=(1,1/2)
supersymmetry in N=2 supersymmetric U(1) gauge theory in non(anti)commutative
N=1 superspace.Comment: 30 pages, LaTeX, V2: a reference adde
Size Dependence of Metal-Insulator Transition in Stoichiometric Fe3O4 Nanocrystals
Magnetite (Fe3O4) is one of the most actively studied materials with a famous
metal-insulator transition (MIT), so-called the Verwey transition at around 123
K. Despite the recent progress in synthesis and characterization of Fe3O4
nanocrystals (NCs), it is still an open question how the Verwey transition
changes on a nanometer scale. We herein report the systematic studies on size
dependence of the Verwey transition of stoichiometric Fe3O4 NCs. We have
successfully synthesized stoichiometric and uniform-sized Fe3O4 NCs with sizes
ranging from 5 to 100 nm. These stoichiometric Fe3O4 NCs show the Verwey
transition when they are characterized by conductance, magnetization, cryo-XRD,
and heat capacity measurements. The Verwey transition is weakly size-dependent
and becomes suppressed in NCs smaller than 20 nm before disappearing completely
for less than 6 nm, which is a clear, yet highly interesting indication of a
size effect of this well-known phenomena. Our current work will shed new light
on this ages-old problem of Verwey transition.Comment: 18 pages, 4 figures, Nano Letters (accepted
Chiral effective potential in non-commutative Wess-Zumino model
We study a structure of holomorphic quantum contributions to the effective
action for noncommutative Wess-Zumino model. Using the symbol
operator techniques we present the one-loop chiral effective potential in a
form of integral over proper time of the appropriate heat kernel. We prove that
this kernel can be exactly found. As a result we obtain the exact integral
representation of the one-loop effective potential. Also we study the expansion
of the effective potential in a series in powers of the chiral superfield
and derivative and construct a procedure for systematic
calculation of the coefficients in the series. We show that all terms in the
series without derivatives can be summed up in an explicit form.Comment: LaTeX, JHEP style, 32 pages, typos corrected, references adde
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