21,710 research outputs found
Detection of charge motion in a non-metallic silicon isolated double quantum dot
As semiconductor device dimensions are reduced to the nanometer scale,
effects of high defect density surfaces on the transport properties become
important to the extent that the metallic character that prevails in large and
highly doped structures is lost and the use of quantum dots for charge sensing
becomes complex. Here we have investigated the mechanism behind the detection
of electron motion inside an electrically isolated double quantum dot that is
capacitively coupled to a single electron transistor, both fabricated from
highly phosphorous doped silicon wafers. Despite, the absence of a direct
charge transfer between the detector and the double dot structure, an efficient
detection is obtained. In particular, unusually large Coulomb peak shifts in
gate voltage are observed. Results are explained in terms of charge
rearrangement and the presence of inelastic cotunneling via states at the
periphery of the single electron transistor dot
Global attractors for Cahn-Hilliard equations with non constant mobility
We address, in a three-dimensional spatial setting, both the viscous and the
standard Cahn-Hilliard equation with a nonconstant mobility coefficient. As it
was shown in J.W. Barrett and J.W. Blowey, Math. Comp., 68 (1999), 487-517, one
cannot expect uniqueness of the solution to the related initial and boundary
value problems. Nevertheless, referring to J. Ball's theory of generalized
semiflows, we are able to prove existence of compact quasi-invariant global
attractors for the associated dynamical processes settled in the natural
"finite energy" space. A key point in the proof is a careful use of the energy
equality, combined with the derivation of a "local compactness" estimate for
systems with supercritical nonlinearities, which may have an independent
interest. Under growth restrictions on the configuration potential, we also
show existence of a compact global attractor for the semiflow generated by the
(weaker) solutions to the nonviscous equation characterized by a "finite
entropy" condition
Large-N phase transition in lattice 2-d principal chiral models
We investigate the large-N critical behavior of 2-d lattice chiral models by
Monte Carlo simulations of U(N) and SU(N) groups at large N. Numerical results
confirm strong coupling analyses, i.e. the existence of a large-N second order
phase transition at a finite .Comment: 12 pages, Revtex, 8 uuencoded postscript figure
SU(N) chiral gauge theories on the lattice
We extend the construction of lattice chiral gauge theories based on
non-perturbative gauge fixing to the non-abelian case. A key ingredient is that
fermion doublers can be avoided at a novel type of critical point which is only
accessible through gauge fixing, as we have shown before in the abelian case.
The new ingredient allowing us to deal with the non-abelian case as well is the
use of equivariant gauge fixing, which handles Gribov copies correctly, and
avoids Neuberger's no-go theorem. We use this method in order to gauge fix the
non-abelian group (which we will take to be SU(N)) down to its maximal abelian
subgroup. Obtaining an undoubled, chiral fermion content requires us to
gauge-fix also the remaining abelian gauge symmetry. This modifies the
equivariant BRST identities, but their use in proving unitarity remains intact,
as we show in perturbation theory. On the lattice, equivariant BRST symmetry as
well as the abelian gauge invariance are broken, and a judiciously chosen
irrelevant term must be added to the lattice gauge-fixing action in order to
have access to the desired critical point in the phase diagram. We argue that
gauge invariance is restored in the continuum limit by adjusting a finite
number of counter terms. We emphasize that weak-coupling perturbation theory
applies at the critical point which defines the continuum limit of our lattice
chiral gauge theory.Comment: 39 pages, 3 figures, A number of clarifications adde
Dynamics of magnetization coupled to a thermal bath of elastic modes
We study the dynamics of magnetization coupled to a thermal bath of elastic
modes using a system plus reservoir approach with realistic magnetoelastic
coupling. After integrating out the elastic modes we obtain a self-contained
equation for the dynamics of the magnetization.
We find explicit expressions for the memory friction kernel and hence, {\em
via} the Fluctuation-Dissipation
Theorem, for the spectral density of the magnetization thermal fluctuations.
For magnetic samples in which the single domain approximation is valid, we
derive an equation for the dynamics of the uniform mode.
Finally we apply this equation to study the dynamics of the uniform
magnetization mode in insulating ferromagnetic thin films.
As experimental consequences we find that the fluctuation correlation time is
of the order of the ratio between the film thickness, , and the speed of
sound in the magnet and that the line-width of the ferromagnetic resonance peak
should scale as where is the magnetoelastic coupling constant.Comment: Revised version as appeared in print. 12 pages 9 figure
Competition between excitonic gap generation and disorder scattering in graphene
We study the disorder effect on the excitonic gap generation caused by strong
Coulomb interaction in graphene. By solving the self-consistently coupled
equations of dynamical fermion gap and disorder scattering rate ,
we found a critical line on the plane of interaction strength and
disorder strength . The phase diagram is divided into two regions: in the
region with large and small , and ; in the
other region, and for nonzero . In particular, there
is no coexistence of finite fermion gap and finite scattering rate. These
results imply a strong competition between excitonic gap generation and
disorder scattering. This conclusion does not change when an additional contact
four-fermion interaction is included. For sufficiently large , the
growing disorder may drive a quantum phase transition from an excitonic
insulator to a metal.Comment: 8 pages, 1 figur
Entanglement Controlled Single-Electron Transmittivity
We consider a system consisting of single electrons moving along a 1D wire in
the presence of two magnetic impurities. Such system shows strong analogies
with a Fabry - Perot interferometer in which the impurities play the role of
two mirrors with a quantum degree of freedom: the spin. We have analysed the
electron transmittivity of the wire in the presence of entanglement between the
impurity spins. The main result of our analysis is that, for suitable values of
the electron momentum, there are two maximally entangled state of the impurity
spins the first of which makes the wire transparent whatever the electron spin
state while the other strongly inhibits the electron transmittivity. Such
predicted striking effect is experimentally observable with present day
technology.Comment: Published version (6 figures
The effects of superconductor-stabilizer interfacial resistance on quench of a pancake coil made out of coated conductor
We present the results of numerical analysis of normal zone propagation in a
stack of coated conductors which imitates a pancake coil.
Our main purpose is to determine whether the quench protection quality of such
coils can be substantially improved by increased contact resistance between the
superconducting film and the stabilizer. We show that with increased contact
resistance the speed of normal zone propagation increases, the detection of a
normal zone inside the coil becomes possible earlier, when the peak temperature
inside the normal zone is lower, and stability margins shrink. Thus, increasing
contact resistance may become a viable option for improving the prospects of
coated conductors for high magnets applications.Comment: 9 pages, 4 figure
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