1,480 research outputs found
Perturbative analysis of anharmonic chains of oscillators out of equilibrium
We compute the first-order correction to the correlation functions of the
stationary state of a stochastically forced harmonic chain out of equilibrium
when a small on-site anharmonic potential is added. This is achieved by
deriving a suitable formula for the covariance matrix of the invariant state.
We find that the first-order correction of the heat current does not depend on
the size of the system. Second, the temperature profile is linear when the
harmonic part of the on-site potential is zero. The sign of the gradient of the
profile, however, is opposite to the sign of the temperature difference of the
two heat baths.Comment: 26 pages, 2 figures, corrected typo
Transitions in sex determination mechanisms through parental antagonism
Parental antagonism (PA) occurs when the fitness effects of a gene depend on the parent from which it is inherited. Such genes may become enriched on sex chromosomes, due to their biased inheritance patterns. Although various sex determination (SD) genes exhibit parent-of-origin effects themselves, and between-parent conflict over offspring sex may affect SD, PA itself has not been considered as a driver of SD transitions. Here, I present a model to investigate the scope for transitions in SD mechanisms through PA. My model assumes an ancestral SD locus linked to a PA gene, as well as an autosomal PA gene in whose vicinity a novel SD gene arises. Transitions between functionally-homologous genes are found to depend on the fitness effects of both PA genes and their linkage to nearby SD genes. Transitions between male and female heterogamety by the invasion of a dominant SD gene are however nearly unconstrained. This also allows for back-and-forth dynamics where the ancestral SD and novel SD genes constantly evolve to be dominant over each other. These results further underline the malleability of SD mechanisms, and the need to consider parent-of-origin effects in driving transitions in SD, through proximate and/or ultimate means
Normal Heat Conductivity in a strongly pinned chain of anharmonic oscillators
We consider a chain of coupled and strongly pinned anharmonic oscillators
subject to a non-equilibrium random forcing. Assuming that the stationary state
is approximately Gaussian, we first derive a stationary Boltzmann equation. By
localizing the involved resonances, we next invert the linearized collision
operator and compute the heat conductivity. In particular, we show that the
Gaussian approximation yields a finite conductivity
, for the anharmonic coupling
strength.Comment: Introduction and conclusion modifie
Real-time Text Queries with Tunable Term Pair Indexes
Term proximity scoring is an established means in information retrieval for improving result quality of full-text queries. Integrating such proximity scores into efficient query processing, however, has not been equally well studied. Existing methods make use of precomputed lists of documents where tuples of terms, usually pairs, occur together, usually incurring a huge index size compared to term-only indexes. This paper introduces a joint framework for trading off index size and result quality, and provides optimization techniques for tuning precomputed indexes towards either maximal result quality or maximal query processing performance, given an upper bound for the index size. The framework allows to selectively materialize lists for pairs based on a query log to further reduce index size. Extensive experiments with two large text collections demonstrate runtime improvements of several orders of magnitude over existing text-based processing techniques with reasonable index sizes
Spin Coherence and N ESEEM Effects of Nitrogen-Vacancy Centers in Diamond with X-band Pulsed ESR
Pulsed ESR experiments are reported for ensembles of negatively-charged
nitrogen-vacancy centers (NV) in diamonds at X-band magnetic fields
(280-400 mT) and low temperatures (2-70 K). The NV centers in synthetic
type IIb diamonds (nitrogen impurity concentration ~ppm) are prepared with
bulk concentrations of cm to cm
by high-energy electron irradiation and subsequent annealing. We find that a
proper post-radiation anneal (1000C for 60 mins) is critically
important to repair the radiation damage and to recover long electron spin
coherence times for NVs. After the annealing, spin coherence times of T~ms at 5~K are achieved, being only limited by C nuclear spectral
diffusion in natural abundance diamonds. At X-band magnetic fields, strong
electron spin echo envelope modulation (ESEEM) is observed originating from the
central N nucleus. The ESEEM spectral analysis allows for accurate
determination of the N nuclear hypefine and quadrupole tensors. In
addition, the ESEEM effects from two proximal C sites (second-nearest
neighbor and fourth-nearest neighbor) are resolved and the respective C
hyperfine coupling constants are extracted.Comment: 10 pages, 5 figure
Surface code architecture for donors and dots in silicon with imprecise and nonuniform qubit couplings
A scaled quantum computer with donor spins in silicon would benefit from a
viable semiconductor framework and a strong inherent decoupling of the qubits
from the noisy environment. Coupling neighbouring spins via the natural
exchange interaction according to current designs requires gate control
structures with extremely small length scales. We present a silicon
architecture where bismuth donors with long coherence times are coupled to
electrons that can shuttle between adjacent quantum dots, thus relaxing the
pitch requirements and allowing space between donors for classical control
devices. An adiabatic SWAP operation within each donor/dot pair solves the
scalability issues intrinsic to exchange-based two-qubit gates, as it does not
rely on sub-nanometer precision in donor placement and is robust against noise
in the control fields. We use this SWAP together with well established global
microwave Rabi pulses and parallel electron shuttling to construct a surface
code that needs minimal, feasible local control.Comment: Published version - more detailed discussions, robustness to
dephasing pointed out additionall
Critical issues in the formation of quantum computer test structures by ion implantation
The formation of quantum computer test structures in silicon by ion
implantation enables the characterization of spin readout mechanisms with
ensembles of dopant atoms and the development of single atom devices. We
briefly review recent results in the characterization of spin dependent
transport and single ion doping and then discuss the diffusion and segregation
behaviour of phosphorus, antimony and bismuth ions from low fluence, low energy
implantations as characterized through depth profiling by secondary ion mass
spectrometry (SIMS). Both phosphorus and bismuth are found to segregate to the
SiO2/Si interface during activation anneals, while antimony diffusion is found
to be minimal. An effect of the ion charge state on the range of antimony ions,
121Sb25+, in SiO2/Si is also discussed
Detection of low energy single ion impacts in micron scale transistors at room temperature
We report the detection of single ion impacts through monitoring of changes
in the source-drain currents of field effect transistors (FET) at room
temperature. Implant apertures are formed in the interlayer dielectrics and
gate electrodes of planar, micro-scale FETs by electron beam assisted etching.
FET currents increase due to the generation of positively charged defects in
gate oxides when ions (121Sb12+, 14+, Xe6+; 50 to 70 keV) impinge into channel
regions. Implant damage is repaired by rapid thermal annealing, enabling
iterative cycles of device doping and electrical characterization for
development of single atom devices and studies of dopant fluctuation effects
- …