Ground state spin and Coulomb blockade peak motion in chaotic quantum dots

We investigate experimentally and theoretically the behavior of Coulomb blockade (CB) peaks in a magnetic field that couples principally to the ground-state spin (rather than the orbital moment) of a chaotic quantum dot. In the first part, we discuss numerically observed features in the magnetic field dependence of CB peak and spacings that unambiguously identify changes in spin S of each ground state for successive numbers of electrons on the dot, N. We next evaluate the probability that the ground state of the dot has a particular spin S, as a function of the exchange strength, J, and external magnetic field, B. In the second part, we describe recent experiments on gate-defined GaAs quantum dots in which Coulomb peak motion and spacing are measured as a function of in-plane magnetic field, allowing changes in spin between N and N+1 electron ground states to be inferred.Comment: To appear in Proceedings of the Nobel Symposium 2000 (Physica Scripta

Some Factors Influencing the Life Span of Golden Hamsters

The golden hamster has found increased use as a laboratory animal over the last two decades. It is of particular interest because it hibernates although its periods of dormancy are short compared to those of other hibernators (1). The complete spectrum of physiological norms should be determined for this common animal just as they were for the laboratory rat. In the list of normal values for the hamster there is little published information on life span (2). This paper will present records of life spans of 126 hamsters kept under controlled laboratory conditions. Approximately 43% of the colony were maintained with a daily light cycle in a coldroom (6 ± 2 ° C.) for about 4 months each winter. Other conditions of the experiment have been described earlier in detail (1). For the purposes of this analysis the cold-exposed group was treated as a homogenous population, in spite of the fact that some of the animals hibernated. This combining of animals was due to the fact that there was such variation in the total duration of hibernation over the winter periods. Some hamsters hibernated for one day, others were in hibernation for a total of 95 days. The systematic pattern of results justifies this approach to this analysis. To be specific, the data treated in this paper look as if cold-exposure with or without hibernation produced the same effects upon the animals, in most respects. Furthermore, the hibernators were distributed nearly equally among four groups of cold-exposed animals. Later analyses will attempt to consider the influence of hibernation as a separate factor. We will ref.er in this report only to the cold-exposed group: this means a mixed group of males and females of two strains of animals, most of which were coldexposed 4 months, but a few of which received only 3 months of cold-exposure at 6° C. and a few weeks at 16° C. About 12 of the males and 10 of the females hibernated for variable periods of time

Defect-induced condensation and central peak at elastic phase transitions

Static and dynamical properties of elastic phase transitions under the influence of short--range defects, which locally increase the transition temperature, are investigated. Our approach is based on a Ginzburg--Landau theory for three--dimensional crystals with one--, two-- or three--dimensional soft sectors, respectively. Systems with a finite concentration $n_{\rm D}$ of quenched, randomly placed defects display a phase transition at a temperature $T_c(n_{\rm D})$, which can be considerably above the transition temperature $T_c^0$ of the pure system. The phonon correlation function is calculated in single--site approximation. For $T>T_c(n_{\rm D})$ a dynamical central peak appears; upon approaching $T_c(n_{\rm D})$, its height diverges and its width vanishes. Using an appropriate self--consistent method, we calculate the spatially inhomogeneous order parameter, the free energy and the specific heat, as well as the dynamical correlation function in the ordered phase. The dynamical central peak disappears again as the temperatur is lowered below $T_c(n_{\rm D})$. The inhomogeneous order parameter causes a static central peak in the scattering cross section, with a finite $k$ width depending on the orientation of the external wave vector ${\bf k}$ relative to the soft sector. The jump in the specific heat at the transition temperatur of the pure system is smeared out by the influence of the defects, leading to a distinct maximum instead. In addition, there emerges a tiny discontinuity of the specific heat at $T_c(n_{\rm D})$. We also discuss the range of validity of the mean--field approach, and provide a more realistic estimate for the transition temperature.Comment: 11 pages, 11 ps-figures, to appear in PR

Gauge dependenceof the order parameter anomalous dimension in the Ginzburg-Landau model and the critical fluctuations in superconductors

The critical fluctuations of superconductors are discussed in a fixed dimension scaling suited to describe the type II regime. The gauge dependence of the anomalous dimension of the scalar field is stablished exactly from the Ward-Takahashi identities. Its fixed point value gives the $\eta$ critical exponent and it is shown that $\eta$ is gauge independent, as expected on physical grounds. In the scaling considered, $\eta$ is found to be zero at 1-loop order, while $\nu\approx 0.63$. This result is just the 1-loop values for the XY model obtained in the fixed dimension renormalization group approach. It is shown that this XY behavior holds at all orders. The result $\eta=\eta_{XY}$ should be contrasted with the negative values frequently reported in the literature.Comment: EuroLaTex, 7 pages, 2 figures, reference updated; version to be published in Europhysics Letter

Conservation-laws-preserving algorithms for spin dynamics simulations

We propose new algorithms for numerical integration of the equations of motion for classical spin systems with fixed spatial site positions. The algorithms are derived on the basis of a mid-point scheme in conjunction with the multiple time staging propagation. Contrary to existing predictor-corrector and decomposition approaches, the algorithms introduced preserve all the integrals of motion inherent in the basic equations. As is demonstrated for a lattice ferromagnet model, the present approach appears to be more efficient even over the recently developed decomposition method.Comment: 13 pages, 2 figure

Spin and Conductance-Peak-Spacing Distributions in Large Quantum Dots: A Density Functional Theory Study

We use spin-density-functional theory to study the spacing between conductance peaks and the ground-state spin of 2D model quantum dots with up to 200 electrons. Distributions for different ranges of electron number are obtained in both symmetric and asymmetric potentials. The even/odd effect is pronounced for small symmetric dots but vanishes for large asymmetric ones, suggesting substantially stronger interaction effects than expected. The fraction of high-spin ground states is remarkably large.Comment: 4 pages, 3 figure

Concentration and mass dependence of transport coefficients and correlation functions in binary mixtures with high mass-asymmetry

Correlation functions and transport coefficients of self-diffusion and shear viscosity of a binary Lennard-Jones mixture with components differing only in their particle mass are studied up to high values of the mass ratio $\mu$, including the limiting case $\mu=\infty$, for different mole fractions $x$. Within a large range of $x$ and $\mu$ the product of the diffusion coefficient of the heavy species $D_{2}$ and the total shear viscosity of the mixture $\eta_{m}$ is found to remain constant, obeying a generalized Stokes-Einstein relation. At high liquid density, large mass ratios lead to a pronounced cage effect that is observable in the mean square displacement, the velocity autocorrelation function and the van Hove correlation function

Exploring the Impact of Knowledge and Social Environment on Influenza Prevention and Transmission in Midwestern United States High School Students

We used data from a convenience sample of 410 Midwestern United States students from six secondary schools to develop parsimonious models for explaining and predicting precautions and illness related to influenza. Scores for knowledge and perceptions were obtained using two-parameter Item Response Theory (IRT) models. Relationships between outcome variables and predictors were verified using Pearson and Spearman correlations, and nested [student within school] fixed effects multinomial logistic regression models were specified from these using Akaike’s Information Criterion (AIC). Neural network models were then formulated as classifiers using 10-fold cross validation to predict precautions and illness. Perceived barriers against taking precautions lowered compliance with the CDC recommended preventative practices of vaccination, hand washing quality, and respiratory etiquette. Perceived complications from influenza illness improved social distancing. Knowledge of the influenza illness was a significant predictor for hand washing frequency and respiratory etiquette. Ethnicity and gender had varying effects on precautions and illness severity, as did school-level effects: enrollment size, proficiency on the state’s biology end-of-course examination, and use of free or reduced lunch. Neural networks were able to predict illness, hand hygiene, and respiratory etiquette with moderate success. Models presented may prove useful for future development of strategies aimed at mitigation of influenza in high school youths. As more data becomes available, health professionals and educators will have the opportunity to test and refine these models

Control and Detection of Singlet-Triplet Mixing in a Random Nuclear Field

We observe mixing between two-electron singlet and triplet states in a double quantum dot, caused by interactions with nuclear spins in the host semiconductor. This mixing is suppressed by applying a small magnetic field, or by increasing the interdot tunnel coupling and thereby the singlet-triplet splitting. Electron transport involving transitions between triplets and singlets in turn polarizes the nuclei, resulting in striking bistabilities. We extract from the fluctuating nuclear field a limitation on the time-averaged spin coherence time T2* of 25 ns. Control of the electron-nuclear interaction will therefore be crucial for the coherent manipulation of individual electron spins.Comment: 4 pages main text, 4 figure

Vortex Origin of Tricritical Point in Ginzburg-Landau Theory

Motivated by recent experimental progress in the critical regime of high-$T_c$ superconductors we show how the tricritical point in a superconductor can be derived from the Ginzburg-Landau theory as a consequence of vortex fluctuations. Our derivation explains why usual renormalization group arguments always produce a first-order transition, in contrast to experimental evidence and Monte Carlo simulations.Comment: 4 pages,1 figur