Electrical read-out of the local nuclear polarization in the quantum Hall effect

It is demonstrated that the now well-established `flip-flop' mechanism of spin exchange between electrons and nuclei in the quantum Hall effect can be reversed. We use a sample geometry which utilizes separately contacted edge states to establish a local nuclear spin polarization --close to the maximum value achievable-- by driving a current between electron states of different spin orientation. When the externally applied current is switched off, the sample exhibits an output voltage of up to a few tenths of a meV, which decays with a time constant typical for the nuclear spin relaxation. The surprizing fact that a sample with a local nuclear spin polarization can act as a source of energy and that this energy is well above the nuclear Zeeman splitting is explained by a simple model which takes into account the effect of a local Overhauser shift on the edge state reconstruction.Comment: Submitted to Phys. Rev. Let

Waveform sampling using an adiabatically driven electron ratchet in a two-dimensional electron system

We utilize a time-periodic ratchet-like potential modulation imposed onto a two-dimensional electron system inside a GaAs/Al$_x$Ga$_{1-x}$As heterostructure to evoke a net dc pumping current. The modulation is induced by two sets of interdigitated gates, interlacing off center, which can be independently addressed. When the transducers are driven by two identical but phase-shifted ac signals, a lateral dc pumping current $I(\phi)$ results, which strongly depends on both, the phase shift $\phi$ and the waveform $V(t)$ of the imposed gate voltages. We find that for different periodic signals, the phase dependence $I(\phi)$ closely resembles $V(t)$. A simple linear model of adiabatic pumping in two-dimensional electron systems is presented, which reproduces well our experimental findings.Comment: 3 figure

Observation of two relaxation mechanisms in transport between spin split edge states at high imbalance

Using a quasi-Corbino geometry to directly study electron transport between spin-split edge states, we find a pronounced hysteresis in the I-V curves, originating from slow relaxation processes. We attribute this long-time relaxation to the formation of a dynamic nuclear polarization near the sample edge. The determined characteristic relaxation times are 25 s and 200 s which points to the presence of two different relaxation mechanisms. The two time constants are ascribed to the formation of a local nuclear polarization due to flip-flop processes and the diffusion of nuclear spins.Comment: Submitted to PR

Performance Limitations of Flat Histogram Methods and Optimality of Wang-Landau Sampling

We determine the optimal scaling of local-update flat-histogram methods with system size by using a perfect flat-histogram scheme based on the exact density of states of 2D Ising models.The typical tunneling time needed to sample the entire bandwidth does not scale with the number of spins N as the minimal N^2 of an unbiased random walk in energy space. While the scaling is power law for the ferromagnetic and fully frustrated Ising model, for the +/- J nearest-neighbor spin glass the distribution of tunneling times is governed by a fat-tailed Frechet extremal value distribution that obeys exponential scaling. We find that the Wang-Landau algorithm shows the same scaling as the perfect scheme and is thus optimal.Comment: 5 pages, 6 figure

All-optical formation of a Bose-Einstein condensate for applications in scanning electron microscopy

We report on the production of a F=1 spinor condensate of 87Rb atoms in a single beam optical dipole trap formed by a focused CO2 laser. The condensate is produced 13mm below the tip of a scanning electron microscope employing standard all-optical techniques. The condensate fraction contains up to 100,000 atoms and we achieve a duty cycle of less than 10s.Comment: 5 pages, 4 figure

Substitutions of red meat, poultry and fish and risk of myocardial infarction

AbstractRed meat has been suggested to be adversely associated with risk of myocardial infarction (MI), but previous studies have rarely taken replacement foods into consideration. We aimed to investigate optimal substitutions between and within the food groups of red meat, poultry and fish for MI prevention. We followed up 55 171 women and men aged 50–64 years with no known history of MI at recruitment. Diet was assessed by a validated 192-item FFQ at baseline. Adjusted Cox proportional hazard models were used to calculate hazard ratios (HR) and 95 % CI for specified food substitutions of 150 g/week. During a median follow-up time of 13·6 years, we identified 656 female and 1694 male cases. Among women, the HR for replacing red meat with fatty fish was 0·76 (95 % CI 0·64, 0·89), whereas the HR for replacing red meat with lean fish was 1·00 (95 % CI 0·89, 1·14). Similarly, replacing poultry with fatty but not lean fish was inversely associated with MI: the HR was 0·81 (95 % CI 0·67, 0·98) for fatty fish and was 1·08 (95 % CI 0·92, 1·27) for lean fish. The HR for replacing lean with fatty fish was 0·75 (95 % CI 0·60, 0·94). Replacing processed with unprocessed red meat was not associated with MI. Among men, a similar pattern was found, although the associations were not statistically significant. This study suggests that replacing red meat, poultry or lean fish with fatty fish is associated with a lower risk of MI.</jats:p

Substitution of meat and fish with vegetables or potatoes and risk of myocardial infarction

AbstractRed meat has been suggested to be adversely associated with risk of myocardial infarction (MI), whereas vegetable consumption has been found to be protective. The aim of this study was to investigate substitutions of red meat, poultry and fish with vegetables or potatoes for MI prevention. We followed up 29 142 women and 26 029 men in the Danish Diet, Cancer and Health study aged 50–64 years with no known history of MI at baseline. Diet was assessed by a validated 192-item FFQ at baseline. Adjusted Cox proportional hazard models were used to calculate hazard ratios (HR) and 95 % CI for MI associated with specified food substitutions of 150 g/week. During a median follow-up of 13·6 years, we identified 656 female and 1694 male cases. Among women, the HR for MI when replacing red meat with vegetables was 0·94 (95 % CI 0·90, 0·98). Replacing fatty fish with vegetables was associated with a higher risk of MI (HR 1·23; 95 % CI 1·05, 1·45), whereas an inverse, statistically non-significant association was found for lean fish (HR 0·93; 95 % CI 0·83, 1·05). Substituting poultry with vegetables was not associated with risk of MI (HR 1·00; 95 % CI 0·90, 1·11). Findings for substitution with potatoes were similar to findings for vegetables. Among men, a similar pattern was observed, but the associations were weak and mostly statistically non-significant. This study suggests that replacing red meat with vegetables or potatoes is associated with a lower risk of MI, whereas replacing fatty fish with vegetables or potatoes is associated with a higher risk of MI.</jats:p

Manifestation of the bulk phase transition in the edge energy spectrum in a two dimensional bilayer electron system

We use a quasi-Corbino sample geometry with independent contacts to different edge states in the quantum Hall effect regime to investigate the edge energy spectrum of a bilayer electron system at total filling factor $\nu=2$. By analyzing non-linear $I-V$ curves in normal and tilted magnetic fields we conclude that the edge energy spectrum is in a close connection with the bulk one. At the bulk phase transition spin-singlet - canted antiferromagnetic phase $I-V$ curve becomes to be linear, indicating the disappearance or strong narrowing of the $\nu=1$ incompressible strip at the edge of the sample.Comment: 5 pages, 5 figure

Role of phason-defects on the conductance of a 1-d quasicrystal

We have studied the influence of a particular kind of phason-defect on the Landauer resistance of a Fibonacci chain. Depending on parameters, we sometimes find the resistance to decrease upon introduction of defect or temperature, a behavior that also appears in real quasicrystalline materials. We demonstrate essential differences between a standard tight-binding model and a full continuous model. In the continuous case, we study the conductance in relation to the underlying chaotic map and its invariant. Close to conducting points, where the invariant vanishes, and in the majority of cases studied, the resistance is found to decrease upon introduction of a defect. Subtle interference effects between a sudden phason-change in the structure and the phase of the wavefunction are also found, and these give rise to resistive behaviors that produce exceedingly simple and regular patterns.Comment: 12 pages, special macros jnl.tex,reforder.tex, eqnorder.tex. arXiv admin note: original tex thoroughly broken, figures missing. Modified so that tex compiles, original renamed .tex.orig in source