We study a lattice bipolaron on a staggered triangular ladder and triangular and hexagonal lattices with both long-range electron-phonon interaction and strong Coulomb repulsion using a novel continuous-time quantum Monte Carlo algorithm to solve the two-particle Coulomb-Fröhlich model. The algorithm is preceded by an exact integration over phonon degrees of freedom, and as such is extremely efficient. The bipolaron effective mass and radius are computed. Bipolarons on lattices constructed from triangular plaquettes have a novel crablike motion, and are small but very light over a wide range of parameters. We discuss the conditions under which such particles may form a Bose-Einstein condensate with high transition temperature, proposing a route to room temperature superconductivity.

The thermoelectric and physical properties of superlattices consisting of modulation doped Ge quantum wells inside Si 1Ày Ge y barriers are presented, which demonstrate enhancements in the thermoelectric figure of merit, ZT, and power factor at room temperature over bulk Ge, Si 1Ày Ge y , and Si/Ge superlattice materials. Mobility spectrum analysis along with low temperature measurements indicate that the high power factors are dominated by the high electrical conductivity from the modulation doping. Comparison of the results with modelling using the Boltzmann transport equation with scattering parameters obtained from Monte Carlo techniques indicates that a high threading dislocation density is also limiting the performance. The analysis suggests routes to higher thermoelectric performance at room temperature from Si-based materials that can be fabricated using micro-and nano-fabrication techniques. V C 2013 AIP Publishing LLC.

We have recently discovered a mistake in the strong coupling, long-range interaction results in this paper, changing Figs. 9-12. The corrected figures are shown below. We also note that ␥ computed by Eq. ͑23͒ should read 0.32͑0͒ for the triangular lattice and 0.33͑4͒ for the square lattice with Fröhlich interaction. We note that the Lang-Firsov limit and spectra now agree even better with our numerical results at large ͑Ͼ5͒ and , Fig. 10. Numerical masses of lattice Fröhlich polarons ͑Fig. 9͒ in the intermediate coupling and adiabicity regimes are instrumental for better understanding of transport properties and angle-resolved photo-emission spectra of cuprate superconductors, where the finite-range Fröhlich interaction with c-axis polarized optical phonons is important. 1,2 The conclusions of the paper are unchanged. FIG. 9. Inverse mass of the Fröhlich polaron on the square and triangular lattices when = 1. Inset: approaching the strong coupling limit at large .

BackgroundThousands of air bubbles enter the cerebral circulation during cardiac surgery, but whether high numbers of bubbles explain post-operative cognitive decline is currently controversial. This study estimates the size distribution of air bubbles and volume of air entering the cerebral arteries intra-operatively based on analysis of transcranial Doppler ultrasound data.MethodsTranscranial Doppler ultrasound recordings from ten patients undergoing heart surgery were analysed for the presence of embolic signals. The backscattered intensity of each embolic signal was modelled based on ultrasound scattering theory to provide an estimate of bubble diameter. The impact of showers of bubbles on cerebral blood-flow was then investigated using patient-specific Monte-Carlo simulations to model the accumulation and clearance of bubbles within a model vasculature.ResultsAnalysis of Doppler ultrasound recordings revealed a minimum of 371 and maximum of 6476 bubbles entering the middle cerebral artery territories during surgery. This was estimated to correspond to a total volume of air ranging between 0.003 and 0.12 mL. Based on analysis of a total of 18667 embolic signals, the median diameter of bubbles entering the cerebral arteries was 33 μm (IQR: 18 to 69 μm). Although bubble diameters ranged from ~5 μm to 3.5 mm, the majority (85%) were less than 100 μm. Numerous small bubbles detected during cardiopulmonary bypass were estimated by Monte-Carlo simulation to be benign. However, during weaning from bypass, showers containing large macro-bubbles were observed, which were estimated to transiently affect up to 2.2% of arterioles.ConclusionsDetailed analysis of Doppler ultrasound data can be used to provide an estimate of bubble diameter, total volume of air, and the likely impact of embolic showers on cerebral blood flow. Although bubbles are alarmingly numerous during surgery, our simulations suggest that the majority of bubbles are too small to be harmful.

Do the complex processes of angiogenesis during organism development ultimately lead to a near optimal coronary vasculature in the organs of adult mammals? We examine this hypothesis using a powerful and universal method, built on physical and physiological principles, for the determination of globally energetically optimal arterial trees. The method is based on simulated annealing, and can be used to examine arteries in hollow organs with arbitrary tissue geometries. We demonstrate that the approach can generate in silico vasculatures which closely match porcine anatomical data for the coronary arteries on all length scales, and that the optimized arterial trees improve systematically as computational time increases. The method presented here is general, and could in principle be used to examine the arteries of other organs. Potential applications include improvement of medical imaging analysis and the design of vascular trees for artificial organs.