Casimir interactions in graphene systems

The non-retarded Casimir interaction (van der Waals interaction) between two free standing graphene sheets as well as between a graphene sheet and a substrate is determined. An exact analytical expression is given for the dielectric function of graphene along the imaginary frequency axis within the random phase approximation for arbitrary frequency, wave vector, and doping.Comment: 4 pages, 4 figure

BCS-BEC crossover in bilayers of cold fermionic polar molecules

We investigate the quantum and thermal phase diagram of fermionic polar molecules loaded in a bilayer trapping potential with perpendicular dipole moment. We use both a BCS-theory approach that is most reliable at weak coupling and a strong-coupling approach that considers the two-body bound dimer states with one molecule in each layer as the relevant degree of freedom. The system ground state is a Bose-Einstein condensate (BEC) of dimer bound states in the low-density limit and a paired superfluid (BCS) state in the high-density limit. At zero temperature, the intralayer repulsion is found to broaden the regime of BCS-BEC crossover and can potentially induce system collapse through the softening of roton excitations. The BCS theory and the strongly coupled dimer picture yield similar predictions for the parameters of the crossover regime. The Berezinskii-Kosterlitz-Thouless transition temperature of the dimer superfluid is also calculated. The crossover can be driven by many-body effects and is strongly affected by the intralayer interaction which was ignored in previous studies

Dielectric function and plasmons in graphene

The electromagnetic response of graphene, expressed by the dielectric function, and the spectrum of collective excitations are studied as a function of wave vector and frequency. Our calculation is based on the full band structure, calculated within the tight-binding approximation. As a result, we find plasmons whose dispersion is similar to that obtained in the single-valley approximation by Dirac fermions. In contrast to the latter, however, we find a stronger damping of the plasmon modes due to inter-band absorption. Our calculation also reveals effects due to deviations from the linear Dirac spectrum as we increase the Fermi energy, indicating an anisotropic behavior with respect to the wave vector of the external electromagnetic field

Dynamical polarization, screening, and plasmons in gapped graphene

The one-loop polarization function of graphene has been calculated at zero temperature for arbitrary wavevector, frequency, chemical potential (doping), and band gap. The result is expressed in terms of elementary functions and is used to find the dispersion of the plasmon mode and the static screening within the random phase approximation. At long wavelengths the usual square root behaviour of plasmon spectra for two-dimensional (2D) systems is obtained. The presence of a small (compared to a chemical potential) gap leads to the appearance of a new undamped plasmon mode. At greater values of the gap this mode merges with the long-wavelength one, and vanishes when the Fermi level enters the gap. The screening of charged impurities at large distances differs from that in gapless graphene by slower decay of Friedel oscillations ($1/r^2$ instead of $1/r^3$), similarly to conventional 2D systems.Comment: 8 pages, 8 figures, v2: to match published versio

Subsumption Reduces Dataset Dimensionality Without Decreasing Performance of a Machine Learning Classifier

When Features in a High Dimension Dataset Are Organized Hierarchically, There is an Inherent Opportunity to Reduce Dimensionality. Since More Specific Concepts Are Subsumed by More General Concepts, Subsumption Can Be Applied Successively to Reduce Dimensionality. We Tested Whether Sub-Sumption Could Reduce the Dimensionality of a Disease Dataset Without Impairing Classification Accuracy. We Started with a Dataset that Had 168 Neurological Patients, 14 Diagnoses, and 293 Unique Features. We Applied Subsumption Repeatedly to Create Eight Successively Smaller Datasets, Ranging from 293 Dimensions in the Largest Dataset to 11 Dimensions in the Smallest Dataset. We Tested a MLP Classifier on All Eight Datasets. Precision, Recall, Accuracy, and Validation Declined Only at the Lowest Dimensionality. Our Preliminary Results Suggest that When Features in a High Dimension Dataset Are Derived from a Hierarchical Ontology, Subsumption is a Viable Strategy to Reduce Dimensionality.Clinical Relevance - Datasets Derived from Electronic Health Records Are Often of High Dimensionality. If Features in the Dataset Are based on Concepts from a Hierarchical Ontology, Subsumption Can Reduce Dimensionality

Zetos: Ein Kulturbelastungssystem für trabekuläre Knochen. Untersuchung eines Belastungssignals mit verschiedenen Intensitäten an bovinen Knochenzylindern

Das Ziel dieses Versuches war es, den Einfluss verschiedener Intensitäten (1000, 1500, 2000, 3000, 4000µstrain) eines physiologischen Belastungssignals (Springen) auf trabekulären Knochen zu untersuchen. Dazu wurde der Zetos: Ein Kultur- und Belastungssystem für trabekulären Knochen verwendet. 48 bovine Knochenexplantate wurden randomisiert und in 6 Gruppen (je 8) eingeteilt. 5 Belastungsgruppen und eine Kontrollgruppe. Die Knochenexplantate wurden für 26 Tage kultiviert (DMEM HighGlucose Medium) und an 23 Tagen mechanisch belastet. Vor jeder Belastung wurde täglich (belastete Gruppen) und alle 3 Tage (Kontrollgruppe) die Steifigkeit (Young's Modulus) gemessen. Bei Auswertung der Steifigkeitsmessung gab es signifikante Unterschiede (p<0,05) zwischen der Kontrollgruppe und den Belastungsgruppen ab 1500µstrain Maximalbelastung. In den Gruppen mit 1500, 2000, 3000µstrain konnte ein kontinuierlicher Steifigkeitsanstieg über den gesamten Versuchszeitraum festgestellt werden. Die Gruppe mit 4000µstrain erreichte schon nach sehr kurzer Zeit (8.Belastungstag) ein Steifigkeitsmaximum. Dies blieb bis zum Versuchende konstant. Die gemessenen Osteoiddicken in dieser Gruppe waren aber nicht größer als in den Gruppen mit 2000 und 3000µstrain. Die Gruppe mit 2000µstrain zeigte den höchsten Anteil neu gebildeten Osteoids. Diese 3 genannten Gruppen unterschieden sich aber signifikant (p<0,05) von der Kontrollgruppe. Zudem konnte eine Korrelation zwischen ansteigender Intensität und Osteoidzunahme beobachtet werden (r = 0,87). Die histologische Auswertung erfolgte an nicht entkalktem Knochen und zeigte gut erhaltene trabekuläre Strukturen sowie ebenso gut erhaltene Zellmorphologien

Few-body bound states in dipolar gases and their detection

We consider dipolar interactions between heteronuclear molecules in a low-dimensional setup consisting of two one-dimensional tubes. We demonstrate that attraction between molecules in different tubes can overcome intratube repulsion and complexes with several molecules in the same tube are stable. In situ detection schemes of the few-body complexes are proposed. We discuss extensions to the case of many tubes and layers, and outline the implications of our results on many-body physics.Comment: Published versio