Strong Correlations Produce the Curie-Weiss Phase of Nax_{x}CoO2_2

Within the t-J model we study several experimentally accessible properties of the 2D-triangular lattice system Na$_x$CoO$_2$, using a numerically exact canonical ensemble study of 12 to 18 site triangular toroidal clusters as well as the icosahedron. Focusing on the doping regime of $x\sim0.7$, we study the temperature dependent chemical potential, specific heat, magnetic susceptibility and the dynamic Hall coefficient $R_H(T,\omega)$ as well as the magnetic field dependent thermopower. We find a crossover between two phases near $x \sim 0.75$ in susceptibility and field suppression of the thermopower arising from strong correlations. An interesting connection is found between the temperature dependence of the diamagnetic susceptibility and the Hall-coefficient. We predict a large thermopower enhancement, arising from {\em transport corrections} to the Heikes-Mott formula, in a model situation where the sign of hopping is reversed from that applicable to Na$_x$CoO$_2$.Comment: 5 pages, 4 figure

Numerical investigation of novel microwave applicators based on zero-order mode resonance for hyperthermia treatment of cancer

This paper characterizes three novel microwave applicators based on zero-order mode resonators for use in hyperthermia treatment of cancer. The radiation patterns are studied with numerical simulations in muscle tissue-equivalent model at 434 MHz. The relative performance of the applicators is compared in terms of reflection coefficient, current distribution, power deposition (SAR) pattern, effective field size in 2D and 3D tissue volumes, and penetration depth. One particular configuration generated the most uniform SAR pattern, with 25% SAR covering 84 % of the treatment volume extending to 1 cm depth under the aperture, while remaining above 58% coverage as deep as 3 cm under the aperture. Recommendations are made to further optimize this structure

Short-coherence length superconductivity in the Attractive Hubbard Model in three dimensions

We study the normal state and the superconducting transition in the Attractive Hubbard Model in three dimensions, using self-consistent diagrammatics. Our results for the self-consistent $T$-matrix approximation are consistent with 3D-XY power-law critical scaling and finite-size scaling. This is in contrast to the exponential 2D-XY scaling the method was able to capture in our previous 2D calculation. We find the 3D transition temperature at quarter-filling and $U=-4t$ to be $T_c=0.207t$. The 3D critical regime is much narrower than in 2D and the ratio of the mean-field transition to $T_c$ is about 5 times smaller than in 2D. We also find that, for the parameters we consider, the pseudogap regime in 3D (as in 2D) coincides with the critical scaling regime.Comment: 4 pages, 5 figure

Kinetic Antiferromagnetism in the Triangular Lattice

We show that the motion of a single hole in the infinite $U$ Hubbard model with frustrated hopping leads to weak metallic antiferromagnetism of kinetic origin. An intimate relationship is demonstrated between the simplest versions of this problem in 1 and 2 dimensions, and two of the most subtle many body problems, namely the Heisenberg Bethe ring in 1-d and the 2-dimensional triangular lattice Heisenberg antiferromagnet.Comment: 10 pages, 2 figures, 5 supplementary figures; Figures fixe

Subjective localization of electrocutaneous stimuli

Studying the perception of spatiotemporal stimulus patterns in various modalities may yield important information on the way in which humans process sensory information. The perception of tactile and nociceptive cutaneous stimulus patterns have been studied by Stolle et al. [1] and Trojan et al. [2][4] respectively. Among other things, both authors studied subjective localization of single stimuli. In Trojan et al. [4], two types of mislocalization patterns were observed for nociceptive single stimuli when comparing the localization reports with the stimulus locations: (1) overall proximal or distal displacement and (2) expansion or contraction of the stimulus area.\ud It is unknown whether tactile and nociceptive stimuli at the same skin site are perceived as being at the same site. Therefore, comparing the spatial perception of tactile and nociceptive cutaneous stimuli may provide new insights into their processing. This comparison can only be successfully made by applying nociceptive and tactile stimuli at the same skin site in the same experiment. This can be done by using a device which has recently been developed at our institute and which we refer to as the bimodal stimulation electrode [3]. \ud Recording the perceived locations of stimuli can be done by letting subjects report these on a scale. The most intuitive scale for this is the stimulated arm itself. However, this would bias the perception of stimulus location by providing visual information of the electrode locations. The goal of the present research was to (1) create and (2) test a setup which allows subjects to report perceived stimulus locations on their own arm without seeing the electrode positions. This was achieved by building a setup consisting of a touch screen (Provision Visboard) which presents a digital image of the subject’s own arm (without electrodes) and which is positioned over this arm after the electrodes have been attached. Subjects can report the localizations by pointing at the screen using a pointer

Decoherence in weak localization II: Bethe-Salpeter calculation of Cooperon

This is the second in a series of two papers (I and II) on the problem of decoherence in weak localization. In paper I, we discussed how the Pauli principle could be incorporated into an influence functional approach for calculating the Cooperon propagator and the magnetoconductivity. In the present paper II, we check and confirm the results so obtained by diagrammatically setting up a Bethe-Salpeter equation for the Cooperon, which includes self-energy and vertex terms on an equal footing and is free from both infrared and ultraviolet divergencies. We then approximately solve this Bethe-Salpeter equation by the Ansatz C(t) = C^0 (t) e^{-F(t)}, where the decay function F(t) determines the decoherence rate. We show that in order to obtain a divergence-free expression for the decay function F(t), it is sufficient to calculate C^1 (t), the Cooperon in the position-time representation to first order in the interaction. Paper II is independent of paper I and can be read without detailed knowledge of the latter.Comment: 18 pages, 3 figures. This is the second of a series of two papers on decoherence. The first introduces an influence functional approach, the second obtains equivalent results using a diagrammatic Bethe-Salpeter equation. For a concise summary of the main results and conclusions, see Section II of the first pape

Clusters under strong VUV pulses: A quantum-classical hybrid-description incorporating plasma effects

The quantum-classical hybrid-description of rare-gas clusters interacting with intense light pulses which we have developed is described in detail. Much emphasis is put on the treatment of screening electrons in the cluster which set the time scale for the evolution of the system and form the link between electrons strongly bound to ions and quasi-free plasma electrons in the cluster. As an example we discuss the dynamics of an Ar147 cluster exposed to a short VUV laser pulse of 20eV photon energy.Comment: 8 pages, 9 figure

Two-Dimensional Bosonization from Variable Shifts in the Path Integral

A method to perform bosonization of a fermionic theory in (1+1) dimensions in a path integral framework is developed. The method relies exclusively on the path integral property of allowing variable shifts, and does not depend on the explicit form of Greens functions. Two examples, the Schwinger model and the massless Thirring model, are worked out.Comment: 4 page

Optimal randomized multilevel algorithms for infinite-dimensional integration on function spaces with ANOVA-type decomposition

In this paper, we consider the infinite-dimensional integration problem on weighted reproducing kernel Hilbert spaces with norms induced by an underlying function space decomposition of ANOVA-type. The weights model the relative importance of different groups of variables. We present new randomized multilevel algorithms to tackle this integration problem and prove upper bounds for their randomized error. Furthermore, we provide in this setting the first non-trivial lower error bounds for general randomized algorithms, which, in particular, may be adaptive or non-linear. These lower bounds show that our multilevel algorithms are optimal. Our analysis refines and extends the analysis provided in [F. J. Hickernell, T. M\"uller-Gronbach, B. Niu, K. Ritter, J. Complexity 26 (2010), 229-254], and our error bounds improve substantially on the error bounds presented there. As an illustrative example, we discuss the unanchored Sobolev space and employ randomized quasi-Monte Carlo multilevel algorithms based on scrambled polynomial lattice rules.Comment: 31 pages, 0 figure