Absence of long-range order in a spin-half Heisenberg antiferromagnet on the stacked kagome lattice

We study the ground state of a spin-half Heisenberg antiferromagnet on the stacked kagome lattice by using a spin-rotation-invariant Green's-function method. Since the pure two-dimensional kagome antiferromagnet is most likely a magnetically disordered quantum spin liquid, we investigate the question whether the coupling of kagome layers in a stacked three-dimensional system may lead to a magnetically ordered ground state. We present spin-spin correlation functions and correlation lengths. For comparison we apply also linear spin wave theory. Our results provide strong evidence that the system remains short-range ordered independent of the sign and the strength of the interlayer coupling

High-Order Coupled Cluster Calculations Via Parallel Processing: An Illustration For CaV4_4O9_9

The coupled cluster method (CCM) is a method of quantum many-body theory that may provide accurate results for the ground-state properties of lattice quantum spin systems even in the presence of strong frustration and for lattices of arbitrary spatial dimensionality. Here we present a significant extension of the method by introducing a new approach that allows an efficient parallelization of computer codes that carry out ``high-order'' CCM calculations. We find that we are able to extend such CCM calculations by an order of magnitude higher than ever before utilized in a high-order CCM calculation for an antiferromagnet. Furthermore, we use only a relatively modest number of processors, namely, eight. Such very high-order CCM calculations are possible {\it only} by using such a parallelized approach. An illustration of the new approach is presented for the ground-state properties of a highly frustrated two-dimensional magnetic material, CaV$_4$O$_9$. Our best results for the ground-state energy and sublattice magnetization for the pure nearest-neighbor model are given by $E_g/N=-0.5534$ and $M=0.19$, respectively, and we predict that there is no N\'eel ordering in the region $0.2 \le J_2/J_1 \le 0.7$. These results are shown to be in excellent agreement with the best results of other approximate methods.Comment: 4 page

Psychische Folgen von Patientenübergriffen auf Mitarbeiter: Prospektive und retrospektive Daten

Zusammenfassung: Hintergrund: Die psychischen Folgen von Patientenübergriffen auf Mitarbeiter psychiatrischer Einrichtungen sind bisher kaum mit validierten und etablierten Instrumenten aus der Forschung über posttraumatische Belastungsstörungen (PTBS) untersucht worden. Methode: In einer prospektiven Längsschnittstudie sowie in einer retrospektiven Untersuchung wurden 46 bzw. 88 Mitarbeiter aus Einrichtungen der Psychiatrie und des Gesundheitswesens nach einem erlittenen Patientenübergriff hinsichtlich der psychischen Folgen befragt. Es wurden anerkannte Instrumente aus der PTBS-Forschung eingesetzt (PCL-C, IES-R). Ergebnisse: In den ersten Wochen nach dem Übergriff erfüllten 17% der Studienteilnehmer der prospektiven Studiedie Kriterien für die Diagnose einer PTBS nach DSM-IV. Im Verlauf von weiteren 2 und 6Monaten sank der Anteil von Betroffenen mit einer PTBS-Diagnose auf 11%. In der retrospektiven Studiewurde bei 3% der Befragten im Mittel 1,5Jahren nach dem Übergriff noch das Vollbild einer PTBS festgestellt. Schlussfolgerung: Patientenübergriffe können posttraumatische Belastungsstörungen bei Mitarbeitern psychiatrischer Einrichtungen auslösen. Einrichtungen, in denen solche Vorfälle geschehen, müssen eine adäquate Nachsorge für die betroffenen Mitarbeiter organisiere

Machine learning classification: case of Higgs boson CP state in H to tau tau decay at LHC

Machine Learning (ML) techniques are rapidly finding a place among the methods of High Energy Physics data analysis. Different approaches are explored concerning how much effort should be put into building high-level variables based on physics insight into the problem, and when it is enough to rely on low-level ones, allowing ML methods to find patterns without explicit physics model. In this paper we continue the discussion of previous publications on the CP state of the Higgs boson measurement of the H to tau tau decay channel with the consecutive tau^pm to rho^pm nu; rho^pm to pi^pm pi^0 and tau^pm to a_1^pm nu; a_1^pm to rho^0 pi^pm to 3 pi^pm cascade decays. The discrimination of the Higgs boson CP state is studied as a binary classification problem between CP-even (scalar) and CP-odd (pseudoscalar), using Deep Neural Network (DNN). Improvements on the classification from the constraints on directly non-measurable outgoing neutrinos are discussed. We find, that once added, they enhance the sensitivity sizably, even if only imperfect information is provided. In addition to DNN we also evaluate and compare other ML methods: Boosted Trees (BT), Random Forest (RF) and Support Vector Machine (SVN).Comment: 1+20 pages, 9 figures, 6 tables, extended content and improved readabilit

Interfaces Within Graphene Nanoribbons

We study the conductance through two types of graphene nanostructures: nanoribbon junctions in which the width changes from wide to narrow, and curved nanoribbons. In the wide-narrow structures, substantial reflection occurs from the wide-narrow interface, in contrast to the behavior of the much studied electron gas waveguides. In the curved nanoribbons, the conductance is very sensitive to details such as whether regions of a semiconducting armchair nanoribbon are included in the curved structure -- such regions strongly suppress the conductance. Surprisingly, this suppression is not due to the band gap of the semiconducting nanoribbon, but is linked to the valley degree of freedom. Though we study these effects in the simplest contexts, they can be expected to occur for more complicated structures, and we show results for rings as well. We conclude that experience from electron gas waveguides does not carry over to graphene nanostructures. The interior interfaces causing extra scattering result from the extra effective degrees of freedom of the graphene structure, namely the valley and sublattice pseudospins.Comment: 19 pages, published version, several references added, small changes to conclusion

ORFEUS II echelle spectra: Absorption by H_2 in the LMC

We present the first detection of molecular hydrogen (H_2) UV absorption profiles on the line of sight to the LMC. The star LH 10:3120 in the LMC was measured with the ORFEUS telescope and the Tuebingen echelle spectrograph during the space shuttle mission of Nov./Dec. 1996. 16 absorption lines from the Lyman band are used to derive the column densities of H_2 for the lowest 5 rotational states in the LMC gas. For these states we find a total column density of N(H_2)=6.6 x 10^18$ cm^-2 on this individual line of sight. We obtain equivalent excitation temperatures of T < 50 K for the rotational ground state and T = 470 K for 0 < J < 6 by fitting the population densities of the rotational states to theoretical Boltzmann distributions. We conclude that UV pumping dominates the population of the higher rotational levels, as known from the H_2 gas in the Milky Way. (Research supported in part by the DARA)Comment: Astronomy & Astrophysics, Letter, in pres