15,555 research outputs found
The spin 1/2 Heisenberg star with frustration II: The influence of the embedding medium
We investigate the spin 1/2 Heisenberg star introduced in J. Richter and A.
Voigt, J. Phys. A: Math. Gen. {\bf 27}, 1139 (1994). The model is defined by
; , . In extension to the Ref. we consider a more general
describing the properties of the spins surrounding the
central spin . The Heisenberg star may be considered as an essential
structure element of a lattice with frustration (namely a spin embedded in a
magnetic matrix ) or, alternatively, as a magnetic system with a
perturbation by an extra spin. We present some general features of the
eigenvalues, the eigenfunctions as well as the spin correlation of the model. For being a linear chain, a square
lattice or a Lieb-Mattis type system we present the ground state properties of
the model in dependence on the frustration parameter .
Furthermore the thermodynamic properties are calculated for being a
Lieb--Mattis antiferromagnet.Comment: 16 pages, uuencoded compressed postscript file, accepted to J. Phys.
A: Math. Ge
Analytical ground state for the three-band Hubbard model
For the calculation of charge excitations as those observed in, e.g.,
photo-emission spectroscopy or in electron-energy loss spectroscopy, a correct
description of ground-state charge properties is essential. In strongly
correlated systems like the undoped cuprates this is a highly non-trivial
problem. In this paper we derive a non-perturbative analytical approximation
for the ground state of the three-band Hubbard model on an infinite, half
filled CuO_2 plane. By comparison with Projector Quantum Monte Carlo
calculations it is shown that the resulting expressions correctly describe the
charge properties of the ground state. Relations to other approaches are
discussed. The analytical ground state preserves size consistency and can be
generalized for other geometries, while still being both easy to interpret and
to evaluate.Comment: REVTeX, 8 pages, 6 figures, to appear in Phys. Rev.
Implementation of liquid culture for tuberculosis diagnosis in a remote setting: lessons learned.
Although sputum smear microscopy is the primary method for tuberculosis (TB) diagnosis in low-resource settings, it has low sensitivity. The World Health Organization recommends the use of liquid culture techniques for TB diagnosis and drug susceptibility testing in low- and middle-income countries. An evaluation of samples from southern Sudan found that culture was able to detect cases of active pulmonary TB and extra-pulmonary TB missed by conventional smear microscopy. However, the long delays involved in obtaining culture results meant that they were usually not clinically useful, and high rates of non-tuberculous mycobacteria isolation made interpretation of results difficult. Improvements in diagnostic capacity and rapid speciation facilities, either on-site or through a local reference laboratory, are crucial
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
A solvable model of a random spin-1/2 XY chain
The paper presents exact calculations of thermodynamic quantities for the
spin-1/2 isotropic XY chain with random lorentzian intersite interaction and
transverse field that depends linearly on the surrounding intersite
interactions.Comment: 14 pages (Latex), 2 tables, 13 ps-figures included, (accepted for
publication in Phys.Rev.B
Spin interference effects in ring conductors subject to Rashba coupling
Quantum interference effects in rings provide suitable means for controlling
spin at mesoscopic scales. Here we apply such control mechanisms to coherent
spin-dependent transport in one- and two-dimensional rings subject to Rashba
spin-orbit coupling. We first study the spin-induced modulation of unpolarized
currents as a function of the Rashba coupling strength. The results suggest the
possibility of all-electrical spintronic devices. Moreover, we find signatures
of Berry phases in the conductance previously unnoticed. Second, we show that
the polarization direction of initially polarized, transmitted spins can be
tuned via an additional small magnetic control flux. In particular, this
enables to precisely reverse the polarization direction at half a flux quantum.
We present full numerical calculations for realistic two-dimensional ballistic
microstructures and explain our findings in a simple analytical model for
one-dimensional rings.Comment: 8 pages, 5 figures. Submitted to Phys. Rev. B, final versio
Breeding of resistant strawberry cultivars for organic fruit production – Diallel crossing strategies and resistance tests for Botrytis cinerea and Xanthomonas fragariae
Organic strawberry production suffers from high yield losses caused by numerous fungal and bacterial diseases. Two of the most important diseases are the grey mould disease caused by Botrytis cinerea Pers. (teleomorph Botryotinia fuckeliana), and the bacterial angular leaf spot disease caused by Xanthomonas fragariae (Kennedy & King). Beside cultivation methods and organic plant protection measures, the development of resistant cultivars seems to be the most promising strategy in order to improve the productivity in organic strawberry cultivation. Therefore, we established resistance tests to determine resistant and susceptible strawberry cultivars and breeding selections. In a first run, 40 different cultivars and selections were tested for their susceptibility towards B. cinerea by artificial inoculation of fruits and leaves and evaluation of the disease symptoms. Plants of 40 cultivars were tested for susceptibility to X. fragariae by artificial inoculation in the greenhouse. In a diallel crossing approach, 12 commonly cultivated strawberry cultivars have been crossed reciprocally and propagated in a field trial. Important characteristics of the progeny such as ripening time, yield, morphological traits and occurrence of diseases have been evaluated for a period of two consecutive years and lead to the determination of general (GCA) and specific (SCA) combining abilities. Together with the results of the resistance tests we identified a set of genotypes that show resistant characteristics towards B. cinerea and might be suitable for use in organic cultivation systems. Furthermore, they can be used for targeted breeding experiments in the future
Localized-magnon states in strongly frustrated quantum spin lattices
Recent developments concerning localized-magnon eigenstates in strongly
frustrated spin lattices and their effect on the low-temperature physics of
these systems in high magnetic fields are reviewed. After illustrating the
construction and the properties of localized-magnon states we describe the
plateau and the jump in the magnetization process caused by these states.
Considering appropriate lattice deformations fitting to the localized magnons
we discuss a spin-Peierls instability in high magnetic fields related to these
states. Last but not least we consider the degeneracy of the localized-magnon
eigenstates and the related thermodynamics in high magnetic fields. In
particular, we discuss the low-temperature maximum in the isothermal entropy
versus field curve and the resulting enhanced magnetocaloric effect, which
allows efficient magnetic cooling from quite large temperatures down to very
low ones.Comment: 21 pages, 10 figures, invited paper for a special issue of "Low
Temperature Physics " dedicated to the 70-th anniversary of creation of
concept "antiferromagnetism" in physics of magnetis
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