21,834 research outputs found
Preparation and photoemission investigation of bulk-like a-Mn films on W(110)
We report the successful stabilization of a thick bulk-like, distorted
-Mn film with (110) orientation on a W(110) substrate. The observed
overstructure for the Mn film with respect to the original W(110)
low-energy electron diffraction pattern is consistent with the presented
structure model. The possibility to stabilize such a pseudomorphic Mn film is
supported by density functional total energy calculations. Angle-resolved
photoemission spectra of the stabilized -Mn(110) film show weak
dispersions of the valence band electronic states in accordance with the large
unit cell.Comment: 11 pages; 5 figure
Bacterial reduction of N-oxides of tobacco- specific nitrosamines (TSNA)
1 Contrary to established metabolic pattern, a recent investigation of NNK metabolism produced in rat urine higher levels of 4-(methylnitrosamino)-1-(3-pyridyl)-1- butanone (NNK) and 4-(methylnitrosamino)-1-(3-pyri dyl)-1-butanol (NNAL) than their N-oxides, suggesting that reconversion of N-oxides could occur after urine formation. 2 To verify the possible role of bacteria in the reduction of NNK-N-oxide and NNAL-N-oxide to their respective parent compounds, NNK and NNAL, in smokers with urinary tract infection (UTI), the N-oxides were isolated from the urine of rats treated with 5-3HNNK and individually incubated at 37°C with ten bacterial species in sterile human urine under different pH regimens. After incubation with the bacteria, aliquots of culture media were analyzed by high pressure liquid chromatography (HPLC) with radiochemical detection. 3 Escherichia coli, Enterobacter cloacae, Klebsiella pneumoniae and Proteus mirabilis possessed varying capacity to regenerate NNK and NNAL from their N- oxides while others showed no detectable reductive capability within 24 h. 4 This result constitutes the first experimental evidence that in tobacco users with concomitant UTI, bacterial regeneration of the procarcinogenic NNK and NNAL from their N-oxides could occur in the bladder leading to increased carcinogen burden in these individuals
Next Generation Higgs Bosons: Theory, Constraints and Discovery Prospects at the Large Hadron Collider
Particle physics model building within the context of string theory suggests
that further copies of the Higgs boson sector may be expected. Concerns
regarding tree-level flavor changing neutral currents are easiest to allay if
little or no couplings of next generation Higgs bosons are allowed to Standard
Model fermions. We detail the resulting general Higgs potential and mass
spectroscopy in both a Standard Model extension and a supersymmetric extension.
We present the important experimental constraints from meson-meson mixing,
loop-induced decays and LEP2 direct production limits. We
investigate the energy range of valid perturbation theory of these ideas. In
the supersymmetric context we present a class of examples that marginally aids
the fine-tuning problem for parameter space where the lightest Higgs boson mass
is greater than the Standard Model limit of 114 GeV. Finally, we study collider
physics signatures generic to next generation Higgs bosons, with special
emphasis on signal events, and describe the capability of
discovery at the Large Hadron Collider.Comment: 43 pages, 12 figures; v3: minor corrections, published in Physical
Review
Application of a trace formula to the spectra of flat three-dimensional dielectric resonators
The length spectra of flat three-dimensional dielectric resonators of
circular shape were determined from a microwave experiment. They were compared
to a semiclassical trace formula obtained within a two-dimensional model based
on the effective index of refraction approximation and a good agreement was
found. It was necessary to take into account the dispersion of the effective
index of refraction for the two-dimensional approximation. Furthermore, small
deviations between the experimental length spectrum and the trace formula
prediction were attributed to the systematic error of the effective index of
refraction approximation. In summary, the methods developed in this article
enable the application of the trace formula for two-dimensional dielectric
resonators also to realistic, flat three-dimensional dielectric microcavities
and -lasers, allowing for the interpretation of their spectra in terms of
classical periodic orbits.Comment: 13 pages, 12 figures, 1 tabl
Experimental Observation of Localized Modes in a Dielectric Square Resonator
We investigated the frequency spectra and field distributions of a dielectric
square resonator in a microwave experiment. Since such systems cannot be
treated analytically, the experimental studies of their properties are
indispensable. The momentum representation of the measured field distributions
shows that all resonant modes are localized on specific classical tori of the
square billiard. Based on these observations a semiclassical model was
developed. It shows excellent agreement with all but a single class of measured
field distributions that will be treated separately.Comment: 6 pages, 5 figures, 1 tabl
Quantum Phase Transitions in Spin Systems
We discuss the influence of strong quantum fluctuations on zero-temperature
phase transitions in a two-dimensional spin-half Heisenberg system. Using a
high-order coupled cluster treatment, we study competition of magnetic bonds
with and without frustration. We find that the coupled cluster treatment is
able to describe the zero-temperature transitions in a qualitatively correct
way, even if frustration is present and other methods such as quantum Monte
Carlo fail.Comment: 8 pages, 12 Postscipt figures; Accepted for publication in World
Scientifi
SrCu(PO): A real material realization of the 1D nearest neighbor Heisenberg chain
We present evidence that crystalline Sr_2Cu(PO_4)_2 is a nearly perfect
one-dimensional (1D) spin-1/2 anti-ferromagnetic Heisenberg model (AHM) chain
compound with nearest neighbor only exchange. We undertake a broad theoretical
study of the magnetic properties of this compound using first principles (LDA,
LDA+U calculations), exact diagonalization and Bethe-ansatz methodologies to
decompose the individual magnetic contributions, quantify their effect, and fit
to experimental data. We calculate that the conditions of one-dimensionality
and short-ranged magnetic interactions are sufficiently fulfilled that Bethe's
analytical solution should be applicable, opening up the possibility to explore
effects beyond the infinite chain limit of the AHM Hamiltonian. We begin such
an exploration by examining some extrinsic effects such as impurities and
defects
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
Ground-state phase diagram of the spin-1/2 square-lattice J1-J2 model with plaquette structure
Using the coupled cluster method for high orders of approximation and Lanczos
exact diagonalization we study the ground-state phase diagram of a quantum
spin-1/2 J1-J2 model on the square lattice with plaquette structure. We
consider antiferromagnetic (J1>0) as well as ferromagnetic (J1<0)
nearest-neighbor interactions together with frustrating antiferromagnetic
next-nearest-neighbor interaction J2>0. The strength of inter-plaquette
interaction lambda varies between lambda=1 (that corresponds to the uniform
J1-J2 model) and lambda=0 (that corresponds to isolated frustrated 4-spin
plaquettes). While on the classical level (s \to \infty) both versions of
models (i.e., with ferro- and antiferromagnetic J1) exhibit the same
ground-state behavior, the ground-state phase diagram differs basically for the
quantum case s=1/2. For the antiferromagnetic case (J1 > 0) Neel
antiferromagnetic long-range order at small J2/J1 and lambda \gtrsim 0.47 as
well as collinear striped antiferromagnetic long-range order at large J2/J1 and
lambda \gtrsim 0.30 appear which correspond to their classical counterparts.
Both semi-classical magnetic phases are separated by a nonmagnetic quantum
paramagnetic phase. The parameter region, where this nonmagnetic phase exists,
increases with decreasing of lambda. For the ferromagnetic case (J1 < 0) we
have the trivial ferromagnetic ground state at small J2/|J1|. By increasing of
J2 this classical phase gives way for a semi-classical plaquette phase, where
the plaquette block spins of length s=2 are antiferromagnetically long-range
ordered. Further increasing of J2 then yields collinear striped
antiferromagnetic long-range order for lambda \gtrsim 0.38, but a nonmagnetic
quantum paramagnetic phase lambda \lesssim 0.38.Comment: 10 pages, 15 figure
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