818 research outputs found
Antiferromagnetic ordering in the Kondo lattice system YbFeSi
Compounds belonging to the RFeSi series exhibit unusual
superconducting and magnetic properties. Although a number of studies have been
made on the first reentrant antiferromagnet superconductor TmFeSi,
the physical properties of YbFeSi are largely unexplored. In this
work, we attempt to provide a comprehensive study of bulk properties such as,
resistivity, susceptibility and heat-capacity of a well characterized
polycrystalline YbFeSi. Our measurements indicate that Yb
moments order antiferromagnetically below 1.7 K. Moreover, the system behaves
as a Kondo lattice with large Sommerfeld coefficient () of 0.5~J/Yb mol
K at 0.3 K, which is well below T. The absence of superconductivity
in YbFeSi down to 0.3 K at ambient pressure is attributed to the
presence of the Kondo effect.Comment: 10 pages, 3 figures, tex document. A fuller version has appeared in
PRB. Here we have omitted the figures showing the crystal structure and the
fitting of the X-ray pattern. Also the table with the lattice parameters
obtained from fitting has been remove
Unusual Ground State Properties of the Kondo-Lattice Compound Yb2Ir3Ge5
We report sample preparation, structure, electrical resistivity, magnetic
susceptibility and heat capacity studies of a new compound YbIrGe.
We find that this compound crystallizes in an orthorhombic structure with a
space group PMMN unlike the compound CeIrGe which crystallizes in
the tetragonal IBAM (UCoSi type) structure. Our resistivity
measurements indicate that the compound YbIrGe behaves like a
typical Kondo lattice system with no ordering down to 0.4 K. However, a
Curie-Weiss fit of the inverse magnetic susceptibility above 100 K gives an
effective moment of only 3.66 which is considerably less than the
theoretical value of 4.54 for magnetic Yb ions. The value of
= -15.19 K is also considerably higher indicating the presence of
strong hybridization. An upturn in the low temperature heat capacity gives an
indication that the system may order magnetically just below the lowest
temperature of our heat capacity measurements (0.4 K). The structure contains
two sites for Yb ions and the present investigation suggests that Yb may be
trivalent in one site while it may be significantly lower (close to divalent)
in the other.Comment: 9 pages, 4 figures. submitted to Phys. Rev.
Exclusive Radiative B-Decays in the Light-Cone QCD Sum Rule Approach
We carry out a detailed study of exclusive radiative rare -decays in the
framework of the QCD sum rules on the light cone, which combines the
traditional QCD sum rule technique with the description of final state vector
mesons in terms of the light-cone wave functions of increasing twist. The
decays considered are: and the corresponding decays of the mesons, and . Based on our estimate of the transition
form factor F_1^{B \to K^*\pg}(0) =0.32\pm0.05, we find for the branching
ratio , which is in
agreement with the observed value measured by the CLEO collaboration. We
present detailed estimates for the ratios of the radiative decay form factors,
which are then used to predict the rates for the exclusive radiative B-decays
listed above. This in principle allows the extraction of the CKM matrix element
from the penguin-dominated CKM-suppressed radiative decays when they
are measured. We give a detailed discussion of the dependence of the form
factors on the -quark mass and on the momentum transfer, as well as their
interrelation with the CKM-suppressed semileptonic decay form factors in , which we also calculate in our approach.Comment: 32 pages, 10 uuencoded figures, LaTeX, preprint CERN-TH 7118/9
Phylogeny and taxonomy of obscure genera of microfungi
The recently generated molecular phylogeny for the kingdom Fungi, on which a new classification scheme is based, still suffers from an under representation of numerous apparently asexual genera of microfungi. In an attempt to populate the Fungal Tree of Life, fresh samples of 10 obscure genera of hyphomycetes were collected. These fungi were subsequently established in culture, and subjected to DNA sequence analysis of the ITS and LSU nrRNA genes to resolve species and generic questions related to these obscure genera. Brycekendrickomyces (Herpotrichiellaceae) is introduced as a new genus similar to, but distinct from Haplographium and Lauriomyces. Chalastospora is shown to be a genus in the Pleosporales, with two new species, C. ellipsoidea and C. obclavata, to which Alternaria malorum is added as an additional taxon under its oldest epithet, C. gossypii. Cyphellophora eugeniae is newly described in Cyphellophora (Herpotrichiellaceae), and distinguished from other taxa in the genus. Dictyosporium is placed in the Pleosporales, with one new species, D. streliziae. The genus Edenia, which was recently introduced for a sterile endophytic fungus isolated in Mexico, is shown to be a hyphomycete (Pleosporales) forming a pyronellea-like synanamorph in culture. Thedgonia is shown not to represent an anamorph of Mycosphaerella, but to belong to the Helotiales. Trochophora, however, clustered basal to the Pseudocercospora complex in the Mycosphaerellaceae, as did Verrucisporota. Vonarxia, a rather forgotten genus of hyphomycetes, is shown to belong to the Herpotrichiellaceae and Xenostigmina is confirmed as synanamorph of Mycopappus, and is shown to be allied to Seifertia in the Pleosporales. Dichotomous keys are provided for species in the various genera treated. Furthermore, several families are shown to be polyphyletic within some orders, especially in the Capnodiales, Chaetothyriales and Pleosporales
Standard and Embedded Solitons in Nematic Optical Fibers
A model for a non-Kerr cylindrical nematic fiber is presented. We use the
multiple scales method to show the possibility of constructing different kinds
of wavepackets of transverse magnetic (TM) modes propagating through the fiber.
This procedure allows us to generate different hierarchies of nonlinear partial
differential equations (PDEs) which describe the propagation of optical pulses
along the fiber. We go beyond the usual weakly nonlinear limit of a Kerr medium
and derive an extended Nonlinear Schrodinger equation (eNLS) with a third order
derivative nonlinearity, governing the dynamics for the amplitude of the
wavepacket. In this derivation the dispersion, self-focussing and diffraction
in the nematic are taken into account. Although the resulting nonlinear
may be reduced to the modified Korteweg de Vries equation (mKdV), it also has
additional complex solutions which include two-parameter families of bright and
dark complex solitons. We show analytically that under certain conditions, the
bright solitons are actually double embedded solitons. We explain why these
solitons do not radiate at all, even though their wavenumbers are contained in
the linear spectrum of the system. Finally, we close the paper by making
comments on the advantages as well as the limitations of our approach, and on
further generalizations of the model and method presented.Comment: "Physical Review E, in press
Influence of Impact Parameter on Thermal Description of Relativistic Heavy Ion Collisions at GSI/SIS
Attention is drawn to the role played by the size of the system in the
thermodynamic analysis of particle yields in relativistic heavy ion collisions
at SIS energies. This manifests itself in the non-linear dependence of K+ and
K- yields in collisions at 1 -- 2 A.GeV on the number of participants. It
is shown that this dependence can be quantitatively well described in terms of
a thermal model with a canonical strangeness conservation. The measured
particle multiplicity ratios (pi+/p, pi-/pi+, d/p, K+/pi+ and K+/K- but not
eta/pi0) in central Au-Au and Ni-Ni collisions at 0.8 -- 2.0 A.GeV are also
explained in the context of a thermal model with a common freeze-out
temperature and chemical potential. Including the concept of collective flow a
consistent picture of particle energy distributions is derived with the flow
velocity being strongly impact-parameter dependent.Comment: revtex, 20 figure
Neutral Pions and Eta Mesons as Probes of the Hadronic Fireball in Nucleus-Nucleus Collisions around 1A GeV
Chemical and thermal freeze-out of the hadronic fireball formed in symmetric
collisions of light, intermediate-mass, and heavy nuclei at beam energies
between 0.8A GeV and 2.0A GeV are discussed in terms of an equilibrated,
isospin-symmetric ideal hadron gas with grand-canonical baryon-number
conservation. For each collision system the baryochemical potential mu_B and
the chemical freeze-out temperature T_c are deduced from the inclusive neutral
pion and eta yields which are augmented by interpolated data on deuteron
production. With increasing beam energy mu_B drops from 800 MeV to 650 MeV,
while T_c rises from 55 MeV to 90 MeV. For given beam energy mu_B grows with
system size, whereas T_c remains constant. The centrality dependence of the
freeze-out parameters is weak as exemplified by the system Au+Au at 0.8A GeV.
For the highest beam energies the fraction of nucleons excited to resonance
states reaches freeze-out values of nearly 15 %, suggesting resonance densities
close to normal nuclear density at maximum compression. In contrast to the
particle yields, which convey the status at chemical freeze-out, the shapes of
the related transverse-mass spectra do reflect thermal freeze-out. The observed
thermal freeze-out temperatures T_th are equal to or slightly lower than T_c,
indicative of nearly simultaneous chemical and thermal freeze-out.Comment: 42 pages, 12 figure
Measurement of the Dalitz plot slope parameters for K- -> pi0 pi0 pi- decay using ISTRA+ detector
The Dalitz plot slope parameters g, h and k for the K- -> pi0 pi0 pi- decay
have been measured using in-flight decays detected with the ISTRA+ setup
operating in the 25 GeV negative secondary beam of the U-70 PS. About 252 K
events with four-momenta measured for the pi- and four involved photons were
used for the analysis. The values obtained g=0.627+/-0.004(stat)+/-0.010(syst),
h=0.046+/-0.004(stat)+/-0.012(syst), k=0.001+/-0.001(stat)+/-0.002(syst) are
consistent with the world averages dominated by K+ data, but have significantly
smaller errors.Comment: LaTeX, 10 pages, 8 eps-figures, update of IHEP 2002-1
Noise-assisted preparation of entangled atoms
We discuss the generation of entangled states of two two-level atoms inside
an optical cavity. The cavity mode is supposed to be coupled to a white noise
with adjustable intensity. We describe how the entanglement between the atoms
inside the cavity arise in such a situation. The entanglement is maximized for
intermediate values of the noise intensity, while it is a monotonic function of
the spontaneous rate. This resembles the phenomenon of stochastic resonance and
sheds more light on the idea to exploit white noise in quantum information
processing.Comment: 4 pages, 4 figure
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