2,284 research outputs found
Strong valence fluctuation in the quantum critical heavy fermion superconductor beta-YbAlB4: A hard x-ray photoemission study
Electronic structures of the quantum critical superconductor beta-YbAlB4 and
its polymorph alpha-YbAlB4 are investigated by using bulk-sensitive hard x-ray
photoemission spectroscopy. From the Yb 3d core level spectra, the values of
the Yb valence are estimated to be ~2.73 and ~2.75 for alpha- and beta-YbAlB4,
respectively, thus providing clear evidence for valence fluctuations. The
valence band spectra of these compounds also show Yb2+ peaks at the Fermi
level. These observations establish an unambiguous case of a strong mixed
valence at quantum criticality for the first time among heavy fermion systems,
calling for a novel scheme for a quantum critical model beyond the conventional
Doniach picture in beta-YbAlB4.Comment: 4 pages, 3 figures, revised version accepted for publication in PR
Alternative selection of processing additives to enhance the lifetime of OPVs
The use of processing additives is known to accelerate the degradation of Organic Photovoltaics (OPVs) and therefore, this paper studies the impact of selecting alternative processing additives for PCPDTBT:PC71BM solar cells in order to improve the stability. The use of naphthalene-based processing additives has been undertaken, which is shown to reduce the initial power conversion efficiency by 23%-42%, primarily due to a decrease in the short-circuit current density, but also fill factor. However, the stability is greatly enhanced by using such additives, with the long term stability (T50%) enhanced by a factor of four. The results show that there is a trade-off between initial performance and stability to consider when selecting the initial process additives. XPS studies have provided some insight into the decreased degradation and show that using 1-chloronaphthalene (ClN) leads to reduced morphology changes and reduced oxidation of the thiophene-ring within the PCPDTBT backbone.HW would like to thank Bangor University for his 125 scholarship enabling his PhD study. JK would like to thank Sêr Cymru national research network in Advanced Engineering and Materials and the Newton Research Collaboration Programme scheme through the Royal Academy for Engineerin
Measurement of K^+ \to \pi^0 \mu^+ \nu \gamma decay using stopped kaons
The K^+ \to \pi^0 \mu^+ \nu \gamma () decay has been
measured with stopped positive kaons at the KEK 12 GeV proton synchrotron. A
sample containing 125 events was obtained. The partial
branching ratio was found to be , which is in good agreement with theoretical predictions.Comment: 12 pages, 3 figures, to be published in Physics Letters
Measurement of direct photon emission in decay using stopped positive kaons
The radiative decay () has
been measured with stopped positive kaons. A sample
containing 4k events was analyzed, and the branching ratio
of the direct photon emission process was determined to be . No interference pattern with internal
bremsstrahlung was observed.Comment: 12 pages, 6 figures, 2 tables, to be published in Phys. Lett.
The Effective Particle-Hole Interaction and the Optical Response of Simple Metal Clusters
Following Sham and Rice [L. J. Sham, T. M. Rice, Phys. Rev. 144 (1966) 708]
the correlated motion of particle-hole pairs is studied, starting from the
general two-particle Greens function. In this way we derive a matrix equation
for eigenvalues and wave functions, respectively, of the general type of
collective excitation of a N-particle system. The interplay between excitons
and plasmons is fully described by this new set of equations. As a by-product
we obtain - at least a-posteriori - a justification for the use of the TDLDA
for simple-metal clusters.Comment: RevTeX, 15 pages, 5 figures in uufiles format, 1 figure avaible from
[email protected]
Generalized Farey trees, transfer Operators and phase transitions
We consider a family of Markov maps on the unit interval, interpolating
between the tent map and the Farey map. The latter map is not uniformly
expanding. Each map being composed of two fractional linear transformations,
the family generalizes many particular properties which for the case of the
Farey map have been successfully exploited in number theory. We analyze the
dynamics through the spectral analysis of generalized transfer operators.
Application of the thermodynamic formalism to the family reveals first and
second order phase transitions and unusual properties like positivity of the
interaction function.Comment: 39 pages, 10 figure
First measurement of the T-violating muon polarization in the decay K^+ --> mu^+ nu gamma
We present the result of the first measurement of the T-violating muon
polarization P_T in the decay K^+ --> mu^+ nu gamma. This polarization is
sensitive to new sources of CP-violation in the Higgs sector. Using data
accumulated in the period 1996-98 we have obtained P_T = (-0.64 +- 1.85(stat)
+- 0.10(syst))x10^{-2} which is consistent with no T-violation in this decay.Comment: 11 pages, 8 figure
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