6,128 research outputs found
Quantum Impurities and the Neutron Resonance Peak in : Ni versus Zn
The influence of magnetic (S=1) and nonmagnetic (S=0) impurities on the spin
dynamics of an optimally doped high temperature superconductor is compared in
two samples with almost identical superconducting transition temperatures:
YBa(CuNi)O (T=80 K) and
YBa(CuZn)O (T=78 K). In the Ni-substituted
system, the magnetic resonance peak (which is observed at E40 meV in
the pure system) shifts to lower energy with a preserved E/T ratio
while the shift is much smaller upon Zn substitution. By contrast Zn, but not
Ni, restores significant spin fluctuations around 40 meV in the normal state.
These observations are discussed in the light of models proposed for the
magnetic resonance peak.Comment: 3 figures, submitted to PR
High energy spin excitations in YBa_2 Cu_3 O_{6.5}
Inelastic neutron scattering has been used to obtain a comprehensive
description of the absolute dynamical spin susceptibility
of the underdoped superconducting cuprate YBa_2 Cu_3 O_{6.5} ()
over a wide range of energies and temperatures ( and ). Spin excitations of two different
symmetries (even and odd under exchange of two adjacent CuO_2 layers) are
observed which, surprisingly, are characterized by different temperature
dependences. The excitations show dispersive behavior at high energies.Comment: 15 pages, 5 figure
Mount St. Helens aerosol evolution
Stratospheric aerosol samples were collected using a wire impactor during the year following the eruption of Mount St. Helens. Analysis of samples shows that aerosol volume increased for 6 months due to gas-to-particle conversion and then decreased to background levels in the following 6 months
Superconductivity-Induced Anomalies in the Spin Excitation Spectra of Underdoped YBa_2 Cu_3 O_{6+x}
Polarized and unpolarized neutron scattering has been used to determine the
effect of superconductivity on the magnetic excitation spectra of YBa_2 Cu_3
O_{6.5} (T_c = 52K) and YBa_2 Cu_3 O_{6.7} (T_c = 67K). Pronounced enhancements
of the spectral weight centered around 25 meV and 33 meV, respectively, are
observed below T_c in both crystals, compensated predominantly by a loss of
spectral weight at higher energies. The data provide important clues to the
origin of the 40 meV magnetic resonance peak in YBa_2 Cu_3 O_7.Comment: LaTex, 4 pages, 4 ps figures. to appear in Phys. Rev. Let
Are Asians comfortable with discussing death in health valuation studies? A study in multi-ethnic Singapore
BACKGROUND
To characterize ease in discussing death (EID) and its influence on health valuation in a multi-ethnic Asian population and to determine the acceptability of various descriptors of death and "pits"/"all-worst" in health valuation.
METHODS
In-depth interviews (English or mother-tongue) among adult Chinese, Malay and Indian Singaporeans selected to represent both genders and a wide range of ages/educational levels. Subjects rated using 0–10 visual analogue scales (VAS): (1) EID, (2) acceptability of 8 descriptors for death, and (3) appropriateness of "pits" and "all-worst" as descriptors for the worst possible health state. Subjects also valued 3 health states using VAS followed by time trade-off (TTO). The influence of sociocultural variables on EID and these descriptors was studied using univariable analyses and multiple linear regression (MLR). The influence of EID on VAS/TTO utilities with adjustment for sociocultural variables was assessed using MLR.
RESULTS
Subjects (n = 63, 35% Chinese, 32% Malay, median age 44 years) were generally comfortable with discussing death (median EID: 8.0). Only education significantly influenced EID (p = 0.045). EID correlated weakly with VAS/TTO scores (range: VAS: -0.23 to 0.07; TTO: -0.14 to 0.11). All subjects felt "passed away", "departed" and "deceased" were most acceptable (median acceptability: 8.0) while "sudden death" and "immediate death" were least acceptable (median acceptability: 5.0). Subjects clearly preferred "all-worst" to "pits" (63% vs. 19%, p < 0.001).
CONCLUSION
Singaporeans were generally comfortable with discussing death and had clear preferences for several descriptors of death and for "all-worst". EID is unlikely to influence health preference measurement in health valuation studies
Leptogenesis from Soft Supersymmetry Breaking (Soft Leptogenesis)
Soft leptogenesis is a scenario in which the cosmic baryon asymmetry is
produced from a lepton asymmetry generated in the decays of heavy sneutrinos
(the partners of the singlet neutrinos of the seesaw) and where the relevant
sources of CP violation are the complex phases of soft supersymmetry-breaking
terms. We explain the motivations for soft leptogenesis, and review its basic
ingredients: the different CP-violating contributions, the crucial role played
by thermal corrections, and the enhancement of the efficiency from lepton
flavour effects. We also discuss the high temperature regime GeV in
which the cosmic baryon asymmetry originates from an initial asymmetry of an
anomalous -charge, and soft leptogenesis reembodies in -genesis.Comment: References updated. Some minor corrections to match the published
versio
Resonant Spin Excitation in an Overdoped High Temperature Superconductor
An inelastic neutron scattering study of overdoped Bi_2Sr_2CaCu_2O_{8+\delta}
$ (T_c = 83 K) has revealed a resonant spin excitation in the superconducting
state. The mode energy is E_res=38 meV, significantly lower than in optimally
doped Bi_2Sr_2CaCu_2O_{8+\delta} (T_c = 91 K, E_ res =43 meV). This
observation, which indicates a constant ratio E_res /k_B T_c \sim 5.4, helps
resolve a long-standing controversy about the origin of the resonant spin
excitation in high-temperature superconductors.Comment: final version: PRL 86, 1610 (2001
Neutron Scattering and the B_{1g} Phonon in the Cuprates
The momentum dependent lineshape of the out-of-phase oxygen vibration as
measured in recent neutron scattering measurements is investigated. Starting
from a microscopic coupling of the phonon vibration to a local crystal field,
the phonon lineshift and broadening is calculated as a function of transfered
momentum in the superconducting state of YBaCuO. It is shown
that the anisotropy of the density of states, superconducting energy gap, and
the electron-phonon coupling are all crucial in order to explain these
experiments.Comment: new figures and discussio
Effect of Nonmagnetic Impurities on the Magnetic Resonance Peak in YBa2Cu3O7
The magnetic excitation spectrum of a YBa_2 Cu_3 O_7 crystal containing 0.5%
of nonmagnetic (Zn) impurities has been determined by inelastic neutron
scattering. Whereas in the pure system a sharp resonance peak at E ~ 40 meV is
observed exclusively below the superconducting transition temperature T_c, the
magnetic response in the Zn-substituted system is broadened significantly and
vanishes at a temperature much higher than T_c. The energy-integrated spectral
weight observed near q = (pi,pi) increases with Zn substitution, and only about
half of the spectral weight is removed at T_c
Magnetic resonance peak and nonmagnetic impurities
Nonmagnetic Zn impurities are known to strongly suppress superconductivity.
We review their effects on the spin excitation spectrum in , as investigated by inelastic neutron scattering measurements.Comment: Proceedings of Mato Advanced Research Workshop BLED 2000. To appear
in Nato Science Series: B Physic
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