20,575 research outputs found
Magnetic excitations in the low-temperature ferroelectric phase of multiferroic YMn2O5 using inelastic neutron scattering
We studied magnetic excitations in a low-temperature ferroelectric phase of
the multiferroic YMn2O5 using inelastic neutron scattering (INS). We identify
low-energy magnon modes and establish a correspondence between the magnon peaks
observed by INS and electromagnon peaks observed in optical absorption [1].
Furthermore, we explain the microscopic mechanism, which results in the
lowest-energy electromagnon peak, by comparing the inelastic neutron spectral
weight with the polarization in the commensurate ferroelectric phase.Comment: 4 pages, 4 figure
Superfluid-insulator transition of the Josephson junction array model with commensurate frustration
We have studied the rationally frustrated Josephson-junction array model in
the square lattice through Monte Carlo simulations of D XY-model. For
frustration , the model at zero temperature shows a continuous
superfluid-insulator transition. From the measurement of the correlation
function and the superfluid stiffness, we obtain the dynamical critical
exponent and the correlation length critical exponent . While the dynamical critical exponent is the same as that for cases
, 1/2, and 1/3, the correlation length critical exponent is surprisingly
quite different. When , we have the nature of a first-order transition.Comment: RevTex 4, to appear in PR
Heavy Carriers and Non-Drude Optical Conductivity in MnSi
Optical properties of the weakly helimagnetic metal MnSi have been determined
in the photon energy range from 2 meV to 4.5 eV using the combination of
grazing incidence reflectance at 80 degrees (2 meV to 0.8 eV) and ellipsometry
(0.8 to 4.5 eV). As the sample is cooled below 100 K the effective mass becomes
strongly frequency dependent at low frequencies, while the scattering rate
developes a linear frequency dependence. The complex optical conductivity can
be described by the phenomenological relation \sigma(\omega,T) \propto
(\Gamma(T)+i\omega)^{-1/2} used for cuprates and ruthenates.Comment: 5 pages, ReVTeX 4, 5 figures in eps forma
Silylation of titanium-containing amorphous silica catalyst: effect on the alkenes epoxidation with H2O2
The surface of a Ti/SiO2 catalyst was silylated using hexamethyldisilazane (HMDS) and tetramethyldisilazane (TMDS) as silylating reagents in vapor phase. The silylation of silanol (Si–OH) on the catalysts was confirmed by diffuse reflectance UV-Vis, DRIFT spectroscopy and solid-state 29Si MAS NMR techniques. Silylation with TMDS improves the catalytic performance of Ti/SiO2 catalyst more significantly than with HMDS and this is mainly due to the steric bulk effects of trimethylsilyl groups from HMDS as compared to dimethylsilyl groups from TMDS. The silylation degree with TMDS was found to be higher than with HMDS and no changes in the silylation degree with TMDS was observed for silylation times longer than 2 h. Despite the silylation route employed, the silylated surface is hydrophobic which enhances not only in hydrogen peroxide efficiency but also and in selectivity to epoxide in the epoxidation of alkenes with H2O2. These effects are more evident when the hydrogen peroxide concentration is higher and particularly for the catalyst modified with TMDS.Peer reviewe
The Value of Early Tumor Size Response to Chemotherapy in Pediatric Rhabdomyosarcoma
Rhabdomyosarcoma is the most common soft tissue sarcoma in childhood. Results of clinical trials, with three-year event-free and overall survival as primary outcomes, often take 7 to 10 years. Identification of an early surrogate biomarker, predictive for survival, is therefore crucial. We conducted a systematic review to define the prognostic value of early tumor size response in children with IRSG group III rhabdomyosarcoma. The search included MEDLINE/EMBASE from inception to 18 November 2020. In total, six studies were included, describing 2010 patients, and assessed by the Quality in Prognosis Studies (QUIPS) instrument. Four studies found no prognostic value for tumor size response, whereas two studies reported a prognostic effect. In these two studies, the survival rate of patients with progressive disease was not separately analyzed from patients with stable disease, potentially explaining the difference in study outcome. In conclusion, our findings support that early progression of disease is associated with poorer survival, justifying adaptation of therapy. However, in patients with non-progressive disease, there is no evidence that the degree of response is a prognostic marker for survival. Because the vast majority of patients do not have progressive disease, early tumor size response should be reconsidered for assessment of treatment efficacy. Therefore, at present, early surrogate biomarkers for survival are still lacking
Disentangling the electronic and phononic glue in a high-Tc superconductor
Unveiling the nature of the bosonic excitations that mediate the formation of
Cooper pairs is a key issue for understanding unconventional superconductivity.
A fundamen- tal step toward this goal would be to identify the relative weight
of the electronic and phononic contributions to the overall frequency (\Omega)
dependent bosonic function, \Pi(\Omega). We perform optical spectroscopy on
Bi2212 crystals with simultaneous time- and frequency-resolution; this
technique allows us to disentangle the electronic and phononic contributions by
their different temporal evolution. The strength of the interaction
({\lambda}~1.1) with the electronic excitations and their spectral distribution
fully account for the high critical temperature of the superconducting phase
transition.Comment: 9 pages, 4 figure
Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in the Globular Cluster M15: II. Kinematical Analysis and Dynamical Modeling
We analyze HST/STIS spectra (see Paper I) of the central region of the dense
globular cluster M15. We infer the velocities of 64 individual stars,
two-thirds of which have their velocity measured for the first time. This
triples the number of stars with measured velocities in the central 1 arcsec of
M15 and doubles the number in the central 2 arcsec. Combined with existing
ground-based data we obtain the radial profiles of the projected kinematical
quantities. The RMS velocity sigma_RMS rises to 14 km/s in the central few
arcsec, somewhat higher than the values of 10-12 km/s inferred previously from
ground-based data. To interpret the results we construct dynamical models based
on the Jeans equation, which imply that M15 must have a central concentration
of non-luminous material. If this is due to a single black hole, then its mass
is M_BH = (3.9 +/- 2.2) x 10^3 solar masses. This is consistent with the
relation between M_BH and sigma_RMS that has been established for galaxies.
Also, the existence of intermediate-mass black holes in globular clusters is
consistent with several scenarios for globular cluster evolution proposed in
the literature. Therefore, these results may have important implications for
our understanding of the evolution of globular clusters, the growth of black
holes, the connection between globular cluster and galaxy formation, and the
nature of the recently discovered `ultra-luminous' X-ray sources in nearby
galaxies. Instead of a single black hole, M15 could have a central
concentration of dark remnants (e.g., neutron stars) due to mass segregation.
However, the best-fitting Fokker-Planck models that have previously been
constructed for M15 do not predict a central mass concentration that is
sufficient to explain the observed kinematics.[ABRIDGED]Comment: 43 pages, LaTeX, with 14 PostScript figures. Astronomical Journal, in
press (Dec 2002). Please note that the results reported here are modified by
the Addendum available at astro-ph/0210158 (Astronomical Journal, in press,
Jan 2003). This second version submitted to astro-ph is identical to first,
with the exception of the preceeding remar
Two-gap superconductivity in MgB: clean or dirty?
A large number of experimental facts and theoretical arguments favor a
two-gap model for superconductivity in MgB. However, this model predicts
strong suppression of the critical temperature by interband impurity scattering
and, presumably, a strong correlation between the critical temperature and the
residual resistivity. No such correlation has been observed. We argue that this
fact can be understood if the band disparity of the electronic structure is
taken into account, not only in the superconducting state, but also in normal
transport
- …