1,823 research outputs found
Suppression of Magnetic Order by Pressure in BaFe2As2
We performed the dc resistivity and the ZF 75As-NMR measurement of BaFe2As2
under high pressure. The T-P phase diagram of BaFe2As2 determined from
resistivity anomalies and the ZF 75As-NMR clearly revealed that the SDW anomaly
is quite robust against P.Comment: 2 pages, 2 figure
Lattice dynamics and the electron-phonon interaction in CaRuO
We present a Raman scattering study of CaRuO, in which we investigate
the temperature-dependence of the lattice dynamics and the electron-phonon
interaction below the metal-insulator transition temperature ({\it T}). Raman spectra obtained in a backscattering geometry with light polarized
in the ab-plane reveal 9 B phonon modes (140, 215, 265, 269, 292, 388,
459, 534, and 683 cm) and 9 A phonon modes (126, 192, 204, 251, 304,
322, 356, 395, and 607 cm) for the orthorhombic crystal structure
(PbcaD). With increasing temperature toward {\it T},
the observed phonon modes shift to lower energies and exhibit reduced spectral
weights, reflecting structural changes associated with the elongation of the
RuO octahedra. Interestingly, the phonons exhibit significant increases in
linewidths and asymmetries for {\it T} {\it T}. These results
indicate that there is an increase in the effective number of electrons and the
electron-phonon interaction strengths as the temperature is raised through {\it
T}, suggesting the presence of orbital fluctuations in the
temperature regime {\it T} {\it T} {\it T}.Comment: 6 pages, 4 figure
X-ray Measurements of the Gravitational Potential Profile in the Central Region of the Abell 1060 Cluster of Galaxies
X-ray spectral and imaging data from ASCA and ROSAT were used to measure the
total mass profile in the central region of Abell 1060, a nearby and relatively
poor cluster of galaxies. The ASCA X-ray spectra, after correcting for the
spatial response of the X-ray telescope, show an isothermal distribution of the
intra-cluster medium (ICM) within at least 12' (or kpc;
km sMpc) in radius of the cluster center. The
azimuthally averaged surface brightness profile from the ROSAT PSPC exhibits a
central excess above an isothermal model. The ring-sorted ASCA GIS
spectra and the radial surface brightness distribution from the ROSAT PSPC were
simultaneously utilized to constrain the gravitational potential profile. Some
analytic models of the total mass density profile were examined. The ICM
density profile was also specified by analytic forms. The ICM temperature
distribution was constrained to satisfy the hydrostatic equilibrium, and to be
consistent with the data. Then, the total mass distribution was found to be
described better by the universal dark halo profile proposed by Navarro, Frenk,
and White (1996;1997) than by a King-type model with a flat density core. A
profile with a central cusp together with a logarithmic radial slope of was also consistent with the data. Discussions are made concerning the
estimated dark matter distribution around the cluster center.Comment: 32 pages. Accepted: ApJ 2000, 35 pages, Title was correcte
Magnetic anisotropy of the spin ice compound Dy2Ti2O7
We report magnetization and ac susceptibility of single crystals of the spin
ice compound Dy2Ti2O7. Saturated moments at 1.8 K along the charasteristic axes
[100] and [110] agree with the expected values for an effective ferromagnetic
nearest-neighbor Ising pyrochlore with local anisotropy, where each
magnetic moment is constrained to obey the `ice-rule'. At high enough magnetic
fields along the [111] axis, the saturated moment exhibits a beaking of the
ice-rule; it agrees with the value expected for a three-in one-out spin
configuration. Assuming the realistic magnetic interaction between Dy ions
given by the dipolar spin ice model, we completely reproduce the results at 2 K
by Monte Carlo calculations. However, down to at least 60 mK, we have not found
any experimental evidence of the long-range magnetic ordering predicted by this
model to occur at around 180 mK. Instead, we confirm the spin freezing of the
system below 0.5 K.Comment: 7 pages, 6 figures, submitted to Phys. Rev.
ASCA PV observations of the Seyfert 2 galaxy NGC 4388: the obscured nucleus and its X-ray emission
We present results on the Seyfert 2 galaxy NGC4388 in the Virgo cluster
observed with ASCA during its PV phase. The 0.5-10 keV X-ray spectrum consists
of multiple components; (1) a continuum component heavily absorbed by a column
density NH = 4E23 cm-2 above 3 keV; (2) a strong 6.4 keV line (EW = 500 eV);
(3) a weak flat continuum between 1 and 3 keV; and (4) excess soft X-ray
emission below 1 keV. The detection of strong absorption for the hard X-ray
component is firm evidence for an obscured active nucleus in this Seyfert 2
galaxy. The absorption corrected X-ray luminosity is about 2E42 erg/s. This is
the first time that the fluorescent iron-K line has been detected in this
object. The flat spectrum in the intermediate energy range may be a scattered
continuum from the central source. The soft X-ray emission below 1 keV can be
thermal emission from a temperature kT = 0.5 keV, consistent with the spatially
extended emission observed by ROSAT HRI. However, the low abundance (0.05 Zs)
and high mass flow rate required for the thermal model and an iron-K line
stronger than expected from the obscuring torus model are puzzling. An
alternative consistent solution can be obtained if the central source was a
hundred times more luminous over than a thousand years ago. All the X-ray
emission below 3 keV is then scattered radiation.Comment: 9 pages, 5 Postscript figures, to be published in MNRA
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