1,072 research outputs found
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
Mechanism of hopping transport in disordered Mott insulators
By using a combination of detailed experimental studies and simple
theoretical arguments, we identify a novel mechanism characterizing the hopping
transport in the Mott insulating phase of CaSrRuO near the
metal-insulator transition. The hopping exponent shows a systematic
evolution from a value of deeper in the insulator to the
conventional Mott value closer to the transition. This behavior,
which we argue to be a universal feature of disordered Mott systems close to
the metal-insulator transition, is shown to reflect the gradual emergence of
disorder-induced localized electronic states populating the Mott-Hubbard gap.Comment: 5 pages, 3 figures, To be published in Physical Review Letter
Active Galaxies and Cluster Gas
Two lines of evidence indicate that active galaxies, principally radio
galaxies, have heated the diffuse hot gas in clusters. The first is the general
need for additional heating to explain the steepness of the X-ray
luminosity--temperature relation in clusters, the second is to solve the
cooling flow problem in cluster cores. The inner core of many clusters is
radiating energy as X-rays on a timescale much shorter than its likely age.
Although the temperature in this region drops by a factor of about 3 from that
of the surrounding gas, little evidence is found for gas much cooler than that.
Some form of heating appears to be taking place, probably by energy transported
outward from the central accreting black hole or radio source. How that energy
heats the gas depends on poorly understood transport properties (conductivity
and viscosity) of the intracluster medium. Viscous heating is discussed as a
possibility. Such heating processes have consequences for the truncation of the
luminosity function of massive galaxies.Comment: 14 pages, 16 fig, Feb 2004 talk for Phil Trans Roy So
Orbital ordering transition in CaRuO observed with resonant x-ray diffraction
Resonant x-ray diffraction performed at the and
absorption edges of Ru has been used to investigate the magnetic and orbital
ordering in CaRuO single crystals. A large resonant enhancement due to
electric dipole transitions is observed at the wave-vector
characteristic of antiferromagnetic ordering. Besides the previously known
antiferromagnetic phase transition at K, an additional phase
transition, between two paramagnetic phases, is observed around 260 K. Based on
the polarization and azimuthal angle dependence of the diffraction signal, this
transition can be attributed to orbital ordering of the Ru electrons.
The propagation vector of the orbital order is inconsistent with some
theoretical predictions for the orbital state of CaRuO.Comment: to appear in PR
High resolution soft X-ray spectroscopy of the elliptical galaxy NGC 5044. Results from the reflection grating spectrometer on-board XMM-Newton
The results from an X-ray spectroscopic study of the giant elliptical galaxy
NGC5044 in the center of a galaxy group are presented. The line dominated soft
X-ray spectra (mainly Fe-L and O VIII Ly_a) from the diffuse gas are resolved
for the first time in this system with the Reflection Grating Spectrometers
on-board XMM-Newton and provide a strong constraint on the temperature
structure. The spectra integrated over 2' (\sim 20kpc) in full-width can be
described by a two temperature plasma model of 0.7keV and 1.1keV. Most of the
latter component is consistent with originating from off-center regions.
Compared to the isobaric cooling flow prediction, the observation shows a clear
cut-off below a temperature of 0.6 +-0.1keV. Furthermore, the Fe and O
abundances within the central 10--20kpc in radius are accurately measured to be
0.55+-0.05 and 0.25+-0.1 times the solar ratios, respectively. The observed
cut-off temperature of this galaxy and other central galaxies in galaxy groups
and clusters are compared with the scale of the galaxy and properties of the
surrounding intra-cluster medium. Based on this comparison, the origin of the
lack of predicted cool emission is discussed.Comment: Accepted for publication in Astronomy & Astrophysic
Possible Multiple Gap Superconductivity with Line Nodes in Heavily Hole-Doped Superconductor KFe2As2 Studied by 75As-NQR and Specific Heat
We report the 75As nuclear quadrupole resonance (NQR) and specific heat
measurements of the heavily hole-doped superconductor KFe2As2 (Tc = 3.5 K). The
spin-lattice relaxation rate 1/T1 in the superconducting state exhibits quite
gradual temperature dependence with no coherence peak below Tc. The
quasi-particle specific heat C_QP/T shows small specific heat jump which is
about 30% of electronic specific heat coefficient just below Tc. In addition,
it suggests the existence of low-energy quasi-particle excitation at the lowest
measurement temperature T = 0.4 K \simeq Tc/10. These temperature dependence of
1/T1 and C_QP/T can be explained by multiple nodal superconducting gap scenario
rather than multiple fully-gapped s_\pm-wave one within simple gap analysis.Comment: 5 pages, 5 figures, to be published in J. Phys. Soc. Jpn. No.8 issue
(2009
X-ray Diagnostics of Thermal Conditions of the Hot Plasmas in the Centaurus Cluster
X-ray data of the Centaurus cluster, obtained with {\it XMM-Newton} for 45
ksec, were analyzed. Deprojected EPIC spectra from concentric thin shell
regions were reproduced equally well by a single-phase plasma emission model,
or by a two-phase model developed by {\it ASCA}, both incorporating cool
(1.7--2.0 keV) and hot ( keV) plasma temperatures. However, EPIC
spectra with higher statistics, accumulated over 3-dimentional thick shell
regions, were reproduced better by the two-phase model than by the singe-phase
one. Therefore, hot and cool plasma phases are inferred to co-exist in the
cluster core region within kpc. The iron and silicon abundances of
the plasma were reconfirmed to increase significantly towards the center, while
that of oxygen was consistent with being radially constant. The implied
non-solar abundance ratios explains away the previously reported excess X-ray
absorption from the central region. Although an additional cool (
keV) emission was detected within kpc of the center, the RGS data
gave tight upper limits on any emission with a tempeartures below
keV. These results are compiled into a magnetosphere model, which interprets
the cool phase as confined within closed magnetic loops anchored to the cD
galaxy. When combined with so-called Rosner-Tucker-Vaiana mechanism which
applies to solar coronae, this model can potentially explain basic properties
of the cool phase, including its temperature and thermal stability.Comment: 53 pages, 11 figures, accepted for publication in Astrophysical
Journa
Pressure-Tuned Collapse of the Mott-Like State in Ca_{n+1}Ru_nO_{3n+1} (n=1,2): Raman Spectroscopic Studies
We report a Raman scattering study of the pressure-induced collapse of the
Mott-like phases of Ca_3Ru_2O_7 (T_N=56 K) and Ca_2RuO_4 (T_N=110 K). The
pressure-dependence of the phonon and two-magnon excitations in these materials
indicate: (i) a pressure-induced collapse of the antiferromagnetic (AF)
insulating phase above P* ~ 55 kbar in Ca_3Ru_2O_7 and P* ~ 5-10 kbar in
Ca_2RuO_4, reflecting the importance of Ru-O octahedral distortions in
stabilizing the AF insulating phase; and (ii) evidence for persistent AF
correlations above the critical pressure of Ca_2RuO_4, suggestive of phase
separation involving AF insulator and ferromagnetic metal phases.Comment: 3 figure
An ASCA Study of the Heavy Element Distribution in Clusters of Galaxies
We perform a spatially resolved X-ray spectroscopic study of a set of 11
relaxed clusters of galaxies observed by the ROSAT/PSPC and ASCA/SIS. Using a
method which corrects for the energy dependent effects of the ASCA PSF based on
ROSAT images, we constrain the spatial distribution of Ne, Si, S and Fe in each
cluster. Theoretical prescriptions for the chemical yields of Type Ia and II
supernovae, then allow determination of the Fe enrichment from both types of
supernovae as a function of radius within each cluster. Using optical
measurements from the literature, we also determine the iron mass-to-light
ratio (IMLR) separately for Fe synthesized in both types of supernovae. For
clusters with the best photon statistics, we find that the total Fe abundance
decreases significantly with radius, while the Si abundance is either flat or
decreases less rapidly, resulting in an increasing Si/Fe ratio with radius.
This result indicates a greater predominance of Type II SNe enrichment at large
radii in clusters. We suggest that the high Si/Fe ratios in the outskirts of
rich clusters may arise from enrichment by Type II SNe released to ICM via
galactic star burst driven winds. Abridged.Comment: 17 pages, ApJ in press (Nov. 2000), a study of systematics is adde
Commensurate Itinerant Antiferromagnetism in BaFe2As2: 75As-NMR Studies on a Self-Flux Grown Single Crystal
We report results of 75As nuclear magnetic resonance (NMR) experiments on a
self-flux grown single crystal of BaFe2As2. A first-order antiferromagnetic
(AF) transition near 135 K was detected by the splitting of NMR lines, which is
accompanied by simultaneous structural transition as evidenced by a sudden
large change of the electric field gradient tensor at the As site. The NMR
results lead almost uniquely to the stripe spin structure in the AF phase. The
data of spin-lattice relaxation rate indicate development of anisotropic spin
fluctuations of the stripe-type with decreasing temperature in the paramagnetic
phase.Comment: 7 pages, 7 figures, accepted for publication in J. Phys. Soc. Jp
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