785 research outputs found
Investigations of the effect of nonmagnetic Ca substitution for magnetic Dy on spin-freezing in Dy2Ti2O7
Physical properties of partially Ca substituted hole-doped Dy2Ti2O7 have been
investigated by ac magnetic susceptibility \chi_ac(T), dc magnetic
susceptibility \chi(T), isothermal magnetization M(H) and heat capacity C_p(T)
measurements on Dy1.8Ca0.2Ti2O7. The spin-ice system Dy2Ti2O7 exhibits a
spin-glass type freezing behavior near 16 K. Our frequency dependent \chi_ac(T)
data of Dy1.8Ca0.2Ti2O7 show that the spin-freezing behavior is significantly
influenced by Ca substitution. The effect of partial nonmagnetic Ca2+
substitution for magnetic Dy3+ is similar to the previous study on nonmagnetic
isovalent Y3+ substituted Dy2-xYxTi2O7 (for low levels of dilution), however
the suppression of spin-freezing behavior is substantially stronger for Ca than
Y. The Cole-Cole plot analysis reveals semicircular character and a single
relaxation mode in Dy1.8Ca0.2Ti2O7 as for Dy2Ti2O7. No noticeable change in the
insulating behavior of Dy2Ti2O7 results from the holes produced by 10% Ca2+
substitution for Dy3+ ions.Comment: 9 pages, 7 figures, 1 tabl
Chandra discovery of luminous supersoft X-ray sources in M81
A Chandra ACIS-S imaging observation of the nearby galaxy M81 (NGC 3031) reveals nine luminous soft X-ray sources. The local environments, X-ray spectral properties, and X-ray light curves of the sources are presented and discussed in the context of prevailing physical models for supersoft sources. It is shown that the sample falls within expectations based on population synthesis models taken from the literature, although the high observed luminosities (Lobs ∼ 2 × 1036-3 × 10 38 ergs s-1 in the 0.2-2.0 keV band) and equivalent blackbody temperatures (Teff ∼ 40-80 eV) place the brightest detected M81 objects at the high-luminosity end of the class of supersoft sources defined by previous ROSAT and Einstein studies of nearby galaxies. This is interpreted as a natural consequence of the higher sensitivity of Chandra to hotter and more luminous systems. Most of the sources can be explained as canonical supersoft sources: accreting white dwarfs powered by steady surface nuclear burning with X-ray spectra well fitted by hot white dwarf local thermodynamic equilibrium atmosphere models. An exceptionally bright source is scrutinized in greater detail since its estimated bolometric luminosity, L bol ∼ 1.5 × 1039 ergs s-1, greatly exceeds theoretical estimates for supersoft sources. This source may be beyond the stability limit and undergoing a phase of mass outflow under extreme conditions. Alternatively, a model in which the observed X-ray spectrum arises from an accretion disk around a black hole of mass ∼1200/(cos i) 1/2 M⊙ (viewed at an inclination angle i) cannot be excluded
Spectrum of elementary and collective excitations in the dimerized S=1/2 Heisenberg chain with frustration
We have studied the low-energy excitation spectrum of a dimerized and
frustrated antiferromagnetic Heisenberg chain. We use an analytic approach,
based on a description of the excitations as triplets above a strong-coupling
singlet ground state. The quasiparticle spectrum is calculated by treating the
excitations as a dilute Bose gas with infinite on-site repulsion. Additional
singlet (S=0) and triplet (S=1) modes are found as two-particle bound states of
the elementary triplets. We have also calculated the contributions of the
elementary and collective excitations into the spin structure factor. Our
results are in excellent agreement with exact diagonalizations and dimer series
expansions data as long as the dimerization parameter is not too small
(), i.e. while the elementary triplets can be treated as localized
objects.Comment: 18 pages, 13 figure
Direct Observation of Field-Induced Incommensurate Fluctuations in a One-Dimensional S=1/2 Antiferromagnet
Neutron scattering from copper benzoate, Cu(C6D5COO)2 3D2O, provides the
first direct experimental evidence for field-dependent incommensurate low
energy modes in a one-dimensional spin S = 1/2 antiferromagnet. Soft modes
occur for wavevectors q=\pi +- dq(H) where dq(H) ~ 2 \pi M(H)/g\mu_B as
predicted by Bethe ansatz and spinon descriptions of the S = 1/2 chain.
Unexpected was a field-induced energy gap , where
as determined from specific heat measurements. At H = 7 T
(g\mu_B H/J = 0.52), the magnitude of the gap varies from 0.06 - 0.3 J
depending on the orientation of the applied field.Comment: 11 pages, 5 postscript figures, LaTeX, Submitted to PRL 3/31/97,
e-mail comments to [email protected]
Line shapes of dynamical correlation functions in Heisenberg chains
We calculate line shapes of correlation functions by use of complete
diagonalization data of finite chains and analytical implications from
conformal field theory, density of states, and Bethe ansatz. The numerical data
have different finite size accuracy in case of the imaginary and real parts in
the frequency and time representations of spin-correlation functions,
respectively. The low temperature, conformally invariant regime crosses over at
to a diffusive regime that in turn connects continuously to
the high temperature, interacting fermion regime. The first moment sum rule is
determined.Comment: 13 pages REVTEX, 18 figure
John Schuster, Descartes-agonistes: Physico-mathematics, method and corpuscular-mechanism, 1618–1633
We report on a 10 ks simultaneous Chandra/HETG-NuSTAR observation of the
Bursting Pulsar, GRO J1744-28, during its third detected outburst since
discovery and after nearly 18 years of quiescence. The source is detected up to
60 keV with an Eddington persistent flux level. Seven bursts, followed by dips,
are seen with Chandra, three of which are also detected with NuSTAR. Timing
analysis reveals a slight increase in the persistent emission pulsed fraction
with energy (from 10% to 15%) up to 10 keV, above which it remains constant.
The 0.5-70 keV spectra of the persistent and dip emission are the same within
errors, and well described by a blackbody (BB), a power-law with an exponential
rolloff, a 10 keV feature, and a 6.7 keV emission feature, all modified by
neutral absorption. Assuming that the BB emission originates in an accretion
disc, we estimate its inner (magnetospheric) radius to be about 4x10^7 cm,
which translates to a surface dipole field B~9x10^10 G. The Chandra/HETG
spectrum resolves the 6.7 keV feature into (quasi-)neutral and highly ionized
Fe XXV and Fe XXVI emission lines. XSTAR modeling shows these lines to also
emanate from a truncated accretion disk. The burst spectra, with a peak flux
more than an order of magnitude higher than Eddington, are well fit with a
power-law with an exponential rolloff and a 10~keV feature, with similar fit
values compared to the persistent and dip spectra. The burst spectra lack a
thermal component and any Fe features. Anisotropic (beamed) burst emission
would explain both the lack of the BB and any Fe components.Comment: 15 pages, 11 figures, Accepted in Ap
The burden of diabetes mellitus during pregnancy in low- and middle-income countries : a systematic review
Peer reviewedPublisher PD
Quasi-1D spin-1/2 Heisenberg magnets in their ordered phase: correlation functions
We study weakly coupled antiferromagnetic spin chains in their ordered phase
by combinining an exact solution of the single-chain problem with an RPA
analysis of the interchain interaction. A single chain is described by a
quantum Sine-Gordon model and dynamical staggered susceptibilities are
determined by employing the formfactor approach to quantum correlation
functions. We consider both antiferromagnetic order encountered in quasi-1D
materials like and spin-Peierls order as found in .Comment: 16 pages of revtex, 12 figure
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