31,034 research outputs found
Long-range and short-range magnetic correlations, and microscopic origin of net magnetization in the spin-1 trimer chain compound CaNi3P4O14
Spin-spin correlations and microscopic origin of net magnetization in the
spin-1 trimer chain compound CaNi3P4O14 have been investigated by powder
neutron diffraction. The present study reveals a 3D long-range magnetic
ordering below 16 K where the magnetic structure consists of ferromagnetic
trimers that are coupled ferromagnetically along the spin-chain. The moment
components along the a and c axes arrange antiferromagnetically. Our study
establishes that the uncompensated moment components along the b axis result in
a net magnetization per unit cell. The magnetic structure, determined in the
present study, is in agreement with the results of recent first principles
calculation; however, it is in contrast to a fascinating experimental
prediction of ferrimagnetic ordering based on the periodicity of the exchange
interactions in CaNi3P4O14. Our study also confirms the presence of broad
diffuse magnetic scattering, due to 1D short-range spin-spin correlations, over
a wide temperature range below ~50 K down to a temperature well below the Tc.
Total neutron scattering analysis by the RMC method reveals that the dominating
spin-spin correlation above Tc is ferromagnetic and along the b axis. The
nearest neighbour spin-spin correlations along the a and c axes are found to be
weakly antiferromagnetic. The nature of the trimer spin structure of the
short-range state is similar to that of the 3D long-range ordered state. The
present investigation of microscopic nature of the magnetic ground state also
explains the condition required for the 1/3 magnetization plateau to be
observed in the trimer spin-chains. In spite of the S=1 trimer chain system,
the present compound CaNi3P4O14 is found to be a good realization of 3D magnet
below the Tc=16 K with full ordered moment values of ~2 mu_B/Ni2+ (1.98 and
1.96 mu_B/Ni2+ for two Ni sites, respectively) at 1.5 K.Comment: 10 pages, 8 figure
Widespread HCN maser emission in carbon-rich evolved stars
Context. HCN is a major constituent of the circumstellar envelopes of
carbon-rich evolved stars, and rotational lines from within its vibrationally
excited states probe parts of these regions closest to the stellar surface. A
number of such lines are known to show maser action. Historically, in one of
them, the 177 GHz line in the -doubled bending mode has
been found to show relatively strong maser action, with results only published
for a single object, the archetypical high-mass loss asymptotic giant branch
(AGB) star IRC+10216. Aims. To examine how common 177 GHz HCN maser emission
is, we conducted an exploratory survey for this line toward a select sample of
carbon-rich asymptotic giant branch stars that are observable from the southern
hemisphere. Methods. We used the Atacama Pathfinder Experiment 12 meter
submillimeter Telescope (APEX) equipped with a new receiver to simultaneously
observe three HCN rotational transitions, the and -doublet components, and the line from
the (0,0,0) ground state. Results. The maser line is
detected toward 11 of 13 observed sources, which all show emission in the
(0,0,0) transition. In most of the sources, the peak intensity of the
line rivals that of the (0,0,0) line; in two sources,
it is even stronger. Except for the object with the highest mass-loss rate,
IRC+10216, the line covers a smaller velocity range
than the (0,0,0) line. Conclusions. Maser emission in the 177 GHz
line of HCN appears to be common in
carbon-rich AGB stars. (Abbreviated)Comment: 12 pages (including appendix), 3 figures / Astronomy & Astrophysics
(in press
^25Mg NMR study of the MgB_2 superconductor
^25Mg NMR spectra and nuclear spin-lattice relaxation time, T_1, have been
measured in polycrystalline ^25MgB_2 with a superconducting transition
temperature T_c = 39.0 K in zero magnetic field. From the first order and
second order quadrupole perturbed NMR spectrum a quadrupole coupling frequency
nu_Q = 222(1.5) kHz is obtained. T_1T = 1090(50) sK and Knight shift K_c =
242(4) ppm are temperature independent in the normal conducting phase. The
^25Mg Korringa ratio equals to 0.95 which is very close to the ideal value of
unity for s-electrons. The comparison of the experimental nu_Q, T_1T, and K_c
with the corresponding values obtained by LDA calculations shows an excellent
agreement for all three quantities.Comment: 4 pages including 4 eps-figures, revtex
Energy Gaps and Kohn Anomalies in Elemental Superconductors
The momentum and temperature dependence of the lifetimes of acoustic phonons
in the elemental superconductors Pb and Nb was determined by resonant spin-echo
spectroscopy with neutrons. In both elements, the superconducting energy gap
extracted from these measurements was found to converge with sharp anomalies
originating from Fermi-surface nesting (Kohn anomalies) at low temperatures.
The results indicate electron many-body correlations beyond the standard
theoretical framework for conventional superconductivity. A possible mechanism
is the interplay between superconductivity and spin- or charge-density-wave
fluctuations, which may induce dynamical nesting of the Fermi surface
Arrays of Josephson junctions in an environment with vanishing impedance
The Hamiltonian operator for an unbiased array of Josephson junctions with
gate voltages is constructed when only Cooper pair tunnelling and charging
effects are taken into account. The supercurrent through the system and the
pumped current induced by changing the gate voltages periodically are discussed
with an emphasis on the inaccuracies in the Cooper pair pumping.
Renormalisation of the Hamiltonian operator is used in order to reliably
parametrise the effects due to inhomogeneity in the array and non-ideal gating
sequences. The relatively simple model yields an explicit, testable prediction
based on three experimentally motivated and determinable parameters.Comment: 13 pages, 9 figures, uses RevTeX and epsfig, Revised version, Better
readability and some new result
Correlation between oxygen isotope effects on the transition temperature and the magnetic penetration depth in high-temperature superconductors close to optimal doping
The oxygen-isotope (^{16}O/^{18}O) effect (OIE) on the in-plane magnetic
penetration depth \lambda_{ab}(0) in optimally-doped YBa_2Cu_3O_{7-\delta} and
La_{1.85}Sr_{0.15}CuO_4, and in slightly underdoped YBa_2Cu_4O_8 and
Y_{0.8}Pr_{0.2}Ba_2Cu_3O_{7-\delta} was studied by means of muon-spin rotation.
A substantial OIE on \lambda_{ab}(0) with an OIE exponent
\beta_O=-d\ln\lambda_{ab}(0)/d\ln M_O\approx - 0.2 (M_O is the mass of the
oxygen isotope), and a small OIE on the transition temperature T_c with an OIE
exponent \alpha_O=-d\ln T_{c}/d \ln M_O\simeq0.02 to 0.1 were observed. The
observation of a substantial isotope effect on \lambda_{ab}(0), even in
cuprates where the OIE on T_c is small, indicates that lattice effects play an
important role in cuprate HTS.Comment: 6 pages, 4 figure
Stellar Associations and their Field East of LMC 4 in the Large Magellanic Cloud
We report about the stellar content and the luminosity and mass functions of
three stellar associations and their field located on the north-east edge of
the super-bubble LMC 4 in the Large Magellanic Cloud.Comment: To be appeared in the meeting Proceedings of ``Modes of Star
Formation and the Origin of Field Populations'', Heidelberg, Germany, October
2000; to be published in the ASP Conference Series, edited by E. K. Grebel
and W. Brandne
Collinear order in a frustrated three-dimensional spin- antiferromagnet LiCuWO
Magnetic frustration in three dimensions (3D) manifests itself in the
spin- insulator LiCuWO. Density-functional band-structure
calculations reveal a peculiar spin lattice built of triangular planes with
frustrated interplane couplings. The saturation field of 29 T contrasts with
the susceptibility maximum at 8.5 K and a relatively low N\'eel temperature
K. Magnetic order below is collinear with the propagation
vector and an ordered moment of 0.65(4) according to
neutron diffraction data. This reduced ordered moment together with the low
maximum of the magnetic specific heat () pinpoint strong
magnetic frustration in 3D. Collinear magnetic order suggests that quantum
fluctuations play crucial role in this system, where a non-collinear spiral
state would be stabilized classically.Comment: published version with supplemental material merged into the tex
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