3,436 research outputs found
Double Exchange in a Magnetically Frustrated System
This work examines the magnetic order and spin dynamics of a double-exchange
model with competing ferromagnetic and antiferromagnetic Heisenberg
interactions between the local moments. The Heisenberg interactions are
periodically arranged in a Villain configuration in two dimensions with
nearest-neighbor, ferromagnetic coupling and antiferromagnetic coupling
. This model is solved at zero temperature by performing a
expansion in the rotated reference frame of each local moment.
When exceeds a critical value, the ground state is a magnetically
frustrated, canted antiferromagnet. With increasing hopping energy or
magnetic field , the local moments become aligned and the ferromagnetic
phase is stabilized above critical values of or . In the canted phase, a
charge-density wave forms because the electrons prefer to sit on lines of sites
that are coupled ferromagnetically. Due to a change in the topology of the
Fermi surface from closed to open, phase separation occurs in a narrow range of
parameters in the canted phase. In zero field, the long-wavelength spin waves
are isotropic in the region of phase separation. Whereas the average spin-wave
stiffness in the canted phase increases with or , it exhibits a more
complicated dependence on field. This work strongly suggests that the jump in
the spin-wave stiffness observed in PrCaMnO with at a field of 3 T is caused by the delocalization of the electrons rather
than by the alignment of the antiferromagnetic regions.Comment: 28 pages, 12 figure
Poly[(μ3-biphenyl-3,3′-dicarboxylÂato)(1,10-phenanthroline)cadmium]
In the title compound, [Cd(C14H8O4)(C12H8N2)]n, the CdII ion is seven-coordinated in a distorted pentaÂgonal–bipyramidal coordination geometry by five O atoms from bridging biphenyl-3,3′-dicarboylate (dpda) ligands and two N atoms from a 1,10-phenanthroline (1,10-phen) ligand. In the crystal, dinuclear units with a Cd⋯Cd separation of 3.8208 (7) Å are observed. Each of these dinuclear units is bridged via 3,3′-bpda in a chelating/chelating and bridging fashion, generating a zigzag chain along the c axis. Neighboring chains are further packed via weak π–π interÂactions between interÂchain parallel 1,10-phen rings [centroid–centroid distance = 3.5197 (9) Å] into a three-dimensional supraÂmolecular architecture
Spin Diffusion in Double-Exchange Manganites
The theoretical study of spin diffusion in double-exchange magnets by means
of dynamical mean-field theory is presented. We demonstrate that the
spin-diffusion coefficient becomes independent of the Hund's coupling JH in the
range of parameters JH*S >> W >> T, W being the bandwidth, relevant to colossal
magnetoresistive manganites in the metallic part of their phase diagram. Our
study reveals a close correspondence as well as some counterintuitive
differences between the results on Bethe and hypercubic lattices. Our results
are in accord with neutron scattering data and with previous theoretical work
for high temperatures.Comment: 4.0 pages, 3 figures, RevTeX 4, replaced with the published versio
Suppression of ferromagnetic spin fluctuations in the filled skutterudite superconductor SrOs4As12 revealed by As-75 NMR-NQR measurements
Motivated by the recent observation of ferromagnetic spin correlations in the filled skutterudite SrFe4As12 [Q.-P. Ding et al., Phys. Rev. B 98, 155149 (2018)], we have carried out As-75 nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements to investigate the role of magnetic fluctuations in the newly discovered isostructural superconductor SrOs4As12 with a superconducting transition temperature of T-c similar to 4.8 K. Knight shift K determined by the NQR spectrum under a small magnetic field (<= 0.5 T) is nearly independent of temperature, consistent with the temperature dependence of the magnetic susceptibility. The nuclear spin-lattice relaxation rate divided by temperature, 1/T1T, is nearly independent of temperature above similar to 50 K and increases slightly with decreasing temperature below the temperature. The temperature dependence is reasonably explained by a simple model where a flat band structure with a small ledge near the Fermi energy is assumed. By comparing the present NMR data with those in SrFe4As12, we found that the values of vertical bar K vertical bar and 1/T1T in SrOs4As12 are smaller than those in SrFe4As12, indicating no obvious ferromagnetic spin correlations in SrOs4As12. From the temperature dependence of 1/T-1 in the superconducting state, an s-wave superconductivity is realized
Unconventional one-magnon scattering resistivity in half metals
Low-temperature resistivity of half-metals is investigated. To date it has
been discussed that the one-magnon scattering process in half-metals is
irrelevant for low-temperature resistivity, due to the fully spin-polarized
electronic structure at the ground state. If one takes into account the
non-rigid-band behavior of the minority band due to spin fluctuations at finite
temperatures, however, the unconventional one-magnon scattering process is
shown to be most relevant and gives T^3 dependence in resistivity. This
behavior may be used as a crucial test in the search for half-metallic
materials which are potentially important for applications. Comparison with
resistivity data of
La_1-x Sr_x MnO_3 as candidates for half-metals shows good agreement.Comment: 4 pages, including 5 eps figures. To be published in J. Phys. Soc.
Jpn. vol. 69 No. 7 (2000
Recombinant Thrombomodulin Has an Antitumor Effect and Enhances the Sensitivity of Gemcitabine Treatment of Pancreatic Cancer via G-protein Coupled Receptor 15
https://openworks.mdanderson.org/sumexp21/1223/thumbnail.jp
The charge ordered state in half-doped Bi-based manganites studied by O and Bi NMR
We present a Bi and O NMR study of the Mn electron spin
correlations developed in the charge ordered state of
BiSrMnO and BiCaMnO. The unusually
large local magnetic field indicates the dominant
character of the lone electron pair of Bi-ions in both compounds. The
mechanism connecting the character of the lone pairs to the high
temperature of charge ordering is still not clarified. The observed
difference in for BiSrMnO to
BiCaMnO is probably due to a decrease in the canting of
the staggered magnetic moments of Mn-ions from. The modification of the
O spectra below demonstrates that the line due to the apical
oxygens is a unique local tool to study the development of the Mn spin
correlations. In the AF state the analysis of the O spectrum of
PrCaMnO and BiSrMnO prompts us to
try two different theoretical descriptions of the charge-ordered state, a
site-centered model for the first manganite and a bond-centered model for the
second one.Comment: 10 pages, 7 figure
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