1,798 research outputs found
Cu-based metalorganic systems: an ab initio study of the electronic structure
Within a first principles framework, we study the electronic structure of the
recently synthesized polymeric coordination compound
Cu(II)-2,5-bis(pyrazol-1-yl)-1,4-dihydroxybenzene (CuCCP), which has been
suggested to be a good realization of a Heisenberg spin-1/2 chain with
antiferromagnetic coupling. By using a combination of classical with ab initio
quantum mechanical methods, we design on the computer reliable modified
structures of CuCCP aimed at studying effects of Cu-Cu coupling strength
variations on this spin-1/2 system. For this purpose, we performed two types of
modifications on CuCCP. In one case, we replaced H in the linker by i) an
electron donating group (NH2) and ii) an electron withdrawing group (CN), while
the other modification consisted in adding H2O and NH3 molecules in the
structure which change the local coordination of the Cu(II) ions. With the
NMTO-downfolding method we provide a quantitative analysis of the modified
electronic structure and the nature of the Cu-Cu interaction paths in these new
structures and discuss its implications for the underlying microscopic model.Comment: 18 pages, 11 figures, final versio
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Polarized proton beam acceleration with a single Siberian Snake in each RHIC ring
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A Study of Polarized Proton Acceleration in J-Parc.
We have studied the feasibility of polarized proton acceleration in rhe J-PARC accelerator facility, consisting of a 400 MeV linac, a 3 GeV rapid cycling synchrotron (RCS) and a 50 GeV synchrotron (MR). We show how the polarization of the beam can be preserved using an rf dipole in the RCS and two superconductive partial helical Siberian snakes in the MR. The lattice of the MR will be modified with the addition of quadrupoles to compensate for the focusing properties of the snakes
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Optimization of the AGS superconducting helical partial snake strength.
Two helical partial snakes, one super-conducting (a.k.a cold snake) and one normal conducting (a.k.a warm snake), have preserved the polarization of proton beam up to 65% in the Brookhaven Alternating Gradient Synchrotron (AGS) at the extraction energy from 85% at injection. In order to overcome spin resonances, stronger partial snakes would be required. However, the stronger the partial snake, the more the stable spin direction tilted producing a stronger horizontal intrinsic resonance. The balance between increasing the spin tune gap generated by the snakes and reducing the tilted stable spin direction has to be considered to maintain the polarization. Because the magnetic field of the warm snake has to be a constant, only the cold snake with a maximum 3T magnetic field can be varied to find out the optimum snake strength. This paper presents simulation results by spin tracking with different cold snake magnetic fields. Some experimental data are also analyzed
First principles electronic structure of spinel LiCr2O4: A possible half-metal?
We have employed first-principles electronic structure calculations to
examine the hypothetical (but plausible) oxide spinel, LiCr2O4 with the d^{2.5}
electronic configuration. The cell (cubic) and internal (oxygen position)
structural parameters have been obtained for this compound through structural
relaxation in the first-principles framework. Within the one-electron band
picture, we find that LiCr2O4 is magnetic, and a candidate half-metal. The
electronic structure is substantially different from the closely related and
well known rutile half-metal CrO2. In particular, we find a smaller conduction
band width in the spinel compound, perhaps as a result of the distinct topology
of the spinel crystal structure, and the reduced oxidation state. The magnetism
and half-metallicity of LiCr2O4 has been mapped in the parameter space of its
cubic crystal structure. Comparisons with superconducting LiTi2O4 (d^{0.5}),
heavy-fermion LiV2O4 (d^{1.5}) and charge-ordering LiMn2O4 (d^{3.5}) suggest
the effectiveness of a nearly-rigid band picture involving simple shifts of the
position of E_F in these very different materials. Comparisons are also made
with the electronic structure of ZnV2O4 (d^{2}), a correlated insulator that
undergoes a structural and antiferromagnetic phase transition.Comment: 9 pages, 7 Figures, version as published in PR
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