323 research outputs found
Superconductivity Driven by the Interband Coulomb Interaction and Implications for the Superconducting Mechanism of MgB2
Superconducting mechanism mediated by interband exchange Coulomb repulsion is
examined in an extended two-band Hubbard models with a wide band crossing the
Fermi level and coexisting with a narrower band located at moderately lower
energy. We apply newly developed path-integral renormalization group method to
reliably calculate pairing correlations. The correlation shows marked
enhancement at moderate amplitudes of the exchange Coulomb repulsion taken
smaller than the on-site repulsion for the narrower band. The pairing symmetry
is s-wave while it has unconventional phases with the opposite sign between the
order parameters on the two bands, in agreement with the mean-field prediction.
Since the band structure of recently discovered superconductor MgB shares
basic similarities with our model, we propose that the present results provide
a relevant clue for the understanding of the superconducting mechanism in
MgB as well as in this class of multi-band materials with good metallic
conduction in the normal state.Comment: 4pages, 2figure
Roles of two successive phase transitions in new spin-Peierls system TiOBr
In this sturdy, we determine the roles of two successive phase transitions in
the new spin-Peierls system TiOBr by electron and synchrotron X-ray diffraction
analyses. Results show an incommensurate superstructure along the h- and
k-directions between Tc1=27K and Tc2=47K, and a twofold superstructure which is
related to a spin-Peierls lattice distortion below Tc1. The diffuse scattering
observed above Tc2 indicates that a structural correlation develops at a high
temperature. We conclude that Tc2 is a second-order lock-in temperature, which
is related to the spin-Peierls lattice distortion with the incommensurate
structure, and that Tc1 is from incommensurate to commensurate phase transition
temperature accompanying the first-order spin-Peierls lattice distortion.Comment: 4 pages, 5 figure
Magnetisation of hole-doped CuO2 spin chains in Sr14-xCaxCu24O41
We report on magnetisation measurements of Sr14-xCaxCu24O41, with 0 <= x <=
12, in magnetic fields up to 16 T. The low temperature magnetic response of the
CuO2 spin chains changes strongly upon doping. For x = 0, the ground state with
nearly independent dimers is confirmed. Reduction of the number of holes in the
chains through Ca-doping leads to an additional contribution to the
magnetisation, which depends linearly on the magnetic field. Remarkably, the
slope of this linear contribution increases with the Ca content. We argue that
antiferromagnetic spin chains do not account for this behaviour but that the
hole dynamics might be involved.Comment: In v2, spelling of author names has been change
Orbital Ordering in ferromagnetic Lu2V2O7
We have observed the orbital ordering in the ferromagnetic Mott-insulator
Lu2V2O7 by the polarized neutron diffraction technique. The orbital ordering
pattern determined from the observed magnetic form factors can be explained in
terms of a linear combination of wave functions |yz>, |zx> and |xy>; |0> =
(1/3)^(1/2) |xy> + (1/3)^(1/2)|yz> + (1/3)^(1/2) |zx> which is proportional to
|(x + y + z)^2 - r^2>; where each orbital is extended toward the center-of-mass
of the V tetrahedron. We discuss the stability of the ferromagnetic Lu2V2O7,
using a Hubbard Hamiltonian with these three orbitals.Comment: 17pages. to be published in J. Phys. Soc. Jpn. 74 (2005
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