4,491 research outputs found
Negative tunnel magnetoresistance and differential conductance in transport through double quantum dots
Spin-dependent transport through two coupled single-level quantum dots weakly
connected to ferromagnetic leads with collinear magnetizations is considered
theoretically. Transport characteristics, including the current, linear and
nonlinear conductance, and tunnel magnetoresistance are calculated using the
real-time diagrammatic technique in the parallel, serial, and intermediate
geometries. The effects due to virtual tunneling processes between the two dots
via the leads, associated with off-diagonal coupling matrix elements, are also
considered. Negative differential conductance and negative tunnel
magnetoresistance have been found in the case of serial and intermediate
geometries, while no such behavior has been observed for double quantum dots
coupled in parallel. It is also shown that transport characteristics strongly
depend on the magnitude of the off-diagonal coupling matrix elements.Comment: 12 pages, 13 figure
Temperature dependence of spinon and holon excitations in one-dimensional Mott insulators
Motivated by the recent angle-resolved photoemission spectroscopy (ARPES)
measurements on one-dimensional Mott insulators, SrCuO and
NaVO, we examine the single-particle spectral weight
of the one-dimensional (1D) Hubbard model at half-filling. We are particularly
interested in the temperature dependence of the spinon and holon excitations.
For this reason, we have performed the dynamical density matrix renormalization
group and determinantal quantum Monte Carlo (QMC) calculations for the
single-particle spectral weight of the 1D Hubbard model. In the QMC data, the
spinon and holon branches become observable at temperatures where the
short-range antiferromagnetic correlations develop. At these temperatures, the
spinon branch grows rapidly. In the light of the numerical results, we discuss
the spinon and holon branches observed by the ARPES experiments on
SrCuO. These numerical results are also in agreement with the
temperature dependence of the ARPES results on NaVO.Comment: 8 pages, 8 figure
Origin of spatial variation of pairing gap in Bi-based high-Tc cuprates
Recently, scanning tunneling microscopy on Bi-2212 cuprate superconductor has
revealed a spatial variation of the energy gap that is directly correlated with
a modulation of the apical oxygen position. We identify two mechanisms by which
out-of-plane oxygens can modulate the pairing interaction within the CuO_2
layer: a covalency between the x^2-y^2 band and apical p-orbital, and a
screening of correlation U by apical oxygen polarization. Both effects strongly
depend on the apical oxygen position and their cooperative action explains the
experiment.Comment: 4 pages, 3 figures, revised version to appear in PR
Enhanced Pairing Correlations near Oxygen Dopants in Cuprate Superconductors
Recent experiments on Bi-based cuprate superconductors have revealed an
unexpected enhancement of the pairing correlations near the interstitial oxygen
dopant ions. Here we propose a possible mechanism -- based on local screening
effects -- by which the oxygen dopants do modify the electronic parameters
within the CuO_2 planes and strongly increase the superexchange coupling J.
This enhances the spin pairing effects locally and may explain the observed
spatial variations of the density of states and the pairing gap.Comment: 4 pages, 4 figures, minor revisions, to appear in PR
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