384 research outputs found
Double exchange model for RuSr_2(Eu,Gd)Cu_2O_8
We propose a double exchange model to describe the RuO_2 planes of
RuSr_2(Eu,Gd)Cu_2O_8. The Ru^+5 ions are described by localized spins, and
additional electrons provided by the superconducting CuO_2 planes are coupled
ferromagnetically to them by Hund rules coupling. We calculate the spin
structure factor, magnetic susceptibility and magnetization as a function of
magnetic field and temperature, using a Monte Carlo algorithm in which the
Ru^+5 spins are treated as classical. Several experiments which seemed in
contradiction with one another are explained by the theory.Comment: 3 pages, 3 figs., submitted to LAW3M conferenc
Effect of covalency and interactions on the trigonal splitting in NaxCoO2
We calculate the effective trigonal crystal field Delta which splits the t2g
levels of effective models for NaxCoO2 as the local symmetry around a Co ion is
reduced from Oh to D3d. To this end we solve numerically a CoO6 cluster
containing a Co ion with all 3d states and their interactions included, and its
six nearest-neighbor O atoms, with the geometry of the system, in which the
CoO6 octahedron is compressed along a C3 axis. We obtain Delta near 130 meV,
with the sign that agrees with previous quantum chemistry calculations, but
disagrees with first-principles results in the local density approximation
(LDA). We find that Delta is very sensitive to a Coulomb parameter which
controls the Hund coupling and charge distribution among the d orbitals. The
origin of the discrepancy with LDA results is discussed.Comment: 6 pages, 2 figure
Comment on "Conductance scaling in Kondo-correlated quantum dots: Role of level asymmetry and charging energy"
In a recent work [L. Merker, S. Kirchner, E. Munoz, and T. A. Costi, Phys.
Rev. B 87, 165132 (2013)], the authors compared results of numerical
renormalization group and a perturbative approach for the dependence on
temperature T and magnetic field B of the conductance through a quantum dot
described by the impurity Anderson model, for small T and B. We show that the
equation used to extract the dependence on B from NRG results is incorrect out
of the particle-hole symmetric case. As a consequence, in the Kondo regime, the
correct NRG results have a weaker dependence on B and the disagreement between
both approaches increase.Comment: 3 pages, 1 figure, references update
Universal out-of-equilibrium transport in Kondo-correlated quantum dots
It is shown that the lesser and greater self energies and Green functions
calculated in a recent work on universal noequilibrium transport in interacting
quantum dots [E. Mu\~noz, C. J. Bolech, and S. Kirchner, Phys. Rev. Lett. 110,
016601 (2013)] are incorrect. The authors start from renormalized second-order
perturbation theory in the Coulomb repulsion in the particle-hole symmetric
impurity Anderson model. The reported lesser self-energy is compared with the
correct one, and with an analytic result valid for small frequency and applied
voltage. As a consequence of the mistakes, the conservation of the current is
not established and the results are unreliable.Comment: 1 page and a few lines, 1 figure, Comment on E. Mu\~noz, C. J.
Bolech, and S. Kirchner, Phys. Rev. Lett. 110, 016601 (2013). In version 3
further precisions are given regarding a simple mistake in the Letter (Ref.
2) and a comment to the use of Ward identities in Ref. 1 is adde
Entangled end states with fractionalized spin projection in a time-reversal-invariant topological superconducting wire
We study the ground state and low-energy subgap excitations of a finite wire
of a time-reversal-invariant topological superconductor (TRITOPS) with
spin-orbit coupling. We solve the problem analytically for a long chain of a
specific one-dimensional lattice model in the electron-hole symmetric
configuration and numerically for other cases of the same model. We present
results for the spin density of excitations in long chains with an odd number
of particles. The total spin projection along the axis of the spin-orbit
coupling is distributed with fractions localized at
both ends, and shows even-odd alternation along the sites of the chain. We
calculate the localization length of these excitations and find that it can be
well approximated by a simple analytical expression. We show that the energy
of the lowest subgap excitations of the finite chain defines tunneling and
entanglement between end states.We discuss the effect of a Zeeman coupling
on one of the ends of the chain only. For , the energy
difference of excitations with opposite spin orientation is ,
consistent with a spin projection . We argue that these physical
features are not model dependent and can be experimentally observed in TRITOPS
wires under appropriate conditions.Comment: 14 pages, 8 Figure
Phase diagram of the extended Hubbard chain with charge-dipole interactions
We consider a modified extended Hubbard model (EHM) which, in addition to the
on-site repulsion U and nearest-neighbor repulsion V, includes polarization
effects in second-order perturbation theory. The model is equivalent to an EHM
with renormalized U plus a next-nearest-neighbor repulsion term. Using a method
based on topological quantum numbers (charge and spin Berry phases), we
generalize to finite hopping t the quantum phase diagram in one dimension
constructed by van den Brink et al. (Phys. Rev. Lett. 75, 4658 (1995)). At
hopping t=0 there are two charge density-wave phases, one spin density-wave
phase and one intermediate phase with charge and spin ordering, depending on
the parameter values. At t \neq 0 the nature of each phase is confirmed by
studying correlation functions. However, in addition to the strong-coupling
phases, a small region with bond ordering appears. The region occupied by the
intermediate phase first increases and then decreases with increasing t, until
it finally disappears for t of the order but larger than U. For small t, the
topological transitions agree with the results of second order perturbation
theory.Comment: 6 pages, 5 figures, two columns latex version. Accepted for
publication in Physical Review B. Mistaken reference 16 has been correcte
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