262 research outputs found
Single-hole dynamics in the half-filled two-dimensional Kondo-Hubbard model
We consider the Kondo lattice model in two dimensions at half filling. In
addition to the fermionic hopping integral and the superexchange coupling
the role of a Coulomb repulsion in the conduction band is investigated.
We find the model to display a magnetic order-disorder transition in the U-J
plane with a critical value of J_c which is decreasing as a function of U. The
single particle spectral function A(k,w) is computed across this transition.
For all values of J > 0, and apart from shadow features present in the ordered
state, A(k,w) remains insensitive to the magnetic phase transition with the
first low-energy hole states residing at momenta k = (\pm \pi, \pm \pi). As J
-> 0 the model maps onto the Hubbard Hamiltonian. Only in this limit, the
low-energy spectral weight at k = (\pm \pi, \pm \pi) vanishes with first
electron removal-states emerging at wave vectors on the magnetic Brillouin zone
boundary. Thus, we conclude that (i) the local screening of impurity spins
determines the low energy behavior of the spectral function and (ii) one cannot
deform continuously the spectral function of the Mott-Hubbard insulator at J=0
to that of the Kondo insulator at J > J_c. Our results are based on both, T=0
Quantum Monte-Carlo simulations and a bond-operator mean-field theory.Comment: 8 pages, 7 figures. Submitted to PR
Boundaries, Cusps and Caustics in the Multimagnon Continua of 1D Quantum Spin Systems
The multimagnon continua of 1D quantum spin systems possess several
interesting singular features that may soon be accessible experimentally
through inelastic neutron scattering. These include cusps and composition
discontinuities in the boundary envelopes of two-magnon continuum states and
discontinuities in the density of states, "caustics", on and within the
continuum, which will appear as discontinuities in scattering intensity. In
this note we discuss the general origins of these continuum features, and
illustrate our results using the alternating Heisenberg antiferromagnetic chain
and two-leg ladder as examples.Comment: 18 pages, 10 figure
Dynamical structure factors of two-leg spin ladder systems
We investigate dynamical properties of two-leg spin ladder systems.
In a strong coupling region, an isolated mode appears in the lowest excited
states, while in a weak coupling region, an isolated mode is reduced and the
lowest excited states become a lower bound of the excitation continuum. We find
in the system with equal intrachain and interchain couplings that due to a
cyclic four-spin interaction, the distribution of the weights for the dynamical
structure factor and characteristics of the lowest excited states are strongly
influenced. The dynamical properties of two systems proposed for are also discussed.Comment: 5 pages, 6 figure
Magnetism of a tetrahedral cluster spin-chain
We discuss the magnetic properties of a dimerized and completely frustrated
tetrahedral spin-1/2 chain. Using a combination of exact diagonalization and
bond-operator theory the quantum phase diagram is shown to incorporate a
singlet-product, a dimer, and a Haldane phase. In addition we consider one-,
and two-triplet excitations in the dimer phase and evaluate the magnetic Raman
cross section which is found to be strongly renormalized by the presence of a
two-triplet bound state. The link to a novel tellurate materials is clarified.Comment: 8 pages, 8 figure
Dynamical structure factors of the magnetization-plateau state in the bond-alternating spin chain with a next-nearest-neighbor interaction
We calculate the dynamical structure factors of the magnetization-plateau
state in the bond-alternating spin chain with a next-nearest-neighbor
interaction. The results show characteristic behaviors depending on the
next-nearest-neighbor interaction and the bond-alternation .
We discuss the lower excited states in comparison with the exact excitation
spectrums of an effective Hamiltonian. From the finite size effects,
characteristics of the lowest excited states are investigated. The
dispersionless mode of the lowest excitation appears in adequate sets of
and , indicating that the lowest excitation is localized
spatially and forms an isolated mode below the excitation continuum. We further
calculate the static structure factors. The largest intensity is located at
for small in fixed . With increasing , the
wavenumber of the largest intensity shifts towards , taking the
incommensurate value.Comment: to appear in Phys. Rev. B (2001
Optical absorption spectra in SrCu_2O_3 two-leg spin ladder
We calculate the phonon-assisted optical-absorption spectra in SrCu_2O_3
two-leg spin-ladder systems. The results for two models proposed for SrCu_2O_3
are compared. In the model including the effects of a cyclic four-spin
interaction, the shoulder structure appears at 978 cm^{-1} and the peak appears
at 1975 cm^{-1} in the spectrum for polarization of the electric field parallel
to the legs. In the other model which describes a pure two-leg ladder, the peak
appears around the lower edge of the spectrum at 1344 cm^{-1}. The feature can
be effective in determining the proper model for SrCu_2O_3.Comment: 5 pages, 5 figures, to appear in PRB vol. 67 (2003
Single-hole dynamics in dimerized and frustrated spin-chains
We present a unified account for the coupled single-hole- and spin-dynamics
in the spin-gap phase of dimerized and frustrated spin-chains and two-leg spin
ladders. Based on the strong dimer-limit of a one-dimensional
--model a diagrammatic approach is presented which employs a
mapping of the spin-Hamiltonian onto a pseudo-fermion bond-boson model. Results
for the single-hole spectrum are detailed. A finite quasi-particle weight is
observed and studied for a variety of system parameters. A comparison with
existing exact diagonalization data is performed and good agreement is found.Comment: 10 pages, 12 figure
Magnetic field effects on the density of states of orthorhombic superconductors
The quasiparticle density of states in a two-dimensional d-wave
superconductor depends on the orientation of the in-plane external magnetic
field H. This is because. in the region of the gap nodes, the Doppler shift due
to the circulating supercurrents around a vortex depend on the direction of H.
For a tetragonal system the induced pattern is four-fold symmetric and, at zero
energy, the density of states exhibits minima along the node directions. But
YBa_2C_3O_{6.95} is orthorhombic because of the chains and the pattern becomes
two-fold symmetric with the position of the minima occuring when H is oriented
along the Fermi velocity at a node on the Fermi surface. The effect of impurity
scattering in the Born and unitary limit is discussed.Comment: 24 pages, 11 Figure
Jordan-Wigner approach to dynamic correlations in spin-ladders
We present a method for studying the excitations of low-dimensional quantum
spin systems based on the Jordan-Wigner transformation. Using an extended
RPA-scheme we calculate the correlation function of neighboring spin flips
which well approximates the optical conductivity of . We
extend this approach to the two-leg --ladder by numbering the spin
operators in a meander-like sequence. We obtain good agreement with the optical
conductivity of the spin ladder compound (La,Ca)CuO for
polarization along the rungs. For polarization along the legs higher order
correlations are important to explain the weight of high-energy continuum
excitations and we estimate the contribution of 4-- and 6--fermion processes.Comment: 15 pages, 16 figure
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