422 research outputs found
Entanglement dynamics and quantum state transport in spin chains
We study the dynamics of a Heisenberg-XY spin chain with an unknown state
coded into one qubit or a pair of entangled qubits, with the rest of the spins
being in a polarized state. The time evolution involves magnon excitations, and
through them the entanglement is transported across the channel. For a large
number of qubits, explicit formulae for the concurrences, measures for
two-qubit entanglements, and the fidelity for recovering the state some
distance away are calculated as functions of time. Initial states with an
entangled pair of qubits show better fidelity, which takes its first maximum
value at earlier times, compared to initial states with no entangled pair. In
particular initial states with a pair of qubits in an unknown state (alpha
up-up + beta down-down) are best suited for quantum state transport.Comment: 4 pages, 3 figure
Near-Boundary and Bulk Regions of a Semi-Infinite Two-Dimensional Heisenberg Antiferromagnet
Using the spin-wave approximation elementary excitations of a semi-infinite
two-dimensional Heisenberg antiferromagnet are considered. The
spectrum consists of bulk modes -- standing spin waves and a
quasi-one-dimensional mode of boundary spin waves. These latter excitations
eject bulk modes from two boundary rows of sites, thereby dividing the
antiferromagnet into two regions with different dominant excitations. As a
result absolute values of nearest-neighbor spin correlations on the edge exceed
the bulk value.Comment: 8 pages, 3 figure
Ferromagnetic spin-polaron on complex lattices
We present a simpler derivation of the exact solution of a spin-polaron in a
ferromagnet and generalize it to complex lattices and/or longer range exchange
interactions. As a specific example, we analyze a two-dimensional MnO-like
lattice (as in the ferromagnetic layers in LaMnO) and discuss the
properties of the resulting spin-polaron in various regimes. At strong
couplings the solution is reminiscent of the Zhang-Rice singlet, however the
electronic wavefunction involved in the singlet is dependent on the momentum of
the singlet, and multiple bands may appear.Comment: 12 pages, 7 figure
A generating functional approach to the Hubbard model
The method of generating functional is generalized to the case of strongly
correlated systems, and applied to the Hubbard model. For the electronic
Green's function constructed for Hubbard operators, an equation using
variational derivatives with respect to the fluctuating fields has been derived
and its multiplicative form has been determined. Corrections for the electronic
self-energy are calculated up to the second order with respect to the parameter
W/U (W width of the band), and a mean field type approximation was formulated,
including both charge and spin static fluctuations. The equations for the
Bose-like Green's functions have been derived, describing the collective modes:
the magnons and doublons. The properties of the poles of the doublon Green's
functions depend on electronic filling. The investigation of the special case
n=1 demonstrates that the doublon Green's function has a soft mode at the wave
vector Q=(pi,pi,...), indicating possible instability of the uniform
paramagnetic phase relatively to the two sublattices charge ordering. However
this instability should compete with an instability to antiferromagnetic
ordering.Comment: 31 pages, 7 figures, to be published in Eur. Phys. J.
Krylov-Bogoliubov-Mitropolsky Averaging Used to Construct Effective Hamiltonians in the Theory of Strongly Correlated Electron Systems
We show that the Krylov-Bogoliubov-Mitropolsky averaging in the canonical
formulation can be used as a method for constructing effective Hamiltonians in
the theory of strongly correlated electron systems. As an example, we consider
the transition from the Hamiltonians of the Hubbard and Anderson models to the
respective Hamiltonians of the t-J and Kondo models. This is a very general
method, has several advantages over other methods, and can be used to solve a
wide range of problems in the physics of correlated systems.Comment: 9 page
Magnetic properties of -FeCr alloy as calculated with the charge and spin self-consistent KKR(CPA) method
Magnetic properties of a FeCr alloy calculated with
the charge and spin self- consistent Korringa-Kohn-Rostoker (KKR) and combined
with coherent potential approximation (KKR-CPA) methods are reported.
Non-magnetic state as well as various magnetic orderings were considered, i.e.
ferromagnetic (FM) and more complex anti-parallel (called APM) arrangements for
selected sublattices, as follows from the symmetry analysis. It has been shown
that the Stoner criterion applied to non-magnetic density of states at the
Fermi energy, is satisfied for Fe atoms situated on all five lattice
sites, while it is not fulfilled for all Cr atoms. In FM and APM states, the
values of magnetic moments on Fe atoms occupying various sites are dispersed
between 0 and 2.5 , and they are proportional to the number of Fe atoms
in the nearest-neighbor shell. Magnetic moments of Cr atoms havin much smaller
values were found to be coupled antiparallel to those of Fe atoms. The average
value of the magnetic moment per atom was found to be that
is by a factor of 4 larger than the experimental value found for a
FeCr sample. Conversely, admitting an anti-
parallel ordering (APM model) on atoms situated on C and D sites, according to
the group theory and symmetry analysis results, yielded a substantial reduction
of to 0.20 $\mu_B$. Further diminution of to 0.15 ,
which is very close to the experimental value of 0.14 , has been
achieved with the KKR-CPA calculations by considering a chemical disorder on
sites B, C and D
Single Electron Spin Decoherence by Nuclear Spin Bath: Linked Cluster Expansion Approach
We develop a theoretical model for transverse dynamics of a single electron
spin interacting with a nuclear spin bath. The approach allows a simple
diagrammatic representation and analytical expressions of different nuclear
spin excitation processes contributing to electron spin decoherence and
dynamical phase fluctuations. It accounts for nuclear spin dynamics beyond
conventional pair correlation models. As an illustration of the theory, we
evaluated the coherence dynamics of a P donor electron spin in a Si crystal.Comment: 37 pages, 13 figure
Ground state of a double-exchange system containing impurities: bounds of ferromagnetism
We study the boundary between ferromagnetic and non-ferromagnetic ground
state of a double-exchange system with quenched disorder for arbitrary relation
between Hund exchange coupling and electron band width. The boundary is found
both from the solution of the Dynamical Mean Field Approximation equations and
from the comparison of the energies of the saturated ferromagnetic and
paramagnetic states. Both methods give very similar results. To explain the
disappearance of ferromagnetism in part of the parameter space we derive from
the double-exchange Hamiltonian with classical localized spins in the limit of
large but finite Hund exchange coupling the model (with classical
localized spins).Comment: 5 pages, 8 eps figures, latex; minor typos correcte
The t-J model on a semi-infinite lattice
The hole spectral function of the t-J model on a two-dimensional
semi-infinite lattice is calculated using the spin-wave and noncrossing
approximations. In the case of small hole concentration and strong
correlations, , several near-boundary site rows appear to be depleted
of holes. The reason for this depletion is a deformation of the magnon cloud,
which surrounds the hole, near the boundary. The hole depletion in the boundary
region leads to a more complicated spectral function in the boundary row in
comparison with its bulk shape.Comment: 8 pages, 5 figure
Superconductivity and superconducting order parameter phase fluctuations in a weakly doped antiferromagnet
The superconducting properties of a recently proposed phenomenological model
for a weakly doped antiferromagnet are analyzed, taking into account
fluctuations of the phase of the order parameter. In this model, we assume that
the doped charge carriers can't move out of the antiferromagnetic sublattice
they were introduced. This case corresponds to the free carrier spectra with
the maximum at , as it was observed in ARPES
experiments in some of the cuprates in the insulating state [1]. The doping
dependence of the superconducting gap and the temperature-carrier density phase
diagram of the model are studied in the case of the pairing
symmetry and different values of the effective coupling. A possible relevance
of the results to the experiments on high-temperature superconductors is
discussed.Comment: 16 pages, 4 figure
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