426 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
Frustrated impurity spins in ordered two-dimensional quantum antiferromagnets
Dynamical properties of an impurity spin coupled symmetrically to sublattices
of ordered 2D Heisenberg quantum antiferromagnet (i.e., frustrated impurity
spin) are discussed at (existence of a small interaction stabilizing
the long range order at is implied). We continue our study on this
subject started in Phys.Rev.B 72, 174419 (2005), where spin-1/2 defect is
discussed and the host spins fluctuations are considered within the spin-wave
approximation (SWA). In the present paper we i) go beyond SWA and ii) study
impurities with spins . It is demonstrated that in contrast to defects
coupled to sublattices asymmetrically longitudinal host spins fluctuations play
important role in the frustrated impurity dynamics. The spectral function, that
is proportional to within SWA, acquires new terms proportional to
and . It is observed that the spin-1/2 impurity
susceptibility has the same structure as that obtained within SWA: the Lorenz
peak and the non-resonant term. The difference is that the width of the peak
becomes larger being proportional to rather than ,
where is the dimensionless coupling parameter. We show that transverse
static susceptibility acquires a new negative logarithmic contribution. In
accordance with previous works we find that host spins fluctuations lead to an
effective one-ion anisotropy on the impurity site. Then defects with
appears to be split. We observe strong reduction of the value of the splitting
due to longitudinal host spins fluctuations. We demonstrate that the dynamical
impurity susceptibility contains Lorenz peaks corresponding to transitions
between the levels, and the non-resonant term.Comment: 17 pages, 7 figures, to appear in PR
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
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
Transverse instabilities of multiple vortex chains in superconductor-ferromagnet bilayers
Using scanning tunneling microscopy and Ginzburg-Landau simulations we
explore vortex configurations in magnetically coupled NbSe-Permalloy
superconductor-ferromagnet bilayer. The Permalloy film with stripe domain
structure induces periodic local magnetic induction in the superconductor
creating a series of pinning-antipinning channels for externally added magnetic
flux quanta. Such laterally confined Abrikosov vortices form quasi-1D arrays
(chains). The transitions between multichain states occur through propagation
of kinks at the intermediate fields. At high fields we show that the system
becomes non-linear due to a change in both the number of vortices and the
confining potential. The longitudinal instabilities of the resulting vortex
structures lead to vortices `levitating' in the anti-pinning channels.Comment: accepted in PRB-Rapid
Disentanglement of the electronic and lattice parts of the order parameter in a 1D Charge Density Wave system probed by femtosecond spectroscopy
We report on the high resolution studies of the temperature (T) dependence of
the q=0 phonon spectrum in the quasi one-dimensional charge density wave (CDW)
compound K0.3MoO3 utilizing time-resolved optical spectroscopy. Numerous modes
that appear below Tc show pronounced T-dependences of their amplitudes,
frequencies and dampings. Utilizing the time-dependent Ginzburg-Landau theory
we show that these modes result from linear coupling of the electronic part of
the order parameter to the 2kF phonons, while the (electronic) CDW amplitude
mode is overdamped.Comment: 4 pages, 3 figures + supplementary material, accepted for publication
in Phys. Rev. Let
Dynamic spin susceptibility in the t-J model
A relaxation-function theory for the dynamic spin susceptibility in the
-- model is presented. By a sum-rule-conserving generalized mean-field
approximation (GMFA), the two-spin correlation functions of arbitrary range,
the staggered magnetization, the uniform static susceptibility, and the
antiferromagnetic correlation length are calculated in a wide region of hole
doping and temperaturs. A good agreement with available exact diagonalization
(ED) data is found. The correlation length is in reasonable agreement with
neutron-scattering experiments on La_{2-\delta}Sr_\delta)CuO_4. Going beyond
the GMFA, the self-energy is calculated in the mode-coupling approximation. The
spin dynamics at arbitrary frequencies and wave vectors is studied for various
temperatures and hole doping. At low doping a spin-wave-type behavior is found
as in the Heisenberg model, while at higher doping a strong damping caused by
hole hopping occurs, and a relaxation-type spin dynamics is observed in
agreement with the ED results. The local spin susceptibility and its (\omega/T)
scaling behavior are calculated in a reasonable agreement with experimental and
ED data.Comment: 13 pages, 14 figure
Vanishing Meissner effect as a hallmark of in-plane FFLO instability in superconductor - ferromagnet layered systems
We demonstrate that in a wide class of multilayered superconductor -
ferromagnet structures (e.g., S/F, S/F/N and S/F/F') the vanishing Meissner
effect signals the appearance of the in-plane Fulde-Ferrell-Larkin-Ovchinnikov
(FFLO) modulated superconducting phase. In contrast to the bulk superconductors
the FFLO instability in these systems can emerge at temperatures close to the
critical one and is effectively controlled by the S layer thickness and the
angle between magnetization vectors in the F/F' bilayers. The predicted FFLO
state reveals through the critical temperature oscillations vs the
perpendicular magnetic field component.Comment: 5 pages, 5 figure
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