97 research outputs found
Magnetic properties of the spin Heisenberg chain with hexamer modulation of exchange
We consider the spin-1/2 Heisenberg chain with alternating spin exchange %on
even and odd sites in the presence of additional modulation of exchange on odd
bonds with period three. We study the ground state magnetic phase diagram of
this hexamer spin chain in the limit of very strong antiferromagnetic (AF)
exchange on odd bonds using the numerical Lanczos method and bosonization
approach. In the limit of strong magnetic field commensurate with the
dominating AF exchange, the model is mapped onto an effective Heisenberg
chain in the presence of uniform and spatially modulated fields, which is
studied using the standard continuum-limit bosonization approach. In absence of
additional hexamer modulation, the model undergoes a quantum phase transition
from a gapped string order into the only one gapless L\"uttinger liquid (LL)
phase by increasing the magnetic field. In the presence of hexamer modulation,
two new gapped phases are identified in the ground state at magnetization equal
to 1/3 and 2/3 of the saturation value. These phases reveal themselves also in
magnetization curve as plateaus at corresponding values of magnetization. As
the result, the magnetic phase diagram of the hexamer chain shows seven
different quantum phases, four gapped and three gapless and the system is
characterized by six critical fields which mark quantum phase transitions
between the ordered gapped and the LL gapless phases.Comment: 21 pages, 5 figures, Journal of Physics: Condensed Matter, 24,
116002, (2012
Magnetic phase diagram of the dimerized spin ladder
The ground-state magnetic phase diagram of a spin two-leg ladder with
alternating rung exchange is
studied using the analytical and numerical approaches. In the limit where the
rung exchange is dominant, we have mapped the model onto the effective quantum
sine-Gordon model with topological term and identified two quantum phase
transitions at magnetization equal to the half of saturation value from a
gapped to the gapless regime. These quantum transitions belong to the
universality class of the commensurate-incommensurate phase transition. We have
also shown that the magnetization curve of the system exhibits a plateau at
magnetization equal to the half of the saturation value. We also present a
detailed numerical analysis of the low energy excitation spectrum and the
ground state magnetic phase diagram of the ladder with rung-exchange
alternation using Lanczos method of numerical diagonalizations for ladders with
number of sites up to N=28. We have calculated numerically the magnetic field
dependence of the low-energy excitation spectrum, magnetization and the on-rung
spin-spin correlation function. We have also calculated the width of the
magnetization plateau and show that it scales as , where critical
exponent varies from in the case of a ladder with isotropic
antiferromagnetic legs to in the case of ladder with
ferromagnetic legs. Obtained numerical results are in an complete agreement
with estimations made within the continuum-limit approach.Comment: 8 pages, 6 figures. arXiv admin note: text overlap with
arXiv:cond-mat/0603153, arXiv:1110.446
Entanglement and quantum correlations in the XX spin- honeycomb lattice
The ground state phase diagram of the dimerized spin-1/2 XX honeycomb model
in presence of a transverse magnetic field (TF) is known. With the absence of
the magnetic field, two quantum phases, namely, the N\'eel and the dimerized
phases have been identified. Moreover, canted N\'eel and the paramagnetic (PM)
phases also emerge by applying the magnetic field. In this paper, using two
complementary numerical exact techniques, Lanczos exact diagonalization, and
Density matrix renormalization group (DMRG) methods, we study this model by
focusing on the quantum correlations, the concurrence, and the quantum discord
(QD) among nearest-neighbor spins. We show that the quantum correlations can
capture the position of the quantum critical points in the whole range of the
ground state phase diagram consistent with previous results. Although the
concurrence and the QD are short-range, informative about long-ranged critical
correlations. In addition, we address a "magnetic-entanglement" behavior that
starts from an entangled field around the saturation field.Comment: 8 pages, 5 figure
Violating Bell inequality using weak coherent states
We present an experimental investigation of two-photon interference using a
continuous-wave laser. We demonstrate the violation of the CHSH inequality
using the phase randomized weak coherent states from a continuous wave laser.
Our implementation serves as an approach to reveal the quantum nature of a
source that is considered to be a classical source.Comment: 6 pages, 2 figure
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