795 research outputs found
Double Time Window Targeting Technique: Real time DMRG dynamics in the PPP model
We present a generalized adaptive time-dependent density matrix
renormalization group (DMRG) scheme, called the {\it double time window
targeting} (DTWT) technique, which gives accurate results with nominal
computational resources, within reasonable computational time. This procedure
originates from the amalgamation of the features of pace keeping DMRG
algorithm, first proposed by Luo {\it et. al}, [Phys.Rev. Lett. {\bf 91},
049701 (2003)], and the time-step targeting (TST) algorithm by Feiguin and
White [Phys. Rev. B {\bf 72}, 020404 (2005)]. Using the DTWT technique, we
study the phenomena of spin-charge separation in conjugated polymers (materials
for molecular electronics and spintronics), which have long-range
electron-electron interactions and belong to the class of strongly correlated
low-dimensional many-body systems. The issue of real time dynamics within the
Pariser-Parr-Pople (PPP) model which includes long-range electron correlations
has not been addressed in the literature so far. The present study on PPP
chains has revealed that, (i) long-range electron correlations enable both the
charge and spin degree of freedom of the electron, to propagate faster in the
PPP model compared to Hubbard model, (ii) for standard parameters of the PPP
model as applied to conjugated polymers, the charge velocity is almost twice
that of the spin velocity and, (iii) the simplistic interpretation of
long-range correlations by merely renormalizing the {\it U} value of the
Hubbard model fails to explain the dynamics of doped holes/electrons in the PPP
model.Comment: Final (published) version; 39 pages, 13 figures, 1 table; 2 new
references adde
Exact diagonalization study of the tunable edge magnetism in graphene
The tunable magnetism at graphene edges with lengths of up to 48 unit cells
is analyzed by an exact diagonalization technique. For this we use a
generalized interacting one-dimensional model which can be tuned continuously
from a limit describing graphene zigzag edge states with a ferromagnetic phase,
to a limit equivalent to a Hubbard chain, which does not allow ferromagnetism.
This analysis sheds light onto the question why the edge states have a
ferromagnetic ground state, while a usual one-dimensional metal does not.
Essentially we find that there are two important features of edge states: (a)
umklapp processes are completely forbidden for edge states; this allows a
spin-polarized ground state. (b) the strong momentum dependence of the
effective interaction vertex for edge states gives rise to a regime of partial
spin-polarization and a second order phase transition between a standard
paramagnetic Luttinger liquid and ferromagnetic Luttinger liquid.Comment: 11 pages, 8 figure
Low ordered magnetic moment by off-diagonal frustration in undoped parent compounds to iron-based high-Tc superconductors
A Heisenberg model over the square lattice recently introduced by Si and
Abrahams to describe local-moment magnetism in the new class of Fe-As high-Tc
superconductors is analyzed in the classical limit and on a small cluster by
exact diagonalization. In the case of spin-1 iron atoms, large enough
Heisenberg exchange interactions between neighboring spin-1/2 moments on
different iron 3d orbitals that frustrate true magnetic order lead to hidden
magnetic order that violates Hund's rule. It accounts for the low ordered
magnetic moment observed by elastic neutron diffraction in an undoped parent
compound to Fe-As superconductors. We predict that low-energy spin-wave
excitations exist at wavenumbers corresponding to either hidden Neel or hidden
ferromagnetic order.Comment: 7 pages, 6 figures, version published in Physical Review Letter
Transition to Landau Levels in Graphene Quantum Dots
We investigate the electronic eigenstates of graphene quantum dots of
realistic size (i.e., up to 80 nm diameter) in the presence of a perpendicular
magnetic field B. Numerical tight-binding calculations and Coulomb-blockade
measurements performed near the Dirac point exhibit the transition from the
linear density of states at B=0 to the Landau level regime at high fields.
Details of this transition sensitively depend on the underlying graphene
lattice structure, bulk defects, and localization effects at the edges. Key to
the understanding of the parametric evolution of the levels is the strength of
the chiral-symmetry breaking K-K' scattering. We show that the parametric
variation of the level variance provides a quantitative measure for this
scattering mechanism. We perform measurements of the parametric motion of
Coulomb blockade peaks as a function of magnetic field and find good agreement.
We thereby demonstrate that the magnetic-field dependence of graphene energy
levels may serve as a sensitive indicator for the properties of graphene
quantum dots and, in further consequence, for the validity of the
Dirac-picture.Comment: 10 pages, 11 figures, higher quality images available on reques
A duality relation for fluid spacetime
We consider the electromagnetic resolution of gravitational field. We show
that under the duality transformation, in which active and passive electric
parts of the Riemann curvature are interchanged, a fluid spacetime in comoving
coordinates remains invariant in its character with density and pressure
transforming, while energy flux and anisotropic pressure remaining unaltered.
Further if fluid admits a barotropic equation of state,
where , which will transform to . Clearly the stiff fluid and dust are dual to each-other
while , will go to flat spacetime. However the n and the deSitter ) universes ar e self-dual.Comment: 5 pages, LaTeX version, Accepted in Classical Quantum Gravity as a
Lette
Uniqueness of a Negative Mode About a Bounce Solution
We consider the uniqueness problem of a negative eigenvalue in the spectrum
of small fluctuations about a bounce solution in a multidimensional case. Our
approach is based on the concept of conjugate points from Morse theory and is a
natural generalization of the nodal theorem approach usually used in one
dimensional case. We show that bounce solution has exactly one conjugate point
at with multiplicity one.Comment: 4 pages,LaTe
Numerical study of critical properties and hidden orders in dimerized spin ladders
Dimerized antiferromagnetic spin-1/2 ladders are known to exhibit a quantum
critical phase transition in the ground state, the existence or absence of
which is dependent on the dimerization pattern of the ladder. The gapped phases
cannot be distinguished by the conventional Landau long-range order parameter.
However, they possess a non-local (hidden) string order parameter, which is
non-zero in one phase and vanishes in the other. We use an exact
diagonalization technique to calculate ground state energies, energy gaps and
string order parameters of dimerized two- and three-leg Heisenberg ladders, as
well as a critical scaling analysis to yield estimates of the critical
exponents nu and beta.Comment: 7 pages, 14 figures. V.2: Extended version to appear in PR
Evolution of Thick Walls in Curved Spacetimes
We generalize our previous thick shell formalism to incorporate any
codimension-1 thick wall with a peculiar velocity and proper thickness bounded
by arbitrary spacetimes. Within this new formulation we obtain the equation of
motion of a spherically symmetric dust thick shell immersed in vacuum as well
as in Friedmann-Robertson-Walker spacetimes.Comment: 8 pages, 1 figur
Exact Solutions of Lovelock-Born-Infeld Black Holes
The exact five-dimensional charged black hole solution in Lovelock gravity
coupled to Born-Infeld electrodynamics is presented. This solution interpolates
between the Hoffmann black hole for the Einstein-Born-Infeld theory and other
solutions in the Lovelock theory previously studied in the literature. The
conical singularity of the metric around the origin can be removed by a proper
choice of the black hole parameters. The thermodynamic properties of the
solution are also analyzed and, in particular, it is shown that the behaviour
of the specific heat indicates the existence of a stability transition point in
the vacuum solutions. We discuss the similarities existing between this
five-dimensional geometry and the three-dimensional black hole. Like BTZ black
hole, the Lovelock black hole has an infinite lifetime.Comment: 9 pages, 7 figures, Revtex. Extended version of the paper published
in Phys. Rev.
Resolving all-order method convergence problems for atomic physics applications
The development of the relativistic all-order method where all single,
double, and partial triple excitations of the Dirac-Hartree-Fock wave function
are included to all orders of perturbation theory led to many important results
for study of fundamental symmetries, development of atomic clocks, ultracold
atom physics, and others, as well as provided recommended values of many atomic
properties critically evaluated for their accuracy for large number of
monovalent systems. This approach requires iterative solutions of the
linearized coupled-cluster equations leading to convergence issues in some
cases where correlation corrections are particularly large or lead to an
oscillating pattern. Moreover, these issues also lead to similar problems in
the CI+all-order method for many-particle systems. In this work, we have
resolved most of the known convergence problems by applying two different
convergence stabilizer methods, reduced linear equation (RLE) and direct
inversion of iterative subspace (DIIS). Examples are presented for B, Al,
Zn, and Yb. Solving these convergence problems greatly expands the
number of atomic species that can be treated with the all-order methods and is
anticipated to facilitate many interesting future applications
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