4,428 research outputs found
Impacts of dust reduction on the northward expansion of the African monsoon during the Green Sahara period
AbstractThe West African Monsoon (WAM) is crucial for the socio-economic stability of millions of people living in the Sahel. Severe droughts have ravaged the region in the last three decades of the 20th century, highlighting the need for a better understanding of the WAM dynamics. One of the most dramatic changes in the West African Monsoon (WAM) occurred between 15000–5000 yr BP, when increased summer rainfall led to the so-called “Green Sahara” and to a reduction in dust emissions from the region. However, model experiments are unable to fully reproduce the intensification and geographical expansion of the WAM during this period, even when vegetation over the Sahara is considered. Here, we use a fully coupled simulation for 6000 yr BP (Mid-Holocene) in which prescribed Saharan vegetation and dust concentrations are changed in turn. A closer agreement with proxy records is obtained only when both the Saharan vegetation changes and dust decrease are taken into account. The dust reduction strengthens the vegetation–albedo feedback, extending the monsoon's northern limit approximately 500 km further than the vegetation-change case only. We therefore conclude that accounting for changes in Saharan dust loadings is essential for improving model simulations of the WAM during the Mid-Holocene
Impurity corrections to the thermodynamics in spin chains using a transfer-matrix DMRG method
We use the density matrix renormalization group (DMRG) for transfer matrices
to numerically calculate impurity corrections to thermodynamic properties. The
method is applied to two impurity models in the spin-1/2 chain, namely a weak
link in the chain and an external impurity spin. The numerical analysis
confirms the field theory calculations and gives new results for the crossover
behavior.Comment: 9 pages in revtex format including 5 embedded figures (using epsf).
To appear in PRB. The latest version in PDF format can be found at
http://fy.chalmers.se/~eggert/papers/DMRGimp.pd
Topological effects at short antiferromagnetic Heisenberg chains
The manifestations of topological effects in finite antiferromagnetic
Heisenberg chains is examined by density matrix renormalization group technique
in this paper. We find that difference between integer and half-integer spin
chains shows up in ground state energy per site when length of spin chain is
longer than , where is a spin-spin correlation
length, for spin magnitude S up to 5/2. For open chains with spin magnitudes
to S=5, we verify that end states with fractional spin quantum numbers
exist and are visible even when the chain length is much smaller than the
correlation length . The end states manifest themselves in the structure
of the low energy excitation spectrum.Comment: 4 pages, 6 figure
The transition between hole-pairs and four-hole clusters in four-leg tJ ladders
Holes weakly doped into a four-leg \tj ladder bind in pairs. At dopings
exceeding a critical doping of four hole clusters are
observed to form in DMRG calculations. The symmetry of the ground state
wavefunction does not change and we are able to reproduce this behavior
qualitatively with an effective bosonic model in which the four-leg ladder is
represented as two coupled two-leg ladders and hole-pairs are mapped on hard
core bosons moving along and between these ladders. At lower dopings,
, a one dimensional bosonic representation for hole-pairs
works and allows us to calculate accurately the Luttinger liquid parameter
\krho, which takes the universal value \krho=1 as half-filling is
approached
Numerical renormalization group study of the 1D t-J model
The one-dimensional (1D) model is investigated using the density matrix
renormalization group (DMRG) method. We report for the first time a
generalization of the DMRG method to the case of arbitrary band filling and
prove a theorem with respect to the reduced density matrix that accelerates the
numerical computation. Lastly, using the extended DMRG method, we present the
ground state electron momentum distribution, spin and charge correlation
functions. The anomaly of the momentum distribution function first
discussed by Ogata and Shiba is shown to disappear as increases. We also
argue that there exists a density-independent beyond which the system
becomes an electron solid.Comment: Wrong set of figures were put in the orginal submissio
Spatially homogeneous ground state of the two-dimensional Hubbard model
We investigate the stability with respect to phase separation or charge
density-wave formation of the two-dimensional Hubbard model for various values
of the local Coulomb repulsion and electron densities using Green-function
Monte Carlo techniques. The well known sign problem is particularly serious in
the relevant region of small hole doping. We show that the difference in
accuracy for different doping makes it very difficult to probe the phase
separation instability using only energy calculations, even in the
weak-coupling limit () where reliable results are available. By contrast,
the knowledge of the charge correlation functions allows us to provide clear
evidence of a spatially homogeneous ground state up to .Comment: 7 pages and 5 figures. Phys. Rev. B, to appear 200
Quantum-fluctuation-induced repelling interaction of quantum string between walls
Quantum string, which was brought into discussion recently as a model for the
stripe phase in doped cuprates, is simulated by means of the
density-matrix-renormalization-group method. String collides with adjacent
neighbors, as it wonders, owing to quantum zero-point fluctuations. The energy
cost due to the collisions is our main concern. Embedding a quantum string
between rigid walls with separation d, we found that for sufficiently large d,
collision-induced energy cost obeys the formula \sim exp (- A d^alpha) with
alpha=0.808(1), and string's mean fluctuation width grows logarithmically \sim
log d. Those results are not understood in terms of conventional picture that
the string is `disordered,' and only the short-wave-length fluctuations
contribute to collisions. Rather, our results support a recent proposal that
owing to collisions, short-wave-length fluctuations are suppressed, but
instead, long-wave-length fluctuations become significant. This mechanism would
be responsible for stabilizing the stripe phase
Quantum-fluctuation-induced collisions and subsequent excitation gap of an elastic string between walls
An elastic string embedded between rigid walls is simulated by means of the
density-matrix renormalization group. The string collides against the walls
owing to the quantum-mechanical zero-point fluctuations. Such ``quantum
entropic'' interaction has come under thorough theoretical investigation in the
context of the stripe phase observed experimentally in doped cuprates. We found
that the excitation gap opens in the form of exponential singularity DeltaE ~
exp(-Ad^sigma) (d: wall spacing) with the exponent sigma =0.6(3), which is
substantially smaller than the meanfield value sigma=2. That is, the excitation
gap is much larger than that anticipated from meanfield, suggesting that the
string is subjected to robust pinning potential due to the quantum collisions.
This feature supports Zaanen's ``order out of disorder'' mechanism which would
be responsible to the stabilization of the stripe phase
A Bosonic Model of Hole Pairs
We numerically investigate a bosonic representation for hole pairs on a
two-leg t-J ladder where hard core bosons on a chain represent the hole pairs
on the ladder. The interaction between hole pairs is obtained by fitting the
density profile obtained with the effective model to the one obtained with the
\tj model, taking into account the inner structure of the hole pair given by
the hole-hole correlation function. For these interactions we calculate the
Luttinger liquid parameter, which takes the universal value as
half filling is approached, for values of the rung exchange between strong
coupling and the isotropic case. The long distance behavior of the hole-hole
correlation function is also investigated. Starting from large , the
correlation length first increases as expected, but diminishes significantly as
is reduced and bound holes sit mainly on adjacent rungs. As the isotropic
case is approached, the correlation length increases again. This effect is
related to the different kind of bonds in the region between the two holes of a
hole pair when they move apart.Comment: 11 page
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