2,788 research outputs found
Induced local spin-singlet amplitude and pseudogap in high cuprates
In this paper we show that local spin-singlet amplitude with d-wave symmetry,
, can be induced by short-range spin correlations even
in the absence of pairing interactions. Fluctuation theory is formulated to
make connection between pseudogap temperature $T^{*}$, pseudogap size
$\Delta_{pg}$ and . In the present scenario for the
pseudogap, the normal state pseudogap is caused by the induced local
spin-singlet amplitude due to short-range spin correlations, which compete in
the low energy sector with superconducting correlations to make go to
zero near half-filling. Calculated falls from a high value onto the
line and closely follows mean-field N\'{e}el temperature .
The calculated is in good agreement with experimental results. We
propose an experiment in which the present scenario can be critically tested.Comment: 5 pages, 3 figure
Anomalous superconductivity and its competition with antiferromagnetism in doped Mott insulators
Proximity to a Mott insulating phase is likely to be an important physical
ingredient of a theory that aims to describe high-temperature superconductivity
in the cuprates. Quantum cluster methods are well suited to describe the Mott
phase. Hence, as a step towards a quantitative theory of the competition
between antiferromagnetism (AFM) and d-wave superconductivity (SC) in the
cuprates, we use Cellular Dynamical Mean Field Theory to compute zero
temperature properties of the two-dimensional square lattice Hubbard model. The
d-wave order parameter is found to scale like the superexchange coupling J for
on-site interaction U comparable to or larger than the bandwidth. The order
parameter also assumes a dome shape as a function of doping while, by contrast,
the gap in the single-particle density of states decreases monotonically with
increasing doping. In the presence of a finite second-neighbor hopping t', the
zero temperature phase diagram displays the electron-hole asymmetric
competition between antiferromagnetism and superconductivity that is observed
experimentally in the cuprates. Adding realistic third-neighbor hopping t''
improves the overall agreement with the experimental phase diagram. Since band
parameters can vary depending on the specific cuprate considered, the
sensitivity of the theoretical phase diagram to band parameters challenges the
commonly held assumption that the doping vs T_{c}/T_{c}^{max} phase diagram of
the cuprates is universal. The calculated ARPES spectrum displays the observed
electron-hole asymmetry. Our calculations reproduce important features of
d-wave superconductivity in the cuprates that would otherwise be considered
anomalous from the point of view of the standard BCS approach.Comment: 13 pages, 7 figure
Pseudogap and photoemission spectra in the attractive Hubbard model
Angle-resolved photoemission spectra are calculated microscopically for the
two-dimensional attractive Hubbard model. A system of self-consistent T-matrix
equations are solved numerically in the real-time domain. The single-particle
spectral function has a two-peak structure resulting from the presense of bound
states. The spectral function is suppressed at the chemical potential, leading
to a pseudogap-like behavior. At high temperatures and densities the pseudogap
diminishes and finally disappears; these findings are similar to experimental
observations for the cuprates.Comment: 5 pages, 4 figures, published versio
Pseudogap and high-temperature superconductivity from weak to strong coupling. Towards quantitative theory
This is a short review of the theoretical work on the two-dimensional Hubbard
model performed in Sherbrooke in the last few years. It is written on the
occasion of the twentieth anniversary of the discovery of high-temperature
superconductivity. We discuss several approaches, how they were benchmarked and
how they agree sufficiently with each other that we can trust that the results
are accurate solutions of the Hubbard model. Then comparisons are made with
experiment. We show that the Hubbard model does exhibit d-wave
superconductivity and antiferromagnetism essentially where they are observed
for both hole and electron-doped cuprates. We also show that the pseudogap
phenomenon comes out of these calculations. In the case of electron-doped high
temperature superconductors, comparisons with angle-resolved photoemission
experiments are nearly quantitative. The value of the pseudogap temperature
observed for these compounds in recent photoemission experiments has been
predicted by theory before it was observed experimentally. Additional
experimental confirmation would be useful. The theoretical methods that are
surveyed include mostly the Two-Particle Self-Consistent Approach, Variational
Cluster Perturbation Theory (or variational cluster approximation), and
Cellular Dynamical Mean-Field Theory.Comment: 32 pages, 51 figures. Slight modifications to text, figures and
references. A PDF file with higher-resolution figures is available at
http://www.physique.usherbrooke.ca/senechal/LTP-toc.pd
Short-range spin correlations and induced local spin-singlet amplitude in the Hubbard model
In this paper, from the microscopic Hubbard Hamiltonian we extract the local
spin-singlet amplitude due to short-range spin correlations, and quantify its
strength near half-filling. As a first application of the present approach, we
study a problem of the energy dispersion and its d-wave modulation in the
insulating cuprates, SrCuOCl and CaCuOCl.
Without any adjustable parameters, most puzzling issues are naturally and
quantitatively explained within the present approach.Comment: 6 pages, 3 figure
Many-body Theory vs Simulations for the pseudogap in the Hubbard model
The opening of a critical-fluctuation induced pseudogap (or precursor
pseudogap) in the one-particle spectral weight of the half-filled
two-dimensional Hubbard model is discussed. This pseudogap, appearing in our
Monte Carlo simulations, may be obtained from many-body techniques that use
Green functions and vertex corrections that are at the same level of
approximation. Self-consistent theories of the Eliashberg type (such as the
Fluctuation Exchange Approximation) use renormalized Green functions and bare
vertices in a context where there is no Migdal theorem. They do not find the
pseudogap, in quantitative and qualitative disagreement with simulations,
suggesting these methods are inadequate for this problem. Differences between
precursor pseudogaps and strong-coupling pseudogaps are also discussed.Comment: Accepted, Phys. Rev. B15 15Mar00. Expanded version of original
submission, Latex, 8 pages, epsfig, 5 eps figures (Last one new). Discussion
on fluctuation and strong coupling induced pseudogaps expande
Arabidopsis ABCG14 is essential for the root-to-shoot translocation of cytokinin.
Cytokinins are phytohormones that induce cytokinesis and are essential for diverse developmental and physiological processes in plants. Cytokinins of the trans-zeatin type are mainly synthesized in root vasculature and transported to the shoot, where they regulate shoot growth. However, the mechanism of long-distance transport of cytokinin was hitherto unknown. Here, we report that the Arabidopsis ATP-binding cassette (ABC) transporter subfamily G14 (AtABCG14) is mainly expressed in roots and plays a major role in delivering cytokinins to the shoot. Loss of AtABCG14 expression resulted in severe shoot growth retardation, which was rescued by exogenous trans-zeatin application. Cytokinin content was decreased in the shoots of atabcg14 plants and increased in the roots, with consistent changes in the expression of cytokinin-responsive genes. Grafting of atabcg14 scions onto wild-type rootstocks restored shoot growth, whereas wild-type scions grafted onto atabcg14 rootstocks exhibited shoot growth retardation similar to that of atabcg14. Cytokinin concentrations in the xylem are reduced by similar to 90% in the atabcg14 mutant. These results indicate that AtABCG14 is crucial for the translocation of cytokinin to the shoot. Our results provide molecular evidence for the long-distance transport of cytokinin and show that this transport is necessary for normal shoot development.open118380Ysciescopu
Higgs Structures of Dyonic Instantons
We study Higgs field configurations of dyonic instantons in spontaneously
broken (4+1)-dimensional Yang-Mills theory. The adjoint scalar field solutions
to the covariant Laplace equation in the ADHM instanton background are
constructed in general noncanonical basis, and they are used to study
explicitly the Higgs field configurations of dyonic instantons when the gauge
fields are taken by Jackiw-Nohl-Rebbi instanton solutions. For these solutions
corresponding to small instanton number we then consider in some detail the
zero locus of the Higgs field, which describes the cross section of supertubes
connecting parallel D4-branes in string theory. Also the information on the
Higgs zeroes is used to discuss the residual gauge freedom concerning the
Jackiw-Nohl-Rebbi solutions.Comment: 1+27 pages, 6 figure
Non-perturbative approach to the attractive Hubbard model
A non-perturbative approach to the single-band attractive Hubbard model is
presented in the general context of functional derivative approaches to
many-body theories. As in previous work on the repulsive model, the first step
is based on a local-field type ansatz, on enforcement of the Pauli principle
and a number of crucial sum-rules. The Mermin-Wagner theorem in two dimensions
is automatically satisfied. At this level, two-particle self-consistency has
been achieved. In the second step of the approximation, an improved expression
for the self-energy is obtained by using the results of the first step in an
exact expression for the self-energy where the high- and low-frequency
behaviors appear separately. The result is a cooperon-like formula. The
required vertex corrections are included in this self-energy expression, as
required by the absence of a Migdal theorem for this problem. Other approaches
to the attractive Hubbard model are critically compared. Physical consequences
of the present approach and agreement with Monte Carlo simulations are
demonstrated in the accompanying paper (following this one).Comment: Revtex, 19 page
Chiral magnetoresistance in Pt/Co/Pt zigzag wires
The Rashba effect leads to a chiral precession of the spins of moving
electrons while the Dzyaloshinskii-Moriya interaction (DMI) generates
preference towards a chiral profile of local spins. We predict that the
exchange interaction between these two spin systems results in a 'chiral'
magnetoresistance depending on the chirality of the local spin texture. We
observe this magnetoresistance by measuring the domain wall (DW) resistance in
a uniquely designed Pt/Co/Pt zigzag wire, and by changing the chirality of the
DW with applying an in-plane magnetic field. A chirality-dependent DW
resistance is found, and a quantitative analysis shows a good agreement with a
theory based on the Rashba model. Moreover, the DW resistance measurement
allows us to independently determine the strength of the Rashba effect and the
DMI simultaneously, and the result implies a possible correlation between the
Rashba effect, the DMI, and the symmetric Heisenberg exchange
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