103 research outputs found
A study of Peierls instabilities for a two-dimensional t-t' model
In this paper we study Peierls instabilities for a half-filled
two-dimensional tight-binding model with nearest-neighbour hopping and next
nearest-neighbour hopping at zero and finite temperatures. Two
dimerization patterns corresponding to the same phonon vector are
considered to be realizations of Peierls states. The effect of imperfect
nesting introduced by on the Peierls instability, the properties of the
dimerized ground state, as well as the competition between two dimerized states
for each and temperature , are investigated. It is found: (i). The
Peierls instability will be frustrated by for each of the dimerized
states. The Peierls transition itself, as well as its suppression by , may
be of second- or first-order. (ii). When the two dimerized states are
considered jointly, one of them will dominate the other depending on parameters
and . Two successive Peierls transitions, that is, the system passing
from the uniform state to one dimerized state and then to the other take place
with decrease of temperature for some values. Implications of our results
to real materials are discussed.Comment: 21 pages with 11 eps figures, to appear in Eur. Phys. J.
Imperfect nesting and Peierls instability for a two-dimensional tight-binding model
Based on a half-filled two-dimensional tight-binding model with
nearest-neighbour and next nearest-neighbour hopping the effect of imperfect
Fermi surface nesting on the Peierls instability is studied at zero
temperature. Two dimerization patterns corresponding to a phonon vector are considered. It is found that the Peierls instability will be
suppressed with an increase of next nearest-neighbour hopping which
characterizes the nesting deviation. First and second order transitions to a
homogeneous state are possible. The competition between the two dimerized
states is discussed.Comment: 17 pages, 10 eps figure
Doping dependence of the electron-doped cuprate superconductors from the antiferromagnetic properties of the Hubbard model
Within the Kotliar-Ruckenstein slave-boson approach, we have studied the
antiferromagnetic (AF) properties for the --- model applied to
electron-doped cuprate superconductors. Due to inclusion of spin fluctuations
the AF order decreases with doping much faster than obtained in the
Hartree-Fock theory. Under an intermediate {\it constant} the calculated
doping evolution of the spectral intensity has satisfactorily reproduced the
experimental results, without need of a strongly doping-dependent as argued
earlier. This may reconcile a discrepancy suggested in recent studies on
photoemission and optical conductivity.Comment: 5 pages, 4 eps figures, minor improvement, references added, to
appear in Phys. Rev.
Study of gossamer superconductivity and antiferromagnetism in the t-J-U model
The d-wave superconductivity (dSC) and antiferromagnetism are analytically
studied in a renormalized mean field theory for a two dimensional t-J model
plus an on-site repulsive Hubbard interaction . The purpose of introducing
the term is to partially impose the no double occupancy constraint by
employing the Gutzwiller approximation. The phase diagrams as functions of
doping and are studied. Using the standard value of and
in the large limit, we show that the antiferromagnetic (AF) order emerges
and coexists with the dSC in the underdoped region below the doping
. The dSC order parameter increases from zero as the doping
increases and reaches a maximum near the optimal doping . In
the small limit, only the dSC order survives while the AF order disappears.
As increased to a critical value, the AF order shows up and coexists with
the dSC in the underdoped regime. At half filing, the system is in the dSC
state for small and becomes an AF insulator for large . Within the
present mean field approach, We show that the ground state energy of the
coexistent state is always lower than that of the pure dSC state.Comment: 7 pages, 8 figure
Bidirectional-Convolutional LSTM Based Spectral-Spatial Feature Learning for Hyperspectral Image Classification
This paper proposes a novel deep learning framework named
bidirectional-convolutional long short term memory (Bi-CLSTM) network to
automatically learn the spectral-spatial feature from hyperspectral images
(HSIs). In the network, the issue of spectral feature extraction is considered
as a sequence learning problem, and a recurrent connection operator across the
spectral domain is used to address it. Meanwhile, inspired from the widely used
convolutional neural network (CNN), a convolution operator across the spatial
domain is incorporated into the network to extract the spatial feature.
Besides, to sufficiently capture the spectral information, a bidirectional
recurrent connection is proposed. In the classification phase, the learned
features are concatenated into a vector and fed to a softmax classifier via a
fully-connected operator. To validate the effectiveness of the proposed
Bi-CLSTM framework, we compare it with several state-of-the-art methods,
including the CNN framework, on three widely used HSIs. The obtained results
show that Bi-CLSTM can improve the classification performance as compared to
other methods
Theory of antiferromagnetism in the electron-doped cuprate superconductors
On the basis of the Hubbard model, we present the formulation of
antiferromagnetism in electron-doped cuprates using the fluctuation-exchange
approach. Taking into account the spin fluctuations in combination with the
impurity scattering effect due to the randomly distributed dopant-atoms, we
investigate the magnetic properties of the system. It is shown that the
antiferromagnetic transition temperature, the onset temperature of the
pseudogap formation, the single particle spectral density, and the staggered
magnetization obtained by the present approach are in very good agreement with
the experimental results. The distribution function in momentum space at very
low temperature is observed to differ significantly from that of the Fermi
liquid. Also, we find zero-energy peak in the density of states (DOS) of the
antiferromagnetic phase. This DOS peak is sharp in the low doping regime, and
disappears near the optimal doping where the AF order becomes weak.Comment: 12 pages, 19 figure
Fermi surface evolution in the antiferromagnetic state for the electron-doped t-t'-t''-J model
By use of the slave-boson mean-field approach, we have studied the
electron-doped t-t'-t''-J model in the antiferromagnetic (AF) state. It is
found that at low doping the Fermi surface (FS) pockets appear around
and , and upon increasing doping the other ones will
form around . The evolution of the FS with
doping as well as the calculated spectral weight are consistent with the
experimental results.Comment: Fig. 4 is updated, to appear in Phys. Rev.
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