417 research outputs found
From local to nonlocal Fermi liquid in doped antiferromagnets
The variation of single-particle spectral functions with doping is studied
numerically within the t-J model. It is shown that corresponding self energies
change from local ones at the intermediate doping to strongly nonlocal ones for
a weakly doped antiferromagnet. The nonlocality shows up most clearly in the
pseudogap emerging in the density of states, due to the onset of short-range
antiferromagnetic correlations.Comment: 4 pages, 3 Postscript figures, revtex, submitted to Phys.Rev.Let
Theory of Underdoped Cuprates
We develop a slave-boson theory for the t-J model at finite doping which
respects an SU(2) symmetry -- a symmetry previously known to be important at
half filling. The mean field phase diagram is found to be consistent with the
phases observed in the cuprate superconductors, which contains d-wave
superconductor, spin gap, strange metal, and Fermi liquid phases. The spin gap
phase is best understood as the staggered flux phase, which is nevertheless
translationally invariant for physical quantities. The electron spectral
function shows small Fermi pockets at low doping which continuously evolve into
the large Fermi surface at high doping concentrations.Comment: 4 pages, latex(revtex,epsf), 3 figure
Photoemission Spectra in t-J Ladders with Two Legs
Photoemission spectra for the isotropic two-leg t-J ladder are calculated at
various hole-doping levels using exact diagonalization techniques. Low-energy
sharp features caused by short-range antiferromagnetic correlations are
observed at finite doping levels close to half-filling, above the naive Fermi
momentum. These features should be observable in angle-resolved photoemission
experiments. In addition, the formation of a d-wave pairing condensate as the
ratio J/t is increased leads to dynamically generated spectral weight for
momenta close to where the -order parameter is large.Comment: 9 pages, RevTex, to be published in Phys. Rev. B (RC
On single and double soft behaviors in NLSM
In this paper, we study the single and double soft behaviors of tree level
off-shell currents and on-shell amplitudes in nonlinear sigma model(NLSM). We
first propose and prove the leading soft behavior of the tree level currents
with a single soft particle. In the on-shell limit, this single soft emission
becomes the Adler's zero. Then we establish the leading and sub-leading soft
behaviors of tree level currents with two adjacent soft particles. With a
careful analysis of the on-shell limit, we obtain the double soft behaviors of
on-shell amplitudes where the two soft particles are adjacent to each other. By
applying Kleiss-Kuijf (KK) relation, we further obtain the leading and
sub-leading behaviors of amplitudes with two nonadjacent soft particles.Comment: 41 pages, 6 tables, 9 figures, minor revised, more content about
nonadjacent double soft limit, update the reference
Superconductivity in the Pseudogap State due to Fluctuations of Short-Range Order
We analyze the anomalies of superconducting state (s and d-wave pairing) in a
simple model of pseudogap state, induced by fluctuations of short - range order
(e.g. antiferromagnetic), based on the model Fermi surface with "hot patches".
We derive a system of recursion relations for Gorkov's equations which take
into account all diagrams of perturbation theory for electron interaction with
fluctuations of short-range order. Then we find superconducting transition
temperature and gap behavior for different values of the pseudogap width and
correlation lengths of short-range order fluctuations. In a similar
approximation we derive the Ginzburg-Landau expansion and study the main
physical characteristics of a superconductor close to the transition
temperature, both as functions of the pseudogap width and correlation length of
fluctuations. Results obtained are in qualitative agreement with a number of
experiments on underdoped HTSC-cuprates.Comment: 18 pages, 12 figures, RevTeX 3.0, minor misprints corrected, to
appear in JET
Dynamical Properties of Two Coupled Hubbard Chains at Half-filling
Using grand canonical Quantum Monte Carlo (QMC) simulations combined with
Maximum Entropy analytic continuation, as well as analytical methods, we
examine the one- and two-particle dynamical properties of the Hubbard model on
two coupled chains at half-filling. The one-particle spectral weight function,
, undergoes a qualitative change with interchain hopping
associated with a transition from a four-band insulator to a two-band
insulator. A simple analytical model based on the propagation of exact rung
singlet states gives a good description of the features at large . For
smaller , is similar to that of the
one-dimensional model, with a coherent band of width the effective
antiferromagnetic exchange reasonably well-described by renormalized
spin-wave theory. The coherent band rides on a broad background of width
several times the parallel hopping integral , an incoherent structure
similar to that found in calculations on both the one- and two-dimensional
models. We also present QMC results for the two-particle spin and charge
excitation spectra, and relate their behavior to the rung singlet picture for
large and to the results of spin-wave theory for small .Comment: 9 pages + 10 postscript figures, submitted to Phys.Rev.B, revised
version with isotropic t_perp=t data include
Superconductivity in a Toy Model of the Pseudogap State
We analyze superconducting state (both s and d - wave) in a simple exactly
solvable model of pseudogap state, induced by short - range order fluctuations
(e.g. antiferromagnetic), which is based upon model Fermi - surface with "hot
patches". It is shown that superconducting energy gap averaged over these
fluctuations is non zero even for the temperatures larger than mean - field T_c
of superconducting transition in a sample as a whole. For temperatures T>T_c
superconductivity apparently exists within separate regions ("drops"). We study
the spectral density and the density of states and demonstrate that
superconductivity signals itself in these already for T>T_c, while at T_c
itself nothing special happens from this point of view. These anomalies are in
qualitative agreement with a number experiments on underdoped cuprates.Comment: 12 pages, 6 figures, RevTeX 3.0, Postscript figures attache
Models of the Pseudogap State of Two-Dimensional Systems
We analyze a number of ``nearly exactly'' solvable models of electronic
spectrum of two-dimensional systems with well-developed fluctuations of short
range order of ``dielectric'' (e.g. antiferromagnetic) or ``superconducting''
type, which lead to the formation of anisotropic pseudogap state on certain
parts of the Fermi surface. We formulate a recurrence procedure to calculate
one-electron Green's function which takes into account all Feynman diagrams in
perturbation series and is based upon the approximate Ansatz for higher-order
terms in this series. Detailed results for spectral densities and density of
states are presented. We also discuss some important points concerning the
justification of our Ansatz for higher-order contributions.Comment: 22 pages, 15 figures, RevTeX 3.0, Postscript figures attache
Ginzburg-Landau Expansion in a Toy Model of Superconductor with Pseudogap
We propose a toy model of electronic spectrum of two-dimensional system with
``hot-patches'' on the Fermi surface, which leads to essential renormalization
of spectral density (pseudogap). Within this model we derive Ginzburg-Landau
expansion for both s-wave and d-wave Cooper pairing and analyze the influence
of pseudogap formation on the basic properties of superconductors.Comment: 14 pages, 14 figures, RevTeX 3.0, Postscript figures attached, some
changes in the explanation of the model, published in JETP 115, No.2, (1999
Screened-interaction expansion for the Hubbard model and determination of the quantum Monte Carlo Fermi surface
We develop a systematic self-consistent perturbative expansion for the self
energy of Hubbard-like models. The interaction lines in the Feynman diagrams
are dynamically screened by the charge fluctuations in the system. Although the
formal expansion is exact-assuming that the model under the study is
perturbative-only if diagrams to all orders are included, it is shown that for
large-on-site-Coulomb-repulsion-U systems weak-coupling expansions to a few
orders may already converge. We show that the screened interaction for the
large-U system can be vanishingly small at a certain intermediate electron
filling; and it is found that our approximation for the imaginary part of the
one-particle self energy agrees well with the QMC results in the low energy
scales at this particular filling. But, the usefulness of the approximation is
hindered by the fact that it has the incorrect filling dependence when the
filling deviates from this value. We also calculate the exact QMC Fermi
surfaces for the two-dimensional (2-D) Hubbard model for several fillings. Our
results near half filling show extreme violation of the concepts of the band
theory; in fact, instead of growing, Fermi surface vanishes when doped toward
the half-filled Mott-Hubbard insulator. Sufficiently away from half filling,
noninteracting-like Fermi surfaces are recovered. These results combined with
the Luttinger theorem might show that diagrammatic expansions for the
nearly-half-filled Hubbard model are unlikely to be possible; however, the
nonperturbative part of the solution seems to be less important as the filling
gradually moves away from one half. Results for the 2-D one-band Hubbard model
for several hole dopings are presented. Implications of this study for the
high-temperature superconductors are also discussed.Comment: 11 pages, 12 eps figures embedded, REVTeX, submitted to Phys. Rev. B;
(v2) minor revisions, scheduled for publication on November 1
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