562 research outputs found
Effects of dimers on cooperation in the spatial prisoner's dilemma game
We investigate the evolutionary prisoner's dilemma game in structured
populations by introducing dimers, which are defined as that two players in
each dimer always hold a same strategy. We find that influences of dimers on
cooperation depend on the type of dimers and the population structure. For
those dimers in which players interact with each other, the cooperation level
increases with the number of dimers though the cooperation improvement level
depends on the type of network structures. On the other hand, the dimers, in
which there are not mutual interactions, will not do any good to the
cooperation level in a single community, but interestingly, will improve the
cooperation level in a population with two communities. We explore the
relationship between dimers and self-interactions and find that the effects of
dimers are similar to that of self-interactions. Also, we find that the dimers,
which are established over two communities in a multi-community network, act as
one type of interaction through which information between communities is
communicated by the requirement that two players in a dimer hold a same
strategy.Comment: 12 pages and 3 figure
Optical and transport properties in doped two-leg ladder antiferromagnet
Within the t-J model, the optical and transport properties of the doped
two-leg ladder antiferromagnet are studied based on the fermion-spin theory. It
is shown that the optical and transport properties of the doped two-leg ladder
antiferromagnet are mainly governed by the holon scattering. The low energy
peak in the optical conductivity is located at a finite energy, while the
resistivity exhibits a crossover from the high temperature metallic-like
behavior to the low temperature insulating-like behavior, which are consistent
with the experiments.Comment: 13 pages, 5 figures, accepted for publication in Phys. Rev. B65
(2002) (April 15 issue
Systematic Cu-63 NQR studies of the stripe phase in La(1.6-x)Nd(0.4)Sr(x)CuO(4) for 0.07 <= x <= 0.25
We demonstrate that the integrated intensity of Cu-63 nuclear quadrupole
resonance (NQR) in La(1.6-x)Nd(0.4)Sr(x)CuO(4) decreases dramatically below the
charge-stripe ordering temperature T(charge). Comparison with neutron and X-ray
scattering indicates that the wipeout fraction F(T) (i.e. the missing fraction
of the integrated intensity of the NQR signal) represents the charge-stripe
order parameter. The systematic study reveals bulk charge-stripe order
throughout the superconducting region 0.07 <= x <= 0.25. As a function of the
reduced temperature t = T/T(charge), the temperature dependence of F(t) is
sharpest for the hole concentration x=1/8, indicating that x=1/8 is the optimum
concentration for stripe formation.Comment: 10 pages of text and captions, 11 figures in postscript. Final
version, with new data in Fig.
Enhancement of pair correlation in a one-dimensional hybridization model
We propose an integrable model of one-dimensional (1D) interacting electrons
coupled with the local orbitals arrayed periodically in the chain. Since the
local orbitals are introduced in a way that double occupation is forbidden, the
model keeps the main feature of the periodic Anderson model with an interacting
host. For the attractive interaction, it is found that the local orbitals
enhance the effective mass of the Cooper-pair-like singlets and also the pair
correlation in the ground state. However, the persistent current is depressed
in this case. For the repulsive interaction case, the Hamiltonian is
non-Hermitian but allows Cooper pair solutions with small momenta, which are
induced by the hybridization between the extended state and the local orbitals.Comment: 11 page revtex, no figur
Microscopic theory of weak pseudogap behavior in the underdoped cuprate superconductors I: General theory and quasiparticle properties
We derive in detail a novel solution of the spin fermion model which is valid
in the quasi-static limit pi T<<omega_sf, found in the intermediate
(pseudoscaling) regime of the magnetic phase diagram of cuprate
superconductors, and use it to obtain results for the temperature and doping
dependence of the single particle spectral density, the electron-spin
fluctuation vertex function, and the low frequency dynamical spin
susceptibility. The resulting strong anisotropy of the spectral density and the
vertex function lead to the qualitatively different behavior of_hot_ (around
k=(pi,0)) and_cold_ (around k=(pi/2,pi/2)) quasiparticles seen in ARPES
experiments. We find that the broad high energy features found in ARPES
measurements of the spectral density of the underdoped cuprate superconductors
are determined by strong antiferromagnetic (AF) correlations and incoherent
precursor effects of an SDW state, with reduced renormalized effective coupling
constant. The electron spin-fluctuation vertex function, i.e. the effective
interaction of low energy quasiparticles and spin degrees of freedom, is found
to be strongly anisotropic and enhanced for hot quasiparticles; the
corresponding charge-fluctuation vertex is considerably diminished. We thus
demonstrate that, once established, strong AF correlations act to reduce
substantially the effective electron-phonon coupling constant in cuprate
superconductors.Comment: REVTEX with EPS figures, uses multicol.sty, epsfig,sty, psfig.st
4f-spin dynamics in La(2-x-y)Sr(x)Nd(y)CuO(4)
We have performed inelastic magnetic neutron scattering experiments on
La(2-x-y)Sr(x)Nd(y)CuO(4) in order to study the Nd 4f-spin dynamics at low
energies. In all samples we find at high temperatures a quasielastic line
(Lorentzian) with a line width which decreases on lowering the temperature. The
temperature dependence of the quasielastic line width Gamma/2(T) can be
explained with an Orbach-process, i.e. a relaxation via the coupling between
crystal field excitations and phonons. At low temperatures the Nd-4f magnetic
response S(Q,omega) correlates with the electronic properties of the
CuO(2)-layers. In the insulator La(2-y)Nd(y)CuO(4) the quasielastic line
vanishes below 80 K and an inelastic excitation occurs. This directly indicates
the splitting of the Nd3+ ground state Kramers doublet due to the static
antiferromagnetic order of the Cu moments. In La(1.7-x)Sr(x)Nd(0.3)CuO(4) with
x = 0.12, 0.15 and La(1.4-x)Sr(x)Nd(0.6)CuO(4) with x = 0.1, 0.12, 0.15, 0.18
superconductivity is strongly suppressed. In these compounds we observe a
temperature independent broad quasielastic line of Gaussian shape below T about
30 K. This suggests a distribution of various internal fields on different Nd
sites and is interpreted in the frame of the stripe model. In
La(1.8-y)Sr(0.2)Nd(y)CuO(4) (y = 0.3, 0.6) such a quasielastic broadening is
not observed even at lowest temperature.Comment: 8 pages, 10 figures included, to appear in Phys. Rev.
Hidden Order in the Cuprates
We propose that the enigmatic pseudogap phase of cuprate superconductors is
characterized by a hidden broken symmetry of d(x^2-y^2)-type. The transition to
this state is rounded by disorder, but in the limit that the disorder is made
sufficiently small, the pseudogap crossover should reveal itself to be such a
transition. The ordered state breaks time-reversal, translational, and
rotational symmetries, but it is invariant under the combination of any two. We
discuss these ideas in the context of ten specific experimental properties of
the cuprates, and make several predictions, including the existence of an
as-yet undetected metal-metal transition under the superconducting dome.Comment: 12 pages of RevTeX, 9 eps figure
Stability of metallic stripes in the extended one-band Hubbard model
Based on an unrestricted Gutzwiller approximation (GA) we investigate the
stripe orientation and periodicity in an extended one-band Hubbard model. A
negative ratio between next-nearest and nearest neighbor hopping t'/t, as
appropriate for cuprates, favors partially filled (metallic) stripes for both
vertical and diagonal configurations. At around optimal doping diagonal
stripes, site centered (SC) and bond centered (BC) vertical stripes become
degenerate suggesting strong lateral and orientational fluctuations. We find
that within the GA the resulting phase diagram is in agreement with experiment
whereas it is not in the Hartree-Fock approximation due to a strong
overestimation of the stripe filling. Results are in agreement with previous
calculations within the three-band Hubbard model but with the role of SC and BC
stripes interchanged.Comment: 10 pages, 8 figure
Hamiltonian Description of Composite Fermions: Magnetoexciton Dispersions
A microscopic Hamiltonian theory of the FQHE, developed by Shankar and myself
based on the fermionic Chern-Simons approach, has recently been quite
successful in calculating gaps in Fractional Quantum Hall states, and in
predicting approximate scaling relations between the gaps of different
fractions. I now apply this formalism towards computing magnetoexciton
dispersions (including spin-flip dispersions) in the , 2/5, and 3/7
gapped fractions, and find approximate agreement with numerical results. I also
analyse the evolution of these dispersions with increasing sample thickness,
modelled by a potential soft at high momenta. New results are obtained for
instabilities as a function of thickness for 2/5 and 3/7, and it is shown that
the spin-polarized 2/5 state, in contrast to the spin-polarized 1/3 state,
cannot be described as a simple quantum ferromagnet.Comment: 18 pages, 18 encapsulated ps figure
Stripes, Pseudogaps, and Van Hove Nesting in the Three-band tJ Model
Slave boson calculations have been carried out in the three-band tJ model for
the high-T_c cuprates, with the inclusion of coupling to oxygen breathing mode
phonons. Phonon-induced Van Hove nesting leads to a phase separation between a
hole-doped domain and a (magnetic) domain near half filling, with long-range
Coulomb forces limiting the separation to a nanoscopic scale. Strong
correlation effects pin the Fermi level close to, but not precisely at the Van
Hove singularity (VHS), which can enhance the tendency to phase separation. The
resulting dispersions have been calculated, both in the uniform phases and in
the phase separated regime. In the latter case, distinctly different
dispersions are found for large, random domains and for regular (static)
striped arrays, and a hypothetical form is presented for dynamic striped
arrays. The doping dependence of the latter is found to provide an excellent
description of photoemission and thermodynamic experiments on pseudogap
formation in underdoped cuprates. In particular, the multiplicity of observed
gaps is explained as a combination of flux phase plus charge density wave (CDW)
gaps along with a superconducting gap. The largest gap is associated with VHS
nesting. The apparent smooth evolution of this gap with doping masks a
crossover from CDW-like effects near optimal doping to magnetic effects (flux
phase) near half filling. A crossover from large Fermi surface to hole pockets
with increased underdoping is found. In the weakly overdoped regime, the CDW
undergoes a quantum phase transition (), which could be obscured
by phase separation.Comment: 15 pages, Latex, 18 PS figures Corrects a sign error: major changes,
esp. in Sect. 3, Figs 1-4,6 replace
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