132 research outputs found
Striped superconductors in the extended Hubbard model
We present a minimal model of a doped Mott insulator that simultaneously
supports antiferromagnetic stripes and d-wave superconductivity. We explore the
implications for the global phase diagram of the superconducting cuprates. At
the unrestricted mean-field level, the various phases of the cuprates,
including weak and strong pseudogap phases, and two different types of
superconductivity in the underdoped and the overdoped regimes, find a natural
interpretation. We argue that on the underdoped side, the superconductor is
intrinsically inhomogeneous -- striped coexistence of of superconductivity and
magnetism -- and global phase coherence is achieved through Josephson-like
coupling of the superconducting stripes. On the overdoped side, the state is
overall homogeneous and the superconductivity is of the classical BCS type.Comment: 5 pages, 3 eps figures. Effect of t' on stripe filling + new
references are adde
Quantum Phase Diagram of the t-Jz Chain Model
We present the quantum phase diagram of the one-dimensional - model
for arbitrary spin (integer or half-integer) and sign of the spin-spin
interaction , using an {\it exact} mapping to a spinless fermion model
that can be solved {\it exactly} using the Bethe ansatz. We discuss its
superconducting phase as a function of hole doping . Motivated by the new
paradigm of high temperature superconductivity, the stripe phase, we also
consider the effect the antiferromagnetic background has on the - chain
intended to mimic the stripe segments.Comment: 4 pages, 2 figure
Origem, disseminação, morfologia e fenologia.
Origem e disseminação; Morfologia: Semente, Talo e coroa, Folha, Flor, Desenvolvimento floral e polinização, Fruto; Fenologi
Electron-Doped Manganese Perovskites: The Polaronic State
Using the Lanczos method in linear chains we study the ground state of the
double exchange model including an antiferromagnetic super-exchange in the low
concentration limit. We find that this ground state is always inhomogeneous,
containig ferromagnetic polarons. The extention of the polaron spin distortion,
the dispersion relation and their trapping by impurities, are studied for
diferent values of the super exchange interaction and magnetic field. We also
find repulsive polaron polaron interaction.Comment: 4 pages, 6 embedded figure
Innovación de educación emocional en el ocio educativo: el Método La Granja
This paper presents an innovative methodology in the emotional education field, La Granja, which was developed during a summer camp celebrated in 2017 in Santa María de Palautordera (Barcelona-Spain). It is a non-formal education experience proposed to promote learning from the emotions and experiences. The main goal is to assess the effect of this method on 127 children, between 8 and 14 years of age, in terms of improving their emotional competencies, as well as their perception of the usefulness of what they have learned. It follows a pragmatic paradigm that combines data collected by different approaches. It includes the application of the Emotional Development Questionnaire (CDE-9-13 and CDE-SEC, of the GROP) before and at the end of the summer camp, plus the ad hoc survey with open questions as post-test. In the global emotional competence, as well as its five dimensions: awareness, emotional regulation, emotional autonomy, social competencies, and life and wellbeing competencies, the scores increase, after their participation in the summer camp. These differences were significant in all cases except in the last dimension. According to the lexicometric analysis and the content of the open questions, there are coincidences between both quantitative observations and the meaning given to the experience in terms of emotional competencies. The most relevant discussion and conclusions are based on the importance of reflection as a learning enhancer, the duration and intensity of the interventions and the alignment of all the resources used that interact synergistically around the achievement of the objectives, topics broadly included in the scientific literature
Charge dynamics in the Mott insulating phase of the ionic Hubbard model
We extend to charge and bond operators the transformation that maps the ionic
Hubbard model at half filling onto an effective spin Hamiltonian. Using these
operators we calculate the amplitude of the charge density wave in different
dimensions. In one dimension, the charge-charge correlations at large distance
d decay as 1/(d^3 ln^{3/2}d), in spite of the presence of a charge gap, as a
consequence of remaining charge-spin coupling. Bond-bond correlations decay as
(-1)^d 1/(d ln^{3/2}d) as in the usual Hubbard model.Comment: 4 pages, no figures, submitted to Phys. Rev. B printing errors
corrected and some clarifications adde
Exact ground states for the four-electron problem in a two-dimensional finite Hubbard square system
We present exact explicit analytical results describing the exact ground
state of four electrons in a two dimensional square Hubbard cluster containing
16 sites taken with periodic boundary conditions. The presented procedure,
which works for arbitrary even particle number and lattice sites, is based on
explicitly given symmetry adapted base vectors constructed in r-space. The
Hamiltonian acting on these states generates a closed system of 85 linear
equations providing by its minimum eigenvalue the exact ground state of the
system. The presented results, described with the aim to generate further
creative developments, not only show how the ground state can be exactly
obtained and what kind of contributions enter in its construction, but
emphasize further characteristics of the spectrum. On this line i) possible
explications are found regarding why weak coupling expansions often provide a
good approximation for the Hubbard model at intermediate couplings, or ii)
explicitly given low lying energy states of the kinetic energy, avoiding double
occupancy, suggest new roots for pairing mechanism attracting decrease in the
kinetic energy, as emphasized by kinetic energy driven superconductivity
theories.Comment: 37 pages, 18 figure
Spectral density for a hole in an antiferromagnetic stripe phase
Using variational trial wave function based on the string picture we study
the motion of a single mobile hole in the stripe phase of the doped
antiferromagnet. The holes within the stripes are taken to be static, the
undoped antiferromagnetic domains in between the hole stripes are assumed to
have alternating staggered magnetization, as is suggested by neutron scattering
experiments. The system is described by the t-t'-t''-J model with realistic
parameters and we compute the single particle spectral density.Comment: RevTex-file, 9 PRB pages with 15 .eps and .gif files. To appear in
PRB. Hardcopies of figures (or the entire manuscript) can be obtained by
e-mail request to: [email protected]
Charge and spin excitations of insulating lamellar copper oxides
A consistent description of low-energy charge and spin responses of the
insulating Sr_2CuO_2Cl_2 lamellar system is found in the framework of a
one-band Hubbard model which besides includes hoppings up to 3^{rd}
nearest-neighbors. By combining mean-field calculations, exact diagonalization
(ED) results, and Quantum Monte Carlo simulations (QMC), we analyze both charge
and spin degrees of freedom responses as observed by optical conductivity,
ARPES, Raman and inelastic neutron scattering experiments. Within this
effective model, long-range hopping processes flatten the quasiparticle band
around . We calculate also the non-resonant A_{1g} and B_{1g} Raman
profiles and show that the latter is composed by two main features, which are
attributed to 2- and 4-magnon scattering.Comment: 6 pages, 3 figures, To be published in PRB (july
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