2,308 research outputs found
Dimerized ground states in spin-S frustrated systems
We study a family of frustrated anti-ferromagnetic spin- systems with a
fully dimerized ground state. This state can be exactly obtained without the
need to include any additional three-body interaction in the model. The
simplest members of the family can be used as a building block to generate more
complex geometries like spin tubes with a fully dimerized ground state. After
present some numerical results about the phase diagram of these systems, we
show that the ground state is robust against the inclusion of weak disorder in
the couplings as well as several kinds of perturbations, allowing to study some
other interesting models as a perturbative expansion of the exact one. A
discussion on how to determine the dimerization region in terms of quantum
information estimators is also presented. Finally, we explore the relation of
these results with a the case of the a 4-leg spin tube which recently was
proposed as the model for the description of the compound
CuClDCSO, delimiting the region of the parameter space
where this model presents dimerization in its ground state.Comment: 10 pages, 9 figure
Phase diagram study of a dimerized spin-S zig-zag ladder
The phase diagram of a frustrated spin- zig-zag ladder is studied through
different numerical and analytical methods. We show that for arbitrary ,
there is a family of Hamiltonians for which a fully-dimerized state is an exact
ground state, being the Majumdar-Ghosh point a particular member of the family.
We show that the system presents a transition between a dimerized phase to a
N\'eel-like phase for , and spiral phases can appear for large . The
phase diagram is characterized by means of a generalization of the usual Mean
Field Approximation (MFA). The novelty in the present implementation is to
consider the strongest coupled sites as the unit cell. The gap and the
excitation spectrum is analyzed through the Random Phase Approximation (RPA).
Also, a perturbative treatment to obtain the critical points is discussed.
Comparisons of the results with numerical methods like DMRG are also presented.Comment: 14 pages, 6 figures. Some typos were corrected, and notation was
clarifie
Quantum phases in the frustrated Heisenberg model on the bilayer honeycomb lattice
We use a combination of analytical and numerical techniques to study the
phase diagram of the frustrated Heisenberg model on the bilayer honeycomb
lattice. Using the Schwinger boson description of the spin operators followed
by a mean field decoupling, the magnetic phase diagram is studied as a function
of the frustration coupling and the interlayer coupling .
The presence of both magnetically ordered and disordered phases is
investigated by means of the evaluation of ground-state energy, spin gap, local
magnetization and spin-spin correlations. We observe a phase with a spin gap
and short range N\'eel correlations that survives for non-zero
next-nearest-neighbor interaction and interlayer coupling. Furthermore, we
detect signatures of a reentrant behavior in the melting of N\'eel phase and
symmetry restoring when the system undergoes a transition from an on-layer
nematic valence bond crystal phase to an interlayer valence bond crystal phase.
We complement our work with exact diagonalization on small clusters and
dimer-series expansion calculations, together with a linear spin wave approach
to study the phase diagram as a function of the spin , the frustration and
the interlayer couplings.Comment: 10 pages, 9 figure
Structural studies of mesoporous ZrO-CeO and ZrO-CeO/SiO mixed oxides for catalytical applications
In this work the synthesis of ZrO-CeO and
ZrO-CeO/SiO were developed, based on the process to form
ordered mesoporous materials such as SBA-15 silica. The triblock copolymer
Pluronic P-123 was used as template, aiming to obtain crystalline single phase
walls and larger specific surface area, for future applications in catalysis.
SAXS and XRD results showed a relationship between ordered pores and the
material crystallization. 90% of CeO leaded to single phase homogeneous
ceria-zirconia solid solution of cubic fluorite structure (Fmm). The
SiO addition improved structural and textural properties as well as the
reduction behavior at lower temperatures, investigated by XANES measurements
under H atmosphere
Desempenho de genótipos de arroz irrigado em Mato Grosso do Sul.
bitstream/item/65825/1/COT87-2004.pd
Nonlinear transport and oscillating magnetoresistance in double quantum wells
We study the evolution of low-temperature magnetoresistance in double quantum
wells in the region below 1 Tesla as the applied current density increases. A
flip of the magneto-intersubband oscillation peaks, which occurs as a result of
the current-induced inversion of the quantum component of resistivity, is
observed. We also see splitting of these peaks as another manifestation of
nonlinear behavior, specific for the two-subband electron systems. The
experimental results are quantitatively explained by the theory based on the
kinetic equation for the isotropic non-equilibrium part of electron
distribution function. The inelastic scattering time is determined from the
dependence of the inversion magnetic field on the current.Comment: 20 pages, 10 figure
Phase diagram study of a two-dimensional frustrated antiferromagnet via unsupervised machine learning
We apply unsupervised learning techniques to classify the different phases of
the antiferromagnetic Ising model on the honeycomb lattice. We
construct the phase diagram of the system using convolutional autoencoders.
These neural networks can detect phase transitions in the system via `anomaly
detection', without the need for any label or a priori knowledge of the phases.
We present different ways of training these autoencoders and we evaluate them
to discriminate between distinct magnetic phases. In this process, we highlight
the case of high temperature or even random training data. Finally, we analyze
the capability of the autoencoder to detect the ground state degeneracy through
the reconstruction error.Comment: 11 pages, 15 figure
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