14,125 research outputs found
Nanoscopic processes of Current Induced Switching in thin tunnel junctions
Full text linkIn magnetic nanostructures one usually uses a magnetic field to commute
between two resistance (R) states. A less common but technologically more
interesting alternative to achieve R-switching is to use an electrical current,
preferably of low intensity. Such Current Induced Switching (CIS) was recently
observed in thin magnetic tunnel junctions, and attributed to electromigration
of atoms into/out of the insulator. Here we study the Current Induced
Switching, electrical resistance, and magnetoresistance of thin
MnIr/CoFe/AlO/CoFe tunnel junctions. The CIS effect at room temperature
amounts to 6.9% R-change between the high and low states and is attributed to
nanostructural rearrangements of metallic ions in the electrode/barrier
interfaces. After switching to the low R-state some electro-migrated ions
return to their initial sites through two different energy channels. A low
(high) energy barrier of 0.13 eV (0.85 eV) was estimated. Ionic
electromigration then occurs through two microscopic processes associated with
different types of ions sites/defects. Measurements under an external magnetic
field showed an additional intermediate R-state due to the simultaneous
conjugation of the MR (magnetic) and CIS (structural) effects.Comment: 6 pages, 4 figure
Emergent Nesting of the Fermi Surface from Local-Moment Description of Iron-Pnictide High-Tc Superconductors
Get PDFWe uncover the low-energy spectrum of a t-J model for electrons on a square
lattice of spin-1 iron atoms with 3dxz and 3dyz orbital character by applying
Schwinger-boson-slave-fermion mean-field theory and by exact diagonalization of
one hole roaming over a 4 x 4 x 2 lattice. Hopping matrix elements are set to
produce hole bands centered at zero two-dimensional (2D) momentum in the
free-electron limit. Holes can propagate coherently in the t-J model below a
threshold Hund coupling when long-range antiferromagnetic order across the d+ =
3d(x+iy)z and d- = 3d(x-iy)z orbitals is established by magnetic frustration
that is off-diagonal in the orbital indices. This leads to two hole-pocket
Fermi surfaces centered at zero 2D momentum. Proximity to a commensurate
spin-density wave (cSDW) that exists above the threshold Hund coupling results
in emergent Fermi surface pockets about cSDW momenta at a quantum critical
point (QCP). This motivates the introduction of a new Gutzwiller wavefunction
for a cSDW metal state. Study of the spin-fluctuation spectrum at cSDW momenta
indicates that the dispersion of the nested band of one-particle states that
emerges is electron-type. Increasing Hund coupling past the QCP can push the
hole-pocket Fermi surfaces centered at zero 2D momentum below the Fermi energy
level, in agreement with recent determinations of the electronic structure of
mono-layer iron-selenide superconductors.Comment: 41 pages, 12 figures, published versio
Quantum Double Models coupled with matter: an algebraic dualisation approach
Full text linkIn this paper, we constructed a new generalization of a class of discrete
bidimensional models, the so called Quantum Double Models, by introduce matter
qunits to the faces of the lattice that supports these models. This new
generalization can be interpreted as the algebraic dual of a first, where we
introduce matter qunits to the vertices of this same lattice. By evaluating the
algebraic and topological orders of these new models, we prove that, as in the
first generalization, a new phenomenon of quasiparticle confinement may appear
again: this happens when the co-action homomorphism between matter and gauge
groups is non-trivial. Consequently, this homomorphism not only classifies the
different models that belong to this new class, but also suggests that they can
be interpreted as a 2-dimensional restriction of the 2-lattice gauge theories.Comment: 18 pages, 8 figures; submitted to publicatio
Quadriciclo agrícola - Solução para a agricultura familiar.
Get PDFbitstream/item/54208/1/ADM11011.pd
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