7,238 research outputs found
Scattering by PT-symmetric non-local potentials
A general formalism is worked out for the description of one-dimensional
scattering by non-local separable potentials and constraints on transmission
and reflection coefficients are derived in the cases of P, T, or PT invariance
of the Hamiltonian. The case of a solvable Yamaguchi potential is discussed in
detail.Comment: 11 page
Nanoscopic processes of Current Induced Switching in thin tunnel junctions
In 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
El estado inicial durante el acondicionamiento del acero para la electrodeposición de Zinc en medio ácido
En el presente trabajo se estudia el acondicionamiento de electrodos de acero para la electrodeposición de zinc a partir de soluciones de ZnCl2 + NH4Cl. El control del “estado inicial”, determinado por el potencial de acondicionamiento, Ei, y la densidad de corriente inicial anódica, ja,i, son de fundamental importancia para el proceso de electrodeposición. Los valores del potencial a circuito abierto, ECA, son afectados por los iones presentes tanto para soluciones de base de NH4Cl y de (NH4)2SO4, como para soluciones de electrodeposición. La comparación de las cronoamperometrías en el potencial inicial, Ei, muestra que este debe ser levemente más positivo que el ECA. Se encuentra que si se parte siempre de un valor de ja,i ≤ 100 (μAcm-2) estacionario y con el electrodo pulido siempre en iguales condiciones, se aseguran las condiciones de reproducibilidad experimental en las diferentes técnicas de electrodeposición.Fil:Mahmud, Z. Instituto Nacional de Tecnología Industrial (INTI). Procesos SuperficialesFil:Gordillo, G. Universidad de Buenos Aires (UBA). Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química FísicaFil:Ventura D'Alkaine, C. Universidade Federal de São Carlos (UFSCar
Topological mass mechanism and exact fields mapping
We present a class of mappings between models with topological mass mechanism
and purely topological models in arbitrary dimensions. These mappings are
established by directly mapping the fields of one model in terms of the fields
of the other model in closed expressions. These expressions provide the
mappings of their actions as well as the mappings of their propagators. For a
general class of models in which the topological model becomes the BF model the
mappings present arbitrary functions which otherwise are absent for
Chern-Simons like actions. This work generalizes the results of [1] for
arbitrary dimensions.Comment: 11 page
Electromigration in thin tunnel junctions with ferromagnetic/nonmagnetic: nanoconstrictions, local heating, and direct and wind forces
Current Induced Resistance Switching (CIS) was recently observed in thin
tunnel junctions with ferromagnetic (FM) electrodes \emph{i.e} FM/I/FM. This
effect was attributed to electromigration of metallic atoms in
nanoconstrictions in the insulating barrier (I). Here we study how the CIS
effect is influenced by a thin non-magnetic (NM) Ta layer, deposited just below
the AlO insulating barrier in tunnel junctions of the type FM/NM/I/FM
(FM=CoFe). Enhanced resistance switching occurs with increasing maximum applied
current (\Imax), until a plateau of constant CIS is reached for \Imax\sim65
mA (CIS60%) and above. However, such high electrical currents also lead
to a large (9%) irreversible resistance decrease, indicating barrier
degradation. Anomalous voltage-current characteristics with negative derivative
were also observed near \pm\Imax and this effect is here attributed to
heating in the tunnel junction. One observes that the current direction for
which resistance switches in FM/NM/I/FM (clockwise) is opposite to that of
FM/I/FM tunnel junctions (anti-clockwise). This effect will be discussed in
terms of a competition between the electromigration contributions due to the so
called direct and wind forces. It will be shown that the direct force is likely
to dominate electromigration in the Ta (NM) layers, while the wind contribution
likely dominates in the CoFe (FM) layers
Electrodeposición de cinc en medio ácido: Influencia de aniones y aditivos
Fil:Mahmud, Z. Instituto Nacional de Tecnología Industrial (INTI). Procesos SuperficialesFil:Gordillo, G. Universidad de Buenos Aires (UBA). Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química FísicaFil:Ventura D'Alkaine, C. Universidade Federal de São Carlos (UFSCar
Control de aditivo tiourea en la solución de electrodeposición de cinc por EIS
En este trabajo se analiza la influencia de la concentración de tiourea en el proceso de deposición de cinc por medio de técnicas electroquímicas. Se ha encontrado una concentración óptima del aditivo en cuanto a que los depósitos son de más calidad y resultan ser más refinados y nivelados. Como se sabe, una forma de protección contra la corrosión de los metales utilizados como sustratos, es mediante recubrimientos metálicos que le dan al sustrato o metal de base protección o más valor (en el caso de metales preciosos como el oro o la plata o el cromado decorativo) o le mejoran notoriamente sus propiedades de resistencia mecánica y / o la resistencia contra la corrosión en servicio (cincados, cromados ingenieriles). Una de las técnicas de obtención de recubrimientos es mediante la electrodeposición. Los aditivos, en general son sustancias orgánicas que se agregan a la solución de electrodeposición para modificar la calidad de los acabados metálicos. Son fórmulas bajo patente y se desconoce cual es la función del aditivo para cada proceso. El uso correcto del aditivo puede resultar en la producción de un recubrimiento nivelado, brillante y con buena resistencia contra la corrosión. En los estudios voltamétricos hemos encontrado que la reducción del Zn 2+ se acelera en presencia de tiourea. Mediante la Espectroscopia de Impedancia Electroquímica, EIS, hemos hallado la cantidad óptima de aditivo para un acabado más decorativo. Se ha encontrado que la resistencia a la corrosión disminuye en presencia detiourea. Mientras que la resistencia a la corrosión se incrementa, cuando las concentraciones de tiourea son próximas al valor óptimo.Fil:Mahmud, Z. Instituto Nacional de Tecnología Industrial (INTI). Procesos SuperficialesFil:Gordillo, G. Universidad de Buenos Aires (UBA). Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química FísicaFil:Gassa, L. Universidad Nacional de La Plata (UNLP). Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasFil:Ventura D'Alkaine, C. Universidade Federal de São Carlos (UFSCar
Control of zinc plating solutions, determining the optimum concentration of thiourea additive by electrochemical techniques
In this work we have analyzed the effect of thiourea concentration in the acid zinc deposition process. For these purpose, we have used the electrochemical techniques (cathodic voltammetry and Electrochemistry Impedance Spectroscopy, EIS). It allows determining the optimal concentration thiourea additive. In this range, we have obtained better finishes, more refined and decoratives layers. We have found in voltammetric studies that the electroreduction of Zn 2+ in the presence of thiourea is accelerated. The EIS studies has shown the “electrodeposition resistance” decrease in the presence of thiourea. The addition of more thiourea in the solution, in the optimum concentration, the electrodeposition resistance to the electrodeposition process is increased. The main contribution of this research and its scientific contribution is the use of electrochemical techniques for determining the optimal concentration of thiourea additive in the zinc electroplating solution in chloride acid mediaFil:Mahmud, Z. Instituto Nacional de Tecnología Industrial (INTI). Procesos SuperficialesFil:Gordillo, G. Universidad de Buenos Aires (UBA). Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química FísicaFil:Gassa, L. Universidad Nacional de La Plata (UNLP). Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasFil:Ventura D'Alkaine, C. Universidade Federal de São Carlos (UFSCar
Electrical current-driven pinhole formation and insulator-metal transition in tunnel junctions
Current Induced Resistance Switching (CIS) was recently observed in thin
tunnel junctions (TJs) with ferromagnetic (FM) electrodes and attributed to
electromigration of metallic atoms in nanoconstrictions in the insulating
barrier. The CIS effect is here studied in TJs with two thin (20 \AA)
non-magnetic (NM) Ta electrodes inserted above and below the insulating
barrier. We observe resistance (R) switching for positive applied electrical
current (flowing from the bottom to the top lead), characterized by a
continuous resistance decrease and associated with current-driven displacement
of metallic ions from the bottom electrode into the barrier (thin barrier
state). For negative currents, displaced ions return into their initial
positions in the electrode and the electrical resistance gradually increases
(thick barrier state). We measured the temperature (T) dependence of the
electrical resistance of both thin- and thick-barrier states ( and R
respectively). Experiments showed a weaker R(T) variation when the tunnel
junction is in the state, associated with a smaller tunnel contribution.
By applying large enough electrical currents we induced large irreversible
R-decreases in the studied TJs, associated with barrier degradation. We then
monitored the evolution of the R(T) dependence for different stages of barrier
degradation. In particular, we observed a smooth transition from tunnel- to
metallic-dominated transport. The initial degradation-stages are related to
irreversible barrier thickness decreases (without the formation of pinholes).
Only for later barrier degradation stages do we have the appearance of metallic
paths between the two electrodes that, however, do not lead to metallic
dominated transport for small enough pinhole radius.Comment: 10 pages, 3 figure
Charge Fluctuations in Soliton Anti-Soliton Systems Without Conjugation Symmetry
We construct the charge operator and discuss the limits of their eigenvalues
as the separation between background soliton and anti-solitons goes to infinity
and analyze the fluctuations of the charge. This is performed in a (1+1)D model
with charge conjugation breaking
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