89 research outputs found
Parametric oscillator based on non-linear vortex dynamics in low resistance magnetic tunnel junctions
Radiofrequency vortex spin-transfer oscillators based on magnetic tunnel
junctions with very low resistance area product were investigated. A high power
of excitations has been obtained characterized by a power spectral density
containing a very sharp peak at the fundamental frequency and a series of
harmonics. The observed behaviour is ascribed to the combined effect of spin
transfer torque and Oersted-Amp\`ere field generated by the large applied
dc-current. We furthermore show that the synchronization of a vortex
oscillation by applying a ac bias current is mostly efficient when the external
frequency is twice the oscillator fundamental frequency. This result is
interpreted in terms of a parametric oscillator.Comment: 4 pages, 4 figure
Three-dimensional Josephson-junction arrays in the quantum regime
We study the quantum phase transition properties of a three-dimensional
periodic array of Josephson junctions with charging energy that includes both
the self and mutual junction capacitances. We use the phase fluctuation algebra
between number and phase operators, given by the Euclidean group E_2, and we
effectively map the problem onto a solvable quantum generalization of the
spherical model. We obtain a phase diagram as a function of temperature,
Josephson coupling and charging energy. We also analyze the corresponding
fluctuation conductivity and its universal scaling form in the vicinity of the
zero-temperature quantum critical point.Comment: 9 pages, LATEX, three PostScript figures. Submitted to Phys. Rev.
Let
Decrease in antibiotic resistance of Streptococcus pneumoniae between 2003 and 2009 in France and changes in serotype distribution: Ongoing survey of the French Pneumococcus Network
SPICE modelling of magnetic tunnel junctions written by spin-transfer torque
International audienceSpintronics aims at extending the possibility of conventional electronics by using not only the charge of the electron, but also its spin. The resulting spintronic devices, combining the front-end CMOS technology of electronics with a magnetic backend technology, employ Magnetic Tunnel Junctions (MTJs) as core elements. With the intent of simulating a circuit without fabricating it first, a reliable MTJ electrical model which is applicable to the standard SPICE (Simulation Program with Integrated Circuit Emphasis) simulator is required. Since such a model was lacking so far, we present an accurate MTJ SPICE model whose magnetic state is written by using the Spin-Transfer Torque (STT) effect. This model has been developed in C language and validated on the Cadence Virtuoso Platform with Spectre simulator. Its operation is similar to those of the standard BSIM (Berkeley Short-channel IGFET Model) SPICE model of the MOS transistor and fully compatible with the SPICE electrical simulator. In order to illustrate the model performance, we studied the tunneling conductance and STT-driven magnetization dynamics by comparing our simulation results with theoretical macrospin calculations and results found in the literature
Spin-current vortices in current-perpendicular-to-plane nanoconstricted spin-valves
The charge and spin diffusion equations taking into account spin-flip and
spin-transfer torque were numerically solved using a finite element method in
complex non-collinear geometry with strongly inhomogeneous current flow. As an
illustration, spin-dependent transport through a non-magnetic nanoconstriction
separating two magnetic layers was investigated. Unexpected results such as
vortices of spin-currents in the vicinity of the nanoconstriction were
obtained. The angular variations of magnetoresistance and spin-transfer torque
are strongly influenced by the structure geometry.Comment: 11 pages, 5 figure
Theory of the Diamagnetism Above the Critical Temperature for Cuprates
Recently experiments on high critical temperature superconductors has shown
that the doping levels and the superconducting gap are usually not uniform
properties but strongly dependent on their positions inside a given sample.
Local superconducting regions develop at the pseudogap temperature () and
upon cooling, grow continuously. As one of the consequences a large diamagnetic
signal above the critical temperature () has been measured by different
groups. Here we apply a critical-state model for the magnetic response to the
local superconducting domains between and and show that the
resulting diamagnetic signal is in agreement with the experimental results.Comment: published versio
Marginal Fermi liquid analysis of 300 K reflectance of Bi2Sr2CaCu2O8+x
We use 300 K reflectance data to investigate the normal-state electrodynamics
of the high temperature superconductor BiSrCaCuO
over a wide range of doping levels. The data show that at this temperature the
free carriers are coupled to a continuous spectrum of fluctuations. Assuming
the Marginal Fermi Liquid (MFL) form as a first approximation for the
fluctuation spectrum, the doping-dependent coupling constant can
be estimated directly from the slope of the reflectance spectrum. We find that
decreases smoothly with the hole doping level, from underdoped
samples with ( K) where to overdoped
samples with , ( K) where . An analysis of
the intercept and curvature of the reflectance spectrum shows deviations from
the MFL spectrum symmetrically placed at the optimal doping point . The
Kubo formula for the conductivity gives a better fit to the experiments with
the MFL spectrum up to 2000 cm and with an additional Drude component or
an additional Lorentz component up to 7000 cm. By comparing three
different model fits we conclude that the MFL channel is necessary for a good
fit to the reflectance data. Finally, we note that the monotonic variation of
the reflectance slope with doping provides us with an independent measure of
the doping level for the Bi-2212 system.Comment: 11 pages, 11 figure
Fluctuating diamagnetism in underdoped high temperature superconductors
The fluctuation induced diamagnetism of underdoped high temperature
superconductors is studied in the framework of the Lawrence-Doniach model. By
taking into account the fluctuations of the phase of the order parameter only,
the latter reduces to a layered XY-model describing a liquid of vortices which
can be either thermally excited or induced by the external magnetic field. The
diamagnetic response is given by a current-current correlation function which
is evaluated using the Coulomb gas analogy. Our results are then applied to
recent measurements of fluctuation diamagnetism in underdoped YBCO. They allow
to understand both the observed anomalous temperature dependence of the
zero-field susceptibility and the two distinct regimes appearing in the
magnetic field dependence of the magnetization.Comment: 12 pages, 4 figures included, accepted for publication in PR
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