196 research outputs found
Magnetic Anisotropy Variations and Non-Equilibrium Tunneling in a Cobalt Nanoparticle
We present detailed measurements of the discrete electron-tunneling level
spectrum within nanometer-scale cobalt particles as a function of magnetic
field and gate voltage, in this way probing individual quantum many-body
eigenstates inside ferromagnetic samples. Variations among the observed levels
indicate that different quantum states within one particle are subject to
different magnetic anisotropy energies. Gate-voltage studies demonstrate that
the low-energy tunneling spectrum is affected dramatically by the presence of
non-equilibrium spin excitations
Proximity DC squids in the long junction limit
We report the design and measurement of
Superconducting/normal/superconducting (SNS) proximity DC squids in the long
junction limit, i.e. superconducting loops interrupted by two normal metal
wires roughly a micrometer long. Thanks to the clean interface between the
metals, at low temperature a large supercurrent flows through the device. The
dc squid-like geometry leads to an almost complete periodic modulation of the
critical current through the device by a magnetic flux, with a flux periodicity
of a flux quantum h/2e through the SNS loop. In addition, we examine the entire
field dependence, notably the low and high field dependence of the maximum
switching current. In contrast with the well-known Fraunhoffer-type
oscillations typical of short wide junctions, we find a monotonous gaussian
extinction of the critical current at high field. As shown in [15], this
monotonous dependence is typical of long and narrow diffusive junctions. We
also find in some cases a puzzling reentrance at low field. In contrast, the
temperature dependence of the critical current is well described by the
proximity effect theory, as found by Dubos {\it et al.} [16] on SNS wires in
the long junction limit. The switching current distributions and hysteretic IV
curves also suggest interesting dynamics of long SNS junctions with an
important role played by the diffusion time across the junction.Comment: 12 pages, 16 figure
Direct measurement of the phase coherence length in a GaAs/GaAlAs square network
The low temperature magnetoconductance of a large array of quantum
coherentloops exhibits Altshuler-Aronov-Spivak oscillations which
periodicitycorresponds to 1/2 flux quantum per loop.We show that the
measurement of the harmonics content in a square networkprovides an accurate
way to determine the electron phase coherence length in units of the
lattice length without any adjustableparameters.We use this method to determine
in a network realised from a 2Delectron gas (2DEG) in a GaAS/GaAlAs
heterojunction. The temperaturedependence follows a power law from
1.3 K to 25 mK with nosaturation, as expected for 1D diffusive electronic
motion andelectron-electron scattering as the main decoherence mechanism.Comment: Additional experimental data in version
Alteration of superconductivity of suspended carbon nanotubes by deposition of organic molecules
We have altered the superconductivity of a suspended rope of single walled
carbon nanotubes, by coating it with organic polymers. Upon coating, the normal
state resistance of the rope changes by less than 20 percent. But
superconductivity, which on the bare rope shows up as a substantial resistance
decrease below 300 mK, is gradualy suppressed. We correlate this to the
suppression of radial breathing modes, measured with Raman Spectroscopy on
suspended Single and Double-walled carbon nanotubes. This points to the
breathing phonon modes as being responsible for superconductivity in carbon
nanotubes
Geometry-related magnetic interference patterns in long SNS Josephson junctions
We have measured the critical current dependence on the magnetic flux of two
long SNS junctions differing by the normal wire geometry. The samples are made
by a Au wire connected to W contacts, via Focused Ion Beam assisted deposition.
We could tune the magnetic pattern from the monotonic gaussian-like decay of a
quasi 1D normal wire to the Fraunhofer-like pattern of a square normal wire. We
explain the monotonic limit with a semiclassical 1D model, and we fit both
field dependences with numerical simulations of the 2D Usadel equation.
Furthermore, we observe both integer and fractional Shapiro steps. The magnetic
flux dependence of the integer steps reproduces as expected that of the
critical current Ic, while fractional steps decay slower with the flux than Ic.Comment: 5 pages, 4 figure
Nonequilibrium excitations in Ferromagnetic Nanoparticles
In recent measurements of tunneling transport through individual
ferromagnetic Co nanograins, Deshmukh, Gu\'eron, Ralph et al.
\cite{mandar,gueron} (DGR) observed a tunneling spectrum with discrete
resonances, whose spacing was much smaller than what one would expect from
naive independent-electron estimates. In a previous publication,
\cite{prl_kleff} we had suggested that this was a consequence of nonequilibrium
excitations, and had proposed a ``minimal model'' for ferromagnetism in
nanograins with a discrete excitation spectrum as a framework for analyzing the
experimental data. In the present paper, we provide a detailed analysis of the
properties of this model: We delineate which many-body electron states must be
considered when constructing the tunneling spectrum, discuss various
nonequilibrium scenarios and compare their results with the experimental data
of Refs. \cite{mandar,gueron}. We show that a combination of nonequilibrium
spin- and single-particle excitations can account for most of the observed
features, in particular the abundance of resonances, the resonance spacing and
the absence of Zeeman splitting.Comment: 13 pages, 10 figure
A Model for Ferromagnetic Nanograins with Discrete Electronic States
We propose a simple phenomenological model for an ultrasmall ferromagnetic
grain, formulated in terms of the grain's discrete energy levels. We compare
the model's predictions with recent measurements of the discrete tunneling
spectrum through such a grain. The model can qualitatively account for the
observed features if we assume (i) that the anisotropy energy varies among
different eigenstates of one grain, and (ii) that nonequilibrium spin
accumulation occurs.Comment: 4 pages, 2 figure
A new topological aspect of the arbitrary dimensional topological defects
We present a new generalized topological current in terms of the order
parameter field to describe the arbitrary dimensional topological
defects. By virtue of the -mapping method, we show that the topological
defects are generated from the zero points of the order parameter field , and the topological charges of these topological defects are topological
quantized in terms of the Hopf indices and Brouwer degrees of -mapping
under the condition that the Jacobian . When , it is shown that there exist the crucial case of branch process.
Based on the implicit function theorem and the Taylor expansion, we detail the
bifurcation of generalized topological current and find different directions of
the bifurcation. The arbitrary dimensional topological defects are found
splitting or merging at the degenerate point of field function but
the total charge of the topological defects is still unchanged.Comment: 24 pages, 10 figures, Revte
Textures and Newtonian Gravity
Newtonian theory is used to study the gravitational effects of a texture, in
particular the formation of massive structures.Comment: 4 pages, 4 ps figures, REVTEX, accepted for publication in PR
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