27,925 research outputs found
Super-Poissonian noise in a Coulomb blockade metallic quantum dot structure
The shot noise of the current through a single electron transistor (SET),
coupled capacitively with an electronic box, is calculated, using the master
equation approach. We show that the noise may be sub-Poissonian or strongly
super-Poissonian, depending mainly on the box parameters and the gate. The
study also supports the idea that not negative differential conductance, but
charge accumulation in the quantum dot, responds for the super-Poissonian noise
observed.Comment: 4 Pages, 3 Figure
Dissipation in a superconducting artificial atom due to a single non-equilibrium quasiparticle
We study a superconducting artificial atom which is represented by a single
Josephson junction or a Josephson junction chain, capacitively coupled to a
coherently driven transmission line, and which contains exactly one residual
quasiparticle (or up to one quasiparticle per island in a chain). We study the
dissipation in the atom induced by the quasiparticle tunneling, taking into
account the quasiparticle heating by the drive. We calculate the transmission
coefficient in the transmission line for drive frequencies near resonance and
show that, when the artificial atom spectrum is nearly harmonic, the intrinsic
quality factor of the resonance increases with the drive power. This
counterintuitive behavior is due to the energy dependence of the quasiparticle
density of states
Model for Anisotropic Directed Percolation
We propose a simulation model to study the properties of directed percolation
in two-dimensional (2D) anisotropic random media. The degree of anisotropy in
the model is given by the ratio between the axes of a semi-ellipse
enclosing the bonds that promote percolation in one direction. At percolation,
this simple model shows that the average number of bonds per site in 2D is an
invariant equal to 2.8 independently of . This result suggests that
Sinai's theorem proposed originally for isotropic percolation is also valid for
anisotropic directed percolation problems. The new invariant also yields a
constant fractal dimension for all , which is the same
value found in isotropic directed percolation (i.e., ).Comment: RevTeX, 9 pages, 3 figures. To appear in Phys.Rev.
Are there stable long-range ordered Fe(1-x)Cr(x) compounds?
The heat of formation of Fe-Cr alloys undergoes an anomalous change of sign
at small Cr concentrations. This observation raises the question whether there
are intermetallic phases present in this composition range. Here we report the
discovery of several long-range ordered structures that represent ground state
phases at zero Kelvin. In particular we have identified a structure at 3.7% Cr
with an embedding energy which is 49 meV/Cr atom below the solid solution. This
implies there is an effective long-range attractive interaction between Cr
atoms. We propose that the structures found in this study complete the low
temperature-low Cr region of the phase diagram.Comment: 3 pages, 2 figure
Competing interactions in artificial spin chains
The low-energy magnetic configurations of artificial frustrated spin chains
are investigated using magnetic force microscopy and micromagnetic simulations.
Contrary to most studies on two-dimensional artificial spin systems where
frustration arises from the lattice geometry, here magnetic frustration
originates from competing interactions between neighboring spins. By tuning
continuously the strength and sign of these interactions, we show that
different magnetic phases can be stabilized. Comparison between our
experimental findings and predictions from the one-dimensional Anisotropic
Next-Nearest-Neighbor Ising (ANNNI) model reveals that artificial frustrated
spin chains have a richer phase diagram than initially expected. Besides the
observation of several magnetic orders and the potential extension of this work
to highly-degenerated artificial spin chains, our results suggest that the
micromagnetic nature of the individual magnetic elements allows observation of
metastable spin configurations.Comment: 5 pages, 4 figure
Rank-Deficiency in Indoor MIMO
This paper points out in an analytical way that rankdeficiency in indoor MIMO is typically due to the small size of scattering windows in the NLOS propagation path between the transmitter and the receiver
Third type of domain wall in soft magnetic nanostrips
Magnetic domain walls (DWs) in nanostructures are low-dimensional objects
that separate regions with uniform magnetisation. Since they can have different
shapes and widths, DWs are an exciting playground for fundamental research, and
became in the past years the subject of intense works, mainly focused on
controlling, manipulating, and moving their internal magnetic configuration. In
nanostrips with in-plane magnetisation, two DWs have been identified: in thin
and narrow strips, transverse walls are energetically favored, while in thicker
and wider strips vortex walls have lower energy. The associated phase diagram
is now well established and often used to predict the low-energy magnetic
configuration in a given magnetic nanostructure. However, besides the
transverse and vortex walls, we find numerically that another type of wall
exists in permalloy nanostrips. This third type of DW is characterised by a
three-dimensional, flux closure micromagnetic structure with an unusual length
and three internal degrees of freedom. Magnetic imaging on
lithographically-patterned permalloy nanostrips confirms these predictions and
shows that these DWs can be moved with an external magnetic field of about 1mT.
An extended phase diagram describing the regions of stability of all known
types of DWs in permalloy nanostrips is provided.Comment: 19 pages, 7 figure
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