161,173 research outputs found
Coulomb blockade and Bloch oscillations in superconducting Ti nanowires
Quantum fluctuations in quasi-one-dimensional superconducting channels
leading to spontaneous changes of the phase of the order parameter by ,
alternatively called quantum phase slips (QPS), manifest themselves as the
finite resistance well below the critical temperature of thin superconducting
nanowires and the suppression of persistent currents in tiny superconducting
nanorings. Here we report the experimental evidence that in a current-biased
superconducting nanowire the same QPS process is responsible for the insulating
state -- the Coulomb blockade. When exposed to RF radiation, the internal Bloch
oscillations can be synchronized with the external RF drive leading to
formation of quantized current steps on the I-V characteristic. The effects
originate from the fundamental quantum duality of a Josephson junction and a
superconducting nanowire governed by QPS -- the QPS junction (QPSJ).Comment: 5 pages, 4 figure
Dynamics of coupled vortices in layered magnetic nanodots
The spin dynamics are calculated for a model system consisting of
magnetically soft, layered nanomagnets, in which two ferromagnetic (F)
cylindrical dots, each with a magnetic vortex ground state, are separated by a
non-magnetic spacer (N). This permits a study of the effects of interlayer
magnetostatic interactions on the vortex dynamics. The system was explored by
applying the equations of motion for the vortex core positions. The restoring
force was calculated taking into account the magnetostatic interactions
assuming a realistic surface charge free spin distribution. For tri-layer F/N/F
dots with opposite chiralities and the same core polarizations (lowest energy
state), two eigenmodes are predicted analytically and confirmed via
micromagnetic simulations. One mode is in the sub-GHz range for submicron dot
diameters and corresponds to quasi-circular rotation of the cores about the dot
center. A second mode is in the MHz range corresponding to a small amplitude
rotation of the mean core position. The eigenfrequencies depend strongly on the
geometrical parameters of the system, suggesting that magnetostatic effects
play a dominant role in determining the vortex dynamics.Comment: One PDF file including text and 4 figure
Evaluation of aerothermal modeling computer programs
Various computer programs based upon the SIMPLE or SIMPLER algorithm were studied and compared for numerical accuracy, efficiency, and grid dependency. Four two-dimensional and one three-dimensional code originally developed by a number of research groups were considered. In general, the accuracy and computational efficieny of these TEACH type programs were improved by modifying the differencing schemes and their solvers. A brief description of each program is given. Error reduction, spline flux and second upwind differencing programs are covered
Reply to [arXiv:1105.5147] "Are GRB 090423 and Similar Bursts due to Superconducting Cosmic Strings?"
The GRB outflow driven by superconducting cosmic strings is likely to be an
arc rather than a usually-considered spherical cap. In such a case, the
afterglows of the cosmic string GRBs could be basically consistent with the
observation of the high-redshift GRBs.Comment: 2 pages, 1 figure, to appear in Phys. Rev. Let
New alphabet-dependent morphological transition in a random RNA alignment
We study the fraction of nucleotides involved in the formation of a
cactus--like secondary structure of random heteropolymer RNA--like molecules.
In the low--temperature limit we study this fraction as a function of the
number of different nucleotide species. We show, that with changing ,
the secondary structures of random RNAs undergo a morphological transition:
for as the chain length goes to infinity,
signaling the formation of a virtually "perfect" gapless secondary structure;
while , what means that a non-perfect structure with
gaps is formed. The strict upper and lower bounds are
proven, and the numerical evidence for is presented. The relevance
of the transition from the evolutional point of view is discussed.Comment: 4 pages, 3 figures (title is changed, text is essentially reworked),
accepted in PR
Depairing currents in superconducting films of Nb and amorphous MoGe
We report on measuring the depairing current J_{dp} in thin superconducting
films as a function of temperature. The main difficulties in such measurements
are that heating has to be avoided, either due to contacts, or to vortex flow.
The latter is almost unavoidable since the sample cross-section is usually
larger than the superconducting coherence length \xi_s and the magnetic field
penetration depth \lambda_s. On the other hand, vortex flow is helpful since it
homogenizes the distribution of the current across the sample. We used a pulsed
current method, which allows to overcome the difficulties caused by dissipation
and measured the depairing current in films of thin polycrystalline Nb (low
\lambda_s, low specific resistance \rho) and amorphous Mo_{0.7}Ge_{0.3} (high
\lambda_s, high \rho), structured in the shape of bridges of various width. The
experimental values of J_{dp} for different bridge dimensions are compared with
theoretical predictions by Kupriyanov and Lukichev for dirty limit
superconductors. For the smallest samples we find a very good agreement with
theory, over essentially the whole temperature interval below the
superconducting critical temperature.Comment: 5 pages, 6 figure
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