69 research outputs found
Excess Noise in Biased Superconducting Weak Links
Non-equilibrium excess noise of a short quasi one-dimensional constriction
between two superconductors is considered. A general expression for the
current-current correlation function valid for arbitrary temperatures and bias
voltages is derived. This formalism is applied to a current-carrying quantum
channel with perfect transparency. Contrary to a transparent channel separating
two normal conductors, a weak link between two superconductors exhibits a
finite level of noise. The source of noise is fractional Andreev scattering of
quasiparticles with energies greater than the half-width of the
superconducting gap. For high bias voltages, , the relation
between the zero-frequency limit of the noise spectrum, , and the excess
current reads . As both the excess noise and the excess current vanish linearly in
, %, their ratio being constant.Comment: 8 pages (Latex), 1 figur
Supercurrent noise in quantum point contacts
Spectral density of current fluctuations in a short ballistic superconducting
quantum point contact is calculated for arbitrary bias voltages . Contrary
to a common opinion that the supercurrent flow in Josephson junctions is
coherent process with no fluctuations, we find extremely large current noise
that is {\em caused} by the supercurrent coherence. An unusual feature of the
noise, besides its magnitude, is its voltage dependence: the noise decreases
with increasing , despite the fact that the dc current grows steadily with
. At finite voltages the noise can be qualitatively understood as the shot
noise of the large charge quanta of magnitude equal to the charge
transferred during one period of Josephson oscillations.Comment: 12 pages, revtex, 2 figures by fax/conventional mail upon reques
Adiabatic Dynamics of Superconducting Quantum Point Contacts
Starting from the quasiclassical equations for non-equilibrium Green's
functions we derive a simple kinetic equation that governs ac Josephson effect
in a superconducting quantum point contact at small bias voltages. In contrast
to existing approaches the kinetic equation is valid for voltages with
arbitrary time dependence. We use this equation to calculate
frequency-dependent linear conductance, and dc characteristics with
and without microwave radiation for resistively shunted quantum point contacts.
A novel feature of the characteristics is the excess current
appearing at small voltages. An important by-product of our
derivation is the analytical proof that the microscopic expression for the
current coincides at arbitrary voltages with the expression that follows from
the Bogolyubov-de Gennes equations, if one uses appropriate amplitudes of
Andreev reflection which contain information about microscopic structure of the
superconductors.Comment: 12 Pages, REVTEX 3.0, 3 figures available upon reques
Josephson currents through spin-active interfaces
The Josephson coupling of two isotropic s-wave superconductors through a
small, magnetically active junction is studied. This is done as a function of
junction transparency and of the degree of spin-mixing occurring in the
barrier. In the tunneling limit, the critical current shows an anomalous 1/T
temperature dependence at low temperatures and for certain magnetic
realizations of the junction. The behavior of the Josephson current is governed
by Andreev bound states appearing within the superconducting gap and the
position of these states in energy is tunable with the magnetic properties of
the barrier. This study is done using the equilibrium part of the
quasiclassical Zaitsev-Millis-Rainer-Sauls boundary condition for spin-active
interfaces and a general solution of the boundary condition is found. This
solution is a generalization of the one recently presented by Eschrig [M.
Eschrig, Phys. Rev B 61, 9061 (2000)] for spin-conserving interfaces and allows
an effective treatment of the problem of a superconductor in proximity to a
magnetically active material.Comment: 8 pages + 3 eps figure
Fused eco29kIR- and M genes coding for a fully functional hybrid polypeptide as a model of molecular evolution of restriction-modification systems
<p>Abstract</p> <p>Background</p> <p>The discovery of restriction endonucleases and modification DNA methyltransferases, key instruments of genetic engineering, opened a new era of molecular biology through development of the recombinant DNA technology. Today, the number of potential proteins assigned to type II restriction enzymes alone is beyond 6000, which probably reflects the high diversity of evolutionary pathways. Here we present experimental evidence that a new type IIC restriction and modification enzymes carrying both activities in a single polypeptide could result from fusion of the appropriate genes from preexisting bipartite restriction-modification systems.</p> <p>Results</p> <p>Fusion of <it>eco29kIR </it>and <it>M </it>ORFs gave a novel gene encoding for a fully functional hybrid polypeptide that carried both restriction endonuclease and DNA methyltransferase activities. It has been placed into a subclass of type II restriction and modification enzymes - type IIC. Its MTase activity, 80% that of the M.Eco29kI enzyme, remained almost unchanged, while its REase activity decreased by three times, concurrently with changed reaction optima, which presumably can be caused by increased steric hindrance in interaction with the substrate. <it>In vitro </it>the enzyme preferentially cuts DNA, with only a low level of DNA modification detected. <it>In vivo </it>new RMS can provide a 10<sup>2</sup>-fold less protection of host cells against phage invasion.</p> <p>Conclusions</p> <p>We propose a molecular mechanism of appearing of type IIC restriction-modification and M.SsoII-related enzymes, as well as other multifunctional proteins. As shown, gene fusion could play an important role in evolution of restriction-modification systems and be responsible for the enzyme subclass interconversion. Based on the proposed approach, hundreds of new type IIC enzymes can be generated using head-to-tail oriented type I, II, and III restriction and modification genes. These bifunctional polypeptides can serve a basis for enzymes with altered recognition specificities. Lastly, this study demonstrates that protein fusion may change biochemical properties of the involved enzymes, thus giving a starting point for their further evolutionary divergence.</p
Intense space storms: Critical issues and open disputes
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94671/1/jgra16863.pd
- …