100 research outputs found
Efficiency at maximum power: An analytically solvable model for stochastic heat engines
We study a class of cyclic Brownian heat engines in the framework of
finite-time thermodynamics. For infinitely long cycle times, the engine works
at the Carnot efficiency limit producing, however, zero power. For the
efficiency at maximum power, we find a universal expression, different from the
endoreversible Curzon-Ahlborn efficiency. Our results are illustrated with a
simple one-dimensional engine working in and with a time-dependent harmonic
potential.Comment: 6 pages, 3 figure
Observation of the spontaneous vortex phase in the weakly ferromagnetic superconductor ErNiBC: A penetration depth study
The coexistence of weak ferromagnetism and superconductivity in ErNiBC suggests the possibility of a spontaneous vortex phase (SVP) in which
vortices appear in the absence of an external field. We report evidence for the
long-sought SVP from the in-plane magnetic penetration depth of high-quality single crystals of ErNiBC. In addition to
expected features at the N\'{e}el temperature = 6.0 K and weak
ferromagnetic onset at K, rises to a maximum
at K before dropping sharply down to 0.1 K. We assign the
0.45 K-maximum to the proliferation and freezing of spontaneous vortices. A
model proposed by Koshelev and Vinokur explains the increasing as a consequence of increasing vortex density, and its subsequent decrease
below as defect pinning suppresses vortex hopping.Comment: 5 pages including figures; added inset to Figure 2; significant
revisions to tex
Response time of a normal-superconductor hybrid system under the step-like pulse bias
The response of a quantum dot coupled with one normal lead and a
superconductor lead driven by a step-like pulse bias is studied using the
non-equilibrium Green function method. In the linear pulse bias regime, the
responses of the upwards and downwards bias are symmetric. In this regime the
turn-on time and turn-off time are much slower than that of the normal system
due to the Andreev reflection. On the other hand, for the large pulse bias
, the instantaneous current exhibits oscillatory behaviors with the
frequency . The turn on/off times are in (or shorter than)
the scale of , so they are faster for the larger bias . In
addition, the responses for the upwards and downwards bias are asymmetric at
large . The turn-on time is larger than the turn-off time but the
relaxation time \cite{note1} depends only on the coupling strength and
it is much smaller than the turn-on/off times for the large bias .Comment: 8 pages, 4 figures, accepted for publication in Phys. Rev.
The role of microtubule movement in bidirectional organelle transport
We study the role of microtubule movement in bidirectional organelle
transport in Drosophila S2 cells and show that EGFP-tagged peroxisomes in cells
serve as sensitive probes of motor induced, noisy cytoskeletal motions.
Multiple peroxisomes move in unison over large time windows and show
correlations with microtubule tip positions, indicating rapid microtubule
fluctuations in the longitudinal direction. We report the first high-resolution
measurement of longitudinal microtubule fluctuations performed by tracing such
pairs of co-moving peroxisomes. The resulting picture shows that
motor-dependent longitudinal microtubule oscillations contribute significantly
to cargo movement along microtubules. Thus, contrary to the conventional view,
organelle transport cannot be described solely in terms of cargo movement along
stationary microtubule tracks, but instead includes a strong contribution from
the movement of the tracks.Comment: 24 pages, 5 figure
Possible isotope effect on the resonance peak formation in high-T cuprates
Starting from the three-band Hubbard Hamiltonian we derive an effective
model including electron-phonon interaction of quasiparticles with
optical phonons. Within the effective Hamiltonian we analyze the influence of
electronic correlations and electron-phonon interaction on the dynamical spin
susceptibility in layered cuprates. We find a huge isotope effect on the
resonance peak in the magnetic spin susceptibility, ,
seen by inelastic neutron scattering. It results from both the electron-phonon
coupling and the electronic correlation effects taken into account beyond
random phase approximation(RPA) scheme. We find at optimal doping the isotope
coeffiecient which can be further tested
experimentally.Comment: revised version, new figure is added. Phys. Rev. B 69, 0945XX (2004);
in pres
Interplay of Electron-Phonon Interaction and Electron Correlation in High Temperature Superconductivity
We study the electron-phonon interaction in the strongly correlated
superconducting cuprates. Two types of the electron-phonon interactions are
introduced in the model; the diagonal and off-diagonal interactions which
modify the formation energy of the Zhang-Rice singlet and its transfer
integral, respectively. The characteristic phonon-momentum and
electron-momentum dependence resulted from the off-diagonal coupling
can explain a variety of experiments. The vertex correction for the
electron-phonon interaction is formulated in the SU(2) slave-boson theory by
taking into account the collective modes in the superconducting ground states.
It is shown that the vertex correction enhances the attractive potential for
the d-wave paring mediated by phonon with around
which corresponds to the half-breathing mode of the oxygen
motion.Comment: 14 pages, 13 figure
Tunneling spectroscopy in the magnetic superconductor TmNi2B2C
We present new measurements about the tunneling conductance in the
borocarbide superconductor TmNiBC. The results show a very good
agreement with weak coupling BCS theory, without any lifetime broadening
parameter, over the whole sample surface. We detect no particular change of the
tunneling spectroscopy below 1.5K, when both the antiferromagnetic (AF) phase
and the superconducting order coexist.Comment: Submitted to Phys. Rev. B, Rapid Communication
Manifestation of triplet superconductivity in superconductor-ferromagnet structures
We study proximity effects in a multilayered superconductor/ferromagnet (S/F)
structure with arbitrary relative directions of the magnetization . If
the magnetizations of different layers are collinear the superconducting
condensate function induced in the F layers has only a singlet component and a
triplet one with a zero projection of the total magnetic moment of the Cooper
pairs on the direction. In this case the condensate penetrates the F
layers over a short length determined by the exchange energy . If
the magnetizations are not collinear the triplet component has, in
addition to the zero projection, the projections . The latter component
is even in the momentum, odd in the Matsubara frequency and penetrates the F
layers over a long distance that increases with decreasing temperature and does
not depend on (spin-orbit interaction limits this length). If the thickness
of the F layers is much larger than , the Josephson coupling between
neighboring S layers is provided only by the triplet component, so that a new
type of superconductivity arises in the transverse direction of the structure.
The Josephson critical current is positive (negative) for the case of a
positive (negative) chirality of the vector . We demonstrate that this
type of the triplet condensate can be detected also by measuring the density of
states in F/S/F structures.Comment: 14 pages; 9 figures. Final version, to be published in Phys. Rev.
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