3,766 research outputs found
Macroscopic quantum tunneling and quasiparticle-tunneling blockade effect in s-wave/d-wave hybrid junctions
We have theoretically investigated macroscopic quantum tunneling (MQT) and
the influence of nodal quasiparticles and zero energy bound states (ZES) on MQT
in s-wave/ d-wave hybrid Josephson junctions. In contrast to d-wave/d-wave
junctions, the low-energy quasiparticle dissipation resulting from nodal
quasiparticles and ZES is suppressed due to a quasiparticle-tunneling blockade
effect in an isotropic s-wave superconductor. Therefore, the inherent
dissipation in these junctions is found to be very weak. We have also
investigated MQT in a realistic s-wave/d-wave (Nb/Au/YBCO) junction in which
Ohmic dissipation in a shunt resistance is stronger than the inherent
dissipation and find that MQT is observable within the current experimental
technology. This result suggests high potential of s-wave/d-wave hybrid
junctions for applications in quantum information devices.Comment: 4 pages, 3 figure
Monte Carlo integration on GPU
We use a graphics processing unit (GPU) for fast computations of Monte Carlo
integrations. Two widely used Monte Carlo integration programs, VEGAS and
BASES, are parallelized on GPU. By using plus multi-gluon production
processes at LHC, we test integrated cross sections and execution time for
programs in FORTRAN and C on CPU and those on GPU. Integrated results agree
with each other within statistical errors. Execution time of programs on GPU
run about 50 times faster than those in C, and more than 60 times faster than
the original FORTRAN programs.Comment: 6 pages, 2 figure
Theory of Macroscopic Quantum Tunneling in High-T_c c-Axis Josephson Junctions
We study macroscopic quantum tunneling (MQT) in c-axis twist Josephson
junctions made of high-T_c superconductors in order to clarify the influence of
the anisotropic order parameter symmetry (OPS) on MQT. The dependence of the
MQT rate on the twist angle about the c-axis is calculated by using
the functional integral and the bounce method. Due to the d-wave OPS, the
dependence of standard deviation of the switching current distribution
and the crossover temperature from thermal activation to MQT are found to be
given by and , respectively. We also show
that a dissipative effect resulting from the nodal quasiparticle excitation on
MQT is negligibly small, which is consistent with recent MQT experiments using
BiSrCaCuO intrinsic junctions. These results
indicate that MQT in c-axis twist junctions becomes a useful experimental tool
for testing the OPS of high-T_c materials at low temperature, and suggest high
potential of such junctions for qubit applications.Comment: 15 pages, 8 figures, 1 tabl
Microscopic Theory of Current-Spin Interaction in Ferromagnets
Interplay between magnetization dynamics and electric current in a conducting
ferromagnet is theoretically studied based on a microscopic model calculation.
First, the effects of the current on magnetization dynamics (spin torques) are
studied with special attention to the "dissipative" torques arising from
spin-relaxation processes of conduction electrons. Next, an analysis is given
of the "spin motive force", namely, a spin-dependent 'voltage' generation due
to magnetization dynamics, which is the reaction to spin torques. Finally, an
attempt is presented of a unified description of these effects.Comment: Written in December 2008, published in July 200
Critical Temperature Tc and Charging Energy Ec between B-B layers of Superconducting diboride materials MgB2 in 3D JJA model
The diboride materials MB2 (M = Mg, Be, Pb, etc.) are discussed on the basis
of the 3D Josephson junction array (JJA) model due to Kawabata-Shenoy-Bishop,
in terms of the B-B layers in the diborides analogous to the Cu-O ones in the
cuprates.
We propose a possibility of superconducting materials with the MgB2-type
structure which exhibit higher critical temperature Tc over 39K of MgB2.
We point out a role of interstitial ionic atoms (e.g., Mg in MgB2) as
capacitors between the B-B layers, which reduce the charging coupling energy in
JJA.Comment: 3 pages, 1 figure included; to be published in J. Phys. Soc. Jpn. 70,
No.10 (2001
grc4f v1.0: a Four-fermion Event Generator for e+e- Collisions
grc4f is a Monte-Carlo package for generating e+e- to 4-fermion processes in
the standard model. All of the 76 LEP-2 allowed fermionic final state processes
evaluated at tree level are included in version 1.0. grc4f addresses event
simulation requirements at e+e- colliders such as LEP and up-coming linear
colliders. Most of the attractive aspects of grc4f come from its link to the
GRACE system: a Feynman diagram automatic computation system. The GRACE system
has been used to produce the computational code for all final states, giving a
higher level of confidence in the calculation correctness. Based on the
helicity amplitude calculation technique, all fermion masses can be kept finite
and helicity information can be propagated down to the final state particles.
The phase space integration of the matrix element gives the total and
differential cross sections, then unweighted events are Generated. Initial
state radiation (ISR) corrections are implemented in two ways, one is based on
the electron structure function formalism and the second uses the parton shower
algorithm called QEDPS. The latter can also be applied for final state
radiation (FSR) though the interference with the ISR is not yet taken into
account. Parton shower and hadronization of the final quarks are performed
through an interface to JETSET. Coulomb correction between two intermediate
W's, anomalous coupling as well as gluon contributions in the hadronic
processes are also included.Comment: 30 pages, LaTeX, 5 pages postscript figures, uuencode
Dynamical Coulomb blockade and spin-entangled electrons
We consider the production of mobile and nonlocal pairwise spin-entangled
electrons from tunneling of a BCS-superconductor (SC) to two normal Fermi
liquid leads. The necessary mechanism to separate the two electrons coming from
the same Cooper pair (spin-singlet) is achieved by coupling the SC to leads
with a finite resistance. The resulting dynamical Coulomb blockade effect,
which we describe phenomenologically in terms of an electromagnetic
environment, is shown to be enhanced for tunneling of two spin-entangled
electrons into the same lead compared to the process where the pair splits and
each electron tunnels into a different lead. On the other hand in the
pair-split process, the spatial correlation of a Cooper pair leads to a current
suppression as a function of distance between the two tunnel junctions which is
weaker for effectively lower dimensional SCs.Comment: 5 pages, 2 figure
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