10,546 research outputs found
Superspace formulation and correlation functions of 3d superconformal field theories
We study SCFTs in the superspace formalism and discuss superfields and
on-shell higher spin current multiplets in free SCFTs with and superconformal symmetry. For 3d SCFTs we
determine the superconformal invariants in superspace needed for constructing
3-point functions of higher spin operators, find the non-linear relations
between the invariants and consequently write down all the independent
invariant structures, both parity even and odd, for various 3-point functions
of higher spin operators.Comment: typos corrected, references added. Accepted for publication in JHE
Macroscopic Resonant Tunneling in the Presence of Low Frequency Noise
We develop a theory of macroscopic resonant tunneling of flux in a
double-well potential in the presence of realistic flux noise with significant
low-frequency component. The rate of incoherent flux tunneling between the
wells exhibits resonant peaks, the shape and position of which reflect
qualitative features of the noise, and can thus serve as a diagnostic tool for
studying the low-frequency flux noise in SQUID qubits. We show, in particular,
that the noise-induced renormalization of the first resonant peak provides
direct information on the temperature of the noise source and the strength of
its quantum component.Comment: 4 pages, 1 figur
Decoherence in adiabatic quantum computation
We have studied the decoherence properties of adiabatic quantum computation
(AQC) in the presence of in general non-Markovian, e.g., low-frequency, noise.
The developed description of the incoherent Landau-Zener transitions shows that
the global AQC maintains its properties even for decoherence larger than the
minimum gap at the anticrossing of the two lowest energy levels. The more
efficient local AQC, however, does not improve scaling of the computation time
with the number of qubits as in the decoherence-free case. The scaling
improvement requires phase coherence throughout the computation, limiting the
computation time and the problem size n.Comment: 4 pages, 2 figures, published versio
Large Tunneling Anisotropic Magneto-Seebeck Effect in a CoPt|MgO|Pt Tunnel Junction
We theoretically investigate the Tunneling Anisotropic Magneto-Seebeck effect
in a realistically-modeled CoPt|MgO|Pt tunnel junction using coherent transport
calculations. For comparison we study the tunneling magneto-Seebeck effect in
CoPt|MgO|CoPt as well. We find that the magneto-Seebeck ratio of CoPt|MgO|Pt
exceeds that of CoPt|MgO|CoPt for small barrier thicknesses, reaching 175% at
room temperature. This result provides a sharp contrast to the
magnetoresistance, which behaves oppositely for all barrier thicknesses and
differs by one order of magnitude between devices. Here the magnetoresistance
results from differences in transmission brought upon by changing the tunnel
junction's magnetization configuration. The magneto-Seebeck effect results from
variations in asymmetry of the energy-dependent transmission instead. We report
that this difference in origin allows for CoPt|MgO|Pt to possess strong thermal
magnetic-transport anisotropy.Comment: 6 pages, 6 figure
Nonlocal mixing of supercurrents in Josephson ballistic point contact
We study coherent current states in the mesoscopic superconducting weak link
simultaneously subjected to the order parameter phase difference on the contact
and to the tangential to the junction interface superfluid velocity in the
banks. The Josephson current-phase relation controlled by the external
transport current is obtained. At phase difference close to pi the nonlocal
nature of the Josephson phase-dependent current results in the appearance of
two vortexlike states in the vicinity of the contact.Comment: 4 pages, 6 figures; to be published in Phys. Rev. B; e-mail:
[email protected]
Kinematic Density Waves in Accretion Disks
When thin accretion disks around black holes are perturbed, the main
restoring force is gravity. If gas pressure, magnetic stresses, and radiation
pressure are neglected, the disk remains thin as long as orbits do not
intersect. Intersections would result in pressure forces which limit the growth
of perturbations. We find that a discrete set of perturbations is possible for
which orbits remain non-intersecting for arbitrarily long times. These modes
define a discrete set of frequencies. We classify all long-lived perturbations
for arbitrary potentials and show how their mode frequencies are related to
pattern speeds computed from the azimuthal and epicyclic frequencies. We show
that modes are concentrated near radii where the pattern speed has vanishing
radial derivative. We explore these modes around Kerr black holes as a possible
explanation for the high-frequency quasi-periodic oscillations of black hole
binaries such as GRO J1655-40. The long-lived modes are shown to coincide with
diskoseismic waves in the limit of small sound speed. While the waves have long
lifetime, they have the wrong frequencies to explain the pairs of
high-frequency quasi-periodic oscillations observed in black hole binaries.Comment: 28 pages, 6 figures; extended comparison with diskoseismology; added
reference to astro-ph/060368
Quantum Nondemolition Charge Measurement of a Josephson Qubit
In a qubit system, the measurement operator does not necessarily commute with
the qubit Hamiltonian, so that the readout process demolishes (mixes) the qubit
energy eigenstates. The readout time is therefore limited by such a mixing time
and its fidelity will be reduced. A quantum nondemolition readout scheme is
proposed in which the charge of a flux qubit is measured. The measurement
operator is shown to commute with the qubit Hamiltonian in the reduced
two-level Hilbert space, even though the Hamiltonian contains non-commuting
charge and flux terms.Comment: 4 pages, 3 figures, a paragraph added to describe how the scheme
works in charge regim
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