35,932 research outputs found
Geometric quantum computation using fictitious spin- 1/2 subspaces of strongly dipolar coupled nuclear spins
Geometric phases have been used in NMR, to implement controlled phase shift
gates for quantum information processing, only in weakly coupled systems in
which the individual spins can be identified as qubits. In this work, we
implement controlled phase shift gates in strongly coupled systems, by using
non-adiabatic geometric phases, obtained by evolving the magnetization of
fictitious spin-1/2 subspaces, over a closed loop on the Bloch sphere. The
dynamical phase accumulated during the evolution of the subspaces, is refocused
by a spin echo pulse sequence and by setting the delay of transition selective
pulses such that the evolution under the homonuclear coupling makes a complete
rotation. A detailed theoretical explanation of non-adiabatic geometric
phases in NMR is given, by using single transition operators. Controlled phase
shift gates, two qubit Deutsch-Jozsa algorithm and parity algorithm in a
qubit-qutrit system have been implemented in various strongly dipolar coupled
systems obtained by orienting the molecules in liquid crystal media.Comment: 37 pages, 17 figure
Diffuse Neutron Scattering Study of Magnetic Correlations in half-doped La0.5Ca0.5-xSrxMnO3 (x = 0.1, 0.3 and 0.4) Manganites
The short range ordered magnetic correlations have been studied in half doped
La0.5Ca0.5-xSrxMnO3 (x = 0.1, 0.3 and 0.4) compounds by polarized neutron
scattering technique. On doping Sr2+ for Ca2+ ion, these compounds with x =
0.1, 0.3, and 0.4 exhibit CE-type, mixture of CE-type and A-type, and A-type
antiferromagnetic ordering, respectively. Magnetic diffuse scattering is
observed in all the compounds above and below their respective magnetic
ordering temperatures and is attributed to magnetic polarons. The correlations
are primarily ferromagnetic in nature above T\_N, although a small
antiferromagnetic contribution is also evident. Additionally, in samples x =
0.1 and 0.3 with CE-type antiferromagnetic ordering, superlattice diffuse
reflections are observed indicating correlations between magnetic polarons. On
lowering temperature below T\_N the diffuse scattering corresponding to
ferromagnetic correlations is suppressed and the long range ordered
antiferromagnetic state is established. However, the short range ordered
correlations indicated by enhanced spin flip scattering at low Q coexist with
long range ordered state down to 3K. In x = 0.4 sample with A-type
antiferromagnetic ordering, superlattice diffuse reflections are absent.
Additionally, in comparison to x = 0.1 and 0.3 sample, the enhanced spin flip
scattering at low Q is reduced at 310K, and as temperature is reduced below
200K, it becomes negligibly low. The variation of radial correlation function,
g(r) with temperature indicates rapid suppression of ferromagnetic correlations
at the first nearest neighbor on approaching TN. Sample x = 0.4 exhibits growth
of ferromagnetic phase at intermediate temperatures (~ 200K). This has been
further explored using SANS and neutron depolarization techniques.Comment: 13 pages, 12 figures, To appear in Physical Review
Reflection of light and heavy holes from a linear potential barrier
In this paper we study reflection of holes in direct-band semiconductors from
the linear potential barrier. It is shown that light-heavy hole transformation
matrix is universal. It depends only on a dimensionless product of the light
hole longitudinal momentum and the characteristic length determined by the
slope of the potential and doesn't depend on the ratio of light and heavy hole
masses, provided this ratio is small. It is shown that the transformation
coefficient goes to zero both in the limit of small and large longitudinal
momenta, however the phase of a reflected hole is different in these limits. An
approximate analytical expression for the light-heavy hole transformation
coefficient is found.Comment: 6 pages, 2 figure
Nodeless superconductivity in the cage-type superconductor Sc5Ru6Sn18 with preserved time-reversal symmetry
We report the single-crystal synthesis and detailed investigations of the
cage-type superconductor Sc5Ru6Sn18, using powder x-ray diffraction (XRD),
magnetization, specific-heat and muon-spin relaxation (muSR) measurements.
Sc5Ru6Sn18 crystallizes in a tetragonal structure (space group I41/acd) with
the lattice parameters a = 1.387(3) nm and c = 2.641(5) nm. Both DC and AC
magnetization measurements prove the type-II superconductivity in Sc5Ru6Sn18
with Tc = 3.5(1) K, a lower critical field H_c1 (0) = 157(9) Oe and an upper
critical field, H_c2 (0) = 26(1) kOe. The zero-field electronic specific-heat
data are well fitted using a single-gap BCS model, with superconducting gap =
0.64(1) meV. The Sommerfeld constant varies linearly with the applied magnetic
field, indicating s-wave superconductivity in Sc5Ru6Sn18. Specific-heat and
transverse-field (TF) muSR measurements reveal that Sc5Ru6Sn18 is a
superconductor with strong electron-phonon coupling, with TF-muSR also
suggesting the single-gap s-wave character of the superconductivity.
Furthermore, zero-field muSR measurements do not detect spontaneous magnetic
fields below Tc, hence implying that time-reversal symmetry is preserved in
Sc5Ru6Sn18.Comment: 23 pages, 11 figure
GINZBURG-LANDAU THEORY OF VORTICES IN -WAVE SUPERCONDUCTORS
Ginzburg-Landau theory is used to study the properties of single vortices and
of the Abrikosov vortex lattice in a superconductor. For a single
vortex, the -wave order parameter has the expected four-lobe structure in a
ring around the core and falls off like at large distances. The
topological structure of the -wave order parameter consists of one
counter-rotating unit vortex, centered at the core, surrounded by four
symmetrically placed positive unit vortices. The Abrikosov lattice is shown to
have a triangular structure close to and an oblique structure at lower
temperatures. Comparison is made to recent neutron scattering data.Comment: 4 pages, REVTeX, 3 figures available upon reques
Renormalization group study of the Kondo problem at a junction of several Luttinger wires
We study a system consisting of a junction of N quantum wires, where the
junction is characterized by a scalar S-matrix, and an impurity spin is coupled
to the electrons close to the junction. The wires are modeled as weakly
interacting Tomonaga-Luttinger liquids. We derive the renormalization group
equations for the Kondo couplings of the spin to the electronic modes on
different wires, and analyze the renormalization group flows and fixed points
for different values of the initial Kondo couplings and of the junction
S-matrix (such as the decoupled S-matrix and the Griffiths S-matrix). We
generally find that the Kondo couplings flow towards large and
antiferromagnetic values in one of two possible ways. For the Griffiths
S-matrix, we study one of the strong coupling flows by a perturbative expansion
in the inverse of the Kondo coupling; we find that at large distances, the
system approaches the ferromagnetic fixed point of the decoupled S-matrix. For
the decoupled S-matrix with antiferromagnetic Kondo couplings and weak
inter-electron interactions, the flows are to one of two strong coupling fixed
points in which all the channels are strongly coupled to each other through the
impurity spin. But strong inter-electron interactions, with K_\rho < N/(N+2),
stabilize a multi-channel fixed point in which the coupling between different
channels goes to zero. We have also studied the temperature dependence of the
conductance at the decoupled and Griffiths S-matrices.Comment: Revtex4, 16 pages including 6 figure
Observational Implications of Precessing Protostellar Discs and Jets
We consider the dynamics of a protostellar disc in a binary system where the
disc is misaligned with the orbital plane of the binary, with the aim of
determining the observational consequences for such systems. The disc wobbles
with a period approximately equal to half the binary's orbital period and
precesses on a longer timescale. We determine the characteristic timescale for
realignment of the disc with the orbital plane due to dissipation. If the
dissipation is determined by a simple isotropic viscosity then we find, in line
with previous studies, that the alignment timescale is of order the viscous
evolution timescale. However, for typical protostellar disc parameters, if the
disc tilt exceeds the opening angle of the disc, then tidally induced shearing
within the disc is transonic. In general, hydrodynamic instabilities associated
with the internally driven shear result in extra dissipation which is expected
to drastically reduce the alignment timescale. For large disc tilts the
alignment timescale is then comparable to the precession timescale, while for
smaller tilt angles , the alignment timescale varies as . We discuss the consequences of the wobbling, precession and
rapid realignment for observations of protostellar jets and the implications
for binary star formation mechanisms.Comment: MNRAS, in press. 10 pages. Also available at
http://www.ast.cam.ac.uk/~mbat
SN 2015ba: A type IIP supernova with a long plateau
We present optical photometry and spectroscopy from about a week after
explosion to 272 d of an atypical Type IIP supernova, SN 2015ba, which
exploded in the edge-on galaxy IC 1029. SN 2015ba is a luminous event with an
absolute V-band magnitude of -17.10.2 mag at 50 d since explosion and has
a long plateau lasting for 123 d. The distance to the SN is estimated to
be 34.80.7 Mpc using the expanding photosphere and standard candle
methods. High-velocity H-Balmer components constant with time are observed in
the late-plateau phase spectra of SN 2015ba, which suggests a possible role of
circumstellar interaction at these phases. Both hydrodynamical and analytical
modelling suggest a massive progenitor of SN 2015ba with a pre-explosion mass
of 24-26 M. However, the nebular spectra of SN 2015ba exhibit
insignificant levels of oxygen, which is otherwise expected from a massive
progenitor. This might be suggestive of the non-monotonical link between O-core
masses and the zero-age main-sequence mass of pre-supernova stars and/or
uncertainties in the mixing scenario in the ejecta of supernovae.Comment: 42 pages, 7 pages Appendix, 20 figures, 10 tables, Accepted for
publication in MNRAS, 14-June-201
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