40,906 research outputs found
Microscopic approach to the proton asymmetry in the non-mesonic weak decay of Lambda-hypernuclei
The non--mesonic weak decay of polarized -hypernuclei is studied
with a microscopic diagrammatic formalism in which one- and two-nucleon induced
decay mechanisms, and , are
considered together with (and on the same ground of) nucleon final state
interactions. We adopt a nuclear matter formalism extended to finite nuclei via
the local density approximation. Our approach adopts different
one-meson-exchange weak transition potentials, while the strong interaction
effects are accounted for by a Bonn nucleon-nucleon interaction. We also
consider the two-pion-exchange effect in the weak transition potential. Both
the two-nucleon induced decay mechanism and the final state interactions reduce
the magnitude of the asymmetry. The quantum interference terms considered in
the present microscopic approach give rise to an opposite behavior of the
asymmetry with increasing energy cuts to that observed in models describing the
nucleon final state interactions semi-classically via the intranuclear cascade
code. Our results for the asymmetry parameter in C obtained
with different potential models are consistent with the asymmetry measured at
KEK
The Non-Mesonic Weak Decay of Double-Lambda Hypernuclei: A Microscopic Approach
The non--mesonic weak decay of double-- hypernuclei is studied
within a microscopic diagrammatic approach. Besides the nucleon--induced
mechanism, , widely studied in single-- hypernuclei,
additional hyperon--induced mechanisms, ,
and , are
accessible in double-- hypernuclei and are investigated here. As in
previous works on single-- hypernuclei, we adopt a nuclear matter
formalism extended to finite nuclei via the local density approximation and a
one--meson exchange weak transition potential (including the ground state
pseudoscalar and vector octets mesons) supplemented by correlated and
uncorrelated two--pion--exchange contributions. The weak decay rates are
evaluated for hypernuclei in the region of the experimentally accessible light
hypernuclei Be and B. Our
predictions are compared with a few previous evaluations. The rate for the
decay is dominated by --, -- and
--exchange and turns out to be about 2.5\% of the free decay
rate, , while the total rate for the and decays, dominated by
--exchange, amounts to about 0.25\% of . The
experimental measurement of these decays would be essential for the beginning
of a systematic study of the non--mesonic decay of strangeness
hypernuclei. This field of research could also shed light on the possible
existence and nature of the --dibaryon.Comment: 17 pages, 2 figure
Polynomial algorithms that prove an NP-hard hypothesis implies an NP-hard conclusion
A number of results in Hamiltonian graph theory are of the form implies , where is a property of graphs that is NP-hard and is a cycle structure property of graphs that is also NP-hard. Such a theorem is the well-known Chv\'{a}tal-Erd\"{o}s Theorem, which states that every graph with is Hamiltonian. Here is the vertex connectivity of and is the cardinality of a largest set of independent vertices of . In another paper Chv\'{a}tal points out that the proof of this result is in fact a polynomial time construction that either produces a Hamilton cycle or a set of more than independent vertices. In this note we point out that other theorems in Hamiltonian graph theory have a similar character. In particular, we present a constructive proof of the well-known theorem of Jung for graphs on or more vertices.. \u
On the Coexistence Magnetism/Superconductivity in the Heavy-Fermion Superconductor CePtSi
The interplay between magnetism and superconductivity in the newly discovered
heavy-fermion superconductor CePtSi has been investigated using the
zero-field SR technique. The SR data indicate that the whole muon
ensemble senses spontaneous internal fields in the magnetic phase,
demonstrating that magnetism occurs in the whole sample volume. This points to
a microscopic coexistence between magnetism and heavy-fermion
superconductivity.Comment: Final version, new figure structure, references correcte
Shape analysis on homogeneous spaces: a generalised SRVT framework
Shape analysis is ubiquitous in problems of pattern and object recognition
and has developed considerably in the last decade. The use of shapes is natural
in applications where one wants to compare curves independently of their
parametrisation. One computationally efficient approach to shape analysis is
based on the Square Root Velocity Transform (SRVT). In this paper we propose a
generalised SRVT framework for shapes on homogeneous manifolds. The method
opens up for a variety of possibilities based on different choices of Lie group
action and giving rise to different Riemannian metrics.Comment: 28 pages; 4 figures, 30 subfigures; notes for proceedings of the Abel
Symposium 2016: "Computation and Combinatorics in Dynamics, Stochastics and
Control". v3: amended the text to improve readability and clarify some
points; updated and added some references; added pseudocode for the dynamic
programming algorithm used. The main results remain unchange
Spin transfer torque on magnetic insulators
Recent experimental and theoretical studies focus on spin-mediated heat
currents at interfaces between normal metals and magnetic insulators. We
resolve conflicting estimates for the order of magnitude of the spin transfer
torque by first-principles calculations. The spin mixing conductance
G^\uparrow\downarrow of the interface between silver and the insulating
ferrimagnet Yttrium Iron Garnet (YIG) is dominated by its real part and of the
order of 10^14 \Omega^-1m^-2, i.e. close to the value for intermetallic
interface, which can be explained by a local spin model.Comment: 4 pages, 4 figures, 2 table
Magnetomechanical Torques in Small Magnetic Cantilevers
We study the dnamics of small magnetic cantilevers, either made from Si
covered by a magnetic film or entirely ferromagnetic ones. The
magnetomechanical torques are found to cause line splittings in ferromagnetic
resonance spectra and magnetization reversal facilitated by mechanical degrees
of freedom. We show that the magnetomechanical torques can extend the limits of
detecting and exciting motion at the nanoscale. A "nanomotor" described here
effectively transforms rf magnetic fields into mechanical oscillations. We
furthermore propose to integrate mechanical oscillators into magnetoelectronic
devices that make use of current-induced spin-transfer torques. This opens new
possibilities for electric transducers of nanomechanical motion.Comment: 20 pages, 12 figures; submitted to a special issue of JJAP:
Magnetization Dynamics in Spintronic Structures and Device
A Tale of Two Theories: Quantum Griffiths Effects in Metallic Systems
We show that two apparently contradictory theories on the existence of
Griffiths-McCoy singularities in magnetic metallic systems [1,2] are in fact
mathematically equivalent. We discuss the generic phase diagram of the problem
and show that there is a non-universal crossover temperature range T* < T < W
where power law behavior (Griffiths-McCoy behavior) is expect. For T<T* power
law behavior ceases to exist due to the destruction of quantum effects
generated by the dissipation in the metallic environment. We show that T* is an
analogue of the Kondo temperature and is controlled by non-universal couplings.Comment: 4 pages, 2 figure
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