16,129 research outputs found
Majorana and the quasi-stationary states in Nuclear Physics
A complete theoretical model describing artificial disintegration of nuclei
by bombardment with alpha-particles, developed by Majorana as early as in 1930,
is discussed in detail alongside the basic experimental evidences that
motivated it. By following the quantum dynamics of a state resulting from the
superposition of a discrete state with a continuum one, whose interaction is
described by a given potential term, Majorana obtained (among the other
predictions) the explicit expression for the integrated cross section of the
nuclear process, which is the direct measurable quantity of interest in the
experiments. Though this is the first application of the concept of
quasi-stationary states to a Nuclear Physics problem, it seems also that the
unpublished Majorana's work anticipates by several years the related seminal
paper by Fano on Atomic Physics.Comment: latex, amsart, 13 page
Random wave functions and percolation
Recently it was conjectured that nodal domains of random wave functions are
adequately described by critical percolation theory. In this paper we
strengthen this conjecture in two respects. First, we show that, though wave
function correlations decay slowly, a careful use of Harris' criterion confirms
that these correlations are unessential and nodal domains of random wave
functions belong to the same universality class as non critical percolation.
Second, we argue that level domains of random wave functions are described by
the non-critical percolation model.Comment: 13 page
Semi-Phenomenological Analysis of Dynamics of Nonlinear Excitations in One-Dimensional Electron-Phonon System
The structure of moving nonlinear excitations in one-dimensional
electron-phonon systems is studied semi-phenomenologically by using an
effective action in which the width of the nonlinear excitation is treated as a
dynamical variable. The effective action can be derived from Su, Schrieffer and
Heeger's model or its continuum version proposed by Takayama, Lin-Liu and Maki
with an assumption that the nonlinear excitation moves uniformly without any
deformation except the change of its width. The form of the action is
essentially the same as that discussed by Bishop and coworkers in studying the
dynamics of the soliton in polyacetylene, though some details are different.
For the moving excitation with a velocity , the width is determined by
minimizing the effective action. A requirement that there must be a minimum in
the action as a function of its width provides a maximum velocity. The velocity
dependence of the width and energy can be determined. The motions of a soliton
in p olyacetylene and an acoustic polaron in polydiacetylene are studied within
this formulation. The obtained results are in good agreement with those of
numerical simulations.Comment: 19 pages, LaTeX, 7 Postscript figures, to be published in J. Phys.
Soc. Jpn. vol.65 (1996) No.
Fermion zero modes on vortices in chiral superconductors
The energy levels of the fermions bound to the vortex core are considered for
the general case of chiral superconductors. There are two classes of chiral
superconductivity: in the superconducting state of class I the axisymmetric
singly quantized vortex has the same energy spectrum of bound states as in
s-wave superconductor: E=(n+1/2)\omega_0 with integral n. In the class II the
corresponding spectrum is E=n\omega_0 and thus contains the state with exactly
zero energy. The effect of a single impurity on the spectrum of bound state is
also considered. For the class I the spectrum acquires the double period \Delta
E=2\omega_0 and consists of two equidistant sets of levels in accordance with
A.I. Larkin and Yu.N. Ovchinnikov, Phys. Rev. B57 (1998) 5457. The spectrum is
not influenced by a single impurity if the same approximation is applied for
vortices in the class II superconducting states.Comment: 4 pages, no figures, corrected version accepted in JETP Letter
The influence of chiral surface states on the London penetration depth in SrRuO
The London penetration depth for the unconventional superconductor
SrRuO is analyzed assuming an order parameter which breaks time
reversal symmetry and parity simultaneously. Such a superconducting state
possesses chiral quasiparticle states with subgap energies at the surface. We
show that these subgap states can give a significant contribution to the
low-temperature behavior of the London penetration depth yielding a
power-law even though bulk quasiparticle spectrum is gapped. The presence of
several electron bands gives rise to interband transition among the subgap
surface states and influences the properties of the surface impedance.
Furthermore, the surface states lead also to a non-linear Meissner effect.Comment: 4 pages, 1 figure, the definition of the Nambu field operator
introduced, and some typos correcte
Pigeons Choose to Gamble in a Categorical Discrimination Task
In slot-machine play, near wins are stimuli that visually approximate winning stimuli but deliver no reinforcers. In two experiments, a categorical discrimination task was embedded in a concurrent chain to investigate how near wins affect preference for probabilistic versus certain food. Pecking in variable-interval initial links produced access to a fixed-ratio (FR) 1-FR 1-FR 1 chain. When all links were red, the chain was a “win” that produced access to food. A “clear loss” chain involved three green stimuli, and in a “near win,” key colors during successive FR 1 links were red, red, and green. In Experiment 1, the magnitude and probability of reinforcement were varied across conditions with and without near wins. Response allocation was sensitive to changes in reinforcer magnitude and probability. Generalized matching analyses revealed a consistent bias for probabilistic over certain outcomes, but only when they included near wins. Response rates on near-win trials were also intermediate to that of clear losses and wins. Near-win probability was varied across conditions of Experiment 2 and probability of near wins was positively associated with bias for probabilistic outcomes. The results from both experiments suggest that near wins encourage individuals to choose to gamble by functioning as conditioned reinforcers
Generalized Stacking Fault Energy Surfaces and Dislocation Properties of Silicon: A First-Principles Theoretical Study
The generalized stacking fault (GSF) energy surfaces have received
considerable attention due to their close relation to the mechanical properties
of solids. We present a detailed study of the GSF energy surfaces of silicon
within the framework of density functional theory. We have calculated the GSF
energy surfaces for the shuffle and glide set of the (111) plane, and that of
the (100) plane of silicon, paying particular attention to the effects of the
relaxation of atomic coordinates. Based on the calculated GSF energy surfaces
and the Peierls-Nabarro model, we obtain estimates for the dislocation
profiles, core energies, Peierls energies, and the corresponding stresses for
various planar dislocations of silicon.Comment: 9 figures (not included; send requests to [email protected]
Dynamical Masses of Low Mass Stars in the Taurus and Ophiuchus Star Forming Regions
We report new dynamical masses for 5 pre-main sequence (PMS) stars in the
L1495 region of the Taurus star-forming region (SFR) and 6 in the L1688 region
of the Ophiuchus SFR. Since these regions have VLBA parallaxes these are
absolute measurements of the stars' masses and are independent of their
effective temperatures and luminosities. Seven of the stars have masses
solar masses, thus providing data in a mass range with little data, and of
these, 6 are measured to precision . We find 8 stars with masses in the
range 0.09 to 1.1 solar mass that agree well with the current generation of PMS
evolutionary models. The ages of the stars we measured in the Taurus SFR are in
the range 1-3 MY, and MY for those in L1688. We also measured the
dynamical masses of 14 stars in the ALMA archival data for Akeson~\&~Jensen's
Cycle 0 project on binaries in the Taurus SFR. We find that the masses of 7 of
the targets are so large that they cannot be reconciled with reported values of
their luminosity and effective temperature. We suggest that these targets are
themselves binaries or triples.Comment: 20 page
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