488 research outputs found
Nonperturbative Coherent Population Trapping: An Analytic Model
Coherent population trapping is shown to occur in a driven symmetric
double-well potential in the strong-field regime. The system parameters have
been chosen to reproduce the transition of the
inversion mode of the ammonia molecule. For a molecule initially prepared in
its lower doublet we find that, under certain circumstances, the level
remains unpopulated, and this occurs in spite of the fact that the laser field
is resonant with the transition and intense enough
so as to strongly mix the and ground states. This
counterintuitive result constitutes a coherent population trapping phenomenon
of nonperturbative origin which cannot be accounted for with the usual models.
We propose an analytic nonperturbative model which accounts correctly for the
observed phenomenon.Comment: 5 pages, 2 figure
Majorana: from atomic and molecular, to nuclear physics
In the centennial of Ettore Majorana's birth (1906-1938?), we re-examine some
aspects of his fundamental scientific production in atomic and molecular
physics, including a not well known short communication. There, Majorana
critically discusses Fermi's solution of the celebrated Thomas-Fermi equation
for electron screening in atoms and positive ions. We argue that some of
Majorana's seminal contributions in molecular physics already prelude to the
idea of exchange interactions (or Heisenberg-Majorana forces) in his later
workson theoretical nuclear physics. In all his papers, he tended to emphasize
the symmetries at the basis of a physical problem, as well as the limitations,
rather than the advantages, of the approximations of the method employed.Comment: to appear in Found. Phy
The all-sky GEOS RR Lyr survey with the TAROT telescopes. Analysis of the Blazhko effect
We used the GEOS database to study the Blazhko effect of galactic RRab stars.
The database is continuously enriched by maxima supplied by amateur astronomers
and by a dedicated survey by means of the two TAROT robotic telescopes. The
same value of the Blazhko period is observed at different values of the
pulsation periods and different values of the Blazhko periods are observed at
the same value of the pulsation period. There are clues suggesting that the
Blazhko effect is changing from one cycle to the next. The secular changes in
the pulsation and Blazhko periods of Z CVn are anticorrelated. The diagrams of
magnitudes against phases of the maxima clearly show that the light curves of
Blazhko variables can be explained as modulated signals, both in amplitude and
in frequency. The closed curves describing the Blazhko cycles in such diagrams
have different shapes, reflecting the phase shifts between the epochs of the
brightest maximum and the maximum O-C. Our sample shows that both clockwise and
anticlockwise directions are possible for similar shapes. The improved
observational knowledge of the Blazhko effect, in addition to some
peculiarities of the light curves, have still to be explained by a satisfactory
physical mechanism.Comment: 13 pages, 12 figures, accepted for publication in Astronomical
Journa
The Origin of Degeneracies and Crossings in the 1d Hubbard Model
The paper is devoted to the connection between integrability of a finite
quantum system and degeneracies of its energy levels. In particular, we analyze
in detail the energy spectra of finite Hubbard chains. Heilmann and Lieb
demonstrated that in these systems there are crossings of levels of the same
parameter independent symmetry. We show that this apparent violation of the
Wigner-von Neumann noncrossing rule follows directly from the existence of
nontrivial conservation laws and is a characteristic signature of quantum
integrability. The energy spectra of Hubbard chains display many instances of
permanent (at all values of the coupling) twofold degeneracies that cannot be
explained by parameter independent symmetries. We relate these degeneracies to
the different transformation properties of the conserved currents under spatial
reflections and the particle-hole transformation and estimate the fraction of
doubly degenerate states. We also discuss multiply degenerate eigenstates of
the Hubbard Hamiltonian. The wave functions of many of these states do not
depend on the coupling, which suggests the existence of an additional parameter
independent symmetry.Comment: 25 pages, 12 figure
In defence of the familiar : understanding conservatism in concert selection amongst classical music audiences
Since the establishment of a classical music canon in the 19th century, classical music culture has historically been focused on a stable set of masterpieces by genius composers predominantly from the classical and romantic periods. A small number of composers continue to dominate programming to this day. Many classical music organisations are keen to programme music beyond this narrow repertoire and to showcase new or unfamiliar works. The need to sell tickets, however, is often an obstacle, with organisations far more confident in the ability of big hits to attract large crowds. This article explores the experiences and opinions of classical music concertgoers in relation to familiar and unfamiliar music, providing a number of reasons as to why audiences may choose to hear well-known pieces rather than new works. This paper reports on one strand of a qualitative study with 42 individuals who booked tickets for one of two concert series consisting of core and populist repertoire, respectively. Semi-structured interviews were carried out to explore the reasons for their choices and their experiences of attending live concerts. These interviews showed that most participants did indeed have a clear preference for hearing music that was familiar to them, and only frequent attenders relished the challenge of unknown music. Participants felt that listening to familiar music was usually a more enjoyable experience than hearing something new. They rarely spoke of becoming bored with over-familiar music, perhaps because the live concert experience brings a sense of freshness to even the most familiar work
Dynamic response of mesoscopic metal rings and thermodynamics at constant particle number
We show by means of simple exact manipulations that the thermodynamic
persistent current in a mesoscopic metal ring threaded by a
magnetic flux at constant particle number agrees even beyond linear
response with the dynamic current that is defined via the
response to a time-dependent flux in the limit that the frequency of the flux
vanishes. However, it is impossible to express the disorder average of in terms of conventional Green's functions at flux-independent
chemical potential, because the part of the dynamic response function that
involves two retarded and two advanced Green's functions is not negligible.
Therefore the dynamics cannot be used to map a canonical average onto a more
tractable grand canonical one. We also calculate the zero frequency limit of
the dynamic current at constant chemical potential beyond linear response and
show that it is fundamentally different from any thermodynamic derivative.Comment: 19 pages, postscript (uuencoded, compressed
Quantum tunneling across spin domains in a Bose-Einstein condensate
Quantum tunneling was observed in the decay of metastable spin domains in
gaseous Bose-Einstein condensates. A mean-field description of the tunneling
was developed and compared with measurement. The tunneling rates are a
sensitive probe of the boundary between spin domains, and indicate a spin
structure in the boundary between spin domains which is prohibited in the bulk
fluid. These experiments were performed with optically trapped F=1 spinor
Bose-Einstein condensates of sodium.Comment: 5 pages, 4 figure
Quantum-Classical Transition of the Escape Rate of a Uniaxial Spin System in an Arbitrarily Directed Field
The escape rate \Gamma of the large-spin model described by the Hamiltonian H
= -DS_z^2 - H_zS_z - H_xS_x is investigated with the help of the mapping onto a
particle moving in a double-well potential U(x). The transition-state method
yields in the moderate-damping case as a Boltzmann average of the
quantum transition probabilities. We have shown that the transition from the
classical to quantum regimes with lowering temperature is of the first order
(d\Gamma/dT discontinuous at the transition temperature T_0) for h_x below the
phase boundary line h_x=h_{xc}(h_z), where h_{x,z}\equiv H_{x,z}/(2SD), and of
the second order above this line. In the unbiased case (H_z=0) the result is
h_{xc}(0)=1/4, i.e., one fourth of the metastability boundary h_{xm}=1, at
which the barrier disappears. In the strongly biased limit \delta\equiv 1-h_z
<< 1, one has h_{xc} \cong (2/3)^{3/4}(\sqrt{3}-\sqrt{2})\delta^{3/2}\cong
0.2345 \delta^{3/2}, which is about one half of the boundary value h_{xm} \cong
(2\delta/3)^{3/2} \cong 0.5443 \delta^{3/2}.The latter case is relevant for
experiments on small magnetic particles, where the barrier should be lowered to
achieve measurable quantum escape rates.Comment: 17 PR pages, 16 figures; published versio
Statistical Mechanics of Elastica on Plane as a Model of Supercoiled DNA-Origin of the MKdV hierarchy-
In this article, I have investigated statistical mechanics of a non-stretched
elastica in two dimensional space using path integral method. In the
calculation, the MKdV hierarchy naturally appeared as the equations including
the temperature fluctuation.I have classified the moduli of the closed elastica
in heat bath and summed the Boltzmann weight with the thermalfluctuation over
the moduli. Due to the bilinearity of the energy functional,I have obtained its
exact partition function.By investigation of the system,I conjectured that an
expectation value at a critical point of this system obeys the Painlev\'e
equation of the first kind and its related equations extended by the KdV
hierarchy.Furthermore I also commented onthe relation between the MKdV
hierarchy and BRS transformationin this system.Comment: AMS-Tex Us
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