605 research outputs found
Nonstationary Collisional Dynamics in Determining Nitric Oxide Laser-Induced Flourescence Spectra
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77237/1/AIAA-8783-947.pd
Modified two-potential approach to tunneling problems
One-body quantum tunneling to continuum is treated via the two-potential
approach, dividing the tunneling potential into external and internal parts. We
show that corrections to this approach can be minimized by taking the
separation radius inside the interval determined by simple expressions. The
resulting two-potential approach reproduces the resonance energy and its width,
both for narrow and wide resonances. We also demonstrate that, without losing
its accuracy, the two-potential approach can be modified to a form resembling
the R-matrix theory, yet without any uncertainties of the latter related to the
choice of the matching radius.Comment: 7 two-column pages, 3 figures, extra-explanation added, Phys. Rev. A,
in pres
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
Correlated Persistent Tunneling Currents in Glasses
Low temperature properties of glasses are derived within a generalized
tunneling model, considering the motion of charged particles on a closed path
in a double-well potential. The presence of a magnetic induction field B
violates the time reversal invariance due to the Aharonov-Bohm phase, and leads
to flux periodic energy levels. At low temperature, this effect is shown to be
strongly enhanced by dipole-dipole and elastic interactions between tunneling
systems and becomes measurable. Thus, the recently observed strong sensitivity
of the electric permittivity to weak magnetic fields can be explained. In
addition, superimposed oscillations as a function of the magnetic field are
predicted.Comment: 4 page
Magnetic flux in mesoscopic rings: Quantum Smoluchowski regime
Magnetic flux in mesoscopic rings under the quantum Smoluchowski regime is
investigated. Quantum corrections to the dissipative current are shown to form
multistable steady states and can result in statistical enhancement of the
magnetic flux. The relevance of quantum correction effects is supported v ia
the entropic criterion. A possible application for a qutrit architecture of
quantum information is proposed.Comment: 7 pages, 2 figure
-invariant two-fermion Dirac equation with extended hyperfine operator
For the S-states of muonium and positronium, the hyperfine shifts to the
order of a recently derived two-fermion equation with explicit -invariance are checked against the results of a nonrelativistic reduction,
and the leading shifts are calculated. An additional hyperfine
operator is discovered which can milden the singularity for of the
Dirac hyperfine operator, such that the resulting extended operator can be used
nonperturbatively. The binding correction to magnetic moments is mentioned
Estimating Systemic Risk in the International Financial System
Using a unique and comprehensive dataset, this paper develops and uses three distinct methods to quantify the risk of a systemic failure in the global banking system. We examine a sample of 334 banks (representing 80% of global bank equity) in 28 countries around 6 global financial crises (such as the Asian and Russian crises and September 11, 2001), and show that these crises did not create large probabilities of global financial system failure. First, we show that cumulative negative abnormal returns for the subset of banks not directly exposed to a negative shock (unexposed banks) rarely exceed a few percent. Second, we use structural models to obtain more precise point estimates of the likelihood of systemic failure. These estimates suggest that systemic risk is limited even during major financial crises. For example, maximum likelihood estimation of bank failure probabilities implied by equity prices suggests the Asian crisis induced less than a 1% increase in the probability of systemic failure. Third, we also obtain estimates of systemic risk implied by equity option prices of U.S. and European banks. The largest values are obtained for the Russian crisis and September 11 and these show increases in estimated average default probabilities of only around 1-2%. Taken together our results suggest statistically significant, but economically small, increases in systemic risk around even the worst financial crises of the last 10 years. Although policy responses are endogenous, the low estimated probabilities suggest that the distress of central bankers, regulators and politicians about the events we study may be overstated, and that current policy responses to financial crises and the existing institutional framework may be adequate to handle major macroeconomic events
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
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
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