5,282 research outputs found
Development of UHF radiometer
A wideband multifrequency UHF radiometer was initially developed to operate in the 500 to 710 MHz frequency range for the remote measurement of ocean water salinity. However, radio-frequency interference required a reconfiguration to operate in the single-frequency radio astronomy band of 608 to 614 MHz. Details of the radiometer development and testing are described. Flight testing over variable terrain provided a performance comparison of the UHF radiometer with an L-band radiometer for remote sensing of geophysical parameters. Although theoretically more sensitive, the UHF radiometer was found to be less desirable in practice than the L-band radiometer
Quasi-one-dimensional Bose gases with large scattering length
Bose gases confined in highly-elongated harmonic traps are investigated over
a wide range of interaction strengths using quantum Monte Carlo techniques. We
find that the properties of a Bose gas under tight transverse confinement are
well reproduced by a 1d model Hamiltonian with contact interactions. We point
out the existence of a unitary regime, where the properties of the quasi-1d
Bose gas become independent of the actual value of the 3d scattering length. In
this unitary regime, the energy of the system is well described by a hard rod
equation of state. We investigate the stability of quasi-1d Bose gases with
positive and negative 3d scattering length.Comment: 5 pages, 3 figure
Quantum Monte Carlo study of quasi-one-dimensional Bose gases
We study the behavior of quasi-one-dimensional (quasi-1d) Bose gases by Monte
Carlo techniques, i.e., by the variational Monte Carlo, the diffusion Monte
Carlo, and the fixed-node diffusion Monte Carlo technique. Our calculations
confirm and extend our results of an earlier study [Astrakharchik et al.,
cond-mat/0308585]. We find that a quasi-1d Bose gas i) is well described by a
1d model Hamiltonian with contact interactions and renormalized coupling
constant; ii) reaches the Tonks-Girardeau regime for a critical value of the 3d
scattering length a_3d; iii) enters a unitary regime for |a_3d| -> infinity,
where the properties of the gas are independent of a_3d and are similar to
those of a 1d gas of hard-rods; and iv) becomes unstable against cluster
formation for a critical value of the 1d gas parameter. The accuracy and
implications of our results are discussed in detail.Comment: 15 pages, 9 figure
Stale or Sticky Stock Prices? Non-Trading, Predictability, and Mutual Fund Returns
The observed predictability in indexes and domestic mutual funds has been attributed to stale prices. Market timing of mutual funds exploits this predictability. We show that there are few stale prices for stocks in the top few deciles of market value and that mutual funds concentrate their holding in these deciles. Still, we observe predictability in the returns of portfolios and mutual funds holding these stocks. Much of this predictability is due to stickiness, or momentum, in market returns and not stale prices. Thus, the often suggested use of “fairvalue” accounting will not eliminate the profitability of market timing
The Valuation of Callable Bonds
Callable bond indentures contain provisions that allow the issuing entity to retire the bond at a predetermined price before the maturity of the bond.1 As such a callable bond is often viewed as a combination of an otherwise identical but non-callable bond and an option to call that bond. The writer of the call option is the holder of the bond, and the buyer of the call is the stockholder of the issuing corporation. Thus, the price of a callable bond is the value of the straight bond less the value of the call provision
Dipolar Bose gases: Many-body versus mean-field description
We characterize zero-temperature dipolar Bose gases under external spherical
confinement as a function of the dipole strength using the essentially exact
many-body diffusion Monte Carlo (DMC) technique. We show that the DMC energies
are reproduced accurately within a mean-field framework if the variation of the
s-wave scattering length with the dipole strength is accounted for properly.
Our calculations suggest stability diagrams and collapse mechanisms of dipolar
Bose gases that differ significantly from those previously proposed in the
literature
Feshbach Resonance Cooling of Trapped Atom Pairs
Spectroscopic studies of few-body systems at ultracold temperatures provide
valuable information that often cannot be extracted in a hot environment.
Considering a pair of atoms, we propose a cooling mechanism that makes use of a
scattering Feshbach resonance. Application of a series of time-dependent
magnetic field ramps results in the situation in which either zero, one, or two
atoms remain trapped. If two atoms remain in the trap after the field ramps are
completed, then they have been cooled. Application of the proposed cooling
mechanism to optical traps or lattices is considered.Comment: 5 pages, 3 figures; v.2: major conceptual change
Returns and Volatility of Low-Grade Bonds 1977–1989
This paper examines the risks and returns of long-term low-grade bonds for the period 1977–1989. We find: (1) low-grade bonds realized higher returns than higher-grade bonds and lower returns than common stocks, and low-grade bonds exhibited less volatility than higher-grade bonds due to their call features and high coupons; (2) there is no relation between the age of low-grade bonds and their realized returns; cyclical factors explain much of the observed relation between default rates and bond age; and (3) low-grade bonds behave like both bonds and stocks. Despite this complexity there is no evidence that low-grade bonds are systematically over- or under-priced
Dipolar Bose-Einstein condensates with dipole-dependent scattering length
We consider a Bose-Einstein condensate of polar molecules in a harmonic trap,
where the effective dipole may be tuned by an external field. We demonstrate
that taking into account the dependence of the scattering length on the dipole
moment is essential to reproducing the correct energies and for predicting the
stability of the condensate. We do this by comparing Gross-Pitaevskii
calculations with diffusion Monte Carlo calculations. We find very good
agreement between the results obtained by these two approaches once the dipole
dependence of the scattering length is taken into account. We also examine the
behavior of the condensate in non-isotropic traps
Three particles in an external trap: Nature of the complete J=0 spectrum
Three bosonic, spin-polarized atoms in a spherical oscillator potential
constitutes the simplest nontrivial Bose-Einstein condensate (BEC). The present
paper develops the tools needed to understand the nature of the complete J=0
energy spectrum for this prototype system, assuming a sum of two-body
potentials. The resulting spectrum is calculated as a function of the two-body
scattering length a_sc, which documents the evolution of certain many-body
levels that evolve from BEC-type to molecular-type as the scattering length is
decreased. Implications for the behavior of the condensate excited-state
spectrum and for condensate formation and decay are elucidated. The energy
levels evolve smoothly, even through the regime where the number of two-body
bound states N_b increases by 1, and a_{sc} switches from -infinity to
infinity. We point out the possibility of suppressing three-body recombination
by tuning the two-body scattering length to values that are larger than the
size of the condensate ground state. Comparisons with mean-field treatments are
presented
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