310 research outputs found
Leave Vacant or Rent: The Optimal Decision for Absentee Home Sellers
Home-sellers may not occupy their property when listed for sale. While previous research has analyzed the effect of a vacancy on the sales price of residential properties, no research to date has quantified the economic benefits and costs of renting vis-à-vis leaving a property vacant while listed for sale. Renting a property will produce revenue but perhaps at the cost of a lower sale price and/or longer time on the market. This paper employs data on 55,202 homes sold in the Las Vegas, Nevada area to determine if renting a property increases or decreases the wealth position of home sellers. The results of an empirical test of the model are used to quantify the wealth effect. The findings indicate that renting the property produces a significant reduction in the home seller’s wealth position.
Point-charge electrostatics in disordered alloys
A simple analytic model of point-ion electrostatics has been previously
proposed in which the magnitude of the net charge q_i on each atom in an
ordered or random alloy depends linearly on the number N_i^(1) of unlike
neighbors in its first coordination shell. Point charges extracted from recent
large supercell (256-432 atom) local density approximation (LDA) calculations
of Cu-Zn random alloys now enable an assessment of the physical validity and
accuracy of the simple model. We find that this model accurately describes (i)
the trends in q_i vs. N_i^(1), particularly for fcc alloys, (ii) the magnitudes
of total electrostatic energies in random alloys, (iii) the relationships
between constant-occupation-averaged charges and Coulomb shifts
(i.e., the average over all sites occupied by either or atoms) in the
random alloy, and (iv) the linear relation between the site charge q_i and the
constant- charge-averaged Coulomb shift (i.e., the average over all sites with
the same charge) for fcc alloys. However, for bcc alloys the fluctuations
predicted by the model in the q_i vs. V_i relation exceed those found in the
LDA supercell calculations. We find that (a) the fluctuations present in the
model have a vanishing contribution to the electrostatic energy. (b)
Generalizing the model to include a dependence of the charge on the atoms in
the first three (two) shells in bcc (fcc) - rather than the first shell only -
removes the fluctuations, in complete agreement with the LDA data. We also
demonstrate an efficient way to extract charge transfer parameters of the
generalized model from LDA calculations on small unit cells.Comment: 15 pages, ReVTeX galley format, 7 eps figures embedded using psfig,
to be published in Phys. Rev.
Using Early Data to Illuminate the Pioneer Anomaly
Analysis of the radio tracking data from the Pioneer 10/11 spacecraft at
distances between about 20 - 70 AU from the Sun has consistently indicated the
presence of an unmodeled, small, constant, Doppler blue shift drift of order 6
\times 10^{-9} Hz/s. After accounting for systematics, this drift can be
interpreted as a constant acceleration of a_P= (8.74 \pm 1.33) \times 10^{-8}
cm/s^2 directed towards the Sun, or perhaps as a time acceleration of a_t =
(2.92 \pm 0.44)\times 10^{-18} s/s^2. Although it is suspected that there is a
systematic origin to this anomaly, none has been unambiguously demonstrated. We
review the current status of the anomaly, and then point out how the analysis
of early data, which was never analyzed in detail, could allow a more clear
understanding of the origin of the anomaly, be it a systematic or a
manifestation of unsuspected physics.Comment: 19 pages, 6 figures, 2 tables, additional materia
Using bond-length dependent transferable force constants to predict vibrational entropies in Au-Cu, Au-Pd, and Cu-Pd alloys
A model is tested to rapidly evaluate the vibrational properties of alloys
with site disorder. It is shown that length-dependent transferable force
constants exist, and can be used to accurately predict the vibrational entropy
of substitutionally ordered and disordered structures in Au-Cu, Au-Pd, and
Cu-Pd. For each relevant force constant, a length- dependent function is
determined and fitted to force constants obtained from first-principles
pseudopotential calculations. We show that these transferable force constants
can accurately predict vibrational entropies of L1-ordered and disordered
phases in CuAu, AuPd, PdAu, CuPd, and PdAu. In
addition, we calculate the vibrational entropy difference between
L1-ordered and disordered phases of AuCu and CuPt.Comment: 9 pages, 6 figures, 3 table
Effects of anharmonic strain on phase stability of epitaxial films and superlattices: applications to noble metals
Epitaxial strain energies of epitaxial films and bulk superlattices are
studied via first-principles total energy calculations using the local-density
approximation. Anharmonic effects due to large lattice mismatch, beyond the
reach of the harmonic elasticity theory, are found to be very important in
Cu/Au (lattice mismatch 12%), Cu/Ag (12%) and Ni/Au (15%). We find that
is the elastically soft direction for biaxial expansion of Cu and Ni, but it is
for large biaxial compression of Cu, Ag, and Au. The stability of
superlattices is discussed in terms of the coherency strain and interfacial
energies. We find that in phase-separating systems such as Cu-Ag the
superlattice formation energies decrease with superlattice period, and the
interfacial energy is positive. Superlattices are formed easiest on (001) and
hardest on (111) substrates. For ordering systems, such as Cu-Au and Ag-Au, the
formation energy of superlattices increases with period, and interfacial
energies are negative. These superlattices are formed easiest on (001) or (110)
and hardest on (111) substrates. For Ni-Au we find a hybrid behavior:
superlattices along and like in phase-separating systems, while for
they behave like in ordering systems. Finally, recent experimental
results on epitaxial stabilization of disordered Ni-Au and Cu-Ag alloys,
immiscible in the bulk form, are explained in terms of destabilization of the
phase separated state due to lattice mismatch between the substrate and
constituents.Comment: RevTeX galley format, 16 pages, includes 9 EPS figures, to appear in
Physical Review
The Effect of Lattice Vibrations on Substitutional Alloy Thermodynamics
A longstanding limitation of first-principles calculations of substitutional
alloy phase diagrams is the difficulty to account for lattice vibrations. A
survey of the theoretical and experimental literature seeking to quantify the
impact of lattice vibrations on phase stability indicates that this effect can
be substantial. Typical vibrational entropy differences between phases are of
the order of 0.1 to 0.2 k_B/atom, which is comparable to the typical values of
configurational entropy differences in binary alloys (at most 0.693 k_B/atom).
This paper describes the basic formalism underlying ab initio phase diagram
calculations, along with the generalization required to account for lattice
vibrations. We overview the various techniques allowing the theoretical
calculation and the experimental determination of phonon dispersion curves and
related thermodynamic quantities, such as vibrational entropy or free energy. A
clear picture of the origin of vibrational entropy differences between phases
in an alloy system is presented that goes beyond the traditional bond counting
and volume change arguments. Vibrational entropy change can be attributed to
the changes in chemical bond stiffness associated with the changes in bond
length that take place during a phase transformation. This so-called ``bond
stiffness vs. bond length'' interpretation both summarizes the key phenomenon
driving vibrational entropy changes and provides a practical tool to model
them.Comment: Submitted to Reviews of Modern Physics 44 pages, 6 figure
Magnetic properties of X-Pt (X=Fe,Co,Ni) alloy systems
We have studied the electronic and magnetic properties of Fe-Pt, Co-Pt and
Ni-Pt alloy systems in ordered and disordered phases. The influence of various
exchange-correlation functionals on values of equilibrium lattice parameters
and magnetic moments in ordered Fe-Pt, Co-Pt and Ni-Pt alloys have been studied
using linearized muffin-tin orbital method. The electronic structure
calculations for the disordered alloys have been carried out using augmented
space recursion technique in the framework of tight binding linearized
muffin-tin orbital method. The effect of short range order has also been
studied in the disordered phase of these systems. The results show good
agreements with available experimental values.Comment: 21 pages, 4 eps figures, accepted for publication in Journal of
Physics Condensed Matte
Screened Coulomb interactions in metallic alloys: II Screening beyond the single-site and atomic sphere approximations
A quantitative description of the configurational part of the total energy of
metallic alloys with substantial atomic size difference cannot be achieved in
the atomic sphere approximation: It needs to be corrected at least for the
multipole moment interactions in the Madelung part of the one-electron
potential and energy. In the case of a random alloy such interactions can be
accounted for only by lifting the atomic sphere and single-site approximations,
in order to include the polarization due to local environment effects.
Nevertheless a simple parameterization of the screened Coulomb interactions for
the ordinary single-site methods, including the generalized perturbation
method, is still possible. We obtained such a parameterization for bulk and
surface NiPt alloys, which allows one to obtain quantitatively accurate
effective interactions in this system.Comment: 24 pages, 2 figure
Configurational Thermodynamics of Alloyed Nanoparticles with Adsorbates
Changes in the chemical configuration of alloyed nanoparticle (NP) catalysts induced by adsorbates under working conditions, such as reversal in core–shell preference, are crucial to understand and design NP functionality. We extend the cluster expansion method to predict the configurational thermodynamics of alloyed NPs with adsorbates based on density functional theory data. Exemplified with PdRh NPs having O-coverage up to a monolayer, we fully detail the core–shell behavior across the entire range of NP composition and O-coverage with quantitative agreement to in situ experimental data. Optimally fitted cluster interactions in the heterogeneous system are the key to enable quantitative Monte Carlo simulations and design
Recruiting Injection Drug Users: A Three-Site Comparison of Results and Experiences with Respondent-Driven and Targeted Sampling Procedures
Several recent studies have utilized respondent-driven sampling (RDS) methods to survey hidden populations such as commercial sex-workers, men who have sex with men (MSM) and injection drug users (IDU). Few studies, however, have provided a direct comparison between RDS and other more traditional sampling methods such as venue-based, targeted or time/space sampling. The current study sampled injection drug users in three U.S. cities using RDS and targeted sampling (TS) methods and compared their effectiveness in terms of recruitment efficiency, logistics, and sample demographics. Both methods performed satisfactorily. The targeted method required more staff time per-recruited respondent and had a lower proportion of screened respondents who were eligible than RDS, while RDS respondents were offered higher incentives for participation
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