3,633 research outputs found
Dense sphere packings from optimized correlation functions
Elementary smooth functions (beyond contact) are employed to construct pair
correlation functions that mimic jammed disordered sphere packings. Using the
g2-invariant optimization method of Torquato and Stillinger [J. Phys. Chem. B
106, 8354, 2002], parameters in these functions are optimized under necessary
realizability conditions to maximize the packing fraction phi and average
number of contacts per sphere Z. A pair correlation function that incorporates
the salient features of a disordered packing and that is smooth beyond contact
is shown to permit a phi of 0.6850: this value represents a 45% reduction in
the difference between the maximum for congruent hard spheres in three
dimensions, pi/sqrt{18} ~ 0.7405, and 0.64, the approximate fraction associated
with maximally random jammed (MRJ) packings in three dimensions. We show that,
surprisingly, the continued addition of elementary functions consisting of
smooth sinusoids decaying as r^{-4} permits packing fractions approaching
pi/sqrt{18}. A translational order metric is used to discriminate between
degrees of order in the packings presented. We find that to achieve higher
packing fractions, the degree of order must increase, which is consistent with
the results of a previous study [Torquato et al., Phys. Rev. Lett. 84, 2064,
2000].Comment: 26 pages, 9 figures, 1 table; added references, fixed typos,
simplified argument and discussion in Section IV
Magnon Heat Transport in doped
We present results of the thermal conductivity of and single-crystals which represent model systems for the
two-dimensional spin-1/2 Heisenberg antiferromagnet on a square lattice. We
find large anisotropies of the thermal conductivity, which are explained in
terms of two-dimensional heat conduction by magnons within the CuO planes.
Non-magnetic Zn substituted for Cu gradually suppresses this magnon thermal
conductivity . A semiclassical analysis of
is shown to yield a magnon mean free path which scales
linearly with the reciprocal concentration of Zn-ions.Comment: 4 pages, 3 figure
Hole Localization in Underdoped Superconducting Cuprates Near 1/8th Doping
Measurements of thermal conductivity versus temperature over a broad range of
doping in YBaCuO and HgBaCaCuO
(=1,2,3) suggest that small domains of localized holes develop for hole
concentrations near =1/8. The data imply a mechanism for localization that
is intrinsic to the CuO-planes and is enhanced via pinning associated with
oxygen-vacancy clusters.Comment: 4 pages, 4 eps fig.'s, to be published, Phys. Rev.
The Casimir Effect and the Quantum Vacuum
In discussions of the cosmological constant, the Casimir effect is often
invoked as decisive evidence that the zero point energies of quantum fields are
"real''. On the contrary, Casimir effects can be formulated and Casimir forces
can be computed without reference to zero point energies. They are
relativistic, quantum forces between charges and currents. The Casimir force
(per unit area) between parallel plates vanishes as \alpha, the fine structure
constant, goes to zero, and the standard result, which appears to be
independent of \alpha, corresponds to the \alpha\to\infty limit.Comment: 9 pages, 3 figures, uses aip class and the package floatftm to float
text around figure
Relationship between intact HIV-1 proviruses in circulating CD4+ T cells and rebound viruses emerging during treatment interruption.
Combination antiretroviral therapy controls but does not cure HIV-1 infection because a small fraction of cells harbor latent viruses that can produce rebound viremia when therapy is interrupted. The circulating latent virus reservoir has been documented by a variety of methods, most prominently by viral outgrowth assays (VOAs) in which CD4+ T cells are activated to produce virus in vitro, or more recently by amplifying proviral near full-length (NFL) sequences from DNA. Analysis of samples obtained in clinical studies in which individuals underwent analytical treatment interruption (ATI), showed little if any overlap between circulating latent viruses obtained from outgrowth cultures and rebound viruses from plasma. To determine whether intact proviruses amplified from DNA are more closely related to rebound viruses than those obtained from VOAs, we assayed 12 individuals who underwent ATI after infusion of a combination of two monoclonal anti-HIV-1 antibodies. A total of 435 intact proviruses obtained by NFL sequencing were compared with 650 latent viruses from VOAs and 246 plasma rebound viruses. Although, intact NFL and outgrowth culture sequences showed similar levels of stability and diversity with 39% overlap, the size of the reservoir estimated from NFL sequencing was larger than and did not correlate with VOAs. Finally, intact proviruses documented by NFL sequencing showed no sequence overlap with rebound viruses; however, they appear to contribute to recombinant viruses found in plasma during rebound
Schreier rewriting beyond the classical setting
Using actions of free monoids and free associative algebras, we establish
some Schreier-type formulas involving the ranks of actions and the ranks of
subactions in free actions or Grassmann-type relations for the ranks of
intersections of subactions of free actions. The coset action of the free group
is used to establish the generalization of the Schreier formula to the case of
subgroups of infinite index. We also study and apply large modules over free
associative algebras in the spirit of the paper Olshanskii, A. Yu.; Osin, D.V.,
Large groups and their periodic quotients, Proc. Amer. Math. Soc., 136 (2008),
753 - 759.Comment: 17 page
The Value of Urban Flood Modeling
Floods are important disturbances to urban socio-eco-technical systems and their meteorological drivers are projected to increase through the century due to global climate change. Urban flood models are numerical models that have the capability of representing the features of urban ecosystems and the mechanisms of flooding that impact them. They have the potential to play a critical role in flood risk assessment, operational response, and resilience planning, but existing models remain limited in their capability to represent integrated flooding processes in urban areas and provide the credible quantitative information needed to support risk assessment and resilience practice. Research to advance model development, facilitate intensive watershed monitoring for model parameterization and validation, and support collaboration between researchers and practitioners should be prioritized. This will represent a substantial, expensive effort, but will still be of great value as cities are faced with urgent challenges posed by climate change in coming decades
Low thermal conductivity of the layered oxide (Na,Ca)Co_2O_4: Another example of a phonon glass and an electron crystal
The thermal conductivity of polycrystalline samples of (Na,Ca)Co_2O_4 is
found to be unusually low, 20 mW/cmK at 280 K. On the assumption of the
Wiedemann-Franz law, the lattice thermal conductivity is estimated to be 18
mW/cmK at 280 K, and it does not change appreciably with the substitution of Ca
for Na. A quantitative analysis has revealed that the phonon mean free path is
comparable with the lattice parameters, where the point-defect scattering plays
an important role. Electronically the same samples show a metallic conduction
down to 4.2 K, which strongly suggests that NaCo_2O_4 exhibits a glass-like
poor thermal conduction together with a metal-like good electrical conduction.
The present study further suggests that a strongly correlated system with
layered structure can act as a material of a phonon glass and an electron
crystal.Comment: 5 pages 3 figures, to be published in Phys. Rev.
Separation of Quasiparticle and Phononic Heat Currents in YBCO
Measurements of the transverse (k_{xy}) and longitudinal (k_{xx}) thermal
conductivity in high magnetic fields are used to separate the quasiparticle
thermal conductivity (k_{xx}^{el}) of the CuO_2-planes from the phononic
thermal conductivity in YBa_2Cu_3O_{7-\delta}. k_{xx}^{el} is found to display
a pronounced maximum below T_c. Our data analysis reveals distinct transport
(\tau) and Hall (\tau_H) relaxation times below T_c: Whereas \tau is strongly
enhanced, \tau_H follows the same temperature dependence as above T_c
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