535 research outputs found
Ab Initio Studies of Cellulose I: Crystal Structure, Intermolecular Forces, and Interactions with Water
We have studied the structural, energetic, and electronic properties of crystalline cellulose I using first-principles density functional theory (DFT) with semiempirical dispersion corrections. The predicted crystal structures of both Iα and Iβ phases agree well with experiments and are greatly improved over those predicted by DFT within the local and semilocal density approximations. The cohesive energy is analyzed in terms of interchain and intersheet interactions, which are calculated to be of similar magnitude. Both hydrogen bonding and van der Waals (vdW) dispersion forces are found to be responsible for binding cellulose chains together. In particular, dispersion corrections prove to be indispensable in reproducing the equilibrium intersheet distance and binding strength; however, they do not improve the underestimated hydrogen bond length from DFT. The computed energy gaps of crystalline cellulose are 5.7 eV (Iα) and 5.4 eV (Iβ), whereas localized surface states appear within the gap for surfaces. The interaction of cellulose with water is studied by investigating the adsorption of a single water molecule on the hydrophobic Iβ(100) surface. The formation of hydrogen bond at the water/cellulose interface is shown to depend sensitively on the adsorption site for example above the equatorial hydroxyls or the CH moieties pointing out of the cellulose sheets. VdW dispersion interactions also contribute significantly to the adsorption energy
An exact study of charge-spin separation, pairing fluctuations and pseudogaps in four-site Hubbard clusters
An exact study of charge-spin separation, pairing fluctuations and pseudogaps
is carried out by combining the analytical eigenvalues of the four-site Hubbard
clusters with the grand canonical and canonical ensemble approaches in a
multidimensional parameter space of temperature (T), magnetic field (h),
on-site interaction (U) and chemical potential. Our results, near the average
number of electrons =3, strongly suggest the existence of a critical
parameter U_{c}(T) for the localization of electrons and a particle-hole
binding (positive) gap at U>U_{c}(T), with a zero temperature quantum critical
point, U_{c}(0)=4.584. For U<U_{c}(T), particle-particle pair binding is found
with a (positive) pairing gap. The ground state degeneracy is lifted at
U>U_c(T) and the cluster becomes a Mott-Hubbard like insulator due to the
presence of energy gaps at all (allowed) integer numbers of electrons. In
contrast, for U< U_c(T), we find an electron pair binding instability at finite
temperature near =3, which manifests a possible pairing mechanism, a
precursor to superconductivity in small clusters.
In addition, the resulting phase diagram consisting of charge and spin
pseudogaps, antiferromagnetic correlations, hole pairing with competing
hole-rich (=2), hole-poor (=4) and magnetic (=3) regions in the
ensemble of clusters near 1/8 filling closely resembles the phase diagrams and
inhomogeneous phase separation recently found in the family of doped high T_c
cuprates.Comment: 10 pages, 7 figure
Chromospheric Activity of HAT-P-11: an Unusually Active Planet-Hosting K Star
Kepler photometry of the hot Neptune host star HAT-P-11 suggests that its
spot latitude distribution is comparable to the Sun's near solar maximum. We
search for evidence of an activity cycle in the CaII H & K chromospheric
emission -index with archival Keck/HIRES spectra and observations from the
echelle spectrograph on the ARC 3.5 m Telescope at APO. The chromospheric
emission of HAT-P-11 is consistent with a year activity cycle,
which plateaued near maximum during the Kepler mission. In the cycle that we
observed, the star seemed to spend more time near active maximum than minimum.
We compare the normalized chromospheric emission index of
HAT-P-11 with other stars. HAT-P-11 has unusually strong chromospheric emission
compared to planet-hosting stars of similar effective temperature and rotation
period, perhaps due to tides raised by its planet.Comment: 16 pages, 8 figures; accepted to the Astrophysical Journa
The Early Days of Research on Carbonic Anhydrase
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73862/1/j.1749-6632.1984.tb12310.x.pd
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