1,120 research outputs found
A log analysis study of 10 years of ebook consumption in academic library collections
Even though libraries have been offering eBooks for more than a decade, very little is known about eBook access and consumption in academic library collections. This paper addresses this gap with a log analysis study of eBook access at the library of the University of Waikato. This in-depth analysis covers a period spanning 10 years of eBook use at this university. We draw conclusions about the use of eBooks at this institution and compare the results with other published studies of eBook usage at tertiary institutes
Anisotropic optical properties of single-crystal GdBa2Cu3O7-delta
The optical spectrum of reduced-T(c) GdBa2Cu3O7-delta has been measured for polarizations parallel and perpendicular to the ab plane. The sample was an oxygen-deficient single crystal with a large face containing the c axis. The polarized reflectance from this face was measured from 20-300 K in the spectral region from 30-3000 cm-1, with 300 K data to 30 000 cm-1. Kramers-Kronig analysis was used to determine the spectral dependence of the ab and the c components of the dielectric tensor. The optical properties are strongly anisotropic. The ab-plane response resembles that of other reduced-T(c) materials whereas the c axis, in contrast, shows only the presence of several phonons. There is a complete absence of charge carrier response along c above and below T(c). This observation allows us to set an upper limit to the free-carrier spectral weight for transport perpendicular to the CuO2 planes
The Point of Origin of the Radio Radiation from the Unresolved Cores of Radio-Loud Quasars
Locating the exact point of origin of the core radiation in active galactic
nuclei (AGN) would represent important progress in our understanding of
physical processes in the central engine of these objects. However, due to our
inability to resolve the region containing both the central compact object and
the jet base, this has so far been difficult. Here, using an analysis in which
the lack of resolution does not play a significant role, we demonstrate that it
may be impossible even in most radio loud sources for more than a small
percentage of the core radiation at radio wavelengths to come from the jet
base. We find for 3C279 that percent of the core flux at 15 GHz must
come from a separate, reasonably stable, region that is not part of the jet
base, and that then likely radiates at least quasi-isotropically and is
centered on the black hole. The long-term stability of this component also
suggests that it may originate in a region that extends over many Schwarzschild
radii.Comment: 7 pages with 3 figures, accepted for publication in Astrophysics and
Space Scienc
Trabecular bone score (TBS) as a new complementary approach for osteoporosis evaluation in clinical practice.
Trabecular bone score (TBS) is a recently-developed analytical tool that performs novel grey-level texture measurements on lumbar spine dual X-ray absorptiometry (DXA) images, and thereby captures information relating to trabecular microarchitecture. In order for TBS to usefully add to bone mineral density (BMD) and clinical risk factors in osteoporosis risk stratification, it must be independently associated with fracture risk, readily obtainable, and ideally, present a risk which is amenable to osteoporosis treatment. This paper summarizes a review of the scientific literature performed by a Working Group of the European Society for Clinical and Economic Aspects of Osteoporosis and Osteoarthritis. Low TBS is consistently associated with an increase in both prevalent and incident fractures that is partly independent of both clinical risk factors and areal BMD (aBMD) at the lumbar spine and proximal femur. More recently, TBS has been shown to have predictive value for fracture independent of fracture probabilities using the FRAX® algorithm. Although TBS changes with osteoporosis treatment, the magnitude is less than that of aBMD of the spine, and it is not clear how change in TBS relates to fracture risk reduction. TBS may also have a role in the assessment of fracture risk in some causes of secondary osteoporosis (e.g., diabetes, hyperparathyroidism and glucocorticoid-induced osteoporosis). In conclusion, there is a role for TBS in fracture risk assessment in combination with both aBMD and FRAX
Geometric phases for generalized squeezed coherent states
A simple technique is used to obtain a general formula for the Berry phase
(and the corresponding Hannay angle) for an arbitrary Hamiltonian with an
equally-spaced spectrum and appropriate ladder operators connecting the
eigenstates. The formalism is first applied to a general deformation of the
oscillator involving both squeezing and displacement. Earlier results are shown
to emerge as special cases. The analysis is then extended to multiphoton
squeezed coherent states and the corresponding anholonomies deduced.Comment: 15 page
Anomalous c-axis charge dynamics in copper oxide materials
Within the t-J model, the c-axis charge dynamics of the copper oxide
materials in the underdoped and optimally doped regimes is studied by
considering the incoherent interlayer hopping. It is shown that the c-axis
charge dynamics is mainly governed by the scattering from the in-plane
fluctuation. In the optimally doped regime, the c-axis resistivity is a linear
in temperatures, and shows the metallic-like behavior for all temperatures,
while the c-axis resistivity in the underdoped regime is characterized by a
crossover from the high temperature metallic-like behavior to the low
temperature semiconducting-like behavior, which are consistent with experiments
and numerical simulations.Comment: 6 pages, Latex, Three figures are adde
Effect of a Normal-State Pseudogap on Optical Conductivity in Underdoped Cuprate Superconductors
We calculate the c-axis infrared conductivity in
underdoped cuprate superconductors for spinfluctuation exchange scattering
within the CuO-planes including a phenomenological d-wave pseudogap of
amplitude . For temperatures decreasing below a temperature , a gap for develops in in the
incoherent (diffuse) transmission limit. The resistivity shows 'semiconducting'
behavior, i.e. it increases for low temperatures above the constant behavior
for . We find that the pseudogap structure in the in-plane optical
conductivity is about twice as big as in the interplane conductivity
, in qualitative agreement with experiment. This is a
consequence of the fact that the spinfluctuation exchange interaction is
suppressed at low frequencies as a result of the opening of the pseudogap.
While the c-axis conductivity in the underdoped regime is described best by
incoherent transmission, in the overdoped regime coherent conductance gives a
better description.Comment: to be published in Phys. Rev. B (November 1, 1999
Gamow-Teller Strengths for Neutrino Detectors
This research was sponsored by the National Science Foundation Grant NSF PHY-931478
Superconductivity in the two dimensional Hubbard Model.
Quasiparticle bands of the two-dimensional Hubbard model are calculated using
the Roth two-pole approximation to the one particle Green's function. Excellent
agreement is obtained with recent Monte Carlo calculations, including an
anomalous volume of the Fermi surface near half-filling, which can possibly be
explained in terms of a breakdown of Fermi liquid theory. The calculated bands
are very flat around the (pi,0) points of the Brillouin zone in agreement with
photoemission measurements of cuprate superconductors. With doping there is a
shift in spectral weight from the upper band to the lower band. The Roth method
is extended to deal with superconductivity within a four-pole approximation
allowing electron-hole mixing. It is shown that triplet p-wave pairing never
occurs. Singlet d_{x^2-y^2}-wave pairing is strongly favoured and optimal
doping occurs when the van Hove singularity, corresponding to the flat band
part, lies at the Fermi level. Nearest neighbour antiferromagnetic correlations
play an important role in flattening the bands near the Fermi level and in
favouring superconductivity. However the mechanism for superconductivity is a
local one, in contrast to spin fluctuation exchange models. For reasonable
values of the hopping parameter the transition temperature T_c is in the range
10-100K. The optimum doping delta_c lies between 0.14 and 0.25, depending on
the ratio U/t. The gap equation has a BCS-like form and (2*Delta_{max})/(kT_c)
~ 4.Comment: REVTeX, 35 pages, including 19 PostScript figures numbered 1a to 11.
Uses epsf.sty (included). Everything in uuencoded gz-compressed .tar file,
(self-unpacking, see header). Submitted to Phys. Rev. B (24-2-95
Electron Dynamics in NdCeCuO: Evidence for the Pseudogap State and Unconventional c-axis Response
Infrared reflectance measurements were made with light polarized along the a-
and c-axis of both superconducting and antiferromagnetic phases of electron
doped NdCeCuO. The results are compared to
characteristic features of the electromagnetic response in hole doped cuprates.
Within the CuO planes the frequency dependent scattering rate,
1/, is depressed below 650 cm; this behavior is a
hallmark of the pseudogap state. While in several hole doped compounds the
energy scales associated with the pseudogap and superconducting states are
quite close, we are able to show that in NdCeCuO
the two scales differ by more than one order of magnitude. Another feature of
the in-plane charge response is a peak in the real part of the conductivity,
, at 50-110 cm which is in sharp contrast with the
Drude-like response where is centered at . This
latter effect is similar to what is found in disordered hole doped cuprates and
is discussed in the context of carrier localization. Examination of the c-axis
conductivity gives evidence for an anomalously broad frequency range from which
the interlayer superfluid is accumulated. Compelling evidence for the pseudogap
state as well as other characteristics of the charge dynamics in
NdCeCuO signal global similarities of the cuprate
phase diagram with respect to electron and hole doping.Comment: Submitted to PR
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