1,066 research outputs found
Statistical properties of a localization-delocalization transition induced by correlated disorder
The exact probability distributions of the resistance, the conductance and
the transmission are calculated for the one-dimensional Anderson model with
long-range correlated off-diagonal disorder at E=0. It is proved that despite
of the Anderson transition in 3D, the functional form of the resistance (and
its related variables) distribution function does not change when there exists
a Metal-Insulator transition induced by correlation between disorders.
Furthermore, we derive analytically all statistical moments of the resistance,
the transmission and the Lyapunov Exponent. The growth rate of the average and
typical resistance decreases when the Hurst exponent tends to its critical
value () from the insulating regime.
In the metallic regime , the distributions become independent of
size. Therefore, the resistance and the transmission fluctuations do not
diverge with system size in the thermodynamic limit
The Kast Ground Based UV Spectral Survey of 79 QSOs at Redshift 2 for Lyman Alpha Forest and Metal Absorption
We present a moderate resolution (~1.15 Angstroms/pixel) survey of 79 quasars
obtained using the Kast spectrograph on the Shane 3m telescope at Lick
observatory. The spectra span the wavelength range of 3175-5880 Angstroms, and
have typical signal to noise of 6-20 in the regions of the spectra showing
Lyman alpha forest absorption. The quasars have a mean emission redshift of
z=2.17, and nearly all cover the entire Lyman alpha forest between Lyman alpha
and Lyman beta. Although the quasars were selected to avoid BAL, two quasars in
the survey are BAL, one of which is a new discovery. We list the HI and metal
ions observed in a total of 140 absorption systems. We also identify 526
emission lines, and list their observed wavelengths, along with new redshifts
of the quasars. We determine the rest wavelengths of 3 emission lines or line
blends in the forest to be 1070.95 +/- 1.00, 1123.13 +/- 0.51, and 1175.88 +/-
0.30 Angstroms.Comment: Submitted to the Astronomical Journa
Will the grass be greener on the other side of climate change?
Increasing atmospheric [CO2] is stimulating photosynthesis and plant production, increasing the demand for nitrogen relative to soil supply with declining global foliar nitrogen concentrations as a consequence. The effects of such oligotrophication on the forage quality of sweetveld, mixed veld, and sourveld grasslands in South Africa, which support livestock production and native ungulates, are unknown. Soil characteristics and the herbage quality of an abundant grass are described from baseline historical (mid-1980s) data collected across a sweet-mixed-sour grassland gradient in KwaZulu-Natal. Sourveld occurred on the most acidic, dystrophic soils and exhibited a pronounced decline in leaf nitrogen, digestibility, and other macronutrients during winter, in sharp contrast to sweetveld, on nutrient-rich soils, where forage quality varied little seasonally. In a carbon-enriched, warmer, and most likely drier future climate, we predict that forage quality will not be substantially altered in sweetveld where soil nutrients and temperature are not limiting but that sourveld could become âsourerâ because soil nutrients will be inadequate to match higher plant production promoted by elevated [CO2] and warmer and longer growing seasons. Reassessing historical data and seasonal and spatial monitoring of forage quality will enable assessment of past and future impacts of climate change on grassland forage quality.
Significance:
Grassland forage quality will likely decline with elevated [CO2] and warming, particularly in sourveld.
Climate change could deepen and widen the sourveld winter forage bottleneck, necessitating greater supplementary feeding of livestock
Intergalactic HeII absorption towards QSO 1157+3143
We report the discovery of a further line of sight allowing detection of HeII
Ly alpha absorption by the intergalactic medium. A HST/STIS survey of 32 bright
z ~ 3 quasars selected from the Hamburg Quasar Surveys yielded one detection
toward QSO 1157+3143 (z ~ 3, B ~ 17). A 10 orbit follow-up spectrum reveals a
UV spectrum significantly supressed by two intervening Lyman limit systems at
z=2.77 and 2.94, but with the continuum flux recovering sufficiently shortward
of ~ 1700 A to allow study of the HeII absorption spectrum in the redshift
range 2.75 < z < 2.97. The absorption is characterized by alternating voids and
dense filament structures seen in both HeII and HI. Attempts to model the HeII
opacity in terms of HI Ly alpha forest absorption are not successful in the
voids, suggesting that HeII reionization is not complete between z=2.77 and
2.97 or that an optically thin Lyman limit system with z ~ 0.3 is responsible
for the additional opacity.Comment: 6 pages, 4 figures, accepted by A&
Calculations of the Local Density of States for some Simple Systems
A recently proposed convolution technique for the calculation of local
density of states is described more thouroughly and new results of its
application are presented. For separable systems the exposed method allows to
construct the ldos for a higher dimensionality out of lower dimensional parts.
Some practical and theoretical aspects of this approach are also discussed.Comment: 5 pages, 3 figure
Black Holes in Magnetic Monopoles
We study magnetically charged classical solutions of a spontaneously broken
gauge theory interacting with gravity. We show that nonsingular monopole
solutions exist only if the Higgs vacuum expectation value is less than or
equal to a critical value , which is of the order of the Planck mass.
In the limiting case the monopole becomes a black hole, with the region outside
the horizon described by the critical Reissner-Nordstrom solution. For
, we find additional solutions which are singular at , but which
have this singularity hidden within a horizon. These have nontrivial matter
fields outside the horizon, and may be interpreted as small black holes lying
within a magnetic monopole. The nature of these solutions as a function of
and of the total mass and their relation to the Reissner-Nordstrom
solutions is discussed.Comment: (28 pages
Enhanced suppresion of localization in a continuous Random-Dimer Model
We consider a one-dimensional continuous (Kronig-Penney) extension of the
(tight-binding) Random Dimer model of Dunlap et al. [Phys. Rev. Lett. 65, 88
(1990)]. We predict that the continuous model has infinitely many resonances
(zeroes of the reflection coefficient) giving rise to extended states instead
of the one resonance arising in the discrete version. We present exact,
transfer-matrix numerical calculations supporting, both realizationwise and on
the average, the conclusion that the model has a very large number of extended
states.Comment: 10 pages, 3 Figures available on request, REVTeX 3.0, MA/UC3M/1/9
The Deep Lens Survey Transient Search I : Short Timescale and Astrometric Variability
We report on the methodology and first results from the Deep Lens Survey
transient search. We utilize image subtraction on survey data to yield all
sources of optical variability down to 24th magnitude. Images are analyzed
immediately after acquisition, at the telescope and in near-real time, to allow
for followup in the case of time-critical events. All classes of transients are
posted to the web upon detection. Our observing strategy allows sensitivity to
variability over several decades in timescale. The DLS is the first survey to
classify and report all types of photometric and astrometric variability
detected, including solar system objects, variable stars, supernovae, and short
timescale phenomena. Three unusual optical transient events were detected,
flaring on thousand-second timescales. All three events were seen in the B
passband, suggesting blue color indices for the phenomena. One event (OT
20020115) is determined to be from a flaring Galactic dwarf star of spectral
type dM4. From the remaining two events, we find an overall rate of \eta = 1.4
events deg-2 day-1 on thousand-second timescales, with a 95% confidence limit
of \eta < 4.3. One of these events (OT 20010326) originated from a compact
precursor in the field of galaxy cluster Abell 1836, and its nature is
uncertain. For the second (OT 20030305) we find strong evidence for an extended
extragalactic host. A dearth of such events in the R passband yields an upper
95% confidence limit on short timescale astronomical variability between 19.5 <
R < 23.4 of \eta_R < 5.2. We report also on our ensemble of astrometrically
variable objects, as well as an example of photometric variability with an
undetected precursor.Comment: 24 pages, 12 figures, 3 tables. Accepted for publication in ApJ.
Variability data available at http://dls.bell-labs.com/transients.htm
Suppression of Persistent Currents in 1-D Disordered Rings by Coulomb Interaction
Effects of Coulomb interaction on persistent currents in disordered
one-dimensional rings are numerically investigated. First of all effectiveness
of the Hartree-Fock approximation is established on small systems. Then the
calculations are done for systems with 40 electrons in 100 sites. It is found
that the amplitude of the average persistent current in the diffusive regime is
suppressed as the strength of the Coulomb interaction increases. The
suppression of the current is stronger in larger rings than in smaller ones.
The enhancement of the current by the electron-electron interaction was not
observed in the diffusive regime.Comment: 9 pages (RevTeX), 4 figures available upon request
([email protected]), KCMG-preprint-HK
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