598 research outputs found
A re-analysis of the spectrum of 2206-199
Recently Pettini, Hunstead, Smith and Mar, (PHSM 1991) performed an analysis of the Ly-alpha forest of QSO 2206-199N at very high resolution. On the basis of their observations they concluded that most Doppler parameters b = (square root of 2)(sigma) of Ly-alpha forest lines are below 22 km/s, with a range down to a few km/s and a median of 17 km/s. They also found a strong intrinsic correlation between Doppler parameter b and column density N. These results are in contrast to those of a similar study by Carswell, Lanzetta, Parnell, and Webb (CLPW 1991) at comparable resolution with the same instruments, who find that most of the Ly-alpha lines towards QSO 1100-264 have Doppler parameters above 15 km/s (median b = 34 km/s), and that there is no significant correlation between b and N. Whilst an intrinsic difference between the lines of sight to 2206-199 and to other QSO's can not be excluded a priori, previous disagreement between Doppler parameter estimates obtained by both groups pointed to a potential difference in estimation techniques and in the interpretation of the results. To investigate this possibility, the AAT/UCLES spectrum of 2206-199 obtained by PHSM were reanalyzed. The spectrum was extracted from the raw data and determined the line parameters using the method described by CLPW
The Local Lyman-Alpha Forest: Absorbers in Galaxy Voids
We have conducted pointed redshift surveys for galaxies in the direction of
bright AGN whose HST far-UV spectra contain nearby (cz <~ 30,000 kms), low
column density (12.5 <= log N_{HI} (cm s^{-2}) <= 14.5) Ly-alpha forest
absorption systems. Here we present results for four lines-of-sight which
contain nearby (cz <~ 3000 kms) Ly-alpha absorbers in galaxy voids. Although
our data go quite deep (-13 <= M_{B}(limit) <= -14) out to impact parameters of
100-250 h_{70}^{-1} kpc, these absorbers remain isolated and thus appear to be
truly intergalactic, rather than part of galaxies or their halos. Since we and
others have discovered no galaxies in voids, the only baryons detected in the
voids are in the Ly-alpha ``clouds''. Using a photoionization model for these
clouds, the total baryonic content of the voids is 4.5% +/- 1.5% of the mean
baryon density.Comment: 5 pages, 1 figure, accepted for publication in Astrophysical Journal
Letter
Thermal broadening of the Coulomb blockade peaks in quantum Hall interferometers
We demonstrate that the differential magnetic susceptibility of a fractional
quantum Hall disk, representing a Coulomb island in a Fabry--Perot
interferometer, is exactly proportional to the island's conductance and its
paramagnetic peaks are the equilibrium counterparts of the Coulomb blockade
conductance peaks. Using as a thermodynamic potential the partition functions
of the edge states' effective conformal field theory we find the positions of
the Coulomb blockade peaks, when the area of the island is varied, the
modulations of the distance between them as well as the thermal decay and
broadening of the peaks when temperature is increased. The finite-temperature
estimates of the peak's heights and widths could give important information
about the experimental observability of the Coulomb blockade. In addition, the
predicted peak asymmetry and displacement at finite temperature due to neutral
multiplicities could serve to distinguish different fractional quantum Hall
states with similar zero-temperature Coulomb blockade patterns.Comment: 6 pages, 6 figures; published versio
Underscreened Kondo effect in S=1 magnetic quantum dots: Exchange, anisotropy and temperature effects
We present a theoretical analysis of the effects of uniaxial magnetic
anisotropy and contact-induced exchange field on the underscreened Kondo effect
in S=1 magnetic quantum dots coupled to ferromagnetic leads. First, by using
the second-order perturbation theory we show that the coupling to
spin-polarized electrode results in an effective exchange field
and an effective magnetic anisotropy . Second, we confirm these
findings by using the numerical renormalization group method, which is employed
to study the dependence of the quantum dot spectral functions, as well as
quantum dot spin, on various parameters of the system. We show that the
underscreened Kondo effect is generally suppressed due to the presence of
effective exchange field and can be restored by tuning the anisotropy constant,
when . The Kondo effect can also be restored by
sweeping an external magnetic field, and the restoration occurs twice in a
single sweep. From the distance between the restored Kondo resonances one can
extract the information about both the exchange field and the effective
anisotropy. Finally, we calculate the temperature dependence of linear
conductance for the parameters where the Kondo effect is restored and show that
the restored Kondo resonances display a universal scaling of Kondo
effect.Comment: 13 pages, 9 figures (version as accepted for publication in Physical
Review B
Transport through single-wall metallic carbon nanotubes in the cotunneling regime
Using the real-time diagrammatic technique and taking into account both the
sequential and cotunneling processes, we analyze the transport properties of
single-wall metallic carbon nanotubes coupled to nonmagnetic and ferromagnetic
leads in the full range of parameters. In particular, considering the two
different shell filling schemes of the nanotubes, we discuss the behavior of
the differential conductance, tunnel magnetoresistance and the shot noise. We
show that in the Coulomb diamonds corresponding to even occupations, the shot
noise becomes super-Poissonian due to bunching of fast tunneling processes
resulting from the dynamical channel blockade, whereas in the other diamonds
the noise is roughly Poissonian, in agreement with recent experiments. The
tunnel magnetoresistance is very sensitive to the number of electrons in the
nanotube and exhibits a distinctively different behavior depending on the shell
filling sequence of the nanotube.Comment: 7 pages, 6 figure
Spin-polarized transport through weakly coupled double quantum dots in the Coulomb-blockade regime
We analyze cotunneling transport through two quantum dots in series weakly
coupled to external ferromagnetic leads. In the Coulomb blockade regime the
electric current flows due to third-order tunneling, while the second-order
single-barrier processes have indirect impact on the current by changing the
occupation probabilities of the double dot system. We predict a zero-bias
maximum in the differential conductance, whose magnitude is conditioned by the
value of the inter-dot Coulomb interaction. This maximum is present in both
magnetic configurations of the system and results from asymmetry in cotunneling
through different virtual states. Furthermore, we show that tunnel
magnetoresistance exhibits a distinctively different behavior depending on
temperature, being rather independent of the value of inter-dot correlation.
Moreover, we find negative TMR in some range of the bias voltage.Comment: 9 pages, 7 figures, accepted in Phys. Rev.
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