328 research outputs found
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
The Influence of Magnetic Anisotropy on the Kondo Effect and Spin-Polarized Transport through Magnetic Molecules, Adatoms and Quantum Dots
Transport properties in the Kondo regime of a nanosystem displaying uniaxial
magnetic anisotropy (such as a magnetic molecule, magnetic adatom or quantum
dot coupled to a localized magnetic moment) are analyzed theoretically. In
particular, the influence of spin-polarized transport through a local orbital
of the system and exchange coupling of conduction electrons to the system's
magnetic core on the Kondo effect is discussed. The numerical renormalization
group method is applied to calculate the spectral functions and linear
conductance in the case of the parallel and antiparallel configurations of the
electrodes' magnetic moments. It is shown that both the magnetic anisotropy as
well as the exchange coupling between electrons tunneling through the
conducting orbital and magnetic core play an important role in formation of the
Kondo resonance, leading generally to its suppression. Specific transport
properties of such a system appear also as a nontrivial behavior of tunnel
magnetoresistance. It is also shown that the Kondo effect can be restored by an
external magnetic field in both the parallel and antiparallel magnetic
configurations.Comment: 14 pages with 10 EPS figures (version as accepted for publication in
Physical Review B
Kinetics of N2O production and reduction in a nitrate-contaminated aquifer inferred from laboratory incubation experiments
Knowledge of the kinetics of N2O production and reduction in groundwater is essential for the assessment of potential indirect emissions of the greenhouse gas. In the present study, we investigated this kinetics using a laboratory approach. The results were compared to field measurements in order to examine their transferability to the in situ conditions. The study site was the unconfined, predominantly sandy Fuhrberger Feld aquifer in northern Germany. A special characteristic of the aquifer is the occurrence of the vertically separated process zones of heterotrophic denitrification in the near-surface groundwater and of autotrophic denitrification in depths beyond 2-3 m below the groundwater table, respectively. The kinetics of N2O production and reduction in both process zones was studied during long-term anaerobic laboratory incubations of aquifer slurries using the 15N tracer technique. We measured N2O, N2, NO3-, NO2-, and SO42- concentrations as well as parameters of the aquifer material that were related to the relevant electron donors, i.e. organic carbon and pyrite. The laboratory incubations showed a low denitrification activity of heterotrophic denitrification with initial rates between 0.2 and 13 μg N kg-1 d-1. The process was carbon limited due to the poor availability of its electron donor. In the autotrophic denitrification zone, initial denitrification rates were considerably higher, ranging between 30 and 148 μg N kg-1 d-1, and NO3- as well as N2O were completely removed within 60 to 198 days. N2O accumulated during heterotrophic and autotrophic denitrification, but maximum concentrations were substantially higher during the autotrophic process. The results revealed a satisfactory transferability of the laboratory incubations to the field scale for autotrophic denitrification, whereas the heterotrophic process less reflected the field conditions due to considerably lower N2O accumulation during laboratory incubation. Finally, we applied a conventional model using first-order-kinetics to determine the reaction rate constants k1 for N2O production and k2 for N2O reduction, respectively. The goodness of fit to the experimental data was partly limited, indicating that a more sophisticated approach is essential to describe the investigated reaction kinetics satisfactorily.DF
High-resolution O VI absorption line observations at 1.2 < z < 1.7 in the bright QSO HE 0515-4414
STIS Echelle observations at a resolution of 10 km/s and UVES/VLT
spectroscopy at a resolution of 7 km/s of the luminous QSO HE 0515-4414 (z_em =
1.73, B = 15.0) reveal four intervening O VI absorption systems in the redshift
range 1.2 < z_abs < 1.7 (1.38503, 1.41601, 1.60175, 1.67359). In addition two
associated systems at z = 1.69707 and z = 1.73585 are present. For the first
time high resolution observations allow to measure radial velocities of H I, C
IV and O VI simultaneously in several absorption systems (1.385, 1.674, 1.697)
with the result that significant velocity differences (up to 18 km/s) are
observed between H I and O VI, while smaller differences (up to 5 km/s) are
seen between C IV and O VI. We tentatively conclude that H I, O VI, and C IV
are not formed in the same volumes and that therefore implications on
ionization mechanisms are not possible from observed column density ratios O
VI/H I or O VI/C IV. The number density of O VI absorbers with W_rest > 25 mA
is dN/dz < 10, roughly a factor of 5 less than what has been found by Tripp at
al. (2000) at low redshift. An estimate of the cosmological mass-density of the
O VI-phase yields Omega_b(O VI) = 0.0003 h^{-1}_{75} for [O/H] = -1 and an
assumed ionization fraction O VI/O = 0.2. This corresponds to an increase by
roughly a factor of 15 between z = 1.5 (this work) and the value found by Tripp
et al. (2000) at z = 0.21, if the same oxygen abundance [O/H] = -1 is assumed.
Agreement with the simulations by Dave et al. (2001) can be obtained, if the
oxygen abundance increases by a factor of 3 over the same redshift interval.Comment: 8 pages, 1 figure, accepted for publication in A&
On the Causality and Stability of the Relativistic Diffusion Equation
This paper examines the mathematical properties of the relativistic diffusion
equation. The peculiar solution which Hiscock and Lindblom identified as an
instability is shown to emerge from an ill-posed initial value problem. These
do not meet the mathematical conditions required for realistic physical
problems and can not serve as an argument against the relativistic
hydrodynamics of Landau and Lifshitz.Comment: 6 page
Groundwater N2O emission factors of nitrate-contaminated aquifers as derived from denitrification progress and N2O accumulation
We investigated the dynamics of denitrification and nitrous oxide (N2O) accumulation in 4 nitrate (NO3-) contaminated denitrifying sand and gravel aquifers of northern Germany (Fuhrberg, Sulingen, Thulsfelde and Gottingen) to quantify their potential N2O emission and to evaluate existing concepts of N2O emission factors. Excess N-2 - N-2 produced by denitrification - was determined by using the argon (Ar) concentration in groundwater as a natural inert tracer, assuming that this noble gas functions as a stable component and does not change during denitrification. Furthermore, initial NO3- concentrations (NO3- that enters the groundwater) were derived from excess N-2 and actual NO3- concentrations in groundwater in order to determine potential indirect N2O emissions as a function of the N input. Median concentrations of N2O and excess N-2 ranged from 3 to 89 mu g N L-1 and from 3 to 10 mg N L-1, respectively. Reaction progress (RP) of denitrification was determined as the ratio between products (N2O-N + excess N-2) and starting material (initial NO3- concentration) of the process, characterizing the different stages of denitrification. N2O concentrations were lowest at RP close to 0 and RP close to 1 but relatively high at a RP between 0.2 and 0.6. For the first time, we report groundwater N2O emission factors consisting of the ratio between N2O-N and initial NO3--N concentrations (EF1). In addition, we determined a groundwater emission factor (EF2) using a previous concept consisting of the ratio between N2O-N and actual NO3--N concentrations. Depending on RP, EF(1) resulted in smaller values compared to EF(2), demonstrating (i) the relevance of NO3- consumption and consequently (ii) the need to take initial NO3--N concentrations into account. In general, both evaluated emission factors were highly variable within and among the aquifers. The site medians ranged between 0.00043-0.00438 for EF(1) and 0.00092-0.01801 for EF(2), respectively. For the aquifers of Fuhrberg and Sulingen, we found EF(1) median values which are close to the 2006 IPCC default value of 0.0025. In contrast, we determined significant lower EF values for the aquifers of Thulsfelde and Gottingen. Summing the results up, our study supports the substantial downward revision of the IPCC default EF5-g from 0.015 (1997) to 0.0025 (2006).DF
The tunnel magnetoresistance in chains of quantum dots weakly coupled to external leads
We analyze numerically the spin-dependent transport through coherent chains
of three coupled quantum dots weakly connected to external magnetic leads. In
particular, using the diagrammatic technique on the Keldysh contour, we
calculate the conductance, shot noise and tunnel magnetoresistance (TMR) in the
sequential and cotunneling regimes. We show that transport characteristics
greatly depend on the strength of the interdot Coulomb correlations, which
determines the spacial distribution of electron wave function in the chain.
When the correlations are relatively strong, depending on the transport regime,
we find both negative TMR as well as TMR enhanced above the Julliere value,
accompanied with negative differential conductance (NDC) and super-Poissonian
shot noise. This nontrivial behavior of tunnel magnetoresistance is associated
with selection rules that govern tunneling processes and various high-spin
states of the chain that are relevant for transport. For weak interdot
correlations, on the other hand, the TMR is always positive and not larger than
the Julliere TMR, although super-Poissonian shot noise and NDC can still be
observed
VPMS J1342+2840 - an unusual quasar from the variability and proper motion survey
We report the discovery of the highly peculiar, radio-loud quasar VPMS
J1342+2840 (z ~ 1.3) from the variability and proper motion survey. We present
spectroscopic, imaging and photometric observations. The unusual spectrum shows
a strong depression of the continuum over a wide wavelength range in the blue
part without the typical structures of broad absorption line (BAL) troughs. The
image of the quasar is unresolved and there is no evidence for a foreground
object on the line of sight. The broad-band spectral energy distribution is not
consistent with obvious dust reddening with the standard SMC extinction curve.
The downturn of the continuum flux of VPMS J1342+2840 at short wavelengths can
be caused by dust reddening only if the reddening curve is steeper then the SMC
curve in the ultraviolet and is very flat at longer wavelengths. Alternatively,
the dominant spectral features can be explained by low-ionization BALs forming
unusually wide, overlapping absorption troughs.Comment: 4 pages, 3 figures, accepted for publication in A&A Letter
Spin effects in single electron tunneling
An important consequence of the discovery of giant magnetoresistance in
metallic magnetic multilayers is a broad interest in spin dependent effects in
electronic transport through magnetic nanostructures. An example of such
systems are tunnel junctions -- single-barrier planar junctions or more complex
ones. In this review we present and discuss recent theoretical results on
electron and spin transport through ferromagnetic mesoscopic junctions
including two or more barriers. Such systems are also called ferromagnetic
single-electron transistors. We start from the situation when the central part
of a device has the form of a magnetic (or nonmagnetic) metallic nanoparticle.
Transport characteristics reveal then single-electron charging effects,
including the Coulomb staircase, Coulomb blockade, and Coulomb oscillations.
Single-electron ferromagnetic transistors based on semiconductor quantum dots
and large molecules (especially carbon nanotubes) are also considered. The main
emphasis is placed on the spin effects due to spin-dependent tunnelling through
the barriers, which gives rise to spin accumulation and tunnel
magnetoresistance. Spin effects also occur in the current-voltage
characteristics, (differential) conductance, shot noise, and others. Transport
characteristics in the two limiting situations of weak and strong coupling are
of particular interest. In the former case we distinguish between the
sequential tunnelling and cotunneling regimes. In the strong coupling regime we
concentrate on the Kondo phenomenon, which in the case of transport through
quantum dots or molecules leads to an enhanced conductance and to a pronounced
zero-bias Kondo peak in the differential conductance.Comment: topical review (36 figures, 65 pages), to be published in J. Phys.:
Condens. Matte
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