1,017 research outputs found
Kondo effect in coupled quantum dots with RKKY interaction: Finite temperature and magnetic field effects
We study transport through two quantum dots coupled by an RKKY interaction as
a function of temperature and magnetic field. By applying the Numerical
Renormalization Group (NRG) method we obtain the transmission and the linear
conductance. At zero temperature and magnetic field, we observe a quantum phase
transition between the Kondo screened state and a local spin singlet as the
RKKY interaction is tuned. Above the critical RKKY coupling the Kondo peak is
split. However, we find that both finite temperature and magnetic field restore
the Kondo resonance. Our results agree well with recent transport experiments
on gold grain quantum dots in the presence of magnetic impurities.Comment: 4 pages, 5 figure
Simultaneous microsurgical spermatic vein ligation and sclerotherapy - A combined procedure for the treatment of recurrent or persistent varicocele
Objectives: Microsurgical ligation as well as antegrade sclerotherapy have been established in varicocele treatment. The aim of this study was to evaluate whether a combination of microsurgery. and sclerotherapy can: provide a safe and effective treatment of varicocele recurrence or persistence. Methods. Nine patients with, recurrent or persistent varicoceles were operated by means of the combination method. Under microscopic control varix veins were ligated selectively preserving: lymphatics and arteries. Ectopic veins as a possible source for varicocele persistence or recurrence were also ligated. Finally, an intraoperative venography with subsequent sclerotherapy was, performed through one of the dissected veins. Results. Despite: difficult anatomical situations after previous surgical interventions, the operations were perform, ed successfully without any complications. Clinical controls showed varicocele disappearance without damage of the testis. No varicocele recurrence or persistence was observed. Conclusions. This method combines the advantages of both methods. Precision of the microsurgical technique is combined with velocity of sclerotherapy. Thus, it may represent an Interesting alternative to conventional operation methods especially in the treatment of recurrent or persistent varicoceles. Copyright (C) 2001 S. Karger AG, Basel
Two-stage Kondo effect in a four-electron artificial atom
An artificial atom with four electrons is driven through a singlet-triplet
transition by varying the confining potential. In the triplet, a Kondo peak
with a narrow dip at drain-source voltage V_ds=0 is observed. The low energy
scale V_ds* characterizing the dip is consistent with predictions for the
two-stage Kondo effect. The phenomenon is studied as a function of temperature
T and magnetic field B, parallel to the two-dimensional electron gas. The low
energy scales T* and B* are extracted from the behavior of the zero-bias
conductance and are compared to the low energy scale V_ds* obtained from the
differential conductance. Good agreement is found between kT* and |g|muB*, but
eV_ds* is larger, perhaps because of nonequilibrium effects.Comment: 7 pages, 7 figures. Added labels on Fig. 3f and one referenc
Spectral function of the Anderson impurity model at finite temperatures
Using the functional renormalization group (FRG) and the numerical
renormalization group (NRG), we calculate the spectral function of the Anderson
impurity model at zero and finite temperatures. In our FRG scheme spin
fluctuations are treated non-perturbatively via a suitable Hubbard-Stratonovich
field, but vertex corrections are neglected. A comparison with our highly
accurate NRG results shows that this FRG scheme gives a quantitatively good
description of the spectral line-shape at zero and finite temperatures both in
the weak and strong coupling regimes, although at zero temperature the FRG is
not able to reproduce the known exponential narrowing of the Kondo resonance at
strong coupling.Comment: 6 pages, 3 figures; new references adde
Frequency-dependent transport through a quantum dot in the Kondo regime
We study the AC conductance and equilibrium current fluctuations of a Coulomb
blockaded quantum dot. A relation between the equilibrium spectral function and
the linear AC conductance is derived which is valid for frequencies well below
the charging energy of the quantum dot. Frequency-dependent transport
measurements can thus give experimental access to the Kondo peak in the
equilibrium spectral function of a quantum dot. We illustrate this in detail
for typical experimental parameters using the numerical renormalization group
method in combination with the Kubo formalism.Comment: 4 pages, 4 figure
Two-stage Kondo effect in side-coupled quantum dots: Renormalized perturbative scaling theory and Numerical Renormalization Group analysis
We study numerically and analytically the dynamical (AC) conductance through
a two-dot system, where only one of the dots is coupled to the leads but it is
also side-coupled to the other dot through an antiferromagnetic exchange (RKKY)
interaction. In this case the RKKY interaction gives rise to a ``two-stage
Kondo effect'' where the two spins are screened by two consecutive Kondo
effects. We formulate a renormalized scaling theory that captures remarkably
well the cross-over from the strongly conductive correlated regime to the low
temperature low conductance state. Our analytical formulas agree well with our
numerical renormalization group results. The frequency dependent current noise
spectrum is also discussed.Comment: 6 pages, 7 figure
Partial Wave Analysis of Scattering with Nonlocal Aharonov-Bohm Effect and Anomalous Cross Section induced by Quantum Interference
Partial wave theory of a three dmensional scattering problem for an arbitray
short range potential and a nonlocal Aharonov-Bohm magnetic flux is
established. The scattering process of a ``hard shere'' like potential and the
magnetic flux is examined. An anomalous total cross section is revealed at the
specific quantized magnetic flux at low energy which helps explain the
composite fermion and boson model in the fractional quantum Hall effect. Since
the nonlocal quantum interference of magnetic flux on the charged particles is
universal, the nonlocal effect is expected to appear in quite general potential
system and will be useful in understanding some other phenomena in mesoscopic
phyiscs.Comment: 6 figure
Level spacings at the metal-insulator transition in the Anderson Hamiltonians and multifractal random matrix ensembles
We consider orthogonal, unitary, and symplectic ensembles of random matrices
with (1/a)(ln x)^2 potentials, which obey spectral statistics different from
the Wigner-Dyson and are argued to have multifractal eigenstates. If the
coefficient is small, spectral correlations in the bulk are universally
governed by a translationally invariant, one-parameter generalization of the
sine kernel. We provide analytic expressions for the level spacing distribution
functions of this kernel, which are hybrids of the Wigner-Dyson and Poisson
distributions. By tuning the single parameter, our results can be excellently
fitted to the numerical data for three symmetry classes of the
three-dimensional Anderson Hamiltonians at the metal-insulator transition,
previously measured by several groups using exact diagonalization.Comment: 12 pages, 8 figures, REVTeX. Additional figure and text on the level
number variance, to appear in Phys.Rev.
Kondo effect and spin-active scattering in ferromagnet-superconductor junctions
We study the interplay of superconducting and ferromagnetic correlations on
charge transport in different geometries with a focus on both a quantum point
contact as well as a quantum dot in the even and the odd state with and without
spin-active scattering at the interface. In order to obtain a complete picture
of the charge transport we calculate the full counting statistics in all cases
and compare the results with experimental data. We show that spin-active
scattering is an essential ingredient in the description of quantum point
contacts. This holds also for quantum dots in an even charge state whereas it
is strongly suppressed in a typical Kondo situation. We explain this feature by
the strong asymmetry of the hybridisations with the quantum dot and show how
Kondo peak splitting in a magnetic field can be used for spin filtering. For
the quantum dot in the even state spin-active scattering allows for an
explanation of the experimentally observed mini-gap feature.Comment: 14 pages, 7 figures, accepted by PR
Bark beetle population dynamics in the Anthropocene: Challenges and solutions
Tree-killing bark beetles are the most economically important insects in conifer forests worldwide. However, despite N200 years of research, the drivers of population eruptions and crashes are still not fully understood and the existing knowledge is thus insufficient to face the challenges posed by the Anthropocene. We critically analyze potential biotic and abiotic drivers of population dynamics of an exemplary species, the European spruce bark beetle (ESBB) (Ips typographus) and present a multivariate approach that integrates the many drivers governing this bark beetle system. We call for hypothesis-driven, large-scale collaborative research efforts to improve our understanding of the population dynamics of this and other bark beetle pests. Our approach can serve as a blueprint for tackling other eruptive forest insects
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