1,070 research outputs found
Aharonov-Bohm oscillations and resonant tunneling in strongly correlated quantum dots
We investigate Aharonov-Bohm oscillations of the current through a strongly
correlated quantum dot embedded in an arbitrary scattering geometry.
Resonant-tunneling processes lead to a flux-dependent renormalization of the
dot level. As a consequence we obtain a fine structure of the current
oscillations which is controlled by quantum fluctuations. Strong Coulomb
repulsion leads to a continuous bias voltage dependent phase shift and, in the
nonlinear response regime, destroys the symmetry of the differential
conductance under a sign change of the external flux.Comment: RevTex, 5 pages, 3 PostScript figures. Accepted for publication in
Phys. Rev. Let
Spin quantum tunneling in single molecular magnets: fingerprints in transport spectroscopy of current and noise
We demonstrate that transport spectroscopy of single molecular magnets shows
signatures of quantum tunneling at low temperatures. We find current and noise
oscillations as function of bias voltage due to a weak violation of spin
selection rules by quantum tunneling processes. The interplay with Boltzmann
suppression factors leads to fake resonances with temperature-dependent
position which do not correspond to any charge excitation energy. Furthermore,
we find that quantum tunneling can completely suppress transport if the
easy-plane anisotropy has a high symmetry.Comment: 4 pages, 3 figure
Quantum tunneling induced Kondo effect in single molecular magnets
We consider transport through a single-molecule magnet strongly coupled to
metallic electrodes. We demonstrate that for half-integer spin of the molecule
electron- and spin-tunneling \emph{cooperate} to produce both quantum tunneling
of the magnetic moment and a Kondo effect in the linear conductance. The Kondo
temperature depends sensitively on the ratio of the transverse and easy-axis
anisotropies in a non-monotonic way. The magnetic symmetry of the transverse
anisotropy imposes a selection rule on the total spin for the occurrence of the
Kondo effect which deviates from the usual even-odd alternation.Comment: 4 pages, 4 figure
Orbit of the Mercury-Manganese binary 41 Eridani
Context. Mercury-manganese (HgMn) stars are a class of slowly rotating
chemically peculiar main-sequence late B-type stars. More than two-thirds of
the HgMn stars are known to belong to spectroscopic binaries.
Aims. By determining orbital solutions for binary HgMn stars, we will be able
to obtain the masses for both components and the distance to the system.
Consequently, we can establish the position of both components in the
Hertzsprung-Russell diagram and confront the chemical peculiarities of the HgMn
stars with their age and evolutionary history.
Methods. We initiated a program to identify interferometric binaries in a
sample of HgMn stars, using the PIONIER near-infrared interferometer at the
VLTI on Cerro Paranal, Chile. For the detected systems, we intend to obtain
full orbital solutions in conjunction with spectroscopic data.
Results. The data obtained for the SB2 system 41 Eridani allowed the
determination of the orbital elements with a period of just five days and a
semi-major axis of under 2 mas. Including published radial velocity
measurements, we derived almost identical masses of 3.17 +/- 0.07 M_Sun for the
primary and 3.07 +/- 0.07 M_Sun for the secondary. The measured magnitude
difference is less than 0.1 mag. The orbital parallax is 18.05 +/- 0.17 mas,
which is in good agreement with the Hipparcos trigonometric parallax of 18.33
+/- 0.15 mas. The stellar diameters are resolved as well at 0.39 +/- 0.03 mas.
The spin rate is synchronized with the orbital rate.Comment: 5 pages, 4 figure
Real-time renormalization group and cutoff scales in nonequilibrium applied to an arbitrary quantum dot in the Coulomb blockade regime
We apply the real-time renormalization group (RG) in nonequilibrium to an
arbitrary quantum dot in the Coulomb blockade regime. Within one-loop
RG-equations, we include self-consistently the kernel governing the dynamics of
the reduced density matrix of the dot. As a result, we find that relaxation and
dephasing rates generically cut off the RG flow. In addition, we include all
other cutoff scales defined by temperature, energy excitations, frequency, and
voltage. We apply the formalism to transport through single molecular magnets,
realized by the fully anisotropic Kondo model (with three different exchange
couplings J_x, J_y, and J_z) in a magnetic field h_z. We calculate the
differential conductance as function of bias voltage V and discuss a quantum
phase transition which can be tuned by changing the sign of J_x J_y J_z via the
anisotropy parameters. Finally, we calculate the noise S(Omega) at finite
frequency Omega for the isotropic Kondo model and find that the dephasing rate
determines the height of the shoulders in dS(\Omega)/d Omega near Omega=V.Comment: 16 pages, 7 figure
Exemple d'approche pluridisciplinaire dans la caractérisation d'eaux thermales carbo-gazeuses
Une approche pluridisciplinaire est menée à propos d'eaux souterraines carbo-gazeuses : au niveau des relations entre tectonique active et hydrothermalisme; sur les origines des composants aqueux et gazeux (CO2) par analyse de compositions isotopiques (2H, 18O, 3H et 13C) et mises en équation des équilibres carboniques.Trois exemples sont traités dans te Sud-Est de la France : le premier en région de socle métamorphique (émergence thermale chaude); les deux autres en domaine de couverture carbonatée épaisse (source karstique littorale froide et aquifère karstique peu profond).La complémentarité des informations acquises permet de préciser d'une part, le rôle de certaines directions fissurales dans les cheminements souterrains par rapport au contexte sismotectonique régional; d'autre part, la genèse des eaux et leur âge relatif; enfin l'origine du CO2 qui peut se révéler mixte (biogénique-mantellique) ou infracrustal.Se dégage de la sorte une méthodologie d'étude de ces eaux particulières qui mérite d'être plus largement développée.Comparison of fissural, isotopic and hydrochemical data constitutes an approach to geothermal phenomena which has several consequences. The examples which are given are taken from a present regional tectonic context which is characteristic of a drawing close of the African and the European plates. Its maximum horizontal stress is of a N-S SE trend. Consequently the fissural trends EW to ENE - NSW existing on the three areas under survey would play no major role in groundwater flows and rising of deep C.O. Such a significant role would be probably played by the joining of NS to NW trends, which at present act in extension (N-S fractures) or with transverse movements.They are found either at the gazeous and carbonic outlets (bare basement) or in the neighbourhood of these outlets (less than 300 m distant) located in the covering (thickness 300 m). It can thus be assumed that the fissuration is at present continuous between the covering and the basement in the N-S direction. This assumption is in agreement with the present stress field and the associated deformations. So all these data make it possible to emphasize on one hand the role of some fractural directions in groundwater flow according to the regional sismotectonic context, on the other the origins of the waters as well as their relative ages and lastly the origin of the CO2 which can be mixed (biogenic and from the mantle) or infracrusted. Such an approach has several consequences connected with :1) geothermal phenomena : the water temperature and the survey of the hydrothermal flows can be considered as a preliminary survey for deep local hydraulic investigation,2) seismic risks : the fact that the dissolved gas (i.e. CO2) derives its origin from the mantle is an indicator of such risks,3) The survey of deep geological accidents in particular within deep sedimentary caver : a methodology for the study of this particular type of waters which can efficiently be applied elsewhere is thus created
Kondo-transport spectroscopy of single molecule magnets
We demonstrate that in a single molecule magnet (SMM) strongly coupled to
electrodes the Kondo effect involves all magnetic excitations. This Kondo
effect is induced by the quantum tunneling of the magnetic moment (QTM).
Importantly, the Kondo temperature can be much larger than the magnetic
splittings. We find a strong modulation of the Kondo effect as function of the
transverse anisotropy parameter or a longitudinal magnetic field. For both
integer and half-integer spin this can be used for an accurate transport
spectroscopy of the magnetic states in low magnetic fields on the order of the
easy-axis anisotropy parameter. We set up a relationship between the Kondo
effects for successive integer and half-integer spins.Comment: 5 pages, 3 figure
Nonequilibrium Spin Dynamics in the Ferromagnetic Kondo Model
Motivated by recent experiments on molecular quantum dots we investigate the
relaxation of pure spin states when coupled to metallic leads. Under suitable
conditions these systems are well described by a ferromagnetic Kondo model.
Using two recently developed theoretical approaches, the time-dependent
numerical renormalization group and an extended ow equation method, we
calculate the real-time evolution of a Kondo spin into its partially screened
steady state. We obtain exact analytical results which agree well with
numerical implementations of both methods. Analytical expressions for the
steady state magnetization and the dependence of the long-time relaxation on
microscopic parameters are established. We find the long-time relaxation
process to be much faster in the regime of anisotropic Kondo couplings. The
steady state magnetization is found to deviate significantly from its thermal
equilibrium value.Comment: 4 pages, 3 figures, final version as accepted by Physical Review
Letter
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