47 research outputs found
Electronic structure of superposition states in flux qubits
Flux qubits, small superconducting loops interrupted by Josephson junctions,
are successful realizations of quantum coherence for macroscopic variables.
Superconductivity in these loops is carried by --
electrons, which has been interpreted as suggesting that coherent
superpositions of such current states are macroscopic superpositions analogous
to Schr\"odinger's cat. We provide a full microscopic analysis of such qubits,
from which the macroscopic quantum description can be derived. This reveals
that the number of microscopic constituents participating in superposition
states for experimentally accessible flux qubits is surprisingly but not
trivially small. The combination of this relatively small size with large
differences between macroscopic observables in the two branches is seen to
result from the Fermi statistics of the electrons and the large disparity
between the values of superfluid and Fermi velocity in these systems.Comment: Minor cosmetic changes. Published version
Two-channel Kondo effect and renormalization flow with macroscopic quantum charge states
Many-body correlations and macroscopic quantum behaviors are fascinating
condensed matter problems. A powerful test-bed for the many-body concepts and
methods is the Kondo model which entails the coupling of a quantum impurity to
a continuum of states. It is central in highly correlated systems and can be
explored with tunable nanostructures. Although Kondo physics is usually
associated with the hybridization of itinerant electrons with microscopic
magnetic moments, theory predicts that it can arise whenever degenerate quantum
states are coupled to a continuum. Here we demonstrate the previously elusive
`charge' Kondo effect in a hybrid metal-semiconductor implementation of a
single-electron transistor, with a quantum pseudospin-1/2 constituted by two
degenerate macroscopic charge states of a metallic island. In contrast to other
Kondo nanostructures, each conduction channel connecting the island to an
electrode constitutes a distinct and fully tunable Kondo channel, thereby
providing an unprecedented access to the two-channel Kondo effect and a clear
path to multi-channel Kondo physics. Using a weakly coupled probe, we reveal
the renormalization flow, as temperature is reduced, of two Kondo channels
competing to screen the charge pseudospin. This provides a direct view of how
the predicted quantum phase transition develops across the symmetric quantum
critical point. Detuning the pseudospin away from degeneracy, we demonstrate,
on a fully characterized device, quantitative agreement with the predictions
for the finite-temperature crossover from quantum criticality.Comment: Letter (5 pages, 4 figures) and Methods (10 pages, 6 figures
The Kondo Effect in Non-Equilibrium Quantum Dots: Perturbative Renormalization Group
While the properties of the Kondo model in equilibrium are very well
understood, much less is known for Kondo systems out of equilibrium. We study
the properties of a quantum dot in the Kondo regime, when a large bias voltage
V and/or a large magnetic field B is applied. Using the perturbative
renormalization group generalized to stationary nonequilibrium situations, we
calculate renormalized couplings, keeping their important energy dependence. We
show that in a magnetic field the spin occupation of the quantum dot is
non-thermal, being controlled by V and B in a complex way to be calculated by
solving a quantum Boltzmann equation. We find that the well-known suppression
of the Kondo effect at finite V>>T_K (Kondo temperature) is caused by inelastic
dephasing processes induced by the current through the dot. We calculate the
corresponding decoherence rate, which serves to cut off the RG flow usually
well inside the perturbative regime (with possible exceptions). As a
consequence, the differential conductance, the local magnetization, the spin
relaxation rates and the local spectral function may be calculated for large
V,B >> T_K in a controlled way.Comment: 9 pages, invited paper for a special edition of JPSJ "Kondo Effect --
40 Years after the Discovery", some typos correcte
Kondo Effect on Mesoscopic Scale (Review)
Following the discovery of the Kondo effect the bulk transport and magnetic
behavior of the dilute magnetic alloys have been successfully described. In the
last fifteen years new directions have been developed as the study of the
systems of reduced dimensions and the artificial atoms so called quantum dots.
In this review the first subject is reviewed starting with the scanning
tunneling microscope (STM) study of a single magnetic impurity. The next
subject is the reduction of the amplitude of the Kondo effect in samples of
reduced dimension which was explained by the surface magnetic anisotropy which
blocks the motion of the integer spin nearby the surface. The electron
dephasing and energy relaxation experiments are discussed with the possible
explanation including the surface anisotropy, where the situation in cases of
integer and half-integer spins is very different. Finally, the present
situation of the theory of dynamical structural defects is briefly presented
which may lead to two-channel Kondo behavior.Comment: 8 pages, submitted to the JPSJ Special Issue "Kondo effect -- 40
years after the Discovery
Precise measurement of particle-wall hydrodynamic interactions at low Reynolds number using laser interferometry
International audienceThe motion of a spherical particle (with radius 1 to 6 mm) in a viscous fluid is measured using laser interferometry. The typical sensitivity on the measured displacement of the sphere is of the order of 50 nm. The particle is moving on the axis of a closed cylinder. The hydrodynamic interactions between the particle and the walls of the cylinder are compared with the theoretical result of Sano [J. Phys. Soc. Jpn. 56, 2713 (1987)] valid for a very small sphere. The agreement is excellent for the smallest sphere used in the experiment. The experiment also agrees with the result from the theory of lubrication when the sphere is close to a plane end wall. The effect of the particle roughness appears at small distances. Laser interferometry appears as a useful tool to study particle–wall hydrodynamic interactions when the geometry is cumbersome
Activation of N−O σ Bonds with Transition Metals: A Versatile Platform for Organic Synthesis and C−N Bonds Formation
Abstract N−O σ bonds containing compounds are versatile substrates for organic synthesis under transition metal catalysis. Their ability to react through both polar (oxidative addition, formation of metallanitrene, nucleophilic substitution) and radical pathways (single electron transfer, homolytic bond scission) have triggered the development of various synthetic methodologies, particularly toward synthesizing nitrogen‐containing compounds. In this review, we discuss the different modes of activation of N−O bonds in the presence of transition metal catalysts, emphasizing the experimental and computational mechanistic proofs in the literature to help to design new synthetic pathways toward the synthesis of C−N bonds
X-RAY AND MAGNETIZATION STUDIES OF THE GEOMETRICAL PARAMETERS OF THE GRAINS OF A FERROFLUID
On présente une étude par microscopie électronique, par diffusion centrale des rayons X et par aimantation, de la distribution en taille, de la structure et des corrélations magnétiques entre les grains de colloïdes magnétiques. Pour les colloïdes à base d'eau (Fe3O4 dans l'eau), les grains se comportent comme des sphères dures dispersées dans un fluide porteur. Pour les colloïdes à base de toluène (grains de cobalt) on propose un modèle où le grain est constitué d'un noyau de cobalt pur entouré d'une couche non magnétique. Dans les colloïdes de cobalt, des corrélations ont été observées qui mettent en évidence une périodicité dans la distance entre grains. Une forte anisotropie magnétique est observée pour les deux types de colloïdes.The size distribution, structure and magnetic correlations between the grains of magnetic colloids are studied by electron microscopy, small angle scattering of X-rays and magnetic measurements. For water-base colloids (Fe3O4 in water) the grains behave as hard spheres dispersed in a carrier liquid. For toluene-base colloids (Co grains) a model of a grain constituted by a core of pure cobalt and a non magnetic sheath surrounding it is proposed. In toluene-base colloids, correlations have been observed which reveal a periodicity in the intergrain spacing. Strong magnetic anisotropy is reported for both colloids
Interaction between a laser beam and some liquid media
cited By 34International audienceno abstrac
Lubricating motion of a sphere in a conical vessel
International audienceThe final stage of sedimentation of a spherical particle moving along the axis of a conical vessel containing a viscous incompressible fluid is studied both theoretically by lubrication analysis and experimentally by laser interferometry. The particle settling velocity varies like d5/2, where d is the gap. There is an excellent agreement between this result from lubrication theory and experiment, the upper bound being for a gap of about 1/30 radius and the lower practical bound being at the size of the particle roughness
Accurate measurement of hydrodynamic interactions between a particle and walls
International audienceA new experimental setup allows the measurement of hydrodynamic interactions between the walls of a vessel and a particle moving along a three-dimensional trajectory in a viscous fluid. The vertical motion of the particle is measured with an accuracy of 50 nm using laser interferometry, while its horizontal movements are controlled with an accuracy of 20 μm by displacing the vessel in the horizontal direction so as to keep the sphere in the fixed vertical laser beam. Three axisymmetric closed containers are used as examples: two vertical cylinders (with flat and convex lens-shaped lower walls) and a cone. Various effects of combined creeping flow hydrodynamic interactions between the particle and walls are observed