Pascal Simon

Exchange Interaction between Single Magnetic Adatoms

The magnetic coupling between single Co atoms adsorbed on a copper surface is determined by probing the Kondo resonance using low-temperature scanning tunneling spectroscopy. The Kondo resonance, which is due to magnetic correlation effects between the spin of a magnetic adatom and the conduction electrons of the substrate, is modified in a characteristic way by the coupling of the neighboring adatom spins. Increasing the interatomic distance of a Cobalt dimer from 2.56 to 8.1 A we follow the oscillatory transition from ferromagnetic to antiferromagnetic coupling. Adding a third atom to the antiferromagnetically coupled dimer results in the formation of a collective correlated state.

Coherent long-range magnetic bound states in a superconductor

Magnetic atoms embedded in a niobium selenide superconductor are shown to give rise to a long-range coherent bound state extending tens of nanometres.

Nuclear magnetism and electron order in interacting one-dimensional conductors

The interaction between localized magnetic moments and the electrons of a one-dimensional conductor can lead to an ordered phase in which the magnetic moments and the electrons are tightly bound to each other. We show here that this occurs when a lattice of nuclear spins is embedded in a Luttinger liquid. Experimentally available examples of such a system are single wall carbon nanotubes grown entirely from C-13 and GaAs-based quantum wires. In these systems the hyperfine interaction between the nuclear spin and the conduction electron spin is very weak; yet it triggers a strong feedback reaction that results in an ordered phase consisting of a nuclear helimagnet that is inseparably bound to an electronic density wave combining charge and spin degrees of freedom. This effect can be interpreted as a strong renormalization of the nuclear Overhauser field and is a unique signature of Luttinger liquid physics. Through the feedback the order persists up into the millikelvin range. A particular signature is the reduction in the electric conductance by the universal factor of 2.

Measurement of Quantum Noise in a Carbon Nanotube Quantum Dot in the Kondo Regime

The current emission noise of a carbon nanotube quantum dot in the Kondo regime is measured at frequencies ν of the order or higher than the frequency associated with the Kondo effect k(B)T (K)/h, with TK the Kondo temperature. The carbon nanotube is coupled via an on-chip resonant circuit to a quantum noise detector, a superconductor-insulator-superconductor junction. We find for hν ≈ k(B)T(K) a Kondo effect related singularity at a voltage bias eV ≈ hν, and a strong reduction of this singularity for hν ≈ 3k(B)T(K), in good agreement with theory. Our experiment constitutes a new original tool for the investigation of the nonequilibrium dynamics of many-body phenomena in nanoscale devices.

Évaluation prospective des effets du stage d’initiation pratique sur le recrutement des internes d’urologie à Paris : résultats définitifs

Resume Objectifs Evaluer les effets d’un stage d’initiation pratique a l’urologie sur le recrutement des internes parisiens depuis la mise en place de l’Examen National Classant (ENC). Materiels et methodes Depuis 2004, tous les internes nommes en chirurgie a Paris ont participe a un stage d’initiation a l’urologie avant de prendre leur fonction hospitaliere. Un questionnaire leur a ete remis. L’enquete prospective s’est interessee a : l’âge, le sexe, l’existence d’un stage d’urologie pendant l’externat et au souhait d’orientation au sein des 11 disciplines chirurgicales. Les internes ont ete recontactes annuellement pour connaitre l’evolution de leur orientation professionnelle. Resultats Population. Deux cent cinquante cinq internes ont participe dont 145 femmes (56,9%) et 110 hommes (43,1%). L’âge moyen etait de 24,6 ± 5 ans (22-31). 173 internes etaient issus d’un CHU parisien (67,8%) et 82 (32,2%) d’un CHU de province. Cinquante deux internes (21,2%) avaient effectue au moins un stage dans un service d’urologie pendant leur externat. Disciplines chirurgicales. L’orthopedie a ete le plus souvent citee (n=48; 28,9%). L’urologie a ete choisie par 32 internes (12,5%) qui avaient effectue un stage d’externe dans la specialite dans 28 cas. A la fin du stage, 18 internes etaient tentes par l’urologie. A un an, 31 internes134 (91,2%) ont confirme leur choix pour I’urologie. Conclusion La realisation d’un stage pratique intervenant tres tot dans la formation des jeunes chirurgiens est une bonne solution pour les initier a l’urologie. En beneficiant d’un choix eclaire, les internes les plus motives sont incites a rejoindre volontairement notre discipline.

Nonequilibrium frequency-dependent noise through a quantum dot: A real-time functional renormalization group approach

We construct a real time current-conserving functional renormalization group (RG) scheme on the Keldysh contour to study frequency-dependent transport and noise through a quantum dot in the local moment regime. We find that the current vertex develops a non-trivial non-local structure in time, governed by a new set of RG equations. Solving these RG equations, we compute the complete frequency and temperature-dependence of the noise spectrum. For voltages large compared to the

Electron Phase Shift at the Zero-Bias Anomaly of Quantum Point Contacts

The Kondo effect is the many-body screening of a local spin by a cloud of electrons at very low temperature. It has been proposed as an explanation of the zero-bias anomaly in quantum point contacts where interactions drive a spontaneous charge localization. However, the Kondo origin of this anomaly remains under debate, and additional experimental evidence is necessary. Here we report on the first phase-sensitive measurement of the zero-bias anomaly in quantum point contacts using a scanning gate microscope to create an electronic interferometer. We observe an abrupt shift of the interference fringes by half a period in the bias range of the zero-bias anomaly, a behavior which cannot be reproduced by single-particle models. We instead relate it to the phase shift experienced by electrons scattering off a Kondo system. Our experiment therefore provides new evidence of this many-body effect in quantum point contacts.

Nonperturbative phase diagram of interacting disordered Majorana nanowires

We develop a Gaussian variational approach in replica space to investigate the phase diagram of a one-dimensional interacting disordered topological superconducting wire in the strong coupling regime. This method allows for a non-perturbative treatment in the disorder strength, electron- electron interactions and the superconducting pairing amplitude. We find only two stable phases: a topological superconducting phase, and a glassy, non-topological localized phase, characterized by replica symmetry breaking.

Finite-frequency-dependent noise of a quantum dot in a magnetic field

We present a detailed study for the finite-frequency current noise of a Kondo quantum dot in presence of a magnetic field by using a recently developed real time functional renormalization group approach [Phys. Rev. B

Disordered one-dimensional Bose-Fermi mixtures: The Bose-Fermi glass

\mathbf{83}