Spatio-temporal spin noise spectroscopy
le mardi 18 février 2020 à 14h00
Séminaire nano-électronique quantique
Personne à contacter : Robert Whitney ()
Lieu : Salle Rémy Lemaire K223, Institut Néel
Résumé :
In experimental measurements the noise is not necessarily a complication and from it one can extract valuable information regarding its sources. Spin noise spectroscopy (SNS) is a powerful technique to get information about dynamics of various systems (atomic gases, conduction electrons, localized states in semiconductors…) based on their spin fluctuations. As powerful as the technique is, SNS only addresses time spin correlations, but not spatial correlations. Thus valuable informations on spin transport or many-body spin interactions are missed.
In this seminar, I shall start with an introduction to SNS from the principles to applications. Then I will present recent advances we made in developing an all-optical probe of spatiotemporal spin fluctuations based on a new SNS approach (1). This new technique gives access to spatial correlations and spin motion. We thus investigated electron spin dynamics in CdTe by measuring simultaneously the electron spin relaxation rate and the spin diffusion constant. In addition, the sensitivity of the SNS allows us to observe the effect of the local nuclear field on the evolution of the electron spin and to estimate the electron spin correlation time on a given donor site. We then face to an unprecedented puzzle we hope to solve.
(1) S. Cronenberger, C. Abbas, D. Scalbert and H.Boukari, Phys. Rev. Lett. 123 017401 (2019)
Stability of the Laughlin phase against long-range interactions
le mercredi 19 février 2020 à 11h00
Séminaire LPMMC
Personne à contacter : Vincent Rossetto ()
Lieu : Salle de lecture 2, maison des Magistères
Résumé : The Laughlin wave function is at the basis of the current understanding
of the fractional quantum Hall effect, nevertheless, many of its
fundamental properties are yet to be understood. I will present a model
for its response, within Laughlin's ansatz, to variations of the
external potential and of the long-range part of the interparticle
interaction.
Our main result is that the minimal energy within Laughlin's
approximation is asymptotically attained, in the large particle number
limit, by states exhibiting uncorrelated quasi-holes on top of the
Laughlin wave function.
This is a joint project with Nicolas Rougerie.
Heat current and energy reactance in a driven quantum system
le mardi 17 mars 2020 à 14h00
Séminaire nano-électronique quantique
Personne à contacter : Robert Whitney ()
Lieu : Salle Rémy Lemaire K223, Institut Néel
Résumé :
We explore the relation between the heat current and the temperature variation of a lead coupled to a driven quantum system. We compare the microscopic definition of heat current to the macroscopic derived from the free electron gas theory which contains explicitly the measured electron temperature. We find that the microscopic definition of heat current is consistent with the variation of temperature of the lead even in such a strongly out-of-equilibrium system. Moreover we look at the total instantaneous power and we find that there is a non-vanishing contribution coming from the coupling term as already found in other studies in the adiabatic regime. Extending those results to finite frequency allows us to characterize its nature and show that it arises from the accumulation of energy in the contacts. Nevertheless this energy does not contribute to the dissipated/absorbed power, therefore justifying the name of energy reactance in analogy with classical circuits. We consider the specific case of a single level quantum dot coupled to leads in the hundreds of mK regime, driving amplitude of the order of tens of μV and driving frequencies of the order of few tenths of GHz.
Phase Signature of Topological Transition in Josephson Junctions
le mardi 31 mars 2020 à 14h00
Séminaire nano-électronique quantique
Personne à contacter : Robert Whitney ()
Lieu : Salle Rémy Lemaire K223, Institut Néel
Résumé :
Topological transition transforms common superconductivity into an exotic phase of matter, which holds promise for fault-tolerant quantum computing. A hallmark of this transition is the emergence of Majorana states. While two-dimensional semiconductor/superconductor heterostructures are desirable platforms for topological superconductivity, direct phase measurements as the fingerprint of the underlying topological transition are conspicuously missing. On gate tunable Josephson junctions made on epitaxial Al/InAs, we observe a closing and a reopening of the superconducting gap with increasing in-plane magnetic field. Since our junctions are embedded into a phase-sensitive SQUID, we are able to measure a π-jump in the superconducting phase across the junction coincident with the closing and reopening of the superconducting gap. Theoretical simulations confirm this transition is topological and compatible with the emergence of Majorana states while the magnetic field angle dependence of the transition further constrain this scenario. Remarkably, in each junction, this topological transition can be controlled by changing the gate voltage. These findings reveal versatile two-dimensional platforms for scalable topological quantum computing.
Laboratoire de Physique et Modélisation des Milieux Condensés
