arXiv subjects

Thibaut Jonckheere

Publications and source records attributed to Thibaut Jonckheere.

45 records · Page 3Linked to original sources

arXiv2008

Colossal spin fluctuations in a molecular quantum dot magnet with ferromagnetic electrodes

We study electronic transport through a magnetic molecule with an intrinsic spin $S$ coupled to two magnetic electrodes, in the incoherent regime. The molecule is modeled as a single resonant level with large Coulomb repulsion (no double occupancy). The molecular spin is isotropic and it interacts with the electronic spin through an exchange interaction. Using an alternative method to the usual master equation approach, we are able to obtain analytical formulas for various physical quantities of interest, such as the mean current and the current fluctuations, but also the mean value of $J_z$ -the $z$ component of the total spin on the molecule- and its fluctuations. This allows us to understand how the electronic current between the magnetized electrodes can control the polarization of the molecular spin. We observe in particular that the fluctuations of $J_z$ reach unexpectedly high values.

cond-mat.mes-hall↗

arXiv2007

Electron polarizability of crystalline solids in quantizing magnetic fields and topological gap numbers

A theory of the static electron polarizability of crystals whose energy spectrum is modified by quantizing magnetic fields is presented. It is argued that The polarizability is strongly affected by non-dissipative Hall currents induced by the presence of crossed electric and magnetic fields: these can even change its sign. Results are illustrated in detail for a two dimensional square lattice. The polarizability and the Hall conductivity are respectively linked to the two topological quantum numbers entering the so--called Diophantine equation. These numbers could in principle be detected in actual experiments.

cond-mat.mes-hall↗

arXiv2007

Controllable pi junction in a Josephson quantum-dot device with molecular spin

We consider a model for a single molecule with a large frozen spin sandwiched in between two BCS superconductors at equilibrium, and show that this system has a $π$ junction behavior at low temperature. The $π$ shift can be reversed by varying the other parameters of the system, e.g., temperature or the position of the quantum dot level, implying a controllable $π$ junction with novel application as a Josephson current switch. We show that the mechanism leading to the $π$ shift can be explained simply in terms of the contributions of the Andreev bound states and of the continuum of states above the superconducting gap. The free energy for certain configuration of parameters shows a bistable nature, which is a necessary pre-condition for achievement of a qubit.

cond-mat.supr-con↗

arXiv2006

Quantum dot dephasing by fractional quantum Hall edge states

We consider the dephasing rate of an electron level in a quantum dot, placed next to a fluctuating edge current in the fractional quantum Hall effect. Using perturbation theory, we show that this rate has an anomalous dependence on the bias voltage applied to the neighboring quantum point contact, which originates from the Luttinger liquid physics which describes the Hall fluid. General expressions are obtained using a screened Coulomb interaction. The dephasing rate is strictly proportional to the zero frequency backscattering current noise, which allows to describe exactly the weak to strong backscattering crossover using the Bethe-Ansatz solution.

cond-mat.mes-hall↗

arXiv2006

An electronic Mach-Zehnder interferometer in the Fractional Quantum Hall effect

We compute the interference pattern of a Mach-Zehnder interferometer operating in the fractional quantum Hall effect. Our theoretical proposal is inspired by a remarkable experiment on edge states in the Integer Quantum Hall effect (IQHE). The Luttinger liquid model is solved via two independent methods: refermionization at nu=1/2 and the Bethe Ansatz solution available for Laughlin fractions. The current differs strongly from that of single electrons in the strong backscattering regime. The Fano factor is periodic in the flux, and it exhibits a sharp transition from sub-Poissonian (charge e/2) to Poissonian (charge e) in the neighborhood of destructive interferences.

cond-mat.mes-hall↗

arXiv2006

Time-controlled charge injection in a quantum Hall fluid

We consider the injection of a controlled charge from a normal metal into an edge state of the fractional quantum Hall effect, with a time-dependent voltage $V(t)$. Using perturbative calculations in the tunneling limit, and a chiral Luttinger liquid model for the edge state, we show that the electronic correlations prevent the charge fluctuations to be divergent for a generic voltage pulse $V(t)$. This is in strong contrast with the case of charge injection in a normal metal, where this divergence is present. We show that explicit formul{ae} for the mean injected charge and its fluctuations can be obtained using an adiabatic approximation, and that non perturbative results can be obtained for injection in an edge state of the FQHE with filling factor $ nu=1/3$. Generalization to other correlated systems which can be described with the Luttinger liquid model, like metallic Carbon nanotube, is given.

cond-mat.mes-hall↗

arXiv2005

Magnetic Field Enhanced Coherence Length in Cold Atomic Gases

We study the effect of an external magnetic field on coherent backscattering of light from a cool rubidium vapor. We observe that the backscattering enhancement factor can be {\it increased} with $B$. This surprising behavior shows that the coherence length of the system can be increased by applying a magnetic field, in sharp contrast with ususal situations. This is mainly due to the lifting of the degeneracy between Zeeman sublevels. We find good agreement between our experimental data and a full Monte-Carlosimulation, taking into account the magneto-optical effects and the geometry of the atomic cloud.

cond-mat.mes-hall↗

arXiv2001

Weak localization of light by cold atoms: the impact of quantum internal structure

Since the work of Anderson on localization, interference effects for the propagation of a wave in the presence of disorder have been extensively studied, as exemplified in coherent backscattering (CBS) of light. In the multiple scattering of light by a disordered sample of thermal atoms, interference effects are usually washed out by the fast atomic motion. This is no longer true for cold atoms where CBS has recently been observed. However, the internal structure of the atoms strongly influences the interference properties. In this paper, we consider light scattering by an atomic dipole transition with arbitrary degeneracy and study its impact on coherent backscattering. We show that the interference contrast is strongly reduced. Assuming a uniform statistical distribution over internal degrees of freedom, we compute analytically the single and double scattering contributions to the intensity in the weak localization regime. The so-called ladder and crossed diagrams are generalized to the case of atoms and permit to calculate enhancement factors and backscattering intensity profiles for polarized light and any closed atomic dipole transition.

quant-ph↗

arXiv2000

Multiple scattering of light by atoms in the weak localization regime

Coherent backscattering is a multiple scattering interference effect which enhances the diffuse reflection off a disordered sample in the backward direction. Classically, the enhanced intensity is twice the average background under well chosen experimental conditions. We show how the quantum internal structure of atomic scatterers leads to a significantly smaller enhancement. Theoretical results for double scattering in the weak localization regime are presented which confirm recent experimental observations.

physics.atom-ph↗