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Search Results: 1 - 10 of 228713 matches for " Jean-No?l Fuchs "
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Charge density wave in graphene: magnetic-field-induced Peierls instability
Jean-Nol Fuchs,Pascal Lederer
Physics , 2006, DOI: 10.1140/epjst/e2007-00235-3
Abstract: We suggest that a magnetic-field-induced Peierls instability accounts for the recent experiment of Zhang et al. in which unexpected quantum Hall plateaus were observed at high magnetic fields in graphene on a substrate. This Peierls instability leads to an out-of-plane lattice distortion resulting in a charge density wave (CDW) on sublattices A and B of the graphene honeycomb lattice. We also discuss alternative microscopic scenarios proposed in the literature and leading to a similar CDW ground state in graphene.
Spontaneous parity breaking of graphene in the quantum Hall regime
Jean-Nol Fuchs,Pascal Lederer
Physics , 2006, DOI: 10.1103/PhysRevLett.98.016803
Abstract: We propose that the inversion symmetry of the graphene honeycomb lattice is spontaneously broken via a magnetic field dependent Peierls distortion. This leads to valley splitting of the $n=0$ Landau level but not of the other Landau levels. Compared to quantum Hall valley ferromagnetism recently discussed in the literature, lattice distortion provides an alternative explanation to all the currently observed quantum Hall plateaus in graphene.
Geometric phase in Stückelberg interferometry
Lih-King Lim,Jean-Nol Fuchs,Gilles Montambaux
Physics , 2014, DOI: 10.1103/PhysRevA.91.042119
Abstract: We study the time evolution of a two-dimensional quantum particle exhibiting an energy spectrum, made of two bands, with two Dirac cones, as e.g. in the band structure of a honeycomb lattice. A force is applied such that the particle experiences two Landau-Zener transitions in succession. The adiabatic evolution between the two transitions leads to St\"uckelberg interferences, due to two possible trajectories in energy space. In addition to well-known dynamical and Stokes phases, the interference pattern reveals a geometric phase which depends on the chirality (winding number) and the mass sign associated to each Dirac cone, as well as on the type of trajectory (parallel or diagonal with respect to the two cones) in parameter space. This geometric phase reveals the coupling between the bands encoded in the structure of the wavefunctions.
Bloch-Zener oscillations across a merging transition of Dirac points
Lih-King Lim,Jean-Nol Fuchs,Gilles Montambaux
Physics , 2012, DOI: 10.1103/PhysRevLett.108.175303
Abstract: Bloch oscillations are a powerful tool to investigate spectra with Dirac points. By varying band parameters, Dirac points can be manipulated and merged at a topological transition towards a gapped phase. Under a constant force, a Fermi sea initially in the lower band performs Bloch oscillations and may Zener tunnel to the upper band mostly at the location of the Dirac points. The tunneling probability is computed from the low energy universal Hamiltonian describing the vicinity of the merging. The agreement with a recent experiment on cold atoms in an optical lattice is very good.
Geometry of Bloch states probed by Stückelberg interferometry
Lih-King Lim,Jean-Nol Fuchs,Gilles Montambaux
Physics , 2015,
Abstract: Inspired by recent experiments with cold atoms in optical lattices, we consider a St\"uckelberg interferometer for a particle performing Bloch oscillations in a tight-binding model on the honeycomb lattice. The interferometer is made of two avoided crossings at the saddle points of the band structure (i.e. at M points of the reciprocal space). This problem is reminiscent of the double Dirac cone St\"uckelberg interferometer that was recently studied in the continuum limit [Phys. Rev. Lett. 112, 155302 (2014)]. Although the two problems share similarities -- such as the appearance of a geometric phase shift -- lattice effects, not captured by the continuum limit, make them truly different. The particle dynamics in the presence of a force is described by the Bloch Hamiltonian $H(\boldsymbol{k})$ defined from the tight-binding Hamiltonian and the position operator. This leads to many interesting effects for the lattice St\"uckelberg interferometer: a twisting of the two Landau-Zener tunnelings, saturation of the inter-band transition probability in the sudden (infinite force) limit and extended periodicity or even non-periodicity beyond the first Brillouin zone. In particular, St\"uckelberg interferometry gives access to the overlap matrix of cell-periodic Bloch states thereby allowing to fully characterize the geometry of Bloch states, as e.g. to obtain the quantum metric tensor.
Inter-band tunneling near the merging transition of Dirac cones
Jean-Nol Fuchs,Lih-King Lim,Gilles Montambaux
Physics , 2012, DOI: 10.1103/PhysRevA.86.063613
Abstract: Motivated by a recent experiment in a tunable graphene analog [L. Tarruell et al., Nature 483, 302 (2012)], we consider a generalization of the Landau-Zener problem to the case of a quadratic crossing between two bands in the vicinity of the merging transition of Dirac cones. The latter is described by the so-called universal hamiltonian. In this framework, the inter-band tunneling problem depends on two dimensionless parameters: one measures the proximity to the merging transition and the other the adiabaticity of the motion. Under the influence of a constant force, the probability for a particle to tunnel from the lower to the upper band is computed numerically in the whole range of these two parameters and analytically in different limits using (i) the Stueckelberg theory for two successive linear band crossings, (ii) diabatic perturbation theory, (iii) adiabatic perturbation theory and (iv) a modified Stueckelberg formula. We obtain a complete phase diagram and explain the presence of unexpected probability oscillations in terms of interferences between two poles in the complex time plane. We also compare our results to the above mentioned experiment.
Mass and chirality inversion of a Dirac cone pair in Stückelberg interferometry
Lih-King Lim,Jean-Nol Fuchs,Gilles Montambaux
Physics , 2013, DOI: 10.1103/PhysRevLett.112.155302
Abstract: We show that a St\"{u}ckelberg interferometer made of two massive Dirac cones can reveal information on band eigenstates such as the chirality and mass sign of the cones. For a given spectrum with two gapped cones, we propose several low-energy Hamiltonians differing by their eigenstates properties. The corresponding inter-band transition probability is affected by such differences in its interference fringes being shifted by a new phase of geometrical origin. This phase can be a useful bulk probe for topological band structures realized with artificial crystals.
Casimir forces between defects in one-dimensional quantum liquids
Alessio Recati,Jean-Nol Fuchs,Claudia Sofia Peca,Wilhelm Zwerger
Physics , 2005, DOI: 10.1103/PhysRevA.72.023616
Abstract: We discuss the effective interactions between two localized perturbations in one-dimensional (1D) quantum liquids. For non-interacting fermions, the interactions exhibit Friedel oscillations, giving rise to a RKKY-type interaction familiar from impurity spins in metals. In the interacting case, at low energies, a Luttinger liquid description applies. In the case of repulsive fermions, the Friedel oscillations of the interacting system are replaced, at long distances, by a universal Casimir-type interaction which depends only on the sound velocity and decays inversely with the separation. The Casimir-type interaction between localized perturbations embedded in a fermionic environment gives rise to a long range coupling between quantum dots in ultracold Fermi gases, opening a novel alternative to couple qubits with neutral atoms. We also briefly discuss the case of bosonic quantum liquids in which the interaction between weak impurities turns out to be short ranged, decaying exponentially on the scale of the healing length.
Geometrical engineering of a two-bands Chern insulator in two dimensions with arbitrary topological index
Doru Sticlet,Frederic Piéchon,Jean-Nol Fuchs,Pavel Kalugin,Pascal Simon
Physics , 2012, DOI: 10.1103/PhysRevB.85.165456
Abstract: Two-dimensional 2-bands insulators breaking time reversal symmetry can present topological phases indexed by a topological invariant called the Chern number. Here we first propose an efficient procedure to determine this topological index. This tool allows in principle to conceive 2-bands Hamiltonians with arbitrary Chern numbers. We apply our methodology to gradually construct a quantum anomalous Hall insulator (Chern insulator) which can be tuned through five topological phases indexed by the Chern numbers {0,+/-1,+/-2}. On a cylindrical finite geometry, such insulator can therefore sustain up to two edge states which we characterize analytically. From this non-trivial Chern insulator and its time reversed copy, we build a quantum spin Hall insulator and show how the phases with a +/-2 Chern index yield trivial Z2 insulating phases.
The clinical impact of [18F]-FDG Pet during the opening year of a Pet centre
Talbot, Jean-Nol;
Brazilian Archives of Biology and Technology , 2002, DOI: 10.1590/S1516-89132002000500009
Abstract: we have evaluated the clinical impact of fdg-pet on patient staging and management during the opening year of our pet centre in france. a questionnaire, translation in french of the questionnaire used recently in california, was sent to the referring physician of each of the 476 patients who had at least one routine fdg-pet examination during the year 2000. of 348 responses (response rate = 73%), the disease was upstaged in 26% of the cases and downstaged in 9%. inter-modality management changes (change from a scheduled therapeutic modality for a different one) were reported in 37% of the cases and intra-modality changes in 9%. those modification rates were respectively 38% and 7% in recurrence of colorectal cancer (153 patients), 47% and 7% in lung cancer (118 patients), 16% and 23% in lymphoma (43 patients), 25% and 6% in the staging of head and neck cancers (32 patients).when comparing with the similar studies performed in california, there were no significant differences between the rates of inter-modality management changes. in contrast, intra-modality management changes were less frequent in our survey, except for lymphoma. globally, the clinical impact of fdg pet was similar, with a higher response rate to our survey (73% versus 35%); it was above the mean 31% rate of therapeutic modification derived from a recent tabulated summary in over 3400 patients.
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