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Search Results: 1 - 10 of 324937 matches for " S. Gambarelli "
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Science in court - experts and advisers as post-academic science communicators
Gambarelli Licia
JCOM : Journal of Science Communication , 2003,
Quantum simulations and experiments on Rabi oscillations of spin qubits: intrinsic {\sl vs} extrinsic damping
H. De Raedt,B. Barbara,S. Miyashita,K. Michielsen,S. Bertaina,S. Gambarelli
Physics , 2011, DOI: 10.1103/PhysRevB.85.014408
Abstract: Electron Paramagnetic Resonance experiments show that the decay of Rabi oscillations of ensembles of spin qubits depends noticeably on the microwave power and more precisely on the Rabi frequency, an effect recently called "driven decoherence". By direct numerical solution of the time-dependent Schr\"odinger equation of the associated many-body system, we scrutinize the different mechanisms that may lead to this type of decoherence. Assuming the effects of dissipation to be negligible ($T_1=\infty$), it is shown that a system of dipolar-coupled spins with -- even weak-- random inhomogeneities is sufficient to explain the salient features of the experimental observations. Some experimental examples are given to illustrate the potential of the numerical simulation approach.
Spin-Orbit Qubits of Rare-Earth-Metal Ions in Axially Symmetric Crystal Fields
Sylvain Bertaina,J. H. Shim,S. Gambarelli,B. Z. Malkin,B. Barbara
Physics , 2009, DOI: 10.1103/PhysRevLett.103.226402
Abstract: Contrary to the well known spin qubits, rare-earth qubits are characterized by a strong influence of crystal field due to large spin-orbit coupling. At low temperature and in the presence of resonance microwaves, it is the magnetic moment of the crystal-field ground-state which nutates (for several $\mu$s) and the Rabi frequency $\Omega_R$ is anisotropic. Here, we present a study of the variations of $\Omega_R(\vec{H}_{0})$ with the magnitude and direction of the static magnetic field $\vec{H_{0}}$ for the odd $^{167}$Er isotope in a single crystal CaWO$_4$:Er$^{3+}$. The hyperfine interactions split the $\Omega_R(\vec{H}_{0})$ curve into eight different curves which are fitted numerically and described analytically. These "spin-orbit qubits" should allow detailed studies of decoherence mechanisms which become relevant at high temperature and open new ways for qubit addressing using properly oriented magnetic fields.
Decoherence window and electron-nuclear cross-relaxation in the molecular magnet V 15
J. H. Shim,S. Bertaina,S. Gambarelli,T. Mitra,A. Müller,E. I. Baibekov,B. Z. Malkin,B. Tsukerblat,Bernard Barbara
Physics , 2010, DOI: 10.1016/j.jmr.2010.12.015
Abstract: Rabi oscillations in the V_15 Single Molecule Magnet (SMM) embedded in the surfactant DODA have been studied at different microwave powers. An intense damping peak is observed when the Rabi frequency Omega_R falls in the vicinity of the Larmor frequency of protons w_N, while the damping time t_R of oscillations reaches values 10 times shorter than the phase coherence time t_2 measured at the same temperature. The experiments are interpreted by the N-spin model showing that t_R is directly associated with the decoherence via electronic/nuclear spin cross-relaxation in the rotating reference frame. It is shown that this decoherence is accompanied with energy dissipation in the range of the Rabi frequencies w_N - sigma_e < Omega_R < w_N, where sigma_e is the mean super-hyperfine field (in frequency units) induced by protons at SMMs. Weaker damping without dissipation takes place outside this dissipation window. Simple local field estimations suggest that this rapid cross-relaxation in resonant microwave field observed for the first time in SMMV_15 should take place in other SMMs like Fe_8 and Mn_12 containing protons, too.
Spin Pumping and Inverse Spin Hall Effect in Germanium
J. -C. Rojas-Sánchez,M. Cubukcu,A. Jain,C. Vergnaud,C. Portemont,C. Ducruet,A. Barski,A. Marty,L. Vila,J. -P. Attané,E. Augendre,G. Desfonds,S. Gambarelli,H. Jaffrès,J. -M. George,M. Jamet
Physics , 2013, DOI: 10.1103/PhysRevB.88.064403
Abstract: We have measured the inverse spin Hall effect (ISHE) in \textit{n}-Ge at room temperature. The spin current in germanium was generated by spin pumping from a CoFeB/MgO magnetic tunnel junction in order to prevent the impedance mismatch issue. A clear electromotive force was measured in Ge at the ferromagnetic resonance of CoFeB. The same study was then carried out on several test samples, in particular we have investigated the influence of the MgO tunnel barrier and sample annealing on the ISHE signal. First, the reference CoFeB/MgO bilayer grown on SiO$_{2}$ exhibits a clear electromotive force due to anisotropic magnetoresistance and anomalous Hall effect which is dominated by an asymmetric contribution with respect to the resonance field. We also found that the MgO tunnel barrier is essential to observe ISHE in Ge and that sample annealing systematically lead to an increase of the signal. We propose a theoretical model based on the presence of localized states at the interface between the MgO tunnel barrier and Ge to account for these observations. Finally, all of our results are fully consistent with the observation of ISHE in heavily doped $n$-Ge and we could estimate the spin Hall angle at room temperature to be $\approx$0.001.
Spin-pumping into surface states of topological insulator α-Sn, spin to charge conversion at room temperature
J. -C. Rojas-Sánchez,S. Oyarzun,Y. Fu,A. Marty,C. Vergnaud,S. Gambarelli,L. Vila,M. Jamet,Y. Ohtsubo,A. Taleb-Ibrahimi,P. Le Fèvre,F. Bertran,N. Reyren,J. -M. George,A. Fert
Physics , 2015,
Abstract: We present experimental results on the conversion of a spin current into a charge current by spin pumping into the Dirac cone with helical spin polarization of the elemental topological insulator (TI) {\alpha}-Sn[1-3]. By angle-resolved photoelectron spectroscopy (ARPES) we first confirm that the Dirac cone at the surface of {\alpha}-Sn (0 0 1) layers subsists after covering with Ag. Then we show that resonant spin pumping at room temperature from Fe through Ag into {\alpha}-Sn layers induces a lateral charge current that can be ascribed to the Inverse Edelstein Effect[4-5]. Our observation of an Inverse Edelstein Effect length[5-6] much longer than for Rashba interfaces[5-10] demonstrates the potential of the TI for conversion between spin and charge in spintronic devices. By comparing our results with data on the relaxation time of TI free surface states from time-resolved ARPES, we can anticipate the ultimate potential of TI for spin to charge conversion and the conditions to reach it.
Equilibrium Solution in a Game between a Cooperative and its Members
Cesarino Bertini,Gianfranco Gambarelli,Antonino Scarelli,Zoltán Varga
AUCO Czech Economic Review , 2011,
Abstract: In the paper a game-theoretical model is set up to describe the conflict situation in which the members of a marketing cooperative may take advantage of an external market price, higher than that offered by the cooperative. Under appropriate conditions on the penalty strategy of the cooperative, the faithfulness of all members will provide a Nash equilibrium for the considered game, which at the same time also is an attractive solution, with the cooperative as a distinguished player.
Crossover from spin accumulation into interface states to spin injection in the germanium conduction band
A. Jain,J. -C. Rojas-Sanchez,M. Cubukcu,J. Peiro,J. C. Le Breton,E. Prestat,C. Vergnaud,L. Louahadj,C. Portemont,C. Ducruet,V. Baltz,A. Barski,P. Bayle-Guillemaud,L. Vila,J. -P. Attané,E. Augendre,G. Desfonds,S. Gambarelli,H. Jaffrès,J. -M. George,M. Jamet
Physics , 2012, DOI: 10.1103/PhysRevLett.109.106603
Abstract: Electrical spin injection into semiconductors paves the way for exploring new phenomena in the area of spin physics and new generations of spintronic devices. However the exact role of interface states in spin injection mechanism from a magnetic tunnel junction into a semiconductor is still under debate. In this letter, we demonstrate a clear transition from spin accumulation into interface states to spin injection in the conduction band of $n$-Ge. We observe spin signal amplification at low temperature due to spin accumulation into interface states followed by a clear transition towards spin injection in the conduction band from 200 K up to room temperature. In this regime, the spin signal is reduced down to a value compatible with spin diffusion model. More interestingly, we demonstrate in this regime a significant modulation of the spin signal by spin pumping generated by ferromagnetic resonance and also by applying a back-gate voltage which are clear manifestations of spin current and accumulation in the germanium conduction band.
Rare-earth solid-state qubits
Sylvain Bertaina,Serge Gambarelli,Alexandra Tkachuk,Igor Kurkin,Boris Malkin,Anatole Stepanov,Bernard Barbara
Physics , 2007, DOI: 10.1038/nnano.2006.174
Abstract: Quantum bits (qubits) are the basic building blocks of any quantum computer. Superconducting qubits have been created with a 'top-down' approach that integrates superconducting devices into macroscopic electrical circuits [1-3], whereas electron-spin qubits have been demonstrated in quantum dots [4-6]. The phase coherence time (Tau2) and the single qubit figure of merit (QM) of superconducting and electron-spin qubits are similar -- Tau2 ~ microseconds and QM ~10-1000 below 100mK -- and it should be possible to scale-up these systems, which is essential for the development of any useful quantum computer. Bottom-up approaches based on dilute ensembles of spins have achieved much larger values of tau2 (up to tens of ms) [7, 8], but these systems cannot be scaled up, although some proposals for qubits based on 2D nanostructures should be scalable [9-11]. Here we report that a new family of spin qubits based on rare-earth ions demonstrates values of Tau2 (~ 50microseconds) and QM (~1400) at 2.5 K, which suggests that rare-earth qubits may, in principle, be suitable for scalable quantum information processing at 4He temperatures.
Enhanced spin pumping by antiferromagnetic IrMn thin films around the magnetic phase transition
Lamprini Frangou,Simon Oyarzun,Stephane Auffret,Laurent Vila,Serge Gambarelli,Vincent Baltz
Physics , 2015,
Abstract: We report measurements of a spin pumping effect owing to fluctuating IrMn antiferromagnets. Spin injection by a precessing NiFe ferromagnet into IrMn spin sinks is used and enhanced damping is observed around the IrMn magnetic phase transition. Our data are compared to a recent theory and converted into interfacial spin mixing conductance enhancements. By spotting the spin pumping peak, we also determined the thickness dependence of the IrMn critical temperature and deduced the characteristic length for the spin-spin interactions, which has been inaccessible to experiments.
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