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Search Results: 1 - 10 of 297434 matches for " J. Jegoudez "
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X-ray Investigation of the Magneto-elastic Instability of alpha'-NaV2O5
S. Ravy,J. Jegoudez,A. Revcolevschi
Physics , 1998, DOI: 10.1103/PhysRevB.59.R681
Abstract: We present an X-ray diffuse scattering study of the pretransitional structural fluctuations of the magneto-elastic transition in alpha'-NaV2O5. This transition is characterized by the appearance below Tsp~35K of satellite reflections at the reduced wave vector (1/2,1/2,1/4). A large regime of structural fluctuations is measured up to 90 K. These fluctuations are three dimensional between Tsp and ~50K and quasi-one dimensional above ~60K. At 40 K the anisotropy ratio is found to be (xib :xia :xic)= (3.8 : 1.8 : 1), which reveals the importance of transverse interactions in the stabilization of the low temperature phase. We discuss our results within the framework of recent theories dealing with the simultaneous occurrence of a charge ordering, a spin gap and a lattice distortion in this intriguing compound.
Infrared study of spin-Peierls compound alpha'-NaV2O5
D. Smirnov,J. Leotin,P. Millet,J. Jegoudez,A. Revcolevschi
Physics , 1998, DOI: 10.1016/S0921-4526(98)00633-4
Abstract: Infrared reflectance of alpha'-NaV2O5 single crystals in the frequency range from 50 cm-1 to 10000 cm-1 was studied for a, b and c-polarisations. In addition to phonon modes identification, for the a-polarised spectrum a broad continuum absorption in the range of 1D magnetic excitation energies was found. The strong near-IR absorption band at 0.8 eV shows a strong anisotropy with vanishing intensity in c-polarisation. Activation of new phonons due to the lattice dimerisation were detected below 35K as well as pretransitional structural fluctuations up to 65K.
Using electronic structure changes to map the H-T phase diagram of alpha'-NaV2O5
A. B. Sushkov,J. L. Musfeldt,S. A. Crooker,J. Jegoudez,A. Revcolevschi
Physics , 2001, DOI: 10.1103/PhysRevB.63.220401
Abstract: We report polarized optical reflectance studies of \alpha'-NaV2O5 as a function of temperature (4-45 K) and magnetic field (0-60 T). Rung directed electronic structure changes, as measured by near-infrared reflectance ratios \Delta R(H)=R(H)/R(H=0 T), are especially sensitive to the phase boundaries. We employ these changes to map out an H-T phase diagram. Topological highlights include the observation of two phase boundaries slightly below T_{SG}, enhanced curvature of the 34 K phase boundary above 35 T, and, surprisingly, strong hysteresis effects of both transitions with applied field.
Inversion symmetry in the spin-Peierls compound NaV2O5
A. Meetsma,J. L. de Boer,A. Damascelli,J. Jegoudez,A. Revcolevschi,T. T. M. Palstra
Physics , 1998,
Abstract: At room-temperature NaV2O5 was found to have the centrosymmetric space group Pmmn. This space group implies the presence of only one kind of V site in contrast with previous reports of the non-centrosymmetric counterpart P21mn. This indicates a non-integer valence state of vanadium. Furthermore, this symmetry has consequences for the interpretation of the transition at 34 K, which was ascribed to a spin-Peierls transition of one dimensional chains of V4+.
The symmetry problem in NaV2O5
A. Damascelli,D. van der Marel,J. Jegoudez,G. Dhalenne,A. Revcolevschi
Physics , 1998, DOI: 10.1016/S0921-4526(98)00648-6
Abstract: We discuss the symmetry of NaV2O5 in the high temperature phase on the basis of optical conductivity data. Conclusive information cannot be obtained by studying the optically allowed lattice vibrations. However, intensity and polarization of the electronic excitations give a direct indication for a broken-parity electronic ground-state. This is responsible for the detection of charged bi-magnons in the optical spectrum.
Optical spectroscopic study of the interplay of spin and charge in NaV2O5
A. Damascelli,C. Presura,D. van der Marel,J. Jegoudez,A. Revcolevschi
Physics , 1999, DOI: 10.1103/PhysRevB.61.2535
Abstract: We investigate the temperature dependent optical properties of NaV2O5, in the energy range 4meV-4eV. The symmetry of the system is discussed on the basis of infrared phonon spectra. By analyzing the optically allowed phonons at temperatures below and above the phase transition, we conclude that a second-order change to a larger unit cell takes place below 34 K, with a fluctuation regime extending over a broad temperature range. In the high temperature undistorted phase, we find good agreement with the recently proposed centrosymmetric space group Pmmn. On the other hand, the detailed analysis of the electronic excitations detected in the optical conductivity, provides direct evidence for a charge disproportionated electronic ground-state, at least on a locale scale: A consistent interpretation of both structural and optical conductivity data requires an asymmetrical charge distribution on each rung, without any long range order. We show that, because of the locally broken symmetry, spin-flip excitations carry a finite electric dipole moment, which is responsible for the detection of direct two-magnon optical absorption processes for E parallel to the a axis. The charged-magnon model, developed to interpret the optical conductivity of NaV2O5, is described in detail, and its relevance to other strongly correlated electron systems, where the interplay of spin and charge plays a crucial role in determining the low energy electrodynamics, is discussed.
Charged Magnons in NaV2O5
A. Damascelli,D. van der Marel,M. Grueninger,C. Presura,T. T. M. Palstra,J. Jegoudez,A. Revcolevschi
Physics , 1998, DOI: 10.1103/PhysRevLett.81.918
Abstract: We investigated the temperature-dependent optical conductivity of NaV2O5 in the energy range 4 meV-4 eV. The intensities and the polarization dependence of the detected electronic excitations give a direct indication for a broken-parity electronic ground-state and for a non-centrosymmetric crystal structure of the system in the high-temperature phase. A direct two-magnon optical absorption process, proposed in this Letter, is in quantitative agreement with the optical data. By analyzing the optically allowed phonons at various temperatures above and below the phase transition, we conclude that a second-order change to a larger unit cell takes place below 34 K.
Charge Ordering and Spin Dynamics in NaV2O5
B. Grenier,O. Cepas,L. P. Regnault,J. E. Lorenzo,T. Ziman,J. P. Boucher,A. Hiess,T. Chatterji,J. Jegoudez,A. Revcolevschi
Physics , 2000, DOI: 10.1103/PhysRevLett.86.5966
Abstract: We report high-resolution neutron inelastic scattering experiments on the spin excitations of NaV2O5. Below Tc, two branches associated with distinct energy gaps are identified. From the dispersion and intensity of the spin excitation modes, we deduce the precise zig-zag charge distribution on the ladder rungs and the corresponding charge order (about 0.6). We argue that the spin gaps observed in the low-T phase of this compound are primarily due to the charge transfer.
X-ray anomalous scattering investigations on the charge order in $α^\prime$-NaV$_2$O$_5$
S. Grenier,A. Toader,J. E. Lorenzo,Y. Joly,B. Grenier,S. Ravy,L. P. Regnault,H. Renevier,J. Y. Henry,J. Jegoudez,A. Revcolevschi
Physics , 2001, DOI: 10.1103/PhysRevB.65.180101
Abstract: Anomalous x-ray diffraction studies show that the charge ordering in $\alpha^\prime$-NaV$_2$O$_5$ is of zig-zag type in all vanadium ladders. We have found that there are two models of the stacking of layers along \emph{c-}direction, each of them consisting of 2 degenerated patterns, and that the experimental data is well reproduced if the 2 patterns appears simultaneously. We believe that the low temperature structure contains stacking faults separating regions corresponding to the four possible patterns.
Calibration and First light of the Diabolo photometer at the Millimetre and Infrared Testa Grigia Observatory
A. Benoit,F. Zagury,N. Coron,M. De Petris,F. -X. Desert,M. Giard,J. -P. Bernard,J. -P. Crussaire,G. Dambier,P. de Bernardis,J. Delabrouille,A. De Luca,P. de Marcillac,G. Jegoudez,J. -M. Lamarre,J. Leblanc,J. -P. Lepeltier,B. Leriche,G. Mainella,J. Narbonne,F. Pajot,R. Pons,J. -L. Puget,S. Pujol,G. Recouvreur,G. Serra,V. Soglasnova,J. -P. Torre,B. Vozzi
Physics , 1999, DOI: 10.1051/aas:2000129
Abstract: We have designed and built a large-throughput dual channel photometer, Diabolo. This photometer is dedicated to the observation of millimetre continuum diffuse sources, and in particular, of the Sunyaev-Zel'dovich effect and of anisotropies of the 3K background. We describe the optical layout and filtering system of the instrument, which uses two bolometric detectors for simultaneous observations in two frequency channels at 1.2 and 2.1 mm. The bolometers are cooled to a working temperature of 0.1 K provided by a compact dilution cryostat. The photometric and angular responses of the instrument are measured in the laboratory. First astronomical light was detected in March 1995 at the focus of the new Millimetre and Infrared Testa Grigia Observatory (MITO) Telescope. The established sensitivity of the system is of 7 mK_RJ s^1/2$. For a typical map of at least 10 beams, with one hour of integration per beam, one can achieve the rms values of y_SZ ~ 7 10^-5 and the 3K background anisotropy Delta T/T ~ 7 10^-5, in winter conditions. We also report on a novel bolometer AC readout circuit which allows for the first time total power measurements on the sky. This technique alleviates (but does not forbid) the use of chopping with a secondary mirror. This technique and the dilution fridge concept will be used in future scan--modulated space instrument like the ESA Planck mission project.
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