Magnon Bose condensation in symmetry breaking magnetic field

Magnon Bose condensation (BC)in the symmetry breaking magnetic field is a result of unusual form of the Zeeman energy, which has terms linear in the spin-wave operators and terms mixing excitations differ in the Wave-vector of the magnetic structure. The following examples are considered: simple easy-plane tetragonal antiferromagnets (AF), frustrated AF family$R_2Cu O_4$ where $R=Pr,Nd$ etc. and cubic magnets with the Dzyaloshinskii-Moriya interaction ($Mn Si$ etc.). In all cases the BC becomes important when the magnetic field becomes comparable with the spin-wave gap. The theory is illustrated by existing experimental results.Comment: Submitted to J. of Phys. Condens. Matter (Proceedings of International Conference "Highly Frustrated Magnets", Osaka (Japan), August 2006). 8 pages, 5 figure

Superexchange coupling and spin susceptibility spectral weight in undoped monolayer cuprates

A systematic inelastic neutron scattering study of the superexchange interaction in three different undoped monolayer cuprates (La_2CuO_4, Nd_2CuO_4 and Pr_2CuO_4) has been performed using conventional triple axis technique. We deduce the in-plane antiferromagnetic (AF) superexchange coupling $J$ which actually presents no simple relation versus crystallographic parameters. The absolute spectral weight of the spin susceptibility has been obtained and it is found to be smaller than expected even when quantum corrections of the AF ground state are taken into account.Comment: 13 pages, 1 table, 3 figure

Silicon resonant microcantilevers for absolute pressure measurement

This work is focused on the developing of silicon resonant microcantilevers for the measurement of the absolute pressure. The microcantilevers have been fabricated with a two-mask bulk micromachining process. The variation in resonance response of microcantilevers was investigated as a function of pressure 10−1-105 Pa, both in terms of resonance frequency and quality factor. A theoretical description of the resonating microstructure is given according to different molecular and viscous regimes. Also a brief discussion on the different quality factors contributions is presented. Theoretical and experimental data show a very satisfying agreement. The microstructure behavior demonstrates a certain sensitivity over a six decade range and the potential evolution of an absolute pressure sensor working in the same rang

Effect of a magnetic field on the long-range magnetic order in insulating Nd2CuO4, nonsuperconducting and superconducting Nd1.85Ce0.15CuO4

We have measured the effect of a c-axis aligned magnetic field on the long-range magnetic order of insulating Nd2CuO4, as-grown nonsuperconducting and superconducting Nd1.85Ce0.15CuO4. On cooling from room temperature, Nd2CuO4 goes through a series of antiferromagnetic (AF) phase transitions with different noncollinear spin structures. In all phases of Nd2CuO4, we find that the applied c-axis field induces a canting of the AF order but does not alter the basic zero-field noncollinear spin structures. Similar behavior is also found in as-grown nonsuperconducting Nd1.85Ce0.15CuO4. These results contrast dramatically with those of superconducting Nd1.85Ce0.15CuO4, where a c-axis aligned magnetic field induces a static, anomalously conducting, long-range ordered AF state. We confirm that the annealing process necessary to make superconducting Nd1.85Ce0.15CuO4 also induces epitaxial, three-dimensional long-range ordered cubic (Nd,Ce)2O3 as an impurity phase. In addition, the annealing process makes a series of quasi two-dimensional superlattice reflections associated with lattice distortions of Nd1.85Ce0.15CuO4 in the CuO2 plane. While the application of a magnetic field will induce a net moment in the impurity phase, we determine its magnitude and eliminate this as a possibility for the observed magnetic field-induced effect in superconducting Nd1.85Ce0.15CuO4.Comment: 12 pages, 10 figures, to be published in Phys. Rev.

SUM_M(2)×\timesUC_C(1) Gauge Symmetry in High TcT_c Superconductivity

The square lattice structure of $CuO_2$ layers and the strongly correlated property of electrons indicate that the high $T_c$ superconductivity in cuprates can be described by a SO$_M$(5) coherent pairing state in which a SU$_M$(2)$\times$U$_C$(1) gauge symmetry is embedded. The spin and charge fluctuations that characterize the low energy magnetic excitations in cuprates are controlled by this intrinsic SU$_M$(2)$\times$U$_C$(1) gauge symmetry.Comment: 4 pages Revtex fil

Coexistence of superconductivity (Tc= 55 K) and antiferromagnetism TN= 230 K) in YBa2CuS06. 66 single crystal

Neutron scattering and susceptibility measurements on a YBa2CU 3O6.55 single crystal show that the antiferromagnetic long range order coexists with superconductivity. The oxygen content has been determined by a complete neutron diffraction structure analysis. Evidence for superconductivity below Tc= 55 K has been obtained from magnetization measurements. Magnetic Bragg scattering has been observed up to TN= 230 K in both the superconducting and the normal phase.Nous montrons par des études de diffusion de neutrons et de susceptibilité sur un monocristal de YBa2Cu3O6.55 qu'un ordre antiferromagnétique à longue distance coexiste avec la supraconductivité. La teneur en oxygène a été mesurée à partir de la détermination complète de la structure par diffraction de neutrons. Des mesures d'aimantation nous ont permis de mettre en evidence la supraconductivité en dessous de T c= 55 K. La diffusion de Bragg magnétique a été observée jusqu'à TN= 230 K à la fois dans la phase supraconductrice et dans la phase normale

PHONON DISPERSION AND TRANSVERSE MODE SOFTENING IN RbFeCl3

Phonon dispersion curves of RbFeCl3 have been measured by neutron scattering. For wave vectors in the hexagonal plane the transverse phonons polarized along Z are found to have very low energies. Furthermore the shape of this branch changes drastically as the temperature is lowered from room temperature to 20K, the zone boundary energy at the K point being decreased by a factor of two. However no Bragg reflection indicative of a phase transition has been detected at this point. These features are interpreted as showing evidence that RbFeCl3 is at the border of a phase transition for which a microscopic mechanism will be discussed

Phonon dispersion and transverse mode softening in RbFeCl

SIGLEAvailable from CEN Saclay, Service de Documentation, 91191 Gif-sur-Yvette Cedex (France) / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Piezoelectric micromachined ultrasonic transducers based on PZT thin films

This paper describes fabrication and characterization results of piezoelectric micromachined ultrasonic transducers (pMUTs) based on 2-mu m-thick Pb(Zr0.53Ti0.47O3) (PZT) thin films. The applied structures are circular plates held at four bridges, thus partially unclamped. A simple analytical model for the fully clamped structure is used as a reference to optimize design parameters such as thickness relations and electrodes, and to provide approximate predictions for coupling coefficients related to previously determined thin film properties. The best coupling coefficient was achieved with a 270-mu m plate and amounted to k(2) = 5.3%. This value compares well with the calculated value based on measured small signal dielectric (epsilon = 1050) and piezoelectric (e(31,f) = 15 Cm-2) properties of the PZT thin film at 100 kV/cm dc bias. The resonances show relatively large Q-factors, which can be partially explained by the small diameters as compared to the sound wavelength in air and in the test liquid (Fluorinert 77). A transmit-receive experiment with two quasi-identical pMUTs was performed showing significant signal transmission up to a distance of 20 cm in air and 2 cm in the test liquid