Field induced biquadratic exchange in hard/soft ferromagnetic bilayers

The appearance of biquadratic exchange coupling between soft Fe and hard SmCo thin layers is found. The remanent magnetization in the Fe layer reorients from parallel to perpendicular with respect to the SmCo easy axis after application of large enough negative field. To explain such an unexpected behavior in contacting ferromagnetic layers a model is proposed based on Slonczewski's fluctuating exchange mechanism. In our samples a partial remagnetization of the hard layer creates fluctuations of the bilinear interactions. The intralayer exchange averaging of the resulting magnetization fluctuations in the soft layer causes the observed biquadratic coupling.     

Very compact tunable solid-state laser utilizing a thin-film organic semiconductor

Optically pumped organic semiconductor lasers are fabricated by evaporation of a thin film of tris(8-hydroxyquinoline) aluminum (Alq(3)) molecularly doped with a laser dye on top of a polyester substrate with an embossed grating structure. We achieve low-threshold, longitudinally monomode distributed-feedback laser operation. By varying the film thickness of the organic semiconductor film, we can tune the wavelength of the surface-emitting laser over 44 nm. The low laser threshold allows the use of a very compact all-solid-state pump laser ( approximately 10 cm long). This concept opens up a way to obtain inexpensive lasers that are tunable over the whole visible range.     

Monopoles in an antiferromagnetic Bose-Einstein condensate

We show that even in three dimensions an antiferromagnetic spin-1 Bose-Einstein condensate, which can, for instance, be created with (23)Na atoms in an optical trap, has not only singular linelike vortex excitations, but also allows for singular pointlike topological excitations, i.e., monopoles similar to the 't Hooft-Polyakov monopoles. We discuss the static and dynamic properties of these monopoles.     

Nonlinear stationary waves with transport current in superconductors

The profile of a nonlinear stationary thermomagnetic wave in the resistive state of superconductors is studied at different transport currents. It is proved that the thermomagnetic wave has an oscillating profile at relatively high values of the transport current in the sample. A shock wave with a monotonic structure corresponds to comparatively weak transport currents. The wave propagation velocity and the wave front width in a superconductor are estimated.     

Inclusive excitation-energy distributions of hot nuclei from 44 MeV/nucleon Ar- and 32 MeV/nucleon Kr-induced reactions

Inclusive neutron multiplicity distributions were measured by means of 4π liquid-scintillator detectors for Ar and Kr-induced reactions at 44 MeV/nucleon and 32 MeV/nucleon, respectively. For all the systems studied, the observed distributions exhibit a bump structure at large multiplicity, corresponding to highly dissipative collisions. For Ar-induced reactions, the excitation energies necessary to explain the most probable neutron multiplicity associated with these dissipative collisions are estimated, the correspondence between excitation energy and neutron multiplicity being calculated in the framework of the statistical model. The so-obtained values of excitation energies, which are systematically lower than those predicted using the massive-transfer picture, are discussed.     

A study of dipolar interactions and dynamic processes of water molecules in tendon by 1H and 2H homonuclear and heteronuclear multiple-quantum-filtered NMR spectroscopy

The effect of proton exchange on the measurement of 1H-1H, 1H-2H, and 2H-2H residual dipolar interactions in water molecules in bovine Achilles tendons was investigated using double-quantum-filtered (DQF) NMR and new pulse sequences based on heteronuclear and homonuclear multiple-quantum filtering (MQF). Derivation of theoretical expressions for these techniques allowed evaluation of the 1H-1H and 1H-2H residual dipolar interactions and the proton exchange rate at a temperature of 24 degrees C and above, where no dipolar splitting is evident. The values obtained for these parameters at 24 degrees C were 300 and 50 Hz and 3000 s-1, respectively. The results for the residual dipolar interactions were verified by repeating the above measurements at a temperature of 1.5 degrees C, where the spectra of the H2O molecules were well resolved, so that the 1H-1H dipolar interaction could be determined directly from the observed splitting. Analysis of the MQF experiments at 1.5 degrees C, where the proton exchange was in the intermediate regime for the 1H-2H dipolar interaction, confirmed the result obtained at 24 degrees C for this interaction. A strong dependence of the intensities of the MQF signals on the proton exchange rate, in the intermediate and the fast exchange regimes, was observed and theoretically interpreted. This leads to the conclusion that the MQF techniques are mostly useful for tissues where the residual dipolar interaction is not significantly smaller than the proton exchange rate. Dependence of the relaxation times and signal intensities of the MQF experiments on the orientation of the tendon with respect to the magnetic field was observed and analyzed. One of the results of the theoretical analysis is that, in the fast exchange regime, the signal decay rates in the MQF experiments as well as in the spin echo or CPMG pulse sequences (T2) depend on the orientation as the square of the second-rank Legendre polynomial.     

TheS=1/2 Heisenberg antiferromagnet on the triangular lattice: Exact results and spin-wave theory for finite cells

We study the ground state properties of theS=1/2 Heisenberg antiferromagnet (HAF) on the triangular lattice with nearest-neighbour (J) and next-nearest neighbour (J) couplings. Classically, this system is known to be ordered in a 120° Néel type state for values-<1/8 of the ratio of these couplings and in a collinear state for 1/8<<1. The order parameter and the helicity /gC of the 120° structure are obtained by numerical diagonalisation of finite periodic systems of up toN=30 sites and by applying the spin-wave (SW) approximation to the same finite systems. We find a surprisingly good agreement between the exact and the SW results in the entire region-<<1/8. It appears that the SW theory is still valid for the simple triangular HAF (=0) although the sublattice magnetisation is substantially reduced from its classical value by quantum fluctuations. Our numerical results for the order parameterM of the collinear order support the previous conjecture of a first order transition between the 120° and the collinear order at 1/8.     

Dissipation, decoherence and preparation effects in the spin-boson system

The dynamics of the reduced density matrix of the driven dissipative two-state system is studied for a general diagonal/off-diagonal initial state. We derive exact formal series expressions for the populations and coherences and show that they can be cast into the form of coupled nonconvolutive exact master equations and integral relations. We show that neither the asymptotic distributions, nor the transition temperature between coherent and incoherent motion, nor the dephasing rate and relaxation rate towards the equilibrium state depend on the particular initial state chosen. However, in the underdamped regime, effects of the particular initial preparation, e.g. in an off-diagonal state of the density matrix, strongly affect the transient dynamics. We find that an appropriately tuned external ac-field can slow down decoherence and thus allow preparation effects to persist for longer times than in the absence of driving. Received 23 October 1998 and Received in final form 26 February 1999     

The effect of zinc ions on the fluorescence spectra of (La, Zn)OF:Sm powder phosphors

This paper reports the fluorescence properties of a new series of (La, Zn) OF:Sm³⁺ powder phosphors. The measured fluorescence spectra were analyzed by computing the color coordinates and relative fluorescence intensity ratios (ED/MD). A bright yellowish-orange color was observed from these powder phosphors under a UV-source due to the confirmation of the transitions ⁴G_5/2→⁶H_5/2, ⁴G_5/2→⁶H_7/2, and ⁴G_5/2→⁶H_9/2. Scanning electron microscopic studies were performed to understand the average grain size of the synthesized powder phosphors.