J/ψ production in PHENIX

Heavy quarkonia production is expected to be sensitive to the formation of a quark gluon plasma (QGP). The PHENIX experiment has measured J/ψ production at  =200 GeV in Au+Au and Cu+Cu collisions, as well as in reference p+p and d+Au runs. J/ψ’s were measured both at mid (|y|<0.35) and forward (1.2<|y|<2.2) rapidity. In this letter, we present the A+A preliminary results and compare them to normal cold nuclear matter expectations derived from PHENIX d+Au and p+p measurements as well as to theoretical models including various effects (color screening, recombination, sequential melting...).     

Magnetoresistance in the ferromagnet/insulator/ferromagnet tunnel junction

Recently experiments and theories show that the tunnel magnetoresistance （TMR） does not only depend on the ferromagnetic metal electrodes but also on the insulator. Considering the rough-scattering effect and spin-flip effect in the insulator, this paper investigates the TMR ratio in a ferromagnet/insulator/ferromagnet （FM/I/FM） tunnelling junction by using Slonczewsik＇s model. A more general expression of TMR ratio as a function of barrier height, interface roughness and spin-flip effect is obtained. In lower barrier case, it shows that the TMR ratio depends on the roughscattering effect and spin-flip effect.     

Cusps in KL→3π decays

The pronounced cusp in K→3π decays which is generated by the pion mass difference is directly related to the ππS-wave scattering lengths. We apply a nonrelativistic effective field theory framework to evaluate the amplitudes for K_L→3π decays in a systematic manner. Electromagnetic effects in the neutral channel K_L→3π⁰ are also discussed.     

Spin Correlations in e+e? Pair Creation by Two-Photons and Entanglement in QED

Spin correlations of e ⁺ e ⁻ pair productions of two colliding photons are investigated and explicit expressions for their corresponding probabilities are derived and found to be energy (speed) dependent, for initially linearly and circularly polarized photons, different from those obtained by simply combining the spins of the relevant particles, for initially polarized photons. These expressions also depend on the angles of spin of e ⁺ (and/or of e ⁻), for initially linearly polarized photons, but not for circularly polarized photons, as a function of the energy. It is remarkable that these explicit results obtained from quantum field theory show a clear violation of Bell’s inequality of Local Hidden Variables theories at all energies beyond that of the threshold one for particle production, in support of quantum field theory in the relativistic regime. We hope that our explicit expression will lead to experiments, of the type described in the bulk of this paper, which can monitor energy (and speed) in polarization correlation experiments.     

High-pT spectra of charged hadrons in Au + Au collisions at √sNN = 9.2 GeV in STAR

The production of hadrons in heavy-ion collisions at high-p _T provides an important information on mechanism of particle formation and constituent energy loss in medium. Such information is needed for search of a Critical Point and signatures of phase transition. Measurements by the STAR Collaboration of charged hadron production in Au + Au collisions at √s _NN = 9.2 GeV over a wide transverse momentum p _T = 0.2−4 GeV/c and at mid-rapidity range are reported. It allows for a first measurement of the spectra for charged hadrons at high p _T at this energy. The spectra demonstrate the dependence on centrality which enhances with p _T . The constituent energy loss and its dependence on transverse momentum of particle, and centrality of collisions are estimated in the z-scaling approach.     

Nuclear effects in J/ψ suppression

We compute the effect on the J/ψ production in nucleus-nucleus coollisions of the absorption of the J/ψ inside the colliding nuclei. Using a value of the absorptive J/ψ-nucleon cross section which reproduces the A-dependence of the J/ψ cross section in hadron-nucleus collisions, we obtain much less suppression than that measured by the NA38 Collaboration. The effect due to a strongly absorbed (A^0.7) component is also discussed. In order to explain the NA38 results one needs a very large contribution (50%) of the latter component.     

Vibrations in Magnet/Superconductor Levitation Systems

The problem of a small magnet levitating above a very thin superconducting disc in the Meissner state is analysed. The dipole-dipole interaction model is employed to derive analytical expressions for the interaction energy, levitation force, magnetic stiffness and frequency of small vibrations about the equilibrium position in two different configurations, i.e. with the magnetic moment parallel and perpendicular to the superconductor. The results show that the frequency of small vibrations decreases with the increasing levitation height for a particular radius of the superconducting disc, which is in good agreement with the experimental results. However, the frequency increases monotonically up to saturation by increasing the radius of the disc for a particular height of the magnet. In addition, the frequency of vibrations is higher when the system is in the vertical configuration than that when the system is in the horizontal configuration.     

Parity violation in the 17/2−–17/2+ doublet in93Tc

The parity-violating mixing of the 17/2^– and 17/2⁺ levels in⁹³Tc nuclei, polarized by the tilted multifoil interaction, was measured by the observation of the forward-backward-ray asymmetry. The nuclear polarization, induced by the tilted multifoils, was measured directly for the neighboring^88,90Zr isomers. The forward to backward asymmetry was determined to be A=(2.5±2.1) 10^–3 which implies a parity violating matrix element ¦H _PV )¦=(4.0±3.7)meV.     

High sum-frequency generation in dielectric/antiferromagnet/Ag sandwich structures

We present a method to increase the sum-frequency(SF) outputs in dielectric/antiferromagnet(AF)/Ag sandwich structures for a fixed input power. Two incident waves simultaneously illuminate the upper surface, one is oblique and the other is normal to it. Numerical calculations based on the SiO2/MnF2/Ag and ZnF2/MnF2/Ag structures show that the SF outputs on the upper film increase a few times as compared to those of a single AF film when the thickness of the AF film is one-quarter of the vacuum wavelength. Moreover, the SF outputs generated near the higher resonant frequency will be higher than those obtained near the lower resonant frequency. An optimum AF film thickness is achieved through investigating its effect on the SF outputs in the two different dielectric sandwich structures.     

Signature inversion in πi13/2⊗νi13/2 structure in 178Ir

High-spin states in ¹⁷⁸Ir were investigated by means of in-beam γ-ray spectroscopy techniques using the multidetector array GASP. Excited states of ¹⁷⁸Ir were populated through the ¹⁵⁹Tb(²⁴Mg, 5n) fusion-evaporation reaction at E(²⁴ Mg) = 131-141 MeV. Several rotational bands were observed. Among them, the πi _13/2⊗νi _13/2 structure has been identified up to spin 36 ?. This band exhibits an anomalous signature splitting and a signature inversion around spin 25 ?. Received: 13 December 2000 / Accepted: 11 April 2001     

Do quasiparticles in high-Tc superconductors condense like molecules of a perfect gas in the bose-Einstein condensation?

Far IR transmission spectra of very thin YBaCuO films in the 10–40 cm^–1 range have now sufficient accuracy and reproducibility to show that the number of quasiparticles at temperature <_c, _c being the transition temperature, do not follow a ⁴ law, but rather a ^1.5 one.     

Interlayer Coupling in Co/Ti/（Gd-Co）/Ti Artificial Layered Ferrimagnet

The magnetic properties and the structure of [Co/Ti/Gd0.36 Co0.64/Ti]4/Co multilayers are investigated by means of torque magnetometer, vibrating sample magnetometer and transverse magneto-optic Kerr effect （TMOKE） measurements and the atomic force microscopy. Due to interlayer exchange interaction, Co and Gd-Co layers form a macroscopic ferrimagnetic system. The change in the sign of the TMOKE hysteresis loops near the compensation temperature and field induced magnetic phase transitions are found. The latter can be characterized by a critical field which shows a linear variation with the temperature. The magnetic properties of these multilayers from many points of view are similar to those of bulk ferrimagnets.     

Pressure effects in AlAs/Inx Ga1-xAs/GaAs resonant tunnelling diodes for application in micromachined sensors

This paper reports the current-voltage characteristics of [001]-oriented AlAs/InxGa1-xAs/GaAs resonant tunnelling diodes （RTDs） as a function of uniaxial external stress applied parallel to the [110] and the [1^-10] orientations, and the output characteristics of the GaAs pressure sensor based on the pressure effect on the RTDs. Under [110] stress, the resonance peak voltages of the RTDs shift to more positive voltages. For [1^-10] stress, the peaks shift toward more negative voltages. The resonance peak voltage is linearly dependent on the [110] and [1^-0] stresses and the linear sensitivities are up to 0.69 mV/MPa, -0.69 mV/MPa respectively. For the pressure sensor, the linear sensitivity is up to 0.37 mV/kPa.     

Four-jet production in e+e−-annihilation

We investigate the four-jet production processes $e^{+}{e}^{?}\to q\overline{q}ggand{e}^{+}{e}^{?}\to q\overline{q}q\overline{q}$ in lowest order QCD perturbation theory. We estimate that four-jet events should be detectable at a rate of about 5% at the highest PETRA energy. The acoplanarity distribution is calculated and compared to nonperturbative effects.     

Spin Hall Magnetoresistance in Pt/BiFeO3 Bilayer

Multiferroic materials are general antiferromagnets with negligibly small net magnetization, which strongly limits their magnetoelectric applications in spintronics. Spin Hall magnetoresistance(SMR) is sensitive to the orientation of the Néel vector, which can be applied for the detection of antiferromagnetic states. Here, we apply SMR on the unique room-temperature antiferromagnetic multiferroic material BiFeO₃(BFO). The angular dependence of SMR in a bilayer of epitaxial BFO(001) an...     

Determination of the probability of existence of pair interactions in the formation of M2tX2t?1Superstructures in MXy nonstoichiometric compounds

An analytical method has been proposed for calculating the probabilities P _i ^(2)(s) of existence of X-X, X−□, and □−□ pair interactions in the nonmetal sublattice of M _2tX_2t−1 superstructures formed in strongly nonstoichiometric compounds MX _y (MX _y □_1−y) and M ₂X _y (MX_y/2□_1−y/2) with a high content of structural vacancies □. The main characteristics necessary for the quantitative determination of the probabilities P_i(2)(s) as functions of the composition, degree of long-range order, symmetry, and structure type have been determined for all the known superstructures M _2tX_2t−1.     

Mixed electrical conduction in Ce1−xCaxO2−x

The solid electrolyte Ce_1?xCa_xO_2-?x with the fluorite-type structure (Ca-doped CeO₂) is a mixed conductor. Conduction occurs predominantly by migration of O^2? ions via oxygen vacancies or by electrons, depending on the departure from stoichiometry. The ionic transference number σ_i/σ_i + σ_e was determined as a function of dopant concentration (0.07?x?0.15), temperature (400–800°C), and oxygen pressure by emf measurements with oxygen concentration cells. It is described by

The entropy term S¹(x) changes from 38.7k for x = 0.07 to 31.7k for x = 0.15; the enthalpy term, 5.42 eV, is independent of x and in excellent accord with semi-empirical calculations.     

Structure change analysis in γ-Fe2O3/carbon composite in the process of electrochemical lithium insertion

It was previously shown that γ-Fe₂O₃/carbon composite synthesized by the aqueous solution method exhibit good high-speed charge/discharge and cycle characteristics as the cathode material in lithium-ion rechargeable batteries. We examined the crystal structure of γ-Fe₂O₃ in γ-Fe₂O₃/carbon composite by X-ray diffraction and the Rietveld analysis before and during electrochemical insertion of lithium ions. Before insertion, the sample has a spinel structure belonging to the Fd3¯m space group with the following iron occupancies: 8a site, 0.92, 16c site, 0 and 16d site, 0.87. During insertion, iron occupancy at 16d site remains virtually constant, at 8a site decreases from 0.92 to 0, and at 16c site increases from 0 to 0.53. These results suggest that, during insertion, iron migrates from 8a to 16c site. In the most highly lithiated sample, iron occupancy at 8a site decreases to 0 and occupancies at 16c and 16d sites were not equalized. Thus, the crystal structure for this sample belongs not to the Fm3¯m space group that represents the rock salt structure, but rather to the Fd3¯m space group.     

I–V characteristics and critical currents in superconducting/ferromagnetic bilayers

Current–voltage (I–V) characteristics and critical current density, J_c, for the onset of vortex motion were measured at different magnetic fields, H, and temperatures, T, in a superconducting (S)/ferromagnetic (F) bilayer and in a single Nb film. We choose Nb as a superconductor and a weak ferromagnetic alloy, Pd_1−xNi_x with x = 16, as F. We found that J_c was smaller for the S/F bilayer with respect to the single Nb film. The result was related to the reduced value of the superconducting order parameter in the bilayer.     

Spin-wave resonance in a multilayer Co1 ? xPx/Co1 ? yPy structure as a method of detection of the Bragg gaps in a spin wave spectrum

The method of spin-wave resonance has been used to detect in multilayer (Co₉₈P₂/Co₉₅P₅) _N structures a modification of the exchange spin wave spectrum due to the formation of the first, second, and third Brillouin zones in a one-dimensional magnon crystal formed by a periodic modulation of the exchange. The band gaps have been measured for wave vectors k _b = π/(d ₁ + d ₂) and 2k _b .