The disordered Bose condensate in two dimensions: application to high-Tc superconductors

We calculate the dynamical conductivity for a weakly disordered Bose condensate in two dimensions. The disorders is due to neutral impurities. We compare the asymptotic laws (for small and large frequencies) for neutral impurities with the ones for charged impurities. Universal functions for the dynamical transport properties are derived. The plasmon density of states shows a linear increase with energy for intermediate energies and a peak structure at larger energies. Our theoretical results are compared with experimental results (far-infrared, electron-energy-loss and Raman spectroscopy) found in the high-T_c superconductor YBa₂Cu₃O_7−δ. The occurrence of a quasi-gap in a disordered Bose condensate is described and discussed in connection with experiments on high-T_c superconductors.     

Beam loading in magnicon deflection cavities

Analysis of the beam-deflection cavity interaction in a magnicon is presented and compared with experiment. For a driven cavity a dispersion relation is obtained wherein the interaction modifies the cold-cavity quality factor and the resonance frequency. In terms of a lumped-parameter equivalent circuit the interaction corresponds to a complex-valued beam admittance Y_b in parallel with the cavity admittance. The response of the gain cavities is modified by the same admittance. In a magnicon, Y_b is a sensitive function of the solenoidal focusing magnetic field B₀, thus providing a convenient means of adjusting the cavity properties in experiments. When the relativistic gyrofrequency is twice the drive frequency, Im Y_b =0 and the beam does not load the cavity. Analytical expressions of the variation of the detuning, instantaneous bandwidth (i.e., loaded quality factor) and gain with B₀ are derived. Simulation results are presented to verify the linear analysis with ideal beams and to illustrate the modifications due to finite beam emittance. Results of the magnicon experiment at the Naval Research Laboratory are examined in the light of the analysis     

Screened potential of a test charge in a two-dimensional Bose condensate

We calculate the screening properties of a boson superlattice with distanced between the planes at temperature zero. The screened Coulomb potentialV _s (r,d) of a test charge located in one plane is calculated in the neighboring plane. It is found that this interplane potential is attractive with a maximal atraction at the intraplane distancer _m:V _s (r _m ,d)<0.V _s (r _m ,d) versusd shows a minimum atd _m , indicating a maximal attraction for anoptimal superlattice period d_m. The attraction could give rise topairing of bosons between different planes.     

Modulations of exchange coupling in oxoferrylporphyrin cation radical complexes detected by Mössbauer and EPR measurements

Oxoferrylporphyrin cation radical complexes were generated using the prophyrin dianions: tetrakis 2,6-dichlorophenyl (TDCPP) and tetrakis 2,4,6-trimethoxyphenyl (TTMPP). Spin coupling between ferryl iron (S=1) and porphyrin radical S′=1/2), ligand field interaction and hyperfine parameters of iron were studied by Mössbauer and EPR measurements and corresponding spin Hamiltonian analyses. Samples of [Fe^IV=0 TDCPP], which had to be prepared in CH₂Cl₂, were “vacuum dried” in order to obtain Mössbauer spectra.     

Instability of layered quantum liquids: 4. Intraplane and interplane correlations

We calculate the local-field correctionsG(q, q _z) f electron superlattices with interlayer distanced in order to describe many-body effects. A generalized Hubbard expression for the local-field correction is derived. The sum-rule version of the self-consistent approach for the local-field correction is developed and used to discuss the effects of exchange and correlation. The RPA-parameterr _s and the ratiod/a ^* determine the many-body effects.a ^* is the effective Bohr radius. The stability region for the Fermi liquid behavior of the layered electron gas is discussed: (i) forda ^* the critical RPA-parameterr _sc1 is determined by exchange andintraplane correlation effects, (ii) forda ^* withr _sc1 correlation effects are small and the instability point is mainly determined by exchange effects, (iii) forda ^* withr _sc<1 exchange andinterplane correlation effects are both important for the instability point. Our results are important for high-T _c superconductors, organic superconductors and semiconductor superlattices.     

Investigation of substitution effects and the phase transition in type-I clathrates RbxCs8-xSn44□2 (1.3≤x≤2.1) using single-crystal X-ray diffraction, Raman spectroscopy, heat capacity and electrical resistivity measurements

The substitution of cations in Rb_xCs_8-xSn₄₄□₂(1.3≤x≤2.1) is reported. The compounds crystallize at room temperature in the space group adopting the type-I clathrate 2×2×2 superstructure with partly ordered framework vacancies (□), whereas at higher temperatures they transform to the primitive, more disordered modification (space group ). The guest atom distributions in the Sn cages on the Rb: Cs ratios is studied by means of single-crystal X-ray diffraction for Rb_2.1(1)Cs_5.8(1)Sn₄₄ at T=293 K (1), Rb_1.42(8)Cs_6.58(8)Sn₄₄ at T=293 K (2a), Rb_1.46(5)Cs_6.54(5)Sn₄₄ at T=373 K (2b) and Rb_1.32(8)Cs_6.68(8)Sn₄₄ at T=293 K (3). The structural order-disorder phase transition influences the electrical resistivity. The hysteresis observed for the electrical resistivity in combination with the symmetric shape of the specific heat anomaly suggests that the transformation is of first-order type and is characterized by an entropy change of about 2.5 J mol^-1 K^-1. The Raman spectrum for the low-temperature modification of 2 is also reported.