Two-phonon bound states in imperfect crystals

The question of the occurrence of two-phonon bound states in imperfect crystals is investigated. It is shown that the anharmonicity mediated two-phonon bound state which is present in perfect crystals gets modified due to the presence of impurities. Moreover, the possibility of the occurrence of a purely impurity mediated two-phonon bound state is demonstrated. The bound state frequencies are calculated using the simple Einstein oscillator model for the host phonons. The two-phonon density of states for the imperfect crystal thus obtained has peaks at the combination and difference frequencies of two host phonons besides the peaks at the bound state frequencies. For a perfect crystal the theory predicts a single peak at the two-phonon bound state frequency in conformity with experimental observations and other theoretical calculations. Experimental data on the two-phonon infrared absorption and Raman scattering from mixed crystals of GA_1−c Al _c P and Ge_1−c Si _c are analysed to provide evidence in support of impurity-mediated two-phonon bound states. The relevance of the zero frequency (difference spectrum) peak to the central peak observed in structural phase transitions, is conjectured; This work is a part of the thesis to be submitted by one of the authors (SS) in partial fulfilment of the degree of Doctor of Philosophy to Utkal University, Bhubaneswar, India.     

Effects of adsorbate molecules on the quadrupolar interaction of framework aluminum atoms in dehydrated zeolite H,Na-Y

The effect of adsorbate molecules on the quadrupolar interaction of framework aluminum atoms with the electric field gradient in dehydrated zeolite H,Na-Y has been studied by (27)Al MAS NMR and (27)Al MQMAS NMR spectroscopy at magnetic fields of 9.4 and 17.6 T. Upon adsorption of molecules interacting with bridging OH groups by hydrogen bonds (acetonitrile and acetone), the quadrupole coupling constant of framework aluminum atoms was found to decrease from 16.0 MHz (unloaded zeolite) to 9.4 MHz. Adsorption of molecules, which cause a proton transfer from the zeolite framework to the adsorbates (ammonia and pyridine), reduces the quadrupole coupling constant to 3.8 MHz for coverages of 0.5-2 molecules per bridging OH group. The experiments indicate that the quadrupole coupling constant of framework aluminum atoms in dehydrated zeolite H,Na-Y reflects the chemical state of adsorbate complexes formed at bridging OH groups. In agreement with earlier investigations it was found that a proton affinity of the adsorbate molecules of PA = 812-854 kJ/mol is necessary to induce a proton transfer from the zeolite framework to the adsorbed compounds. This proton transfer is accompanied by a strong improvement of the tetrahedral symmetry of zeolitic framework AlO(4) tetrahedra and a decrease of the electric field gradient.     

Fluctuation induced magneto-conductivity studies in YBa2Cu3O7−δ + xBaZrO3 composite high-Tc superconductors

Fluctuations on the electrical conductivity of polycrystalline YBa₂Cu₃O_7?δ + xBaZrO₃ (x = 1.0, 2.5, 5.0 and 10.0 wt.%) superconductors were investigated from the resistivity vs. temperature data for zero field and 8 T (Tesla) external magnetic fields. Attempts have been made to identify the optimum inclusion of BaZrO₃ (BZO) in YBa₂Cu₃O_7?δ (YBCO) superconductors. The phase formation, texture and grain alignments were analyzed by XRD and SEM techniques. Then the effects of superconducting fluctuations on the electrical conductivity of granular composite superconductors were studied for zero field and 8 T external magnetic fields. Though inclusions of BZO sub-micron particles are not expected to influence superconducting order-parameter fluctuation (SCOPF) much, the transition from 2D to 3D of the order parameter in the mean-field region depends on the BZO content in the composites. It has been observed that BZO residing at the grain boundary of YBCO matrix influences the tailing region without having significant change in the mean-field critical temperature. In the present work, attention has been focused mostly in the experimental domain relatively above the T_c. It reveals that, 1 wt.% composite exhibits a better superconducting property in comparison with pure YBCO.     

Near‐Unity Photoluminescence Quantum Efficiency for All CsPbX3 (X=Cl,Br, and I) Perovskite Nanocrystals: A Generic Synthesis Approach

In a generic synthesis approach, all three CsPbX₃ (X=Cl, Br and I) perovskite nanocrystals having near unity quantum yields is reported. This has been achieved by injecting the desired amount of preformed alkylammonium halide salts which acted as a dual source providing halide ions and the capping agent to an equimolar amount of non‐halide Pb and Cs precursors in a reaction flask at an optimized reaction temperature. The composition sensitivity of Pb to Cs ratio, high temperature reaction, and injection of ammonium halide remained the key parameters for obtaining the high quantum yields. Details of the reaction process, use of different reagents and setting up the reaction parameters are reported.     

Near‐Unity Photoluminescence Quantum Efficiency for All CsPbX3 (X=Cl,Br, and I) Perovskite Nanocrystals: A Generic Synthesis Approach

In a generic synthesis approach, all three CsPbX₃ (X=Cl, Br and I) perovskite nanocrystals having near unity quantum yields is reported. This has been achieved by injecting the desired amount of preformed alkylammonium halide salts which acted as a dual source providing halide ions and the capping agent to an equimolar amount of non‐halide Pb and Cs precursors in a reaction flask at an optimized reaction temperature. The composition sensitivity of Pb to Cs ratio, high temperature reaction, and injection of ammonium halide remained the key parameters for obtaining the high quantum yields. Details of the reaction process, use of different reagents and setting up the reaction parameters are reported.     

Classical φ6-field theory in (1+1) dimensions. A model for structural phase transitions

The classical φ⁶-field theory in (1+1) dimensions, is considered as a model for the first order structural phase transitions. The equation of motion is solved exactly; and the presence of domain wall (kink) solutions at and below the transition point, in addition to the usual phonon-like oscillatory solutions, is demonstrated. The domain wall solutions are shown to be stable, and their mass and energies are calculated. Above the transition point there exists exotic unstable kink-like solutions which takes the particle from one hill top to the other of the potential. The partition function of the system is calculated exactly using the functional integral method together with the transfer matrix techniques which necessitates the determination of the eigenvalues of a Schrödinger-like equation. Thus the exact free energy is evaluated which in the low temperature limit has a phonon part and a contribution coming from the domain wall excitations. It was shown that this domain wall free energy differs from that calculated by the use of the domain wall phenomenology proposed by Krumhansl and Schrieffer. The exact solutions of the Schrödinger-like equation are also used to evaluate the displacement-displacement, intensity-intensity correlation functions and the probability distribution function. These results are compared with those obtained from the phenomenology as well as the φ⁴-field theory. A qualitative picture of the central peak observed in structural phase transitions is proposed.     

An organically templated Co(II) sulfate with the kagome lattice

An amine-templated cobalt(II) sulfate with the kagome lattice, prepared for the first time, exhibits magnetic properties comparable to those of the analogous Fe(III) compounds.     

Anharmonic oscillator model for first order structural phase transition

Exact solutions for the motion of a classical anharmonic oscillator in the potentialV(φ)=Bφ ² ? |A|φ ⁴ +Cφ ⁶ are obtained in (1 + 1) dimensions. Instanton-like solutions in (imaginary time) which takes the particle from one maximum of the potential to the other are obtained in addition to the usual oscillatory solutions. The energy dependence of the frequencies of oscillation is discussed in detail. This can be used as a model for the first order structural phase transition in the mean field approximation. The high and low temperature behaviour of the static susceptibility is obtained. Finally, a qualitative explanation is offered for the observed central peak in ferroelectrics like SrTiO₂.     

Measurement of the CP violation parameter sin2 phi(1) in B(0)(d) meson decays

We present a measurement of the standard model CP violation parameter sin2 phi(1) (also known as sin2beta) based on a 10.5 fb(-1) data sample collected at the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric e(+)e(-) collider. One neutral B meson is reconstructed in the J/psiK(S), psi(2S)K(S), chi(c1)K(S), eta(c)K(S), J/psiK(L), or J/psipi(0) CP-eigenstate decay channel and the flavor of the accompanying B meson is identified from its charged particle decay products. From the asymmetry in the distribution of the time interval between the two B-meson decay points, we determine sin2 phi(1) = 0.58(+0.32)(-0.34)(stat)+0.09-0.10(syst).