A re-examination of the induction (crystal field) energy contribution to the relative stabilities of the f.c.c. and h.c.p. rare-gas crystal structures

A new calculation of the induction (crystal field) energy of the f.c.c. and h.c.p. structures of the rare-gas crystals of neon and argon gives an energy in favour of f.c.c. which is more than sufficient to counter the summed pair-potential which favours h.c.p. Predicted values of (E _f.c.c. - E _h.c.p.)/E _f.c.c. for neon and argon are 0·9 × 10^-3 and 1·3 × 10^-3 respectively. These values are similar to those that have been obtained for a modified dispersion energy and it is concluded that they should be added to these dispersion energies. Calculations on solid helium show a negligible difference in the induction energy for the two close-packed structures.     

Mean-square atomic displacements in f.c.c. crystals

The mean-square displacements of volume atoms, surface atoms and atoms near a vacancy are calculated for f.c.c. metals and rare gas solids, using a simple series expansion of the Wallis formula. We employ a model with rotationally invariant bond bending forces, unlike the de Launay model used by Masri and Dobrzynski. For atoms near a vacancy and for rare gas solids this method has been applied for the first time. The calculations are done to the R⁴ term in the expansion. The inclusion of the R⁴ term together with temperature-dependent elastic constants give results which are in better agreement with experiment. For rare gas solids Lindemann ratios calculated by this method agree closely with the values obtained from elaborate computations as well as those deduced from entropy data. The present calculation also supports the view that the melting of solids is initiated at the crystal surfaces and in the neighbourhood of vacancies.     

Magnetic properties of CoFeRu alloys in the f.c.c. and h.c.p. phases

Measurements have been made of the magnetization, the Fe⁵⁷ Mössbauer effect, and the crystal structures of Co-rich CoFeRu alloys. The alloys with f.c.c. structure behave in a typical ferromagnetic way, while those with h.c.p. structure show non-saturation of the magnetization in magnetic fields as high as 80 kG, and they show relatively small magnetic moments per atom and a broad single Mössbauer absorption.     

Search for the ρρ enhancement near 1700 MeV/c in the reaction at 8.3 GeV/c

Results are presented from a search for a ρ⁰ρ⁰ enhancement in antiproton annihilations. A ρ⁰ρ⁰ resonance was recently observed in radiative ψ decays, and its existence has been supported by the results of an antiproton experiment at 5.7 GeV/c. No indication of this ρ⁰ρ⁰ enhancement is seen is our data, in direct contradiction with the earlier, lower statistics experiment.     

The density of states of b.c.c. Nb3Au: A comparison with A-15 Nb3Au

When Nb₃Au is converted from the A-15 structure to the b.c.c. allotrope, a sharp decrease in the superconducting transition temperature, T_c, occurs (11.1 to 1.5 K). To study possible causes for this sharp decrease, specific heat measurements have been made on the b.c.c. form of Nb₃Au yielding values of γ=19.2 mJ/mole-K², β=0.274 mJ/mole-K⁴ and θ_D=305 K. These parameters compared with published values for the A-15 Nb₃Au indicate that a sharp drop in the density of states at the Fermi level, N(0), occurs when the A-15 structure is converted to the b.c.c. form.     

Orientational anisotropy of the CN- ions in f.c.c. NaCN: an optical spectroscopy measurement

We present a new method for measuring the anisotropy of the orientational probability function of CN^- ions in a f.c.c. NaCN crystal. The results are discussed and compared with previous neutron diffraction measurements.     

A.c. conductivity in AsF5 doped polyphenylacetylene (PPA)

The a.c. conductivity of semiconducting cis-cisoid polyphenylacetylene (PPA) pellets has been studied at temperatures between 230 and 290 K and frequencies, ?, from 37 to 10⁵ Hz. The a.c. conductivity (σ_a.c.) is found to be strongly temperature dependent. σ_a.c. is proportional to ?^s with s varying from 0.45 to 0.75 as temperature is raised from 230 to 290 K. Both frequency dispersion and strong temperature dependence of σ_a.c. are best explained by the mechanism of hopping conduction in the band-tails.     

Pressure dependence of the Néel temperature of f.c.c. stainless steel

The Néel temperature T_N of f.c.c. stainless steel has been measured as a function of pressure using the Mössbauer effect. A sample with approximately 74 wt.% Fe, doped with ^57Co, and having T_N = 58.4 ± 0.1 K at zero pressure, shows T_N rising with pressure with increasing slope. Our results indicate that T_N is raised to room temperature by a pressure of 175 ± 5 kbar.     

Evidence for tunnelling relaxation in the a.c. conductivity of chalcogenide glasses

The a.c. conductivity of chalcogenide glasses is well fitted at low temperature by the expression Σ_a.c.=Aω^sTⁿ where s=1 and n≤0.5. This expression is consistent with correlated barrier hopping (CBH) of bipolarons between both random and paired defects. However, a study of the temperature and frequency dependence of the capacitance shows that in a-SiO₂ and a-GeSe₂ the capacitance obeys the scaling relation: C=A ln (Tω^?1). This result would suggest that the a.c. conduction mechanism in chalcogenide glasses may be better described by tunneling relaxation than by CBH.     

B.c.c.-f.c.c. allotropy, F-bands,and metallization in xenon and krypton

APW-Xα calculations of a f.c.c.-b.c.c. phase transition in Xe prior to metallization give P ? 950 kbar, contrary to the 820 kbar result of Hama and Matsui; their use of non-touching APW spheres (f.c.c.) may be the crucial difference. Hartree-Fock plus correlation calculations in Kr indicate that f-bands play no role in rare gas crystal metallization.     

An anomalous -ω interference in the reaction ee → 3π

It is shown that in the π⁺π⁻ meson mass-spectrum in the reaction e⁺e⁻ → π⁺π⁻π⁰ at 2E > 1 GeV one should expect the anomalously large effect of the -ω interference. It can be used as a tool for the elucidation of the details of the e⁺e⁻ → Vπ reaction dynamics and as the additional possibility for the determination of the electromagnetic -ω mixing parameters.     

A.c. conductivity and optical absorption in chalcogenide glasses

The energy levels of a simple model describing the doubly occupied defect states in chalcogenide glasses are calculated when strong electron-phonon interaction is supposed. The Kubo formula is used to get a.c. conductivity. The model implies a connection between the power dependence of a.c. conductivity ^s and between the low-energy exponential absorption tail.I would like to thank Dr. E. Majerníková for advice and encouragement throughout this work.     

Hall effect studies in ytterbium through the f.c.c.-h.c.p. transformation

The Hall coefficient (R_H) of polycrystalline Yb showing the f.c.c.-h.c.p. transformation has been determined at 20 koe in the range 1.8–420°K. Both the structural and the associated magnetic transformation would separately be expected to modify R_H, but the evidence suggests that the observed behaviour through the transition arises primarily from the effects of the change in structure. A more detailed interpretation is presently not possible, however, because of the lack of knowledge of the band structure of h.c.p. Yb.     

Upper limits on resonance contributions to proton-proton elastic scattering in the c.m. mass range 2.05-2.85 GeV/ c2

Recently published excitation functions in proton-proton (pp) elastic scattering observables in the laboratory energy range 0.5-2.5GeV provide an excellent data base to establish firm upper limits on the elasticities η_el = Γ_el/Γ_tot of possible isovector resonant contributions to the nucleon-nucleon (NN) system. Such contributions have been predicted to arise from dibaryonic states, with c.m. masses between 2.1-2.9GeV/c², but have not been confirmed experimentally. A method to determine quantitatively the maximum value of η_el compatible with experimental data is presented. We use energy-dependent phase shift fits to the pp data base to model the non-resonant interaction. Based upon the differential cross-section data measured by the EDDA Collaboration an unbiased statistical test is constructed to obtain upper limits on η_el, that exclude larger values with a 99% confidence level. Results in the c.m. mass range 2.05-2.85GeV/c² and total widths of 10-100MeV/c² in the partial waves ¹S₀, ¹D₂, ³P₀, ³P₁, and ³F₃ are presented and discussed.     

Study of the process ee→ππγ in c.m. energy range 600–970 MeV at CMD-2

The cross section of the process e⁺e⁻→π⁰π⁰γ has been measured in the c.m. energy range 600–970 MeV with the CMD-2 detector. The following branching ratios have been determined:

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and

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. Evidence for the ρ⁰→f₀(600)γ decay has been obtained:

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. From a search for the process e⁺e⁻→ηπ⁰γ the following upper limit has been obtained: at 90% CL.     

The decay and saturation of the emission in a.c. EL ZnS:Mn thin films

The decay of the yellow emission of a.c. ZnS:Mn electroluminescent cells is faster at high brightness levels. A corresponding quenching of the brightness is seen. This quenching is observed at much lower current densities than would be expected from the Auger coefficient 2X10^-9 cm³ s^-1.     

How learn the branching ratio X(3872) \to D*^0 \bar D^0 + c.c.

Enfant terrible of charmonium spectroscopy, the resonance X(3872), generated a stream of interpretations and ushered in a new exotic XYZ spectroscopy. In the meantime, many (if not all) characteristics of X(3872) are rather ambiguous. We construct spectra of decays of the resonance X(3872) with good analytical and unitary properties which allows to define the branching ratio of the \(X(3872) \to D*^0 \bar D^0 + c.c.\) decay studying only one more decay, for example, the X(3872) → π⁺π^? J/ψ(1S) decay. We next define the range of values of the coupling constant of the X(3872) resonance with the \(X(3872) \to D*^0 \bar D^0 + c.c.\) system. Finally, we show that our spectra are effective means of selection of models for the resonance X(3872).     

Electronically induced lattice instabilities in b.c.c. Zr

Electronically induced lattice instabilities in metals are discussed by means of an accurate determination of the generalized susceptibility function, χ(q), from a KKR band structure and Fermi surface determined for b.c.c. Zr. We describe how the detailed behavior of χ(q) for b.c.c. Zr may be related to the omega phase upon alloying with Nb or upon application of pressure to the b.c.c. lattice.     

Kaon production in\bar pp interactions at c.m. energies from 200 to 900 GeV

A detailed analysis ofK _s ⁰ production in \(\bar pp\) |<2.5 the average transverse momentum is found to be 0.53±0.07 GeV/c at 200 GeV and 0.62±0.08 GeV/c at 900 GeV, which is an increase with respect to data at c.m. energies below 60 GeV. TheK _s ⁰ production cross sections in inelastic collisions are 29±4 mb at 200 GeV and 63±6 mb at 900 GeV, showing an increase compared to lower energy data. The central kaon density is found to increase as a logarithmic function of energy. At 900 GeV, where statistics are sufficient to allow one to draw conclusions, the average transverse momentum is higher in events with large charged multiplicity than in events with low multiplicity.