On the Bose-Einstein correlation in inelastic collisions of nuclei

The Bose-Einstein correlation function for two pions of same charge produced in a high energy nucleus-nucleus collision is computed in the framework of the reggeon theory. We obtain a coherence parameter λ=1 and a transverse radius of the sourceR _T close to the radius of the smaller of the colliding nuclei.

     

Relaxation times and line widths of isotopically-substituted nitroxides in aqueous solution at X-band

Optimization of nitroxides as probes for EPR imaging requires detailed understanding of spectral properties. Spin lattice relaxation times, spin packet line widths, nuclear hyperfine splitting, and overall lineshapes were characterized for six low molecular weight nitroxides in dilute deoxygenated aqueous solution at X-band. The nitroxides included 6-member, unsaturated 5-member, or saturated 5-member rings, most of which were isotopically labeled. The spectra are near the fast tumbling limit with T₁∼T₂ in the range of 0.50–1.1 μs at ambient temperature. Both spin–lattice relaxation T₁ and spin–spin relaxation T₂ are longer for ¹⁵N- than for ¹⁴N-nitroxides. The dominant contributions to T₁ are modulation of nitrogen hyperfine anisotropy and spin rotation. Dependence of T₁ on nitrogen nuclear spin state m_I was observed for both ¹⁴N and ¹⁵N. Unresolved hydrogen/deuterium hyperfine couplings dominate overall line widths. Lineshapes were simulated by including all nuclear hyperfine couplings and spin packet line widths that agreed with values obtained by electron spin echo. Line widths and relaxation times are predicted to be about the same at 250 MHz as at X-band.     

Relaxation effects in weakly-coupled three spin systems by double resonance

Nuclear magnetic double-resonance spectra of weakly-coupled spin systems exhibit several interesting features. The effect of relaxation on the symmetry properties of double resonance spectra of weakly-coupled spin systems is investigated using spin inversion operators. The double-resonance spectra of the AX₂ type systems formed by the protons in (I) 1,1,2-trichloroethane and in (II) 2,2-dichloroethanol (excluding the hydroxyl proton) exhibit these symmetry effects. In both these molecules relaxation is contributed by several mechanisms. In sample II the effect of the exchanging proton in the hydroxyl group is also considered and the exchange time is estimated.     

1H NMR relaxation investigation of herbicides interacting with photosystem II preparations fromSpinacia oleracea L.

The interaction of three common herbicides, paraquat, acifluorfen and alachlor, with spinach chloroplast photosystem II (PS II) was investigated by measuring¹H nuclear magnetic resonance spin-lattice relaxation rates, transient nuclear Overhauser effect (NOE) and NOE spectroscopy (NOESY) spectra. Binding to PS II was detected by (i) the enhancement of single-selective relaxation rates and (ii) the decrease in the optimal mixing time providing maximal cross-peak intensity in NOESY spectra. Titration of relaxation enhancements was used to calculate the dissociation constants (K _d) from the bound state for paraquat (K _d = 292 ± 71 μM⁻¹) and acifluorfen (K _d = 311 ± 58 μM⁻¹). A similarK _d was apparent for alachlor. Double-selective relaxation rates allowed the isolation of dipolar relaxation terms between selected proton pairs wherefrom dynamic features of the bound state were evaluated. In all cases the motional correlation time of bound herbicide (τ_c = 0.1−0.4 ns at 300 K) was found two orders of magnitude slower than in the free-solution state. In the case of alachlor the E and Z isomers were observed to bind differently to PS II and a change in conformation could be hypothesized.     

Nonreciprocal light birefringence in the R 3B7O13 X Boracites (R=Co, Cu, Ni; X=I, Br)

The field and angular dependences of nonreciprocal birefringence (NB), which is linear in magnetic field B and is due to magnetic-field-induced spatial dispersion, have been studied in the cubic (symmetry class T _d) paraelectric phase of the R ₃B₇O₁₃ X boracites (R=Co, Cu, Ni; X=I, Br) at a wavelength λ=633 nm. It is shown that the NB in crystals with different 3d and halogen ions exhibits the same anisotropy. The relation between the A and g parameters, A=2g, which determine the NB anisotropy, suggests that the microscopic mechanism of the NB is the manifestation of second-order magnetoelectric susceptibility at optical frequencies.     

Influence of magnetic impurities on proton spin relaxation of water in clay

The¹H nuclear magnetic spin relaxation of water in slurry of kaolin clay was investigated in the presence of magnetite (black iron oxide, Fe₃O₄) at 0.2 T and room temperature. The water spectra at high magnetite contents showed two different resonances, presumably from surface-associated water and free interstitial water. The difference in observed resonance frequencies increased as much as 200 ppm with increasing magnetite content. The apparent nuclear magnetic resonance intensity decreased biexponentially as a function of magnetite added. The observedT ₂^* values at low magnetite contents were in accordance with the predicted values from the resonance intensities and the estimated magnetic susceptibilities. TheT ₁ relaxation was multiexponential in character, so a uniform penalty program was used for the analysis of distribution. At 0.2 T for¹H, kaolin slurry containing less than 5.5 ppm magnetite did not differ significantly from magnetite-free clay in the longitudinal relaxation rates of water. However, higher concentrations of magnetite produced features in theT ₁ distribution significantly different from those of magnetite-free clay. TheT ₂ could be approximated by monoexponential relaxation, probably because the fast-decaying components relaxed before they could be recorded. The apparent transverse relaxation ratesR ₂ increased linearly as a function of magnetite content. On the basis of the comparison of spin-echo and Carr-Purcell-Meiboom-Gill data, an empirical relation was derived to describe the signal loss due to diffusion. It can be expressed by a power function of magnetite amount, which is multiplied by the sum of volume-dependent and volume-independent terms.     

Effect of Ca substitution on structural,magnetic and dielectric properties of BiFeO3

Bi_1-xCa_xFeO₃ (x = 0.40, 0.50) compounds are synthesized by conventional solid-state reaction method. The effect of Ca²⁺ substitution is investigated on structural, dielectric and magnetic properties. Rietveld refinement confirms that crystal structures of both the samples are tetragonal with P4mm symmetry. The highest values of remnant magnetization (M_r ) and coercive field (H_c ) are 0.002 emu/g and 0.23 kOe, respectively, for x = 0.50. The electric modulus formalism is used to distinguish and separate out the relaxation process which is dominated by the transport phenomenon. The relaxation process has activation energy ～0.32 eV which is found to be related to both the hopping mechanisms, i.e. electronic and oxide ions.     

19F double resonance in the presence of chemical exchange

The ¹⁹F N.M.R. spectra of CF₂BrCClBr₂ in CS₂ from 20°c to -80°c are reported. Computer-fitting of the experimental spectra using the density matrix formalism of Alexander yields the chemical exchange correlation times and produces a computed internal rotation barrier for trans →gauche of 39·7 ± 0·5 kJ mol^-1 (9·5±0·1 kcal/mole). Results of double resonance studies on the -80°c spectrum upon irradiation of the various peaks with a strong r.f. field are presented. The density matrix equations in the presence of chemical exchange and several relaxation mechanisms were solved to obtain theoretical changes in peak intensities on irradiation. The indirect spin saturation data and their relation to the rate of internal rotation and the relaxation mechanism are discussed. Random field relaxation is shown to be the important relaxation mechanism in the system. The strong irradiation of one component of a strongly coupled spin multiplet with intramolecular exchange causes indirect saturation of the transitions connecting energy levels that have the same m _I values as the one being irradiated whereas the intensities of other transitions are only slightly affected. It appears therefore to be an important complement to the spin-tickling technique in determining energy level diagrams and relative signs of coupling constants.     

The analysis of complex nuclear magnetic resonance spectra

The theoretical scheme described in Part I for the analysis of nuclear resonance spectra of the type ABX _q is extended to systems AB₂X _q where the symmetry is such that there is only one BX coupling constant. It is shown that these spectra can be interpreted in a systematic manner for all q. The spectra allow determination of the relative signs of all three coupling constants and can therefore be used to compare couplings between nuclei of different species (AX and BX) with the coupling between nuclei of the same species (AB). Further systems which could be studied by similar methods are indicated briefly.     

Nuclear Magnetic Resonance Relaxation Study of the Phase Transitions of Rb2CuCl4·2H2O and Cs2MnCl4·2H2O Single Crystals

The physical properties and phase transitions of Rb₂CuCl₄·2H₂O and Cs₂MnCl₄·2H₂O crystals were investigated by performing ⁸⁵Rb, ⁸⁷Rb, and ¹³³Cs nuclear magnetic resonance relaxation analyses. The temperature dependences of the spectra and the spin–lattice relaxation times T ₁ near T _C are related to changes in the symmetry of the octahedrons consisting of four Cl⁻ ions and two water molecules around Rb⁺ or Cs⁺; the forms of these octahedrons are disrupted by the loss of H₂O. The difference between the relaxation times of the two crystals is possibly due to the difference between the electron structures of the Cu and Mn ions. Cu²⁺ has nine valence electrons in its 3d orbital, whereas Mn²⁺ has five valence electrons in its 3d orbital.     

Current commutators for the non-linear σ-model with Wess-Zumino term

We consider equal-time commutation relations of chiralSU(N) _L ×SU(N) _R charge densities in the non-linear σ-model. These commutators are derived using the cocycle formalism and from the usual canonical theory. Both methods give the same result. The charge density commutator of the symmetry currents contains operator valued Schwinger terms arising from the Wess-Zumino term.     

Observation of sharp resistance anomalies near Tc in granular superconductors

Granular films prepared from well-defined In clusters embedded in Kr (? 10-40 at % Kr) show sharp resistance anomalies very close to the superconducting transition temperature. The observed resistance peak(s), which go(es) above the normal state resistance R_N and which can be as high as ? 1.5 R_N, can be explained by a “mesoscopic” effect: Superconducting percolation aggregates of size ξ_ρ, larger than the superconducting coherence length ξ_S, but smaller than the quasiparticle relaxation length λ_Q*, have a superconducting gap, but are not yet able to carry a supercurrent.     

Separation of cross-relaxation and exchange in two-site spin systems without resolved couplings

Variable temperature two-dimensional nuclear Overhauser enhancement experiment (2-D NOESY) is used to extract the rate constants and cross-relaxation rates that contribute to the same cross-peaks in NOESY spectra. Rate constants (k _AB) and cross-relaxation rates (R _AB) for two-site spin systems are related to the ratio between the cross-peak and diagonal peak integrals (F) by the expression:R _AB -k _AB = (1/2τ _m)ln[(1 -F)/(1 +F)], where τ_m is the mixing time. As a model, we investigated the exchange processes in a system of dimer calix[4]arenes of C_4v symmetrical configuration with guest inclusion (benzene or benzene-d₆), where the measurement of exchange processes is hindered by the presence of strong nuclear Overhauser enhancement between protons in adjacent aromatic rings in the cone conformation of the calix[4]arene.     

Singularities in Weyl gravitational fields

The peculiarities of the scalarS ≡R _ijkl R ^ijkl are exhibited for two axially-symmetric static (Weyl) gravitational fields. By examiningS along curved families of trajectories to the Weyl singularities, examples are found which contradict previous claims by Gautreau and Anderson regarding ‘directional singularities’. Proper circumferences about the Bach and Weyl line-mass singularity are also examined. There is no apparent correlation between the source structure and the behaviour ofS from this analysis.     

Crystal field effect on X-ray magnetic circular dichroism spectra at the L<Subscript>2,3</Subscript> absorption edges of mixed-valence Ce and Yb compounds in high magnetic fields

A theoretical study is presented, with an extended single impurity Anderson model, for the crystal field effect on the X-ray magnetic circular dichroism (XMCD) spectra at L_2,3 edges of mixed-valence Ce and Yb compounds in high magnetic fields. The crystal field acting on the 4f electrons is assumed to have cubic symmetry. Due to the competition among the effects of crystal field, mixed valency, and external magnetic field, the magnetic-field-dependence of XMCD spectra exhibits a variety of features; for instance, the branching ratio, R(L₂/L₃), of L₂ and L₃ XMCD intensities of Ce compounds can take R(L₂/L₃) > 1.0 and <1.0, and that of Yb compounds can take R(L₂/L₃) > 0 and <0. It is shown that the magnetic-field-dependence of the total XMCD intensity I(L₂ + L₃) is proportional to the magnetization curve, but that of R(L₂/L₃) gives more precise information on the ground state wavefunction in magnetic fields. A new and useful method to correlate the XMCD spectra, the 4f magnetization and the ground state wavefunction is proposed and used to discuss the relation between I(L₂ + L₃) and the magnetization curve and that between R(L₂/L₃) and the ground state wavefunction.     

Effect of iron doping at Mn-site on complex impedance spectroscopy properties of Nd0.67Ba0.33MnO3 perovskite

The effect of Fe-doping at Mn-site on the structural and electrical properties of Nd_0.67Ba_0.33Mn_1?xFe_xO₃ (0 ≤ x ≤ 0.05) perovskites has been investigated. X-ray diffraction patterns show that the structural parameters change slightly due to the fact that the Fe³⁺ ions replacing the Mn³⁺ have similar ionic radius. The electrical properties of these samples have been investigated using complex impedance spectroscopy technique. a function of the frequency at different temperatures. When increasing the Fe-content, a decrease of dc conductivity was observed throughout the whole explored temperature range and the deduced activation energy values are found to increase from 128 meV for x = 0 to 166 meV for x = 0.05. The curves of the imaginary part of impedance (Z″) show the presence of relaxation phenomenon in our samples. The complex impedance spectra show semicircle arcs at different temperatures and an equivalent circuit of the type of R_g + (R_gb//C_gb) has been proposed to explain the impedance results.     

Theoretical predictions about ENDOR of biradicals in liquid solution

Starting from the equation of motion for the density matrix, ENDOR spectra are computed for a biradical for the first time. The model biradical carries two electronic spins S = 1/2 and two nuclear spins I = 1/2. The relaxation operator is calculated ab initio for three distinct relaxation mechanisms. These mechanisms are the modulation of the isotropic exchange interaction, the hyperfine tensor and the zero-field splitting tensor. The dependence of the ENDOR effect on the amplitudes of the radiofrequency and microwave fields, on the strength of the zero-field splitting tensor, and on the correlation times of the correlation functions is investigated.     

General SU(2)L × SU(2)R × U(1)EM Sigma Model with External Sources, Dynamical Breaking and Spontaneous Vacuum Symmetry Breaking

We give a general SU(2) _L × SU(2) _R × U(1) _EM sigma model with external sources, dynamical breaking and spontaneous vacuum symmetry breaking, and present the general formulation of the model. It is found that σ and π⁰ without electric charges have electromagnetic interaction effects coming from their internal structures. A general Lorentz transformation relative to external sources is derived, using the general Lorentz transformation and the four-dimensional current of nuclear matter of the ground state with J _gauge = 0, we give the four-dimensional general relations between the different currents of nuclear matter systems with J _gauge≠ 0 and those with J _gauge = 0. The relation of the density’s coupling with external magnetic field is derived, which conforms well to dense nuclear matter in a strong magnetic field. We show different condensed effects in strong interaction about fermions and antifermions, and give the concrete scalar and pseudoscalar condensed expressions of σ₀ and π₀ bosons. About different dynamical breaking and spontaneous vacuum symmetry breaking, the concrete expressions of different mass spectra are obtained in field theory. This paper acquires the running spontaneous vacuum breaking value σ′₀, and obtains the spontaneous vacuum breaking in terms of the running σ′₀, which make nucleon, σ and π particles gain effective masses. We achieve both the effect of external sources and nonvanishing value of the condensed scalar and pseudoscalar paticles. It is deduced that the masses of nucleons, σ and π generally depend on different external sources. PACA numbers: 24.10.-i, 11.30.Qc     

E.S.R. evidence for the planarity of the t-butyl radical

Monte Carlo calculations are reported for the radial distribution function g ₂(r; λ) of a fluid in which the intermolecular pair potential is [u ^ref(r) + λu ^p(r)], u ^ref(r) being the Weeks-Chandler-Andersen (WCA) reference fluid, and [u ^ref(r) + u ^p(r)] being the Lennard-Jones (6, 12) fluid. The calculations are performed for λ values in the range 0 to 1, at the state condition ρσ³ = 0·80, kT/ε = 0·719. It is shown that at high densities the perturbation expansion of g ₂(r; λ = 1) about g ₂(r; λ = 0) is rapidly convergent, but that the corresponding expansion for y ₂(r; λ) = exp [βu(r; λ)] × g ₂(r; λ) is not. In addition Monte Carlo estimates of the individual terms that contribute to the first-order perturbation term, (?g ₂/?λ)_λ=0, are presented. It is shown that these terms are individually large, but that (?g ₂/?λ)_λ=0 is small because there is strong cancellation between the various terms. Consequently, the calculation of (?g ₂/?λ)_λ=0 is highly sensitive to the approximation used to evaluate the individual terms.     

Test for the influence of relaxation on Mössbauer spectra of (Fe0.65Ni0.35 1−x Mn x alloys by the sharpening method

A mathematical procedure which gives an opportunity to distinguish between relaxation and static mechanisms of the Mössbauer line broadening is developed. It is based on the method of Mössbauer line sharpening developed recently and allows one to answer unambiguously the question whether or not relaxation manifests itself in Mössbauer measurements by examinign the wings of the spectrum. The procedure was applied to the spectra of (Fe_0.65Ni_0.35)_1–x Mn _x alloys withx=0 (INVAR) andx=0.047, where the mechanism of the line broadening has not been clarified so far. An analysis of the spectra shows that in the temperature range 4 to 300 K no relaxation is observed and the line broadening is mainly caused by the distributions of magnetic hyperfine fields.