Inelastic neutron scattering study of magnetic excitations in the helimagnetic and antiferromagnetic phases of NiBr2

The spin wave dispersion in NiBr₂ has been studied by medium and long wavelength inelastic neutron scattering in the [1 1 0], [1 0 0] and [0 0 1] directions at 4.2 and 30 K, i.e. in the incommensurate helical and collinear antiferromagnetic phases. The values of the intralayer Heisenberg exchange constant J_ij and XY anisotropy constant D at 4.2(30) K are J₀₁ 0.379(1)(0.379(1)), J₀₂ 0.0036(50)(0.0036(50)), J₀₃ - 0.105(5) (?0.105(5)), J′ - 0.0423(50)(?0.389(50))D 0.0364(50)(0.0290(50)), where J′; is the interlayer exchange constant. In fitting the 4.2 K data account is taken of the co-existence of three equivalent domains and of intensity arising from $\text{ω(}\text{q}\text{)}$ and $\text{ω(}\text{q ± k}{}_0\text{)}$ where $\text{k}{}_0$ is the wavevector of the helix. In the low frequency region of the dispersion curve such peaks are resolved. The results reinforce the hypothesis that in zero-field the commensurate-incommensurate phase transition is driven by fluctuations.     

Resonance production in the reaction π±p→Ks0K±p at 30 and 50 GeV/c

The mass and momentum transfer spectra of the charged $\text{K}\text{K}$ system produced in the reaction π^±p→K_s⁰K^±p are analyzed. The data have been collected at the CERN SPS with the Geneva-Lausanne two-arm, non-magnetic spectrometer at 30 and 50 GeV/c incident momenta. The general features of the reactions at these energies and the results of partial-wave analyses of the two kaon system are presented.The channel is dominated by the diffractive production of even spin resonances. The spin 4 recurrence of the A₂(1320) is clearly observed at 2040 MeV (Γ=380 MeV. A new resonance is observed with a mass M=2450MeV and a width Γ=400 MeV; the quantum numbers of this state are found to be I^G(J^PC)=1^?(6++). The analysis also shows the decay of the decay of the meson ?′(1600) through the $\text{K}\text{K}$ channel at both energies.The production amplitudes are determined both as a function of the $\text{K}\text{K}$ effective mass and of the momentum transfer. Isoscalar natural parity exchange is dominant. The energy dependence between 10 and 50 GeV/c is shown to be well described by a Regge pole model based on the f-dominated pomeron hypothesis. We compare the production mechanisms of the 2⁺ resonances A₂(1320) and K¹(1430). Finally, we estimate the $\text{K}\text{K}$ branching ratios of the spin 4 A₂(2040) and spin 6 A₂(2450) resonances.     

Analysis of the high-temperature spin-glass susceptibility: Determination of the local magnetic exchange

New measurements are reported of the magnetic susceptibility above the freezing temperature T_f for noble-metal spin-glasses with $\text{1}\text{2}$ to 6 at.% Fe or Mn. The susceptibility for T_f<T?5 T_f is not Curie-Weiss, but local magnetic correlations manifest themselves and provide a key for the determination of the exchange interactions in spin-glasses. The exchange parameters J_n are resolved up to 5 neighbors for $\text{Au}$Fe, $\text{Cu}$Mn, $\text{Au}$Mn and $\text{Pt}$Mn through a configuration ensemble calculation which includes atomic short range order.     

A comparison of πππ, Kππ, πKKKK system produced in the reactions meson +p→3 mesons + p

A comparison is made of the low-mass three-meson systems (πππ), (Kππ), $\text{(π}\text{K}\text{K}\text{)}$ and $\text{(}\text{K}\text{K}\text{K}\text{)}$ diffractively produced in the reaction meson + proton → three mesons + proton. Several striking similarities and a few important differences are observed: (i) the reactions are consistent with the assumption that the three mesons decay entirely into a 0^? meson and a 0⁺, 1^? or 2⁺ resonance; (ii) the three-meson mass spectra have a peak ≈ 250 MeV above the effective threshold M_eff of the dominant decay mode and then fall off approximately as (mass)^?3;(iii) the average spin 〈J〉 = 0.55 + 1.1 Q_eff, where Q_eff = M - M_eff; (iv) the average orbital angular momentum 〈l〉 increases according to 〈l〉 = 0.75 Q_eff; (v) the three-meson states are produced dominantly in unnatural spin-parity states and no evidence for their being resonant is found; (vi) the only natural spin-parity states found are the well-established 2⁺ resonances A₂ and K¹ (1420); they have similar properties to the non-resonant unnatural parity states except for a dip at t = 0 in the dσ/dt distributions; (vii) both the unnatural and natural spin-parity states are produced mostly by an exchange of natural parity; (viii) there is evidence for two types of production mechanism with different polarization properties, one approximately conserving helicity in the t-channel and the other in the s-channel.     

A study of ππ → NN partial-wave amplitudes using hyperbolic dispersion relations

We investigate $\text{ππ →}\text{N}\text{N}$ partial-wave amplitudes, using a spin separation method based on hyperbolic dispersion relations. Partial-wave amplitudes with J ? 3 are dominant in the pseudophysical region between the ππ and $\text{N}\text{N}$ thresholds, but we find clear evidence for J = 4 and J = 5 contributions from regions near and above the $\text{N}\text{N}$ threshold. We isolate J = 2 and J = 3 partial waves and determine the couplings of f₀(1270) and g (1680). Knowing the high-spin contributions, we are able to eliminate thse and to study s- and p-waves. We find evidence for small p-wave contributions above the ?, having the same sign as the ? contributions. We develop methods for determining the I = J = 0 ππ scattering length a₀⁰ and find a₀⁰ = 0.30 ± 0.15.     

Magnetic hysteresis studies on slow cooled and quenched CuxZn1−xFe2O4 system

Magnetisation for both slow cooled and quenched samples of Cu_xZn_1?xFe₂O₄ as a function of Zn content has shown a maximum at x = 0.6 and a decrease for x0.6. The increase of magnetisation is explained on the basis of Neel's two sub-lattice model while the decrease of magnetisation is explained on a three sub-lattice model where $\text{r}{}_{\mathrm{ex}}\text{=J}{}_{\mathrm{bb}}\text{S}{}_{\mathrm{b}}\text{/J}{}_{\mathrm{ab}}\text{S}{}_{\mathrm{a}}\text{3}\text{4}$. Quenched samples showed higher magnetisation than the slow cooled ones and this increased with the increase of temperature of quenching. The cation transfer between the two sites, characteristic of the temperature, that can be frozen-in seems to govern this. Variation of M_r/M_s with the content of Zn for both slow cooled and quenched samples indicated more impedence to the domain wall motion or higher Zn content. As the temperature of quenching is increased M_r/M_s decreases and this is attributed to detect cluster formation.     

Anhydrous cuprous chloride: The paramagnetic phase

Anhydrous cuprous chloride is monoclinic, space group: C 2/m. We can consider it as being a layered compound consisting of pseudohexagonal layers in the (001) plane. Nevertheless, the symmetry deviation and the uncommon position of the pseudohexagonal environment symmetry axis do not lead to a unaxial anisotropy, contrary to the case of CuF₂ · 2H₂O. If we compare the experimental results with the isotropic quadratic 2d planar Heisenberg model with spin $\text{1}\text{2}$ (lines [1], we find $\text{J}\text{k}\text{= 37}\text{K}$, with J the antiferromagnetic exchange integral.     

Low temperature specific heats of nearly one-dimensional antiferromagnet: CuCl2·2NC5H5

Transition temperature to LRO state was found at T_N=1.14K for nearly one-dimensional antiferromagnet CuCl₂ · 2NC₅H₅. Intra- and inter-chain exchange constants J and J′ were estimated, $\text{kT}{}_{\mathrm{<mtext>N</mtext>}}\text{J}\text{=0.082}$ and $\text{J′}\text{J}\text{=3×10}{}^{\mathrm{?3}}$, respectively. Comparison with those of TMMC implies highly one-dimensional character.     

Quantum renormalizations in one dimensional easy plane ferromagnets

We examine the quantum corrections to the spin wave energies of easy plane ferromagnets. We utilize a formalism developed by Villain, and we obtain the quantum corrections of orders $\text{S(S + 1)}{}^{\mathrm{<mtext>-1</mtext><mtext>2</mtext>}}$. The experimentally determined spin wave spectra for CsNiF₃ is used to find the bare exchange J and anisotropy parameter D. We compare our values with those obtained from other theories, which order the corrections in powers of S^-1 and also neglect higher powers of D/J.     

Transfer of rotational population in CO by IR laser double resonance

A double resonance experiment on CO using two CO lasers (pump on v = 1 → 0, probe on v = 2 → 1) is described. Molecular diffusion has to be considered to reduce the data. The total rotational relaxation rate constant for J = 10 is found to be 1.25 ± 0.15 s^-1 torr^-1. Population transfer seems to proceed for $\text{2}\text{3}$ by |ΔJ| = 1 and for $\text{1}\text{3}\text{by |ΔJ| =2}$.     

Far infrared laser magnetic resonance spectrum of CH

Laser magnetic resonance spectra between 0 and 17 kG have been recorded and analyzed for $\text{(J′ ← J″) = (}\text{7}\text{2}\text{←}\text{5}\text{2}\text{)}$, $\text{(}\text{5}\text{2}\text{←}\text{3}\text{2}\text{)}$, and $\text{(}\text{3}\text{2}\text{←}\text{1}\text{2}\text{)}$ transitions in the CH molecule, using the optically pumped far infrared lasers: 118.8 μm (CH₃OH), 180.7 μm (CD₃OH), 554.4 μm (CH₂CF₂), 561.3 μm (DCOOD), and 567.9 μm (CH₂CHCl). Other transitions in CH were detected with the ¹³CH₃OH laser at 115.8, 149.3, and 203.6 μm. The CH radical was generated in a low-pressure methane and atomic fluorine flame within the laser cavity. Analysis of the M′_J ← M″_J structure yields wavenumbers for the rotational transitions mentioned above of 84.3494, 55.3397, and 17.8376 cm^?1, respectively. Combining results from the M_J analysis with the $\text{J =}\text{1}\text{2}\text{Λ-}\text{doubling}$ interval derived from radioastronomy measurements yields Λ-doubling values for the $\text{J =}\text{3}\text{2}\text{,}\text{5}\text{2}$, and $\text{7}\text{2}$ states of 0.0237, 0.1620, and 0.3759 cm^?1, respectively. Both the rotational intervals and the Λ-doublings are in good agreement with earlier less precise optical results. Analysis of the hyperfine structure yields values for the Frosch and Foley hyperfine parameters of a = +52, b = ?74, c = +52, and d = +43.6 MHz, in good agreement with recent ab initio estimates and radioastronomy measurements.     

Etude des proprietes thermodynamiques d'un groupement tetranucleaire: Application au compose Na3RuO4

The exact eigenvalues spectrum of the spin hamiltonian $\text{H}\text{= ?2}\text{∑}\text{(i, j)}\text{J}{}_{\mathrm{ij}}\text{S}\text{?}{}_{\mathrm{i}}\text{S}\text{?}{}_{\mathrm{j}}$ have been calculated for a tetranuclear cluster formed by four spins 3/2 at the vertices of a lozenge. Two isotrope exchange interactions J₁ and J₂ are able to explain the thermodynamic properties (magnetic susceptibility, entropy, specific heat). A ground state transition from singlet to triplet state occurs when the $\text{J}{}_2\text{J}{}_1$ ratio reaches the value $\text{4}\text{3}$. The magnetic susceptibility data of Na₃RuO₄ fit well with the theoretical values proposed for $\text{J}{}_1\text{K}\text{= (?19,5 K)}$ and J₂/k (?22,5 K).     

A note on the impurity spin magnetisation of dilute alloys

Double time temperature dependent anticommutator Green's function has been used to obtain magnetisation of the impurity spin in s-band host metals. The function φ_Ji which is related to the impurity spin magnetisation shows a break in the magnetisation vs temperature curve when $\text{|(}\text{H}{}_0\text{J}\text{) × (}\text{g}{}_{\mathrm{i}}\text{S}{}_{\mathrm{e}}\text{(S}{}_{\mathrm{e}}\text{+ 1))|}$ lies within 1.50. The effect of crystal field of the host lattice on the impurity spin magnetisation has also been considered.     

s- and p-wave neutron spectroscopy: Part V. Level Spacings in Heavy Nuclei

Total neutron cross sections were measured for natural rubidium and thallium and for the separated isotopes, Pb²⁰⁸, Pb²⁰⁷, Pb²⁰⁶, Tl²⁰³, and Rb⁸⁵. Approximate s-wave resonance spacings were estimated for Pb²⁰⁷, Y⁸⁹, Rb⁸⁷, Rb⁸⁵, Tl²⁰³, and Tl²⁰⁵. D₀ = D_J(2J + 1) = 2D_S(2I + 1) = 50, 24, 8, 4, and 30 kev, respectively, where D_s is the average (s-wave) level spacing for all channels. The spacings (D₀) of Pb²⁰⁶ and Pb²⁰⁸ were found to be of the order of hundreds of kew; there is also evidence that resonance spacings are very wide for Sr⁸⁶ and Ba¹³⁶. It is concluded that, in all compound nuclei with a neutron number silightly less than the magic numbers 50, 82, and 126, the resonance spacings are usually not much less than when the magic number is exceeded slightly. Since neutron excitation energies of these sub-magic nuclei are higher than the average, the observed wide level spacings below the magic numbers must be due to the properties of the nearly closed shells and can not possibly be caused only by low excitation energy of the compound nucleus. In considering these comparisons it is shown that, for s-wave resonances, the relation $\text{D}{}_{\mathrm{J}}\text{=}\text{D}{}_0\text{(2J + 1)}$ is a useful approximation in that a plot of D₀ is a much less erratic function of A than is the observed spacing. We also discuss the Bethe-Hurwitz effect, i.e., the influence on resonance spacing (apart from the 2J + 1 factor) of an unpaired nucleon in the target nucleus. We estimate that $\text{α ≧ 30}$ in the equation $\text{D}{}_0\text{(0)}\text{D}{}_0\text{(W)}\text{=}\text{exp}\text{(αW)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ where D₀(0) is the energy and spin independent spacing parameter, and W is the excitation energy of the compound nucleus.     

The effect of the biquadratic exchange interaction on the curie temperature of the mixed ising ferromagnet

For the mixed Ising ferromagnet with S = 1 and $\text{S =}\text{1}\text{2}$, the effect of the biquadratic exchange interaction J′ on the Curie temperature has been calculated by the use of the Bethe-Peierls approximation and its effect is shown to be large for J′ > 0.     

Susceptibility divergence of the spin 1 Ising model on the Cayley tree

The temperature T₂(1), below which the zero-field isothermal susceptibility diverges for the spin 1 Ising model on the M-generation Cayley tree, in the limit as M → ∞, is located. The general approach follows closely that of Falk for the spin $\text{1}\text{2}$ problem. Pruning and retraction identities are used to find upper and lower bounds on the bond-length dependence of the general two-spin correlation function and thence on the susceptibility. A conjecture is advanced regarding the divergence of the higher-order susceptibilities.     

Exact results for the diluted Ising model with competing interactions

We consider the random-bond Ising model with the exchange integrals J > 0, ?J and 0 with the respective probabilities p, q and r, where p + q + r = 1. We give the exact value of the averaged internal energy and an exact upper bound to the averaged specific heat at temperature T determined by $\text{k}{}_{\mathrm{B}}\text{T =}\text{2J}\text{In[}\text{p}\text{(1 ? p ? r)}\text{]}\text{,}$ where k_B is the Boltsmann constant. We show that all the averaged correlation functions of even spins are non-negative at this temperature.     

Spin of the τ-neutrino and suppression of the decay-mode τ → ντπ

It is pointed out that a zeromass spin $\text{3}\text{2}\text{τ}$-neutrino naturally suppresses τ → ν_τπ and is not in contradiction with known data.     

Temperature jump method in molecular beam relaxation spectrometry applied to catalytic decomposition of CH3OH on polycrystalline Ni foil

A new modification of molecular beam relaxation spectrometry (MBRS) is described: the temperature jump method for studying catalytic surface processes on metal foils. The temperature of the catalyst foil is maintained by direct ohmic heating; a constant particle beam is directed towards the catalyst surface. A jump of the surface temperature caused by a high current pulse generates a response of the fluxes of desorption. The decay of the desorption intensity after the temperature jump contains the relaxation times of the elementary steps involved. The mathematical treatments of unimolecular and bimolecular surface reactions, of sequences of two and three unimolecular steps and of a sequential reaction accompanied by the redesorption of the reactant are given. The application of the new method is shown by a study of the catalytic decomposition of CH₃)OH on polycrystalline Ni: CO and H₂ are the sole reaction products. The limit of the catalytic activity — apart from the low sticking probability of the reactant — must be seen in the abstraction of the first methyl hydrogen from the transient methoxy species. In the temperature range between 320 and 550 K the reaction mechanism can be described as follows:

Rate constants in dependence from surface temperature T are: k₁ = 4.2 × 10⁴ exp$\text{(}\text{?22.4}\text{RT}$$\text{kJ}\text{mol}\text{)}$ s^?1; k₃ = 2.4 × 10⁹ exp$\text{(}\text{?75}\text{RT}$$\text{kJ}\text{mol}\text{)}$ s^?1; k₄ = 1.2 × 10¹³ exp$\text{(}\text{?104}\text{RT}$$\text{kJ}\text{mol}\text{)}$ s^?1; η = 0.2. Typical surface residence times of the intermediates are: 110 ? τ₁ ? 15 ms at 320 ? T ? 450 K; 210 ? τ₃ ? 6 ms at 450 ? T ? 550 K; 98 ? τ₄ ? 6 ms at 450 ? T ? 500 K.