Proprietes magnetiques et etude par effet Mössbauer du thiosilicate Fe2SiS4

Fe₂SiS₄$\text{(a = 12,407}\text{A}\text{?}\text{, b = 7,198}\text{A}\text{?}\text{, c = 5,812}\text{A}\text{?}$, space group Pnma) has the olivine structure; the ferrous ions are located in two kinds of sites: one half in planes of mirror symmetry m, the other half in centers of symmetry. 1?.The magnetic study of this compound by means of magnetization measurements and Mössbauer effect, indicates from 127 K an antiferromagnetic arrangement along oy for the m sites, and an induced partial order of the same kind for the 1? sites. At 33 K, the Fe²⁺ spins in 1? sites are completely ordered and at the same time takes place a rearrangement of the magnetic structure. The observed complex model is analogous to that of Fe₂GeS₄, i.e. antiferromagnetic along 0x and ferrimagnetic along Oz.     

Evidence for assignment of 14.0 MeV state in 11B from 10B(n,n)10B

The differential cross section and polarization for neutrons scattered from ¹⁰B have been measured at E_n = 2.63 MeV (E_x = 13.85 MeV). The results of this experiment and other available neutron scattering data in the range 1 < E_n < 4 MeV are interpreted through a single-level R-matrix calculation over the region 12 < E_x < 15 MeV. Based on this analysis the most probable J^π assignment for the 14.0 MeV level in ¹¹B is $\text{11}\text{2}{}^{\mathrm{+}}$. The anomaly near E_x = 13.1 MeV can only be explained in terms of two overlapping levels having assignments of ($\text{5}\text{2}$, $\text{7}\text{2}$)^? and ($\text{3}\text{2}$, $\text{5}\text{2}$, $\text{7}\text{2}$)⁺.     

Transverse spin correlation function of the one-dimensional spin-12 XY model

The transverse spin pair correlation function p^x_n=<S^x_mS^x_m+n>=<S^x_mS^x_m+n> is calculated exactly in the thermodynamic limit of the system described by the one-dimensional, isotropic, spin-$\text{1}\text{2}$, XY Hamiltonian

$\text{H=?2J}\text{∑}\text{l=1}\text{N}\text{(S}{}^{\mathrm{x}}{}_{\mathrm{l}}\text{S}{}^{\mathrm{x}}{}_{\mathrm{l+1}}\text{+S}{}^{\mathrm{y}}{}_{\mathrm{l}}\text{S}{}^{\mathrm{y}}{}_{\mathrm{l+1}}\text{)}$

. It is found that at absolute zero temperature (T = 0), the correlation function ρ^x_n for n ≥ 0 is given by

$\text{ρ}{}^{\mathrm{x}}{}_{\mathrm{2p}}\text{=}\text{1}\text{4}\text{2}\text{π}{}^{\mathrm{2p}}\text{Π}\text{j=1}\text{p?1}\text{4j}{}^2\text{4j}{}^2\text{?1}{}^{\mathrm{2p?2j}}\text{if}\text{n=2p}$

,

$\text{ρ}{}^{\mathrm{x}}{}_{\mathrm{2p+1}}\text{=±}\text{1}\text{4}\text{2}\text{π}{}^{\mathrm{2p+1}}\text{Π}\text{j=1}\text{p}\text{4j}{}^2\text{4j}{}^2\text{?1}{}^{\mathrm{2p+2j}}\text{if}\text{n=2p+1}$

, where the plus sign applies when J is positive and the minus sign applies when J is negative. From these the asymptotic behavior as n → ∞ of |?^x_n| at T = 0 is derived to be $\text{|ρ}{}^{\mathrm{x}}{}_{\mathrm{n}}\text{| ～}\text{a}\text{n}$ with a = 0.147088?. For finite temperatures, ρ^x_n is calculated numerically. By using the results for ?^x_n, the transverse inverse correlation length and the wavenumber dependent transverse spin pair correlation function are also calculated exactly.     

Correlation functions of the transverse Ising chain at the critical field for large temporal and spatial separations

We study the behavior of 〈σ₀^x(t)σ_n^x(0)〉 and 〈σ₀^y(t)σ_n^y(0)〉 for the transverse Ising chain at the critical magnetic field at T = 0. Explicit results are obtained for the three distinct regions where t → ∞ and n → ∞with $\text{0 ?}\text{n}\text{t}\text{<1}$, $\text{1 <}\text{n}\text{t}$, or $\text{t = n + n}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{(}\text{z}\text{2}\text{)}$ where z is fixed of order one. In this latter region the general Painlevé V solution is shown to reduce to a Painlevé II function. We use our results to discuss the general problem of long-time behavior of Toda equations with slowly decaying initial values.     

Lithium mobility in the layered oxide Li1−xCoO2

The Li⁺-ion chemical diffusion coefficient in the layered oxide Li_0.65CoO₂ has been measured to be $\text{D}\text{?}\text{= 5 × 10}{}^{\mathrm{?12}}$ m² s^?1 by three independent techniques: (1) from the Warburg prefactor, (2) from the transition frequency for semi-infinite to finite diffusion lengths in steady-state ac-impedence measurements and (3) from a modified Tubandt method that uses ac-impedance data to distinguish interfacial and surface-layer resistances from the bulk resistance of the sample. This value and a small increase in D? with (1 ? x) in Li_1?xCoO₂, 0.45 < (1 ? x) < 0.80, compare favorably with the $\text{D}\text{?}\text{= 5}\text{to}\text{7 × 10}{}^{-12}\text{m}{}^2\text{s}{}^{-1}$ obtained by Honders for this system with pulse techniques. A qualitative discussion is presented as to why this composition dependence and why D? for this system is a factor of five larger than that for Li⁺-ion diffusion in Li_xTiS₂.     

Electrical conductivity of (Bi,Pb)2MO4 (M = Pd,Pt) linear chain compounds

Partial oxidation of Pd in Bi₂PdO₄ is achieved by substitution of Pb²⁺ for Bi³⁺ up to Bi₁₉₁Pb₀₀₉PdO₄, partial oxidation is necessary to stabilize the isostructural Pt compound, Bi_1?xPb_xPtO₄ within the range 0.33 ? x ? 0.52. In both cases, the tetragonal cell c parameter, therefore metal-metal distance $\text{(d}{}_{\mathrm{<mtext>M?M</mtext>}}\text{=}\text{c}\text{2}\text{)}$, decreases linearly with increasing mean oxidation degree (MOD) of transition metal atom For the insulator B1₂CuO₄, no substitution occurs Powder electrical conductivity measurements of the partially oxidized compounds show that these materials are semiconductors Platinum compounds exhibit relatively high conductivities $\text{(σ?10 (Ω}\text{cm}\text{)}{}^{\mathrm{?1}}\text{)}$ and low activation energies (?0 02 eV) with small variations with x Palladium compounds exhibit lower conductivities which linearly increases with MOD These electronic properties are comparable with those of the most one-dimensional Pt or Pd chain conductors.     

Defect structure of the Spinel,Ni2.44Ti0.77O4

Cation-excess spinels with composition Ni_2(1+x)Ti_(1?x)O₄ (0.16 ≤ x ≤ 1) form a limited solid solution between the rocksalt (NiO) and the hypothetical spinel (Ni₂TiO₄) structure types at temperatures above 1375°C. A quenched product of this series, Ni_2.44Ti_0.77O₄ ($\text{Fd3m;}\text{Z}\text{= 8; ρ}{}_{\mathrm{<mtext>calc</mtext>}}\text{= 5.6}\text{g}\text{/}\text{cm}{}^3\text{; a = 8.339}\text{A}\text{?}$), has been refined with X-ray intensity data in three models. Results indicate partial occupancy of the tetrahedral site (8a) and a second octahedral site (16c). Substitution of one tetrahedral metal atom for two octahedral atoms in random units of the NiO structure accounts for the additional metal atoms over the M₃O₄ stoichiometry. The best agreement with the model is obtained when all Ti₄₊ partially occupies the tetrahedral site.     

Electrical properties of narrow gap semiconductor PtSb2

Results of measurements of conductivity, Hall and Seebeck coefficients of tellurium doped n-type crystals of platinum antimonide are presented. The Hall coefficient and the Seebeck coefficient undergo sign inversion twice, below and above room temperature. The detailed analysis of the experimental results revealed that below 200 K PtSb₂ can be described by a simple conduction and valence band model. The energy gap E_g = (110?0.15 × T) (meV), the electron conductivity mass m_n^c/m₀ = 0.35, acoustic phonon limited electron mobility 〈μ_a〉_n = 3 × 10⁶ T^{?$\text{3}\text{2}$} ($\text{cm}{}^2\text{V · s}$) and mobility ratio 〈μ_a〉_n/〈μ_a〉_p = 0.4 are determined. However, at higher temperatures a more complicated valence band model is needed to account for the experimental results. The arguments for the existence of subsidiary valleys in the valence band are presented.     

Spin dynamics in the impurity-doped one-dimensional heisenberg paramagnet (CH3)4NMn(1-x)CuXCl3: Nuclear magnetic relaxation

Proton spin-lattice relaxation rate has been measured at room temperature in the impurity-doped (CH₃)₄NMn_(1-x)Cu_xCl₃ for X = 0.04, 0.1 and 0.17. The result for Hz. snfc;chain-axis as a function of resonance frequency clearly shows that the spectral density of the spin fluctuations in the impure system remains to have the characteristics 1-D diffusive term (ω^{-$\text{1}\text{2}$}), with a slower rate of the spin diffusion in accordance with the theory by Richards. The result for H⊥ chain-axis indicates, however, the existence of a singularity of the fluctuations near ω = 0.     

Analysis of the (ν1 + ν2) and (ν2 + ν3) combination bands of ozone

The fine structures of the (ν₁ + ν₂) and (ν₂ + ν₃) combination bands of ozone in the 5.7-μm region have been recorded and analyzed. The two vibrational states are coupled through Coriolis and second-order distortion terms. The interaction has been treated by the numerical diagonalization of the secular determinant for the two coupled states. With the centrifugal distortion parameters fixed to the ground state values, the following constants have been obtained: ν₁ + ν₂ = 1796.26₆, A₁₁₀ = 3.610₄, B₁₁₀ = 0.4414₅, $\text{C}{}^1{}_{110}\text{= 0.3902}{}_9$, ν₂ + ν₃ = 1726.52₆, A₀₁₁ = 3.553₇, B₀₁₁ = 0.4398₂, $\text{C}{}^1{}_{011}\text{= 0.3884}{}_4$, Y₁₃ = ?0.46₆, and X₁₃ = ?0.01₀ cm^?1. In addition, the following anharmonic constants have been obtained: x₁₂ = ?7.82₁ and x₂₃ = ?16.49₄ cm^?1. The value of the dipole moment ratio, $\text{R =}\text{〈011|μ}{}_{\mathrm{z}}\text{|0〉}\text{〈110|μ}{}_{\mathrm{x}}\text{|0〉}$, is 1.30 ± 0.10.     

Hall to drift mobility ratio in Ga1−xAlxAs alloys

Hall concentration measurements as a function of temperature ($\text{750 ? T ? 77}\text{K}$) on n-type epitaxial layers of Ga_1?xAl_xAs in the composition range $\text{0 ? x ? 0.78}$, have been used to evaluate the composite dependence of the Hall to drift mobility ratio in this alloy at 300 K. This ratio is found to be close to unity for alloy compositions $\text{0 ? x ? 0.25}$ and $\text{0.6 ? x ? 0.78}$, but attains a maximum value of 3.8 at x = 0.42 due to the multiconduction Hall effect.     

Energy gaps in the fractional quantum hall effect

Based on a density-of-states N(E) ∝ (E − μ₀)² Poisson's equation is solved perpendicular to the current direction in the two-dimensional Hall-plate. By comparing the result with the structure obtained for an Ising-like chain with occupation numbers $\text{σ = 0,}\text{1}\text{2}$, 1 to account for vanishing energy gaps for filling factors $\text{v =}\text{m}\text{n}$ with n even, the energy gaps for n odd are found essentially to be proportional to $\text{B}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{n}{}^{\mathrm{-2}}$ in accordance with recent results.     

Energies of the n2S12 and n2D32,52 states of Cs

Using the technique of Doppler-free two-photon spectroscopy the energies of the n²S_{$\text{1}\text{2}$} (12 ? n ? 35) and n²D_{$\text{3}\text{2}$,$\text{5}\text{2}$} (11 ? n ? 48) states of CsI have been measured with a thermionic detector. The absorption spectrum of molecular iodine was used as a reference giving us a total estimated accuracy of about 2 × 10^-7. Taking also into account the energies of the 7–11 ²S_{$\text{1}\text{2}$} and 5 and 6 ²D_{$\text{3}\text{2}$,$\text{5}\text{2}$} states measured by other authors and using an extended Ritz-formula we found the ionization limit to be E_i = 31406.468 ± 0.006 cm^-1.     

Specific heat of (C6H11NH3) CuCl3 (CHAC), a system of ferromagnetic chains

The heat capacity of (C₆H₁₁NH₃) CuCl₃ (CHAC) has been measured for 0.45 < T < 60 K. Three-dimensional ordering is observed at T = 2.214 K. The data in the paramagnetic region can be described by a ferromagnetic $\text{S =}\text{1}\text{2}$ Heisenberg linear chain model system with J/k = +45 ± 5K.     

Polarization transfer in the 2H(d,n)3He reaction at θ = 0°

The vector polarization transfer coefficient K_y^y′ and the tensor analyzing power A_zz have been measured for the ²H($\text{d}$$\text{n}$)³He reaction at θ = 0° over an incident deuteron energy range from 1 to 15 MeV in 0.5 MeV steps. The results agree with the previous ²H($\text{d}$$\text{n}$)³He measurements of Simmons et al. and are nearly identical to the ²H($\text{d}$$\text{n}$)³H measurements of Clegg et al. in the region of overlap. The present results provide an accurate and complete set of the observables necessary to use the ²H($\text{d}$,$\text{n}$)³He reaction as a source of polarized neutrons.     

Evolution of the modulated antiferromagnetic structure of CeAl2 in the presence of nonmagnetic impurities

Neutron diffraction studies show that the alloy system Ce_1-xLa_xAl₂ with x = 0.1 and 0.2 orders in the same incommensurate modulated antiferromagnetic structure as the pure compound CeAl₂. The existence and concentration dependence of a commensurate extra reflection ($\text{5}\text{2}$$\text{3}\text{2}$$\text{1}\text{2}$) is interpreted as being due to a modulation quenching of the CeAl₂ structure. This is in contrast to the suggestion that the magnetic order of CeAl₂ can be described by a triple-q structure.     

Magnetic structure of the canted 1D-antiferromagnet NaBaFe2F9

NaBaFe₂F₉ crystallizes with the Ba₂CoFeF₉ structural type (space group P2₁/n, Z = 4). In this structure, infinite cis double [M₂F₉]^3? chains of corner sharing octahedra are isolated one from each other by sodium and barium ions. At 4.5 K, the cell parameters (Å) are found to be: a = 7.3236(3), b = 17.4525(7), c = 5.4586(2), β = 91.840(3)°. Antiferromagnetic interactions dominate but, below T_N ? 19 K, a parasitic ferromagnetic component appears with σ_r(4.5K) = 0.03(1) μ_B·mole^?.The magnetic structure (C_xF_yC_z mode) was foreseen by the macroscopic theory of Bertaut and established from neutron powder diffraction. Using the Rietveld profile refinement method, the 4.5 K spectrum was analysed at $\text{λ =}\text{1.909}\text{A}\text{?}\text{(R}{}_{\mathrm{<mtext>Profile</mtext>}}\text{=}\text{0.093}\text{, R}{}_{\mathrm{<mtext>Nucl</mtext>}}\text{=}\text{0.053}\text{, R}{}_{\mathrm{<mtext>Mag</mtext>}}\text{=}\text{0.126}\text{)}$ and the 4.5–35K difference spectrum at $\text{λ =}\text{2.518}\text{A}\text{?}\text{(R}{}_{\mathrm{<mtext>Mag</mtext>}}\text{=}\text{0.049}\text{, R}{}_{\mathrm{<mtext>Profile</mtext>}}\text{=}\text{0.092}\text{)}$. The magnetic moments on two Fe³⁺ sites, lie mainly in the (0 1 0) plane, approximately at 45° from the a- and c-axes. They form a pseudo G-type antiferromagnetic arrangement (μ(Fe1) = 3.60(4) μ_B;μ(Fe2) = 3.61(6)μ_B).     

Investigation of the metastable state of Na2[Fe(CN)5NO] · 2H2O by optical spectroscopy. II The electronic spectra of the ground and metastable states

The absorption spectra of Na₂[Fe(CN)₅NO] · 2H₂O were measured in the visible region in the range of 3400–7000 Å. In the metastable state, an additional absorption band in the long wavelength range is observed and the transition $\text{2}\text{b}{}_{\mathrm{<mtext>2</mtext>}}\text{(d}{}_{\mathrm{xy}}\text{)→}\text{7}\text{e(π}{}^1\text{?NO}\text{)}$ becomes weaker in the excited state indicating a population of the π¹(NO)-orbital. The laser excited emission spectrum shows a broad luminescence beginning at the excitation line $\text{λ =}\text{5145}\text{A}\text{?}\text{(}\text{19,436 cm}{}^{-1}\text{)}$ with a maximum at about 6250 Å (16,000 cm^-1). A strong sharp luminescence at about 7836 Å is registered and may be assigned to a transition 3b₁(d_x²?y²) or 5a₁(d_z²) to the antibonding π¹(NO)- orbital. Further the broad luminescence is superimposed by a series of sharp spikes. These sharp spikes can also be observed for several days, when the laser is switched off, and are depending on the crystal orientation.     

Neutron diffraction study of the layered compounds MnPSe3 and FePSe3

In the insulating compounds MnPSe₃ (1) and FePSe₃ (2) the divalent transition metal ions form planar honeycomb lattices. A neutron diffraction study revealed a collinear antiferromagnetic order below T_N = 74 ± 2 K (1) and T_N = 119 ± 1 K (2) with the corresponding wavevectors k = [000] (1) and $\text{k = [}\text{1}\text{2}\text{0}\text{1}\text{2}\text{]}$ (2). In MnPSe₃ the magnetic moments (m₀ = 4.74 μ_B) lie within the basal plane and in FePSe₃ (m₀ = 4.9 μ_B) they are pointing along the c-axis. The collinear structures are determined by the dominating intralayer interactions between first (J₁), second (J₂) and third neighbours (J₃) which in MnPSe₃ are all antiferromagnetic whereas in FePSe₃J₁ is ferromagnetic and J₂ and J₃ are antiferromagnetic.     

Non-perturbative QCD effects explain the ΔI = 12 rule in the chiral limit

It is shown that in the chiral limit, the enhancement of the $\text{ΔI=}\text{1}\text{2}$ transitions for mesons is explained by the large size of non-perturbative QCD matrix elements. For $\text{〈}\text{s}\text{s〉 = 〈}\text{u}\text{u〉}$ we obtain ∣M(K_S⁰ → 2π⁰) ∣ =(5.2±0.6) × 10^?7m_K in excellent agreement with experiment.