Intrinsic V- and Vo centers in single crystal SrO

The intrinsic V^- center (a positive hole trapped at a cation vacancy) has been observed in single crystals of SrO following x-irradiation at 77 K. The ESR spectrum corresponds to a spin-$\text{1}\text{2}$ defect having <100> axial symmetry with g₆ = 2.0012(3) and g_⊥ = 2.0703(3). The concentration of V^- centers was enhanced significantly by heating crystals in oxygen at 1000°C and rapidly quenching. V^o centers (two holes trapped at a cation vacancy) were also observed after x-irradiation at low temperature, with g₆ = 2.0011(3), g_⊥ =2.0751(3), and D = 380.4(5) MHz.     

ESR of a Mn-S nearest neighbour complex in GaP

ESR measurements on a manganese-sulphur nearest neighbour complex having axial symmetry along 〈111〉 axes are reported. The spin Hamiltonian parameters are found to be g = 2.002 ± 0.001, A = (-57 ±1) × 10^-4 cm^-1, D = (-306 ± 2) × 10^-4 cm^-1 and (a − F) = (+12 ±2) × 10^-4 cm^-1. The formation of these complexes is discussed. briefly.     

EPR of trivalent iron centers in SrF<Subscript>2</Subscript>: Fe crystals

Paramagnetic centers of three types are found in SrF₂: Fe(0.2 at. %) crystals. Two types are observed in the untreated crystals, and the third type appears only in the crystals irradiated by x-rays. The EPR spectra of one type of centers in a nonirradiated crystal and of the centers that appear after irradiation are described by the orthorhombic Hamiltonians with an effective spin S _eff = 5/2. In both cases, the centers are observed at 4.2 and 77 K. The principal axes of the spin Hamiltonians for them are along the 〈001〉, 〈1 \(\overline 1 \) 0〉, and 〈110〉 axes. However, the fine-structure parameters of their EPR spectra differ significantly. An analysis of the superhyperfine structure (SHFS) of the EPR spectra shows that the radiation center forms through substitution of a Fe²⁺ ion for a Sr²⁺ cation. Under x-ray irradiation, the Fe²⁺ ion transforms into the Fe³⁺(⁶ A _1g ) state and is displaced to an off-center position along the C ₂ axis of its coordination cube. The absence of a SHFS in the EPR spectra of the orthorhombic centers in a nonirradiated crystal makes it impossible to determine their molecular structure unambiguously. The most probable model is proposed for this structure. The EPR spectra of centers of the third type were observed only at 4.2 K, and the structure of these centers was not studied.     

Electron paramagnetic resonance of Er<Superscript>3+</Superscript> ions in a polycrystalline α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

The EPR spectra of rare-earth Er³⁺ ions in a polycrystalline corundum α-Al₂O₃ synthesized by the sol-gel technology were revealed. It is shown that the EPR spectra belong to the Er³⁺ ions in the ground state corresponding to the lower Stark sublevel of the ⁴ I _15/2 term and can be described by the spin Hamiltonian of axial symmetry with an effective spin S = 1/2 and the g tensor with components g _‖ = 12.176 and g _⊥ = 4.14. The average value of the g tensor (〈g〉 = 6.82) corresponds to the Γ₇ state in a cubic field. Erbium is assumed to substitute for aluminum in the Al₂O₃ corundum crystal. The local symmetry C ₃ of the Al³⁺ ion remains despite the pronounced expansion of the lattice around the Er³⁺ ion.     

Determination of the constantg ωρπ from QCD sum rules

The ωρπ coupling constant is calculated using a modified form of sum rules for the vertex function 〈0|T(J _μ(x),J _ν(0))|π〉 accounting for the axial anomaly. The resultg _ωρπ=16 GeV^?1 is in good agreement with the estimates of the Vector Meson Dominance model. We show that the standard procedure gives forg _ωρπ a considerably smaller value compared to the experimental number.     

Etude par resonance paramagnetique electronique de sulfures de molybdene et de tungstene

Chalcogenides MeS_3−x (Me = Mo, W; 0 ⩽ x ⩽ 1) have been prepared by the thermal decomposition of tetrathiometallates. These compounds are largely amorphous; desulfurization as well as crystalline organization occur when the heat treatment temperature (TT) increases. At 1000°C a perfect hexagonal arrangement can be obtained (x = 1). ESR spectra of amorphous species having x in the range 0–1 have been recorded. A part of the ESR signals is ascribed to sulfur chains whilst the other part is attributed to Me (V) species. The intensity of both series of ESR lines is a decreasing function of TT. The spectra are characteristic of Me (V) in an axial symmetry environment. At TT ≅ 1000°C a narrow ESR signal remains, which is assigned to electron spin centers of impurities (g-value close to that of the free electron). The results are explained either in terms of continuous structural evolution of the solid during heat treatment or in terms of a biphasic mixture of amorphous MoS₃ and MoS₂. The variation of the spin concentration as a function of TT is reported and compared with the results previously obtained from a study of the transport properties of these solids. In all cases the values obtained for Me = W are smaller than those observed on Me = Mo.     

ESR study of some Cu(II)-4-substituted-2-Thiazolylhydrazone complexes

The Cu(II) complexes with 2-N-acetyl-salicylidene-hydrazino-4-chlor-methyl thiazole (L.) and 2-N-acetyl salicylidene-hydrazioo-4-thiazolyl acetic ester (L_II) were prepared and investigated by ESR measurements. The powder ESR spectrum at room temperature of CuL_IICl is quasi-isotropic (g=2.113), while for CuL_IICl is characteristic of monomeric species with axial symmetry (g _II)=2.234,g ₁=2.073). The isotropic ESR spectra of the CuLCl compounds in DMSO solution suggest the presence of pseudo-tetrahedral monomeric species. The anisotropic spectra were obtained for adsorbed CuLCl DMSO solutions on NaY zeolite. The parallel hyperfine structure shows the coexistence of two magnetic nonequivalent monomeric species. The estimation of some LCAO-MO coefficients using Kivelson and Neiman’s approximation reveals a dominant ionic character for copper-ligand bonds.     

Symmetry and structure of N–O shallow donor complexes in silicon

Shallow donors in silicon related to nitrogen–oxygen complexes have been investigated by piezospectroscopy of their hydrogenic transitions in the far infrared. Complete stress dependences up to 0.25 GPa were obtained for the 1s→2p₀ and 1s→2p_± transitions of the most prominent members of the (N, O)-family, N–O-3 and N–O-5. Very unusual for shallow donors in silicon, the symmetry of the ground state wave function is T₂-like. The lifting of orientational degeneracy for stress in the 〈1 0 0〉, 〈1 1 1〉, and 〈1 1 0〉 directions is compatible with a C_2v defect symmetry. Data from the other species of the (N, O)-family are indicative for the same symmetry. The microscopic structure of these centers, in part contradictory to present theoretical models, is discussed.     

ESR of X-ray induced centre in CaF2:Gd crystals

An ESR centre with orthorhombic symmetry was induced in CaF₂:Gd crystals by X-ray irradiation after heat treatment at 900°C. The observed ESR spectrum has resolved shfs in particular crystal directions. Spin Hamiltonian analysis indicates that the ESR centre is a hole trapped at an oxygen substituted for a fluorine site and interacts mainly with two nearby fluorine ions located along the crystal [001] direction. The determined spin Hamiltonian parameters are g_xx = 2.0036, g_yy = 2.0159, g_zz = 2.0306, A_xx = 22.5, A_yy = 5.6 and A_zz = 5.6 (G), respectively. A probable model is discussed by considering both the angular dependence of the ESR lines and the local charge neutrality around the ESR centre.     

Near-infrared absorption of MgF2:Fe2+

The multiphonon sideband of the spin allowed transition 6T_2g:A₁ ? 2〉 → 6E_g:B₃ ? 2〉 within the 3d⁶ orbital of Fe²⁺ in D_2h symmetry was analyzed with respect to the one phonon absorption and density of states of the coupled phonon modes. Temperature dependence of the zero-phonon line shows, that the axial anisotropy of the S = 2 spin multiplet in the excited B₃ state is about 0.9 cm^-1.     

Light-induced charge transfer processes in Mn-doped Bi12GeO20 and Bi12SiO20 single crystals

The optical absorption and ESR spectra of Bi₁₂GeO₂₀ and B₁₂SiO₂₀ doped with Mn have been measured before and after illumination with visible light. Uniaxial stress measurements on a sharp line observed at 8026 cm^?1 were performed. The observed ESR spectrum is a superposition of six lines resulting from the hyperfine interaction of manganese ions in tetrahedral positions. The g-factor and hyperfine constant are g = 1.999 ± 0.003 and A = 78 Gs. Analysis of the light-induced absorption spectrum leads to the conclusion that a small hole polaron bound to an Mn impurity at a tetrahedral site is responsible for the very broad absorption band which appears after illumination. The sharp line is interpreted as due to a transition inside the Mn⁺ center in tetrahedral coordination. Bands in the region 10,000–16,000 cm^?1 are due to Mn³⁺ centers in interstitial positions, whose symmetry can be treated to a first approximation as tetragonal. The following crystal field parameters for this center were found: B = 565 cm^?1, Dq = 1400 cm^?1, Dt = ?330 cm^?1, Ds = 4170 cm^?1 and C = 2260 cm^?1. The illumination conditions which are needed for homogeneous coloration of the sample are also discussed.     

Chiral symmetry breaking and condensates in the rishon model

The rishon model is studied in the limit g_c → 0, α → 0 when its global flavour symmetry is SU(6) × SU(6) × U(1) analogous to six massless flavour QCD. Recently it was shown that the ad hoc breaking SU(6) × SU(6) → SU(3) × SU(3) allows the anomaly constraint to be satisfied. In this paper this is shown to be but one of several successful patterns of chiral symmetry breaking. The condensates required to perform these breakings are fully discussed. A plausibility argument based on single gauge boson exchange is presented which determines the condensate uniquely to be 〈(v_LV_L)³〉 corresponding to the original breaking above. The same argument applies to QCD, which is argued to differ in its chiral behaviour due to the large intrinsic masses of the quarks. The implications of the above condensate and pattern of chiral symmetry breaking for the rishon model include the prediction of integer charged colour octet fermions, a naive mass formula m_e = 2m_u ? m_d, new insight into the parity-violating condensate 〈(v_Lv_L)²(v_Rv_R)〉 and the prediction of 52 new pseudos whose masses are estimated.     

Non-spherical magnetic moment in MnAlGe

The magnetic structure factors of MnAlGe (space groupP4/nmm) measured with polarised neutrons have been expressed in terms of the magnetic moment of the Mn atom (site symmetry tetrahedral with tetragonal distortion), the Bessel transforms 〈j _n〉 of the Mn radial functions and the fractional occupancies of the moment density in the various crystal field orbitals. The measured structure factors were least-squares fitted with the theoretical expression involving 〈j _n〉 appropriate to the Mn⁰, Mn⁺ and Mn²⁺ atoms. The best fit was got using Mn⁰ transforms, yielding 1·45µ _B as the Mn magnetic moment. The fractional occupancies of the moment density in the crystal field orbitalsA _1g,B _1g E _g andB _2g were obtained. This analysis shows the magnetic moment to be highly non-spherical with a large fractional occupancy (38%) in theA _1g orbital directed along the tetragonal axis while the fractional occupancies ofB _1g andB _2g are found to be 31% and 30% respectively. The fractional occupancy of the moment in theE _g orbital directed towards the Ge and Al atoms is very low (1%). The spatially averaged moment density of Mn in MnAlGe is more diffuse than that of Mn I and Mn II in isostructural Mn₂Sb.     

Possibility of utilizing the D-3He fuel cycle with 3He production in a spherical tokamak reactor

Possible operating regimes of a spherical tokamak reactor based on the D-³He fuel cycle with ³He production are considered. The parameters of the plasma and magnetic system are calculated for several versions corresponding to the high power efficiency (with a power gain factor in plasma of Q = 20) in a reactor with an aspect ratio of A = 1.5. According to calculations, for an axial magnetic field in vacuum of B ₀ = 2 T, a plasma radius of a = 3 m, an average 〈β〉 value of 0.53, and a plasma temperature of 〈T〉 = 48 keV, the reactor power can reach P _fus = 500 MW. In order to achieve a power of P _fus = 1500 MW in a reactor with a = 2 m, 〈β〉 = 0.36, and 〈T〉 = 40 keV, the magnetic field should be increased to B ₀ = 5 T.     

Electron paramagnetic resonance in Y1−cH1.92:Erc

We have investigated the EPR of isotopically enriched ¹⁶⁸Er³⁺ in Y_1?cH_1.92:Er_c where c = 100 and 1400 ppm, at both 1.4 and 9 GHz and between 1.5 and 50 K. Resonance lines were observed from Er³⁺ ions in both sites of cubic symmetry and sites of axial symmetry. We determine the numbers of Er³⁺ in cubic, and C_4v axial symmetry to be in the ratio 2:1. The cubic site resonance line is at g = 6.85 ± 0.07 and is attributed to a Γ₇ doublet. The linewidth has a linear thermal broadening of 3.9 ± 0.05 gauss K^-1 below circa 7 K. From the nonlinear thermal broadening above this temperature we determine the first excited state, in the cubic crystal field scheme, to be a Γ₈ at 35 ± 10 K above the Γ₇ ground state. We have investigated the origins of the (T = 0) residual linewidth for the ions in cubic symmetry, and conclude there to be a small but significant contribution due to unresolved transferred hyperfine structure from the surrounding hydrogen nuclei.     

Brillouin scattering observation of phonon renormalization in KC1:CN−

The phonon-defect interaction in KC1:CN^? has been studied in the 16 GHz frequency range using the Brillouin scattering technique. Brillouin spectra of KC1:CN^? at 4.2 K show a defect induced phonon velocity renormalization. The measurements are consistent with a 〈111〉 oriented CN^? dipole in KC1 with a tunneling level of T_2g symmetry at 2.6 cm^?1 and a coupling constant of 1.5 × 10^?21 cm³.     

Size effect on multiferroicity of GdMn2O5 nanorods

Four GdMn₂O₅ nanorod samples of various axial lengths (〈L_C〉) along the c axis were synthesized. The antiferromagnetic and ferroelectric ordering disappeared as 〈L_C〉 decreased to 66 and 55 nm. Various ferroic critical sizes were observed for the two types of domain sizes. Between T = 18 and 26 K, a charge ordering X-ray diffraction peak appeared at 〈L_C〉 = 79 nm. This peak was associated with structural distortion and axial length. The broken multiferroicity of the GdMn₂O₅ nanorods limits their practical application. For applications in memory devices, the estimated maximal capacity is approximately 650 Gbits/in².     

ESR study of vanadyl ion in tripotassium citrate

ESR spectra of VO²⁺ doped tripotassium citrate are recorded at room temperature. The observed spectra are fitted to a spin Hamiltonian of orthorhombic symmetry with g_x = 2.001 ± 0.001, g_y = 1.997 ± 0.001, g_z = 1.945 ± 0.001, A_x = (49.0 ± 1) × 10⁻⁴ cm⁻¹, A_y = (66.8 ± 1) × 10⁻⁴ cm⁻¹ and A_z = (168.4 ± 1) × 10⁻⁴ cm⁻¹. The covalency and Fermi contact terms are evaluated and compared with those of other lattices.     

Electron spin resonance in carbon-doped boron nitride

It is shown that carbon impurity is responsible for the yellow color and paramagnetism in boron nitride which has been heated to 1800–2300°C in graphite furnaces. The yellow BN powders exhibited 10-line ESR spectra with spin concentrations up to 10¹⁹/g. Annealing the yellow BN at 1800–2300°C in a carbon-free environment removed the yellow color and reduced the spin concentration below the observable limit of 10¹⁶/g. Such pure white BN was unaffected by prolonged irradiation with u.v., 50 kV X-rays or 1 MeV γ-rays, whereas other less pure types of BN showed a large enhancement of ESR signal after relatively short exposures to such radiation. The ESR spectra from pure white BN doped with C¹³-enriched carbon were identical to those from BN doped with normal carbon. These results suggest that the yellow color in BN is due to an F-center produced by a carbon impurity ionization mechanism. The anisotropy of the ESR was studied in carbon-doped specimens of highly oriented BN. The room temperature hyperfine splitting at 9·151 GHz was found to vary as (a + b cos²θ), where a = 7·85 ± 0·07 G and b = −1·27 ± 0·07 G. The g-value obeyed a similar relation: g = g_⊥ + Δg cos²θ, where g_⊥ = = 2·00321 ± 0·00002 and Δg = g_∥ − g_⊥ = (−9·5 ± 0·2) × 10⁻⁴.