Griffiths inequalities for noninteractingN-vector (classical heisenberg) models and applications to interacting systems

An order relation for tensors is defined. With this ordering it is shown that in noninteractingN-vector models 〈σ_Aσ_B〉?〈σ_A〉〈σ_B〉 is positive. Applications to interacting models include a proof for the alignment of spins and the subadditivity of the free energy.     

Farbzentren rhombischer und monokliner Symmetrien in NaF

A number of zero-phonon absorption lines ofF aggregate color centers is studied inx-irradiated NaF crystals under uniaxial stress. The color centers associated with the lines are found to exhibit rhombic (D_2h, D₂, C_2v; rotation axes along 〈110〉 and 〈100〉) and monoclinic (C_2h, C₂, C_s; rotation axis along 〈110〉) symmetries. The transitions of the rhombic color centers correspond to 〈100〉 and 〈110〉 dipoles, those of the monoclinic centers to 〈112〉 and 〈110〉 dipoles. The most prominent line at 5803 Å is due to a 〈112〉 dipole transition within a monoclinic color center. Several models of centers are discussed.     

Dependence of the Daily <Emphasis Type="Italic">N</Emphasis><Subscript><Emphasis Type="Italic">m</Emphasis></Subscript>F2 Values over Mid-Latitude Stations on the Solar and Geomagnetic Activity

The results of studies of the dependence of the daily electron concentration at maximum of the F2 ionospheric layer in January 2008–2015 on the solar and geomagnetic activity are presented. The solar radio emission flux density indices F_10.7 and geomagnetic activity indices A _p were averaged over 27 days, and 〈F_10.7〉₂₇ and 〈A _p 〉₂₇, respectively, were obtained. Based on the data of three stations, 27-day median (with the middle of January 15) daily N _m F2 variations were obtained for 2008–2015. Based on these data, the following paradox was discovered: in January 2014, when the values of the solar activity index F_10.7 were larger than in 2015, the dailyN _m F2 values were smaller. Averaging over four hours of local daytime (10:00–14:00 LT) gave the daily average January 〈N _m F2〉 values for each selected station for each year. To solve this paradox, a double linear regression of 〈N _m F2〉 on 〈F_10.7〉₂₇ and 〈A _p 〉₂₇ was constructed. Due to this, it was concluded that the contribution of geomagnetic activity to daily January 〈N _m F2〉 values is positive. A comparison of the mean square errors of the linear and double linear regressions for 〈F_10.7〉₂₇ and 〈F_10.7〉₈₁ showed that the use of 〈F_10.7〉₂₇ led to smaller errors than the use of 〈F_10.7〉₈₁.     

Anionische Silberzentren („B-Zentren“) in Alkalihalogeniden

It is proposed that negatively charged silver ions on anion sites are responsible for theB bands found in silver doped alkalihalides. Experimental investigations confirming this model of theB center are presented. Optical absorption and emission ofB centers were measured in seven alkalihalides in the temperature range from 450 to 4 °K. Configuration coordinate diagrams were obtained for KCl, KBr, and KI. A chemical method was used to determine the charge of theB center. Thermal or optical excitation causes theB centers to dissociate into neutral silver atoms andF centers. The Ag⁰ centers are bound to interstitial positions at low temperature. At high temperature they collect together forming colloidal centers. TetragonalB _A centers were formed during optical bleaching of theB band in mixed crystals of the type KCl+ΔNaCl. Their absorption and emission spectra, optical orientation, and polarized emission were investigated. These properties are shown to be similar to those ofF _A centers.B _A fluorescence polarized almost completely in the [100] direction was observed. A Jahn-Teller splitting of theB absorption band was resolved in RbCl at low temperature. The splitting confirms the configuration5s² for the Ag^? ion. The results are compared with those for the isoelectronic centers In⁺ and Sn⁺⁺. Thin films of alkalihalides containing small amounts of silver, copper, or thallium were condensed simultaneously with alkali vapor. New bands in the UV region were found, possibly due to Cu^? and Tl^? centers.     

Optical Fresnel transformation and quantum tomography

Corresponding to optical Fresnel transformation characteristic of a ray transfer matrix (A,B,C,D),AD-BC=1, there exists Fresnel operator F(A,B,C,D) in quantum optics, we show that under the Fresnel transformation the pure-state position density ∣x〉〈x∣ becomes density operator ∣x〉_s,rs,r〈x∣, which is just the Radon transform of the Wigner operator, i.e.,

     

Magnetic structure and crystal field in cerium compounds with CsCl-type structure

Neutron diffraction and spectroscopy experiments have been performed on the cubic (CsCl-type structure) compounds CeAg, CeZn and CeMg. CeAg orders ferromagnetically with a moment μ_F= 0.8μ_B at 1.7 K. CeZn and CeMg are antiferromagnets; at 4.2 K their magnetic moments are 1.95 μ_B (CeZn) and 1.85 μ_B (CeMg). In the ordered state, they reveal a strong quadratic distortion, the (c/a ? 1) value reaching 1.9% in CeAg at 1.7 K; in CeAg additional atoms shifts seem to be present in the tetragonal cell. Crystal field parameters have been determined as |A₄ 〈r₄〉| = 117 K (CeAg), 74 K (CeZn) and 117 K (CeMg); the consistence of the results with all the previous studies is discussed.     

Isotope shift and hyperfine structure of the transition5d6s 2 2 D 3/2-5d6s6p 4 F 3/2 of Lu175 and Lu176

Using a tunable single mode dye laser the isotope shift of the 573.7 nm-line between the isotopes Lu¹⁷⁵ and Lu¹⁷⁶ has been determined to be IS(176?175, 573.7 nm)=?394(5) MHz yielding a change of mean square nuclear radii ofδ〈r ²〉=0.022(5) fm². In addition from our measurements the following values of the hyperfine splitting constantsA andB could be deduced Lu¹⁷⁶ 5d6s6p ⁴ F _3/2:A=?651.4(0.3) MHz,B=2,494(4) MHz 5d6s ^{2 2} D _3/2:A=138.0(0.3) MHz,B=2,131(3) MHz Lu¹⁷⁵ 5d6s6p ⁴ F _3/2:A=?924.7(0.5) MHz,B=1,767(4) MHz.     

Type I FA (Rb, Cs) and II FA (Li, Na) tunable laser activities and donor-acceptor properties of O and O at KCl (001) surface: first principle calculations

Type I F_A (Rb⁺, Cs⁺) and II F_A (Li⁺, Na⁺) tunable laser activities, adsorptivity and donor-acceptor properties of O and O⁻ adsorbates at the flat surface of KCl crystal were investigated using an embedded cluster model and ab initio methods of molecular electronic structure calculations. Ion clusters were embedded in a simulated Coulomb field that closely approximates the Madelung field of the host surface, and the nearest neighbor ions to the defect site were allowed to relax to equilibrium. Based on the calculated Stokes shifted optical transition bands, F_A tunable laser activities were found to be inversely proportional to the size of the dopant cation (Li⁺, Na⁺, Rb⁺, Cs⁺) relative to the host cation (K⁺). This relation was explained in terms of the axial perturbation of the impurity cation. The probability of orientational bleaching attributed to the RES saddle point ion configuration along the 〈110〉 axis was found to be inversely proportional to the size of the dopant cation, with activation energy barriers of ca. 0.44-3.34 eV. Surface relaxation energies of type II F_A centers were more important than those of type I F_A centers. In terms of defect formation energies, the products of type II F_A center imperfection were more stable than those of type I F_A. The difference between F or F_A band energies and exciton bands depended almost exclusively on the size of the positive ion species. As far as the adsorptivity of O and O⁻ is concerned, the results confirm that surface imperfection enhances the adsorption energies by ca. 4.38-16.37 eV. O and O⁻ penetrate through the defect-containing surface. The energy gap between the adsorbate and the defect containing surface and the donor-acceptor property of adsorbate play the dominant role in the course of adsorbate substrate interactions and the results were explained in terms of electrostatic potential curves and Mulliken population analysis.     

The Hd-center interaction with a Rb+ ion during irradiation and thermal annealing in kBr

The role of Rb⁺ ions on defect formation in KBr has been studied. The impurity suppresses colorability due to X-raying at 6 K, but does not result in the formation of any centers characteristic to Rb⁺ ions at this temperature. A new optical absorption band peaked at 3.19 eV is produced only by thermal annealing of irradiated KBr:Rb. This band is annealed in parallel to the annealing of the F band in a stage at 55 K, obeying second-order rate equation with an activation energy of 0.102 eV. This band is ascribed to the H_A(Rb⁺)-center. Calculation is made on the elastic interaction energy between the H-center and a Rb⁺ ion, to show that the interaction along 〈110〉 is repulsive, whereas that along 〈001〉 is attractive. Based on this result of calculation, the structure and the formation mechanism of the H_A(Rb⁺) center, and origin of suppression of colorability at 6 K are discussed. The difference in the interaction of the interstitial atom with Rb⁺ during its dynamical motion and thermal motion is emphasized.     

Stress-optical studies of VI B transition metal dichalcogenides

The effect of a uniaxial stress on the excitonic optical spectra are studied for MoS₂, WS₂, MoSe₂ and WSe₂. Stress dichroism appears in the A′, B′ excitons in diselenides, while it is absent in the A, B excitons in the four compounds. The A′, B′ excitons shift oppositely to the A, B excitons, indicating that A, B and A′, B′ are not pair excitons split by interlayer interaction.     

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.     

Formation time of F centers in alkali halides

It is argued that in most alkali halides an appreciable fraction of the self-trapped excitons may undergo non-radiative transitions from vibrationally excited states of B_3u to the A_1g state during the relaxation of the self-trapped excitons created by ionizing radiation. Numerical calculations show that the non-radiative transition probabilities, from vibrational levels of B_3u near or above the crossing point of the A_1g and B_3u potential curves to the A_1g state, are consistent with the observed formation times of F centers in various alkali halides. The exceptional case of KI is also discussed.     

Electron trapping and untrapping during the dark decay of flash-produced KC1 F′ and F′ > A(Na) bands between 290–350°K

Both the thermal lifetimes of the F′(F′_A) centers and the ratios of the electron trapping cross-sections of (F_A) to α(α_A) centers were derived from the decay kinetics of flash-produced F′(F′_A) bands. In particular, the ratios dropped steeply with the temperature possibly due to a thermally activated de-excitation following excited state pre-trapping of the electron by the anion vacancy.     

Magnetic ordering above room temperature in the sigma-phase of Fe66V34

Magnetic properties of four sigma-phase Fe_100−xV_x samples with 34.4?x?55.1 were investigated by Mössbauer spectroscopy and magnetic measurements in the temperature interval 4.2-300 K. Four magnetic quantities, viz. hyperfine field, Curie temperature, magnetic moment and susceptibility, were determined. The sample containing 34.4 at% V was revealed to exhibit the largest values found up to now for the sigma-phase for average hyperfine field, 〈B〉=12.1 T, average magnetic moment per Fe atom, 〈μ〉=0.89 μ_B, and Curie temperature, T_C=315.3 K. The quantities were shown to be strongly correlated with each other. In particular, T_C is linearly correlated with 〈μ〉 with a slope of 406.5 K/μ_B, as well as 〈B〉 is so correlated with 〈μ〉, yielding 14.3 T/μ_B for the hyperfine coupling constant.     

Spin-selective electron tunneling from excited FA centers to F+ centers in CaO

From pulsed phosphorescence microwave double resonance experiments it is established that electron tunneling between F_A and F⁺ centers in CaO occurs from F_A centers in the excited ³A₁ state. Only the radiative sub-levels within this ³A₁ state act as precursor states in the electron transfer process. The tunneling rate at 1.2 K is determined to be 1.5±0.2s^-1.     

Optical and thermal properties of F and F aggregate centers in KF

A detailed investigation has been carried out of the optical and thermal properties of centers such as F, F_A, M and M_A in pure, Na-doped and Li-doped KF. Most centers exhibit properties which are qualitatively similar to those produced in other potassium halides with the notable exception of the F_A (Na) center, which has both Type I and Type II behavior.     

Composite structure of the V1 band in strongly sodium-doped KBr

A detailed analysis was made of the thermal annealing of V₁ band in strongly Na-doped KBr crystal. It was found that two components (406 and 415 nm) exist in V₁ band region in such crystals. The 415 nm component, which increases with a decrease of the 406 nm component and shows a 〈110〉 type dichroism, has been assigned to be H_AA and/or H_A'A band.     

Superparamagnetism and spin-glass-like behavior in (La1−xTbx)2/3Ca1/3MnO3 Perovskite

The magnetic and magnetotransport properties of a series of (La_1−xTb_x)_2/3Ca_1/3MnO₃ oxides have been investigated. Substitutions ranging from x=0.0 to 0.4 have been used. In the largest 〈r_A〉 region, only ferromagnetism is observed. As 〈r_A〉 decreases, there is a transition from ferromagnetic to spin-glass order. As 〈r_A〉 decreases further, the ferromagnetism will completely disappear and a direct transition from superparamagnetic to spin-glass order will occur on cooling.     

Franck-Condon factors and R-centroids for the BΣ → AΠ band system of CN

Franck-Condon factors and R-centroids for the transition from the B²Σ electronic state of CN to the A²Π state have been calculated. The validity of the Morse potential energy function for this system has been studied by using the calculated values of α_e. Also, the ratios 〈rⁿ〉/〈rⁿ⁻¹〉 have been obtained to check the applicabili ty of the R-centroid approximation.     

Crystalline fields,exchange interactions and spin reorientations in TmxHo1−xFe2 systems,studied by a Yb170 Mössbauer probe

Mössbauer studies of the 84.3 keV gamma ray of Yb¹⁷⁰ emitted from Yb in Tm_xHo_1?xFe₂ at various temperatures were performed. These yield the crystalline field parameters (A₄〈r⁴〉 = 36 ± 5 K, A₆〈r⁶〉 = -3 ± 2K, the exchange field (μ_BH_exch = 116 ± 4 K) and the Yb³⁺ free ion hyperfine constants (H_4f = 4100 ± 150 kOe, eqQ = 2400 ± 250 MHz and H (conduction electrons) = 350±100 kOe). Spin relaxation phenomena observed in TmFe₂ at low temperatures give a value of ～ 0.03 for |?(E_F)J_sf|. Spectra observed in Tm_0.2Ho_0.8Fe₂ in the spin reorientation transition region indicate that the transition is of second order.