q-analog ofA m−1 ⊕A n−1 ⊂A mn−1

A natural embeddingA _m–1 A _n–1 A _mn–1 for the corresponding quantum algebras is constructed through the appropriate comultiplication on the generators of each of theA _m–1 andA _n–1 algebras. The above embedding is proved in theq-boson realization by means of the isomorphism between theA _q ^– (mn ~ ⁿ A _q ^– (m) ~ ^m A _q ^– (n)algebras.     

Substrate effect on the electronic structure in GaxIn1−xAsySb1−y/GaSb and GaxIn1−xAsySb1−y/InAs

We present a theoretical study of the substrate effect on electronic structure in cubic Ga_xIn_1−xAs_ySb_1−y lattice-matched to GaSb and InAs. Our calculations are based on the empirical pseudopotential formalism within the virtual crystal approximation where the effect of disorder is taken into account. We show that the electronic band structure of the quaternary alloy Ga_xIn_1−xAs_ySb_1−y is altered by the change of substrate for the entire range of alloy compositions x. Moreover, we find that at Ga concentrations (x≤0.8), the ionicity is less important when Ga_xIn_1−xAs_ySb_1−y is lattice-matched to GaSb instead of InAs. The information will be useful for the choice of substrate in the composition range 0–1 and hence for the determination of the lattice-matching conditions for Ga_xIn_1−xAs_ySb_1−y quaternary materials.     

Magnetic susceptibility studies of Zn1−xFexSe and Zn1−xFexS

The magnetic susceptibility of diluted magnetic semiconductors Zn_1−xFe_xSe and Zn_1−xFe_xS has been calculated within a crystal field model and by applying the extended nearest-neighbour pair approximation. The results are compared with available experimental data. It appears that a good agreement between theory and experiment is obtained for Zn_1−xFe_xSe taking the long-range exchange comprising several nearer interionic distances, whereas in Zn_1−xFe_xS the exchange interaction with nearest-neighbours alone gives a better solution.     

High-spin states in the T z = −1/2 nucleus 55Ni

High-spin states of the isospin T _z = ? 1/2 nucleus ⁵⁵Ni have been identified for the first time by means of the reaction ²⁸Si(³⁶Ar,2αn) at 143 MeV beam energy. The Gammasphere array together with ancillary detectors was used to detect γ rays in coincidence with evaporated light particles. The level scheme of ⁵⁵Ni comprising four transitions is compared to its mirror partner ⁵⁵Co and shell-model calculations in the fp shell.     

(T1u+T1g)⊗(hg+τ1u)vibronic interaction and superconductivity in C 60 n− fullerides

The closeness of low-lying T_1u and T_1g levels of C ₆₀ ^? could enable their mixing under an odd parity vibration of (T_{1 u} + T_{1 g} ? (h_g + τ_{1 u})type. In addition, the two levels are susceptible to Jahn-Teller interaction due to five-fold degenerate h_g vibrations. This complex problem of (T_1u+T_1g)?(h_g+τ_1u) vibronic interaction is transformed to a form similar to T_2g ? (ε_g + τ_2g) vibronic problem of octahedral symmetry. The problem is analysed in an infinite coupling model and compared with the experimental spectroscopic results for the C ₆₀ ^? radical. The resulting parameters are used to calculate the pair-binding energy and superconducting transition temperature in C ₆₀ ^n? fullerides. Vibronic mixing with the T_1g level is found to be responsible for maximising the pair-binding energy at the doping level n=3. It is also found to be an important source of T_c enhancement.     

Spinglass behaviour of Zn1−xMnxSe and Zn1−xMnxTe semimagnetic semiconductors

The magnetic susceptibility of Zn_1−xMn_xSe (0.02 ≤ × ≤ 0.53) and Zn_1−xMn_xTe (0.07 ≤ × ≤ 0.21) was investigated in the temperature range 10 mK < T < 40 K. A paramagnetic spinglass transition was observed in the whole concentration range. The concentration dependence of the freezing temperature T_f was found to be compatible with a radial dependence of the exchange interaction between manganese ions of the type J(R) ∝ R^−6.8. It appears from the available data that this radial dependence is rather universal for all II–VI wide gap semimagnetic semiconductors. A comparison is also made with other semimagnetic semiconductors and the physical exchange mechanism is discussed.     

Unique features of the magnetic susceptibility of Hg1−xMnxTe1−ySey

The magnetic susceptibility of the new semimagnetic solid solutions Hg_1–xMn_xTe_1–ySe_y (0Mn = f(T) are caused by the presence within the specimen of Mn-Te-Mn-Te, Mn-Se-Mn-Se, and Mn-Te-Mn-Se type clusters, in which indirect exchange action of an antiferromagnetic character occurs among the Mn atoms by means of the Te or Se atoms.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 60–62, April, 1991.     

Superconductivity of (Sn1−z Pbz)1−x InxTe alloys

The temperature dependences of the electrical resistivity of (Sn_1?z Pb_z)_1?x In_xTe alloys with different concentrations of lead (z=0–0.60) and indium (x=0.03–0.20) were studied at temperatures T=0.4–4.2 K in magnetic fields from zero to H=15 kOe. A resistivity drop of no less than three-four orders of magnitude was observed in this range of alloy compositions. Application of a magnetic field above a critical level resulted in a recovery of the sample resistivity to the original value. The observed resistivity drop is identified with a superconducting transition. The critical parameters of the superconducting transition (T _c and H _c2) were determined at the drop to one half the normal resistivity level. Experimental dependences of the critical supercon-ducting-transition temperature T _c and of the second critical magnetic field H _c2 on the contents of lead (z) and indium (x) were measured. The data obtained confirm a strong localization of the In impurity states and are evidence of the extrinsic nature of superconductivity in the class of materials under study. It was established that as the Pb content in (Sn_1?z Pb_z)_1?x In_xTe increases, T _c and H _c2 decrease as the Fermi level E _F (fixed in the In impurity resonance band) leaves the Δ extremum and the superconductivity breaks down when E _F leaves the LΣ saddle point in the valence-band energy spectrum.     

An−1(1) reflection K-matrices

We investigate the possible regular solutions of the boundary Yang–Baxter equation for the vertex models associated with the A_n−1⁽¹⁾ affine Lie algebra. We have classified them in two classes of solutions. The first class consists of n(n−1)/2 K-matrix solutions with three free parameters. The second class are solutions that depend on the parity of n. For n odd there exist n reflection K-matrices with 2+[n/2] free parameters. It turns out that for n even there exist n/2 K-matrices with 2+n/2 free parameters and n/2 K-matrices with 1+n/2 free parameters.     

The effect of frenkel defects on the electronic structure of 1T−TiSe2

Using a tight binding formalism the wave vector resolved electronic density of states has been calculated in the coherent potential approximation for an 1 T–TiSe₂ crystal containing Frenkel defects. Additional structure originating from disorder is discussed. An extra peak in the A direction at 1.5 eV binding energy can be traced back to a resonance caused by titanium vacancies.     

Defects and transport in SrFe1−xCoxO3−δ

The non-stoichiometry and chemical diffusion coefficient of SrFe_1−xCo_xO_3-δ have been measured by steady state and transient thermogravimetry in the temperature range 750–1200 °C at different oxygen partial pressures. At high oxygen partial pressures, the chemical diffusion coefficient was in the range 1·10⁻⁴ – 7·10⁻⁴ cm²/s. This, combined with high concentration of disordered vacancies make these materials perhaps the fastest solid oxygen ion diffusers known at high temperatures and high oxygen partial pressures. However, due to the high concentration of defects in SrFe_1−xCo_xO_3-δ the compound transforms from a cubic (disordered) perovskite to a brownmillerite type of structure under reduced oxygen partial pressures below approx. 900 °C. Due to this phase transition, the mobility of oxygen vacancies in SrFe_1−xCo_xO_3-δ decreases up to about an order of magnitude at 850 °C. We also observe an ordering effect at 1000 °C, although smaller in size, and this is suggested to be due to short range ordering of four-coordinated polyhedra of Fe. For possible use as oxygen separation membranes, phase stability against sulphur and carbon containing atmospheres is also discussed with respect to the formation of carbonates and sulphates. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Superconductivity in CeO1−xFxFeAs with upper critical field of 94 T

We have successfully synthesized Ce based oxypnictide superconductors with fluorine doping (CeO_1?xF_xFeAs) by a two step solid state reaction method. Detailed XRD and EDX confirm the crystal structure and chemical compositions. We observe that an extremely high H_c2(0) of 94 T can be achieved in the x = 0.1 composition. This increase in H_c2(0) is accompanied by a decrease in transition temperature (38.4 K in x = 0.1 composition) from 42.5 K for the x = 0.2 phase. The in-plane Ginzburg–Landau coherence length is estimated to be ～27 Å at x = 0.2 suggesting a moderate anisotropy in this class of superconductors. The Seebeck coefficient confirms the majority carrier to be electrons and strong dominance of electron–electron correlations in this multiband superconductor.     

LaAg x In1−x

The structures of LaAg _x In_1–x alloys withx=0.75, 0.89 are determined by neutron diffraction on powder samples. The space group isI4/mmm (D _4h ^/17 ). The lattice constants splitting, the order parameter and the mean square vibrational amplitudes of the atoms are given in the temperature range from 20 KT300 K.     

Dielectric properties of NaCl1−xBr and KCl1−xBrx mixed ionic solution

In the present paper a three-body potential model (TBPM) has been employed for the analysis of dielectric behaviour of NaCl-NaBr and and KCl-KBr mixed crystals with varying compositions. The physical properties like dielectric constants ( ₀ and ), optic mode frequencies ( _OLO and _TO), effective charge parameter (e _s ^* ), optic mode Grüneisen parameters and strain derivatives of ₀ and dielectric constants have been calculated. The results achieved in the present study are found in fairly good agreement with the available experimental data. The results obtained by previous investigators are also shown for the sake of comparison.     

Dielectric and piezoelectric properties of (Bi1−x−yNdxNa1−y)0.5BayTiO3 lead-free ceramics

New lead-free (Bi_1−x−yNd_xNa_1−y)_0.5Ba_yTiO₃ ceramics were prepared by a conventional ceramic technique and their dielectric and piezoelectric properties were studied. X-ray diffraction studies reveal that Nd³⁺ and Ba²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure, and a morphotropic phase boundary (MPB) of rhombohedral and tetragonal phases is formed at 0.04 < y < 0.10. The partial substitutions of Nd³⁺ and Ba²⁺ decrease effectively the coercive field E_c and increase significantly the remanent polarization P_r. Because of lower E_c, larger P_r and the formation of the MPB, the piezoelectric properties of the ceramics are significantly enhanced at x/y = 0.02/0.06: d₃₃ = 150 pC/N and k_p = 30.5%. The ceramics exhibit relaxor characteristic, which is probably resulted from the cation disordering in the 12-fold coordination sites. The depolarization temperature T_d shows a strong compositional dependence and reaches a minimum value at the MPB. The temperature dependences of the ferroelectric and dielectric properties suggest that the ceramics may contain both the polar and non-polar regions near the depolarization temperature T_d, which cause the polarization hysteresis loop become deformed near/above T_d.     

Martensitic transformation in La1−x Yb x Ag1−y In y

The structural phase transformation of La_1–x Yb _x Ag_1–y In _y has been studied on single crystals by low temperature Laue-technique. The martensitic transformation in this pseudobinary intermetallic alloy has to be characterized as a weak orthorhtombic distortion of a single I centered unit cell (c/a1.04;a/b1.006) and a collective slipping or twinning of these cells that gives a fixed orientation between the remaining cubic room temperature structure and the martensitic phase. Above room temperature exists an order-disorder transformation from the CsCl-B2 structure to an at room temperature metastable W-A2 structure. There is no dramatic change in the physical properties of this alloy by substituting La by Yb, so we may approximate our results to LaAg_1–y In _y .This paper is dedicated to Prof. Dr. S. Methfessel on the occasion of his 60^th birthday     

Optical properties of Li1−xNb1−xEu2xO3 solid solutions

Abstract

The effects of europium substitution on crystalline solubility, structural changes and optical properties of LiNbO₃ is reported. Li_1?xNb_1?xEu_2xO₃ solid solutions exist over a very limited range of europium concentrations (x ≤ 0.01). The solid solutions were characterized by X-ray powder diffraction and density measurements. Within the range of compositions of the solid solutions, the optical properties (emission and excitation spectra) of the Eu³⁺ ions have been characterized. The obtained optical data indicate that two equally abundant europium luminescent species exist in the stoichiometric speciments; these correspond to the Eu³⁺ ions occupying the Li⁺ and Nb⁵⁺ sites, in agreement with the replacement mechanism which is inferred from density measurements. Other Eu³⁺ luminescent species which appear to be closely related with lithium deficiency were found to exist in nonstoichiometric samples, in addition to those which are present in the stoichiometric materials.     

Pathologies of the large-N limit for RPN−1, CPN−1, QPN−1 and mixed isovector/isotensor σ-models

We compute the phase diagram in the N→∞ limit for lattice RP^N−1, CP^N−1 and QP^N−1 σ-models with the quartic action, and more generally for mixed isovector/isotensor models. We show that the N=∞ limit exhibits phase transitions that are forbidden for any finite N. We clarify the origin of these pathologies by examining the exact solution of the one-dimensional model: we find that there are complex zeros of the partition function that tend to the real axis as N→∞. We conjecture the correct phase diagram for finite N as a function of the spatial dimension d. Along the way, we prove some new correlation inequalities for a class of N-component σ-models, and we obtain some new results concerning the complex zeros of confluent hypergeometric functions.