超冷原子向异核四聚物分子A3B的绝热转化

提出了利用Efimov共振辅助的受激拉曼绝热通道(ER-STIRAP) 过程实施超冷原子向异核四聚物分子A₃B转化的理论方案, 得到了转化过程中中间态分别为同核Efimov三聚物A₃和 异核Efimov三聚物A₂B两种途径下系统的暗态解, 证实了ER-STIRAP技术对超冷异核四聚物分子A₃B合成的可行性和有效性. 研究了外场参数, 包括缔合光脉冲的强度、脉宽、磁耦合强度及其失谐量等对A₃B形成的影响. 对两种不同中间态的转化途径进行比较发现, 与中间态为异核Efimov三聚物A₂B的途径相比, 经历中间态为同核Efimov三聚物A₃的途径时系统实现最终四聚物分子A₃B的产率更高. 另外, 还讨论了系统内禀的非线性和中间态的自发辐射损失对异核四聚物分子合成的影响.     

A<Subscript>4</Subscript> symmetry and lepton masses and mixing

Stimulated by Ma’s idea, which explains the tribimaximal neutrino mixing by assuming an A₄ flavor symmetry, a lepton mass matrix model is investigated. A Frogatt–Nielsen-type model is assumed, and the flavor structures of the masses and mixing are caused by the VEVs of SU(2)_L singlet scalars φ_i ^u and φ_i ^d (i=1,2,3), which are assigned to 3 and (1 ,1 ’,1 ”) of A₄, respectively. Possible charged lepton and neutrino mass spectra and mixing are investigated.     

Effects of Ca substitution on the phonon anomalies in the Raman scattering of YBa<Subscript>2</Subscript>Cu<Subscript>4</Subscript>O<Subscript>8</Subscript>

We report on the temperature dependence of the frequencies and linewidths in the phonon Raman scattering for Y _1-y Ca y Ba ₂ Cu ₄ O ₈ system ( y = 0 - 0.15). The phonon anomalies above T _c of the frequencies are observed for the out-of-phase O(2)-O(3) A _g and O(4) A _g modes, and these onset temperatures decrease with increasing Ca content. These features are consistent with the spin-gap behaviors associated with Ca doping reported previously. Furthermore, we find that the smaller gap exists near or just above the Ba phonon frequency at in the undoped samples and its energy increases with Ca doping. Received 9 June 1999     

Ab Initio Calculation of the Lowest Singlet and Triplet Excited States of the N<Subscript>2</Subscript> Molecule

Ab initio calculations of the adiabatic potential curves and matrix elements of radial nonadiabatic coupling of the N₂ molecule for the states related to dissociation limits I–V were performed. The most important spectral characteristics of the adiabatic states agreed well with the available experimental and theoretical data. The diabatic states were constructed. The diabatic quantum defects and radial matrix elements of the configuration interaction of the dissociative and Rydberg configurations whose states converge to the ground state \(X^{2}\Sigma{_g^+}\) and the first electronically excited state A²Π_u of the \(\rm{N_2^+}\) ion were calculated. The possibility of using the results for calculating the cross sections and rate constants of dissociative recombination and associative ionization within the framework of the multichannel quantum defect theory was discussed.     

Linear and nonlinear optical spectroscopy of gadolinium iron borate GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The optical spectra and the second-harmonic generation (SHG) are studied in a noncentrosymmetric GdFe₃(BO₃)₄ magnet. In the region of weak absorption (α～20–400 cm^?1) below ～3 eV, three absorption bands are distinguished, which can be unambiguously assigned to forbidden electronic transitions from the ground ⁶A₁ state of the Fe³⁺ ion to its excited states ⁴T₁(～1.4 eV), ⁴T₂(～2 eV), and ⁴A₁, ⁴E(～2.8 eV). Intense absorption begins in the region above 3 eV (α～2–4×10⁵ cm^?1), where two bands at ～4.0 and 4.8 eV are observed, which are caused by allowed electric dipole charge-transfer transitions. The spectral features of SHG in the 1.2–3.0-eV region are explained by a change in the SHG efficiency caused by a change in the phase mismatch. It is shown that in the weak absorption region, phase matching can be achieved for SHG.     

Magnetic phases in Mn<Subscript>1</Subscript>_<Subscript>x</Subscript>Fe<Subscript>x</Subscript>WO<Subscript>4</Subscript> studied by neutron powder diffraction

The magnetic structures of Mn_1-xFe_xWO₄ with x = 0.0, 0.16, 0.21, 0.225, 0.232, 0.24, 0.27, 0.29, and 1.0 were refined from neutron powder diffraction data. The magnetic phase diagram could be completed in the coexistence range of different magnetic structures up to x = 0.29. For the magnetic state at 1.5 K a commensurate antiferromagnetic structure with a propagation vector = (±1/4, 1/2, 1/2) was found for x ⩽ 0.22 while the magnetic spins order with = (1/2, 0, 0) for x ≥ 0.22. In the latter phase, additionally, weak magnetic reflections indexed to an incommensurate ordering with = (- 0.214, 1/2, 0.457) occur in the diffraction pattern up to x = 0.29 indicating the occurence of a reentrant phase. For 0.12 ⩽ x ⩽ 0.29 the low temperature phases are separated from a magnetic high temperature phase showing only magnetic reflections indexed to a spin arrangement with = (1/2, 0, 0). The magnetic phase diagram is discussed qualitatively considering random superexchange between the statistically distributed Mn²⁺- and Fe²⁺-ions in the coexistence range 0.12 ⩽ x ⩽ 0.29 of different magnetic structures related to those of pure MnWO₄ and FeWO₄. Received 9 October 2002 Published online 14 March 2003     

Influence of magnetic ordering on electronic structure of Tb<Superscript>3+</Superscript> ion in TbFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> crystal

Optical absorption spectra of trigonal crystal TbFe₃(BO₃)₄ have been studied in the region of ⁷F₆ → ⁵D₄ transition in Tb³⁺ ion depending on temperature (2–220 K) and on magnetic field (0–60 kOe). Splitting of the Tb³⁺ excited states, both under the influence of the external magnetic field and effective exchange field of the Fe-sublattice, have been determined. Landé factors of the excited states have been found. Stepwise splitting of one of the absorption lines has been discovered in the region of the Fe-sublattice magnetic ordering temperature. This is shown to be due to the abrupt change of equilibrium geometry of the local Tb³⁺ ion environment only in the excited state of the Tb³⁺ ion. In general, the magnetic ordering is accompanied by temperature variations of the Tb³⁺ local environment in the excited states. The crystal field splitting components have been identified. In particular, it has been shown that the ground state (in D ₃ symmetry approximation) consists of two close singlet states of A ₁ and A ₂ type, which are split and magnetized by effective exchange field of the Fe-sublattice. Orientations of magnetic moments of the excited electronic states relative to that of the ground state have been experimentally determined in the magnetically ordered state of the crystal. A pronounced shift of one of absorption lines has been observed in the vicinity of the TbFe₃(BO₃)₄ structural phase transition. The temperature interval of coexistence of the phases is about 3 K.     

Long- and short-range order in the Pd<Subscript>6</Subscript>B monoclinic superstructure and M<Subscript>6</Subscript>X<Subscript>5</Subscript> and M<Subscript>6</Subscript>X allied superstructures

Symmetry analysis of the Pd₆B monoclinic superstructure (space group C2/c) formed in the cubic (with the B1 structure) solid solution of boron in palladium (PdB _y ) has been carried out. The formation of this superstructure proceeds as a first-order phase transition via the disorder-order channel including nine nonequivalent superstructure vectors of four stars {k ₁₀}, {k ₄}, {k ₃}, and {k ₀}. For the Pd₆B monoclinic super-structure (space group C2/c), the distribution function for boron atoms is calculated and the interval of admissible values of the long-range order parameters is defined. It is shown that the transition channel determined in this way coincides with the channel in which the M₆X monoclinic superstructure (space group C2) is formed; therefore, the Pd₆B superstructure can also be described in space group C2 to the same degree of accuracy. The higher symmetry of the monoclinic model (space group C2/c) suggests that it describes the structure of the Pd₆B phase (Pd₆B□₅), as well as of mutually inverse phases M₆X□₅ and M₆X₅□, more adequately than the model with space group C2. It is shown that superstructures of the M₆X□₅ type (space groups C2/c, C2, C2/m, and P3₁) and inverse superstructures of the M₆X₅□ type with the same space groups have the positions of the nearest surrounding of metal atoms by two types of nonmetallic sublattice sites located in the first and second coordination spheres.     

Pressure-induced modifications of crystal and magnetic structure of oxygen deficient La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3-d</Subscript> manganites

The crystal and magnetic structures of the oxygen deficient manganites La_0.7Sr_0.3MnO_3-d (d = 0.15, 0.20) have been studied by means of powder neutron diffraction over the 0–5.2 GPa pressure and 10–290 K temperature ranges. La_0.7Sr_0.3MnO_2.85 exhibits a coexistence of rhombohedral and tetragonal (I4/mcm) crystal structures and below T_g ~ 50 K a spin glass state is formed. La_0.7Sr_0.3MnO_2.80 exhibits a tetragonal (I4/mcm) crystal structure. Below T_g ~ 50 K a phase separated magnetic state is formed, involving coexistence of C-type AFM domains with spin glass domains. In both compounds the crystal structure and magnetic states remain stable upon compression. The factors leading to the formation of different magnetic states in La_0.7Sr_0.3MnO_3-d (d = 0.15, 0.20) and their specific high pressure behavior, contrasting with that of the stoichiometric A_0.5Ba_0.5MnO₃ (A = Nd, Sm) compounds showing pressure-induced suppression of the spin glass state and the appearance of the FM state, are analysed.     

Formation of boride phases from individual powder components during the mechanical synthesis of Ni<Subscript>80</Subscript>Mo<Subscript>7</Subscript>B<Subscript>13</Subscript> alloy

Ni₈₀Mo₇B₁₃ nanocrystalline alloy containing a Ni(Mo,B) face-centered cubic (FCC) solid solution of the substitutional-interstitial type was obtained by high-energy ball milling of the component mixtures. In the temperature range 400–700°C, the metastable solid solution Ni(Mo,B) decomposes, leading to the formation of metastable FCC Ni(Mo) and HCP MoB₄ phases. Upon isothermal annealing at 1000°C for1 h, the alloy transforms into the stable state and contains the equilibrium phases FCC Ni(Mo), cubic Ni₂₁Mo₂B₆, and orthorhombic Ni₃B.     

Thermoelectric Properties of the TlBiS<Subscript>2</Subscript>-PbS Alloys

High efficiency of thermoelectric conversion can be achieved by using materials with a high Seebeck coefficient, high electrical conductivity, and low thermal conductivity. Mass-difference-scattering of the phonons is one of the most effective way for reducing the thermal conductivity in bulk thermoelectric materials. Investigations of transport phenomena in (TlBiS₂)_1-x (2PbS)_x alloys system have shown that in solid solutions of the (A³B⁵C ₂ ⁶ )_1-x (2A⁴B⁶)_x type at cation substitution according to scheme 2A⁴⁽⁺²⁾ A ³⁽⁺¹⁾ + B⁵⁽⁺³⁾ occurs a strong decrease of the lattice thermal conductivity. In the vicinity of x = 0. 50 the lattice part of thermal conductivity of (TlBiS₂)_1-x (2PbS)_x alloys decreases down to 0. 26 W/mK, i. e., it approaches the theoretical minimum. As a result, the thermoelectric figure of merit for these alloys ( 25%) exceeds the respective value for lead sulfide at room temperature.     

Magnetocaloric effect in the Sm<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript> manganite

The magnetocaloric effect ΔT has been studied by a direct method in two samples of the manganite Sm_0.55Sr_0.45MnO₃, namely, a single crystal (sample A) and a ceramic sample (sample C). The temperature dependences of the ΔT effect of both samples exhibit a maximum at T _max = 143.3 K for the sample A and T _max = 143 K for the sample C. In these maxima, the values of the ΔT effect are 0.8 and 0.4 K in the magnetic field H = 14.2 kOe for the samples A and C, respectively. In addition, the ΔT(T) curve of the sample A has a minimum at T _min = 120 K, in which ΔT = −0.1 K. The maximum value of the ΔT effect increases with an increase in the magnetic field H in the range of magnetic fields up to 14.2 kOe, and the rate of this increase at H > 8 kOe is higher than that at H < 8 kOe. These features of the ΔT effect are explained by the presence of ferromagnetic and antiferromagnetic A- and CE-type clusters in the samples.     

Response to the Comment: “On the computation of molecular auxiliary functions<Emphasis Type="Italic">A</Emphasis><Subscript>n</Subscript>and B<Subscript>n</Subscript>”

The Comment ‘on the computation of auxiliary functionsA _n(p) and B_n(pt)’ (FEHarris, Pramana - J. Phys. 61, C779 (2003)) is analysed in the arbitrary range of parametersn,p andpt. It is shown that our downward recursion approach forB _n(pt) in the range (n/pt) > 1 is more efficient than the well-known upward recursion method, and the upward recursion procedure forA _n(p) does not have merit for smaller non-zero values ofp (p < 001).     

Superposition of M<Subscript>5</Subscript>X<Subscript>5</Subscript> superstructures and X-ray diffraction in TiO<Subscript>1.0</Subscript> titanium monoxide

The interpretation of diffraction spectra of ordered high-temperature phases of solid solutions and strongly nonstoichiometric compounds is discussed. It has been shown that variations of the intensities of superstructure reflections, which cannot be explained within simple ordering models, can be due to the superposition of superstructures with different symmetries in the matrix of the basis crystal structure. Using an example of atom–vacancy ordering in TiO_1.0 titanium monoxide, a model of the order–order transition state formed by the superposition of low-temperature monoclinic (space group A2/m (C2/m)) and high-temperature cubic (space group Pm3?m) M₅X₅ superstructures has been proposed. It has been shown that the transition state is thermodynamically equilibrium and should be implemented instead of the M₅X₅ cubic superstructure. The transition state model can be considered as an M_(5–i)X_(5–i) superstructure (i = 1, 14/18, 11/18) with the monoclinic symmetry (space group P1m1).     

Comment: “On the computation of molecular auxiliary functions<Emphasis Type="Italic">A</Emphasis><Subscript>n</Subscript>and<Emphasis Type="Italic">B</Emphasis><Subscript>n</Subscript>”

Guseinov, Mamedov, Kara and Orbay (Pramana - J. Phys. 56, 691 (2001)) propose methods for evaluating the molecular auxiliary functionsA _n(p) andB _n(pt) for the range 17 ≤n ≤ 60 and 25 ≤pt ≤ 60. However, their procedure forA _n(p) is not new, and that forB _n(pt) is less efficient for their target range than another well-known method. Their approach does have merit for smaller non-zero values ofpt. Two minor errors in table 1 of their paper are also identified.     

Magnetization and specific heat of the dimer system CuTe<Subscript>2</Subscript>O<Subscript>5</Subscript>

We report on magnetization and specific heat measurements on single-crystalline CuTe₂O₅. The experimental data are directly compared to theoretical results for two different spin structures, namely an alternating spin-chain and a two-dimensional (2D) coupled dimer model, obtained by Das et al. [Phys. Rev. B 77, 224437 (2008)]. While the analysis of the specific heat does not allow to distinguish between the two models, the magnetization data is in good agreement with the 2D coupled dimer model.     

Structural and magnetic phase transitions in Pr<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at high pressures

The crystal structure and Raman spectra of Pr_0.7Ca_0.3MnO₃ manganite at high pressures of up to 30 GPa and the magnetic structure at pressures of up to 1 GPa have been studied. A structural phase transition from the orthorhombic phase of the Pnma symmetry to the high-pressure orthorhombic phase of the Imma symmetry has been observed at P ∼ 15 GPa and room temperature. Anomalies of the pressure dependences of the bending and stretching vibrational modes have been observed in the region of the phase transition. A magnetic phase transition from the initial ferromagnetic ground state (T _C = 120 K) to the A-type antiferromagnetic state (T _N = 140 K) takes place at a relatively low pressure of P = 1 GPa in the low-temperature region. The structural mechanisms of the change of the character of the magnetic ordering have been discussed.     

Conformational analysis of <Emphasis Type="Italic">o</Emphasis>-allylphenol by density functional and IR spectroscopy methods

The two-dimensional cyclic potential energy surfaces for internal rotation of the allyl substituent and its vinyl fragment in o-allylphenol (o-APh) depending on the OH group orientation relative to the allyl substituent were constructed by a B3LYP/6-31G method. It is shown that o-APh exists in the gas phase as a mixture of eight non-planar rotamers (A, B, C, D, E, F, G, and H) and their eight optical isomers (A ₁, B ₁, C ₁, D ₁, E ₁, F ₁, G ₁, and H ₁). An intramolecular H-bond (IHB) O–H...π occurs only in four rotamers (A, B, A ₁, and B ₁). The content of such rotamers in the gas phase is 47.2% (as calculated by the B3LYP/cc-pVTZ method). Taking into account the solvation effect in the polarizable continuum model (PCM) for a solution of o-APh in cyclohexane decreases the total content of rotamers with an IHB (A and B) to 37.7%. The ratio of rotamers with OH groups bonded by an IHB and with free OH groups that is predicted theoretically agrees with the value measured experimentally from IR spectra of o-APH in CCl₄ solution.     

Conditions favoring the polar weak ferromagnetic state in BiFeO<Subscript>3</Subscript>-type multiferroics

The crystal structure and magnetic properties of Bi_{1 − x} A _x FeO_{3 − x/2} (A = Ca, Sr, Pb, Ba), Bi_{1 − x} A _x (Fe_{1 − x} Ti _x )O₃, and Bi_{1 − x} A _x (Fe_{1 − x/2}Nb _x/2)O₃ solid solutions have been studied. It is shown that the homogeneous polar weak ferromagnetic state occurs in the vicinity of a morphotropic phase boundary in the systems where dopant ions lead to the reduction of the unit cell volume in the polar phase. In the case of A = Ca, the non-polar phase also exhibits weak ferromagnetism and the spontaneous magnetizations in the polar and nonpolar phases differ only slightly.     

Luminescence and scintillation properties of Y<Subscript>3</Subscript>A<Subscript>l5</Subscript>O<Subscript>12</Subscript>:Ce single crystals and single-crystal films

Luminescence and scintillation properties of Y₃A_l5O₁₂:Ce single crystals grown from the melt by the Czochralski and horizontal directed crystallization methods in various gas media and Y₃A_l5O₁₂:Ce single-crystal films grown by liquid-phase epitaxy from a melt solution based on a PbO-B₂O₃ flux have been comparatively analyzed. The strong dependence of scintillation properties of Y₃A_l5O₁₂:Ce single crystals on their growth conditions and concentrations of Y_Al antisite defects and vacancy defects has been established. Vacancy defects are involved in Ce³⁺ ion emission excitation as the centers of intrinsic UV luminescence and trapping centers. It has been shown that Y₃A_l5O₁₂:Ce single-crystal films are characterized by faster scintillation decay kinetics than single crystals and a lower content of slow components in Ce³⁺ ion luminescence decay during high-energy excitation due to the absence of Y_Al antisite defects in them and low concentration of vacancy defects. At the same time, the light yield of Y₃A_l5O₁₂:Ce single-crystal films is comparable to that of single crystals grown by directed crystallization due to the quenching effect of the Pb²⁺ ion impurity as a flux component and is slightly lower (∼25%) than the light yield of single crystals grown by the Czochralski method.