Bi1-xYbxFeO3的结构相变与磁性

用溶胶-凝胶法制备了Bi_1-xYb_xFeO₃(0≤x≤0.2)粉晶样品,并用X射线衍射光谱和拉曼光谱对其结构进行了研究. 结果表明, 在x=0.1～0.125, 发生了结构相变, 由菱形R3c结构变为正交Pnma结构,对应的应该是从铁电相到顺电相的转变. 在结构相变边界,磁化强度达到最大值. 其原因是在低浓度掺杂区域,随着Yb³⁺的掺杂浓度的增大,BiFeO₃的自旋螺旋结构受到抑制,部分未被抵消的磁矩被释放出来,到了相变边界,自旋螺旋结构受到的抑制达到最大,磁矩被完全释放出来. 随着x的继续增大,磁化强度开始逐渐减小,这是由于逐渐形成了良好的反铁磁序排列.     

Exactly solvable model of a structural phase transition

An exactly solvable d-dimensional model for the structural phase transition with long-range anharmonic interaction is considered. Classical as well as purely quantum regimes are discussed within the framework of the method of approximating Hamiltonians.     

Structural instability in high-T c superconducting oxides

Two broad internal friction (attenuation as well) peaks in YBCO measured at around 120 K and 250 K are found to exhibit the characteristics of a first-order phase transition. X-ray diffraction indicates no symmetry change on cooling from room temperature but only a downward jump of the lattice parameters was observed. This is referred to as a phase-like transition (PLT) and further confirmed by stress-strain, specific heat and Debye-Waller factor measurements. There always occurs a phase-like transition at 10-30 K above T _c in YBCO, BSCCO and TBCCO which disappears in the non-superconducting phases. Ultrasonic studies on single-crystal BSCCO reveal a pronounced elastic anisotropy in the c-plane and a velocity minimum associated with PLT. The overall trend of elastic stiffening below room temperature and some discrepancies and questions in elasticity measurements so far observed for high-T _c oxides have been explained or clarified in terms of an abnormal fast change of lattice parameters.     

High T c,structural instabilities and electronic properties of Nb3Ge

Abstract

A number of Nb₃Ge samples with rather high T _c,on (T _c,on ? 22.5 K) were prepared by CVD and characterized by the electrical resistance, X-ray diffraction and scanning electron microscope patterns. It was found that the crystal growth patterns are roughly divided into two categories, (i) a particles-pattern in a nearly single phase of Nb₃Ge and (ii) a network pattern with a fairly large amount of the Nb₅Ge₃ phase but with rather a large resistance ratio and a small resistivity at T ? T _c,on. The latter type of samples shows generally higher T _c,on. It is deduced that the origin of high T _c,on comes mainly from a deformation of the atomic potential at the Nb-site into an anharmonic and rather unstable shape due to the coexistence of the Nb₅Ge₃ phase. The mechanism of the formation of different patterns is discussed on the basis of the theory of non-equilibrium thermodynamics of Prigogine.     

Raman study of KMnF3 perovskite crystals doped by Na+

A study of the influence of cationic Na⁺ substitution in the archetype KMnF₃ perovskite crystal was performed using the Raman method. The Raman spectra of mixed K_1-xNa_xMnF₃ crystals with x = 0.029, 0.048 and 0.065 were recorded versus temperature and fully interpreted in terms of a “one mode” behaviour. In addition to the soft mode not completely vanishing close to Tc, attention was especially paid to evidence of static and dynamical disorder. From this point of view the behaviour of the hard Raman modes versus temperature has been studied together with two unexpected Raman bands in the cubic phase. The interpretation has been made within the more general framework of structural disorder existing in such perovskites with anisotropic interactions.     

Phase transition in the ternary fluorides Li3Alf6 and Li3CrF6

The phase transitions of two ternary fluorides have been reinvestigated by thermal and X-ray diffraction analyses. Four phase transitions have been detected at T ₁ = 483 ± 5 K, T ₂ = 548 ± 5 K, T ₃ = 765 ± 5 K and T ₄ = 867 ± 5 K for Li₃AlF₆. Only two transformations occurred at T ₁ = 550 ± 5 K and T ₂ = 660 ± 10 K for Li₃CrF₆. Dielectric and optical measurements performed respectively on ceramics and single crystals revealed a ferroelectric behaviour coupling partially with ferroelastic coupling for the two investigated fluorides.     

Integration of structural and functional magnetic resonance imaging improves mild cognitive impairment detection

The identification of mild cognitive impairments (MCI) via either structural magnetic resonance imaging (sMRI) or functional MRI (fMRI) has great potential due to the non-invasiveness of the techniques. Furthermore, these techniques allow longitudinal follow-ups of single subjects via repeated measurements. sMRI- or fMRI-based biomarkers have been adopted separately to diagnose MCI; however, there has not been a systematic effort to integrate sMRI- and fMRI-based features to increase MCI detection accuracy. This study investigated whether the detection of MCI can be improved via the integration of biomarkers identified from both sMRI and fMRI modalities. Regional volume sizes and neuronal activity levels of brains from MCI subjects were compared with those from healthy controls and used to identify biomarkers from sMRI and fMRI data, respectively. In the subsequent classification phase, MCI was automatically detected using a support vector machine algorithm that employed the identified sMRI- and fMRI-based biomarkers as an input feature vector. The results indicate that the fMRI-based biomarkers provided more information for detecting MCI than the sMRI-based biomarkers. Moreover, the integrated feature sets using the sMRI- and fMRI-based biomarkers consistently showed greater detection accuracy than the feature sets based only on the fMRI-based biomarkers. The results demonstrate that integration of sMRI and fMRI modalities can provide supplemental information to improve the diagnosis of MCI relative to either the sMRI or fMRI modalities alone.     

Pressure Study on New Superconducting Germanium Clathrates

We investigate the low temperature properties of the recently discovered clathrates Ba 6 Ge 25 and Na 2 Ba 4 Ge 25 by tuning both materials with hydrostatic pressure. At ambient pressure, Ba 6 Ge 25 undergoes a two-step structural phase transition between 230 K and 180 K from metallic behavior to a high-resistivity state. A superconducting transition occurs at T_{C}\approx 0.24\,\hbox{K} out of the resulting bad metal ( \rho_{0}\approx 1.5\,\hbox{m}\Omega\;\hbox{cm} ). With increasing pressure, the structural phase transition is shifted to lower temperature but T C increases drastically. T C reaches a maximum value of 3.85 K at the critical pressure p_{C}\approx 2.8\,\hbox{GPa} , where the structural distortion is completely suppressed and the system exhibits metallic behavior. On replacing 1/3 of the Ba atoms with Na (Na 2 Ba 4 Ge 25 ), no structural transformation is observed below room temperature, and the superconducting transition temperature is higher (T_{C}(p = 0) \approx 1.05\,\hbox{K}) than in the undoped case at ambient pressure but decreases slightly with increasing pressure.     

Pricing Defaultable Bonds in a Markov Modulated Market

We address the problem of pricing defaultable bonds in a Markov modulated market. Using Merton's structural approach we show that various types of defaultable bonds are combination of European type contingent claims. Thus pricing a defaultable bond is tantamount to pricing a contingent claim in a Markov modulated market. Since the market is incomplete, we use the method of quadratic hedging and minimal martingale measure to derive locally risk minimizing derivative prices, hedging strategies and the corresponding residual risks. The price of defaultable bonds are obtained as solutions to a system of PDEs with weak coupling subject to appropriate terminal and boundary conditions. We solve the system of PDEs numerically and carry out a numerical investigation for the defaultable bond prices. We compare their credit spreads with some of the existing models. We observe higher spreads in the Markov modulated market. We show how business cycles can be easily incorporated in the proposed framework. We demonstrate the impact on spreads of the inclusion of rare states that attempt to capture a tight liquidity situation. These states are characterized by low risk-free interest rate, high payout rate and high volatility.