混合团簇Cu13-nAgn(n≤13)基态结构的遗传算法研究

在混合团簇基态结构优化的遗传算法方案中增加了交换算子,结合Gupta紧束缚模型势研究了Cu     

单层GaSe表面Fe原子吸附体系电子自旋性质调控

Ⅲ族金属单硫化物因其优越的光电和自旋电子特性而备受关注,实现对其自旋性质的有效调控是发展器件应用的关键.本文采用密度泛函理论系统地研究了GaSe表面Fe原子吸附体系的几何构型及自旋电子特性.Fe/GaSe体系中Fe吸附原子与最近邻Ga,Se原子存在较强的轨道耦合效应,使体系呈现100%自旋极化的半金属性.其自旋极化贡献主要来源于Fe-3d电子的转移及Fe-3d,Se-4p和Ga-4p轨道杂化效应.对于Fe双原子吸附体系,两Fe原子之间的自旋局域导致原本从Fe转移至GaSe的自旋极化电荷量减少,从而费米能级附近的单自旋通道转变为双自旋通道,费米能级处的自旋极化率转变为0.研究结果揭示了Fe_n/GaSe吸附体系自旋极化特性的形成和转变机制,可为未来二维自旋纳米器件的设计与构建提供参考.     

Magneto-volume effect and invar phenomena in ferromagnetic metals

The magneto-volume effect is discussed in terms of the recent unified theory of ferromagnetic metals based on general spin fluctuations. Explanations are given for the large negative thermal expansion observed in weakly ferromagnetic metals and for the invar effect in some f.c.c. alloys. Large thermal expansion in nearly ferromagnetic metals is also pointed out.     

Phase transition and anisotropy of thermal expansion in TlInS2

Linear expansion coefficients parallel and perpendicular to the layer plane of TlInS₂ layer crystal were measured in the temperature range 20–250 K at T { 200 K a strong anomaly in α⊥ behaviour was observed—the value of α6 increased abruptly up to the 200 × 10^?6 K^?1 due to phase transition in this crystal. It was shown that there is no anomaly in the behaviour of linear expansion coefficient α_z.dfnc;. This fact allowed to conclude that the phase transition in TlInS₂ is caused by changes in interlayer distances.     

Magneto-volume effect in invar-type amorphous Fe—B alloys

Amorphous Fe_100-xB_x(11.5 ≦ x ≦ 22) alloys having the invar characteristics were prepared by a single roller quenching method to investigate the magneto-volume effect. Forced volume magnetostriction, ?ω/?H, increased remarkably with decreasing boron content and the maximum value obtained was about 90 × 10^?10 Oe^?1 at 11.5 at.% boron. The estimated value of the pressure dependence of the Curie temperature, ?T_c/?P, was considerably large, being comparable to those of crystalline FeNi invar alloys. The ?T_c/?P curve plotted as a function of T_c approximately fitted Wohlfarth's expression.     

Specific heat and thermal expansion in the heavy RECu2 compounds

Specific heat and thermal expansion measurements in the heavy RECu₂ compounds reveal several magnetic transitions in most of the studied compounds. Broad Schottky-type anomalies are observed above and below the Néel temperature. For ErCu₂ the magnetic entropy, the magnetic volume, the crystal field splitting and its relative volume dependence have been determined.     

Abnormal positive thermal expansion in Mo substituted ZrW2O8

ZrW₂O₈ displays the unusual property of an isotropic bulk contraction in volume as a function of temperature. We report here on the positive thermal expansion (PTE) property caused by substituting Mo for W sites in ZrW₂O₈ at room temperature. The room temperature crystal structure of ZrW_2−xMo_xO₈ compounds, which were synthesized using a low temperature route, could be divided into ordered phase with α-ZrW₂O₈ structure (0≤x≤0.5) and disordered phase with β-ZrW₂O₈ (0.5<x≤1.5) and c-ZrMo₂O₈ structure (1.5<x≤2). ZrW_2−xMo_xO₈ adopting β-ZrW₂O₈ structure shows abnormal PTE property at room temperature due to the existence of water molecules, while ZrW_2−xMo_xO₈ adopting the other two structures (α-ZrW₂O₈ and c-ZrMo₂O₈) shows negative thermal expansion (NTE) property from room temperature until decomposition for α-ZrW₂O₈ structure or trigonal phase transition for c-ZrMo₂O₈ structure. The reason lies in the fact that the oxygen migration caused by the Mo substitution leads to the re-arrangement of W(Mo)O₄ tetrahedra lying along the 3-fold axis, only particular W/Mo ratio (0.5<x≤1.5) produces a special crystal structure, which allows the entrance of water molecules.     

Anomalous thermal expansion of CeAl3 due to valence mixing

The lattice parameters of the compounds CeAl₃, LaAl₃, PrAl₃, and GdAl₃ have been measured with x-rays in the range $\text{5}\text{K}{}^{\mathrm{\le }}\text{T}{}^{\mathrm{\le }}\text{300}\text{K}$. In spite of their similarity, a significant difference between CeAl₃ and the other compounds is found in the relative volume change $\text{Δ}\text{V}\text{V}\text{(}\text{T}\text{)}$. This is attributed to an increasing admixture of Ce⁴⁺ with decreasing temperature. Furthermore, cristalline-field effects are observed in the case of PrAl₃.     

Unusual thermal conductivity of the negative thermal expansion material, ZrW2O8

In order to investigate the relationship between negative thermal expansion and other thermal properties, the thermal conductivity of the α-phase of ZrW₂O₈ has been determined from 1.9 to 390 K. In addition, the heat capacity was measured from 1.9 to 300 K. The thermal conductivity of ZrW₂O₈ is low, glass-like and close to its theoretical minimum value. The phonon-phonon coupling of the highly anharmonic low-frequency modes which are responsible for negative thermal expansion in ZrW₂O₈ appears to be highly efficient, leading to short phonon mean free paths and exceptionally low thermal conductivity.     

Low temperature thermal expansion of NaNO2 along the ferroelectric b-axis

Measurements of the low temperature thermal expansion of ferroelectric NaNO₂ along the ferroelectric b-axis have been performed with a three terminal capacitance dilatometer, both in single domain and multidomain state.A contribution, due to domain walls, is present when the sample is in multidomain state. An Einstein contribution with ?_E ～ 120 K is present in both domain-states of the sample.     

Magnetic anomalies of thermal expansion in (rare-earth) Ni2 intermetallic compounds

The thermal expansion of eight RENi₂ intermetallic compounds (RE = La, Nd, Sm, Tb, Dy, Ho, Er, Tm) and YNi₂ have been measured between 3.8 and approx. 300 K. The anomalies found at the paramagnetic-magnetically ordered phase transition have allowed an accurate determination of the Curie temperatures. From the low temperature comparison of the thermal expansion of the light RE compounds with that of LaNi₂, and of the heavy RE compounds with that of YNi₂, the Debye temperatures have been obtained. In the case of the compounds DyNi₂, TbNi₂ and HoNi₂ the contributions from crystal field and exchange interactions to the spontaneous volume strain at the magnetically ordered phase have been calculated, the agreement between experiment and calculation being good.     

ZrV2-xPxO7固溶体的相变与热膨胀性质的研究

本文采用固相烧结法制备了ZrV2-xPxO7@=0,0．2,0．4,0．6,0．8,1）系列材料．粉末X射线衍射（XRD）分析表明,所制备材料的结构为单一立方相．应用变温拉曼光谱研究该材料相变,变温拉曼光谱研究结果表明,材料起始相变温度随着P5＋替代V5＋量的增加逐渐降低,x=0,0．4,0．8,1对应的相变温度分别为383K,363K,273K,213K．热膨胀测试结果表明：随着P替代量的增加正一负膨胀转变温度先降低后增加,x=0,0．2,0．4,0．6,0．8,1对应的正一负膨胀转变温度分别为429K,403K,372K,390K,398K和435K．本文指出了该系列材料存在两个相变过程,为设计和制备ZrV2O7基室温附近的负热膨胀材料奠定了良好的基础．     

Magnetostrictive strain and thermal expansion in NdCo5 in the spin orientation temperature region

Magnetostrictive strain and thermal expansion measurements in an oriented assembly of NdCo₅ particles along the c-axis were carried out between 77 and 330 K. The measurement strains in the c-derection and transverse to the c-direction exhibited anomalous behavior at 285 and 245 K, which are assoiciated with the spin reorientation process in this temperature range. The estimated magnetostrictive energy $\text{1}\text{2}\text{E(λ}{}_{\mathrm{c}}\text{- λ⊥}{}_{\mathrm{c}}\text{)}{}^2$ is comparable to the anisotropy energy at 285 K. There is also a pronouced anomaly in ther thermal expansion in the spin reorientation temperature region.     

The anomalous thermal expansion of VO2 crystal with the rutile structure

The large coefficient of thermal expansion parallel to the c-axis in the rutile phase of VO₂ is explained using the phonon-softening model of the metal-insulator transition.     

Application of the thermal boson expansion to the Heisenberg antiferromagnet MnF2

We apply the thermal boson expansion which has been presented in refs. [1–3] to the Heisenberg isotropic antiferromagnet, submitted to an external field. Considering only spin waves with wave vectors up to a certain cut-off, the magnetization and the principal susceptibilities are calculated. The results are compared with the experimental data on MnF₂.     

Phase-matched second-harmonic generation due to thermal expansion of TlGaSe2 layered crystal

Second-harmonic generation effects have been investigated in TlGaSe₂ layered crystal over a temperature range where ferroelectric phase exists. Pronounced periodical changes of the second-harmonic signal with temperature have been discovered. The observed effect is explained within the phase synchronism condition which changes with the temperature. The main mechanism of the thermal expansion of the sample in the observed phenomenon is explained.     

Electrical and photocatalytic properties of Na2Ti6O13 nanobelts prepared by molten salt synthesis

Single-crystalline Na₂Ti₆O₁₃ nanobelts were prepared on large-scale by molten salt synthesis at 825 °C for 3 h. The obtained nanobelts have typical width of less than 200 nm and thickness of 10-30 nm, and length up to 10 μm. The growth direction of the nanobelts was determined to be along [0 1 0]. Electrical transport property of an individual nanobelt was measured at room temperature and ambient atmosphere, and results showed that the nanobelts are semiconductor. Na₂Ti₆O₁₃ nanobelts exhibited good photocatalytic efficiency for the degradation of RhB under UV irradiation.     

Heat capacity and thermal expansion of CdCr2Se4 and CdCr2S4

The thermodynamic properties of the spinel ferromagnetic compounds CdCr₂Se₄ and CdCr₂S₄ have been investigated by making heat capacity and thermal expansion measurements on single crystals. For both compounds, the ferromagnetic transition is marked by λ-type thermal anomalies, and the results provide a pressure dependence of the transition temperatures that is in agreement with direct measurements. Below the transition, CdCr₂S₄ shows an anomalous heat-capacity contribution and negative thermal expansion, which are in contrast to the conventional behavior found in CdCr₂Se₄.     

Electrical behavior of BaZr0.1Ti0.9O3 and BaZr0.2Ti0.8O3 thin films

BaZr_0.1Ti_0.9O₃ and BaZr_0.2Ti_0.8O₃ (BZT) thin films were deposited on Pt/Ti/LaAlO₃ (1 0 0) substrates by radio-frequency magnetron sputtering, respectively. The films were further annealed at 800 °C for 30 min in oxygen. X-ray diffraction θ-2θ and Φ-scans showed that BaZr_0.1Ti_0.9O₃ films displayed a highly (h 0 0) preferred orientation and a good cube-on-cube epitaxial growth on the LaAlO₃ (1 0 0) substrate, while there are no obvious preferential orientation in BaZr_0.2Ti_0.8O₃ thin films. The BaZr_0.1Ti_0.9O₃ films possess larger grain size, higher dielectric constant, larger tunability, larger remanent polarization and coercive electric field than that of BaZr_0.2Ti_0.8O₃ films. Whereas, BaZr_0.1Ti_0.9O₃ films have larger dielectric losses and leakage current density. The results suggest that Zr⁴⁺ ion can decrease dielectric constant and restrain non-linearity. Moreover, the enhancement in dielectric properties of BaZr_0.1Ti_0.9O₃ films may be attributed to (1 0 0) preferred orientation.     

Reduced dielectric loss and leakage current in CaCu3Ti4O12/SiO2/CaCu3Ti4O12 multilayered films

The CaCu₃Ti₄O₁₂/SiO₂/CaCu₃Ti₄O₁₂ (CCTO/SiO₂/CCTO) multilayered films were prepared on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition method. It has been demonstrated that the dielectric loss and the leakage current density were significantly reduced with the increase of the SiO₂ layer thickness, accompanied with a decrease of the dielectric constant. The CCTO film with a 20 nm SiO₂ layer showed a dielectric loss of 0.065 at 100 kHz and the leakage current density of 6×10⁻⁷ A/cm² at 100 kV/cm, which were much lower than those of the single layer CCTO films. The improvement of the electric properties is ascribed to two reasons: one is the improved crystallinity; the other is the reduced free carriers in the multilayered films.