The phase transition in VO2

The electronic d-states in VO₂ are investigated by a semi-empirical MO LCAO method. The calculations of clusters for the crystal structures above and below the phase transition point show that the energy gap is mainly formed by the crystal distortions. The energies calculated agree well with the ESCA experimental estimations and explain the large gap values obtained from the optical data.     

Fast pulse measurements of the dielectric constant of semiconducting VO2

By using a pulse technique in the picosecond range, we have obtained the dielectric constant of semiconducting VO₂ parallel (?∥ = 18.3) and perpendicular (?⊥=39) to the pseudo tetragonal c-axis. We have also looked for changes in the dielectric constant in the neighbourhood of the non metal-metal transition.     

Light-induced ultrafast phase transitions in VO2 thin film

Vanadium dioxide shows a passive and reversible change from a monoclinic insulator phase to a metallic tetragonal rutile structure when the sample temperature is close to and over 68 °C. As a kind of functional material, VO₂ thin films deposited on fused quartz substrates were successfully prepared by the pulsed laser deposition (PLD) technique. With laser illumination at 400 nm on the obtained films, the phase transition (PT) occurred. The observed light-induced PT was as fast as the laser pulse duration of 100 fs. Using a femtosecond laser system, the relaxation processes in VO₂ were studied by optical pump-probe spectroscopy. Upon a laser excitation an instantaneous response in the transient reflectivity and transmission was observed followed by a relatively longer relaxation process. The alteration is dependent on pump power. The change in reflectance reached a maximum value at a pump pulse energy between 7 and 14 mJ/cm². The observed PT is associated with the optical interband transition in VO₂ thin film. It suggests that with a pump laser illuminating on the film, excitation from the d_θ,? - state of valence band to the unoccupied excited mixed d_θ,?-π* - state of the conduction band in the insulator phase occurs, followed by a resonant transition to an unoccupied excited mixed d_θ,?-π* - state of the metallic phase band.     

VO2金属-绝缘体相变机理的研究进展

VO₂是一种热致相变金属氧化物. 在341 K附近, VO₂发生由低温绝缘体相到高温金属相的可逆转变, 同时伴随着光学、电学和磁学等性质的可逆突变, 这种独特的性质使得VO₂在光电开关材料、智能玻璃、存储介质材料等领域有着广阔的应用前景. 因此, VO₂金属-绝缘体可逆相变一直是人们的研究热点, 但其相变机理至今未有定论. 首先, 简要概述了VO₂相变时晶体结构和能带结构的变化情况: 从晶体结构来讲, 相变前后VO₂从低温时的单斜相VO₂(M)转变为高温稳定的金红石相VO₂(R), 在一定条件下此过程也可能伴随着亚稳态单斜相VO₂(B)与四方相VO₂(A)的产生; 从能带结构来看, VO₂处于低温单斜相时, 其d_//能带和π^*能带之间存在一个禁带, 带宽约为0.7 eV, 费米能级恰好落在禁带之间, 表现出绝缘性, 而在高温金红石相时, 其费米能级落在π^*能带与d_//能带之间的重叠部分, 因此表现出金属导电性. 其次, 着重总结了VO₂相变物理机理的研究现状. 主要包括: 电子关联驱动相变、结构驱动相变以及电子关联和结构共同驱动相变的3种理论体系以及支撑这些理论体系的实验结果. 文献报道争论的焦点在于, VO₂是否是Mott绝缘体以及结构相变与MIT相变是否精确同时发生. 最后, 展望了VO₂材料研究的发展方向.     

基于FTO/VO2/FTO结构的VO2薄膜电压诱导相变光调制特性

采用直流磁控溅射和后退火工艺在掺氟的SnO₂(FTO)导电玻璃衬底上制备VO₂薄膜, 研究了不同退火时间和不同比例的氮氧气氛对VO₂薄膜性能的影响, 对VO₂薄膜的结晶取向、表面形貌、表面元素的相对含量和透过率随波长变化进行了测试分析, 结果表明在最佳工艺条件下制备得到了组分相对单一的VO₂薄膜. 基于FTO/VO₂/FTO结构在VO₂薄膜两侧的透明导电膜上施加电压并达到阈值电压时, 观察到了明显的电流突变. 当接触面积为3 mm×3 mm时, 阈值电压为1.7 V, 阈值电压随接触面积的增大而增大. 与不加电压的情况相比, FTO/VO₂/FTO结构在电压作用下高低温的红外透过率差值可达28%, 经反复施加电压, 该结构仍保持性能稳定, 具有较强的电致调控能力.     

The model of ultrafast light-induced insulator-metal phase transition in VO2

Ultrafast light-induced insulator-metal phase transitions (PT) in VO₂ thin films was studied with use of a pump-probe technique. The theoretical and experimental study of PT kinetics shows that the PT could be realized via an intermediate state. The relaxation processes after optical pumping are dependent on pump energy. The excitonic controlled model for such type of PT is proposed. The main channel for the ultrafast light-induced PT is the resonant transition between excited states of correlated vibronic Wannier-Mott excitons (WME) in insulator phase and the unoccupied excited states in metallic phase. During this process an equilibrium local distortion occurred. According to the proposed model the experimental observation of the drastic temperature- and pump power- dependent relaxation processes could be interpreted.     

The metal-insulator transition in VO2

A model of the phase transition in VO₂ is presented which is based on phonon softening effects and gives quantitative agreement with experiment.     

Coulomb interactions in metallic VO2

The intra-cationic coulomb energy in the metallic phase of VO₂ is estimated using a model in which the observed spin susceptibility and its temperature dependence arise from correlation modified exchange enhancement of the spin susceptibility of the d - subbands.     

Structural phase transitions in semiconducting SrTiO3

From ultrasonic velocity measurements we found that in SrTiO₃ the temperature for the structural phase transition is lowered in samples reduced in hydrogen, and increased in Nb-doped samples. In the reduced samples the shift in T_c is proportional to the concentration of oxygen vacancies. The results can qualitatively be understood in terms of local changes in the d_?/p_σ-hybridization of Ti-3d and 0–2p orbitals.     

One-electron dispersion relations in metallic VO2

One-electron dispersion relations are presented for those bands in VO₂ involved in the metallic conduction which characterises the rutile phase. The density of states at the Fermi energy (0.56 eV) is found to be 2.89 eV^-1 per cation, and the metallic state to be stable against Mott-insulation.     

Phase transitions in Fe doped VO2

Resistivity measurements were performed on single crystals of V_1-χFe_χO₂ (0 ? χ < 0.03). Anomalies in the ln σ vs T^-1 curve indicate the presence of different structural phases. The results of these experiments and previous ⁵⁷Fe Mössbauer data are discussed in terms of a model recently proposed by Pouget et al. for the structural phases in the system V_1-χCr_χO₂     

电场诱导二氧化钒绝缘-金属相变的研究进展

二氧化钒(VO_2)是电子强关联体系的典型代表,其晶体结构在特定阈值的温度、电场、光照和压力等物理场作用下会发生由单斜金红石结构向四方金红石结构的可逆转变,从而引发绝缘-金属相变.其中,电场诱导VO_2绝缘-金属相变后的电导率可提高2-5个数量级,在可重构缝隙天线、太赫兹辐射以及智能电磁防护材料等领域具有广阔的应用前景,成为近年来人们的研究热点.首先,简要概述了VO_2发生绝缘-金属相变时晶体结构和能带结构的变化,进而从电场诱导VO_2绝缘-金属相变的研究方法、响应时间、临界阈值场强调控以及相变机理几个方面系统总结和评述了近年来国内外学者在该领域的重要发现和研究进展.最后,指出了当前VO_2绝缘-金属相变研究存在的问题,并展望了未来的发展方向.     

First order antiferromagnetic phase transition in MnS2

The antiferromagnetic structure of MnS₂ has been re-examined by neutron diffraction from a single crystal. The phase transition has been found to be of the first order without any detectable thermal hysteresis. Application of uniaxial stress tends to convert the first order transition into a second order one.     

Thermal expansivity in semiconducting NbO2

Measurements have been made of sample lengths L_a and L_c parallel to the a- and c-crystallographic directions, respectively, in semiconducting, distorted-rutile (C⁶_4h) structure NbO₂ between 133 K and 360 K using a fused silica LVDT dilatometer system. L_a changes very slightly and goes through a minimum as a function of temperature, T, while L_c increases monotonically with T in a normal manner. This behavior is compared with that of rutile-structure compounds. The present data do not cause significant changes in the elastic stiffness moduli which were deduced previously from ultrasonic transit times using lengths extra-polated from lattice parameter data above room T. Thus the discrepancy between the elastic and calorimetric Debye temperatures reported in the literature remains. Thermal expansion coefficients and Grüneisen anharmonicity parameters are deduced. The expansion coefficients are analyzed into a quasicubic component and a term depending on the anisotropy of the Grüneisen function.     

二氧化钒纳米棒的相变行为

研究了VO₂(M)纳米棒的金属绝缘体相变(MIT)行为．在VO₂(M)纳米棒的DSC分析曲线上发现了两个MIT,分别位于低温和高温区．低温MIT总是伴随出现VO₂(B)纳米棒,而独立的高温MIT出现在纯VO₂(M)纳米棒．分析和讨论了这两个MIT的机制．     

First order magnetic phase transition in tetragonal NpCu2Si2

Magnetization and Np²³⁷ Mössbauer studies of the tetragonal compounds NpM₂Si₂ (M = Cr, Mn, Fe, Co, Ni, Cu) were performed. NpMn₂Si₂ is ferromagnetic. All other compounds order antiferromagnetically. Only in NpCu₂Si₂ the Mössbauer studies reveal a first order magnetic phase transition at T_N = 34 K. It is interpreted in terms of Blume's model, originally developed for cubic UO₂.     

Evolution of the d∥ band across the metal-insulator transition in VO2

We studied the evolution of the electronic structure of VO₂ across the metal-insulator transition. The electronic structure was calculated using the standard TB-LMTO-ASA method. The calculated DOS was compared to previous photoemission and X-ray absorption spectra. The electronic structure is discussed in terms of the usual molecular-orbital scheme. In the metallic phase, the d_∥ band appears at the bottom of the V 3d bands and crosses the Fermi level. In the insulating phase, the d_∥ band is split around 2 eV opening a pseudo band gap at the Fermi level. The largest effect of the splitting appears in the unoccupied part of the d_∥ band. The calculated value of the splitting accounts for 77% of the experimental value, 2.6 eV. The results suggest that electron-lattice interaction seems to be the dominant factor in the splitting of the d_∥ band.     

Capacitance discharge in VO2 threshold switching devices

Threshold switching in a polycrystalline VO₂ thin film was studied by pulse operation. By varying the geometrical dimensions of the device, we have investigated the influence of device's electrical capacitance on the switching event. This capacitance discharge produces in fact a transient temperature increase which can be sufficient to melt the active material or the electrodes.     

Electron spin resonance in chromium doped VO2

Electron spin resonance has been studied in V_1?xCr_xO₂ crystals in the monoclinic insulating phase M₁(x<0.3%). Two distinct substitutional Cr³⁺ centers are observed. The two centers differ in their axial character which we interpret as due to different charge compensation mechanisms. One of the centers is preponderant at higher Cr concentrations (x>0.1%) and its spectrum can be interpreted by assuming charge compensation by a nearest neighbour V⁵⁺.     

Magnetism in nanoparticles of semiconducting FeSi2

Iron disilicide has been found to exhibit superparamagnetism in nanoparticles, even though no magnetic ordering occurs in bulk. The unexpected behavior was attributed, based on magnetic studies, to chemical disorder. A lack of sextet-type signals in Mössbauer spectra supports that the observed magnetic order is confined to only a very small fraction of magnetic Fe ions. Moreover, quadrupole-splitting and isomer-shift parameters reveal a significant amount of Fe in a short-range α-FeSi₂ structure, while XRD suggests an overall β-FeSi₂ structure. Such a compositional heterogeneity is also reflected in a calorimetrically obtained spin-glass-like anomaly at low temperatures. Meanwhile, as the particle size decreases, specific heat and Sommerfeld constant are enhanced due to lattice softening and the emergence of surface charge density of states, respectively, in nanoparticles.