Effect of the mechanical stretching on the ferroelectric properties of a (VDF/TrFE) (75/25) copolymer film

The transition phase behaviour of a vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymer film was studied after being subjected to different mechanical stretching levels in both longitudinal (L) and transversal (T) directions relatively to the initial extrusion direction. Both ferroelectric-paraelectric (FE-PE) and melting transitions were detected in the films by differential scanning calorimetry, that were not affected by L stretching. This suggests that sliding mechanism along the c-axis of such films during plastic deformation influences slightly the all-trans chain conformations and the CF₂ dipole orientation. On the other hand, the FE-PE peak decreases in area and temperature upon T stretching, indicating a damage of the ferroelectric phase. However, the melting peak maintains unchanged. This fact provides evidence that the mechanical stretching do not change the total degree of crystallinity. The results suggest that the stretching induces a FE-PE phase transformation, without affecting the degree of crystallinity.     

Terahertz signatures of the exciton formation dynamics in non-resonantly excited semiconductors

A microscopic theory for the induced terahertz (THz) absorption of semiconductors is applied to study the time-dependent system response after non-resonant optical excitation. The formation of excitonic populations from an interacting electron-hole plasma is analyzed and the characteristic THz signatures are computed. Good qualitative agreement with recent experiments is obtained.     

Low-temperature thermodynamic properties of a one-dimensional disordered electron lattice system

A new method of low-temperature thermodynamic calculation of a one-dimensional generalized Wigner crystal on a disordered host-lattice is proposed. This method is based on the system statistical sum presentation in terms of modified transfer-matrixes. A gapless structure of the elementary excitations spectrum of the system under consideration is found.     

创新主体研究:整合的观点及中国的实践

本文以创新主体为主线 ,分析了不同时期创新理论中由于创新的不同特征而对创新主体的不同认识 ,并因应现代创新的整合特征提出对整合创新三个层次的理解 ,进而对我国创新主体的演变作了分析 ,提出在我国推进整合创新主体建设的重要意义。     

Impedance spectroscopy of a poly(p-phenylenevinylene)-based light-emitting diode device

A poly-p-phenylenevinylene (PPV)-based light-emitting diode (LED) device was studied by means of comprehensive ac impedance measurements. Separate regions of frequency-dependent responses, the bulk and depleted (interface) regions, with distinct electrical properties, were identified and characterized. The ac J-E characteristics revealed the space-charge limited conduction mechanism, which was specifically assigned to the depleted region. Besides, a direct estimate of the trap density in the bulk region was obtained from the applied voltage dependence of the charge concentration.     

High-temperature soft magnetic properties of Co-doped nanocrystalline alloys

In situ hysteresis measurements were used to follow the evolution of soft magnetic properties of various Co containing Finemet- and Hitperm-type nanocrystalline materials. This work is aimed at obtaining the proper chemical composition of nanocrystalline alloys, which can be applied for highest temperatures and is less affected by the inevitable decoupling phenomena, which is common to all two-phase nanocrystalline systems. Experimental results pointed out that in the Co-added Finemet-type alloys the decoupling temperature can be increased by about 50–70 K as compared to Co-free Finemet alloy. Hitperm alloys, close to equiatomic Fe/Co ratio, have much higher decoupling temperature, but their coercivity is an order of magnitude larger at low temperatures compared to the Co-free alloys so their advantage is evident above 400 °C only.     

Mössbauer characterization of tin dopant ions in the antiferromagnetic ilmenite MnTiO3

Mössbauer parameters of ¹¹⁹Sn diamagnetic dopant cations in an antiferromagnetic compound having the ilmenite structure are for the first time reported. The spectra reveal a well resolved hyperfine splitting pattern of combined magnetic and quadrupole interactions (at 5 K, δ=0.19 mm/s, H₁=52.5 kOe, eV_ZZQ_3/2=−0.80 mm/s, θ≈0°). This spectral component whose contribution (A₁=82%) represents more than four fifths of the total amount of the dopant (Sn/(Mn+Ti)=1/200) is assigned to Sn(IV) ions located in the bulk of MnTiO₃, on the Mn(II) site, and with a Mn(II) vacancy in their nearest surrounding. Two spectral components with minor contributions are also observed: one of them (H₂≈25 kOe, A₂=8%) can be assigned to Sn(IV) ions, in the MnTiO₃ lattice as well, on a site where they exhibit a weaker spin polarization (this site could be the Ti(IV) one) and the other (H₃=0 kOe, A₃=10%) to SnO₂ or/and Ti_1−xSn_xO₂ clusters. The Néel temperature of MnTiO₃ probed by the ¹¹⁹Sn dopant (T_N=69±2 K) agrees well with the values previously provided by ESR and antiferromagnetic resonance measurements. Variation of H₁ with temperature follows close the Brillouin function for S=5/2. No perturbation appears in the Mössbauer spectra around T=90 K where a broad peak, characteristic of 2D magnetic interactions, is observed on the static magnetic susceptibility curve.     

Characterization of the Fermi surface of the organic superconductor - by measurements of Shubnikov-de Haas and angle-dependent magnetoresistance oscillations and by electronic band-structure calculations

The electronic structure of the quasi two-dimensional (2D) organic superconductor -(ET)₂SF₅CH₂CF₂SO₃ was examined by measuring Shubnikov-de Haas (SdH) and angle-dependent magnetoresistance (AMRO) oscillations and by comparing with electronic band-structure calculations. The SdH oscillation frequencies follow the angular dependence expected for a 2D Fermi surface (FS), and the observed fundamental frequency shows that the 2D FS is 5% of the first Brillouin zone in size. The AMRO data indicate that the shape of the 2D FS is significantly non-circular. The calculated electronic structure has a 2D FS in general agreement with experiment. From the temperature and angular dependence of the SdH amplitude, the cyclotron and band effective masses were estimated to be and ,where g is the conduction electron g factor and the free electron mass. The band effective mass is estimated to be from the calculated electronic band structure. Received: 3 March 1997 / Revised: 5 May 1997 / Received in final form: 5 November 1997 / Accepted: 10 November 1997     

Exciton in wurtzite GaN/AlxGa1−xN coupled quantum dots

Based on effective-mass approximation, we present a three-dimensional study of the exciton in GaN/Al_xGa_1−xN vertically coupled quantum dots (QDs) by a variational approach. The strong built-in electric field due to the piezoelectricity and spontaneous polarization is considered. The relationship between exciton states and structural parameters of wurtzite GaN/Al_xGa_1−xN coupled QDs is studied in detail. Our numerical results show that the strong built-in electric field in the GaN/Al_xGa_1−xN strained coupled QDs leads to a marked reduction of the effective band gap of GaN QDs. The exciton binding energy, the QD transition energy and the electron-hole recombination rate are reduced if barrier thickness L^AlGaN is increased. The sizes of QDs have a significant influence on the exciton state and interband optical transitions in coupled QDs.     

Mechanism of intense blue photoluminescence in silica wires

The photoluminescence (PL) properties of our silica wires were investigated with PL, PL excitation and PL decay. A high brightness photoluminescence band at 2.8 eV with a shoulder around 3.0 eV was observed in our silica wires. Two PL excitation bands for the 2.8 eV emission were observed at 4.77 and 3.37 eV. The 3.37 eV excitation band is reported for the first time. The characteristic of the blue PL in our silica wires was different from that of the well-known 2.7 eV PL in bulk silica material, suggesting a negation of previous attribution of blue emission in silica nanowires. The mechanism of the PL was also discussed.