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861.
862.
863.
864.
We review recent results on the behaviour of the dielectric function of cylindrical nano‐organic materials at very low frequencies in a magnetic field. For cylindrical structures — such as carbon nanotubes — the polarisability is shown to be a discontinuous function of a longitudinal magnetic field where plateau‐like regions are separated by sudden jumps or peaks. A relation is pointed out between each discontinuity in the polarisability and the cross‐over between ground and first excited states induced by the magnetic field. This one to one correspondence suggests to use measurements of the dielectric function in an applied magnetic field in order to obtain informations about the electronic structures of cylindrical nanostructures. In addition, it is shown, by studying finite graphene layers, that the measurement of the polarisability in a magnetic field could be a powerful way for detecting possible edge‐states in amorphous carbon materials such as activated carbon fibres. Finally, the importance of the electron‐electron interaction is emphasised by discussing examples of strongly interacting electrons on rings or cylinders, in the limit of infinite interaction. 相似文献
865.
With the development of ultralarge scale integrated circuit, new interlayer dielectrics with low dielectric constant for multilevel interconnections are required, instead of conventional SiO2 films. For the sake of seeking perfect dielectrics, amorphous fluoropolymer (AF) thin film with a thickness of about 0.9μm has been prepared by spin-coating method, following the principle of phase separation. By capacitance-voltage (C-V) measurements the dielectric constant of the thin film is equal to 1.57 at 1 MHz, which is attributed to numerous pores contained in the film matrix. X-ray photoelectron spectroscopy (XPS) spectra show that after annealing, about 71% CF3 groups in the AF film have decomposed into CF2, CF, etc. This leads to the increase of CF2 groups by three times and CF groups by 8% in the AF film. In a word, compared with the film without being annealed, about 25% carbon, 7% fluorine and 12% oxygen atoms will be lost after annealing at 400℃ for 30min. 相似文献
866.
EFFECT OF DIELECTRIC CONSTANT ON THE EXCITON GROUND STATE ENERGY OF CdSe QUANTUM DOTS 总被引:1,自引:0,他引:1
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The B-spline technique is used in the calculation of the exciton ground state energy based on the effective mass approximation (EMA) model. The exciton is confined in CdSe microspherical crystallites with a finite-height potential wall (dots). In this approach, (a) the wave function is allowed to penetrate to the outside of the dots; (b) the dielectric constants of the quantum dot and the surrounding material are considered to be different; and (c) the dielectric constant of the dots are size-dependent. The exciton energies as functions of radii of the dots in the range 0.5-3.5 nm are calculated and compared with experimental and previous theoretical data. The results show that: (1) The exciton energy is convergent as the radius of the dot becomes very small. (2) A good agreement with the experimental data better than other theoretical results is achieved. (3) The penetration (or leaking) of the wave function and the difference of the dielectric constants in different regions are necessary for correcting the Coulomb interaction energy and reproducing experimental data. (4) The EMA model with B-spline technique can describe the status of excition confined in quantum dot very well. 相似文献
867.
J. Dec W. Kleemann Th. Woike R. Pankrath 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(4):627-632
The complex dielectric susceptibility of Sr
0.61
Ba
0.39
Nb
2
O
6
:Ce
3+
(SBN61:Ce) has been measured at frequencies and temperatures before and after poling. The relaxor behaviour with large polydispersivity observed above the ferroelectric phase transition
temperature, T
c
= 360 and 340 K for x
(Ce) = 0 and 0.0066, respectively, is perfectly modeled within the framework of Chamberlin's dynamically correlated domain approach.
Below T
c
the dynamic nanodomain state crosses over into a ferroelectric state with polydispersive domain wall dynamics at very low
frequencies. Presumably SBN61:Ce belongs to the three-dimensional random field Ising rather than to the dipole glass universality
class.
Received 1 October 1999 相似文献
868.
A series of Ba1-xSrxTi1-yZryO3 (0≤x≤0.5, 0≤y≤0.4) and BA1?xZnxTi1?ySnyO3 (0≤x≤0.3, 0≤y≤0.3) solid solutions were synthesized by low-temperature/low-pressure hydrothermal method below 170°C, 0.8
MPa. XRD pattern and cell parameters-composition figures of these prepared powders demonstrated that they are completely miscible
solid solutions based on BaTiO3. Furthermore, TEM showed that they have a shape of uniform, substantially spherical particles with an average particle size
of 70 nm in diameter. The sintered ceramics of those powders doped by Sr2+ and Zr4+ or Zn2+ and Sn4+ have dielectric constant twelve times higher than and dielectric loss 1/6 those of pure BaTiO3 phase at room temperature. 相似文献
869.
E. Kogan M. Auslender M. Kaveh 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(3):373-376
Localization and dephasing of conduction electrons in a low carrier density ferromagnet due to scattering on magnetic fluctuations
is considered. We claim the existence of the “mobility edge”, which separates the states with fast diffusion and the states
with slow diffusion; the latter is determined by the dephasing time. When the “mobility edge” crosses the Fermi energy a large
and sharp change of conductivity is observed. The theory provides an explanation for the observed temperature dependence of
conductivity in ferromagnetic semiconductors and manganite pyrochlores.
Received 17 January 1999 and Received in final form 12 March 1999 相似文献
870.
From selective to wide-band light reflection: a simple thermal diffusion in a glassy cholesteric liquid crystal 总被引:1,自引:0,他引:1
M. Mitov A. Boudet P. Sopéna 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(3):327-330
Due to their helicoidal structure, cholesteric liquid crystals exhibit remarkable optical properties. Selective light reflection
occurs when the pitch (repeat distance) is of the order of the wavelength of incident light propagating along the helix axis.
The wavelength bandwidth, due to the optical anisotropy, is typically limited to 50 nm which is insufficient for some applications
(full-colors displays, for example). By introducing a pitch gradient in the helix during a novel two-step process in a cholesteric
glass, we show that reflection may occur over a wavelength bandwidth greater than 300 nm. First, the reflection bandwidth
is adjusted by thermal annealing. Then, the optical properties are permanently stored by quenching the viscous material to
a glass at room temperature. The two steps, pitch gradient establishment and film hardening, are independently controlled.
The present process exhibits some reversibility and properties intrinsic to the glassy state are gained: laser-writing high
resolution full-color images on solid films for image recording or high-density optical data-storage are indeed conceivable.
Received 17 December 1998 相似文献