Theoretical investigation of the local lattice structure of Mn 2+ ion doped in tetragonal K 2ZnF 4 crystal

The complete energy matrices for a d⁵configuration ion in a tetragonal ligand-field has been constructed on the basis of the complete set of basis of d⁵configuration (252 dimension), and the relationship between the low-symmetry EPR parameters b₂ ⁰ ,b₄ ⁰ and the local distortion parameters has been established based on the complete energy matrices. As an application, we have studied the EPR parameters and the local lattice structure of Mn²⁺ ion doped in tetragonal K₂ZnF₄ system. The calculation indicated that the local lattice structure around a tetragonal Mn²⁺ ion center has an expansion distortion. Simultaneously, the local lattice structure parameters R₁ =2.0727 ?, R₂ =2.0801 ? at room temperature (295 K) and R₁ ^′ = 2.0439 ?, R₂ ^′ =2.05478 ? at low temperature (4.2 K) are determined.     

Studies on electrical and the dielectric properties in MS structures

In this study, we investigated frequency dependent electrical and dielectric properties of metal-semiconductor (MS) structures using capacitance-voltage (C-V) and conductance-voltage (G/ω-V) characteristics in the frequency range 100 kHz-10 MHz in the room temperature. The dielectric constant (ε′), dielectric loss (ε″), dielectric loss tangent (tan δ) and ac electrical conductivity (σ_ac) were calculated from the C-V and G/ω-V measurements and plotted as a function of frequency. In general, ε′, ε″ and tan δ values decreased with increasing the frequency, while σ_ac increased with increasing frequency. Furthermore, the voltage and frequency dependence of series resistance were calculated from the C-V and G/ω-V measurements and plotted as functions of voltage and frequency. The distribution profile of R_S-V gives a peak in the depletion region at low frequencies and disappears with increasing frequencies. Also, series resistance values decreased with increasing frequency. The experimental results show that both frequency dependent electrical and dielectric parameters were strongly frequency and voltage dependent.     

High-resolution optical study of HoHo pairs in LiY1−xHoxF4 crystals

High-resolution spectra of holmium-doped LiYF₄ crystals at low temperatures were investigated. It was shown that weak lines observed near some main lines in the spectra belong to the Ho³⁺Ho³⁺ pair centers. These satellites can be explained by two types of pairs with the magnetic dipole coupling within each of them, provided that a change of the crystal field due to lattice distortion is taken into account.     

Dielectric response in ferroelectric BaTiO3

The far-infrared optical and dielectric properties of ferroelectric perovskite titanate powder BaTiO₃ are reported. The terahertz time-domain spectroscopy (THz-TDS) measurement reveals that the low frequency dielectric response of BaTiO₃ is closely related to the lowest pair of transverse optical (TO) and longitudinal optical (LO) modes near at 180 cm⁻¹, which is verified by Raman spectroscopy. This result provides a better understanding of the relation of low-frequency dielectric function with the optical phonon mode for ferroelectric materials. Combining terahertz TDS with Raman spectra, the overall low frequency optical phonon response of BaTiO₃ is presented in an extended spectral range from 6.7 to 1200 cm⁻¹.     

Effect of grain boundary on resistive magnetodielectric property of polycrystalline <Emphasis Type="Italic">γ</Emphasis>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>

We have investigated the role of the grain boundary on the resistive magnetodielectric property of polycrystalline γ-Fe₂O₃ through impedance spectroscopy measurements. Depending on the sample preparation temperature, the dielectric constant of γ-Fe₂O₃ is significantly different especially at low frequencies (<10⁴ Hz) and high temperatures (>200 K). The value of the magnetodielectric effect at a specific frequency and the resonance frequency for the maximized magnetodielectric effect are different, although polycrystalline γ-Fe₂O₃ samples show a quite similar magnetoresistance. Through the experimentally obtained resistance ratio between the grain and the grain boundary, we can reproduce the magnetodielectric curves based on the Maxwell–Wagner model and the measured magnetoresistance.     

Controllable off-plane deflection of cantilevers for multiple scanning proximity probe arrays

The presented work reports the fabrication of 4×32 arrays of fully addressable proximity probes, which are initially and controllably off-plane deflected (bent). Such a deflection is required for the simultaneous approach and scanning of all cantilevers. It is realized by deposition of the silicon cantilevers with Si₃N₄ film inducing tensile stress. ANSYS simulations are used to calculate the off-plane deflection for different thickness of the cantilever and the Si₃N₄ layer and compared with experimentally obtained values. Cantilever arrays with set bending up to 50 μm, employing LPCVD silicon nitride film with a tensile stress of 750 MPa are fabricated.     

Biased magnetization reversal in bi-phase multilayer microwires

We report on the magnetic behaviour of a novel family of two-magnetic-phase multilayer microwires consisting of: (i) a bistable FeSiB glass-coated amorphous microwire as soft nucleus, and (ii) a polycrystalline CoNi outer microtube as harder layer. Such bi-phase microwires are prepared by combined quenching and drawing plus sputtering and electroplating techniques. The stray field produced by the harder outer layer after premagnetizing it to saturation is used to bias the magnetization reversal process of the soft nucleus via dipolar magnetostatic coupling. A detailed analysis of the asymmetric low-field magnetization reversal process of the soft nucleus is presented together with the study of the fluctuating switching field and its asymmetric behaviour. The study of the domain wall characteristics under the presence of a nucleation coil at one end of the microwire allows us to draw conclusions on the role of the bias field generated by the premagnetized hard outer layer.     

Single-Walled Carbon Nanotubes Acting as Controllable Transport Channels

The motion and equilibrium distribution of water molecules adsorbed inside neutral and negatively charged singlewalled carbon nanotubes (SWNTs) have been studied using molecular dynamics simulations (MDSs) at room temperature based on CHARMM (Chemistry at HARvard Molecular Mechanics) potential parameters. We find that water molecules have a conspicuous electropism phenomenon and regular tubule patterns inside and outside the charged tube wall. The analyses of the motion behaviour of water molecules in the radial and axial directions show that by charging the SWNT, the adsorption efficiency is greatly enhanced, and the electric field produced by the charged SWNTs prevents water molecules from flowing out of the nanotube. However, water molecules can travel through the neutral SWNT in a fluctuating manner. This indicates that by electrically charging and uncharging the SWNTs, one can control the adsorption and transport behaviour of polar molecules in SWNTs for using as a stable storage medium or long transport channels. The transport velocity can be tailored by changing the charge on the SWNTs, which may have a further application as modulatable transport channels.     

The role of photoelectronic processes in the formation of a fluorescent plume by 248-nm laser irradiation of single crystal NaNO3

 Visible fluorescent “plumes” are readily produced when nominally transparent ionic materials are exposed to pulsed UV laser irradiation. Over a wide range of laser fluences where plumes are observed, however, the photon and electron densities are inadequate to support multiphoton ionization and inverse bremsstrahlung, which are often used to explain plasma production and excitation of atomic spectral lines. We present evidence that the great majority of charged particles (electrons and positive ions) comprising the plume at the onset of formation in defect-laden NaNO₃ are emitted directly from the surface. A model is described wherein the required electron energy to excite and eventually ionize neutral atoms is provided by electrostatic interactions in the expanding plume. The time evolution of the “overlap” between the expanding charge cloud and thermally emitted neutrals accounts for the time evolution of the atomic line emissions after the laser pulse. Received: 15 August 1996/Accepted: 16 August 1996