Multiphased (NaCl)x(KCl)y−x(KBr)1−y single crystals: ac conductivity activation energy versus bulk properties

The ac electrical measurements have been just reported for alkali halide mixed crystals that were melt grown from NaCl, KCl and KBr starting materials. They showed a nonlinear variation of all the electrical parameters with the bulk composition. In this short paper, we show that these mixed systems, depending on their major constituent, are classified into three categories in each of which, the activation energy for the ac conductivity increases linearly with BΩ, where B is the isothermal bulk modulus and Ω the mean volume per atom. In addition, the resulting three straight lines are formed to have almost the same slope.     

Thermal, structural and complex impedance analysis of Mn modified BaTiO3 electroceramic

The polycrystalline samples of BaTi_1−xMn_xO₃ (BMT) ferroelectric ceramics with x=0, 0.04, 0.07, 0.10 have been prepared using a solid-state reaction technique. The calcination temperature of the samples was optimized by thermal gravimetric analysis and repeated firing. Preliminary structural study using X-ray diffraction technique at room temperature suggests that structure of Mn modified compounds (BMT) change into orthorhombic crystal system from tetragonal crystal system. The field emission scanning electron micrographs (FE-SEM) show uniform grain distribution throughout the surface of the samples. Complex impedance analysis (CIS) has been carried out to investigate the electrical properties of BMT. The real and imaginary part of complex impedance plots exhibit semicircle(s) in the complex plane. The bulk resistance of the material decreases with rise in temperatures similar to a semiconductor. The variation of bulk ac conductivity with frequency shows that the compounds exhibit dispersive type electrical conductivity.     

Spin-wave dispersion in ferromagnetic semiconductor superlattices and thin films in the narrow-band limit

Bulk and surface magnetic excitations of the semi-infinite ferromagnetic semiconductor (FMS) superlattices and thin films described by Heisenberg and s-d model are analyzed using the transfer matrix method, developed in our previous work. Results are discussed in the narrow-band limit. The spin-wave frequencies for the semi-infinite narrow-band semiconductors are analyzed in both low- and high-frequency regions. Energies of localized excitations are compared to the bulk and the results of Green function formalism. Depending on the parameters of the system, the surface spin waves appear as “acoustical” and “optical”, and there are only some quantitative difference in the high-frequency region, comparing our method and the Green function method. In the framework of the same methodology, bulk and surface magnetic excitations of more complicated superlattices and thin films made of the FMS superlattices are analyzed in terms of dependence of the system parameters. It is shown that the s-d interaction governs the behavior of the systems. Dependence on bulk and surface parameters is discussed.     

Metastable phenomena in lutetium nitrate crystal

The ac conductivities along the c-axis of the lutetium nitrate crystal were measured successively in the frequency range from 0.5 to 10⁵ Hz at temperatures from 290 to 210 K. The metastable behaviors of the conductivity were found in the temperature region above 200 K. The fluctuation was given by the successive measurements of the time series of the ac conductivities and represented by non-Gaussian probability distribution function. The effect of the metastable singularity on Raman scattering spectra was observed in the metastable temperature region.     

Bulk and surface localized modes on a magnetic nonlinear impurity

We study localized modes on a single magnetic impurity positioned in the bulk or at the surface of a one-dimensional chain, in the presence of a magnetic field B acting at the impurity site. The strong on-site nonlinear interaction U between two electrons of opposite spin at the impurity site, modelled here as a nonlinear local term, and the presence of the external field induce a strong correlation between parallel and antiparallel spin bound states. We find that, for an impurity in the bulk, a localized vector mode (with up and down spin components) is always possible for any given value of U and B, while for a surface impurity, a minimum value of both, U and B is needed to create a vector mode. In this case, up to two localized modes are possible, but only one of them is stable. The presence of the surface seems to destabilize the bulk mode in the parameter region U∼B, creating a “forbidden strip” region in parameter space, bounded by U=B+V and U=B−V, approximately.     

A new analysis of ac conductivity data in single crystal β-alumina

The low temperature ac conductivity of single crystal Na β-alumina, in the region of the bulk, intragranular response shows very non Debye-like behaviour. In the Z¹ plane, a broadened asymmetric arc is observed. Plots of M″ against frequency show broad, asymmetric peaks. Dispersions occur in plots of Y′, Y″, ?′ and ?″ against frequency. We use a simple equivalent circuit, comprising a frequency dependent admittance in parallel with a Debye-like parallel RC element, which models well the ac response of β-alumina in all four complex formalisms in terms of four independent parameters.     

EXAFS study of size dependence of atomic structure in palladium nanoparticles

Dependence of atomic structure of Palladium nanoparticles on supports Al₂O₃ and SiO₂ upon their size, changed from 1.3 to 10.5 nm, was studied by Pd K-edge EXAFS. To determine the structure of the interior (core) and the near surface regions of nanoparticle, the fitting technique of the Fourier-transforms F(R) of spectra was used, which enabled to overcome instabilities of the obtained structural parameters values. The processing of experimental data was performed using results of the study of features formation in │F(R)│ of Pd K-EXAFS in Pd foil. By this approach it was revealed that the local structure of Pd atoms in the core is similar to fcc structure of bulk Pd, irrespective of size. The percentage of Pd atoms, which can be attributed to the core, upon the particles size was determined and the obtained dependence was described by the “cluster size equation”. In the near surface region of nanoparticles, nearest-neighbors Pd–Pd distances show a large Debye–Waller parameters and the mean bond length slightly contracted for nanoparticles of sizes less than ~2 nm. The effect of small structural distortions in the vicinity of absorbing Pd atom in the near surface region was studied using the cluster model of nanoparticle.     

Effect of grain boundaries on paraconductivity of YBa2Cu3Ox

To investigate the effect of grain boundaries on paraconductivity of YBa₂Cu₃O_x, melt-textured and c-axis oriented thin films with controlled grain boundaries (superconducting transition width, ΔT, varying between 0.54 and 2.85 K) were prepared, and dc-conductivity has been measured as a function of temperature. In the logarithmic plots of excess-conductivity (Δσ) and reduced temperature (?), starting from low values of ?, we have observed three different regions namely critical region, mean field region and short wave fluctuation region. A correlation is observed between the range of critical region and ΔT, which is found to increase with ΔT. While for ΔT values smaller than 2.5 K only static critical region is observed, for higher ΔTs both static and dynamic critical regions are observed. In the mean field region a crossover from 3D to 2D was observed for all the samples. At ? values larger than 0.24, the excess-conductivity decreased sharply as ?⁻³, which suggested the existence of the short wave fluctuations.     

AC conductivity and dielectric behavior of bulk Furfurylidenemalononitrile

AC conductivity and dielectric behavior for bulk Furfurylidenemalononitrile have been studied over a temperature range (293–333 K) and frequency range (50–5×10⁶ Hz). The frequency dependence of ac conductivity, σ_ac, has been investigated by the universal power law, σ_ac(ω)=Aω^s. The variation of the frequency exponent (s) with temperature was analyzed in terms of different conduction mechanisms, and it was found that the correlated barrier hopping (CBH) model is the predominant conduction mechanism. The temperature dependence of σ_ac(ω) showed a linear increase with the increase in temperature at different frequencies. The ac activation energy was determined at different frequencies. Dielectric data were analyzed using complex permittivity and complex electric modulus for bulk Furfurylidenemalononitrile at various temperatures.     

Magnetization reversal in bulk and thin films of the ferrimagnetic ErCo0.50Mn0.50O3 perovskite

We present herein a comparison of the magnetic properties of bulk ceramics and thin films of the ferrimagnetic ErCo_0.50Mn_0.50O₃ compound. Epitaxial thin films were deposited onto (1 0 0) SrTiO₃ substrates by pulsed-laser ablation while bulk ceramics were prepared by solid state reaction. When cooling under low applied fields, a spin reversal is observed in both thin film and bulk due to the competition between two magnetic sublattices (Co/Mn and Er) coupled by a negative exchange interaction. Original features are observed in the M(H) loops for bulk materials: abrupt jumps at 4 T due to a reorientation of domains, while in the low field region, the increasing and decreasing branches of the magnetization intersect each other. In the thin film, the ordering temperature increased from 69 to 75 K, and the ZFC anomaly (AF transition) became sharper, compared to the bulk specimen. The oxygen content and the microstructure are crucial to observe the intersection of the magnetization branches.     

Dynamic dielectric response of a quantum wire superlattice in interaction with a nonlocal host medium

We study nonlocality effects of a bulk plasmalike dielectric medium on the plasmon spectrum of a one-dimensional (1D) quantum wire superlattice in interaction with the 3D nonlocal host bulk plasma, by carrying out a closed-form analytic determination of the inverse dielectric function κ for the joint nanostructure system within the random phase approximation (RPA), in which we treat nonlocality of the 1D superlattice in the RPA and that of the bulk medium in the hydrodynamic model. By examining the frequency poles of κ (i.e., the dispersions relations), we show analytically that coupled plasmon modes of the interacting 1D superlattice-3D nonlocal host are damped in high frequencies (damping is pronounced near resonance region) and that nonlocality of the host medium introduces nonlocal low frequency (real) modes into the spectrum, which have cutoff frequencies for finite wave vector values. In order to describe the impact of nonlocality effects more clearly, we also examine the spectrum numerically.     

Dielectric behavior of nano-structured and bulk Li Ni Zn ferrite samples

The ac conductivity and dielectric properties of spinel ferrite nanoparticles of Li_0.1(Ni_1−xZn_x)_0.8Fe_2.1O₄ (x=0.0–1.0) prepared by the chemical co-precipitation method were investigated as functions of frequency and temperature by using a complex impedance technique. Parts of the precipitated powders were pressed into a disk-shape and were sintered at 1473 K for 2 h to increase the particle size to the bulk scale (dimensions >100 nm). The ac conductivity of the samples increases with increasing temperature, ensuring the semiconducting behavior of both nano and bulk samples, in agreement with the Koops model to describe heterogeneous structures. The significant decrease in ac conductivity σ′_ac, dielectric constant, and dielectric loss of the as-prepared nanosamples compared to their bulk counterparts is correlated to the small size of the grain compared to the grain boundary size. This might be useful for many applications requiring the reduction of eddy current effects.     

Study of magnetotransport in double-layered La1.4Ca1.6Mn2O7 manganite: Presence of nano-ferromagnetic domains in paramagnetic matrix

Double-layered manganite La_1.4Ca_1.6Mn₂O₇ has been synthesized using the solid-state reaction method. It had a metal-to-insulator transition at temperature T_M1≈127 K. The temperature dependence of ac susceptibility showed a broad ferromagnetic transition. The two-dimensional (2D)-ferromagnetic ordering temperature (T_C2) was observed as ≈245 K. The temperature dependence of its low-field magnetoresistance has been studied. The low-field magnetoresistance of double-layered manganite, in the temperature regions between T_M1 and T_C2, has been found to follow 1/T⁵. The observed behaviour of temperature dependence of resistivity and low-field magnetoresistance has been explained in terms of two-phase model where ferromagnetic domains exist in the matrix of paramagnetic regions in which spin-dependent tunneling of charge carriers occurs between the ferromagnetic correlated regions. Based on the two-phase model, the dimension of these ferromagnetic domains inside the paramagnetic matrix has been estimated as ∼12 Å.     

Theory of enhanced dynamical photo-thermal bi-stability effects in cuprous oxide/organic hybrid heterostructure

We theoretically demonstrate the formation of multiple bi-stability regions in the temperature pattern on the interface between a cuprous oxide quantum well and DCM2:CA:PS organic compound. The Frenkel molecular exciton of the DCM2 is brought into resonance with the 1S quadrupole Wannier-Mott exciton in the cuprous oxide by “solvatochromism” with CA. The resulting hybrid is thermalized with the surrounding helium bath. This leads to strongly non-linear temperature dependence of the laser field detuning from the quadrupole exciton energy band which is associated with the temperature-induced red shift of the Wannier exciton energy. A numerical up- and down-scan for the detuning reveals hysteresis-like temperature distribution. The obtained multiple bi-stability regions are at least three orders of magnitude bigger (meV) than the experimentally observed bi-stability in bulk cuprous oxide (μeV ). The effective absorption curve exhibits highly asymmetrical behavior for the Frenkel-like (above the 1S energy) and Wannier-like (below the 1S energy) branches of the hybrid.     

Dispersion relation for surface-localized magnetic polaritons and magnetostatic waves in antiferromagnetic superlattice

In addition to the general dispersion equation for surface-localized magnetic polaritons and magnetostatic waves, which propagate in the system antiferromagnetic superlattice-antiferromagnet, we derive a simplified result for the long-wavelength limit λ?L (L is the period of superlattice) when the superlattice is found to behave like an anisotropic bulk medium (effective-medium approach). The dispersion curves and frequency region of the existence of the surface magnetic polaritons and magnetostatic waves are presented numerically for several values of the external magnetic field and for different antiferromagnetic materials.     

Anisotropic ac dissipation at the surface of mesoscopic superconductors

In this work we study the ac dissipation of mesoscopic superconductors at microwave frequencies using the time dependent Ginzburg-Landau equations. Our numerical simulations show that the ac dissipation is strongly dependent on the orientation of the ac magnetic field (h_ac) relative to the dc magnetic field (H_dc). When h_ac is parallel to H_dc we observe that each vortex penetration event produces a significant suppression of the ac losses because the imaginary part of the ac susceptibility as a function of H_dc increases before the penetration of vortices, and then it decreases abruptly after vortices have entered into the sample. In the second case, when h_ac is perpendicular to H_dc, we observe that the jumps in dissipation occur at the same values of H_dc but are much smaller than in the parallel configuration. The behavior of the dissipation in the perpendicular configuration is similar to previous results obtained in recent microwave experiments using mesoscopic lithographed squares of Pb [A.D. Hernández, O. Arés, C. Hart, D. Domínguez, H. Pastoriza, A. Butera, J. Low Temp. Phys. 135 (2004) 119].     

Structure, thermal and transport properties of PVAc-LiClO4 solid polymer electrolytes

The lithium ion conducting solid polymer electrolytes (SPE) based on PVAc-LiClO₄ of various compositions were prepared by solution casting technique. Structure and surface morphology characterization were studied by X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) measurements, respectively. Thermal and conductivity behavior of polymer-salt complexes were studied by employing differential scanning calorimetry (DSC) and ac impedance measurements, respectively. XRD and SEM analyses indicate the amorphous nature of the polymer-salt complexes. DSC measurements show decrease in T_g with the increase in LiClO₄ concentrations. The bulk conductivity of the PVAc:LiClO₄ polymer electrolytes was found to vary between 7.6×10⁻⁷ and 6.2×10⁻⁵ S cm⁻¹ at 303 K with the increase in salt concentration. The temperature dependence of the polymer electrolyte complexes appear to obey Arrhenius law.     

Resistance switching in Fe-doped P0.7Ca0.3MnO3 thin films

An enhanced electric-pulse-induced resistance (EPIR) switching effect is observed in the Ag/P_0.7Ca_0.3Mn_1 − xFe_xO₃/YBa₂Cu₃O₇ (Ag/PCMFO/YBCO) and Ag/(PCMFO/PCMO)/YBCO structures. Unlike in the PCMO-based EPIR devices, where the Ag/PCMO interface plays a crucial role, both the Ag/PCMFO interface and the bulk PCMFO are found to have significant contributions to the EPIR switching of the PCMFO-based device. A possible explanation is to extend the pulse-driven oxygen ion/vacancy motion model at the metal/PCMO interface region to the bulk PCMFO.     

Intrinsic surface binding energy shifts in Yb metal

We report on the four-peak structure observed in the region of 4f photoemission in Yb metal, using synchrotron radiation in the energy range 70–200 eV. We conclude, contrary to previous reports, that the doublet associated with surface emissions results from an intrinsic surface shift on clean regions of the surface. We also demonstrate that the observed structure is consistent with earlier XPS measurements, and we set an upper limit on the width of the bulk peaks.     

Growth and characterization of pure and potassium iodide-doped zinc tris-thiourea sulphate (ZTS) single crystals

Single crystals of pure and potassium iodide (KI)-doped zinc tris-thiourea sulphate (ZTS) were grown from aqueous solutions by the slow evaporation method. The grown crystals were transparent. The lattice parameters of the grown crystals were determined by the single-crystal X-ray diffraction technique. The grown crystals were also characterized by recording the powder X-ray diffraction pattern and by identifying the diffracting planes. The FT-IR spectrum was recorded in the range 400-4500 cm⁻¹. Second harmonic generation (SHG) was confirmed by the Kurtz powder method. The thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) studies reveal that the materials have good thermal stability. Atomic absorption studies confirm the presence of dopant in ZTS crystals. The electrical measurements were made in the frequency range 10²-10⁶ Hz and in the temperature range 40-130 °C along a-, b- and c-directions of the grown crystals. The present study shows that the electrical parameters viz. dc conductivity, dielectric constant, dielectric loss factor and ac conductivity increase with increase in temperature. Activation energy values were also determined for the ac conduction process in grown crystals. The dc conductivity, dielectric constant, dielectric loss factor and ac conductivity of KI-doped ZTS crystal were found to be more than those of pure ZTS crystals.