Complete 'melting' of charge order in hydrothermally grown Pr0.57Ca0.41Ba0.02MnO3 nanowires

Nanowires of Pr0.57Ca0.41Ba0.02MnO3 (PCBM) (diameter similar to 80-90 nm and length similar to 3.5 mu m) were synthesized by a low reaction temperature hydrothermal method. Single-phase nature of the sample was confirmed by XRD experiments. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the morphology and microstructures of the nanowires. While the bulk PCBM is known to exhibit charge order (CO) below 230 K along with a ferromagnetic transition at 110 K, SQUID measurements on the nanowires of PCBM show that the charge order is completely absent and a ferromagnetic transition occurs at 115 K. However, the magnetization in the nanowires is observed to be less compared to that in the bulk. This observation of the complete 'melting' of the charge order in the PCBM nanowires is particularly significant in view of the observation of only a weakening of the CO in the nanowires of Pr0.5Ca0.5MnO3. Electron paramagnetic resonance experiments were also carried out on the PCBM nanowires using an X-band EPR spectrometer. Characteristic differences were observed in the line width of nanowires when compared with that of the bulk.

Low loss and frequency (1 kHz–1 MHz) independent dielectric characteristics of 3BaO–3TiO2–B2O3 glasses

X-ray powder diffraction along with differential thermal analysis carried out on the as-quenched samples in the 3BaO–3TiO2–B2O3 system confirmed their amorphous and glassy nature, respectively. The dielectric constants in the 1 kHz–1 MHz frequency range were measured as a function of temperature (323–748 K). The dielectric constant and loss were found to be frequency independent in the 323–473 K temperature range. The temperature coefficient of dielectric constant was estimated using Havinga’s formula and found to be 16 ppm K−1. The electrical relaxation was rationalized using the electric modulus formalism. The dielectric constant and loss were 17±0.5 and 0.005±0.001, respectively at 323 K in the 1 kHz–1 MHz frequency range which may be of considerable interest to capacitor industry.

Dielectric properties of Li2O–3B2O3 glasses

The frequency and temperature dependences of the dielectric constant and the electrical conductivity of the transparent glasses in the composition Li2O–3B2O3 were investigated in the 100 Hz–10 MHz frequency range. The dielectric constant and the loss in the low frequency regime were electrode material dependent. Dielectric and electrical relaxations were, respectively, analyzed using the Cole–Cole and electric modulus formalisms. The dielectric relaxation mechanism was discussed in the framework of electrode and charge carrier (hopping of the ions) related polarization using generalized Cole–Cole expression. The frequency dependent electrical conductivity was rationalized using Jonscher’s power law. The activation energy associated with the dc conductivity was 0.80±0.02 eV, which was ascribed to the motion of Li+ ions in the glass matrix. The activation energy associated with dielectric relaxation was almost equal to that of the dc conductivity, indicating that the same species took part in both the processes. Temperature dependent behavior of the frequency exponent (n) suggested that the correlated barrier hopping model was the most apposite to rationalize the electrical transport phenomenon in Li2O–3B2O3 glasses. These glasses on heating at 933 K/10 h resulted in the known nonlinear optical phase LiB3O5.

Effect of indium doping on the electrical switching behaviour of Ge–Te glasses

Bulk Ge15Te85−x In x (1 ≤ x ≤ 11) series of glasses have been found to exhibit a threshold switching behaviour for an input current of 2 mA. An initial decrease is seen in the switching voltages (V T) with the addition of indium, which is due to the higher metallicity of indium. An increase is seen in V T above 3 at.% of indium, which proceeds until 8 at.%, with a change in slope (lower to higher) seen around 7 at.%. Beyond x = 8, a reversal in trend is exhibited in the variation of V T, with a well-defined minimum around x = 9 at.%. Based on the composition dependence of V T, it is proposed that Ge15Te85−x In x glasses exhibit an extended rigidity percolation threshold. The composition, x = 3, at which the V T starts to increase and the composition, x = 7, at which a slope change is exhibited correspond to the onset and completion, respectively, of the extended stiffness transition. Thermal studies and photoconductivity measurements also support the idea of an extended rigidity percolation in Ge15Te85−x In x glasses. In addition, the minimum seen in V T at x = 9 is associated with the chemical threshold (CT) of this glassy system.

Thermal properties of 3BaO–3TiO2–B2O3 glasses

Transparent glasses in the system 3BaO–3TiO2–B2O3 (BTBO) were fabricated via the conventional melt-quenching technique. The as-quenched samples were confirmed to be non-crystalline by differential thermal analysis (DTA). Thermal parameters were evaluated using non-isothermal DTA experiments. The Kauzmann temperature was found to be 759 K based on heating-rate-dependent glass transition and crystallization temperatures. A theoretical relation for the temperature-dependent viscosity is proposed for these glasses and glass-ceramics.

Electrical Switching and Other Properties of Chalcogenide Glasses

Abstract | Electrical switching which has applications in areas such as information storage, power control, etc is a scientifically interesting and technologically important phenomenon exhibited by glassy chalcogenide semiconductors. The phase change memories based on electrical switching appear to be the most promising next generation non-volatile memories, due to many attributes which include high endurance in write/read operations, shorter write/read time, high scalability, multi-bit capability, lower cost and a compatibility with complementary metal oxide semiconductor technology.Studies on the electrical switching behavior of chalcogenide glasses help us in identifying newer glasses which could be used for phase change memory applications. In particular, studies on the composition dependence of electrical switching parameters and investigations on the correlation between switching behavior with other material properties are necessary for the selection of proper compositions which make good memory materials.In this review, an attempt has been made to summarize the dependence of the electrical switching behavior of chalcogenide glasses with other material properties such as network topological effects, glass transition & crystallization temperature, activation energy for crystallization, thermal diffusivity, electrical resistivity and others.

Non-linear electrical response of chalcogenide glasses: Memory state phenomena

Investigations on the switching behaviour of arsenic-tellurium glasses with Ge or Al additives, yield interesting information about the dependence of switching on network rigidity, co-ordination of the constituents, glass transition & ambient temperature and glass forming ability.

Non-resonant rf absorption evidence for reentrant melting of vortex lattice in Bi2Sr2CaCu2O8 single crystals

We have studied the magnetic field dependent rf (20 MHz) losses in Bi2Sr2CaCu2O8 single crystals in the low field and high temperature regime. Above HCl the dissipation begins to decrease as the field is increased and exhibits a minimum at HM>HCl. For H>HM the loss increases monotonically. We attribute the decrease in loss above HCl to the stiffening of the vortex lines due to the attractive electromagnetic interaction between the 2D vortices (that comprise the vortex line at low fields) in adjacent CuO bilayers. The minimum at HM implies that the vortex lines are stiffest and hence represents a transition into vortex solid state from the narrow vortex liquid in the vicinity of HCl. The increase in loss for H>HM marks the melting of the vortex lattice and hence a second transition into vortex liquid regime. We discuss our results in the light of recent theory of reentrant melting of the vortex lattice by G. Blatter et al. (Phys. Rev. B 54, 72 (1996)).

Thermodynamic Models for the Size-dependent Melting of Nanoparticles: Different Hypotheses

A careful comparison of the experimental results reported in the literature reveals different variations of the melting temperature even for the same materials. Though there are different theoretical models, thermodynamic model has been extensively used to understand different variations of size-dependent melting of nanoparticles. There are different hypotheses such as homogeneous melting (HMH), liquid nucleation and growth (LNG) and liquid skin melting (LSM) to resolve different variations of melting temperature as reported in the literature. HMH and LNG account for the linear variation where as LSM is applied to understand the nonlinear behaviour in the plot of melting temperature against reciprocal of particle size. However, a bird's eye view reveals that either HMH or LSM has been extensively used by experimentalists. It has also been observed that not a single hypothesis can explain the size-dependent melting in the complete range. Therefore we describe an approach which can predict the plausible hypothesis for a given data set of the size-dependent melting temperature. A variety of data have been analyzed to ascertain the hypothesis and to test the approach.

Anisotropy induced crossover from weakly to strongly first order melting of two dimensional solids

Melting and freezing transitions in two dimensional (2D) systems are known to show highly unusual characteristics. Most of the earlier studies considered atomic systems: the melting of 2D molecular solids is still largely unexplored. In order to understand the role of anisotropy as well as multiple energy and length scales present in molecular systems, here we report computer simulation studies of melting of 2D molecular systems. We computed a limited portion of the solid-liquid phase diagram. We find that the interplay between the strength of isotropic and anisotropic interactions can give rise to rich phase diagram consisting of isotropic liquid and two crystalline phases-honeycomb and oblique. The nature of the transition depends on the relative strength of the anisotropic interaction and a strongly first order melting turns into a weakly first order transition on increasing the strength of the isotropic interaction. This crossover can be attributed to an increase in stiffness of the solid phase free energy minimum on increasing the strength of the anisotropic interaction. The defects involved in melting of molecular systems are quite different from those known for the atomic systems.

Nonequilibrium ``Melting'' of a Charge Density Wave Insulator via an Ultrafast Laser Pulse

We employ an exact solution of the simplest model for pump-probe time-resolved photoemission spectroscopy in charge-density-wave systems to show how, in nonequilibrium, the gap in the density of states disappears while the charge density remains modulated, and then the gap reforms after the pulse has passed. This nonequilibrium scenario qualitatively describes the common short-time experimental features in TaS2 and TbTe3, indicating a quasiuniversality for nonequilibrium ``melting'' with qualitative features that can be easily understood within a simple picture.

Effect of Surface Condition of Electrode on the Surface Melting of Material by Laser-Guided Micro-Arc Discharge

This paper studies the surface melting in the atmosphere by YAG laser-guided micro-arc discharge. In three kinds of surface conditions (free, oiled, and polyethylene covered), we try to control the diameter and the power density of discharge pit. It is found that the power density of 3 x 10(6) W/cm(2) of discharge pit on the oiled surface is moderate to form the melted layer thicker than that of the others, adapting to strengthen the surface of material, and the power density of 1.07 x 10(7) W/cm(2) of discharge pit on the polyethylene-covered surface is highest to form the deepest discharge pit among them, adapting to remove the material.

Surface-induced melting of metal nanoclusters

We investigate the size effect on melting of metal nanoclusters by molecular dynamics simulation and thermo dynamic theory based on Kofman's melt model. By the minimization of the free energy of metal nanoclusters with respect to the thickness of the surface liquid layer, it has been found that the nanoclusters of the same metal have the same premelting temperature T-pre = T-0 - T-0(gamma(su) - gamma(lv) - gamma(sl))/(rhoLxi) (T-0 is the melting point of bulk metal, gamma(sv) the solid-vapour interfacial free energy, gamma(sl) the liquid-vapour interfacial free energy, gamma(sl),l the solid-liquid interfacial free energy, p the density of metal, L the latent heat of bulk metal, and xi the characteristic length of surface-interface interaction) to be independent of the size of nanoclusters, so that the characteristic length of a metal can be obtained easily by T-pre, which can be obtained by experiments or molecular dynamics (MD) simulations. The premelting temperature T-pre of Cu is obtained by AID simulations, then xi is obtained. The melting point T-cm is further predicted by free energy analysis and is in good agreement with the result of our MD simulations. We also predict the maximum premelting-liquid width of Cu nanoclusters with various sizes and the critical size, below which there is no premelting.

Size effects on the melting of nickel nanowires: a molecular dynamics study

The melting process of nickel nanowires are simulated by using molecular dynamics with the quantum Sutten-Chen many-body force field. The wires studied were approximately cylindrical in cross-section and periodic boundary conditions were applied along their length; the atoms were arranged initially in a face-centred cubic structure with the [0 0 1] direction parallel to the long axis of the wire. The size effects of the nanowires on the melting temperatures are investigated. We find that for the nanoscale regime, the melting temperatures of Ni nanowires are much lower than that of the bulk and are linear with the reciprocal of the diameter of the nanowire. When a nanowire is heated up above the melting temperature, the neck of the nanowire begins to arise and the diameter of neck decreases rapidly with the equilibrated running time. Finally, the breaking of nanowire arises, which leads to the formation of the spherical clusters. (C) 2004 Elsevier B.V. All rights reserved.