Study of the peak effect phenomenon in single crystals of 2H-NbSe2

The weakly pinned single crystals of the hexagonal 2H-NbSe₂ compound have emerged as prototypes for determining and characterizing the phase boundaries of the possible order-disorder transformations in the vortex matter. We present here a status report based on the ac and dc magnetization measurements of the peak effect phenomenon in three crystals of 2H-NbSe₂, in which the critical current densities vary over two orders of magnitude. We sketch the generic vortex phase diagram of a weakly pinned superconductor, which also utilizes theoretical proposals. We also establish the connection between the metastability effects and pinning.     

Solvent-Free Green Synthesis of Azines and Their Conversion to 2,5-Disubstituted-1,3,4-thiadiazoles

A solvent-free, clean, and efficient method has been developed for the synthesis of 2,5-disubstituted-1,3,4-thiadiazoles via azines. This approach exploits the synthetic potential of clean reactions and offers many advantages such as excellent product yields, easy isolation of products, and ecofriendly benign reaction conditions. The newly synthesized compounds were analyzed by infrared, ¹H NMR, ¹³C NMR, and elemental analysis.     

Crystal engineering of primary cubanecarboxamides: Repetitive formation of an unexpected N-H...O hydrogen-bonded network

The unusual N-H.O hydrogen bond pattern in a family of primary cubanecarboxamides is described. 4-Chloro-1-cubanecarboxamide, 1, and the corresponding bromo and iodo derivatives, 2 and 3, form the "shallow-glide" hydrogen-bonded motif instead of the usual 5.1 A translated ribbon pattern, more characteristic of primary amides. This behavior is also seen, somewhat unexpectedly, for cubanecarboxamide, 4, but more or less unsurprisingly for 1,4-cubanedicarboxamide, 5. This repetitive occurrence of the same hydrogen bond pattern is of significance in crystal engineering wherein synthon robustness is measured in terms of such repetitivity. The cubyl group is directly responsible for the appearance of the shallow-glide motif in this family in preference to the 5.1 A translation pattern for two reasons: (1) steric--it is too large to fit in a 5.1 A translated structure and (2) electronic--its carbon acidity is sufficient to result in the appearance of C-H.O hydrogen bonds to the C=O group, disrupting any putative 5.1 A translated structure. Such a molecule --> supermolecule relationship is of value in crystal engineering strategies.     

The influence of a transverse magnetic field on a subnormal glow discharge in air

In subnormal glow discharge under d.c. excitation at different pressure in a varying transverse magnetic field (0 to 30 G) some measurements have been carried out for various initial average tube currents. The voltage across the discharge increases and average tube current and residual current decreases in the magnetic field. With the help of Beckman’s expression [4] for the axial field and the electron density distribution in a transverse magnetic field the observed variation of current and voltage can be satisfactorily explained. The variation of axial electric field with transverse magnetic field can be represented to a fair degree of accuracy by the derived equation. The behaviour of residual current with magnetic field has been observed in these oscillations.     

Integral equation formulation for buoyancy-driven convection problems

An integral equation formulation for buoyancy-driven convection problems is developed and illustrated. Buoyancy-driven convection in a bounded cylindrical geometry with a free surface is studied for a range of aspect ratios and Nusselt numbers. The critical Rayleigh number, the nature of the cellular motion, and the heat transfer enhancement are computed using linear theory. Green's functions are used to convert the linear problem into linear Fredholm integral equations. Theorems are proved which establish the properties of the eigenvalues and eigenfunctions of the linear integral operator which appears in these equations.     

Oblate versus Prolate Electron Density of Lanthanide Ions: A Design Criterion for Engineering Toroidal Moments? A Case Study on {LnIII6} (Ln=Tb,Dy, Ho and Er) Wheels

We report four new complexes based on a {Ln^III₆} wheel structure, three of which possess a net toroidal magnetic moment. The four examples consist of {Tb^III₆} and {Ho^III₆} wheels, which are rare examples of non Dy^III based complexes possessing a toroidal magnetic ground state, and a {Dy^III₆} complex which improves its toroidal structure upon lowering the crystallographic symmetry from trigonal (R ) to triclinic (P ). Notably the toroidal moment is lost for the trigonal {Er^III₆} analogue. This suggests the possibility of utilizing the popular concept of oblate and prolate electron density of the ground state M_J levels of lanthanide ions to engineer toroidal moments.     

Determination of isoniazid and hydrazine by capillary electrophoresis with amperometric detection at a Pt-particle modified carbon fiber microelectrode

Capillary electrpphoresis (CE)/electrochemical detection (EC) for the simultane-ous determination of hydrazine and isoniazid has been developed.The electrochemical method uses a novel modified electrode dispersed with ultrafine platinum particles on the surface of a 30μm carbon fiber microelectrode.The unique characteristic of the Pt-particles modified carbon fiber microelectrode is its excellent stability.The current measurement for hydrazine is more sensitive than that of isoniazid.Selective determination of trace amount of free hydrazine in isoniazid and its formulation can be achieved at applied potential of 0.5V.     

Investigating the vortex melting phenomenon in BSCCO crystals using magneto-optical imaging technique

Using a novel differential magneto-optical imaging technique we investigate the phenomenon of vortex lattice melting in crystals of Bi₂Sr₂CaCu₂O₈ (BSCCO). The images of melting reveal complex patterns in the formation and evolution of the vortex solid-liquid interface with varying field (H)/temperature (T). We believe that the complex melting patterns are due to a random distribution of material disorder/inhomogeneities across the sample, which create fluctuations in the local melting temperature or field value. To study the fluctuations in the local melting temperature/field, we have constructed maps of the melting landscape T _m(H, r), viz., the melting temperature (T _m) at a given location (r) in the sample at a given field (H). A study of these melting landscapes reveals an unexpected feature: the melting landscape is not fixed, but changes rather dramatically with varying field and temperature along the melting line. It is concluded that the changes in both the scale and shape of the landscape result from the competing contributions of different types of quenched disorder which have opposite effects on the local melting transition.     

Amphiphilic behavior of an apparently non-polar calixarene

The self-assembly properties of a non-polar calixarene have been investigated at the air-water interface, in water and in the solid state showing that this molecule behaves as an amphiphilic molecule.     

Slow Magnetic Relaxation and Single‐Molecule Toroidal Behaviour in a Family of Heptanuclear {CrIIILnIII6} (Ln=Tb,Ho, Er) Complexes

The synthesis, magnetic properties, and theoretical studies of three heterometallic {Cr^IIILn^III₆} (Ln=Tb, Ho, Er) complexes, each containing a metal topology consisting of two Ln₃ triangles connected via a Cr^III linker, are reported. The {CrTb₆} and {CrEr₆} analogues display slow relaxation of magnetization in a 3000 Oe static magnetic field. Single‐crystal measurements reveal opening up of the hysteresis loop for {CrTb₆} and {CrHo₆} molecules at low temperatures. Ab initio calculations predict toroidal magnetic moments in the two Ln₃ triangles, which are found to couple, stabilizing a con‐rotating ferrotoroidal ground state in Tb and Ho examples and extend the possibility of observing toroidal behaviour in non Dy^III complexes for the first time.