S V Bhat

Absorption of electromagnetic radiation by superconducting YBa2Cu3O7: an oxygen-induced phenomenon

In the superconducting state, YBa2Cu3O7 absorbs electromagnetic radiation over a wide range of frequencies (8 MHz-9 GHz). The absorption is extremely sensitive to temperature, particle size and the magnetic field and depends crucially on the presence of oxygen. A possible explanation for the phenomenon based on the formation of Josephson junctions is suggested.

An electron paramagnetic resonance study of Pr0.6Ca0.4MnO3 across the charge-ordering transition

We report the first electron paramagnetic resonance studies of single crystals and powders of Pr0.6Ca0.4MnO3 in the 300-4.2 K range, covering the charge-ordering transition (Tco) at ~240 K and antiferromagnetic transition (TN) at ~170 K. The asymmetry parameter for the Dysonian single-crystal spectra shows an anomalous increase at Tco. Below Tco the g-value increases continuously, suggesting a gradual strengthening of the orbital ordering. The linewidth undergoes a sudden increase at Tco and continues to increase down to TN. The intensity increases as the temperature is decreased until Tco is reached, due to the renormalization of the magnetic susceptibility arising from the build-up of ferromagnetic correlations.

Investigating thermal stability of structural defects and its effect on d0 ferromagnetism in undoped SnO2

The effect of annealing on structural defects and d(0) ferromagnetism in SnO2 nanoparticles prepared by solution combustion method is investigated. The as-synthesized SnO2 nanoparticles were annealed at 400-800 degrees C for 2 h, in ambient conditions. The crystallinity, size, and morphology of the samples were studied using x-ray diffraction and transmission electron microscopy studies. The annealing results in grain growth due to coarsening as well as reduction in oxygen vacancies as confirmed by Raman spectroscopy, photoluminescence spectroscopy, and x-ray photoelectron spectroscopy. All the as synthesized and annealed samples exhibit room temperature ferromagnetism (RTFM) with distinct hysteresis loops and the saturation magnetization as high as similar to 0.02 emu/g in as-synthesized samples. However, the saturation magnetization is drastically reduced with increasing annealing temperature. Further the presence of singly charged oxygen vacancies (V-o(-) signal at g-value 1.99) is confirmed by electron paramagnetic resonance studies, which also diminish with increasing annealing temperature. The observed diminishing RTFM and simultaneous evidences of diminishing O vacancies clearly indicate that RTFM is driven by defects in oxide lattice and confirms primary role of oxygen vacancies in inducing ferromagnetic ordering in metal oxide semiconductors. The study also provides improved fundamental understanding regarding the ambiguity in the origin of intrinsic RTFM in semiconducting metal oxides and projects their technological application in the field of spintronics

An “EPR” study of YBa2Cu3O7 and related high-temperature superconductors

Besides ag ≈ 2 signal which disappears on cooling to the superconductingTc, YBa2Cu3O7 and related oxides show a near-zero-field signal in the superconducting state with certain unusual features attributable to a “superconducting glassy state”.

Molecular ferromagnetism in C60·TDAE

Abstract C 60 ·TDAE = tetrakis(dimethylamino)ethylene) with a T c of ∼ 16K is probably the best example of an organic molecular ferromagnet known to date. Based on ESR studies, we demonstrate that C 60 ·TDAE is a quasi one-dimensional spin one-half Heisenberg ferromagnet, with the C 60 units being entirely responsible for the magnetism. C 60 -C 60 interactions responsible for superconductivity in alkali metal doped C 60 seem to be essential for the magnetism of C 60 ·TDAE as well.

Preparation, structure, microwave absorption and other properties of the 125K superconductor Tl2Ca2Ba2Cu3O10+δ

It is shown that Tl2Ca2Ba2Cu3O10+I´ (2223), the n=3 member of the Tl2O2. Canâ��1Ba2CunO2n+2 family shows a Tc (zero-resistance) of 125K (onset 140K) only when it is prepared by the sealed tube ceramic method starting from the 1313 composition. The structure is orthorhombic (Image compared to 30A� of 2122), but electron diffraction patterns show two possible orthorhombic structures. Lattice images show the expected local structure and also the presence of dislocations and intergrowths. Both 2223 and 2122 oxides absorb microwaves (9.1GHz) intensely in the superconducting state, with some hysteresis. XPS measurements show Cu mainly in the 1+ state, suggesting the important role of oxygen holes.

Nonresonant microwave absorption studies of surface passivation of superconducting YBa2Cu3O7−δ thin films

The surfaces of laser ablated thin films of YBa2Cu3O7−δ have been passivated with about 100 A thick textured layer of Ca0.95Sr0.025Ba0.025Zr0.98Ta0.01Ti0.01O3. It is shown that this low loss dielectric material preserves the quality of the surface and also prolongs the aging process. The films (both passivated and as‐deposited) have been studied for degradation on exposure to atmosphere and also on dipping directly in water. The technique of nonresonant microwave absorption is used to study the effects and extent of degradation in these films.

Magnetic study of an amorphous conducting polyaniline

We show that newly found BF3-doped polyaniline, although highly conducting, remains amorphous. Magnetic studies reveal many unusual properties, while suggesting that the intrinsic conductivity of this system is significantly larger than all other known forms of conducting polyaniline, establishing it as an interesting class of highly conducting amorphous polymer.

Bi2−xPbx(Ca, Sr)n+1CunO2n+4+δ (n = 1, 2, 3, and 4) family of superconductors

Abstract Superconducting oxides of the Bi 1.5 Pb 0.5 (Ca, Sr) n +1 Cu n O 2 n +4+δ series with n = 1, 2, 3 and 4 have been characterized. The superconducting transition temperature increases markedly with n up to n = 3, but the T c of the n = 4 member is not much higher than that of the n = 3 member. The T c does not change significantly in Bi 2− x Pb x CaSr 2 Cu 2 O 8+δ with x (0.1 x ≤ 0.5).

An electron paramagnetic resonance study of electron?hole asymmetry in charge ordered Pr1?xCaxMnO3 (x = 0.64, 0.36)

We present and compare the results of temperature-dependent electron paramagnetic resonance (EPR) studies on