M. D. Sastry

Superconducting transition temperature of single-phase Tl-2223: Crucial role of Ca-vacancies and Tl-content

Abstract Detailed investigations on the synthesis and phase-indentification of a number of Tl-Ca-Ba-Cu oxides show that single-phase Tl-2223 superconductor with the best transition temperature ( R = 0 at 130 K) can be prepared from a thallium-deficient starting composition, namely Tl ≈1.0 Ca 2 Ba 2 Cu 3 O x . A number of other starting compositions containing Tl: Ca T c -values between 115 K and 130 K. It is argued that the existence of vacancies at Ca-sites is crucial to the production of pure 2223-phase and the highest T c (130 K) is obtained when the number of such vacancies reaches an optimum. Starting compositions deficient in Tl lead to such a possibility by aiding the migration of some Ca-ions to vacant Tl-sites. Under standardised preparation conditions single-phase Tl-2223 structure is obtained for a combined nominal stoichiometry 3 ⩽ (Tl + Ca) ⩽ 4.

Photoacoustic response of CdS quantum dots in nafion

Quantum confinement effects on the excitonic transitions in CdS quantum dots embedded in the polymer matrix Nafion are studied using the technique of photoacoustic spectroscopy. The characteristic blueshift in the absorption onset with decreasing quantum dot size is in agreement with that observed in the optical absorption spectra. Excitonic transitions are better resolved in the photoacoustic spectra when compared to the corresponding optical absorption spectra. The results are discussed in relation to existing theoretical models.

Insensitivity of Tc to high K substitution in the Tl-Ba-Ca-Cu-O system

Abstract We report that substitution of potassium for thallium upto as high as 70% in the Tl-Ba-Ca-Cu-O superconducting system does not adversely affect the T c(zero) which can be as high as 123 K for 50% K and 118 K for 70% K containing compounds. The major lines in the X-ray diffraction patterns of the 50% K substituted compounds are quitte similar to those in the pure Tl containing compound. Possibility of complete substitution of Tl by K was also explored and the results discussed.

Unique signatures of microwave absorption of superconducting YBa2Cu3O7−x

Abstract Microwave absorption in the 9 GHz range using EPR technique in the superconducting phase of YBa 2 Cu 3 O 7− x and related compounds has been investigated as a function of temperature and magnetic field. The microwave absorption in the superconducting phase is found to occur due to three different processes. These are manifested in the observation of: (i) An intense low field absorption, (ii) a field-independent absorption and (iii) an absorption-like feature in the intensity versus temperature plot in the close vicinity of T c . This feature appeared more prominently when the sample was cooled at high (∼few KG) magnetic fields. The low field absorption had exhibited saturation behaviour with increasing microwave power and temperature dependent line position. These are some of the features normally observed for ‘resonance’ absorptions. The field-independent absorption is identified as due to the imaginary part of the a.c. diamagnetic susceptibility of superconducting YBa 2 Cu 3 O 7− x . The mechanism for the third process is not yet understood.

Electron Paramagnetic Resonance study of magnetic ordering in MnPS3, Mn0.79PS3(4,4'-bipy)0.42 and Mn0.84PS3(1,10-Phen)0.64 compounds

Abstract We report Electron Paramagnetic Resonance (EPR) studies on intercalation ferromagnets Mn0.79PS3(4,4′-bipy)0.42 and Mn0.84PS3(1,10-phen)0.64 and their antiferromagnetic host MnPS3, in the temperature range 300–15 K. The nature of the organic guest cation appears to have a strong influence on the magnetic behaviour of this inorganic-organic hybrid system. The continued appearance of the exchange-narrowed sharp line, characteristic of the paramagnetic phase, even at 15 K suggests that the bulk of the intercalate ferromagnets in this system do not attain a completely ordered state.

Ferromagnetism at 19K in a bimetallic compound based on 1,2-propanediamine ligand

Abstract We report the synthesis, magnetic characteristics and electron paramagnetic resonance (EPR) data on the new hexacyanometallate-based bimetallic molecular compound [Ni(pn)2]3. [Fe(CN)6]2. 12H2O, where pn = 1,2-propanediamine, which shows bulk ferromagnetic order below 19K. It is a soft magnet exhibiting a rather weak hysteresis. The EPR spectra show a fine structure, characteristic of an S = 3/2 state, in a very narrow temperature region just above T C that is suggestive of the formation and ordering of S = 3/2 ferromagnetically coupled states of state of Ni2+ ( S = 1) and FeIII ( S = 3/2) ions.

Coexistence of Spin Fluctuations and Magnetic Order in (Nbu4)2Mn2 [Cu(Opba)]3: An Epr Evidence

Abstract EPR investigations have been carried outboth in the first and second harmonic modes on the organic ferromagnet (NBu4)2Mn2[Cu(opba)]3 with Tc = 22 K. The data was collected in the temperature range 300 – 15 K. At temperatures much higher than Tc (T ≥ 100 K), the spectrum consists of a single line at g = 2.00 exhibiting Curie behaviour. Upon cooling below 100 K, the line shows strong temperature dependent g - shift in resonance field to higher value by 50 G at 15 K. This is taken as evidence for the onset of uncompensated antiferromagnetic spin correlations well above Tc. Evidence was also obtained for spin fluctuations which got slowed down with lowering of temperature. The correlation time at 15 K is greater that 10−10 sec.

Synthesis of single phase Tl-2223 superconductors: How much thallium do we really need?

This paper describes the synthesis and microwave absorption studies of single phase Tl-2223 compound, withTc=(129 ± 1) K. The single phase compound has been synthesized using a precursor matrix method. It has been found that Tl-deficient starting composition gives the best results.

Microwave absorption studies of diluted high-temperature superconductors : delineation of superconductor-insulator-superconductor and superconductor-normal-superconductor junctions

Abstract With a view to investigating the nature of weak links in granular superconductors at very dilute level, we have studied the low-field microwave absorption signal (below about 50 G) in the electron paramagnetic resonance mode of detection in samples of Tl2BaCa2Cu2O8+δ and YBa2Cu3O7-δ at different dilution levels in the Al2O3 matrix. The signal has been monitored as a function of temperature as well as concentration. The temperature dependence of the signal intensity has been understood by applying a model in which microwave absorption is assumed to be caused by the field- and tempera ture-dependent currents in the Josephson links. It is shown that the parameter ηo quantifying the strength of coupling between superconducting grains can be calculated from the value of temperature at which the intensity I pp(T) (the subscript pp indicates a peak-to-peak value), goes through a maximum. Using the ηo value so obtained, the fitting for I pp(T) fhroughout the temperature range, was used to distinguish superconductor-insulator-superconductor (S-I-S) junctions from superconductor-normal-superconductor (S-N-S) junctions. In bulk samples the signal due to S-I-S junctions is found to predominate, whereas the relative contribution due to S-N-S junctions predominates on dilution. In extremely diluted samples, the surviving junctions were found to have only S-N-S character.

Studies on ion sorption property and stability of YBa2Cu3O7−x in aqueous and mixed solvent media

Abstract Ion sorption behaviour and stability of YBa 2 Cu 3 O 7−x in water and methanol-water mixture was studied by chemical analyses of the solutions and oxide and X-ray analyses of the oxide after treatment. Ba is preferentially leached from YBa 2 Cu 3 O 7−x by water,whereas in methanol-water mixture leaching of Ba is considerably reduced. The 1-2-3 phase of Yue5f8Baue5f8Cuue5f8O could not be stabilised by increasing the concentration of methanol in methanol-water mixture. In presence of dissolved NaCl and CaCl 2 in 20 v% methanol-water mixture, the leaching of Ba was further reduced and the residual solid contained mainly the 1-2-3 phase of Yue5f8Baue5f8Cuue5f8O. It was possible to incorporate Ca into the YBa 2 Cu 3 O 7−x matrix using CaCl 2 dissolved in 20 v% methanol-water mixture. The consequences of treating YBa 2 Cu 3 O 7−x with solutions of salts dissolved in methanol-water mixture on its superconducting property are discussed.