Amitava Sen Gupta

Mo¨ssbauer studies of Sn in the high T c superconductor YBa 1.8Sn 0.2Cu 3O 9−y

Abstract Mo¨ssbauer measurements have revealed that the Sn in YBa 1.8 Sn 0.2 Cu 3 O 9−y occupies two sites, possibly those of Ba and Y. Marked variations in the intensity of one of the resonance lines, most probably corresponding to Sn in Ba sites, are observed in the superconducting transition region 115 to 82 K, indicating changes in the lattice rigidity. This implies the important role of phonons in the superconductivity phenomenon, which seems to occur in the Ba—Cu—O cuboctahedron.

High temperature superconductivity in Y-Ba-Cu-O and Y-Ba-Sr-Cu-O systems

Abstract Mixed metal oxides Y-Ba-Cu-O and Y-Ba-Sr-Cu-O have been prepared in varying compositions and by changing process parameters. Superconductivity with critical temperatures in the range of 70 to 100 K is reported.

Superconductivity of Y Ba2 Cu3 O9−y containing zirconium

Abstract The effect of Zr incorporation on the superconductivity of YBa 2 Cu 3 O 9−y has been investigated. A systematic study made to replace the three cation sites reveals that the partial replacement of Y and both Y and Ba by Zr results in the formation of superconducting multiphases, whereas Cu replacement leads only to the single superconducting phase of usual 123 composition. The data obtained indicate that variation in the Cu 2+ Cu 3+ ratio does not affect superconductivity of these compounds significantly.

NPLI Cesium Atomic Fountain Frequency Standard: Preliminary Results

National Physical Laboratory India has developed a cesium atomic fountain frequency standard. The fountain has now become operational and is ready for complete frequency evaluation. In this paper, we report for the first time, the preliminary results obtained with this fountain, which include Ramsey fringes, frequency stability analysis, with an H-maser, and magnetic field mapping in the atomic flight region.

A novel low noise regenerative divide-by-four circuit

Discusses a novel design of a self starting regenerative divider that permits division by 3, 4, 5, 6... instead of the usual 2. This is accomplished by having the loop oscillate simultaneously at two different frequencies, e.g., at f/4 and 3f/4. A prototype of the divide-by-four circuit has been constructed for an input frequency of 400 MHz. This divider exhibits very low phase noise L(1 kHz) = -161dBc/Hz and L(100 kHz) = -169 dBc/Hz,, which is at least 8 dB lower than its constituent parts. Simple modifications of the feedback loop of the prototype enable it to divide by 3, 5 and 8. Operation at divide by 10 appears feasible.

Parallel configuration for conjugate regenerative dividers

In this paper we discuss an alternate design of a self-starting conjugate regenerative divider (CRD), which uses two parallel loops instead of one series loop as in conventional regenerative divider. This is accomplished by making the first loop oscillate at one conjugate frequency while the second loop oscillates at the other conjugate frequency, for example /spl nu//4 and 3/spl nu//4. As before the two conjugate frequencies are harmonically related and sum to the input frequency. The advantages of this new configuration are that one can optimize each loop for phase shift, gain, and noise performance and that the individual amplifiers do not have to span the entire bandwidth of the two conjugate frequencies. A prototype of the divide-by-four circuit has been constructed for input frequencies of 400 MHz as well as for 10 GHz.

Progress in building of cesium fountain frequency standard at NPL, India

We describe the current state of progress of the Cesium fountain frequency standard development at the National Physical Laboratory India (NPLI). The optical set-up of the fountain needed to capture, cool, manipulate and detect the cesium atoms is discussed in detail. The concept and design of physical structure of the fountain is also described. In addition, some of the recent results on cooling and trapping of cesium atoms are reported.

Substitutional effect on superconducting transition of Y-Ba-Cu-O

Superconducting Y-Ba-Cu-O system has been studied. The effect of changing compositions as well as the constituents on the superconducting transition temperature is reported.

Systematic shifts in the frequency of NPLI-CsF1

At CSIR-National Physical Laboratory, India (NPLI) we have developed first generation of Cesium atomic fountain (NPLI-CsF1) primary frequency standard (PFS). NPLI-CsF1 is now fully operational and its frequency is now being evaluated along with all systematic and statistical uncertainties. The present experimental data conforms that fountain frequency is stable to few parts in 1015 at less than one day averaging time. The fountain frequency is compared with the Hydrogen MASER (H-MASER) which is contributing to TAI already and hence fountain frequency is traceable to TAI. Recent evaluation results were communicated to BIPM and published in circular T. Typical stability of NPLI CsF1 is ~7 × 10-13 τ-1/2. The present performance of fountain is limited by the measurement uncertainty of cold collisional shift.

A novel technique for precise phase and frequency measurement

A novel technique for precise phase and frequency measurement is described in this paper. A primary frequency standard, namely, Cs atomic fountain has already been developed at CSIR-NPL. Six commercial Cs atomic beam clocks along with a H-Maser are continuously inter-compared and constitute a stable time scale. H-Maser frequency is being evaluated with respect to the Cs fountain on regular basis. For evaluating the frequencies of these different frequency sources precisely, a stable, low noise and very accurate frequency measurement instrument is required. All digital implementation of dual mixer time difference technique is a step towards more precise measurement and evaluation of frequencies and will provide better stability to the timescale.