• A K Grover

      Articles written in Pramana – Journal of Physics

    • Electromagnetic response of a single phase Y-Ba-Cu-O compound superconducting in the 90 K range

      V Nagarajan P L Paulose A K Grover S K Dhar E V Sampathkumaran

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      The results of x-ray diffraction, electrical resistance, DC and AC magnetization studies on two specimens of compositions Y2.1Ba3.9Cu6O14−δ and Y1.8Ba4.2Cu6O14−δ are presented. Our studies confirm that La3Ba3Cu6O14 type orthorhombic phase is responsible for superconductivity in the 90 K range. The heat treatment that yields high quality samples with sharp transition in electrical resistance as well as in AC magnetic susceptibility measurements is described. Magnetic response just belowTc is found to be sensitive to the measuring field.

    • Anomalous magnetic hysteresis loops and smallHc1 values in highTc superconductors

      A K Grover C Radhakrishnamurty P Chaddah G Ravi Kumar G V Subbarao

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      We have studied the hysteresis loops of RBa2Cu3O7 (R=Gd, Ho and Y) and detected anomalies in some of them. The observed anomalies support a recent prediction by Ravi Kumar and Chaddah based on an extension of Bean’s model. The anomalies indicate lowHc1 values and we have confirmed this by studying the onset of low-field hysteresis in less than 10 Oe at 77 K for these highTc superconductors.

    • Measurement and understanding of magnetization in AC and DC fields and the determination of intragrainHc1 in highTc RBa2Cu3O7 superconductors

      A K Grover C Radhakrishnamurty P Chaddah G Ravi Kumar G V Subba Rao

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      We present DC and low frequency AC magnetization measurements on various RBa2Cu3O7 superconductors. We identify features intrinsic to these compounds, and establish the features originating from intergranular links in sintered pellets. The isothermal magnetization curves, and the temperature dependence of magnetization in field-cooled and zero field-cooled states are shown to be consistent with the calculations done following a recent extension of Bean’s model. Low field anomalies predicted within this model are observed, and yieldHc1 values of a few Oe. These values are shown to be consistent with the temperature variation of magnetization. A comparison is made with the other existing data and it is demonstrated that earlier quoted values ofHc1 are gross overestimates.

    • Thermomagnetic history effects in niobium and its implication for Hc1 in highTc superconductors

      A K Grover P L Paulose P Chaddah G Ravikumar

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      The existence of a remanent magnetization (Mrem) on switching off the field of a field cooled (FC) sample of a highTc superconductor is often reported. It has recently been argued thatMrem should equal the difference in FC and zero field cooled (ZFC) magnetizations (MFCMZFC) in hard superconductors and this has been demonstrated to hold in single crystals of YBCO at 4.2K over a limited range ofH values. We report the detailed magnetization measurements under various thermomagnetic histories (of whichMrem is one special case) on two specimens of Nb, which show different extents of flux trapping. We find that there are in general three regions inH, T space, corresponding toMrem+MZFCMFC=0,Mrem<(MFCMZFC) andMrem>(MFCMZFC). At anyT, the equality holds forH<Hc1(T), and forHHc2 (MFCMZFC) asymptotically vanishes and thereMrem>(MFCMZFC). We show that there exists an intermediate region in all hard superconductors, whereMrem<(MFCMZFC). The range over which this situation persists, however, depends on the degree of irreversibility in a sample. We can explain qualitatively all the history dependent magnetization data in terms of the critical state model. We point out an inconsistency in an earlier analysis to determineHc1(T) from such data in YBCO. We also propose a new criterion for putting limits onHc1(T) in hard superconductors.

    • Comparison of irreversibility temperatures determined via DC and AC magnetization techniques in conventional superconductors

      A K Grover S Ramakrishnan Ravi Kumar P L Paulose S K Malik P Chaddah

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      A comparison has been made of irreversibility temperature determined by four different methods in few specimens of lead (type-I) and niobium (type-II). The merger ofMZFC(T) andMFC(T) curves giveTr(H) values lower than those evident from vanishing the hysteresis in isothermal DC magnetization. The identification of peak temperature inxH(T) data withTr(H) is appropriate only if the contribution from changes in the normal state electrodynamics can be isolated and the peak is narrow. The appearance of differential paramagnetic effect inxH(T) data is adequate to imply reversibility, however, its efficacy to precisely locate irreversibility line remains to be established.

    • Magnetization studies in niobium and YBa2Cu3O7 samples

      B V B Sarkissian A K Grover G Balakrishnan Ravi Kumar P L Paulose R Vijayaraghavan V Sankaranarayanan C K Subramanian

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      The results of experimental studies on hysteresis in magnetization, thermomagnetic history effects, anomalous variations in magnetic hysteresis curves and the decay rates of magnetization obtained under different thermomagnetic histories in specimens of conventional and high temperature superconductors are presented. The Bean’s critical state model is considered adequate to explain magnetic behaviour in conventional hard superconductors. The similarity in the general features of the results of different experiments on specimens of the two families of superconductors underscores the efficacy of the said model to understand some aspects of the macroscopic magnetic response of high temperature superconductors as well. For instance, the isothermal magnetization hysteresis loop which comprises of magnetization curves along forward (−Hmax to +Hmax) and reverse (+Hmax to −Hmax) paths define an envelop within which all isothermal magnetization data along different thermomagnetic histories lie. There exist inequality relationship between various field values identified asHpeak,HI,HII etc. in isothermal magnetization hysteresis as well as magnetic relaxation data. The entire field span of an isothermal magnetization hysteresis data set can be considered to comprise of three parts corresponding to (Mrem(H)−MFC(H)+MZFC(H)) being equal to, less than or greater than zero, whereMrem(H) are the remanent magnetization values obtained on reducing field to zero after having the specimen in different applied field (H) values. There are, however some situations amongst thermomagnetic history effects in specimens which show incomplete flux trapping on field cooling, where the critical state model has been found inadequate.

    • Magnetic phase diagram of anisotropic layered superconductors via magnetization measurements forHc in Bi2212

      A K Grover F Iga Y Yamaguchi N Aoki K Ino N Goyal Y Nishihara

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      The Bi2Sr2CaCu2O8 system is viewed as an archetypal of superconductors modelled as Josephson coupled CuO2 bilayers. The isothermal and temperature dependent DC and AC magnetization measurements forHc in a single crystal of Bi2212 have been performed. Qualitative changes are observed to occur over a narrow range of temperature values before reaching the superconducting-normal transition. The observed behaviour can be ascribed to the rapid variation in the strength of the coupling between the superconducting CuO2 planes (i.e., bilayers in the case of Bi2212). Strongly coupled planes behave like a 3D superconductor, whereas weakly coupled planes have a two component response attributable to 2D planes and interplanar couplings. We believe that this paper is a plethora of new findings. Our observations imply that resistivity across the planes becomes zero earlier than that within the planes. A new line (designated asH2D(T)) above which the change in the electromagnetic response is dominated by quasi 2D-planes has been determined for the first time. This paper also contains the first observation of Differential Diamagnetic Effect (DDE) in the In-phase AC susceptibility data which signals the onset (atT2D(H)) of dominance of response from 2D-planes. In addition to a host of interesting thermomagnetic history effects which are a consequence of interplay between the diamagnetic responses from the two components, a comparison of irreversibility lines (of the 3D state) determined by different methods on the same specimen of a HTSC is also being presented for the first time. We have come across Paramagnetic Meissner Effect (PME), first recognized in ceramic samples of Bi2212, in the temperature region of dimensional crossover in our single crystal sample, whichinter-alia confirms our labelling of the two component behaviour. A schematic phase diagram summarizing the various transformations that can occur nearTc in the electromagnetic response of an anisotropic layered system has been drawn.

    • Geometry effect on the nonlinear magnetic response of YBa2Cu3O7−y

      Parshant Batham S B Roy Shailendra Kumar A K Pradhan A K Grover P Chaddah Ram Prasad N C Soni

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      Results are presented for study of nonlinear magnetization of a sintered YBa2Cu3O7−y pellet of different thickness of a sample subjected to very low magnetic field. On cooling the sample belowTc in zero field a change, in the oscillatory structure of harmonics in increasing dc field is observed in very low ac magnetic field. The effect of finiteness of the sample on the oscillatory structure and on the hysteresis of harmonics is also studied. The results are explained qualitatively.

    • A study of the plasticity in the vortex matter across the second magnetization peak in a YBCO crystal via measurements of minor hysteresis loops

      D Pal S Ramakrishnan A K Grover D Dasgupta Bimal K Sarma

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      Results of an investigation of the path dependence of the critical current density Jc due to the plastic deformation of the flux line lattice in a weakly pinned YBa2Cu3O7-δ (YBCO) crystal for H‖c are reported. The procedure of minor hysteresis loops has been used to explore the path dependence of Jc and the metastability effects. Contrary to the behavior observed in low Tc systems. in YBCO it is found that at low temperatures, the multivaluedness in Jc(H) could persist beyond the notional peak field Hp, at which the anomalous variation in Jc(H) reaches its maximum value.

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