• P Chaddah

      Articles written in Pramana – Journal of Physics

    • Effect of correlations on electron momentum density in liquid metals

      P Chaddah

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      A scheme to calculate the electron momentum density in simple liquid metals, with the effect ofboth electron correlations and ionic potentials included, is given. This scheme is applied to the case of liquid aluminium. The results are substantially different from calculations considering only the ion potentials, and also from the results for a homogeneous electron gas of corresponding density.

    • Disorder in superconductors—a study on Cu2Mo6S8

      P Chaddah

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      Some of the recent work on disorder-induced changes inTc is reviewed. Shock-pressures induce a disorder uncomplicated by antisite disorder typical of particle irradiation, and have generated interest because of the shock-synthesis of A-15 Nb3Si. In this paper we present our results on laser-induced shock-damage, and compare it with the results on V3Si and the results on particle irradiation of Chevrel phase superconductors.

    • 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.

    • Magnetic field dependence of critical current density in YBa2Cu3O7

      P K Mishra G Ravi Kumar P Chaddah B A Dasannacharya M K Malik

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      We have measured the transport critical current densityJcof sintered YBa2Cu3O7, in various applied fields up to 185 Oe at 77 K. We find a sharp decay ofJcwith magnetic field. We show that this sharp decay is consistent with the low field hysteresis results of Groveret al. We argue that the observed field dependence is not caused by intragranular weak links.

    • Low field anomaly in magnetization curves

      G Ravi Kumar P Chaddah

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      We present a calculation of the isothermal magnetization hysteresis curves appropriate to highTcsuperconductors. We discuss the nature of the low field anomaly as one goes from this strong pinning case to the weak pinning case. We show that the shape of the equilibrium (thermodynamic) magnetization curve is recovered in the limit ofJcapproaching zero.

    • 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.

    • Magnetization curves of hard superconductor samples with non-zero demagnetization factor

      K V Bhagwat P Chaddah

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      We present exact solutions of Bean’s critical state model for some sample shapes having non-zero demagnetization factorN. Virgin and hysteresis magnetization curves are obtained for samples in the shape of (i) a sphere (ii) a spheroid (iii) a cylinder of circular cross-section with its axis perpendicular to the field and (iv) a cylinder of elliptical cross section with its axis perpendicular to the field. Some interesting features seen in these first solutions forN ≠ 0 are discussed.

    • Field and temperature dependence of intergranular hysteresis in YBa2Cu3O7

      Shailendra Kumar G Ravikumar P K Mishra P Chaddah B A Dasannacharya Ram Prasad N C Soni

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      We have studied the variation of low field magnetization hysteresis in YBa2Cu3O7 as a function of the maximum magnetic field applied during a hysteresis cycle (1 G<Hmax<7.3 G) and also as a function of temperature (77 K<T<95 K). The remnant magnetization is studied as a function ofHmax andT and the measured dependences are explained using the extended critical state model. The potential of this technique as a contactless method of probing the temperature dependence ofJc is discussed.

    • Studies of irreversible magnetization in superconductors—a review

      P Chaddah

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      Magnetic measurements in the superconducting state of the high temperature superconductors have been characterized by the feature of irreversibility. Similar effects have been known in the conventional type II superconductors for about three decades now, and have been studied in great detail during the last few years. Recent studies of magnetic irreversibilities, in both conventional and high temperature superconductors, will be reviewed here. Thermally-activated relaxation accompanies such irreversibilities, and studies on flux-creep will also be reviewed.

      This review shall cover the measurement of isothermal magnetization curves, of ac susceptibility, of thermo-magnetic history effects in the magnetization at a particular field and temperature, and of flux creep. An understanding of these in terms of Bean’s celebrated macroscopic model shall be discussed. We shall also cover measurements that confirm the existence of weak links in ceramic high-temperature, as well as in conventional multifilamentary, 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.

    • Field-cooled measurements of harmonic generation in magnetization of high-Tc superconductors

      S B Roy Shailendra Kumar A K Pradhan P Chaddah Ram Prasad N C Soni Kalyan Adhikary

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      We present measurements of harmonic generation in the magnetization of sintered pellets of YBa2Cu3O7 and Bi1.7Pb0.3Sr2Ca2Cu3O10 as a function of DC field. The DC field is applied in the field-cooled mode. Measurements are made at 77 K for various values of the AC field amplitude. A comparison is made with calculations done within the critical state model.

    • 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.

    • Interesting normal state and superconducting properties of the intermediate valence compound CeRu2

      S B Roy P Chaddah

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      Superconductivity in CeRu2 was discovered 40 years ago, and was extensively studied because alloying with magnetic elements showed the coexistence of superconductivity and magnetic order. The normal state of CeRu2 has been of interest because of its intermediate valuence character. The superconducting state has been studied extensively because of its paramagnetic nature and anomalous pinning properties. This review presents the present status of knowledge, and discusses the puzzling features of CeRu2.

    • Supercooling across first-order phase transitions in vortex matter

      P Chaddah S B Roy

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      Hysteresis in cycling through first-order phase transitions in vortex matter, akin to the well-studied phenomenon of supercooling of water, has been discussed in literature. Hysteresis can be seen while varying either temperature T or magnetic field H (and thus the density of vortices). Our recent work on phase transitions with two control variables shows that the observable region of metastability of the supercooled phase would depend on the path followed in H-T space, and will be larger when T is lowered at constant H compared to the case when H is lowered at constant T. We discuss the effect of isothermal field variations on metastable supercooled states produced by field-cooling. This path dependence is not a priori applicable to metastability caused by reduced diffusivity or hindered kinetics.

    • Calculation of minor hysteresis loops under metastable to stable transformations in vortex matter

      P Chaddah M Manekar

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      We present a model in which metastable supercooled phase and stable equilibrium phase of vortex matter coexist in different regions of a sample. Minor hysteresis loops are calculated with the simple assumption of the two phases of vortex matter having field-independent critical current densities. We use our earlier published ideas that the free energy barrier separating the metastable and stable phases reduces as the magnetic induction moves farther from the first order phase transition line, and that metastable to stable transformations occur in local regions of the sample when the local energy dissipation exceeds a critical value. Previously reported anomalous features in minor hysteresis loops are reproduced, and calculated field profiles are presented.

    • Studies on magnetic-field-induced first-order transitions

      P Chaddah

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      We shall discuss magnetization and transport measurements in materials exhibiting a broad first-order transition. The phase transitions would be caused by varying magnetic field as well as temperature, and we concentrate on ferro- to antiferromagnetic transitions in magnetic materials. We distinguish between metastable supercooled phases and metastable glassy phase.

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