Produção Nacional
Granular behavior in polycrystalline Sm 2 − x Ce <…
1994; American Physical Society; Volume: 50; Issue: 14 Linguagem: Inglês
10.1103/physrevb.50.10080
ISSN1095-3795
AutoresR. F. Jardim, L. Ben‐Dor, D. Stroud, M. B. Maple,
Tópico(s)Iron-based superconductors research
ResumoThis work reports a systematic study of polycrystalline samples of ${\mathrm{Sm}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathit{y}}$ (0.15\ensuremath{\le}x\ensuremath{\le}0.18) obtained from a sol-gel precursor and subjected to different cooling rates after reduction. A double resistive superconducting transition is a common feature of all samples studied, suggesting that this is an intrinsic property of these polycrystalline compounds. At an upper temperature ${\mathit{T}}_{\mathit{c}\mathit{i}}$, there is a fairly sharp drop in the magnitude of the electrical resistivity, which is followed by a well-defined plateau down to a lower temperature ${\mathit{T}}_{\mathit{c}\mathit{j}}$. At this temperature, Josephson coupling between superconducting islands is believed to complete the transition to the zero resistance state. From the compositional dependence of electrical resistivity, we infer that ${\mathit{T}}_{\mathit{c}\mathit{i}}$ decreases slightly from x=0.15 through x=0.18, while ${\mathit{T}}_{\mathit{c}\mathit{j}}$ shows a maximum for x=0.16. With increasing excitation current, no significant changes in the behavior of the electrical resistivity between ${\mathit{T}}_{\mathit{c}\mathit{i}}$ and ${\mathit{T}}_{\mathit{c}\mathit{j}}$ are observed, while a dramatic broadening and a shift of the transition at ${\mathit{T}}_{\mathit{c}\mathit{j}}$ towards lower temperatures are found. Magnetic-susceptibility measurements reveal appreciable diamagnetism just below the coupling temperature ${\mathit{T}}_{\mathit{c}\mathit{j}}$ suggesting that superconducting properties are really confined to small regions, with size comparable to the London penetration depth.The average size of these regions are estimated to be between \ensuremath{\sim}6 and 300 \AA{}, in good agreement with recent estimates obtained from magnetoresistance measurements on polycrystalline ${\mathrm{Sm}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathit{y}}$ (0.14\ensuremath{\le}x\ensuremath{\le}0.17) samples and with both neutron-diffraction studies and M\"ossbauer spectroscopy measurements in the isomorphic compound ${\mathrm{Nd}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathit{y}}$ (0.0\ensuremath{\le}x\ensuremath{\le}0.18). All the observed macroscopic properties, as well as the absence of a peak in the specific heat at ${\mathit{T}}_{\mathit{c}\mathit{i}}$, are discussed within the framework of a granular superconductor model. In addition, we give qualitative arguments suggesting the importance of charging effects in the macroscopic properties of these polycrystalline samples.
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