Electron-Doped High T c Superconductors
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2) F1' ions for O2" ions; and (3) Ga3+ or In3+ ions for Cu2+ ions. The presently known electron-doped superconductors and their onset critical températures are summarized in Table I. The Ln 2 . x M x Cu0 4 . y electron-doped superconductors hâve the same chemical formula as the Ln2.xMxCu04_y holedoped compounds, the class of materials in which évidence for high température superconductivity was first reported by Bednorz and Miiller.1 The Ln2.xMxCuCVj, hole-doped superconductors are obtained by substituting divalent alkaline earth éléments Ca, Sr, and Ba, or monovalent Na, for trivalent La in the parent compound La2CuC*4.y, an insulator in which the Cu 2+ magnetic moments order antiferromagnetically below ~300 K.1112 As more holes in the Cu0 2 planes
Table I. The T'-Phase Ln2Cu04-Based Electron-Doped Superconductors and Thelr Onset Superconducting Critical Températures Tc. 1. Ln^M.CuO^; x » 0.15; y « 0.02 Ln M Tc Onset (K) Référence Pr Nd Sm Eu Pr Nd Sm
Ce Ce Ce Ce Th Th Th
22 24 19 13 23 19 7
n2Cu04.,^Xx Ln X Tc Onset (K) Nd
F
27
[3] [3] [3] [7] [7] [4] [8] Réfère [5]
3. Ln2.,M,Cu1.IM'I04^; x + z =» 0.15 Ln M M' Tc Onset (K) Référence Nd Ce Ga 25 [9] Nd Ce In 25 [10]
are generated by increasing the concentration x of the alkaline earth or Na soluté, the material evolves from an insulator into a métal, the Cu2+ antiferromagnetism is suppressed, and the compound becomes superconducting with a maximum Tc and maximum flux expulsion (Meissner effect) at * = 0.15 for divalent alkaline earth solutés. Similarly, the Ln2.xMxCu04.y electron-doped superconductors are obtained by substituting tetravalent Ce or Th for the trivalent Ln in the parent compounds Ln2Cu04.y, insulators in which the Cu2+ magnetic moments order antiferromagnetically below ~250 K.12'15 Increasing the concentration x of the Ce or Th soluté apparently introduces more électrons into the Cu0 2 planes, producing an insulator-metal transition, suppressing the Cu2+ antiferromagnetism, and yielding superconductivity with a maximum Tc and maximum flux expulsion at x - 0.15. Thèse results suggest there is an electron-hole symmetry in the occurrence of the insulator-metal transition, magnetism, and superconductivity in copper oxides as a function of the concentration of charge carriers in the Cu0 2 planes. This is illustrated in Figure 1 where the temperature-dopant concentration (T - x) phase diagrams delineating the régions of superconductivity and antiferromagnetic ordering of the Cu2+ ions are displayed for the holedoped La 2 . x Sr x CuÔ 4 . y and electrondoped Nd2_xCexCu04.y Systems. The superconducting phase boundaries for the La2.xSrxCu0.t.y and Nd^Ce^CuO^ Systems are from Torrance et al.16 and Ayoub et al.,17 respectively, while the magnetic phase boundaries are based on muon spin résonance studies by Uemura and co-workers.12,13 The régions in the figure denoted "N-type" and "Ptype" refer to électron and hole carriers, respectively, as inferred from the sign of the Hall coefficient measured on ceramic samples between 80 K and 300 K.18'19 The symmet
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