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Many exotic phenomena in strongly correlated electron systems, such as unconventional superconductivity, metal-insulator transition, and quantum criticality, take place in the intermediate regime between localized and itinerant electronic state. To understand the electronic behaviors near the localized-to-itinerant crossover remains a challenging problem in condensed matter physics. The Ru5+ cubic pyrochlores A2Ru2O7 (A=Cd, Cd, Hg) constitute such a system that the Ru-4d electrons acquire characters of both itinerancy and localization. In addition, the magnetic Ru5+ ions that are situated on the vertices of corner-shared tetrahedral lattice are expected to experience strong geometrical frustration given an antiferromagnetic (AF) arrangement. In this work, we investigate the cubic pyrochlore Cd2Ru2O7, which develops a peculiar metallic state below the AF transition. We synthesize a series of Pb-doped Cd2-xPbxRu2O7 (0 x 2) polycrystalline samples under high-pressure condition, and study the effects of Pb doping on their crystal structure and physical properties. Although the Pb2Ru2O7 pyrochlore is a Pauli paramagnetic metal, we find that the substitution of 10% Pb2+ for Cd2+ in Cd1.8Pb0.2Ru2O7 converts the metallic state of Cd2Ru2O7 into an insulating ground state, in a manner similar to the consequence of exerting hydrostatic pressure or substituting 10% Ca2+ for Cd2+ ions as we found recently. We propose that the electronic state of Cd2Ru2O7 be located at the itinerancy to localization crossover, and the introduction of chemical disorder via Pb2+ substitution may enhance the localized character and induce the metal-to-insulator transition. Our results further demonstrate that the cubic Ru5+ pyrochlore oxides offer an important paradigm for studying the exotic physics of correlated electrons on the border of (de)localization in the presence of strong geometrical frustration.
[1] Goodenough J B 2001 Localized to Itinerant Electronic Transition in Perovskite Oxides, In Structure and Bonding (Vol. 98) (Berlin:Springer)
[2] Bednorz J G, Muller K A 1986 Z. Phys. B 64 189
[3] Imada M, Fujimori A, Tokura Y 1998 Rev. Mod. Phys. 70 1039
[4] Morosan E, Natelson D, Nevidomskyy A H, Si Q 2012 Adv. Mater. 24 4896
[5] Gegenwart P, Si Q, Steglich F 2008 Nat. Phys. 4 186
[6] Miyazaki M, Kadono R, Satoh K H, Hiraishi M, Takeshita S, Koda A, Yamamoto A, Takagi H 2010 Phys. Rev. B 82 094413
[7] Munenaka T, Sato H 2006 J. Phys. Soc. Jpn. 75 103801
[8] Taniguchi T, Munenaka T, Sato H 2009 J. Phys.:Conf. Ser. 145 012017
[9] Gardner J S, Gingras M J P, Greedan J E 2010 Rev. Mod. Phys. 82 53
[10] Wang R, Sleight A W 1998 Mater. Res. Bull. 33 1005
[11] Yamamoto A, Sharma P A, Okamoto Y, Nakao A, Katori H A, Niitaka S, Hashizume D, Takagi H 2007 J. Phys. Soc. Jpn. 76 043703
[12] Klein W, Kremer R K, Jansen M 2007 J. Mater. Chem. 17 1356
[13] Duijin J V, Ruiz-Bustos R, Daoud-Aladine A 2012 Phys. Rev. B 86 214111
[14] Tachibana M, Kohama Y, Shimoyama T, Harada A, Taniyama T, Itoh M, Kawaji H, Atake T 2006 Phys. Rev. B 73 193107
[15] Shannon R D 1976 Acta Cryst. A 32 751
[16] Mott N F 1969 Phil. Mag. 19 835
[17] Mott N F 1967 Adv. Phys. 16 49
[18] Fritzsche H 1971 Solid State Comm. 9 1813
[19] Mandrus D, Thompson J R, Gaal R, Forro L, Bryan J C, Chakoumakos B C, Woods L M, Sales B C, Fishman R S, Keppens V 2001 Phys. Rev. B 63 195104
[20] Kim B J, Jin H, Moon S J, Kim J Y, Park B G, Leem C S, Yu J, Noh T W, Kim C, Oh S J, Park J H, Durairaj V, Cao G, Rotenberg E 2008 Phys. Rev. Lett. 101 076402
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[1] Goodenough J B 2001 Localized to Itinerant Electronic Transition in Perovskite Oxides, In Structure and Bonding (Vol. 98) (Berlin:Springer)
[2] Bednorz J G, Muller K A 1986 Z. Phys. B 64 189
[3] Imada M, Fujimori A, Tokura Y 1998 Rev. Mod. Phys. 70 1039
[4] Morosan E, Natelson D, Nevidomskyy A H, Si Q 2012 Adv. Mater. 24 4896
[5] Gegenwart P, Si Q, Steglich F 2008 Nat. Phys. 4 186
[6] Miyazaki M, Kadono R, Satoh K H, Hiraishi M, Takeshita S, Koda A, Yamamoto A, Takagi H 2010 Phys. Rev. B 82 094413
[7] Munenaka T, Sato H 2006 J. Phys. Soc. Jpn. 75 103801
[8] Taniguchi T, Munenaka T, Sato H 2009 J. Phys.:Conf. Ser. 145 012017
[9] Gardner J S, Gingras M J P, Greedan J E 2010 Rev. Mod. Phys. 82 53
[10] Wang R, Sleight A W 1998 Mater. Res. Bull. 33 1005
[11] Yamamoto A, Sharma P A, Okamoto Y, Nakao A, Katori H A, Niitaka S, Hashizume D, Takagi H 2007 J. Phys. Soc. Jpn. 76 043703
[12] Klein W, Kremer R K, Jansen M 2007 J. Mater. Chem. 17 1356
[13] Duijin J V, Ruiz-Bustos R, Daoud-Aladine A 2012 Phys. Rev. B 86 214111
[14] Tachibana M, Kohama Y, Shimoyama T, Harada A, Taniyama T, Itoh M, Kawaji H, Atake T 2006 Phys. Rev. B 73 193107
[15] Shannon R D 1976 Acta Cryst. A 32 751
[16] Mott N F 1969 Phil. Mag. 19 835
[17] Mott N F 1967 Adv. Phys. 16 49
[18] Fritzsche H 1971 Solid State Comm. 9 1813
[19] Mandrus D, Thompson J R, Gaal R, Forro L, Bryan J C, Chakoumakos B C, Woods L M, Sales B C, Fishman R S, Keppens V 2001 Phys. Rev. B 63 195104
[20] Kim B J, Jin H, Moon S J, Kim J Y, Park B G, Leem C S, Yu J, Noh T W, Kim C, Oh S J, Park J H, Durairaj V, Cao G, Rotenberg E 2008 Phys. Rev. Lett. 101 076402
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