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H. C. Ku Department of Physics, National Tsing Hua University, Hsinchu, Taiwan 300, R.O.C.

Superconductivity and Anomalous Magnetic Properties of the New Ru Ca 2 RCu 2 O 8+  (R = Pr-Gd) System. H. C. Ku Department of Physics, National Tsing Hua University, Hsinchu, Taiwan 300, R.O.C. with: B. N. Lin, P. C. Guan, Y. C. Lin, T. Y. Chiu, M. F. Tai. Introduction.

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H. C. Ku Department of Physics, National Tsing Hua University, Hsinchu, Taiwan 300, R.O.C.

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  1. Superconductivity and Anomalous Magnetic Properties of the New RuCa2RCu2O8+ (R = Pr-Gd) System H. C. Ku Department of Physics, National Tsing Hua University, Hsinchu, Taiwan 300, R.O.C. with: B. N. Lin, P. C. Guan, Y. C. Lin, T. Y. Chiu, M. F. Tai

  2. Introduction • Magnetic Superconductivity in RuSr2RCu2O8+ • Ru-1212 system (R = Sm, Eu, Gd) • -tetragonal TlBa2CaCu2O7+-type (1212) structure • - weak ferromagnetic metal Tm ~ 135 K • high-Tc superconductor Tc ~ 0-65 K • *sensitive to oxygen  - hole concentration • *coexistence with ferromagnetic order (Tc < Tm) • Bauernfeind et al., Physica C 254, 151 (1995) • Bernhard et al., PRB 59, 14099 (1999) • Awana et al., Physica C 357 (2001)

  3. RuSr2GdCu2O8+d Tetragonal 1212-type P4/mmm Sr Gd CuO5 RuO6

  4. RuSr2GdCu2O8+d Tm = 133 K, Tc = 16-48 K Bernhard et al., PRB 59, 14009 (1999) R(T) v(T)(ZFC) Mm(T)(FC) S(T)

  5. Normal or abnormal ferromagnetic magnetic hysteresis curve M(Ba)for T < Tm = 135 K? Current oxygen-deficient sample superconducting Tc ~ 10 K superconducting signal? Bernhard et al., PRB 59, 14009 (1999)

  6. ZFC/FC susceptibilitySuperconducting diamagnetic signal? Current oxygen-deficient sample superconducting Tc ~ 10 K superconducting signal? Cordero et al., PRB 67, 144519 (2003)

  7. Superconducting diamagnetic signal appears only invery low applied fields due to strong ferromagnetic background Papageorgiou & Braun, PRB 66, 104509 (2002) Mg(T)(FC)

  8. Tc only in R = Sm, Eu, Gd of RuSr2RCu2O8+d With oxygen effect - hole concentration

  9. Question • What about the RuCa2RCu2O8+ system • (R = La, Ce, Pr, Nd, Sm, Eu, Gd)? • First try R = Pr : RuCa2PrCu2O8+

  10. Results and Discussion • RuCa2PrCu2O8+ • RuCa2-xSrxPrCu2O8+system (2  x  0) • RuCa2RCu2O8+ system • (R = La, Ce, Pr, Nd, Sm, Eu, Gd) • Sample preparation: • Direct one-step, oxygen annealed

  11. Distorted orthorhombic variation of tetragonal 1212 structure: ao ~ bo ~ 2atco ~ 2ct

  12. Abnormal low temperature magnetic behavior at different measuring times oxygen diffusion effect

  13. Normal ZFC/FC susceptibility and magnetic hysteresis for standard high-Tc superconductors without magnetic order

  14. Magnetization Mm(T) in different applied fields Anomalous FC/ZFC behavior

  15. Normal paramagnetic behavior for T > Tm &Mgnetic order (weak-ferromagnetic or spin-glass?) for Tm > T > Tc

  16. Weak magnetization relaxation at T= 45 K < Tm(itinerant weak ferromagnetism almost spin-glass-like)

  17. Spin-glass-like weak hysteresis m(Ba) and relaxation m(t) for paramagnetic Ca1-xSrxRuO3 x = 0 & 0.2Lin et al., JMMM, in press (2004)

  18. Lattice distortion of CaRuO3 compared with SrRuO3 Orthorhombic (pseudo-cubic) a ~ b ~ c/2 Orthorhombic (distorted)

  19. Compare the magnetic phase diagram of Ca1-xSrxRuO3system with RuA2RCu2O8 with the same RuO6 configuration

  20. Ru4/5+ 4d4/d3 in RuO6 • -4dnint2g bandsdue to strong RuO6octahedral crystal field splitting Dcf and weak Hund’ rule exchange coupling JH • strong Ru 4dxy,yz,zx-O 2px,y,z hybridization • large on-site Coulomb repulsion Udd • Long-range magnetic order • (or short-range spin glass) Charge transfer D Coulomb repulsion Udd eg On-site Coulomb repulsion energy Udd = E(dn-1) + E(dn+1) - 2E(dn) Charge transfer energy D = E(dn+1L) – E(dn) d t2g

  21. Origin of weak, itinerant ferromagnetic order - self-doing with anisotropic Ru 4dxy,yz,zx-O 2px,y,z hybridization drives the resulting mixed-valent system metallic and ferromagnetic via double exchange interaction - lower Tm ~ 50 K for RuCa2PrCu2O8 as compared with Tm ~ 135 K for RuSr2GdCu2O8 (in more robust local moment regime) indicates weaker double exchange coupling strength Jde due to RuO6 distortion

  22. Complex magnetic flux dynamics for T < Tc < Tmin magnetization due to interplay between superconductivity and itinerant ferromagnetic order

  23. FC and ZFC magnetization at 10 K show complex interplay between superconductivity and itinerant ferromagnetic order

  24. Low field (1 G) time relaxation M(t) at 10 K < Tc < Tm

  25. 1-G and 1-kG time relaxation M(t) at 10 K < Tc < Tm

  26. RuCa2PrCu2O8+ • distorted orthorhombic variation of tetragonal 1212 • structure: ao ~ bo ~ 2atco ~ 2ct • Ru4/5+ itinerant weak ferromagnetic metal close to • spin-glass with Tm ~ 50 K • superconductor Tc ~ 37-40 K • *sensitive to oxygen  -- hole concentration), • *coexistence with weak magnetic order • anomalous ZFC (Meissner) & FC (field expulsion) • behavior due to interplay with ferromagnetic order

  27. Powder x-ray diffraction patterns oftetragonal RuSr2PrCu2O8+d

  28. Powder x-ray diffraction patterns for RuCa2-xSrxPrCu2O8+d

  29. Low temperature spin-glass-like behaviorfor tetragonal RuSr2PrCu2O8+dRef: Awana et al. Physica C357-360, 121 (2001).

  30. Low temperature spin-glass-like behaviorfor tetragonal RuSr2PrCu2O8+d

  31. Weak magnetization relaxation at T= 10 K < Tmspin-glass-likefor tetragonal RuSr2PrCu2O8+d

  32. Field-cooled (FC) susceptibility for RuCa2-xSrxPrCu2O8+d

  33. Magnetic behavior for RuCa2-xSrxPrCu2O8+d (x = 0, 0.1)

  34. RuCa2-xSrxPrCu2O8+ system • orthorhombic 1212 structure only near RuCa2PrCu2O8+ • (x ~ 0) • multiphase samples for 0 < x < 2 • diamagnetic, superconducting signal observed only • for x  0.2 • spin-glass-like behavior for tetragonal RuSr2PrCu2O8+ • (x = 2)

  35. Powder x-ray diffraction patterns for RuCa2RCu2O8+d (R = Pr, Gd)

  36. Magnetic behaviorfor RuCa2RCu2O8+d (R = Pr, Gd)

  37. Powder x-ray diffraction patternsfor RuCa2RCu2O8+d

  38. Field-cooled (FC) susceptibilityfor RuCa2-xSrxPrCu2O8+d (R = La, Ce, Pr, Nd, Sm, Eu, Gd)

  39. Zero-field-cooled (ZFC) susceptibility for RuCa2-xSrxPrCu2O8+d (R = La, Ce, Nd, Sm, Eu, Gd)

  40. RuCa2RCu2O8+ system • orthorhombic 1212 phase in R = Pr, Nd, Sm, Eu, Gd • magnetic order Tm ~ 45-50 K for R = Pr-Gd • superconductivity Tc ~ 25-40 K for R = Pr-Gd • with anomalous ZFC and FC behavior due to magnetic • order • weak paramagnetic spin-glass-like behavior signal for • R = La, Ce

  41. Conclusions • Superconductivity are observed in the • RuCa2RCu2O8+d (R = Pr, Nd, Sm, Eu, Gd) • system with the distorted orthorhombic • 1212-type structure • Structural and anomalous magnetic • properties are strongly correlated with the • RuO6 distortion and Ru 4d-O2p hybridization • withweak, anisotropic double-exchange • coupling strength due to distortion

  42. Unit for magnetic measurements Total magnetic moment m (emu) [Gaussian system] Volume magnetization Mv (emu/cm3 G) Mass magnetization Mg (emu/g = G•cm3/g) Molar magnetizationMm (emu/mol = G•cm3/mol) (MmMg • molar formula unit weight) Magnetic moment per formula unit m (mB/f.u.) (m Mm/(No • mB) Volume magnetic susceptibility v  Mv/Ba (emu/cm3•G = 1) Mass magnetic susceptibility g  Mg/Ba (emu/g•G = cm3/g) Molar magnetic susceptibilitym (emu/mol•G = cm3/mol) (mg• molar formula unit weight) [good for SI system with g  Mg/0Ba]

  43. Magnetic behavior cm(T) for bulk and powder samples Diamagnetic signal, bulk superconductivity

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