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Phase Diagram of Ruthenate: Ca 2-x Sr x RuO 4 (CSRO) (0.0<x<2.0) Biao Hu

Phase Diagram of Ruthenate: Ca 2-x Sr x RuO 4 (CSRO) (0.0<x<2.0) Biao Hu. Outline: . 1. Introduction to Ruthenates. 2. Phase diagram of Ca 2-x Sr x RuO 4. 3. Lattice dynamics in Ca 2-x Sr x RuO 4. 4. Summary. 1. Introduction to Ruthenates.

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Phase Diagram of Ruthenate: Ca 2-x Sr x RuO 4 (CSRO) (0.0<x<2.0) Biao Hu

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  1. Phase Diagram of Ruthenate:Ca2-xSrxRuO4(CSRO)(0.0<x<2.0)Biao Hu

  2. Outline: 1. Introduction to Ruthenates 2. Phase diagram of Ca2-xSrxRuO4 3. Lattice dynamics in Ca2-xSrxRuO4 4. Summary

  3. 1. Introduction to Ruthenates Transition metal oxides (TMOs): strong coupling between charge, lattice, orbital, and spin. Ruddlesden-Popper (RP) series (Sr,Ca)n+1RunO3n+1 n=1,Sr2RuO4(SC), Ca2RuO4(AF insulator); n=2,Sr3Ru2O7,Ca3Ru2O7; n=3,Sr4Ru3O10 Crystal structures for various n. T site is Ru. G. Cao et al. Matl. Sci. Eng. B 63,76 (1999)

  4. Ru electronic configuration: [Kr]4d75s1 Rutherate (Ru4+) (a) (b)

  5. 2. Phase diagram of Ca2-xSrxRuO4 P: paramagnetic, CAF: canted antiferromagnetic, M: magnetic, SC: superconducting, -M: metallic phase, –I : insulating phase. • (0≤x＜0.2) AF insulating ground state; • (0.2≤x＜0.5) Magnetic metallic (M-M) region; • (0.5≤x≤2) Paramagnetic metal. Does there have some critical points? S. Nakatsuji et al Phys. Rev. Lett. 84, 2666 (2000)

  6. High-temperature tetragonal to a low-temperature orthorhombic phase(xc≈0.5) To (open diamond) is the temperature below which the in-plane susceptibility starts to show twofold anisotropy. xc≈0.5 is the instability point at absolute zero temperature, which perhaps is the quantum critical point of the second-order structural transition. S. Nakatsuji et al Phys. Rev. B 62, 6458 (2000)

  7. Temperature dependence of the anisotropy ratio of the in-plane susceptibilities: for Ca2-xSrxRuO4 with x=0.2, 0.3, 0.4, and 0.5. The inset shows the appearance of the in-plane anisotropy at To. To as 220K for x=0.3, 150K for 0.4, and below 1.8K for 0.5 What about the lattice dynamics in CSRO family?

  8. 3. Lattice dynamics in Ca2-xSrxRuO4 RuO6 rotation and tilt Rotation: The RuO6 octahedron rotates around the long axis (c axis) with an angle . Tilt: The RuO6 octahedron tilts around an axis lying in the RuO2 plane; the tilt angle between the octahedron basal planes and the a, b planes ; the angle between the Ru-O(2) bond and the long axis.

  9. 3. Lattice dynamics in Ca2-xSrxRuO4 Structural stability of Sr2RuO4 Low-frequency part of the phonon dispersion along [110]. Only the branches corresponding to the , , and representations are shown. rotational mode: The rotation of the octahedron around the c axis represents a zone-boundary mode. tilt mode: The structural instability is reflected in a low-frequency zone boundary mode. Sr2RuO4 crystal structure M. Braden et al. PRB 57, 1236 (1998)

  10. 3. Lattice dynamics in Ca2-xSrxRuO4 The rotation and tilt mode frequencies and widths as a function of temperature. The left side is the results for the tilt around an in-plane axis; The right side is the c-axis rotation mode.

  11. 3. Lattice dynamics in Ca2-xSrxRuO4 Symmetry degeneration due to rotation and tilt (a) (b) Schematic pictures showing the group-subgroup relations. M. Braden et al. PRB 58, 847 (1998)

  12. 3. Lattice dynamics in Ca2-xSrxRuO4 Structural phase diagram Phase diagram of Ca2-xSrxRuO4 including the different structural and magnetic phases and the occurrence of the maxima in the magnetic susceptibility. O. Friedt et al. PRB 63, 174432 (2001)

  13. (a) Raman Scattering on Sr2RuO4 P1 and P2 correspond to the symmetry-allowed vibrations along c axis in the tetragonal phase; P1 ~ 200cm-1(24.8meV) is the in-phase motion of Sr and apical oxygen O(2) , and P2 ~550cm-1(68.15meV) is the vibration of the apical oxygen O(2) . S. Sakita et al. PRB 63, 134520 (2001)

  14. Surface phonon in Sr2RuO4 by HREELS Surface dipole active optic phonon mode for K2NiF4 structure Ismail et al. PRB 67, 035407 (2003) Dipole active optical phonon Sr2RuO4

  15. Ca1.9Sr0.1RuO4 surface phonon T=200K,observed phonon mode; In Ca1.9 only A1g mode. R. Moore et al. Phys.Stat.Sol.(b) 241 2363 (2004)

  16. Surface structural analysis of Sr2RuO4 by LEED I(V) A lattice distortion characterized by rigid RuO6 octahedra rotation of ，not present in the bulk;(a) Structure model of the surface reconstruction(top view on surface); (b) p4gm plane group symmetry with surface unit cell. R. Matzdorf et al. PRB 65, 085404 (2002)

  17. LEED I-V structural analysis on Ca1.5Sr0.5RuO4(001) Result: The surface octahedra rotating angle of is the same as that in the bulk . But the Ca/Sr ions in the surface display a large displacement inward . Final structure obtained for the Ca1.5(001) surface V.B. Nascimento et al PRB 75, 035408 (2007)

  18. Surface phase diagram for Ca2-xSrxRuO4(From Rob’s thesis) Solid lines denote structural phase transitions; Dashed line indicates onset of tilt instability; Light orange and green regions are projections based on current trends; Light red region indicates insulating phases.No structural phase boundary is indicated between metallic and insulating phases for x<0.2.

  19. 5. Summary (a) From the analysis to phase diagram Ca2-xSrxRuO4, there exists a quantum critical point at x=0.5. (b) The substitution of Ca2+ for Sr2+ will generate a different structure involving a static rotation and tilt of the RuO6 octahedral. Rotational and tilt play a significant role in lattice dynamics for Ca2-xSrxRuO4 compounds.

  20. Thank you for attention!

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