Phase Diagram of Ruthenate:Ca2-xSrxRuO4 (CSRO) (0.0<x<2.0)

Biao Hu

Outline:

1. Introduction to Ruthenates

3. Lattice dynamics in Ca2-xSrxRuO4

2. Phase diagram of Ca2-xSrxRuO4

4. Summary

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)

1. Introduction to RuthenatesTransition metal oxides (TMOs): strong coupling between charge, lattice, orbital, and spin.

Ru electronic configuration: [Kr]4d75s1

Rutherate (Ru4+)

(a) (b)

2. Phase diagram of Ca2-xSrxRuO4

P: paramagnetic, CAF: canted antiferromagnetic, M: magnetic, SC: superconducting, -M: metallic phase, –I : insulating phase.

S. Nakatsuji et al Phys. Rev. Lett. 84, 2666 (2000)

(I) (0≤x＜ 0.2) AF insulating ground state;

(II) (0.2≤x＜ 0.5) Magnetic metallic (M-M) region;

(III) (0.5≤x≤2) Paramagnetic metal.

Does there have some critical points?

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.

S. Nakatsuji et al Phys. Rev. B 62, 6458 (2000)

xc≈0.5 is the instability point at absolute zero temperature, which perhaps is the quantum critical point of the second-order structural transition.

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?

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.

1O

2O

3. Lattice dynamics in Ca2-xSrxRuO4

Low-frequency part of the phonon dispersion along [110]. Only the branches corresponding to the , , and representations are shown.

1 34

tilt mode: The structural instability is reflected in a low-frequency zone boundary mode.

rotational mode: The rotation of the octahedron around the c axis represents a zone-boundary mode.

3

4

Structural stability of Sr2RuO4

Sr2RuO4 crystal structureM. Braden et al. PRB 57, 1236 (1998)

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.

3. Lattice dynamics in Ca2-xSrxRuO4

Schematic pictures showing the group-subgroup relations.

Symmetry degeneration due to rotation and tilt

M. Braden et al. PRB 58, 847 (1998)

(a)

(b)

3. Lattice dynamics in Ca2-xSrxRuO4

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)

3. Lattice dynamics in Ca2-xSrxRuO4

Structural phase diagram

(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)

Surface dipole active optic phonon mode for K2NiF4 structure

Surface phonon in Sr2RuO4 by HREELS

Dipole active optical phonon Sr2RuO4

Ismail et al. PRB 67, 035407 (2003)

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)

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.

5.25.8

4522 R

R. Matzdorf et al. PRB 65, 085404 (2002)

LEED I-V structural analysis on Ca1.5Sr0.5RuO4(001)

Final structure obtained for the Ca1.5(001) surface

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 .

512 43.12

A03.013.0 V.B. Nascimento et al PRB 75, 035408 (2007)

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.

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.

Thank you for attention!