Phase Diagram of Ruthenate: Ca2xSrxRuO4 CSRO 0'0x2'0 Biao Hu

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Phase Diagram of RuthenateCa2-xSrxRuO4 (CSRO)
(0.0ltxlt2.0) Biao Hu
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Outline
1. Introduction to Ruthenates
2. Phase diagram of Ca2-xSrxRuO4
3. Lattice dynamics in Ca2-xSrxRuO4
4. Summary
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1. Introduction to Ruthenates
Transition metal oxides (TMOs) strong coupling
between charge, lattice, orbital, and spin.
Ruddlesden-Popper (RP) series (Sr,Ca)n1RunO3n1
n1,Sr2RuO4(SC), Ca2RuO4(AF
insulator) n2,Sr3Ru2O7,Ca3Ru2O7 n3,Sr4Ru3O10 C
rystal structures for various n. T site is Ru.
G. Cao et al. Matl. Sci. Eng. B 63,76 (1999)
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Ru electronic configuration Kr4d75s1
Rutherate (Ru4)
(a)
(b)
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2. Phase diagram of Ca2-xSrxRuO4
P paramagnetic, CAF canted antiferromagnetic,
M magnetic, SC superconducting, -M
metallic phase, I insulating phase.

• (0xlt0.2) AF insulating ground state
• (0.2xlt0.5) Magnetic metallic (M-M) region
• (0.5x2) Paramagnetic metal.

Does there have some critical points?
S. Nakatsuji et al Phys. Rev. Lett. 84, 2666
(2000)
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High-temperature tetragonal to a low-temperature
orthorhombic phase (xc0.5)
To (open diamond) is the temperature below which
the in-plane susceptibility starts to show
twofold anisotropy.
xc0.5 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)
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Temperature dependence of the anisotropy ratio of
the in-plane susceptibilities for Ca2-xSrxRuO4
with x0.2, 0.3, 0.4, and 0.5. The inset shows
the appearance of the in-plane anisotropy at To.
To as 220K for x0.3, 150K for 0.4,
and below 1.8K for 0.5
What about the lattice dynamics in CSRO family?
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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.
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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)
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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.
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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)
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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)
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(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)
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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
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Ca1.9Sr0.1RuO4 surface phonon
T200K,observed phonon mode In Ca1.9 only A1g
mode.
R. Moore et al. Phys.Stat.Sol.(b) 241 2363 (2004)
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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)
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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)
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Surface phase diagram for Ca2-xSrxRuO4 (From
Robs 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 xlt0.2.
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5. Summary
(a) From the analysis to phase diagram
Ca2-xSrxRuO4, there exists a quantum critical
point at x0.5.
(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.
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Thank you for attention!