Transcript Document

Phase Diagram of Ruthenate:
Ca2-xSrxRuO4 (CSRO) (0.0<x<2.0)
Biao Hu
Outline:
1. Introduction to Ruthenates
2. Phase diagram of Ca2-xSrxRuO4
3. Lattice dynamics in Ca2-xSrxRuO4
4. Summary
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)
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.
(I) (0≤x＜0.2) AF insulating
ground state;
(II) (0.2≤x＜0.5) Magnetic
metallic (M-M) region;
Does there have some critical (III) (0.5≤x≤2) Paramagnetic
metal.
points?
S. Nakatsuji et al Phys. Rev. Lett. 84, 2666 (2000)
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 secondorder structural transition.
S. Nakatsuji et al Phys. Rev. B 62, 6458 (2000)
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?
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;
  O1 the tilt angle
between the octahedron basal
planes and the a, b planes ;
  O2 the angle between
the Ru-O(2) bond and the
long axis.
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 1 , 3 ,
and  4 representations are
shown.
3 rotational mode: The rotation of the octahedron
around the c axis represents a zone-boundary mode.
 4 tilt mode: The structural instability is reflected
in a low-frequency zone boundary mode.
M. Braden et al. PRB 57, 1236 (1998)
Sr2RuO4 crystal
structure
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
Symmetry degeneration due to
rotation and tilt
(a)
(b)
Schematic pictures showing the group-subgroup relations.
M. Braden et al. PRB 58, 847 (1998)
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)
(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 inphase 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 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
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 8.5  2.5 ，
not present in the bulk;(a) Structure model
of the surface reconstruction(top view on
surface); (b) p4gm plane group symmetry
with 2  2 R45 surface unit cell.


R. Matzdorf et al. PRB 65, 085404 (2002)
LEED I-V structural analysis on Ca1.5Sr0.5RuO4(001)
Result:
Final structure obtained for
the Ca1.5(001) surface
V.B. Nascimento et al PRB 75,
035408 (2007)
The surface octahedra rotating angle

of 12 5 is the same as that in the

bulk 12.43. But the Ca/Sr ions in the
surface display a large displacement

inward 0.13  0.03 A .
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!