physics of the 3d simple cubic perovskites

1
Evolution of the orbital Peierls state with
doping Neutron scattering in Y1-xCaxVO3
C. Ulrich1, J. Fujioka2, G. Khaliullin1, M.
Reehuis3, K. Schmalzl4, A. Ivanov4, K. Hradil5,
S. Miyasaka2, Y.Tokura2, and B. Keimer1
1Max-Planck-Institut für Festkörperforschung,
Stuttgart, Germany 2University of Tokyo, Tokyo,
Japan 3Hahn-Meitner-Institut, Berlin,
Germany 4Institut Laue-Langevin, Grenoble,
France 5FRM II, Munich, Germany
physics of the 3d simple cubic perovskites spin,
charge, orbital degrees of freedom
YVO3 3d2 two magnetic phases (C- and G-type)
Krakow, 20. June 2008
2
Crystal Structure
3D - perovskite structure
VO6
octahedra
Y
Pm3m
ideal cubic perovskite
Tilt and rotation of the VO6 octahedra
Pbnm
GdFeO3 - type distortion
Distortion of the VO6 octahedra
3
YVO3 Elastic Neutron Scattering
m0 1.72 mB / V3-ion
m0 1.05 mB / V3-ion
4
YVO3 Low Temperature Phase
conventional orbital order
5
YVO3 High Temperature Phase
C-type
reduced magnetic moment total m0 1.05 mB/V3

• total ordered moment
• 2.00 mB (free ion moment)
• 1.72 mB (ordered moment of
• the G-type phase)
• canting angle 160 out of plane

C-type mx 0.49 mB my 0.89 mB
G-type mz 0.30 mB
C. Ulrich et al., PRL 91, 257202 (2003).
6
YVO3 High Temperature Phase
overall collapse of the magnon band width (20
meV C-type vs. 35 meV G-type)
band width larger in the ferromagnetic
c-direction than in the antiferromagnetic
ab-plane (assuming Goodenough-Kanamori rules one
would expect the opposite)
Excelent fit obtained by using three exchange
parameters Jab 2.6 meV, Jc1 -2.2 meV, Jc2
-4 meV gt dimerization of exchange bonds along
the c-axis
C. Ulrich et al., PRL 91, 257202 (2003).
7
xz
yz
xz / yz
xy
undistorted
xy
xy
Pbnm phase
G-type
C-type
c
77 K
116 K
210 K
Khaliullin et al., PRL 86, 3879 (2001).
8
Orbital Peierls State
Orbital Peierls State


Consequence of the orbital fluctuations
C-type phase 77 K lt T lt 118 K
c
strong ferro.
weak ferro.
strong ferro.
G. Khaliullin et al., PRL 86, 3879 (2001). C.
Ulrich et al., PRL 91, 257202 (2003). P. Horsch
et al., PRL 91,257203 (2003). A.M. Oles et al.,
PRB 75, 184434 (2007).
structural evidence for the dimerized phase Pb11
A.A. Tsvetkov et al., PRB 69, 075110 (2004).
9
LaVO3 Inelastic Neutron Scattering
Tstruc. 145 K Pbnm monoclinic P21/a
Tmag. 143 K C-type, spins within the
ab-plane
magnon
phonon
no splitting into an optical and acoustic magnon
branch exchange parameters Jab 6.5 meV
Jc - 4.0 meV
10
LaVO3 Inelastic Neutron Scattering
L.D. Tung, D.M.K. Paul, University of Warwick,
UK
LaVO3 Tmag. 136 K C-type
ILL experimental report
11
Doping dependence of the Orbital Peierls State
Y1-xCaxVO3
Effect of doping 2 t2g Þ 1 t2g
S. Miyasaka, PRL 85, 5388 (2000). J. Fujioka, PRB
72, 024460 (2005). Y. Tokura, University of Tokyo
G-type phase disappears at 1.5 Ca doping C-type
mag. phase (Orbital Peierls phase) is robust 50
Ca-doping Metal Insulator Transition
12
Doping dependence of the Orbital Peierls State
Y1-xCaxVO3
Magnetic phase transition temperatures decrease
with doping C-type phase magnetic structure
at T 85 K almost unchanged G-type phase
magnetic moment decreases with doping
13
Doping dependence of the Orbital Peierls State
Y1-xCaxVO3 C-type phase
Y1-xCaxVO3 5 3.7 K
14
Doping dependence of the Orbital Peierls State
Y1-xCaxVO3
IN22 ILL-Grenoble 2
PUMA FRMII-Munich 5
Magnon
Magnon
Phonon
raw data taken at the IN22/ILL-Grenoble and
Puma/FRMII-Munich data were taken above and
below the magnetic phase transition
15
Doping dependence of the Orbital Peierls State
new Si-monochromator IN20/ILL
16
Doping dependence of the Orbital Peierls State
(0.5,0.5,1.25)
(0.5,0.5,1) spin gap
slight increase in the spin wave energies
Orbital Peierls State is confirmed and even
more robust
17
YVO3 G-type phase
Y1-xCaxVO3 1 G-type phase
Magnetic structure and spin wave dispersion as
in YVO3
perfect orbitally ordered state
18
YVO3 G-type phase
Y1-xCaxVO3 1 CG-mixed phase
(1/2, 1/2, 0) C-type magnetic Bragg peak

• Depending in the cooling history
• - pure G-type phase at T 2 K
• - C-type / G-type mixed phase

Hysteresis between the C-type and G-type phase
19
YVO3 CG-mixed phase
Y1-xCaxVO3 1 CG-mixed phase
spin gap
C-type
G-type
Magnon branches of the C-type phase and G-type
phase coexist Spin gap demonstrates microscopic
interaction between both phases
20
Conclusions
Y1-xCaxVO3 x 0 1 2 5
magnetic structure and dynamics measured by
neutron scattering
Orbital fluctuations
C-type phase is stabilized by
realization of spin-orbital chains in 3D insulator
entropy driven orbital Peierls state identified
Effects of Ca-doping
G-type orbitally ordered phase disappears
rapidly
C-type phase orbital Peierls state is stabilized
C/G-mixed phase with a microscopic interaction
21
LaVO3 Inelastic Neutron Scattering
Tstruc. 145 K Pbnm monoclinic P21/a
Tmag. 143 K C-type, spins within the
ab-plane