Evolution of Orbital Peierls State with Doping in Y1-xCaxVO3

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. e - O rbital g 2 2 3z - r 2 2 x - y x - y 2 2 Jahn - Teller Aufspaltung 2 2 2 2 3 3 z z - - r r y z x z t – O rbital x x y y 2g yz xy xz tetragonal cubicsplitting splitting 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 => dimerization of exchange bonds along the c-axis C. Ulrich et al., PRL 91, 257202 (2003).

7. Resonating Orbitals in the YVO3 t orbital pseudospin = ½, G-type phase: T < 77 K strong JT effect orbitally ordered phase C-type phase: 77 K < T < 116 K J J å å = = + + t t t t + + SE SE H H ( ( S S S S 1 1 )( )( ) ) 1 1 + + + + i i i i 1 1 i i i i 1 1 4 4 2 2 i i static xy orbital stabilizes the AF magnetic order in plane orbital fluctuations along the c-axis between xz and yz orbital xz yz xz / yz xy undistorted xy xy Pbnmphase G-type C-type c 77 K 116 K 210 K Khaliullin et al., PRL 86, 3879 (2001).

8. orbital singlet spin triplet Orbital Peierls State Orbital Peierls State Consequence of the orbital fluctuations C-type phase: 77 K < T < 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. Oleś 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 Pbnmmonoclinic 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. YCaVO3 1 % 65 K YCaVO3 2 % 2 K 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-Munich5 % 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 Pbnmmonoclinic P21/a Tmag. = 143 K C-type, spins within the ab-plane