1. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Electronically smectic-like phase in a nearly half-doped manganite J. A. Fernandez-Baca Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA INTERNATIONAL WORKSHOP ON SELF-ORGANIZED STRONGLY CORRELATED ELECTRON SYSTEMS Seillac, France May 29-31, 2006

2. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY F. Ye (ORNL) P. Dai (Univ. of Tennessee, ORNL) J. W. Lynn (NIST) H. Kawano-Furukawa (Ochanomizu University, Japan) H. Yoshizawa (ISSP-University of Tokyo, Japan) Y. Tomioka (CERC, Japan) Y. Tokura (U. of Tokyo, CERC, Japan Collaborators

3. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Outline  Motivation: Charge and orbital ordering in the CMR manganites are associated with the insulating state in the manganite perovskites.  “CE-type” CO/OO and AF ordering is prevalent and not so far energetically from the metal state  Need to better understand CE-type CO/OO and AF order  Neutron studies in Pr 1-x (Ca 1-y Sr y ) x MnO 3 near x=0.5 Evidence of weakly coupled 1D spin chains (smetic-like) above TN  Discussion Ye et al Phys. Rev. B 72, 212404 (2005)

4. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY CMR perovskite manganites:Substitution of Ln 3+ for Ca 2+ results in mixed valence Mn Mn: B site : perovskite structure Ln 1-x A x MnO 3 : perovskite structure Ln 3+ =Lanthanide (La,Pr,Nd…) A 2+ =Ca,Sr,Ba… A site

5. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Insulator-Metal transition coupled with the Paramagnetic- Ferromagnetic (PM-FM) transition P. Schiffer et al, PRL (1995) Rich phase diagram: Metallic state is associated with ferromagnetism due to Double exchange

6. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Pr 0.5 Sr 0.5 MnO 3 Rich phase diagram: Insulating state is associated with Charge/orbital Ordering (CO-OO) CO-OO promotes AF and insulating behavior 50% Mn 3+ with 50% Mn 4+ CE-type CO-OO Tomioka et al, PRL 74

7. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Rich phase diagram: Insulating state is associated with Charge/orbital Ordering (CO-OO) CO-OO promotes AF and insulating behavior 50% Mn 3+ with 50% Mn 4+ CE-type CO-OO Tomioka et al, PRL 74 The prevalent CO-OO and AF order is of the so-called CE-type originally proposed by Goodenough (Phys. Rev. 1955)

8. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY CO-OO peak at [1/4,1/4] Neutron scattering of CE-type CO-OO

9. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Pr 1-x (Ca 1-y Sr y ) x MnO 3 y=0.20, x=0.45 Tomioka and Tokura PRB 66, 104416 (2002) Ye et al (unpublished) Charge/orbital ordering CO-OO can be melted by an external magnetic field

10. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Dai et al., PRL (2000) Short range CO/OO fluctuations in LCMO30 above T C CO-OO peak at [1/4,1/4] CMR seems to result from the competition of the tendency to CE CO/OO and DE

11. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Trying to understand the CE-type CO-OO van den Brink et al. PRL (1999) Anisotropy of short range DE Competition between Kinetic Energy Magnetic Exchange Energy Orbital degeneracy Goodenough (1955) charge ordering at high temperature ( ~ covalent bond energy) magnetic ordering at much lower temperature (exchange interaction).

12. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Popović et al., PRL (2002) Jahn-Teller Distortion Onsite Coulomb repulsion Mn 3+ /Mn 4+ picture ??? Trying to understand the CE-type CO-OO Zener polarons (Daoud- Aladine) Trapping of electrons in Mn pairs (Zener polarons) Mn 3+ /Mn 4+ picture challenged Resonant x-ray experiments suggets magnetic moments at Mn 3+ and Mn 4+ not so different Magnetic powder diffraction, small difference in the magnetic moments

13. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Trying to understand the CE-type CO-OO Important questions Origin of CE-type structure Role of magnetism What about Mn 3+ and Mn 4+, are they really different? (in magnitude, in character) Are there really zigzag chains of Mn moments?

14. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Case study: CO-OO Pr 0.55 (Ca 1-y Sr y ) 0.45 MnO 3 (PCSMO45) Tomioka et al. (2002) (Pr,Ca)MnO 3 (Pr,Sr)MnO 3

15. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY CE type structure in real and reciprocal space Mn 4+ AFM(½,0,0) Mn 3+ AFM(¼,¼,0) Mn 3+ CO-OO(¼,¼,0)+(2,0,0) Magnetic form factor

16. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Charge/Orbital Ordering of PCSMO45 (Synchrotron X-ray) T=160K CO-OO peak at large equivalent [1/4,1/4] T=130K T=240K

17. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Resistivity, CO-OO and AF Ordering in PCSMO45

18. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Anisotropic AFM fluctuation in PCSMO45 Highly anisotropic Suggest low-dimensional behavior (1D, smectic)

19. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Isotropic AFM fluctuations in RbMnF 3 PM to AFM transition in perovskite RbMnF 3 spin system is isolated from the lattice Paramagnetic ordering (diffuse scattering) above T N Tucciarone, et al. Phys. Rev. B (1972) Isotropic, Lorentzian shape Spin-correlation only function of distance

20. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Comparison of Mn3+ and Mn4+ in PCSMO45 Mn 3+ Mn 4+ Scattering from Mn 3+ and Mn 4+ have distinct orbital character F. Ye, et al., PRB (2005)

21. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY AFM from Mn 3+ shows strong hysteresis AFM from Mn 4+ nearly no hysteresis Comparison of AFM (Mn 3+ and Mn 4+ ) in PCSMO45

22. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Discussion  Conventional wisdom (Goodenough)  charge ordering at high temperature ( ~ covalent bond energy)  magnetic ordering at much lower temperature (exchange interaction).  Observations do not support this picture. These types of ordering occur at temperatures T CO and T N close to each other.  Region between T CO and T N is quite unique (remarkable)  Charge/orbital and magnetic orderings likely have a common origin.  Hints of low-Dimensional AF (1D, smectic) behavior  Mn 3+ and Mn 4+ sites have distinctive orbital character  Observations support van den Brink, Solovyev approach: CE ordering explained from kinetic energy, exchange interactions and orbital degeneracy.

23. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Evolution of CE-type CO-OO and magnetic ordering First observation of quasi long-range 1D spin chains T N < T < T CO T < T N

24. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Discussion  Quasi long range 1D zigzag spin changes are the “building blocks” of the CE-structure  Consistent with the new “soft phase” proposed by Milward et al (Nature 2005), Dagotto et al (Science, 2005)

25. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY General feature in “ strongly correlated electron ” system S. A. Kivelson, Nature (1998) charge/spin order in cuprates (Stripes) Tranquada, Nature (2004)