Electron correlation and magnetism in 4 d transition metal oxides

1. Workshop on Electronic Structure of  Emerging Materials Theory & Experiment Electron correlation and magnetism in 4d transition metal oxides Kalobaran Maiti TIFR, Mumbai Collaborators: Ravi Shankar Singh V.R.R. Medicherla

2. eg2 t2g6 3d Oh de Boer and Verwey, Proc. Phys. Soc. A49, 59 (1937). NiO Ni2+O2- : N2+ [Ar] 3d 8 Insulator !!! Band gap = 4eV The Concept of the electron-electron Coulomb repulsion was introduced by N.F. Mott (1949) to explain the insulating behavior of NiO.

3. Electron correlation is often described using Hubbard model: A solid consisting of large number of atoms with 10-fold degenerate d-band !!!  difficult to solve

4. Dynamical Mean Field Theory (DMFT) A model calculation Captures most of the features However Parameters are inconsistent with experiment. Line shape are often different from experimental spectral functions. A. Georges et al. Rev. Mod. Phys. 68, 13 (1996). Various other efforts such as LDA+U, LDA+DMFT etc…

5. Energy d n+1 EF Metal d n-1 UHB U EF EF LHB Mott insulator U/W >> 1 d n Electron Correlation Correlated metal; U/W < 1

6. EF U Spectral functions in transition metal oxides Incoherent feature (UHB) EF Coherent feature Incoherent feature (LHB)

7. CaVO3 (surf.) LDA Coherent feature Incoherent feature SrVO3 (surf.) LDA Coherent feature CaVO3 (bulk) Incoherent feature Increasing SrVO3 (bulk) U/W CaVO3 and SrVO3

8. 4d 3d 4d orbitals in 4d TMO are more extended that 3d orbitals in 3d TMO. Expectations: Correlation effects  less important Ab initio approaches  more successful. Coupling to the lattice increases

9. Crystal Structure of SrRuO3 and CaRuO3 In SrRuO3: Ru-O(1)-Ru = 167.6o Ru-O(2)-Ru=159.7o In CaRuO3: Ru-O(1)-Ru = 149.6o Ru-O(2)-Ru=149.8o • No long range order in CaRuO3 BUT SrRuO3 is ferromagnetic !! • CaRuO3 is a non-Fermi liquid but SrRuO3 is NOT !!!

10. Ru 4d band • XP vs. UP spectra: • Lineshape in XPS is different • Dominant peak in XP spectra at ~ 0.6 eV (similar to LDA results) • Bulk sensitivity is higher in XP measurements than UP. Surface and bulk electronic structures are different

11. Surface spectra is significantly different with a peak at ~ 1eV Coherent feature in the bulk is strong compared to incoherent feature Although U/W is expected to be larger at surface compared to bulk, surface peak is at lower energy compared to bulk incoherent feature !!!!

12. U = 0.6 + 0.1 eV Optical response calculated from band structure results

13. Sr1-xCaxRuO3 : PRB70, 134426 (2004) CaRuO3 : PRB60, 1448 (1999)

14. TC = 165 K in SrRuO3 • No long range order in CaRuO3 • Slope is similar  magnetic moment is similar

15. Magnetism It is already known that ab initio approaches are often successful to determine the magnetic ground state properties even in highly correlated 3d systems. We employ Full Potential Linearized Augmented Plane Wave Method (FLAPW) within the local spin density approximations (LSDA) to investigate the magnetic ground state properties in these compounds.

16. Non-magnetic solution SrRuO3 CaRuO3

17. SrRuO3 CaRuO3

18. Ca-O covalency Ca-O covalency plays the key role in determining the crystal structure

19. Satellites Satellites Satellites in the core levels of A-atoms in ABO3 • Satellites appear in the core level spectra • Satellite-main peak separation is higher in CaRuO3 compared to that in SrRuO3 1.3 eV • A-O overlap is significant that allows charge transfer • A-O covalency is stronger in CaRuO3 1.8 eV

20. Spin polarized solution SrRuO3 CaRuO3 J=0.5 eV J=0.4 eV J=0.25 eV J=0.25 eV

21. Energies (in meV) and magnetic moments (mB) from band structure calculations CaRuO3 str SrRuO3 str Ferromagnetic solution is more stable in the cubic structure Probability of long range order reduces with the increase in distortion thereby decrease in Ru-O-Ru bond angle

22. Temperature dependence of PE spectra The CaRuO3 spectra remains almost the same to low temperatures In SrRuO3, the coherent feature intensity reduces significantly below TC = 165 K Down spin states move above eF. However, CaRuO3 also possess similar magnetic moment !!!

23. Temperature evolution in CaSrRuO3 Surface spectra Bulk spectra

24. Summary • Electron correlation strength is weak (U < 1 eV) in 4d systems. • U is similar in CaRuO3 and SrRuO3 • Ferromagnetism in SrRuO3 can be described using ab initio approaches. • Ca-O covalency plays the key role in determining the crystal structure. • Long range order presumably appears due to the coupling of local spins via conduction electrons (e.g. Rare earths such as Gd, Tb, Dy etc.) • Decrease in Ru-O-Ru angle reduces the itineracy of the conduction electrons and may be the origin of the absence of long range order in CaRuO3.

25. SrTi1-xRuxO3 • SrTiO3 is a band insulator • SrRuO3 is a ferromagnetic metal • Ti remains 4+ throughout the series • Long range order is not lost even at 40% substitutions • Magnetic moment increases with decrease in Ru-content

26. Ferromagnetic solution of SrRu0.5Ti0.5O3 Magnetism is well described by the ab initio method

27. Valence band Electron correlation strength is essentially the same across the series

28. Thank you