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What drives the insulating state in ultrathin films of an “uncorrelated” metal?

What drives the insulating state in ultrathin films of an “uncorrelated” metal?. Priya Mahadevan S.N. Bose National Centre for Basic Sciences, Kolkata. Collaborators: F. Aryasetiawan, AIST-RICS Janotti, UCSB T. Sasaki, NIMS. Mahadevan et al. (unpublished). Why ultrathin films?.

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What drives the insulating state in ultrathin films of an “uncorrelated” metal?

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  1. What drives the insulating state in ultrathin films of an “uncorrelated” metal? Priya Mahadevan S.N. Bose National Centre for Basic Sciences, Kolkata • Collaborators: • F. Aryasetiawan, AIST-RICS • Janotti, UCSB • T. Sasaki, NIMS Mahadevan et al. (unpublished)

  2. Why ultrathin films? • An alternate to current semiconductor technology – oxide electronics • Harness the multifunctionality of oxide materials From a physics point of view one finds unusual physics qualitatively different from the bulk of the same systems in this regime.

  3. Our system of choice … A 4d oxide – a set of systems which one believe are not so correlated SrRuO3 ( This is both ferromagnetic and metallic ) Would one/few monolayers of the system be metallic and show ferromagnetism ?

  4. And the answer existed from experiment … System: SrRuO3 grown on SrTiO3 Bulk is ferromagnetic + metallic Metal From Prof. Fujimori’s group Toyota et al. APL (2005) Insulator

  5. And we first try to understand the bulk Ru Projection of the perovskite unit cell O Sr Ru-O-Ru angle of 162 degrees

  6. The Method • Plane-wave, spin-polarized PAW (first proposed by Bloechl) calculations using VASP • GGA PW-91 exchange functional • Calculations for the ultrathin films were carried out for a symmetric slab (discussed later) with atleast 4 unit cells of vacuum.

  7. And the basic electronic structure is … Dominated by Ru d and O p states t2g And the ground state is found to be ferromagnetic eg

  8. We do have a good description of the electronic structure within GGA, But how important are electronic correlation effects?

  9. Determination of U The fully screened Coulomb interaction is given by W = [ 1 – v P] -1 v v is the bare Coulomb interaction P is the non-interacting polarization For d electrons say, P = Pd + Pr  On the d manifold, U = [ 1 – v Pr ] -1 v The fully screened interaction is then, W = [ 1 – U Pd] -1 U F. Aryasetiawan et al., PRB 70, 195104 (2004)

  10. How successful is this method? F. Aryasetiawan et al. PRB 74, 125106 (2006) Present work

  11. Effect of U on the bulk electronic structure (lattice distortions) Primary distortion is GdFeO3 - type

  12. Effect of U on the bulk electronic structure (magnetism) FM stability is measured with respect to the A type antiferromagnetic state

  13. Effect of U on the bulk electronic structure &magnetism Half-metallic state opens up (exists from U=2.5) At the brink of a transition to a half-metallic state

  14. And for the ultrathin films … SrRuO3 grown on SrTiO3 RuO2 Considered a 15 – layer slab SrO • The two are lattice-matched, so in-plane strain effects are small TiO2 SrO A 2x2 reconstruction at each layer TiO2 SrO TiO2 SrO TiO2 SrO TiO2 SrO TiO2 SrO TiO2 RuO2

  15. Why is the bulk metallic? • Ru has a d4 configuration • SRO has a large crystal field splitting t2g (dn) t2g (up) Why is the surface insulating? Metal-insulator transition induced by a non-magnetic transition ? dxz,dyz

  16. And the electronic structure at the surface is … Ferromagnetic and metallic dxz,dyz

  17. Why don’t we see the nonmagnetic transition? dxy dxz,dyz • Significant interaction between Ru atoms, which is aided by the distortion • of the out-of-plane Oxygen In-plane bond length = 1.97 A, same as Ti-O bond length Ru O O O Out of plane bond length = 1.95 A

  18. How do we engineer narrower bands and drive the transition? • With U on Ru (but relevant U is small 2.5-3 eV • In an antiferromagnetic arrangement of Ru – “coupling weaker than FM” • “aided” by lattice distortions This summarizes what happens!

  19. Why is the surface insulating? In-plane bond length = 1.97 A, same as Ti-O bond length Ru O O O Out of plane bond length = 2.13 A Metal-insulator transition induced by orbital-ordering + spin-state transition dxy d3z2-r2 dxz,dyz

  20. How do the energetics change with U? FM AFM NM (All energies in meV) U=0 -104 0 0 U=3 0 -291 +498 A value of U in the range of 2.5-3 eV is sufficient to bring about the transition to the AFM state. This estimate of U is found to be in agreement with ab-initio estimate of 2.5 eV.

  21. And the effect of U on the surface … Insulating and antiferromagnetic

  22. To conclude Ultrathin films enable us to move into a new regime of physics for these “uncorrelated” 4d oxides. Thank you for your attention

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