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A. Perucchi

Optical properties of (SrMnO 3 ) n /(LaMnO 3 ) 2n superlattices: an insulator-to-metal transition observed in the absence of disorder. A. Perucchi. SISSI, the IR beamline of the ELETTRA Storage Ring. Synchrotron Infrared Source for Spectroscopy and Imaging.

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A. Perucchi

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  1. Optical properties of (SrMnO3)n/(LaMnO3)2n superlattices: an insulator-to-metal transition observed in the absence of disorder A. Perucchi

  2. SISSI, the IR beamline of the ELETTRA Storage Ring Synchrotron Infrared Source for Spectroscopy and Imaging

  3. SISSI - Solid State Physics activities Electrodynamics at High Pressures THz Superconducting Gaps (MgB2, V3Si, Ba(Fe,Co)2As2) Insulator to Metal Transitions (VO2, V2O3, V3O5, NiS2, etc.) Charge-Density-Waves (CeTe3, LaTe2) Superconductivity (BaFe2As2)

  4. Optical Properties of (LaMnO3)2n/(SrMnO3)n • L. Baldassarre • S. Lupi • P. Calvani • A. Nucara • L. Maritato • P. Orgiani • D.G. Schlom • C. Adamo

  5. Outline • Basic concepts on Manganites Double-Exchange, Jahn-Teller polarons, optical conductivity • (LMO)2n/(SMO)n SuperLattices (SL) • Optical properties of n=1 compound Understanding the role of disorder in LSMO alloys • Optical properties of large period SLs Appearance of novel “bulk” electronic states

  6. Colossal Magnetoresistance (CMR) manganites O Mn R,A R1-xAxMnO3 P. Schiffer, Phys. Rev. Lett. 75, 3336 (1995)

  7. Electronic Structure and Phase Diagram Mn3+ Jahn-Teller LaMnO3 (Mn3+) SrMnO3 (Mn4+) Y. Tokura, Rep. Prog. Phys 69, 797 (2006).

  8. CMR Models Double-Exchange model C Zener, Phys Rev 82, 403 (1951); PW Anderson and H Hasegawa, Phys Rev 100, 675 (1955) Phys Rev Lett 74, 5144 (1995) • DE explains the PI-FM transition, but fails in predicting • the right Curie temperature (TcDE~103 K vs TcExp~102 K) • the resistivity values (above Tc: DE~10-3.cm vs Exp~10-2.cm ) Double-Exchange + Jahn-Teller polarons

  9. CMR and Phase Separation Zhang et al., Science 298, 805 (2002) Phase Separation as an essential CMR ingredient Role of disorder as a source of nucleation centers see Poster from A. Pineiro on Tuesday Sarma et al., Phys Rev 93, 097202 (2004) Dagotto, New J Phys 7, 67 (2005)

  10. Optical conductivity NORMAL METAL “BAD” METAL (Strongly correlated metals, Polaronic metals, etc.) MIR bands indicate that a localization mechanism (mass enhancement) is at play MIR band

  11. LSMO optical properties La0.825Sr0.175MnO3 cleaved single crystal Takenaka et al., Phys. Rev B 60, 13011 (1999) 40 nm La2/3Sr1/3MnO3 on STO Haghiri-Gosnet et al., Phys. Rev B 78, 115118 (2008)

  12. Interfaces and Superlattices • Designing materials with novel electronic states at the interface between two different oxides as in (LAO/STO), (LTO/LAO), etc. • Addressing CMR and the physics of DE in the absence of substitutional disorder. • The (LMO)2n/(SMO)n SL series mimics the doping content of La2/3Sr1/3MnO3 alloys Smadici et al., 2007

  13. Tuning the MIT in (LaMnO3)2n/(SrMnO3)n A peak in the resistivity is always found at the Curie Temperature!!! Double-Exchange physics Adamo PRB 2009

  14. Optical reflectivity of 20 nm (LMO)2n/(SMO)n on STO IR I0 R=IR/I0

  15. Optical properties of the multilayer vacuum (n=1, k=0) sample (n, k) STO substrate (nSTO, kSTO) vacuum (n=1, k=0)

  16. The Lorentz-Drude model

  17. Data fitting

  18. Hartinger et al. (2004) (LMO)2/(SMO)1 parameters • 1 Drude term • 1 MIR band • 2 T-independent HOs with if

  19. Comparing n=1 SL with alloys AP et al., Nano Letters 10, 4819 (2010) dc conductivity ~ 104.cm Tcurie ~ 350 K m*/mb ~ 7 MIR band softening edge in1 at ~ 1000 cm-1 The electronic properties of (LMO)2/(SMO)1 SL are fully equivalent to those of the corresponding La2/3Sr1/3MnO3 Adamo PRB 2009 • The n=1 SL has homogeneous electronic density • Disorder probably plays a very limited role in the corresponding LSMO alloy

  20. Reflectivity of n=1,3,5 and 8 compounds AP et al., Nano Letters 10, 4819 (2010)

  21. Optical conductivity AP et al., Nano Letters 10, 4819 (2010)

  22. (LMO)2n/(SMO)n parameters The overall free carrier spectral weight diminishes with n The agreement between resistivity measurements and dc conductivity worsens with increasing n: Role of perpendicular paths in the resistivity Dong et al. (2008)

  23. Large period SLs, end-members, and alloys Adamo PRB 2009 SrMnO3 In site Mn4+ transitions: t2g-eg La1-xSrxMnO3 Mn3+ to Mn4+ transitions: 1/2 Jahn-Teller LaMnO3 In site Mn3+ transitions: eg-eg (Jahn-Teller) The presence of a mid-IR band signals mixed Mn valencies. Its sizable spectral weight can not be attributed to interfacial Mn3+-Mn4+ transitions alone AP et al., Nano Letters 10, 4819 (2010)

  24. Conclusions Homogeneous electronic state for short period SLs Similarities between n=1 SL and corresponding alloy (reduced role for disorder) Optical characterization of the Metal to Insulator transition with increasing n Novel “bulk” (not limited to interface) electronic states in large period SLs

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