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Optical properties of (SrMnO 3 ) n /(LaMnO 3 ) 2n superlattices: an insulator-to-metal transition observed in the absence of disorder A. Perucchi

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

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SISSI - Solid State Physics activities Electrodynamics at High Pressures THz Superconducting Gaps (MgB 2, V 3 Si, Ba(Fe,Co) 2 As 2 ) Insulator to Metal Transitions (VO 2, V 2 O 3, V 3 O 5, NiS 2, etc.) Charge-Density-Waves (CeTe 3, LaTe 2 ) Superconductivity (BaFe 2 As 2 )

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Optical Properties of (LaMnO 3 ) 2n /(SrMnO 3 ) n L. Baldassarre S. Lupi P. Calvani A. Nucara L. Maritato P. Orgiani D.G. Schlom C. Adamo

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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

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Colossal Magnetoresistance (CMR) manganites R 1-x A x MnO 3 P. Schiffer, Phys. Rev. Lett. 75, 3336 (1995) Mn O R,A

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Electronic Structure and Phase Diagram Y. Tokura, Rep. Prog. Phys 69, 797 (2006). Jahn-Teller Mn 3+ LaMnO 3 (Mn 3+ ) SrMnO 3 (Mn 4+ )

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

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

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

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LSMO optical properties Takenaka et al., Phys. Rev B 60, 13011 (1999) Haghiri-Gosnet et al., Phys. Rev B 78, 115118 (2008) 40 nm La 2/3 Sr 1/3 MnO 3 on STO La 0.825 Sr 0.175 MnO 3 cleaved single crystal

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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 La 2/3 Sr 1/3 MnO 3 alloys Smadici et al., 2007

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Tuning the MIT in (LaMnO 3 ) 2n /(SrMnO 3 ) n Adamo PRB 2009 A peak in the resistivity is always found at the Curie Temperature!!! Double-Exchange physics

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Optical reflectivity of 20 nm (LMO) 2n /(SMO)n on STO I0I0 IRIR R=I R /I 0

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

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The Lorentz-Drude model

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Data fitting

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

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Comparing n=1 SL with alloys 1.dc conductivity ~ 10 4 .cm 2.T curie ~ 350 K 3.m*/m b ~ 7 4.MIR band softening 5.edge in 1 at ~ 1000 cm -1 Adamo PRB 2009 The electronic properties of (LMO) 2 /(SMO) 1 SL are fully equivalent to those of the corresponding La 2/3 Sr 1/3 MnO 3 The n=1 SL has homogeneous electronic density Disorder probably plays a very limited role in the corresponding LSMO alloy AP et al., Nano Letters 10, 4819 (2010 )

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

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

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(LMO) 2n /(SMO) n parameters Dong et al. (2008) 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

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Large period SLs, end-members, and alloys LaMnO 3 In site Mn 3+ transitions: e g -e g ( Jahn-Teller ) SrMnO 3 In site Mn 4+ transitions: t 2g -e g La 1-x Sr x MnO 3 Mn 3+ to Mn 4+ transitions: 1/2 Jahn-Teller The presence of a mid-IR band signals mixed Mn valencies. Its sizable spectral weight can not be attributed to interfacial Mn 3+ -Mn 4+ transitions alone Adamo PRB 2009 AP et al., Nano Letters 10, 4819 (2010 )

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Conclusions 1.Homogeneous electronic state for short period SLs 2.Similarities between n=1 SL and corresponding alloy (reduced role for disorder) 3.Optical characterization of the Metal to Insulator transition with increasing n 4.Novel “bulk” (not limited to interface) electronic states in large period SLs

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