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

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Optical properties of (SrMnO3)n/(LaMnO3)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 (MgB2, V3Si,
Ba(Fe,Co)2As2)
Insulator to Metal Transitions (VO2, V2O3, V3O5,
NiS2, etc.)
Charge-Density-Waves (CeTe3, LaTe2)
Superconductivity (BaFe2As2)
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Optical Properties of (LaMnO3)2n/(SrMnO3)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 n1 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
O
Mn
R,A
R1-xAxMnO3
P. Schiffer, Phys. Rev. Lett. 75, 3336 (1995)
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Electronic Structure and Phase Diagram
Mn3
Jahn-Teller
LaMnO3 (Mn3)
SrMnO3 (Mn4)
Y. Tokura, Rep. Prog. Phys 69, 797 (2006).
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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)
Phys Rev Lett 74, 5144 (1995)

• DE explains the PI-FM transition, but fails in
  predicting
• the right Curie temperature (TcDE103 K vs
  TcExp102 K)
• the resistivity values (above Tc ?DE10-3 ?.cm
  vs ?Exp10-2 ?.cm )

Double-Exchange Jahn-Teller polarons
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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)
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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
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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)
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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 La2/3Sr1/3MnO3 alloys

Smadici et al., 2007
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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
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Optical reflectivity of 20 nm (LMO)2n/(SMO)n on
STO
IR
I0
RIR/I0
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Optical properties of the multilayer
vacuum (n1, k0)
sample (n, k)
STO substrate (nSTO, kSTO)
vacuum (n1, k0)
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The Lorentz-Drude model
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Data fitting
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(LMO)2/(SMO)1 parameters
Hartinger et al. (2004)

• 1 Drude term
• 1 MIR band
• 2 T-independent HOs

with
if
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Comparing n1 SL with alloys
AP et al., Nano Letters 10, 4819 (2010)

1. dc conductivity 104 ?.cm
2. Tcurie 350 K
3. m/mb 7
4. MIR band softening
5. 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 n1 SL has homogeneous electronic density
• Disorder probably plays a very limited role in
  the corresponding LSMO alloy

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Reflectivity of n1,3,5 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
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)
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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)
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Conclusions

1. Homogeneous electronic state for short period SLs
2. Similarities between n1 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