Optical properties of (SrMnO3)n/(LaMnO3)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

SISSI - Solid State Physics activities

Electrodynamics at High PressuresTHz Superconducting Gaps

(MgB2, V3Si, Ba(Fe,Co)2As2)Insulator to Metal Transitions(VO2, V2O3, V3O5, NiS2, etc.)

Charge-Density-Waves(CeTe3, LaTe2)

Superconductivity(BaFe2As2)

Optical Properties of (LaMnO3)2n/(SrMnO3)n

•L. Baldassarre

•S. Lupi•P. Calvani•A. Nucara

•L. Maritato•P. Orgiani

•D.G. Schlom•C. Adamo

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

Colossal Magnetoresistance (CMR) manganites

R1-xAxMnO3P. Schiffer, Phys. Rev. Lett. 75, 3336 (1995)

Mn

O

R,A

€

MR =ρ (H) − ρ(0)

ρ(0)≈102

Electronic Structure and Phase Diagram

Y. Tokura, Rep. Prog. Phys 69, 797 (2006).

Jahn-Teller

Mn3+

LaMnO3

(Mn3+)

SrMnO3

(Mn4+)

CMR Models

€

tij = bij cosϑ ij

2

Double-Exchange modelC 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 (TcDE~103 K vs TcExp~102 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

CMR and Phase SeparationZhang 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 centerssee Poster from A. Pineiro on

Tuesday

Optical conductivity

6000

4000

2000

0

σ1(

. )cm

-1

50 10x3

403020100

(Frequency cm-1)

6000

4000

2000

0

σ1(

. )cm

-1

50 10x3

403020100

(Frequency cm-1)

€

ω p2 =

4πNe2

mb

€

ω pDrude2

=4πNe2

m *

€

ω pTot 2

=ωpDrude2

+ωpMIR 2

=4πNe2

mb

€

m * /mb =ωp

Tot 2

ωpDrude2

MIR bands indicate that a localization mechanism (mass

enhancement) is at play

MIR band

NORMAL METAL

“BAD” METAL(Strongly correlated metals, Polaronic metals, etc.)

LSMO optical properties

Takenaka et al., Phys. Rev B 60, 13011 (1999)

Haghiri-Gosnet et al., Phys. Rev B 78, 115118 (2008)

40 nm La2/3Sr1/3MnO3 on STO

La0.825Sr0.175MnO3 cleaved single

crystal

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

Tuning the MIT in (LaMnO3)2n/(SrMnO3)n

Adamo PRB 2009

A peak in the resistivity is always found at the Curie Temperature!!! Double-Exchange physics

Optical reflectivity of 20 nm (LMO)2n/(SMO)n on STO

I0

IR

R=IR/I0

1.0

0.8

0.6

0.4

0.2

0.02000150010005000

Frequency (cm-1

)

n=110 K400 K

SrTiO3

Optical properties of the multilayer

vacuum (n=1, k=0)

sample (n, k)

vacuum (n=1, k=0)

STO substrate (nSTO, kSTO)

€

r∧

ij =n∧

i− n∧

j

n∧

i+ n∧

j

€

t∧

ij =2n

∧

j

n∧

i+ n∧

j

€

r∧

1234 =r∧

12+ r∧

23 exp{2iδ2} + r∧

34 exp{2i(δ2 + δ3)} + r∧

12 r∧

23 r∧

34 exp{2iδ3}

1+ r∧

12 r∧

23 exp{2iδ2} + r∧

23 r∧

34 exp{2iδ3} + r∧

12 r∧

34 exp{2i(δ2 + δ3)}

€

t∧

1234 =t∧

12 t∧

23 t∧

34 exp{2i(δ2 + δ3)}

1+ r∧

12 r∧

23 exp{2iδ2} + r∧

23 r∧

34 exp{2iδ3} + r∧

12 r∧

34 exp{2i(δ2 + δ3)}

€

δp = β p + iα pdp /2 = 2πdp (np + ikp ) /λ 0

The Lorentz-Drude model

€

ε(ω) =1−ωpD

2

ω2 + iγDω+

4πN je2

m

1

(ω j2 −ω2) − iγ jωj

∑

ωj

1

γj

ω

ε1

ε2

ωpj

ε1, ε2

ε0

€

N~

= ε~

(ω)

€

ε1 = n2 − k 2,

ε2 = 2nk

€

σ~

= iω

4π(1−ε

~

)

€

σ1 =ωε2

4π,

σ 2 = (1−ε1)ω

4π

Data fitting

0.8

0.6

0.4

0.2

0.0

R(ω)

80006000400020000

(Frequency cm-1)

14x103

12

10

8

6

4

2

0

σ1(⋅

)cm-1

80006000400020000

(Frequency cm-1)

10 K400 K

0.8

0.6

0.4

0.2

0.0

R(ω)

35 10x3

302520151050

(Frequency cm-1)

14x103

12

10

8

6

4

2

0

σ1(⋅

)cm-1

35 10x3

302520151050

(Frequency cm-1)

10 K400 K

(LMO)2/(SMO)1 parameters

2

4

6

10-4

2

4

6

10-3

2

4

6

()·cm

4003002001000

( )Temperature K

8000

6000

4000

2000

0

ω0MIR

(cm

-1)

4003002001000

( )Temperature K

2.0 10x21

1.5

1.0

0.5

0.0

ND (

cm-3)

10 10x21

8

6

4

2

0

NMIR (

cm-3)

€

m * /mb =ωp

Tot 2

ωpDrude2 =

ND + NMIR

ND

≈ 7€

Nme

mb

= ND + NMIR ≈ 5 ⋅1021cm−3

N ≈1/3⋅NMn

NMn ≈ 7 ⋅1021cm−3

mb ≈ 0.5me

with

if

Hartinger et al. (2004)

14x103

12

10

8

6

4

2

0

σ1(⋅

)cm-1

80006000400020000

(Frequency cm-1)

10 K400 K

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

Comparing n=1 SL with alloys

8000

6000

4000

2000

0

σ1(⋅

)cm-1

80006000400020000

(Frequency cm-1)

10 K400 K

La2/3 Sr1/3 MnO3

La0.825 Sr0.175 MnO3

1. dc conductivity ~ 104 .cm

2. Tcurie ~ 350 K3. m*/mb ~ 74. MIR band softening5. edge in σ1 at ~

1000 cm-1

Adamo PRB 2009

The electronic properties of (LMO)2/(SMO)1 SL are fully equivalent to those of the corresponding La2/3Sr1/3MnO3

• 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)

Reflectivity of n=1,3,5 and 8 compounds

1.0

0.5

12008004000

Frequency (cm-1

)

1.0

0.5

1.0

0.5

R(ω)

12008004000

1.0

0.5

0.0

1.0

0.5

=10 T K

=1n =3n

=8n

=8n=5n=3n=1n

=5n

SrTiO3

10 K200 K300 K400 K

AP et al., Nano Letters 10,

4819 (2010)

Optical conductivity

1000

0

80006000400020000

Frequency (cm-1

)

2000

1000

0

3000

2000

1000

0

σ1(.)cm

-1

1000

0

4000

3000

2000

1000

0

σ1(.

)cm-1

1600012000800040000

(Frequency cm-1)

=10 T K

=1n

=3n

=8n

10 K200 300400

=5n

=8n=5n=3n=1n

AP et al., Nano Letters 10,

4819 (2010)

(LMO)2n/(SMO)n parameters

5004003002001000

Temperature (K)

12x103

10

8

6

4

2

0

ω0MIR

(cm

-1)

4003002001000

( )Temperature K

2.0 10x21

1.5

1.0

0.5

0.0

ND (

cm-3)

10-4

10-3

10-2

10-3

10-2

10-1

(

)·cm

10-3

10-2

10-1

100

10-3

10-2

10-1

100

10 10x21

8

6

4

2

0

NMIR (

cm-3)

=1n

=3n

=5n

=8n

=8n=5n=3n=1n

a

b

c

d

e

f

g

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

Large period SLs, end-members, and alloys2000

1500

1000

500

0

σ1(.)cm

-1

2500020000150001000050000

(Frequency cm-1)

=10 T K=16n=8n

LaMnO3 La0.9 Sr0.1 MnO3

SrMnO3

LaMnO3

In site Mn3+ transitions:eg-eg (Jahn-Teller)

SrMnO3

In site Mn4+ transitions:t2g-eg

La1-xSrxMnO3

Mn3+ to Mn4+ 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 Mn3+-Mn4+ transitions alone

Adamo PRB 2009

AP et al., Nano Letters 10,

4819 (2010)

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