Phonon behavior in compressively strained EuTiO3 thin films ...Phonon behavior in compressively...
1
Phonon behavior in compressively strained EuTiO 3 thin films Fourier transform infrared (FTIR) spectrometer Bruker IFS 113v V. Goian 1 , S.Kamba 1 , X. Martí 2 J. H. Lee 3 , R. W. Ulbricht 3 , C. Fennie 3 , D.G. Schlom 3 K. Rushchanskii 4 , M.Ležaić 4 A. Fuith 5 , W. Schranz 5 1 Institute of Physics, ASCR,Na Slovance 2, 18221 Prague 8, Czech Republic 2 Charles University, Faculty of Mathematics and Physics, Ke Karlovu 5, 121 16, Prague 6, Czech Republic 3 Dep. of Materials Science and Engineering, Cornell University, Ithaca, New York 14853-1501, USA 4 Institut für Festkörperforschung, Forschungszentrum Jülich GmbH 52425, Jülich and JARA-FIT, Germany 5 Physics of Functional Materials, Faculty of Physics, University of Vienna, Boltzmanngasse 5 A-1090, Austria Samples and method of preparation: • 100 nm & 22 nm thick compressively (-0.9%) strained films of EuTiO 3 deposited on (001)LaAlO 3 and (001)(LaAlO 3 ) 0.29 (SrAl 1/2 Ta 1/2 O 3 ) 0.71 (abbreviated as LSAT) by reactive molecular beam epitaxy (MBE); • 200 nm thick compressively (-0.3%) strained film of EuTiO 3 deposited on (110)NdGaO 3 by pulse laser deposition (PLD) Model for the substrate Generalized four-parameter damped oscillator model , , 2 2 2 2 * 3 LOj TOj LOj TOj j TOj TOj LOj LOj i i -transverse and longitudinal eigenfrequencies -transverse and longitudinal damping constants 2 * 3 * 3 1 ) ( 1 ) ( ) ( R Model for the film: Sum of damped Lorentz oscillators plus two coupled oscillators Reflectance of the film on substrate * 3 * 2 * 3 * 2 23 * 2 * 2 12 2 23 12 2 23 12 123 2 123 ) ( ; 1 1 ) ( ; 1 ) ( ) ( ) ( * 2 * 2 r r e r r e r r r r R c id c id Fresnel coefficients in the case of normal incidence where d, ω - thickness of the film and frequency of light, respectively n j j j i s i i s s i i s i s 3 2 2 2 2 2 2 2 2 1 2 2 1 2 1 1 2 2 1 2 2 2 2 2 1 * ) ( ) )( ( 2 ) ( ) ( oscillator strength of the j-th mode j , j , j - dielectric contribution, eigenfrequency, damping constant of the j-th polar phonon, respectively - imaginary coupling constant between TO1 and TO2, - high-frequency permittivity from the electronic polarization 2 j j j s Phonon behavior in bulk EuTiO 3 ceramics 0 100 200 300 400 500 600 80 100 120 140 160 Barrett fit Cochran fit SM 95% density TO2 Polar Phonon Frequency (cm -1 ) Temperature (K) 0 100 200 300 400 500 600 400 600 800 1000 1200 1400 1600 Plasma Frequency (cm -1 ) Temperature (K) p1 p2 p2 EuTiO 3 Plasma frequency is defined as j j pj EuTiO 3 phonon dispersion branches Antiferrodistortive instability at BZ boundary!!! Reflectance spectra and phonon behavior in EuTiO 3 on LaAlO 3 , NdGaO 3 and LSAT 100 200 300 400 500 600 0.0 0.2 0.4 0.6 0.8 0.0 0.2 0.4 0.6 0.8 EuTiO 3 on (110)NdGaO 3 Substrate 10 K 10 K 300 K 300 K Reflectance Wavenumber (cm -1 ) Spliting of TO1 TO2 TO4 Spliting of TO1 EuTiO 3 on (001)NdGaO 3 , d= 200 nm 300 K 10 K (001) NdGaO 3 300 K 10 K Reflectance TO2 TO4 0.0 0.2 0.4 0.6 0.8 1.0 10 K 300 K Substrate 300 K 10 K Spliting of TO1 Reflectance TO2 TO4 EuTiO 3 on (100)LaAlO 3 0 50 100 150 200 250 300 80 90 100 110 160 180 200 EuTiO 3 on(110)NdGaO 3 EuTiO 3 on(110)NdGaO 3 TO (cm -1 ) Temperature (K) E||(001) TO1c TO1b TO1a TO2a TO2b b 80 90 100 110 160 180 200 TO (cm -1 ) E||(110) a TO1a TO1b TO1c TO2a TO2b 0 50 100 150 200 250 300 80 85 90 95 100 105 110 115 120 125 EuTiO 3 /LaAlO 3 TO1b TO1c TO1 (cm -1 ) Temperature (K) EuTiO 3 /LaAlO 3 EuTiO 3 / LSAT TO1c TO1b EuTiO 3 on NdGaO 3 E||[110] TO1c TO1b TO1a TO1 bulk EuTiO 3 Temperature dependences of two lowest phonon frequencies in EuTiO 3 bulk ceramics. Temperature dependence of mode plasma frequencie of the soft Slater-type TO1 mode, the Last-type TO2 mode and the Axe TO4 mode obtained from IR reflectivity fits of ceramics. Observation Cochran law for soft mode frequency ω SM Barrett formula ) ( c SM T T A where T c is critical temperature and A is a parameter ] ) 2 coth( ) 2 [( 0 1 1 T T T T A SM where T 1 is the temperature where the quantum fluctuations start to play role and T 0 is critical temperature Temperature dependence of static dielectric constant for EuTiO 3 /LaAlO 3 and EuTiO 3 /NdGaO 3 compared with bulk EuTiO 3 ceramics. Temperature dependence of polar phonon frequencies in a) EuTiO 3 on LSAT and in b) EuTiO 3 on LaAlO 3 Activation of new modes below 150 K at approx. 102 cm -1 in EuTiO 3 /LaAlO 3 and at 109 cm -1 in EuTiO 3 /LSAT are seen. Temperature dependence of optical phonon frequencies in EuTiO 3 on NdGaO 3 for polarizations of IR beam a) E||[110] and b) E||[001]. A new mode is activated below 150 K around 98 cm -1 . Temperature dependence of TO1 frequencies mode and newly activated phonons in the low frequency region for bulk EuTiO 3 , EuTiO 3 /LaAlO 3 , EuTiO 3 /NdGaO 3 and EuTiO 3 /LSAT. New mode appears near 100 cm -1 in all thin films at 150 K. a) Polarized reflectance spectra of LaAlO 3 and EuTiO 3 /LaAlO 3 . b) Polarized reflectance spectra of NdGaO 3 and EuTiO 3 /NdGaO 3 with IR electric field oriented parallel to the [001]-direction of the substrate at 300 K and 10 K. c) Polarized reflectance spectra of NdGaO 3 and EuTiO 3 /NdGaO 3 at 300 K and 10 K with polarization E oriented parallel to the [110] direction of the substrate The modes coming from EuTiO 3 are indicated by arrows. Real and imaginary parts of permittivity in EuTiO 3 /NdGaO 3 for E||[110]. Origin of newly activated phonon: a) Inhomogeneous strain in the thin film can create splitting of TO1 mode. This explanation can work in EuTiO 3 /NdGaO 3 and EuTiO 3 /LaAlO 3 , where inhomogeneous strain of 0.3 % and 0.95 %, respectively, was observed instead of theoretical -2.9 % and 1.1 %. However, this explanation does not work in homogeneously strained EuTiO 3 /LSAT (strain 0.9%), where the same phonon was observed as in previous two samples. b) Antiferrodistortive (AFD) phase transition, i.e. doubling of unit cell due to oxygen octahedra rotation and tilting causes folding of Brillouin zone. New mode originally from BZ boundary can be activated in IR spectra, if the film will be polar. If the film undergoes AFD transition at 150 K, the phonon becomes polar only in ferroelectric phase like in tensile strained EuTiO 3 /DyScO 3 . However, the compressive strain is not satisfactory for ferroelectric order, so the question remains why the phonon is IR active? Conclusions: Ab initio calculation shows a phonon instabilities near BZ boundary → bulk EuTiO 3 should undergo an antiferrodistortive phase transition (AFD). Temperature dependence of Young’s modulus and thermal expansion exhibit anomalies near 35 o C. Are these anomalies due to AFD transition? TO1 mode is highly stiffened in EuTiO 3 on LSAT and EuTiO 3 on LaAlO 3 and less in EuTiO 3 on NdGaO 3 due to presence of the strain. We can distinguish two components of polar TO1 phonon: first one is coming from strained part of the film (higher frequency) and second from relaxed part. A newly activated phonon is observed near 150 K around 105 cm -1 in all thin films. This mode could have different origin. 1) it can be due to inhomogenous strain in partially strained thin films or 2) can be a signature of polar AFD phase. 0 100 200 300 1 10 100 100 K 50 K 10 K " Wavenumbers (cm -1 ) 300 K 250 K 150 K -100 0 100 200 300 ' EuTiO 3 on NdGaO 3 E|| [110] -150 -100 -50 0 50 100 150 200 3.4 3.6 3.8 4.0 4.2 4.4 -2.0x10 -3 -1.5x10 -3 -1.0x10 -3 -5.0x10 -4 0.0 5.0x10 -4 1.0x10 -3 1.5x10 -3 2.0x10 -3 E (GPa) Temperature ( o C) Young's modulus l/l 35 o C Temperature dependence of Young’s modulus and thermal expansion. An anomaly is observed at 35 o C In the cooling curve and also in heating curve. These data were measured by Dynamical Mechanical Analysis (DMA7-Perkin Elmer). 0 50 100 150 200 250 300 80 100 120 140 160 180 545 550 TO (cm -1 ) Temperature (K) TO1b TO2a TO4 TO1a TO2b TO1 in bulk EuTiO 3 b) EuTiO 3 /LaAlO 3 , d=100 nm 80 90 100 110 120 160 170 TO1 in bulk EuTiO 3 TO (cm -1 ) EuTiO 3 on LSAT d= 22 nm TO1b TO1a a) TO2 The eigen-displacements of a) Slater b) Last and c) Axe modes a) b) c) 3 - substrate 2 – thin film 1 – air, vacuum 0 50 100 150 200 250 300 100 150 200 250 300 350 Dielectric constant Temperature(K) EuTiO 3 on (100)LaAlO 3 EuTiO 3 on NdGaO 3 , E||[001] EuTiO 3 on NdGaO 3 , E||[110] EuTiO 3 bulk ceramic 0 50 100 150 200 250 300 40 60 80 100 120 140 160 TO (cm -1 ) Temperature (K) EuTiO 3 /DyScO 3 E||[110] of DyScO 3 TO1 soft mode TO2 T C T AFD coupled TO1 and TO2 phonons _ Temperature dependence of polar phonon frequencies in EuTiO 3 tensile strained film deposited on DyScO 3 . (after Ref[3]) Related references: 1. T. Katsufuji and H. Takagi, PRB, 64, 054415(2001); 2. V. Goian et al., EPJ B, 71, 429(2009); 3. J. H. Lee et al. Nature, 466, 954 (2010) 4. D.G. Schlom at al. Annu. Rev. Matter. Res., 37, 589(2007);
Transcript of Phonon behavior in compressively strained EuTiO3 thin films ...Phonon behavior in compressively...
Phonon behavior in compressively strained EuTiO3 thin films
Fourier transform infrared (FTIR) spectrometer Bruker IFS 113v
V. Goian1, S.Kamba1, X. Martí2J. H. Lee3, R. W. Ulbricht3, C. Fennie3, D.G. Schlom3
K. Rushchanskii4, M.Ležaić4
A. Fuith5, W. Schranz51Institute of Physics, ASCR,Na Slovance 2, 18221 Prague 8, Czech Republic
2Charles University, Faculty of Mathematics and Physics, Ke Karlovu 5,121 16, Prague 6, Czech Republic
3Dep. of Materials Science and Engineering, Cornell University, Ithaca, New York 14853-1501, USA
4Institut für Festkörperforschung, Forschungszentrum Jülich GmbH 52425, Jülichand JARA-FIT, Germany
5Physics of Functional Materials, Faculty of Physics, University of Vienna, Boltzmanngasse 5 A-1090, Austria
Samples and method of preparation:• 100 nm & 22 nm thick compressively (-0.9%) strained films of EuTiO3 deposited on (001)LaAlO3 and (001)(LaAlO3)0.29(SrAl1/2Ta1/2O3)0.71 (abbreviated as LSAT) by reactive molecular beam epitaxy (MBE);• 200 nm thick compressively (-0.3%) strained film of EuTiO3 deposited on (110)NdGaO3 by pulse laser deposition (PLD)
Model for the substrateGeneralized four-parameter damped oscillator model
,
,
22
22*3
LOjTOj
LOjTOj
j TOjTOj
LOjLOj
ii
-transverse and longitudinaleigenfrequencies-transverse and longitudinal damping constants
2
*3
*3
1)(
1)()(
R
Model for the film:
Sum of damped Lorentz oscillators plus two coupled oscillators
Reflectance of the film on substrate
*3
*2
*3
*2
23*2
*2
122
2312
2
2312123
2123
)(;11)(;
1)(
)()(
*2
*2
rrerr
errr
rR
cid
cid
Fresnel coefficients in the case of normal incidence
where d, ω - thickness of the film and frequency
of light, respectively
n
jj
j
is
iissiisis
32222
222
2122
1
21122
12222
21*
)())((2)()(
oscillator strength of the j-th mode
j, j, j - dielectric contribution, eigenfrequency, damping constant of the j-th polar phonon, respectively
- imaginary coupling constant between TO1 and TO2, - high-frequency permittivity from the electronic polarization
2jjjs
Phonon behavior in bulk EuTiO3 ceramics
0 100 200 300 400 500 600
80
100
120
140
160
Barrett fit Cochran fit
SM95% density
TO2
Pol
ar P
hono
n Fr
eque
ncy
(cm
-1)
Temperature (K)
0 100 200 300 400 500 600
400
600
800
1000
1200
1400
1600
Pla
sma
Freq
uenc
y (c
m-1)
Temperature (K)
p1
p2
p2
EuTiO3
Plasma frequency is defined as jjpj
EuTiO3 phonon dispersion branches
Antiferrodistortive instability at BZ boundary!!!
Reflectance spectra and phonon behavior in EuTiO3 on LaAlO3, NdGaO3 and LSAT
100 200 300 400 500 6000.0
0.2
0.4
0.6
0.8
0.0
0.2
0.4
0.6
0.8
EuTiO3 on
(110)NdGaO3 Substrate
10 K 10 K 300 K 300 K
Ref
lect
ance
Wavenumber (cm-1)
Spliting of TO1
TO2 TO4
Spliting of TO1
EuTiO3 on (001)NdGaO
3,
d= 200 nm 300 K 10 K
(001) NdGaO3
300 K 10 KR
efle
ctan
ce
TO2
TO4
0.0
0.2
0.4
0.6
0.8
1.0
10 K 300 K
Substrate 300 K 10 K
Spliting of TO1
Ref
lect
ance
TO2TO4
EuTiO3 on (100)LaAlO3
0 50 100 150 200 250 30080
90
100
110
160180200
EuTiO3 on(110)NdGaO3
EuTiO3 on(110)NdGaO3
TO
(cm
-1)
Temperature (K)
E||(001)
TO1c
TO1b
TO1a
TO2aTO2b
b
80
90
100
110160180200
TO
(cm
-1)
E||(110)aTO1a
TO1b
TO1c
TO2a
TO2b
0 50 100 150 200 250 300
808590
95100
105110115
120125
EuTiO3 /LaAlO3
TO1b
TO1c
TO
1(cm
-1)
Temperature (K)
EuTiO3 /LaAlO3
EuTiO3/ LSAT
TO1c
TO1b EuTiO3 on NdGaO3
E||[110]
TO1c
TO1b
TO1aTO1 bulk EuTiO3
Temperature dependences of two lowest phononfrequencies in EuTiO3 bulk ceramics.
Temperature dependence of mode plasma frequencie of the soft Slater-type TO1 mode,the Last-type TO2 mode and the Axe TO4 mode
obtained from IR reflectivity fits of ceramics.
ObservationCochran law for soft modefrequency ωSM
Barrett formula
)( cSM TTA where Tc is critical temperature and A is a parameter
])2
coth()2
[( 011 TT
TTASM where T1 is the temperature where the quantum fluctuations start to play roleand T0 is critical temperature
Temperature dependence of static dielectric constant for EuTiO3/LaAlO3 and EuTiO3/NdGaO3 compared with bulkEuTiO3ceramics.
Temperature dependence of polar phonon frequencies in a) EuTiO3 on LSAT and in b) EuTiO3 on LaAlO3Activation of new modes below 150 K at approx. 102 cm-1 in EuTiO3/LaAlO3and at 109 cm-1 in EuTiO3/LSAT are seen.
Temperature dependence of optical phonon frequencies in EuTiO3 on NdGaO3 for polarizations of IR beam a) E||[110] and b) E||[001]. A new mode is activated below 150 K around 98 cm-1.
Temperature dependence of TO1 frequencies mode and newly activated phonons in the low frequency region for bulk EuTiO3, EuTiO3/LaAlO3, EuTiO3/NdGaO3 and EuTiO3/LSAT. New mode appears near 100 cm-1 in all thin films at 150 K.
a) Polarized reflectance spectra of LaAlO3 and EuTiO3/LaAlO3.b) Polarized reflectance spectra of NdGaO3 and EuTiO3/NdGaO3
with IR electric field oriented parallel to the [001]-direction of the substrate at 300 K and 10 K.
c) Polarized reflectance spectra of NdGaO3 and EuTiO3/NdGaO3at 300 K and 10 K with polarization E oriented parallel to the [110] direction of the substrate The modes coming from EuTiO3 are indicated by arrows.
Real and imaginary parts of permittivity in EuTiO3/NdGaO3 for E||[110].
Origin of newly activated phonon:a) Inhomogeneous strain in the thin film can create splitting of TO1 mode. This explanation
can work in EuTiO3/NdGaO3 and EuTiO3/LaAlO3, where inhomogeneous strain of 0.3 % and 0.95 %, respectively, was observed instead of theoretical -2.9 % and 1.1 %. However, this explanation does not work in homogeneously strained EuTiO3/LSAT (strain 0.9%), where the same phonon was observed as in previous two samples.
b) Antiferrodistortive (AFD) phase transition, i.e. doubling of unit cell due to oxygen octahedrarotation and tilting causes folding of Brillouin zone. New mode originally from BZ boundary can be activated in IR spectra, if the film will be polar. If the film undergoes AFD transition at 150 K, the phonon becomes polar only in ferroelectric phase like in tensile strained EuTiO3/DyScO3. However, the compressive strain is not satisfactory for ferroelectric order, so the question remains why the phonon is IR active? Conclusions:
Ab initio calculation shows a phonon instabilities near BZ boundary → bulk EuTiO3 should undergo an antiferrodistortive phase transition (AFD). Temperature dependence of Young’s modulus and thermal expansion exhibit anomalies near 35 oC. Are these anomalies due to AFD transition?
TO1 mode is highly stiffened in EuTiO3 on LSAT and EuTiO3 on LaAlO3 and less in EuTiO3 on NdGaO3 due to presence of the strain. We can distinguish two components of polar TO1 phonon: first one is coming from strained part of the film (higher frequency) and second from relaxed part.
A newly activated phonon is observed near 150 K around 105 cm-1 in all thin films. This mode could have different origin. 1) it can be due to inhomogenous strain in partially strained thin films or 2) can be a signature of polar AFD phase.
0 100 200 300
1
10
100
100 K 50 K 10 K
"
Wavenumbers (cm-1)
300 K 250 K 150 K
-100
0
100
200
300
'
EuTiO3 on NdGaO3
E|| [110]
-150 -100 -50 0 50 100 150 200
3.4
3.6
3.8
4.0
4.2
4.4
-2.0x10-3
-1.5x10-3
-1.0x10-3
-5.0x10-4
0.0
5.0x10-4
1.0x10-3
1.5x10-3
2.0x10-3
E (G
Pa)
Temperature (oC)
Young's modulus
l/l
35 oC
Temperature dependence of Young’s modulus andthermal expansion. An anomaly is observed at 35 oCIn the cooling curve and also in heating curve. These data were measured by Dynamical Mechanical Analysis (DMA7-Perkin Elmer).
0 50 100 150 200 250 30080
100
120
140
160
180545550
TO
(cm
-1)
Temperature (K)
TO1b
TO2a
TO4
TO1a
TO2b
TO1 in bulk EuTiO3
b)EuTiO3/LaAlO3, d=100 nm
80
90
100
110
120
160
170
TO1 in bulk EuTiO3
TO
(cm
-1)
EuTiO3 on LSATd= 22 nm
TO1b
TO1a
a)
TO2
The eigen-displacements of a) Slater b) Last and c) Axe modes
a) b) c)
3 - substrate
2 – thin film1 – air, vacuum
0 50 100 150 200 250 300
100
150
200
250
300
350
Die
lect
ric c
onst
ant
Temperature(K)
EuTiO3 on (100)LaAlO3
EuTiO3 on NdGaO3, E||[001] EuTiO3 on NdGaO3, E||[110] EuTiO3 bulk ceramic
0 50 100 150 200 250 300
40
60
80
100
120
140
160
TO
(cm
-1)
Temperature (K)
EuTiO3/DyScO3
E||[110] of DyScO3
TO1 soft mode
TO2
TCTAFD
coupled TO1 and TO2phonons _
Temperature dependence of polar phonon frequencies in EuTiO3 tensile strained film deposited on DyScO3. (after Ref[3])
Related references:1. T. Katsufuji and H. Takagi, PRB,
64, 054415(2001);2. V. Goian et al., EPJ B, 71,
429(2009); 3. J. H. Lee et al. Nature, 466, 954
(2010) 4. D.G. Schlom at al. Annu. Rev.
Matter. Res., 37, 589(2007);
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