1. No category

Optical study of novel perovskitic oxides, with focus
on their lattice and electronic properties
W.S.Mohamed
Prof. Paolo Calvani
Dott. Alessandro Nucara
Dipartimento di Fisica, Sapienza, Università di Roma
XXVII Cycle (October 30, 2014)
Interest in Perovskite Manganites
 the colossal magnetoresistance (CMR) effect (1,2)
 Manganites exhibit rich and interesting physical properties related to the interplay
between lattice distortions, transport properties, and magnetic ordering.
1- G. H. Jonker and J. H. Van Santen, Physica 16 (1950).
2- E.O. Wollan and W.C. Koehler. Phys. Rev. 100 (1955)
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Applications of manganites
Using a spin valve
structure
Magnetic field
sensors
Using the CMR
effect in a film
Using a SrTiO3 gate
Electric field
effect devices
as a microwave
CMR sensor
Using a
ferroelectric gate
Bolometric uncooled infrared (IR) sensors using
Bolometric devices
the metal {insulator transition at
the Curie
temperature}
solid oxide fuel cells
(SOFCs)
W.S.Mohamed
as cathodes.
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Multiferroics and Magnetoelectricity
Definition : Materials which present at least 2 of the 3 properties
Ferroelasticity, ferroelectricity, ferromagnetism
Key Issue : Coupling among P, M, and e
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Applications of Multiferroic Materials
Fundamental
physics
understanding the origin of the
coupling
studying physics of magnetic
oxides
transducers, sensors
Technological
applications
spintronics (spin-filter, spintransistor)
information storage technology
(Read/ write electronics)
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Experimental Methods
 UV-VIS spectroscopy (200-800 nm) (electronic excitations)
 High energy photons absorbed by bound electrons.
 Change of the electronic states of the molecules By transfer of electrons
from their ground state orbitals to the higher energy excited state orbitals.
 IR spectroscopy (1µm-1000 µm) (phonon excitations)
 Excitation of vibrational motions of atoms in a crystal.
 Excitation of free electrons in metals (Drude model).
 Excitation of polarons .
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Experimental technique: UV-VIS Spectroscopy
I ( )
R( )  R
I ( )
0
Schematic diagram of the optical setup for reflection measurements.
T ( )
T ( )  R
I ( )
0
Photography of the experimental UV-VIS
setup mounted at the IRS Group Lab. of the
Physics Department in Rome
Schematic diagram of the optical setup for transmission measurements.
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Experimental technique: FTIR
I ( )
R( )  R
I ( )
0
where I0 and IR are the intensities of the
incident and reflected beams, respectively.
Photography of the experimental FTIR setup mounted at
the IRS Group Lab. of the Physics Department in Rome.
W.S.Mohamed
Sketch of a Michelson interferometer
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Optical study of Insulator to Metal transition in LaxMnO3-δ thin films
Motivation
 In La deficient manganite films, Mn2+ ions substitute the La at the A site of the lattice,
therefore, Manganese of three formal valence (Mn2+, Mn3+, Mn4+) are present in the lattice.
indeed TIMT is sensitive to both the La/Mn ratio and the oxygen concentration.
 In the present work we have measured the optical spectra of three LaxMnO3-δ films with x <
1, and one with x > 1, with the aim to study the effect of La non-stoichiometry on the
electrodynamics of LaMnO3 .
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Reflectivity Spectra of LaxMnO3-δ thin films
Frequency range
(40 - 45000 cm-1)
Temperature range
(20 - 300 K)
in the far infrared all
the reflectivity spectra
are dominated by the
phonons
of
STO
substrate
infrared reflectivity spectra of
SrTiO3 single crystal*
The film with La excess is insulating at all
temperatures and does not present spectral features
much different from those reported in the literature
for the stoichiometric material.
* B.G. Almeida, et al., Thin Solid Films 513 (2006) 275–282 .
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Optical conductivity
 1( )  ( / 4 )e 2 ( )
Metallicity evidence
The drude absorption
increases steadly for
decreasing temperature
for x<1 samples.
broad absorption
bands appear in
the
midinfrared
which
strongly
change
with
temperature
No drude term
Typical of a good insulator at
all temperatures for x>1
sample.
Real part of the optical conductivity of the bare LMO films.
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
MIR-1 and MIR-2 Bands
Evolution of the midinfrared bands MIR-1 and MIR-2 of the Drude absorption for
lowering temperature in the thin LaxMnO3 film with x = 0.88
broad absorption bands called MIR-1
and MIR-2 appear in the midinfrared
which
strongly
change
with
temperature ,intensity and peak
frequency
the insulator-to-metal transition is driven
by a continuous transfer of spectral weight
from a ”hard” band MIR-2 to a softer band
(MIR-1) which acts as a ”reservoir” for the
Drude term of the free carriers.
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Conclusion
 La- deficient Mn films undergo an insulator-to-metal transition (IMT) like those
doped by divalent ions, but also present peculiar properties
 The metallization is monitored through the increase of the Drude term and a
transfer of spectral weight from a ”hard” midinfrared band MIR-2, to a ”soft”
midinfrared band MIR-1.
 The Drude term in the metallic phase is at least as narrow as in the best bulk
manganites, and therefore these systems can be interesting for the applications.
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Optical spectra of LaMn0.5Ga0.5O3 : A contribution to the assignment of
the electronic transition in manganites
Motivation
 Ga substitution is a clean tool to study the effect of a replacement of Mn+3 by anonmagnetic (diamagnetic) ion without introducing lattice distortion and any additional
magnetic exchange interaction due to the replacement.
 We have measured the transmittance of LaMn0.5Ga0.5O3 in the visible and near UV range,
from 300 to 10 K. The aim was to shed light on the orgin of the electronic bands in the
LaMnO3 family through a comparison with the corresponding spectra collected previously
on the parent compound.
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Transmittance Spectra of LaMn0.5Ga0.5O3-δ thin film
A broad feature between 10 000
and 25 000 cm-1
An intense Absorption band above
35 000 cm-1
Transmittance of LMGO+LSAT
temperatures from 10 K to 300 K
at
different
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Optical conductivity
 ()  e 2 (). / 60
A strong band at higher frequency, S5
(dashed line) had to be included. This is
probably the tail of the intense bands
around 5 eV due to the charge transfer O–
Mn transitions.
Four contributions are found to describe
the optical conductivity of LMGO. Two of
them, s1 and s2, are centered around 13
000 cm-1 and 16000 cm-1 respectively. A
third contribution,s3, is detected at 20 700
cm-1. These three bands account for the
strong absorbtion at 2 eV. S4 high
frequency part of the conductivity.
Optical conductivity of LMGO and LMO at 10 K (a) and
250 K (b) with the individual DL contributions in LMGO
(6).
W.S.Mohamed
A. Nucara, et al., Eur.Phys. J. B 7, 10945 (2011).
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Conclusion
 Optical spectra of LaMn0.5Ga0.5O3 thin film with 50% Ga substituted deposited by
Pulsed Laser Deposition on a LSAT substrate is an effective tool to study the origin of
the electronic optical absorption in manganites, which is still a controversial topic.
 Two out of the four bands detected have been assigned to the intersite d–d
transitions between Mn+3 ions. The other ones were ascribed to the p–d Mn – O
charge-transfer transitions.
 We are thus led to a“mixed” interpretation where both a Mott–Hubbard and a
charge-transfer approach concur to interpret the electronic spectrum of those
manganites.
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Infrared phonon dynamics of Ba2CuGe2O7 and BiMnO3
Infrared phonon spectrum of the tetragonal helimagnet Ba2CuGe2O7
Motivation
 In the present work the lattice dynamics of Ba2CuGe2O7 , a compound which develops
Dzyaloshinsky-Moriya (DM) helical magnetism below TN= 3.2 K, has been studied by
measuring the infrared reflectivity of a single crystal with the radiation polarized both in the
the ab plane and along the c axis of its tetragonal cell, from 7 K to 300 K.
H DM  DAB .(S A  S B )
non-centrosymmetric tetragonal structure
W.S.Mohamed
Dzyaloshinskii - Moriya (DM)
interaction due to spin – orbital
coupling
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Reflectivity spectra
The reflectivity spectra are
typical of an insulating crystal
with a comb of phonon lines in
the far infrared.
Reflectivity of Ba2CuGe2O7, in the far-infrared range of frequencies at 7 K (solid line) and 300 K (dashed line),
with the radiation polarized in the ab plane (a) and along the c axis (b).
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Optical Conductivity
Factor Group Analysis
R  10 A1  6 A2  7 B1  11B2  19E
fourteen transverse optical (TO) phonon modes with the
radiation polarized in the ab plane were detected out of
the eighteen E modes predicted by a factor-group analysis
of the unit cell of BCGO.
a) Optical conductivity of Ba2CuGe2O7 , in the far infrared range of frequencies at 7 K (solid line) and 300 K (dashed line),
with the radiation polarized in the ab plane. b) Shell-model results for the vibrational modes of the ab plane. The bar
length is the calculated strength Sj [th] in cm−2
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Optical Conductivity
Factor Group Analysis
R  10 A1  6 A2  7 B1  11B2  19E
all the ten B2 modes
predicted along the c axis
Optical conductivity of Ba2CuGe2O7 , in the far infrared range of frequencies at 7 K (solid line) and 300 K (dashed
line), with the radiation polarized along the c axis. b) Shell-model results for the vibrational modes along the c axis.
The bar length is the calculated strength Sj [th] in cm-1
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Infrared phonon dynamics of Ba2CuGe2O7 and BiMnO3
Far-infrared spectra of the multiferroic BiMnO3
Motivation

Bismuth manganite is remarkable because strong coupling between magnetic, electric and
structural order parameters occurs at once.

only a few infrared studies on BiMnO3 are known to date.

a profound understanding of the magnetoelectric coupling mechanism in BiMnO3 is still an
important and necessary issue

We have investigated the spectra in the mid and far infrared region with the aim to reveal
the spectral structures arising from the strong correlation between magnetic and lattice
parameters and their evolution with temperature.
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Far- Infrared spectra of BiMnO3
IR reflectivity spectra of bulk BiMnO3 taken at
Temperature dependence of the optical conductivity
different temperatures.
spectra of bulk BiMnO3, in the far infrared range of
frequencies
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Drude-Lorentz analysis of the Infrared
phonons
23 oscillators to fit 10 K data
18 oscillators to fit the 300 K data
Factor group analysis
the non-centrosymmetric C2 space group
(57 phonons are both IR and Raman active)
C 2  29 A( z, x 2 , y 2 , z 2 , xy)  31B( x, y, yz, xz)
the centrosymmetric space group C2/c,
(27 phonons are IR active)
the optical conductivity (solid lines) of BiMnO 3, in the far
C 2 / C  14 A g ( x , y , z , xy)  14 Au ( z )  16Bg ( xz, yz)  16Bg ( x, y)
2
2
infrared range with the fitting curves (dotted lines) which
2
provide the parameters listed in table 7-4. at (a) 10 K and
(b) 300 K.
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Conclusion
 The lattice vibration of Ba2CuGe2O7 was studied using infrared reflectivity measurements
with polarized radiation
 The optical conductivity extracted from R(ω) has been fit by a sum of Lorentzians, and their
parameters have been compared with the results of shell-model calculations.
 The number of the observed phonon lines is lower (for the ab plane) or equal (for the c axis)
to that predicted for the P4̀21m cell of Ba2CuGe2O7
 The spectra confirm that the tetragonal symmetry is conserved down to the lowest
temperatures with no appreciable orthorhombic distortion
 the Optical conductivity analysis of BiMnO3 infrared phonons support the centrosymetric
C2/c space group structure of BiMnO3.
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
List of Publications
 W. S. Mohamed, P. Maselli, P. Calvani, L. Baldassarre, P. Orgiani, A. Galdi, L. Maritato,
and A. Nucara “Optical study of the insulator-to-metal transition in LaxMnO3 thin films"
, Mater. Res. Express 1 036406 (2014).
 A. Nucara, W. S. Mohamed, L. Baldassarre, S. Koval, J. Lorenzana, R. Fittipaldi, G.
Balakrishnan,A. Vecchione and P. Calvani ,“Infrared phonon spectrum of the tetragonal
helimagnet Ba2CuGe2O7” ,Phy.Rev. B 90, 014304 (2014).
 A. Nucara , F. Miletto Granozio, W.S. Mohamed , A. Vecchione, R. Fittipaldi ,P.P. Perna ,
M. Radovic , F.M. Vitucci , P. Calvani, “Optical spectra of LaMn0.5Ga0.5O3: A contribution
to the assignment of the electronic transitions in manganites” Physica B 433 102–106
(2014).
W.S.Mohamed
Optical study of novel perovskitic oxides, with
focus on their lattice and electronic properties
Thank you for your attention
W.S.Mohamed
Optical study of novel perovskitic oxides, with focus on
their lattice and electronic properties