By Wael Saad Mohamed Ahmed

171 Pages·2014·5.12 MB·English

“
Optical study of novel perovskitic oxides, with
”
focus on their lattice and electronic properties
Doctoral School in Physics
PhD in Physics - XXVII Cycle
By
Wael Saad Mohamed Ahmed

ID number 1506361
Thesis Advisor Thesis co-Advisor
Prof. Paolo Calvani Dr. Alessandro Nucara
Coordinator
Prof. Massimo Testa

A thesis submitted in partial fulfilment of the requirements for the degree of
Doctor of Philosophy in Physics
October 2014

Thesis not yet defended

Wael Saad Mohamed Ahmed, “Optical study of novel perovskitic oxides, with focus
on their lattice and electronic properties”.
Ph.D. thesis. Sapienza – University of Rome
©2014 ISBN: 000000000-0
Version: 4 October 2014
Email:[email protected]

“Optical study of novel perovskitic oxides, with focus on their lattice and electronic properties” i
To my parents, my beloved wife

ii “Optical study of novel perovskitic oxides, with focus on their lattice and electronic properties”
Table of Contents

Table of Contents .............................................................................................. iii
Outline ............................................................................................................... v
Acknowledgment .............................................................................................. vi
Abstract ............................................................................................................ vii
I Basic concepts ...............................................................................................ix
Chapter 1:Basics of optical spectroscopy ....................................................... 11
1.1 Introduction of Optical Spectroscopy ..................................................... 11
1.2 Optical functions ..................................................................................... 15
1.3 Transmittance, reflectance and absorption coefficient ........................... 17
1.4 Kramers Kronig Relations ...................................................................... 18
1.5 Generalization of Reflectance and Transmittance to any angle of
incidence .......................................................................................................... 21
Bibliography....................................................................................................26
Chapter 2: Phenomenology of Manganites and of selected
multiferroics.....................................................................................................27
2.1 Manganites .............................................................................................. 27
2.2 Colossal magnetoresistance (CMR) in manganites ................................ 28
2.3 Structural properties of manganites ........................................................ 30
2.4 Electronic properties of manganites ........................................................ 31
2.5 LaMnO and its phase digram ................................................................. 33
3
2.6 Optical properties of manganites ............................................................ 36
2.7 Multiferroics ............................................................................................ 43
2.8 Optical spectra of multiferroic manganites ............................................ 46
2.9 Multiferroic Germanates ......................................................................... 49
Bibliography....................................................................................................52
Chapter 3: Backgroun Theory ............................................................................. 59
3.1 Introduction ............................................................................................. 59
3.2 Exchange Interaction ............................................................................... 59
3.3 Crystal field theory and JT effect ............................................................ 65
3.4 Modeling the optical conductivity .......................................................... 67
Bibliography....................................................................................................72

“Optical study of novel perovskitic oxides, with focus on their lattice and electronic properties” iii
Chapter 4: Experimental Methods ................................................................. 73
4.1 Introduction ............................................................................................. 73
4.2 Infrared spectroscopy and experimental setup ........................................ 75
4.3 UV-Vis Spectroscopy .............................................................................. 87
4.4 Thin films and single crystal preparation ................................................ 94
Bibliography....................................................................................................97
II Results and Discussion ............................................................................. 98
Chapter 5: Structural, Electronic and Optical properties of LaMn Ga O .
0.5 0.5 3
..........................................................................................................................99
5.1 Introduction ............................................................................................. 99
5.2 Sample growth and characterization ..................................................... 102
5.3 Transmittance spectra of the LaMn Ga O film ............................... 103
0.5 0.5 3
5.4 Extraction of the bare film optical functions ........................................ 106
5.5 Analysis of the bare film optical conductivity ...................................... 107
5.6 Drude-Lorentz analysis of the Optical Conductivity ............................ 109
5.7 Conclusion ............................................................................................. 115
Bibliography..................................................................................................117
Chapter 6: Optical study of Insulator to Metal transition in La MnO thin
x 3
films......................... ...................................................................................... 119
6.1 Introduction ........................................................................................... 119
6.2 Sample description and experimental procedure .................................. 121
6.3 Reflectivity spectra of the LaMn O films ............................................ 124
x 3
6.4 Extraction of the bare film optical functions ........................................ 125
6.5 Analysis of the bare film optical conductivity ...................................... 126
6.6 Conclusion ............................................................................................. 133
Bibliography..................................................................................................134
Chapter 7: Infrared phonon dynamics of Ba CuGe O and BiMnO ......137
2 2 7 3
7.1 Infrared phonon spectrum of the tetragonal helimagnet Ba CuGe O ...137
2 2 7
7.2 Infrared phonon dynamics of BiMnO multiferroic compound............ 155
3
7.3 Conclusion ............................................................................................. 163
Bibliography..................................................................................................164
Conclusions....................................................................................................167
List of publications ........................................................................................169
iv “Optical study of novel perovskitic oxides, with focus on their lattice and electronic properties”
Outline

This thesis is divided into two main sections. After the Acknowledgments and
the Abstract the reader enters the first section, entitled “Basic Concepts”. This
is divided into four chapters: The first chapter is entitled “Basics of Optical
Spectroscopy” which introduces to the readers the fundamentals of
spectroscopy and the basic concepts of light matter interaction. The second
chapter is entitled “Phenomenology of Manganites and of selected
Multiferroics” .its elaborate on the optical and structural properties of
manganites and multiferroics, repectivley. The third one is entitled
“Background theory” which discusses the main theories related to our
investigation. The fourth chapter is entitled “Experimental Methods” which
illustrates the experimental set-ups for different spectroscopic techniques (for
instances: UV spectroscopy and IR spectroscopy set-ups). Additionally, the
fourth chapter reviews the methodologies that are being used in my thesis.
The second section, “Results and Discussion”, contains three chapters and a
conclusion. This section deals with a detailed spectroscopic study of the four
compounds under study (i.e., LaMnO and LaMnGaO thin films,
3 3
Ba CuGe O and BiMnO bulk samples). For all these ensembles we
2 2 7 3
measured the transmittance and/or reflection. Measurements are taken in the
infrared, visible and ultraviolet spectral range. The range is between 100 and
45000 cm-1 to obtain information on the vibrational, electronic and
multiferroic properties of these correlated perovskites and multiferroics
compounds.

“Optical study of novel perovskitic oxides, with focus on their lattice and electronic properties” v
Acknowledgment
Thanks to Allah who pushed many of the noble people to help me to
accomplish this work. I am deeply indebted to my Ph.D. supervisor
Prof. Paolo Calvani for accepting me as a PhD student in his group at
Physics Department of the Sapienza University in Rome, for giving me the
opportunity to work and learn in one of the most exciting fields in solid
state physics, that of optical spectroscopy, for his supervision and constant
support through these years.
I would like to thank my tutor, Dr. Alessandro Nucara, for his brilliant
guidance and for the excellent collaboration we had. In these three years he
was always willing to help me during experiments and to explain concepts
when they were not clear to me.
I should also give many thanks to my group staff, in particular Dr. Paola
Maselli and Dr. Michele Ortolani for their great support during the
experiments, for discussions about science and for creating a spectacular
working environment.
I should also give many thanks to my group colleagues at the IRS Group,
Valeria Giliberti, Eugenio Calandrini, Odeta Limaj, Fausto D’Apuzzo , Paola
Dipietro , Marta Autore , Irene Lovecchio and Leonetta Baldassarre and many
more for the wonderful working atmosphere and the nice time we had
together.
I would like to thank the Erasmus Mundus European Commission for
providing financial support for 34 months in framework of FFEEBB 2 project
- Erasmus Mundus Action 2 during my PhD program.
I finish with my family; I would like to thank my parents as well as my
beloved wife for their love, support and encouragement, particularly during my
doctoral studies. Without their vital help over many years, nothing would have
been accomplished.
W.S.M.AHMED
vi “Optical study of novel perovskitic oxides, with focus on their lattice and electronic properties”
Abstract
Thesis title: “Optical study of novel perovskitic oxides, with focus on
their lattice and electronic properties”
Candidate: Wael Saad Mohamed Ahmed
Physics department - Sapienza University of Rome

The experimental work reported in this thesis consists in a systematic
investigation of various physical properties of epitaxial perovskite manganese
oxide
s films and single crystals with general formula A B MnO where A = La, Bi
1-x x 3
and B = Ga and the lattice dynamics of Ba CuGe O tetragonal helimagnet
2 2 7
compound. In all these systems we measure the transmittance and/or reflection
in the infrared, visible and ultraviolet spectral range, between 100 and 45000
cm-1 to obtain information on the vibrational, electronic and multiferroic
properties of those compounds.
We have measured the transmittance of LaMn Ga O thin film deposited by
0.5 0.5 3
Pulsed Laser Deposition on a LSAT substrate in the visible and near UV
range, from 300 to 10 K. The aim was to shed light on the origin of the
electronic bands in the LMO family through a comparison with the
corresponding spectra collected previously on the parent compound
LaMnO . Two out of the four bands detected have been assigned to the
3
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.
Lanthanum manganites with a massive concentration of La defects have also
been investigated by optical spectroscopy. They can be stabilized in form of
thin films, by exploiting the structural stress produced by a substrate like
SrTiO . They undergo an insulator-to-metal transition (IMT) like those doped
3
“Optical study of novel perovskitic oxides, with focus on their lattice and electronic properties” vii
by divalent ions, but also present peculiar properties. That process is here
studied by determining the optical conductivity of La MnO films with
x 3−δ
x=0.66, 0.88, 0.98 and 1.10, and with δ ≃ 0, from the far infrared to the near
ultraviolet, and between 20 and 300 K. We find that the IMT is a slow process
which continues down to 100 K at least, more than 250 K below its onset at
the Curie temperature Tc and at the T Measured in dc from a change of
IMT
slope in the resistivity vs. temperature. The metallization is monitored through
the increase of the Drude term and a transfer of spectral weight from a ”hard”
midinfrared band MIR-2 peaked between 3000 and 5000 cm−1 at room
temperature, to a ”soft” midinfrared band MIR-1 at ∼ 1000 cm−1. We have
also observed that, in these La-defective films, the Drude term at low
temperature is much narrower - and therefore the scattering rate considerably
lower - than in chemically doped manganites with comparable dc
conductivity. This favorable feature persists up to room temperature.
Therefore, this finding may be of interest for potential applications of La-
defective films to conductive devices.
Finally, in this thesis, we have studied the lattice dynamics of two multiferroic
compounds, Ba CuGe O and BiMnO , by infrared reflectivity measurements.
2 2 7 3
The number of the observed phonon lines is consistent with that predicted for
the P 42 m cell of Ba CuGe O , and no line splitting has been observed when
1 2 2 7
cooling the sample to 7 K. Therefore, our spectra confirm that the tetragonal
symmetry is conserved down to the lowest temperatures above T , with no
N
appreciable distortion. In collaboration with theorists, the theoretical
frequencies and intensities of about fourty phonon modes have been calculated
and found in good agreement with the observations. The temperature
dependence of the phonons parameters in a BiMnO single crystal was probed
3
by infrared spectroscopy down to 10 K. (i.e., well below the ferromagnetic
transition at T = 100 K.). The Optical conductivity analyses of the infrared
C
phonons support the centrosymetric C2/c space group structure of BiMnO
3
single crystal.

viii “Optical study of novel perovskitic oxides, with focus on their lattice and electronic properties”