KWAME NKRUMAH UNIVERSITY OF SCIENCE AND TECHNOLOGY,
KUMASI
PROFESSORIAL INAUGURAL LECTURE
PROFESSOR ROBERT KWAME NKUM
MSc (Kumasi), DSc (Technion, Haifa), CPhys MInstP
DEAN, FACULTY OF PHYSICAL SCIENCES
COLLEGE OF SCIENCE
TOPIC:
ENTANGLED IN THE MATERIAL WORLD
JANUARY 26, 2007
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ABOUT THE SPEAKER
Professor Robert Kwame Nkum was born on March 13, 1954, at Agona Bobikumah in
the Central Region. He attended the Bobikumah Methodist Primary School between
1961 and 1967 and the Bobikumah Local Authority Middle School between 1967 and
1969. He sat for and passed the Common Entrance in 1969 and gained admission into
the Government (now Ghana) Secondary Technical School (GSTS), Takoradi, with a
Cocoa Marketing Board Scholarship. He completed the General Certificate of Education
(Ordinary) in 1974 with Division 1 Distinction. That same year he was re-admitted into
GSTS where he completed the GCE (Advanced Level) in 1976.
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In 1976, Professor Robert Kwame Nkum gained admission into the then University of
Science and Technology to pursue a four-year degree programme in Physics. He
completed the BSc (Physics) programme in 1980 with a Second Class (Upper Division).
He also completed the two-year MSc (Physics) programme at the University of Science
and Technology in 1983. In September, 1986, he left the shores of Ghana to the
Technion – Israel Institute of Technology, Haifa, Israel, to study for the Doctor of
Science (DSC) degree as a Lady Davis Fellow. He completed the doctorate programme
in August, 1989. During the doctorate research programme, Professor Nkum discovered
and reported for the first time the observation of ferroelectricity in cadmium-zinc-telluride
ternary system. Between November 1, 1990 and October 30, 1992, Professor Nkum
worked with the Condensed Matter Physics group of the Department of Physics,
McMaster University, Hamilton, Ontario, Canada, as a Postdoctoral Fellow.
Professor Robert Kwame Nkum was appointed Assistant Lecturer in Physics in 1983.
He became a Lecturer in 1984 and was promoted to the post of Senior Lecturer in 1993.
In 1998, he was promoted Associated Professor and became a Full Professor in 2003.
Professor Robert Kwame Nkum has served the international and local communities in a
number of capacities. He has refereed a number of articles for publication in journals
such as the Journal of the Ghana Science Association, the Ghana journal of Science,
the Journal of Science and Technology, Journal of Applied Science and Technology,
Canadian Journal of Physics, Journal of Materials Science, Materials Chemistry and
Physics, etc. He has also assessed a number of MSc and PhD theses for various
3
Universities. In addition, Professor Nkum has served on a number of International and
National Committees including the National Committee on the Information and
Communication Technology Policy for Ghana.
At the Kwame Nkrumah University of Science and Technology, Professor Nkum has
served as the Deputy Vice-Dean and Vice-Dean of the then Faculty of Science (now
College of Science), Vice-Dean of the School of Graduate Studies and Head of the
Department of Computer Science. He was a member of the Executive Committee of the
University. He is a member of the Academic Board and the Foundation Dean of the
Faculty of Physical Sciences.
Professor Robert Kwame Nkum is a member of the Ghana Institute of Physics, a
member of the Ghana Science Association, a member of the Materials Research
Society of Ghana, a member of the West Africa Materials Society, a Chattered Physicist
and a Member of the Institute of Physics (UK). He is also a Senior Associate of the
International Centre for Theoretical Physics, Trieste, Italy.
Professor Robert Kwame Nkum has been involved in research in the following areas:
Dielectric and ferroelectric properties of materials, Superconducting and normal-state
properties
of
bismuth-based
systems,
Transport
properties
of
amorphous
semiconductors, and Magnetic properties of materials. He has published forty-five
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papers in International journals including Physical Review Letters, the Physical Review,
Physica C, Solid State Communications, Thin Solid Films, Acta Materiala, Materials
Chemistry and Physics, Superconductor Science and Technology, Journal of Materials
Science, Materials Engineering B, Journal of Applied Physics, Discovery and
Innovation, etc. In addition, he has numerous Conference and Seminar papers.
Professor Nkum is a practicing Christian and an active member of the bethel Methodist
Church, Ayigya. He is a Sunday School Teacher, a Bible Class Leader, a Steward of
the Bethel Society and an accredited Local Preacher. He is also the Chairman of the
Christian Education Committee, Children’s Work Programme Committee and a member
of the Synod of the Kumasi Diocese of the Methodist Church Ghana.
He is married to Mrs. Grace Ama Nkum and they have five children – Marian, Judith,
Philip, Grace and Priscilla.
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Abstract
The world has been influenced by materials. In this lecture, the importance of
investigation of the properties of various kinds of materials has been discussed.
The specialness of humans is not in our ability to fashion tools from the things found
around us, but in our insatiable drive to make our conditions better. Man not satisfied
with carving out gourds to make drinking vessels, went on to invent pottery. We
replaced animal skins with fabrics out of wool, cotton and artificial threads. We now
fashion materials which are fifty times stronger and run engines which are a hundred
times hotter than those just made a few decades ago. Electronic components made
now may be only a millionth of the size of the smallest ones possible a generation ago.
The first fibre optics could not move a beam of light down a city block; now such fibres
regularly carry signals across entire oceans.
Human history has been delineated by our ability to manipulate different forms of
matter: the stone age, the iron age, the bronze age, etc. It is natural for us to be curious
about why the materials which we see around us behave the way they do. Indeed,
Condensed Matter Physics had a very pragmatic orientation in its early days, as Carnot,
Kelvin and others came up with such fundamental concepts as entropy and free energy
in their effort to understand steam engines. Today, condensed matter physics is still the
closest branch of physics to modern technological applications.
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Condensed Matter Physics has had a great impact on the way people live through its
contribution to technology, where devices such as the transistor, semiconductor chip,
microprocessors and computers have found applications.
The role of the condensed matter physicist and the materials scientists in investigating
the properties, synthesising materials and developing related materials cannot be
overemphasized. The materials investigated by the speaker – magnetic materials,
amorphous semiconductors, ferroelectric materials, and superconductors –and their
device applications are presented in this lecture.
The development of a nation depends on the utilization of the many material resources
it has. The speaker therefore recommends the formation of a Materials Research
Society on this campus with various groups taxed to investigate properties of various
materials for applications.
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Preamble
“I am sitting with a philosopher in the garden;
He says again and again ‘I know that that’s a tree’,
pointing to a tree near us.
Someone else arrives and hears this, and I tell him:
‘this fellow isn’t insane. We are only doing philosophy.’”
- Ludwig Wittgenstein
Mr Chairman the Vice Chancellor, Pro Vice Chancellor, Provosts, my colleague Deans,
Heads of Department, Members of Staff of this University, Members of the Clergy,
Invited Guests, Ladies and gentlemen. I feel immensely honoured and privileged to
deliver this inaugural lecture at the Kwame Nkrumah University of Science and
Technology for a number of reasons.
It is an opportunity for me to thank those loved ones and colleagues whose
support over a lifetime has led to this moment for me.
I am very conscious of the work of those who have gone before me – on which
my own work has been built; and
Inaugural Lectures are part of an important research tradition that this University
is building.
8
My talk today is, to some extent, historical; in part, it is a journey through my research
career to date. I hope to convey to you some of that excitement and I shall tell you some
of the hopes and ambitions that I have for the future of research in the group of people I
work with.
Introduction
“You know that we are living in a material world
And I am a material girl.”
When Madonna sang the song titled: Living in a material world (with the above lines),
she might have touched a chord in a lot of people who heard it, just as my topic might
have touched many. And why not?
Are we not living in a material world, a world full of materialistic people with material
needs and material aspirations? The label of our shirts and the make of our car are
filling the vacuum once occupied by anchors such as religion, education and family
name.
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“We have made the material world the map of value. What religion used to do,
what occupation used to do, what bloodlines used to do …….. now objects do it”
- Twitchell
We all know that we cannot live without comfort. In fact, in today’s fast-paced, stressedout world, comforts are actually necessities. A comfortable house, a car to take you
around, a job that you enjoy and make enough out of, a holiday now and then to help
you feel better. What is wrong with all these, Mr. Chairman?
No harm at all! However, the harm lies in letting our aspirations take over our lives to
such an extent that our peace of mind is compromised. The harm does not lie in
thinking that we deserve something and that we should have it; the harm lies in sulking
over someone else’s success in the belief that we deserve it more than they.
Matter at the macroscopic level
Mr. Chairman, all those things we enjoy in life – cars, clothing, houses, furniture, etc –
are made of matter at the macroscopic level and in the condensed state. Human
development has always been associated with the use of materials in the solid state.
The clothes we wear are made of a variety of materials. Our home is made of materials
– mostly manufactured. The glass in the windows, the vinyl sliding, the ceramic
dinnerware, the metal silverware, and everywhere we look we see products made from
many different kinds of materials to satisfy the needs of the product. Since the dawn of
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civilization, materials have been central to our growth, prosperity, security, and quality of
life.
Archaeologists have long accepted the importance of materials to the development of
man and classified time period according to the dominant materials in use. Thus we
have the stone age, bronze age, iron age, etc. The interaction between materials and
the industries that have transformed society was the driving force for the Industrial
Revolution. In today’s world we use such a diversity of materials for such a diversity of
applications, that the importance of materials is rarely appreciated. We use materials in
the form of polymers, metals, ceramics and glasses for applications as far apart as heat
resistant tiles on the space shuttle to integrated circuits in your personal computer.
To get a sense of the special problems associated with understanding the macroscopic
properties of matter, consider a glass of water. The amount of water molecules
contained in this glass of water is of the order of 1023. If one were to write down
Newton’s equations or the Schrodinger’s equation for these molecules, they would be
far too complicated to solve. Even if one could solve them, the solution would be
useless without very precise knowledge of the initial conditions – all of the positions and
the velocities at some given time. If we were to make even a very small error in these
initial conditions, we might find that the solution describes a block of ice. Thus, the
microscopic equations which describe an individual water molecule are of limited use
when it comes to describing the macroscopic properties of matter, as are more refined
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descriptions at the atomic or subatomic scales. In essence, the reductionist approach
fails.
An entirely new set of concepts – which are not inherent in the macroscopic equations –
must be introduced, such as temperature, entropy, and phase. The concept of
temperature, for instance, makes no sense for an individual water molecule, but one
can hardly understand a macroscopic body of water without it. These properties only
emerge at macroscopic scales, which are the most interesting and important scales for
humankind.
In trying to understand the properties of matter, one must contend with the fact that
matter is made up of a larger number of microscopic constituents. As Einstein showed
in his investigation of diffusion about a hundred years ago, therein lies the secrets to
many of the mysterious properties of matter. In considering such a perspective, one is
immediately faced with such questions as WHY IS A SOLID SOLID? If you open the
door to a sauna and try to push against the steam which streams out, the water
molecules near your hand will move out of the way, and your hand (and eventually the
rest of you) will be bathed in steam. This is precisely what you would naively expect of a
collection of molecules, each of which acts fairly independently of the others in moving
around your hand. However, if you try the same thing with a block of ice, the whole
block will move as one. How can this be? The answer is that the water molecules in the
ice block interact strongly with each other in order to form one highly correlated whole in
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which the oxygen and hydrogen atoms sit at sites of a rather rigid lattice. The same is
true of all crystalline solids. Such highly correlated states are said to be condensed; it is
their study which is the subject of condensed matter physics.
Mr. Chairman, as a physicist, I know that even a tiny crystal of common salt contains
billions upon billions of charged atoms (ions) undergoing elaborate, coordinated motions
(phonons) even though the crystal appears to be just lying there doing nothing.
Understanding these structures and motions allows us to predict the physical properties:
salt crystals are cubic in shape, hard and brittle, and melt at very high temperatures. An
even more detailed description, involving the quantum mechanical motions of the
electrons within and between the atoms (ions), is needed to get a full understanding of
the chemical and electrical properties.
What is Condensed Matter Physics?
Condensed matter physics is the fundamental science of solids and liquids. It also deals
with states intermediate between solids and liquids (e.g. liquid crystals, glasses, and
gels), with dense gases and plasmas, and with special quantum states such as
superfluids that exist only at low temperatures.
Of all the branches of Physics, condensed matter physics has the greatest impact on
our daily lives through technological developments. For example, the invention of
transistors and semiconductor chips have led to the widespread use of a variety of
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electronic appliances, telecommunication devices (fax, cellular phones, and modems),
and personal computers. By one estimate, about one third of all physicists identify
themselves as condensed matter physicists.
Many aspects of our daily life benefit from condensed matter research. For example,
plastics are used for everything from furniture to automobile bodies; composite
materials are used in jet turbines and modern tennis rackets; magnetic disks are used in
almost every modern information system; superconducting magnets are used in
magnetic resonance imaging (MRI) tomography for medical diagnostics.
One of the reasons for calling the field “condensed matter physics” is that many of the
concepts and techniques developed for studying solids actually apply to fluid systems.
For instance, the conduction electrons in an electrical conductor form a type of quantum
fluid with essentially the same properties as fluids made up of atoms. In fact, the
phenomenon of superconductivity, in which the electrons condense into a new fluid
phase in which they flow without dissipation, is very closely analogous to the superfluid
phase found in helium-3 at low temperatures.
Having its beginnings in solid state physics, primarily concerned with the electronic
properties of solids, modern condensed matter physics has grown to be the study of
diverse systems such as solids, liquids, superfluids, glasses, polymers, gels colloids,
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neural networks, macromolecules, and indeed any system in which many interacting
basic components lead to complex or qualitatively new cooperate behaviour at the
macroscopic scale. Research in modern condensed matter physics spans the range
from understanding the properties of exotic and artificially fabricated materials, to
fundamental questions concerning ordering, phase transitions, and critical behaviour in
classical and quantum statistical systems.
Today, condensed matter physics remains as one of the most active and exciting
research areas in both basic sciences and technological applications. At the
fundamental level, condensed matter physics is intellectually stimulating due to
continuing discoveries of many new phenomena and the development of many new
concepts. It is the field in which advances in theory can most directly be confronted with
experiments. It has repeatedly served as a source or testing ground for new ideas (e.g.,
Josephson effect, integer and fractional quantum hall effects, Aharanove-Bohn effect,
mechanism of high-Tc superconductors, dissipative quantum physics, mesoscopic
physics, non-linear dynamics). Another unique aspect of condensed matter physics is
its intimate connection with industry.
Mr. Chairman, a sheet of ice, such as that of an ice-skating rink, can support the weight
of a person just as a metal platform. A cloud of water vapour, made up of the very same
water molecules, cannot. Thus, when it comes to understanding the properties of matter
in ordinary circumstances, it may not be particularly useful to understand the difference
15
between a water molecule and an iron atom. The more crucial information is contained
in the difference between the gaseous phase and the solid phase.
There are an enormous number of consequences which will follow from the fact that a
solid, such as a block of ice or the graphite in a pencil, consists of an order of 10 23
atoms which have been condensed into a highly-correlated state in which they act in
unison. One of them is the propagation of longitudinal and transverse sound wave
through the solid. These, in turn, affect the specific heat, thermal conductivity, and
optical properties of solids in detail without first appreciating the consequences of the
condensation phenomenon which makes it solid. There are many more exotic
condensed states with fascinating properties which follow from the particular type of
correlations which give birth to them. Superconductivity and magnetism are two
examples; in these states, the electrons are correlated in such a way that they exhibit a
rigidity akin to that of a solid, but in their ability to repel a magnetic field or maintain a
static magnetisation rather than their ability to support a person’s weight. One of the
main challenges facing physicists is understanding other states of matter involving the
correlated behaviour of a large number of constituents.
As a Condensed Matter Physicist, I have spent most of my life in the material world
investigating the properties of materials which include
(a) Magnetic materials
(b) Amorphous semiconductors
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(c) Ferroelectric materials, and
(d) Superconductors.
These are discussed in this lecture.
Magnetic world
Mr. Chairman, my life in the magnetic world began with the investigation of the
electrical properties of magnetic materials in 1979, as a final year undergraduate
student.
Never before has our daily life and environment been so significantly dependent on
materials with outstanding magnetic properties. Modern life is today in many aspects an
automated world which uses ferro- and ferri-magnetic materials in nearly all important
technical fields (e.g., electrical power, mechanical power, high-power electro-motors,
miniature
motors,
telecommunication,
computer
navigation,
technique,
aviation
magnetic
and
space
high-density
recording,
operations,
automation
micromechanics, sensor techniques, material testing and household applications).
Recent developments in the field of exchange-coupled thin film systems and the new
techniques for the development of nanocrystalline magnetic materials have initiated
numerous activities for the development of advanced magnetic devices for energy
transfer, for high-power and miniature electro motors, for medical applications and for
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the sensor and magnetic recording industry, nowadays known as magnetoelectronics.
Most of the progress achieved so far was due to the discovery of new materials with
extremely low or extremely large magnetocrystalline anisotropy, as well as the tailoring
of high coercivities which are determined by the microstructure.
The search for new materials led the Kumasi group to investigate the properties of Mn
thin films and its various alloys. Electrical resistivity measurements on thermally
evaporated -Mn thin films, -Mn-Fe alloys and -Mn-Ni alloys have been made.
Results
of
magnetoresistance,
spin
fluctuation
resistivity,
ferromagnetic
and
antiferromagnetic studies indicate that the material could be used in making magnetic
tapes and other magnetic devices.
Amorphous world
Mr. Chairman, not all materials we deal with are crystalline. In fact, a question is often
asked as to whether glass, for example, is solid or liquid. If ordinary window glass were
heated and cooled very slowly, it would form crystalline quartz. When it is cooled
quickly, however, the silicon and oxygen atoms get stuck in a random, disordered
configuration instead. Though it is far from equilibrium, it appears to the casual eye to
simply be a transparent brittle solid, not so very different from other covalent solids. On
the other hand, some of its properties, such as its specific heat and thermal conductivity
are quite different. Although it might not be appropriate to think of it as a condensed
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phase, it has macroscopic properties which are as robust as those of matter in
equilibrium.
Devices made of amorphous and disordered materials have become the enabling
technology for generating electricity through thin film photovoltaics. Thin film
photovoltaics can be cost competitive to fossil fuels, for storage of electricity in batteries
for electric and hybrid vehicles, ushering in a new, much needed transportation
revolution, and high density switching and storage media based on phase change
optical and electrical memories, so needed for our information society.
How is it possible that multi-elemental disorder can be the basis for such “revolutionary”
possibilities when it is well-known that the great success of the 20th century, the
transistor is based upon the periodicity of materials with particular emphasis on one
element, silicon? Indeed, with the great success of the transistor based upon the crystal
structure of germanium and silicon, we entered the historical era where achieving
crystalline perfection over a very large distance became the sine qua non of condensed
matter physics.
From materials point of view, the physics that made the transistor possible was based
upon the ability to utilize periodicity mathematically which permitted parts per million
perturbations of the crystalline lattice by substitutional doping. But right from the
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beginning, the plague of their disordered surfaces prevented for a decade the fulfilment
of the field effect transistor. The disorder of the surface states swamped out the
transistor action, emphasizing again Pauli’s statement
“God created the solids, the devil their surfaces.”
The irony was that the solution that made not only the field effect transistor possible but
also the integrated circuit which became the basis for the information age was the
utilization of amorphous silicon dioxide for photolithography and for the gate oxide.
Mr. Chairman, when the idea that there was a new world of interesting physics and
chemistry in minimizing and removing the constraints of periodicity was introduced, one
could understand the resulting consternation of the solid state physicists who had
received their PhD’s by accepting the dogma of periodicity as being the basis of
condensed matter and of the theoretical physicists to whom the control of many
elements was as incomprehensible as the conundrum of many-body theory. The
change from periodicity to local order permitted atomic engineering of materials in a
synthetic manner by opening up new degrees of design freedom. Literally many scores
of new materials could be displayed, new products made and new process dependent
production technology invented.
Disorder is the common theme in the minimization and lifting of lattice constraints which
permits the placing of elements in three-dimensional space where they interact in ways
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that were not previously available. This allowed the use of multi-elements and complex
materials including metals where positional, translational and compositional disorder
removed the restrictions so that new local order environments could be generated which
controlled the physical, electronic and chemical properties of the material.
To put into perspective the principles of disorder, what is required is a metaphor. In
ancient times, the earliest explorers stayed as close to the shore of the sea as possible.
There were many things to discover that way – new people, animals, physical
environment; things could be strange but understandable. However, to explore the great
unknown ocean, they were filled with anxiety, for out there was the end of the world and
where the dragons lay. Navigational skills were needed to avoid dangerous shoals,
utilize favourable currents, etc.
For amorphous tetrahedral materials to be useful, they would have to be as close to the
four-fold coordination of their crystalline cousins as possible, otherwise, the huge
density of states of dangling bonds would prohibit their use.
In order to follow the exploration process, we will intermingle the relevant scientific and
technological approaches with the materials, products and technologies made possible
by utilizing the freedom permitted by disorder to design and atomically engineer local
environments.
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During the past thirty or so years, research in the field of optical materials based on
amorphous
chalcogenide
semiconductors
has
made
significant
advances.
A
chalcogenide semiconductor is one containing a large amount of chalcogen atoms,
sulphur, selenium, and tellurium. Most of this research is driven by applied interest and
this field of research is extremely broad and active. Because of their switching and
memory characteristics, tellurium glasses have received considerable attention for
practical application in solid state electronics. Also, chalcogenide glasses containing
silver (Ag) have attracted a great deal of theoretical and experimental interest. This is
because chalcogenide glasses containing Ag have found application in optical imagine,
information storage, photolithography, integrated and diffractive optics, and in micro
lithography schemes for integrated fabrication. We have observed that the addition of
Ag to As2S3 results in the decrease of dielectric constant of the system which makes it
useful for optical imaging. We have also observed that addition of up to 10 – 15 at. % of
the transition metals into the amorphous chalcogenide films,
increases the room temperature electrical resistivity by 8 – 10 orders of
magnitude,
induces a strong contribution of variable range hopping conduction at low
temperatures,
decreases considerably, the activation energy,
induces relative small variations in the optical band gap, while the energy
dependence of the absorption coefficient becomes less steep,
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reduces drastically the magnitude of the thermopower from a few mV K-1 to a
hundred µV K-1.
All these properties make the doped-As2S3 system a very important material for device
applications.
Ferroelectric world
Mr. Chairman, imagine a student is in the last stages of typing his thesis or an important
document. It is a hot, hazy afternoon in 1980. A thunderstorm brews on the horizon.
Tense and tired, the student forgot to save the document on his disk. Suddenly,
lightning strikes, the computer shuts down and the final chapter is lost.
Wistfully, you long for a day when everything you write is automatically stored in a nonvolatile memory system as soon as you write it and does not vanish if there is a power
outage. What I need to make this happen is a fast, random-access, solid-state memory
that is cheap, reliable and, most important of all, intrinsically non-volatile – that is, it
behaves like a magnetic disc storage system.
And while you are contemplating the electronic future, imagine also that you have an
electronic smart card that you can use for everything from providing emergency crews
with their health records to transacting all your expenses without having to sign a
cheque.
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Now, fast-forward your mind to January, 26, 2007, enter the new world of non-volatile
ferrooelectric random access memories – and let us go on.
Mr. Chairman, all crystal structures can be classified into one of thirty-two possible
forms of crystal symmetry. Eleven of these forms are centrosymmetric; of the remaining
twenty-one non-centrosymmetric groups, twenty are known as piezoelectric, meaning
these materials produce an electric surface charge in response to applied mechanical
stress. In ten of these twenty crystal groups there is a permanent electric dipole, and the
equilibrium of the electrostatic potential caused by this dipole is distorted by mechanical
stress (piezoelectricity) or temperature change (pyroelectricity). Certain pyroelectric
materials can be further classified as ferroelectric materials, as shown in the summary
diagram in Figure 1.
Ferroelectric materials – in particular ceramics – have been commercially important to
the electronics industry for more than fifty years. Prominent examples include lead
zirconium titanate ceramic, the ultrasonic transducer in virtually all sonar and depthsounding systems; and modified barium titanate, which is the dielectric in the ubiquitous
multilayer ceramic capacitor. There are many other examples, ranging from infrared
movement detectors to actuators for autofocusing cameras.
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Figure 1: A schematic tree of the different classes of crystal systems and their property classifications.
The common and defining feature for all ferroelectrics is the presence of a field reorientable spontaneous polarization. Exactly which crystals belong to this group is an
empirical distinction, but many ferroelectrics are characterized by structures
incorporating oxygen polyhedral. The most important of these is the perovskite structure
(Figure 2). This polarization appears as a result of the small, highly charged cation
being displaced into a noncentrosymmetric position as the structure is cooled below the
Curie temperature. The physical significance of this spontaneous polarization is that it
25
confers the largest relative permittivity of any type of capacitor dielectric, as well as a
field-orientable polarization which has been exploited in the manufacture of a range of
piezoelectric and pyroelectric products.
Figure 2: The perovskite structure and the origin of spontaneous polarisation.
Mr. Chairman, the material I started investigating in the ferroelectric world was the
Cd1-xZnxTe solid solution. CdTe and ZnTe are members of the series of compounds
usually indicated as the II-VI compounds. Both CdTe and ZnTe have zinc blende
structure. CdTe and ZnTe form solid solutions for all compositions. Cd1-xZnxTe belongs
to a group of semiconductors (of the form A1-xBxTe) which are of considerable
technological and scientific interest. Cd1-xZnxTe solid solution has promising
applications in a variety of solid state devices such as solar cells, photodetectors, and
light-emitting diodes.
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Though a great deal of effort had been expended in the study of the properties of CdTe
and ZnTe and their mixed crystals Cd1-xZnxTe, no attempt had been made to investigate
whether the mixed crystals could be ferroelectric. This might have been due to the fact
that no material of prototype zinc blende phase was found to be ferroelectric. Probably
due to the same reason, a prediction that a transition which exhibits all the characteristic
features of a ferroelectric is in principle possible in a zinc blende structure, had not
received any attention.
However, some work showed that in Cd0.1Zn0.9Te <111> the Cd atoms have an
abnormally high amplitude of vibration as compared to that of the Te. This could be due
to anharmonicity in the lattice vibrations which had been found to be the source of
ferroelectricity in many materials.
Also, early X-ray diffraction experiments on alloys indicated that, although on the
macroscopic length scale, the alloys retains the overall space group of the parent
materials, the attendant diffuse scattering background suggested that the atomic
structure changes on a microscopic scale. The atomic scale structure of zinc blende
ternary alloys in which substitution occurs on the cation sublattice is such that the cation
sublattice is almost undistorted while the anion sublattice is strongly distorted.
27
With these motivations we investigated and observed the existence of ferroelectricity in
Cd1-xZnxTe. This was the first observation and the report on a material of the prototype
zinc blende structure has led to continued research on such materials in Israel, Europe
and the Americas.
Recently, melt-quenched K(NO3)1-x(ClO3)x solid solutions have been found to be
ferroelectric in the Department of Physics of this University. The optical properties of
such a system indicate that they can be used in many optical devices. Work is in
progress to investigate other properties for device applications.
The polarization of the ferroelectric phase can influence, or can be influenced by,
temperature (pyroelectricity), stress (piezoelectricity) or light (electro-opticity). These
materials also have large dielectric constant and therefore store large amount of electric
charges in smaller areas compared to normal dielectrics. Due to these outstanding
properties, ferreoelectric materials are used for a wide range of functional purposes
from simple capacitors to complicated microwave devices. Major applications can be
divided into six distinct areas, which depend upon different combinations of properties.
Dielectric applications make use of the high dielectric constant, low dispersion in
wide frequency range for compact capacitors layered on multilayers, thick and
thin films. High dielectric constant films are of interest for local capacitors in high
count dynamic random-access memory (DRAM).
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The nonlinear hysteretic response is also of interest for thin film non-volatile
semiconductor memories (FRAM).
Piezoelectric and relaxor ferroelectric compositions are of importance in
transducers for converting electrical signal to mechanical response and vice
versa. Sensor applications make use of the very high piezoelectric constant of
the converse effect, which also permits efficient conversion of electrical to
mechanical response.
Pyroelectric systems rely upon the strong temperature sensitivity of electric
polarization, the pyroelectric effect in ferroelectrics, for the bolometric detection
of long wavelength infra red (IR) radiation.
Ferroelectric materials in a single-crystal form have high optical transparency
and electro-optic properties, particularly when used in conjunction with polarized
light. These properties can be applied in modular switches, wave guide
structures and photo-refractive devices.
Recently, paraelectric state in ferroelectric films extended their application areas
to the microwave fields where they can be used in phase shifters, tunable filters,
varactors, etc. Higher tunability of the dielectric permittivity together with lower
dielectric loss at room temperature are desirable for tunable device applications.
There have been many attempts to find and develop new materials appropriate
for microwave devices.
The future appears promising for the development of a new generation of ferroelectric
devices, some of which will profoundly affect the evolution of the electronics industry
over the next 50 years.
29
Superconducting World
Mr. Chairman, ladies and gentlemen, my journey through the material world has taken
me to low temperature regions where some materials exhibit some interesting
properties, including superconductivity. Superconductivity, which was discovered in
1911, is a phenomenon of great intricacy, diversity and elegance. It is one of the most
interesting and challenging subfields of condensed matter physics. For instance, the
mechanism of high temperature (high-Tc) superconductors remains unresolved despite
two decades of persistent efforts by leading scientists in the world. On the other hand,
significant progress has been made in the technical applications of superconductivity.
In 1911, Onnes began investigating the electrical properties of metals in extremely cold
temperatures. It had been known for many years that the resistance of metals fell when
cooled to low temperatures, but it was not known what limiting value the resistance
would approach, if the temperature were reduced to very close to 0 K. Some scientists,
such as William Kelvin, believed that electrons flowing through a conductor would come
to a complete halt as the temperature approached absolute zero. Other scientists,
including Onnes, felt that a cold wire’s resistance would dissipate. This suggested that
there would be a steady decrease in electrical resistance, allowing for better conduction
of electricity. At some low temperature point, scientists felt that there would be a
levelling off as the resistance reached some ill-defined minimum value allowing the
current to flow with little or no resistance.
30
Onnes passed a current through a very pure mercury wire and measured its resistance
as he steadily lowered the temperature. Much to his surprise, there was no levelling off
of resistance, let alone the stopping of electrons as suggested by Kelvin. At 4.2 K the
resistance suddenly vanished. Current was flowing through the mercury wire and
nothing was stopping it; the resistance was zero (Figure 3). According to Onnes,
“Mercury has passed into a new state, which on account of its extraordinary
electrical properties may be called the superconductive state.”
Figure 3: Variation of non-superconductive metal and superconconducting metal with temperature, T c as
the transition temperature.
31
Onnes recognised the importance of his discovery to the scientific community as well as
its commercial potential. An electrical conductor with no resistance could carry current
any distant with no losses. In one of Onnes’s experiments he started a current flowing
through a loop of lead wire cooled to 4 K. A year later the current was still flowing
without significant current loss. Onnes found that the superconductor exhibited what he
called persistent currents, electric currents that continued to flow without an electric
potential driving them. Onnes had discovered superconductivity, and was awarded the
Nobel Prize in 1913.
Giving the difficulties of working at such “cryogenic” temperatures, superconductivity
remained interesting but of little practical use, though materials were found that became
superconducting at slightly higher temperatures. Theoreticians were fascinated by the
phenomenon because nobody had any idea why it occurred.
The theoretical principles of superconductivity were finally outlined in 1957, when John
Barden, Leon N. Cooper, and J. Schrieffer published a theory that would also win a
Nobel Prize. The “Barden-Cooper-Schrieffer (BCS)” theory suggested that cryogenic
cooling of materials such as niobium suppressed the random thermal noise in their
crystal structure. This allowed quantum mechanical vibrations “phonons” to set a weak
electrical interaction that coupled electrons with opposite spin and momentum together
in “Cooper pairs”, which had zero net spin and momentum.
32
Electrical resistance is caused by the scattering of electrons due to defects, impurities,
and thermal vibrations in the crystal lattice of a conductor. However, the binding of
electrons in Cooper pairs eliminates scattering, and so electrical resistance disappears.
The pair states are no longer obliged to obey the Fermi-Dirac statistics, which enforced
the electrons to occupy high kinetic energy single particle states due to the Pauli
principle. The energy gain of the superconducting state with respect to the normal state
does not result from the small binding energy of the pairs but it is the condensation
energy of the pairs merging into the macroscopic quantum state. It can be measured as
an energy gap for electron excitation into the single particle state. Above a specific
“Curie temperature (Tc)”, thermal vibrations disrupt the Cooper pairs, and the material
becomes resistive again. Intense magnetic fields and currents can also disrupt the pairs
and destroy superconductivity.
Despite
the
development
of
the
BCS
theory,
doing
anything
useful
with
superconductivity remained an uphill struggle. What seemed to be a breakthrough
finally occurred in the 1980s. In September 1986, Alexander Mueller and Georg
Berdnoz, two scientists at an IBM research centre in Zurich, Switzerland, published a
paper describing a cooper-oxide compound that exhibited superconductivity at 35 K, 12
K above the Curie temperature of any superconductivity known at that time. They
published their paper in an obscure German physics journal in hopes that it would not
be noticed. This tactic allowed them time to reinforce their preliminary research without
interference, but still be able to prove the priority of their work if other reports were
published.
33
After many studies, the two scientists became convinced that their findings were
correct. Once their discovery became widely known, a flood of new “high temperature
superconductor (HTS)” materials were discovered. By December 1986, a material had
been discovered with a Tc of 38 K. A year later, in early 1987, a team under Paul C. W.
Chu discovered a compound, “yttrium barium copper oxide” (“YBCO”, pronounced
“ibco”) that had a Tc of 93 K. In the same year, the Bi-based cuprate superconductors
with transition temperatures 80 K and 120 K were discovered.
This moved the Curie temperatures of superconducting materials from the range of
liquid helium temperatures to those of liquid nitrogen temperatures. The reduction in
cooling requirements promised to greatly reduce the cost of superconducting
technology and widen its range of applications. The enthusiasm of researchers in the
field was manifested that year by a special meeting of the American Physical Society in
the Hilton Hotel in New York, crammed with 3,000 physicists, many of whom stayed up
all night discussing the new superconductors. The event became known as the
“Woodstock of Physics”.
Since 1986, over a hundred HTS materials have been discovered. The record T c now
stands at 138 K. This progress has been made even though nobody is exactly sure how
high-temperature superconductivity works.
34
While there is clearly electron pairing mechanism involved, as in the case with the old
“low temperature superconductors (LTS), the phonon-linkage mechanism associated
with Cooper pairs in low-temperature superconductors cannot work at high
temperatures. This is because at such high temperatures thermal vibrations would
quickly break the phonon-linkages. The problem is not the high Tc of up to 138 K under
normal pressures, far above the pre-HTS record of 23 K. However, in contrast to the
“deep” Fermi sea of quasi-free electrons in the case of classical metals where the
Cooper-pair condensed electrons amount only a small part of the valence electron
system (kBTc << EFermi), in these layered cuprate compounds there is only a “shallow”
reservoir of charge carriers (kBTc ~ EFermi) which have to be introduced in the insulating
antiferromagnetic stoichiometric parent compound by appropriate doping. There are
thus generated normal state correspondents to a “bad metal” in which Coulomb
interactions strongly link the charge and spin degrees of freedom. This intrinsic
proximity of metal-insulator, magnetic and superconducting transitions continue to
present a great challenge to theory, which is sensibly more complicated than the
classical superconducting problem. The superconducting instability in cuprate HTS, as
well as in the structurally and chemically related layered cobaltate and ruthenate
compounds, is hence believed to stem predominantly from a magnetic and not from a
photonic interaction as in the case of the classical metallic superconductors where
magnetism plays only the role of an alternative, intrinsically antagonistic long range
order instability. The most popular theory is that the pair coupling occurs due to subtle
magnetic effects created by the HTS lattice, but nobody has been able to explain how it
happens.
35
Finding a better theory for high-temperature superconductivity is not an academic issue.
Understanding what causes the phenomenon will help researchers to address some of
the problems they have encountered working with HTS. For example, magnetic
vortexes set up by the flow of electrical current through HTS have a tendency to drift
through the material, and this drift dissipates energy, or in other words, causes
resistance. The material needs a strong “flux pinning” to ensure the vortexes do not
migrate.
More importantly, a better theoretical understanding may lead to raising the Curie
temperature still further. Researchers believe this is perfectly possible, since a copperoxide compound made with mercury has been shown to superconduct at 164 K when
squeezed to extremely high pressure in a diamond anvil. As a result, one avenue of
research is to modify superconductive materials into configurations resembling those
that they adopt under high pressure.
Practical applications of superconductors have focussed in three areas: electrical power
systems and devices, such as power transmission lines, electric motors, and
transformers; sensitive “superconducting quantum interference device (SQUID)”
sensors; and ultrafast superconducting digital logic components.
Magnetic-levitation is an application where superconductors perform extremely
well. Transport vehicles such as trains can be made to “float” (about 1 cm above
the track) on strong superconducting magnets, virtually eliminating friction
36
between train and its tracts – the MAGLEV technology. This application is based
on the Meissner effect. Some MAGLEV trains can attain a speed of 580 kph.
An area where superconductors can perform a life-saving function is in the field
of biomagnetism. Doctors need a non-invasive means of determining what is
going on inside the human body. By impinging a strong superconducting-derived
magnetic field into the body, hydrogen atoms that exist in the body’s water and
fat molecules are forced to accept energy from the magnetic field. They then
release this energy at a frequency that can be detected and displayed graphically
by a computer. Magnetic Resonance Imaging (MRI) was actually discovered in
the mid 1940s, but the first MRI examination on a human being was not
performed until July 3, 1977. And it took almost five hours to produce one image!
Today’s fast computers process the data in much less time.
Electron generators made with superconducting wire are far more efficient than
conventional generators wound with copper wire. In fact, their efficiency is above
99 % and their size about half that of conventional generators. Other commercial
power projects in the works that employ superconductor technology include high
energy storage to enhance power stability.
These are some of the materials which have kept me in the material world, Mr.
Chairman
Conclusion
Mr. Chairman, ladies and gentlemen, the specialness of humans is not in our ability
to fashion tools from the things found naturally around us. Certain animals do that.
37
Our specialness is in our insatiable drive to improve upon nature. Not satisfied with
carving out gourds to make drinking vessels, we invented pottery. Animal skins were
replaced by fabrics woven out of wool, cotton and artificial threads. We can now
fashion materials that are fifty times stronger and run engines that are a hundred
times hotter than those made just a few decades ago, and electronic components
made may be a millionth of the size of the smallest ones possible a generation ago.
Human history has been delineated by our ability to manipulate different forms of
matter: the stone age, the iron age, the bronze age, etc. It is natural for us to be
curious about why the materials we see around us behave the way they do. They
are clearly basic scientific issues here; they are complemented by practical
motivations. Indeed condensed matter physics had a very pragmatic orientation in its
early days, as Carnot, Kelvin and others came up with such fundamental concepts
as entropy and free energy in their effort to understand steam engines.
Condensed matter physics has had great impact on the way people live through its
contribution to technology, where devices such as the transistor, semiconductor
chip, microprocessors and computers have found applications. Condensed matter
physicists find themselves entangled in the
science of today which is the technology of tomorrow.
38
However, Mr. Chairman, the relationship between science and technology has
sometimes been very complex. This University had a Department of Materials
Science and Engineering. When the Department was being moved from its old
location into the College of Engineering, the Science component of the programme
“got lost on the way”, or was probably offloaded on the way. Thank God, the College
of Science has seriously searched for and found this important item and will submit it
to your office soon.
I recommend the establishment of a programme for the study of Materials Science
and the College of Science will pursue this vigorously. I also recommend the
formation of the Materials Research Society in this University through which all those
involved in the study of materials will share ideas and work together for the good of
our dear nation.
Acknowledgement
Mr. Chairman, before I end it all, I would want to acknowledge the support of a
number of people who have made tremendous input into my life, to make me what I
am and what I stand here to represent.
I could not have been in a better Department than the Department of Physics of this
University. We have been a family where members are challenged and encouraged
39
to excel. I am grateful to my Head of Department. I am also grateful to the Provost of
the College of Science, who also happens to be a member of staff of the Department
of Physics. We completed the BSc Physics programme in the same year.
Professor Francis Boakye led me into the Material World when he supervised my
undergraduate project work in the 1979/80 academic year. We have since formed a
very strong research team. God bless you, my teacher.
Prof. Keshaw Singh pushed me further into the Material World when he taught me
Materials Science at the undergraduate level and also supervised my MSc thesis. I
am his first graduate student in this University. He has been one of my mentors.
Dr. C. P. Ntiforo who was my Head of Department for about twelve years was like a
father to me. He advised me on many issues including my decision to stay in the
Material World. He co-supervised my MSc thesis.
Prof. Raoul Baruch Weil and Prof. Lucien Benguigui of the Department of Physics
and the Solid State Institute of the Technion – Israel Institute of Technology, Haifa,
gave me all the advice I needed to successfully obtain a doctorate in the Material
World.
40
The Lady Davis Fellowship Trust of Jerusalem awarded me a Lady Davis Fellowship
to enable me study at the Technion.
Prof. William Ross Datars of the Department of Physics, McMaster University,
Hamilton, Canada, invited me to spend two years in his laboratory as a Postdoctoral
Fellow. He helped to polish me up as a Condensed Matter Physicist.
The Abdus Salam International Centre for Theoretical Physics, Trieste, Italy, has
provided support for my research work for a number of years by appointing me first
as a Regular Associate and later a Senior Associate of the Centre.
There are some people who have made sure that though I am in this material world I
will not be of this world – those beautiful people called Methodists. The Bishop of the
Kumasi Diocese and my Superintendent Minister have been supportive both
physically and spiritually.
The members of my external family, especially my siblings, have been so supportive
and I thank them. They have also provided the encouragement I needed.
41
My wife and children have been very tolerant with me as I spend long hours in the
laboratory and office and sometimes on my computer when I am even at home. My
children have been my best friends.
I cherish the friendship and the brotherly love of all my mates of the Ghana
Secondary Technical School (GSTS), Takoradi, some of whom are here to share my
joy with me. Some of my seniors and juniors are also here. Thanks for your support,
you GIANTS
Mr. Chairman, ladies and gentlemen, it has been said that
“The reasonable man adapts himself to the world;
the unreasonable one persists in trying to adapt the world to himself.
Therefore, all progress depends on the unreasonable man.”
I pray the scientists and technologists of the Kwame Nkrumah University of Science
and Technology, who our fifty year old country is looking up to for her scientific and
technological development, will be among the unreasonable men and women who
will bring progress to this land of our birth.
Thank you for your attention and may the good Lord bless you all this Jubilee Year.
42
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45