Pioneer of Correlated Electron Materials,
Dreams of New Technologies for a Sustainable
Society: The Magic of “4”
by David Pendlebury, Special correspondent
Professor Yoshinori Tokura is one of Japan's most distinguished
condensed-matter physicists and a global leader in the cutting-edge field
of correlated electron materials.
Correlated electron systems (sometimes called strongly correlated electron
systems) are those in which electron-electron interactions determine the
properties of the material. These interactions—including charges,
spins, and orbitals—can, under certain conditions, produce a wide and
surprising array of electronic phases, which hold promise for a new type of
electronics. Examples of correlated electron materials include
high-temperature (high-Tc) superconductors, spintronics materials, Mott
insulators, and many others.
"One of the triumphs of twentieth century physics was the development of
quantum theories of the behavior of electrons in solids," notes Andrew P.
Mackenzie, Professor of Physics at the University of St. Andrews. "These
theories underpin our understanding of many simple materials, and are
directly responsible for the way in which silicon technology has so
profoundly changed the world in which we live and work. They are reliant,
however, on the key assumption that electron-electron interactions can be
treated in a mean-field approximation.
Professor Yoshinori Tokura is one of Japan's most
distinguished condensed-matter physicists and a global
leader in the cutting-edge field of correlated electron
"The challenge of the twenty-first century will be to understand and
exploit the huge class of materials in which this approximation breaks
down. In these compounds, the position and motion of each electron are
correlated with those of all the others. The correlated electron problem in
solids is one of the most profound quantum mechanical problems faced by
modern physics…The scientific and technological promise of
correlated electron materials is enormous."
Among Professor Tokura's major achievements in the field are the
development of new types of high-Tc
superconductors, the establishment of a general rule for high-Tc
materials design and synthesis, detailed descriptions of colossal
magnetoresistance in oxide materials, especially in manganites, systematic
investigation of the Mott transition in perovskite materials, and the
discovery of gigantic non-linear optical properties in one-dimensional Mott
(see also) earned B.S. and Ph.D. degrees in applied
physics, both from the University of Tokyo, where he currently holds the
rank of Professor. He is also Research Director of the ERATO-JST's
Tokura Multiferroics Project, Group Director of RIKEN's Cross-Correlated
Materials Research Group, and an AIST Fellow of Japan's National
Institute of Advanced Industrial Science and Technology.
In recognition of his outstanding research contributions, Professor Tokura
has been awarded a number of prestigious prizes, including the IBM Japan
Science Prize (1990), the Nissan Science Prize (1998), the Asahi Prize
(2003), and the James C. McGroddy Prize for New Materials of the American
Physical Society (2005).
Today, he ranks among the 10 most-cited physicists of the last three
decades, having published nearly 1,000 research articles which have been
cited close to 45,000 times. His h-index is fast approaching 100,
signifying authorship of 100 papers each cited 100 or more times. Professor
Tokura's 10 most-cited papers are listed in
Table 1 on
the next page.
ScienceWatch.com special correspondent
David Pendlebury had the honor of meeting with Professor Tokura at the
University of Tokyo in late October 2009. The following is an edited
version of their conversation.
In the 1980s, after your graduate work and some
teaching at the University of Tokyo, you went to IBM in San Jose,
California, for a one-year research position. Is that right?
Yes. During 1987 and beginning of 1988.
That was a very important moment in the history of
condensed matter physics.
Yes, this year almost coincided with the period of "high-Tc fever."
Tanaka, my neighbor here, had just confirmed the 1986 results (of J.
Georg Bednorz and K. Alex Müller of IBM, who won the Nobel Prize in
Physics in 1987 for their discovery of cuprate high T-c superconductors).
If I had stayed in Japan, I might have never attacked that problem. But in
the United States, I was completely independent and I began my high-Tc
work. That was very good for me.
You have described the cuprate superconductor as a
wonderful example of a correlated electronic system. What other
materials are good examples of correlated electron materials?
Photo gallery of the interview
with Professor Yoshinori Tokura and David
Actually, all compounds of transition metal oxides—copper, but also
iron or manganese oxide materials. Manganese oxide is the material in which
we showed gigantic magnetoresistance. In correlated electron materials, the
electrons are in a strictly localized state, catched…
I think you have also used the term
Yes, pinned. Actually an electron can behave like a sort of wave in the
solid, but only an electron can stop an electron by their mutual
interaction—their motion is almost freezed out. That is the essence
of correlated electrons.
In the case of the copper oxide
high-Tc superconductors, the frozen electrons that make an insulator
are turned into a metal, and then immediately its state is a high-Tc
superconductor. But in another compound, it's sort of a ferromagnetic
metal. Of course, the result is quite different, but still the common
background is the melting of the electrons within the solid.
So the formula is that the methodologies are quite similar and also the
basic concept is quite common in correlated electron materials.
What is so fascinating about these materials is
the possibility of changing, say, their optical properties just by
exposing them to a magnetic field or other force. It does seem like
Yes. For example, in gigantic magnetoresistance, or what we called colossal
magnetoresistance, when we add a tiny or small magnetic field, then a
completely insulating ceramic suddenly turns into metal. So, it's a sort of
And high-Tc materials are the same. These are originally completely
insulating, but when chemically modified, with only a small change in
composition, they immediately turn into a metal.
So with this kind of the material—I mean the correlated-electron
materials—my favorite word is "emergence." Emergence means many
independent components come together to generate many very surprising
You're referring to quantum effects.
The collapse of the wave function, where the whole
system takes on a particular property?
Indeed. This is best represented by Nobel laureate Philip W. Anderson's
1972 article "More is Different," (Science, 177 :
393–396). I mean the same thing. The entirety of the material's
property cannot be described by the sum of the individual components.
And so that leads to all kinds of possibilities,
such as new kinds for computing and switching devices with these
materials. Is that correct?
I read that you said within a 40-nanometer area
you can have…I forgot the number...
Actually, almost a million electrons.
So, within a 40-nanometer-sized box you can have
one million electrons acting as a single mechanism.
You've been methodical about exploring all the
transition metals in sequence.
Yes. I always say that perhaps the correlated electron materials represent
an important area of science we need to realize our dreams. Of course, our
dreams are not to know the ultimate nature of the universe or such a big
thing as that, but we are trying to obtain very surprising or
unconventional functions or electronic functions in solids. In other words,
the goal is to develop new electronics—not in the narrow sense like
I just read Physics of the Impossible (New York: Doubleday, 2008),
by the US physicist Michio Kaku. I was very impressed by his book, and
actually my goal is to realize the physics of semi-impossible in condensed
matter science, which may lead to innovative and even revolutionary
technologies. So, we aim at very, very fundamental issues. Only a new
concept can lead to a revolution.
With this in mind, I have drawn up a list I call "Innovation 4." If these
kinds of numbers could be realized (see
Table 2), it
would create a revolution in our daily lives through basic discoveries in
In energy transfer, we need 400 Kelvin in order to realize a real room
temperature superconductor. At the moment we have 130 or 140 Kelvin
superconductors. So we need not only three times the effort to achieve
this, but also three times the innovation.
Of course, if you can use liquid nitrogen and then maybe you can make any
power transmission line, but it's still very difficult. High-Tc is a main
target of our research. We are still struggling and often it's not so
successful, but maybe we will one day reach the 400 Kelvin superconductor.
Another case is thermoelectric materials. Our goal is to produce materials
for ultra-low energy consuming electronics. The thermoelectric effect tells
us we can generate electricity from temperature differences. An example is
the air conditioner, which uses a compressor for refrigeration—that's
19th century thermodynamics.
But if we could directly convert electricity to heat conduction, that would
be better. We usually measure this by the so-called thermal figure of merit
(ZT). At the moment, ZT is typically 1 or a little more. But if this value
exceeds 3 or 4, then every compressor can go away and we can immediately
replace it with a direct heat-electricity transformation.
And another case, in terms of energy conversion, is
solar cells. As you know, the efficiency is now at
10%, but for industrial use 40% would be very important. So I think
correlated electron materials may help. I am not sure, but we are
working towards that purpose, and maybe with these correlated electron
materials we can generate a surprising result. We will need a very new
physics. Yes, it's a dream.
In silicon, light pumps out an electron leaving a hole, positively charged
which generates an electric current. But with the use of these new
materials, a photon of light comes in, then we have sort
of metallic state, and the semiconductor or insulator
suddenly turns into a metal. Of course, we have to consider the energy
conservation rule, but still a lot of the electrons can be generated
and extracted, so we may realize a very highly efficient solar cell.
This may be 10 years away.
And batteries, too. This is the problem of energy storage. It's another
dream of mine. The best performance of the present state-of-the-art
batteries is 100 watt-hours per kilogram. If you could increase this
performance three or four times, it would make a great difference in our
mobile computing society.
Our battery technology is classical electrochemistry. So we are thinking
there is a chance to move from the classical concept to more advanced
So, in summary, the items in the Innovation 4 program show we are only
one-third of the way to our ultimate goal in these areas.
The Innovation 4 program is certainly visionary.
What about the recent discovery of iron-based superconductors? How
might this new class of high-Tc materials fit into your
I think the Tc there is almost saturated. Obviously, the
revealed a new physics concept, and that's very important, but I think in
terms of the high-Tc, at 50 Kelvin it is already saturated.
Of course, people say that it has only been one year since this discovery,
so it may increase. In reality, the science community is already mature, so
almost immediately we can make a new series of materials. Maybe one month
nowadays corresponds to the one year of 20 years ago.
Chinese scientists have been very active in
exploring iron-based superconductors and, more generally, in physics
and materials science. Chinese science is growing rapidly, and the
nation is now second in output of papers worldwide according to our
data. What is your impression of research in China today?
Today, although the level of research tends to vary, I think China's
possibilities are vast. I think there are some top-notch scientists working
at a very high level. Tsinghua University has such scientists. I notice
that our group will publish a paper describing a new experimental method or
a new concept, and immediately many Chinese scientists work on the same
subject, verifying our work.
China has a very good machine. They have perhaps helped my citation count!
Many young people in China still want to study in the US, but some are now
studying in Japan or Germany. China is not always on the top, but it has
some very good fundamental research groups, so that will help the country
develop much faster. I think, yes, this country, with its huge population
and some very smart scientists, will rapidly develop.