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JAPAN NANONET BULLETIN
-- 51st Issue -- August 18, 2005
Nanotechnology Researchers Network Center of Japan
Ministry of Education, Culture, Sports, Science and Technology (MEXT)
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IN THIS ISSUE
Nanonet Interview:
"Great potential of electronic phase transition
--Strongly correlated electron: a new way in materials science--"
Yoshinori TOKURA, Professor, Department of Applied Physics, Graduate
School of Engineering, The University of Tokyo
Young Researchers' Introduction:
"Polymer structure control based on crystal engineering for materials
design"
Akikazu MATSUMOTO, Professor, Department of Applied Chemistry,
Graduate School of Engineering, Osaka City University
-- NANO CALENDAR --
For information on nanotechnology related symposiums and conferences
held in the world,
http://www.nanonet.go.jp/english/calendar/
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NANONET INTERVIEW
Great potential of electronic phase transition
--Strongly correlated electron: a new way in materials science--
(Issued in Japanese: Feb.10, 2004)
Yoshinori TOKURA, Professor, Department of Applied Physics, Graduate
School of Engineering, The University of Tokyo
Semiconductor electronics strives to control a single electron. On the
other hand, strongly correlated electronics attempts to control many
electrons at the same time, and Prof. Tokura has been pursuing this
area of research. In a band insulator, which is based on the
electronic structure of an intrinsic semiconductor, there are no
vacant sites into which electrons can hop because the two sites on a
lattice point are occupied by electrons with an up-spin and a down-
spin. In Mott insulators and insulators induced by charge ordering in
a strongly correlated system, even if there is only one electron at a
lattice point, the electron cannot hop into a vacant site because
neighboring electrons repel each other due to Coulombic repulsion.
Therefore, the electrons localize at the lattice points and go into a
crystal-like state. However, since they are actually conduction
electrons, metallic electrical conductivity occurs when the electron's
crystal-like state is disturbed by weak stimulation.
Electrons hop into new adjacent vacant sites when carriers are
injected into an insulator, which disturbs an electron's crystal-like
state. Sequentially, the neighboring electrons hop into the vacant
sites out of which electrons just moved. However, electrons cannot
move freely due to a strong electron correlation effect. When spins
are localized at the lattice points and the spins are in the same
direction as the localized spins, conduction electrons can hop into
vacant sites due to Hund's rule. Even though the spins are aligned in
opposite directions, conduction electrons can move into vacant sites
if the localized spins are flipped by applying a magnetic field. Prof.
Tokura discovered colossal magnetoresistance where the electric
resistance decreases to one thousandths of that currently used by
applying a magnetic field. Antiferromagnetic Mott insulators become
high-temperature superconductors when carriers are injected from
atomic layers, called block layers, that sandwich a conductive plane,
which is known as Tokura Rule. "In a strongly correlated electron
system, even if the electron mass is large, the whole system goes
through a phase transition at the same time. Therefore, this dramatic
phenomenon occurs rapidly," said Prof. Tokura. Strongly correlated
electronics makes use of dramatic changes in properties, such as
insulators becoming metals, or antiferromagnetic materials becoming
ferromagnetic materials. Groups of electrons are caused to change
phases on a sub-picosecond timescale by applying a magnetic or
electric field, external stress or light.
Prof. Tokura has focused on transition metal oxides with a perovskite
structure in developing strongly correlated electronics. In these
crystals, the basic unit is comprised of a transition metal atom, such
as manganese and copper, surrounded by six oxygen atoms with an
octahedral structure. In order to induce a strong electron correlation,
the necessary electron number is perhaps only 10,000 or less. If an
octahedron with side lengths of 0.4 nm has an electron, there are
enough electrons to induce a strong correlation in such a tiny box as
having sides 8 nm in length. Improvements in epitaxial growth
technology made it possible to build perovskite layers consisting of
units 0.4 nm in length and to design strongly correlated materials
which take into account electron charges, spins and orbital
configurations.
Spintronics is one of the fields involved in strongly correlated
electronics. Prof. Tokura says, "Since electrons are almost immobile,
localized electrons cause a material to be magnetic." When sandwich
layered structures of ferromagnetic transition metal oxides and
nonmagnetic insulators are fabricated, tunneling magnetoresistance
(TMR) that an electric current flows parallel to the layers by an
applied magnetic field, can occur. Although devices using this
phenomenon have not been developed yet, the transition temperature
required to obtain TMR has exceeded room temperature. Prof. Tokura's
Sr2CrReO6 has a transition temperature of 615K. Although its TMR
functionality is not yet stable, it has great potential.
Orbitronics is another field of technology in strongly correlated
electronics. The ability of conduction electrons to hop depends on the
anisotropy of columnar shaped orbital and planar shaped orbital at the
crystal lattice points. Electron conductivity is controlled by the
configurations of the orbits. "Electrons arrange themselves in orbital.
Therefore, when an electric field is applied to an unstable orbital
state, the electrons move rapidly. Since light also has an electric
field, it can control the conduction properties as well. It is
surprising that information can be transmitted through a disturbance
in electron orbital, which occurs instantaneously when a material is
exposed to light. In the future, carriers that work on the scale of
angstroms will be needed," says Prof. Tokura.
Prof. Tokura has proposed the field of cross-correlation physics,
which brings about new functionality by utilizing the various degrees
of freedom in strongly correlated electrons. He says, "Now, the value
of things is all-too-one-dimensional. An electron, as an entity itself,
has various degrees of freedom. Spintronics is a combined technology
of charges and spins, and orbitronics is a combined technology of
charges, orbitals and lattices. Thus, combining various properties
will create a new way of thinking in material science." The way to do
this is a brainchild of scientists. "I am not good at computer
graphics, but there are images of up and down spins and beaded
structures of electrons in my head. Although observing something is
important, sometimes new functions can be created just by the
imagination. So, researchers need to be trained to expand their own
imagination. To see electrons actually behave and induce new functions,
as I have done through my imagination, is exciting and enjoyable,"
says Prof. Tokura.
(Interviewer: Kuniko Ishiguro, Cosmopia Inc.)
For more information,
http://www.nanonet.go.jp/english/mailmag/2005/051a.html
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YOUNG RESEARCHERS' INTRODUCTION
Polymer structure control based on crystal engineering for materials
design
(Issued in Japanese: March 2, 2004)
Akikazu MATSUMOTO, Professor, Department of Applied Chemistry,
Graduate School of Engineering, Osaka City University
We can control the structure of a polymer and design organic materials
using polymer crystal engineering. The structures and properties of
crystalline materials are designed using pre-organized molecules
through various intermolecular interactions such as hydrogen bonds, pi
/pi, CH/pi, CH/O, and halogen interactions. We have investigated the
features and mechanisms of the topochemical polymerization of 1,3-
diene monomers, including esters, ammonium, and amide derivatives of
muconic and sorbic acids, which are 1,3-diene mono- and dicarboxylic
acid derivatives, respectively.
We proposed the principles for the topochemical polymerization of
diene monomers based on the crystallographic data accumulated for
various kinds of diene monomers. By combining intermolecular
interactions, it is possible to produce the appropriate molecular
packing for 5 A (angstrom) stacking, which facilitate topochemical
polymerization in the crystalline state.
We can also control aspects of the polymer chain, including tacticity,
molecular weight, and structure, through which a ladder structure can
be produced. In addition, the polymer crystal structures can be
controlled using this method, and by layering polymer crystals,
obtained from topochemical polymerization, an organic intercalation
system can be prepared. A totally solvent-free system for the
synthesis of layered polymer crystals was also developed.
For more information,
http://www.nanonet.go.jp/english/mailmag/2005/051b.html
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Ministry of Education, Culture, Sports, Science and Technology (MEXT)
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