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JAPAN NANONET BULLETIN
-- 58th Issue -- November 24, 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:
"Nanostructures fabricated with polymers
-- Designing high-order structures with block copolymers --"
Seiichi NAKAHAMA, Research Coordinator, National Institute of
Advanced Industrial Science and Technology (AIST)
Young Researchers' Introduction:
"Anomalous Hall Effect and magnetic monopole in momentum space"
Atsushi ASAMITSU, Associate Professor, Cryogenic Research Center,
The University of Tokyo and Group Leader, Tokura Spin Superstructure
Project, Exploratory Research for Advanced Technology (ERATO), Japan
Science and Technology Agency (JST)
-- 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
Nanostructures fabricated with polymers
-- Designing high-order structures with block copolymers --
(Issued in Japanese: April 6, 2004)
Seiichi NAKAHAMA, Research Coordinator, National Institute of
Advanced Industrial Science and Technology (AIST)
Nylon, polyester and other ordinary polymers are made up of polymer
chains of various molecular weights. However, the molecular weight of
the polymers can be made uniform through "anionic living
polymerization". "Block copolymers" composed of different types of
polymer chains and "telechelic polymers" containing terminal
functional groups can also be synthesized using this living
polymerization. Prof. Nakahama has realized his idea - designing high-
order structures of polymers at the nanoscale level - through
accurate control of primary structures using anionic living
polymerization.
A block copolymer, in which the constituent polymer chains of each
block do not mix each other, has various nanoscale-level phase
separated structures. If monomers, from which polymers are made,
contain functional groups, anionic initiators and propagating active-
end anions react with the functional groups to terminate the
polymerization. Thus, block copolymers were for many years synthesized
only from styrene, butadiene and other hydrocarbon monomers without
functional groups.
In the mid 80's, Prof. Nakahama found that functional groups of the
monomer could be protected by replacing them with more stable moieties
and that the protective groups could be removed after polymerization.
He says, "This method is difficult to carry out. If the functional
groups are not protected tightly, polymerization will be blocked. But
if the protection is too tight, it will be very difficult to remove
the protection after polymerization. I examined many combinations of
polymerization conditions and protecting groups to be used. High-order
nanostructures can be designed with primary structures such as polymer
chain lengths and block sequences. If functional groups are introduced
to specific parts of a nanostructure, these parts will have special
functions." He developed a method to synthesize block copolymers from
polymers with functional groups and has been designing the
configurations of many high-order nanostructures and their functions.
Prof. Nakahama has also developed porous membranes with nanoscale
holes from triblock copolymers (with a structure of -(AAA...AAA)-(BBB...
BBB)-(AAA...AAA)-) made of polyisoprene and polystyrene with Si-OR
groups. When a high-order structure of this copolymer is treated with
acid, the Si-OR groups are hydrolyzed and the polystyrene micro domain
is fixed firmly through silicon's cross-linking reaction. Then, the
structure is treated with ozone, and polyisoprene chain is decomposed
and resolved resulting in porous polystyrene membranes containing
silicon. The size of the holes can be changed by altering the
molecular weights of polystyrene and polyisoprene segments in the
copolymer. These membranes may be used as materials for sensors with
suitable holes for specific enzymes.
Prof. Nakahama has also studied peculiar properties of a high-order
structure of polymers. The block copolymer of hydrophilic PHEME (poly
hydroxyethyl methacrylate) and hydrophobic polystyrene has a high-
order structure composed of cylinder-like arrays of polystyrene and
PHEMA filling spaces among the polystyrene arrays. Film of this block
copolymer may be used as an anti-thrombogenic material because it is
highly biocompatible and does not activate blood platelets. Prof.
Nakahama became very interested in the surface of this film, which
gradually becomes hydrophilic when wet, although dry film is usually
hydrophobic because its surface is covered with the polystyrene domain.
Researchers were confused about this phenomenon at the time. They were
unable to study the wet surface of the film with electron microscopes,
whose insides should be kept in a vacuum state when used to check
samples. Prof. Nakahama discovered that the structure of the wet
surface is maintained even after it is stained and dried. He found out
how the hydrophobic surface becomes hydrophilic by observing the
process closely. There are very tiny defects on the polystyrene
surface and water gradually enters these defects, making the inside
PHEMA expand. The expanded PHEMA domain comes out to the surface,
changing it to hydrophilic. He also discovered that the film's surface
returns to a hydrophobic state when dried.
Prof. Nakahama wants young researchers to thoroughly study basic
research. "I hope they will conduct basic studies with courage and
enthusiasm for developing new fields on their own," he says. He also
advises them to learn the engineering approach in addition to the
scientific one. He says, "Science and engineering have been different
historically but with some overlapping. I think young researchers
should make it clear the difference in science and engineering to find
out where they stand when starting a new research."
(Interviewer: Yu Tatsukawa, Cosmopia Inc.)
For more information,
http://www.nanonet.go.jp/english/mailmag/2005/058a.html
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YOUNG RESEARCHERS' INTRODUCTION
Anomalous Hall Effect and magnetic monopole in momentum space
(Issued in Japanese: May 12, 2004)
Atsushi ASAMITSU, Associate Professor, Cryogenic Research Center,
The University of Tokyo and Group Leader, Tokura Spin Superstructure
Project, Exploratory Research for Advanced Technology (ERATO), Japan
Science and Technology Agency (JST)
In real space, magnetism always occurs due to a magnetic dipole, and
a magnetic monopole has not been found to be naturally occurring.
However, a magnetic monopole can appear in the momentum space of
solids due to the anomalous Hall Effect in ferromagnetic metals. Here,
we report experimental results together with first-principle
calculations involving the ferromagnetic metal SrRuO3, which provide
evidence for the existence of a magnetic monopole in momentum space.
Bulk crystals and high quality thin films of SrRuO3 are fabricated
using flux-growth techniques and pulsed laser deposition, respectively.
The anomalous Hall Effect, or the transverse conductivity, sigma_xy
changes with temperature or magnetization, changes sign and has a
fairly large value of -60 ohm^-1cm^-1 at absolute zero (see Fig.1).
Theoretically, sigma_xy is represented as an integral of the gauge
field, which corresponds to the "effective magnetic field" that
an electron feels and comes from the quantal phase, or the so-called
Berry phase, of an electron Bloch wave function. The source, or sink,
of the gauge field corresponds to the magnetic monopole in momentum
space, and its position and distribution quantitatively govern the
anomalous Hall Effect in this system (see Fig.2).
We are convinced that the novel idea of "controlling the monopole in
solids" will be a promising method for phase control in solids as well
as be useful in future applications, such as colossal magnetoresistive
or magneto-optical devices.
For more information,
http://www.nanonet.go.jp/english/mailmag/2005/058b.html
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