Gen Shirane
A brief review of his career at the
time of his 65th birthday in 1989
by John Axe and Steve
Shapiro
Shortly after the discovery of the neutron in 1932 it was demonstrated
that the neutron could be diffracted by a solid. After WW II,
when
research reactors were built, there was a burst of activity in neutron
scattering and the present age dawned in the mid-60's with the arrival
of high flux reactors such as the High Flux Beam Reactor at
Brookhaven.
Gen Shirane's remarkable 30 year career in neutron scattering spans the
entire period of high flux reactors, and he is unequivocally recognized
as the world's leading expert in utilizing neutron scattering in
probing problems in solid state physics. In that period he has
published nearly 500 scientific papers, many of them seminal in
nature.
His contributions to Solid State Physics were recognized in 1973 when
he was awarded the Oliver E. Buckley Prize, the most prestigious award
in condensed matter science given by the American Physical Society, and
the Warren Award presented by the American Crystallographic
Association. Most recently (April, 1989), he was honored by being
elected to the National Academy of Science.
The underlying theme of Gen's work has been the study of phase
transitions. This concerns the study of solids which undergo
changes of
symmetry as the temperature, pressure, or field (electric or magnetic)
is changed. It is worthwhile to note, here, that prior to the
1950's
solids were generally studied in terms of single particle effects and
from that period on physicists began to view interactions in solids in
terms of cooperative, many-body phenomena. Neutron scattering had
already been shown to be uniquely capable of measuring collective
effects in solids, and Gen's early experiments on phase transitions
provided many convincing examples of cooperative behavior of atoms in
solids. His first work focused on phase transitions in
ferroelectric
materials where he collaborated extensively with Chalmers Frazer (now
at DOE). These materials have important technological applications
because the lower symmetry in the ferrolectric phase allows for
piezoelectricity and non-linear optical properties. Gen
co-authored a
book entitled "Ferroelectric Crystals" which is now considered a
classic and is still used extensively by researchers. These
studies led
to Gen's experimentation in other structural phase transitions in the
mid-60's and 70's. The soft mode theory of P. W. Anderson (1960) and W.
Cochran (1959) predicted that the frequency of a lattice vibrational
mode would tend to zero as the transformation temperature is
approached, and the solid would distort to a new state of lower
symmetry. The elegant experiments performed by Gen and his
collaborators, notably J. D. Axe (BNL) and Y. Yamada (ISSP), confirmed
this theory on a wide variety of systems and discovered new features
which were not envisioned in the theory, such as incommensurate
structures. Simultaneously, Gen was studying critical pehnomena
in
magnetic systems and providing a wealth of trusted experimental data
that served as a test of any theory. In this area, as well as
others,
Gen interacted closely with the BNL Solid State theory group, most
notably, M. Blume and V. J. Emery. The beneficial interaction
continues
to this day.
In the early sixties, Gen started using the Brookhaven graphite reactor
and became interested in the magnetic structure of solids. He
performed
the definitive measurements of the collective motions of magnetic
moments in a magnetically ordered solid such as iron. These
excitations, called spin waves, contain the microscopic information
about the magnetic interactions in the solid. Knowledge about
their
energy and dispersion is necessary for an understanding of
magnetism.
His early experiments in Fe, Ni, and Co formed the basis of our present
understanding of magnetism in these elemental solids. Gen
returned to
this field in the mid-80's when questions were raised about the nature
of the paramagnetic state, that is, the state existing at high enough
temperatures where the thermal energy destroys the ordered arrangement
of the magnetic moments. The groups of experiments performed at
Brookhaven are accepted by the scientific community as being the most
thorough and definitive.
Another theme of Gen' s work is his study of magnetic and structural
properties of lower dimensional systems. Although we live in a
three
dimensional world, special solids can be prepared with very anisotropic
interactions such that their properties resemble a one- or
two-dimensional material. Frequently, detailed calculations are
much
easier in one- or two- dimensions than they are in
three-dimensions. In
addition, the cooperative effects depend fundamentally on the
dimensionality of the solid. In the 70's Gen formed a close
collaboration with R. J. Birgeneau (then at Bell Labs, now at MIT) and
extensively studied one- and two-dimensional magnets with a wide
variety of different types of interactions. At about the same
time he
began his studies on structural transformations in lower dimensional
systems along with R. Comes (Orsay) where the electron-lattice
interactions are fundamental. It is widely recognized that his
results
provided the experimental basis for our understanding of lower
dimensional systems.
Around the same period Gen, Bob Birgeneau, along with Roger Cowley
(Oxford), performed some seminal experiments on the study of random
magnetic impurities in antiferromagnetic systems. These
experiments
showed that when a non-magnetic atom is placed in an ordered magnetic
solid, the magnetic properties are altered in a very surprising
way.
The experiments performed at BNL with Gen's help proved the most
reliable in deciding between different and often conflicting theories.
Another unifying thread in Gen's body of work involves the study of the
properties of superconductors. He performed the first
measurements with
neutrons on the effect of the energy gap on phonon linewidths in
strong-coupling superconductors. This is a direct measure of the
amount
of electron-lattice coupling which is one of the key parameters in the
Bardeen-Cooper-Schrieffer theory of superconductivity. He
subsequently
collaborated on neutron studies of the interplay of superconductivity
and magnetism in rare earth ternary solids where the magnetism and the
superconductivity are due to different electronic states. These
experiments demonstrated that magnetic order and superconductivity are
not always incompatible, contrary to prevailing wisdom. This was
a very
important finding.
Currently Gen is actively involved in the study of the new high-Tc
superconductors. All of his past experiences in magnetism,
structural
phase transitions in perovskites, lower dimensional phenomena, and
superconductivity come together beautifully in the series of
experiments he has performed in this rapidly advancing and exciting
field. This is, perhaps, the pinnacle of his long and
distinguished
career. Gen and his collaborators succeeded (while others have
failed!)
in detecting magnetic fluctuations in theYBa2Cu3O7
(called 123) and La2CuO4 (called 214)
superconductors. The significance of his results is that they
essentially define the nature of the charge carriers and, possibly, the
mechanism describing the superconductivity in these high-Tc
materials.
In addition, his studies of the magnetism in the above compounds
provide the first information about the quantum mechanical effects on
the dynamics of lower dimensional solids.
It is important to emphasize the totality and consistency of Gen
Shirane's scientific output over his entire scientific career.
His
energy has never lagged nor have his scientific standards been
compromised: only superior science is acceptable to him. Gen has
served
as head of the Neutron Scattering Group within the Physics Department
at BNL and has trained numerous young neutron scatterers who struggled
to meet his standards. Only the best succeeded, and they now
populate
the world-wide neutron research centers, as well as some of the top
universities. Gen Shirane has few equals!
Last Modified: Wednesday, 26-Jan-2005 16:39:26 EST
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