American Ceramic Society Bulletin

esearch briefs
X-ray microtomography and finite element
analysis to examine in each of the
90 specimens how trabecular scaling
changes in relation to size, and how
bone mechanics change in relation to
the scaling. For materials scientists,
here are the key findings (some of
which may be a little counterintuitive):
• From animal to animal, the bone
volume fraction does not substantially
scale with creature size;
• Although the bone volume fraction
does not increase greatly, the trabeculae
in the femur of larger animals
are thicker, farther apart;
• Also in larger animals, the trabeculae
are less densely connected (the
number per unit volume is considerably
fewer than in small animals; and
• Finite element modeling explains
that scaling does not alter the bulk
stiffness of trabecular bone, but probably
mitigates strain on the scale of the
osteocyctes.
The authors suggest that the differences
in how trabeculae grow in various
animals might be “an interspecific
manifestation of bone tissue’s drive to
maintain mechanical homeostasis. It
appears that changes in geometry are
preferred over increased bone mass.”
They and other researchers note that
this preference “may be an adaption
that limits the physiological cost of producing,
maintaining and moving more
tissue.”
So, what are broader implications? If
one is thinking about how to develop a
“smart material” that could adapt to a
changing environment, there is a lesson
in bones: The modeling and remodeling
of trabeculae and surrounding
internal structures seem to be a massefficient
strategy for dealing with strain.
Elephants do not require thick, dense
bones to support their loads. They just
use their internal capacity to alter their
bone structure.
“We can learn a lot from nature,
such as how nature develops these
strong, lightweight structures,” advises
Shefelbine in a story in The Engineer.
“We could adopt this in design. It could
inform how people develop structural
foams.” In particular, the researchers
say, “This may represent a new
approach to designing cellular solids
for engineered structures of different
scales.”
The IC research team also has created
an open-source computer program
(“BoneJ”) for examining the number,
thickness and spacing of trabeculae as
well as analyses of whole bones.
Visit Imperial College, www3.imperial.ac.uk;
and BoneJ, www.bonej.org/ n
Evidence mounts that
‘pseudogap’ is distinct phase in
superconducting materials
Investigators in the field of hightemperature
superconductors have
been stumped for some time about
what is occurring between when the
temperature of a material drops to the
point (T*) where electrons begin to
form Cooper pairs (T c ) and the critical
temperature for full superconductivity.
Heretofore, this odd transitional region
has been dubbed a “pseudogap,” but
now a collaborative research project has
revealed that three different tests suggest
the pseudogap is actually a distinct
phase.
The collaboration included scientists
from the Lawrence Berkeley National
Laboratory, the University of California
at Berkeley, Stanford University and
the SLAC National Accelerator Lab.
Their results were recently published in
Science (doi:10.1126/science.1198415).
Led by Zhi-Xun Shen, director of
the Stanford Institute for Materials and
Energy Science at SLAC and a professor
of physics at Stanford University, the
group focused only on Pb-Bi2201 (a lead
bismuth strontium lanthanum copper
oxide) because of the materials relatively
wide range between T* and T c .
Previous research supported two separate
theories about the odd pseudogap:
One theory is that it is just a range of
gradual transition to superconductivity,
and the other is that it is a state of
material distinct from superconductiv-
ity and normal “metallicity” with a
quantum critical point.
“Promising as the ‘quantum critical’
paradigm is for explaining a wide range
of exotic materials, high-T c superconductivity
in cuprates has stubbornly
refused to fit the mold. For 20 years, the
cuprates managed to conceal any evidence
of a phase-transition line where
the quantum critical point is supposed
to be found,” says Joseph Orenstein in
a news release from the Berkeley Lab.
Orenstein works in the lab’s Materials
Sciences Division and is a professor of
physics at UC Berkeley, whose group
conducted one of the research team’s
three experiments.
According to the release, the hope
is that once researchers can wrap their
thinking around the concept of a quantum
critical point (X c ), new routes
to superconductivity can be found.
“This is a paradigm shift in the way we
understand high-temperature superconductivity,”
says Ruihua He, lead author
with Makoto Hashimoto. “The involvement
of an additional phase, once
fully understood, might open up new
possibilities for achieving superconductivity
at even higher temperatures
in these materials.” These two worked
with Shen at SIMES and also worked
at Stanford’s Department of Applied
Physics and at Berkeley Lab’s Advanced
Light Source.
One of the tests they conducted
involved angle-resolved photoemission
spectroscopy to track the kinetic
energy and momentum of the emitted
electrons over a temperature range. In
another test, investigators measured
changes in rotations of the plane of
polarization light reflected from the
same Pb-Bi2201 sample under a zero
magnetic field (magneto optical Kerr
effects). The rotations are proportional
to the net magnetization of the sample
at various temperatures. Orenstein’s
group performed the third test, a study
of time-resolved reflectivity of the
Pb-Bi2201 sample.
None of these tests were particularly
novel – except that this time they
22 American Ceramic Society Bulletin, Vol. 90, No. 4