the size of memristance effects increas as the inver square of device size. Strukov et al.2 u a simple model to show how memristance aris naturally in a nanoscale system when electronic and atomic transport are coupled under an external voltage. The authors realize this memristive system by fabricating a layered platinum–titanium-oxide–platinum nanocell device. Here, the hysteretic current–voltage characteristics relate to the drift back and forth of oxygen vacancies in the titanium oxide layer driven by an applied voltage4.
This obrvation provides a wonderfully simple explanation for veral puzzling phe-nomena in nanoscale electronics: current–voltage anomalies in switching; hysteretic conductance; multiple-state conductances (as oppod to the normal instance of just two con-ductance states, ON and OFF); the often mis-characterized ‘negative differential resistance’, in which current decreas as voltage increas in certain nanoscale two-terminal devices; and metal–oxide–miconductor memory structures, in which switching is caud by the formation and breakdown of metal filaments owing to the movement of metal atoms under applied bias.
But what of Moore’s Law? Established by Intel co-founder Gordon Moore in 1965, this empirical rule states that the density of transis-tors on a silicon-bad integrated circuit, and so the attainable computing power, doubles about every 18 months. It has held for more than 40 years, but there is a so
bering conn-sus in the industry that the miniaturization process can continue for only another decade or so.
The memristor might provide a new path onwards and downwards to ever-greater proc-essor density. By fabricating a cross-bar latch, consisting of one signal line crosd by two control lines5, using (two-terminal) memris-tors, the function of a (three-terminal) transis-tor can be achieved with different physics. The two-terminal device is likely to be smaller and more easily addressable than the three-termi-nal one, and more amenable to three-dimen-sional circuit architectures. That could make memristors uful for ultra-den, non-volatile memory devices.
For memristor memory devices to become reality, and to be readily scaled downwards, the efficient and reliable design and fabrica-tion of electrode contacts, interconnects and the active region of the memristor must be assured. In addition, becau (unlike with transistors) signal gain is not possible with a memristor, work needs to be put into obtain-ing high resistance ratios between the ON and OFF states. In all the instances, a deeper understanding of the memristor’s dynamic nature is necessary.
It is often the simple ideas that stand the test of time. But even to consider an alternative to the transi
stor is anathema to many device engi-neers, and the memristor concept will have a steep slope to climb towards acceptance. Some will undoubtedly trivialize the realization of
this ubiquitous nanoscale concept, whereas
rubberduckothers will embrace it only after the demon-
stration of a well-functioning, large-scale array
of the denly packed devices. When that
happens, the race towards smaller devices will
proceed at full steam. ■
James M. T our and T ao He are in the
Departments of Chemistry, Computer Science,
Mechanical Engineering and Materials
Science, and the Smalley Institute for
Nanoscale Science and T echnology, Rice
University, Houston, T exas 77005, USA.
e-mail: tour@rice.edu
1. Chua, L. O. IEEE Trans. Circuit Theory18, 507–519
(1971).
2. Strukov, D. B., Snider, G. S., Stewart, D. R. & Williams, R. S.
Nature453, 80–83 (2008).
3. Chua, L. O. & Kang, S. M. Proc. IEEE64, 209–223
(1976).
4. Yang, J. J. et al. Nature Nanotech. (in the press).
5. Kuekes, P. J., Stewart, D. R. & Williams, R. S. J. Appl. Phys.
97, 034301 (2005).
CLIMATE CHANGE
Natural ups and downs
Richard Wood
The effects of global warming over the coming decades will be modified
by shorter-term climate variability. Finding ways to incorporate the variations will give us a better grip on what kind of climate change to expect. Climate change is often viewed as a phenom-
enon that will develop in the coming cen-
tury. But its effects are already being en,
and the Intergovernmental Panel on Climate
Change recently projected that, even in the
next 20 years, the global climate will warm by
around 0.2 °C per decade for a range of plaus-
ible greenhou-gas emission levels1. Many
organizations charged with delivering water
and energy resources or coastal management
billbillare starting to build that kind of warming
into their planning for the coming decades.
A confounding factor is that, on the
timescales, and especially on the regional
scales on which most planning decisions are
made, warming will not be smooth; instead,
it will be modulated by natural climate varia-
tions. In this issue, Keenly-
side et al. (page 84)2 take a
step towards reliably quan-
tifying what tho ups and
downs are likely to be.
Their starting point is
the ocean. On a time s cale
of decades, this is where
most of the ‘memory’ of the
climate system for previous
states resides. Anomalously
warm or cool patches of
ocean can be quite persist-
ent, sometimes exchanging
uttheat with the atmosphere
only over veral years.
In addition, large ocean-
current systems can move
phenomenal amounts of
heat around the world, and
are believed to vary from
decade to decade3,4.
the girl next doorTo know and predict the
state of the ocean requires
an approach similar to
weather forecasting: one ts up (initializes)
a mathematical model of the climate system
using obrvations of the current state, and
aqtruns it forwards in time for the desired forecast
period. With a given climate model, enough
obrvations to t the ball rolling and a large-
enough computer to move it onwards, the exer-
ci is conceptually straightforward.
But does it actually produce anything uful?
We don’t expect to be able to predict the details
squeezeinof the weather at a particular time veral years
in the future: that kind of predictability runs
out after a week or two. But even predicting,
say, that summers are likely to be unusually wet
during the coming decade would be uful to
many decision-makers. Only recently, with
the study from Keenlyside et al.2 and another
Figure 1 | Heat up? The three possible trends of winter temperature
in northern Europe from 1996 to 2050 were simulated by a climate
model using three different (but plausible) initial states6. The choice
of initial state crucially affects how natural climate variations evolve
on a timescale of decades. But as we zoom out to longer timescales,
the warming trend from greenhou gas begins to dominate, and
the initial state becomes less important. Keenlyside and colleagues2
u obrvations of the a surface temperature to t the initial state
bsbof their model. Their results indicate that, over the coming decade,
natural climate variability may counteract the underlying warming
trend in some regions around the North Atlantic. (Figure courtesy of
A. Pardaens, Met Office Hadley Centre).
–2
2000 2010 2020 2030 2040 2050
Year
2
4
S
u
r
f
a
c
坂田英语培训e
厌倦的英文
t
e
m
p
e
r
a
t
u
r
e
c
h
a
n
g
e
text message(
K
)
Tight squeeze — an apparatus for reducing quantum noi
in lar light.
