Information Technology Reference
In-Depth Information
Heat dissipation is a serious problem in operating supercomputers. Impressive
examples of its consequences were given by the creators of a less powerful
supercomputer system—the employees of the Virginia Polytechnic Institute and
State University (Blacksburg, Virginia, USA), who created a supercomputer for
“merely” $5 million.
The system was assembled from Apple 1100 clusters (the 2 GHz G5 system with
4 GB of memory). The performance of the supercomputer was 17.6 teraflops and
the random access memory 176 TB. But despite the less impressive performance,
the developers wrote that its power consumption would be sufficient for 3,000
homes of average size.
It is these issues—power and heat dissipation—that dramatically increase the
interest in molecular elements that consume much less energy and dissipate much
less heat.
Today semiconductor electronics is deeply enrooted in industrial infrastructure.
Moreover, it optimally meets the demands of the modern society. However, indus-
tries such as supercomputer manufacturing would greatly benefit from the practical
use of molecular components. However, it should be noted that the molecular
components have semiconductor rivals in the same size range. Above all, these
are devices based on quantum wells.
8.1.1 Some Details: Quantum Dots and Cellular Automata
In the early 1970s of the last century a new direction emerged in semiconductor
physics—the study of heterostructures formed by semiconductors of varying com-
position and properties. Especially interesting were the heterostructures with the
spatial dimensionality different from three dimensions, i.e., from the usual solid.
Those may be thin nanometer films or filaments as well as nanometer ensembles of
atoms. Since quantum effects manifest themselves at nanometer scale, these sys-
tems were called quantum wells, quantum wires, and quantum dots. Their remark-
able property is that the nano-sizes of semiconductor structures restrict the
movement of electrons, and therefore the density of electronic states in them is
fundamentally different from the macroscopic body. It is easy to see (Fig.
8.2
) that
quantum effects appear starting from quantum wells, i.e., when the motion of
electrons is limited to nanometer sizes, at least in one dimension.
Without going into detail of extremely interesting properties and practical
applications of quantum wells and quantum wires, let us discuss quantum dots,
which are considered today as one of the possible alternatives to the molecular
elements of computing devices.
Quantum dots are sometimes (particularly in the popular press) called artificial
atoms. In fact, a quantum dot is a collection of atoms with nano-sizes in all three
spatial dimensions. The motion of electrons in such a system can be approximated
by a simple quantum mechanical model known as “particle in a rectangular
potential field.” This model is described by the Schr¨dinger equation:
