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The quest for Superconduction at room temperature - how close are we?
Professor Johan F. Prins
Department of Physics, University of Pretoria, Pretoria 0002.
The quest for superconduction at room temperature
When electrons convey an electrical current in a material, they collide with
vibrating atoms and lose energy, which then appears as heat: the material has a
resistance to current flow. Owing to this resistance, an electric field has to
drive the electrons in order to maintain the current. Onnes discovered
superconduction in 1911 [1], after cooling mercury to 4.2 K (above absolute
zero). When a material turns superconducting, an electrical current can flow
through it without experiencing any resistance. No field is needed to drive the
electrons, and no energy is wasted by heat generation. Obviously, if such
materials could be made to work at room temperature, Electrical Engineering, and
other branches of Science and Engineering, would undergo revolutionary changes.
Although other materials have since been found, which are also superconducting,
the maximum temperature at which this effect could be generated was, for many
years, only 23 K.
In 1986 excitement ensued when certain copper-oxide ceramics were discovered
which superconduct at higher temperatures [2]. The temperature quickly rocketed
from 30 to 150 K, where it has again reached a ceiling; still well below the 300
K required for room temperature operation. The initial enthusiasm, to generate
superconduction at room temperature, has, since, been waning. However, my
present studies, and recent experimental results, have now produced compelling
evidence that it is possible to generate a superconducting phase, at room, and
higher temperatures, consisting entirely of electrons. These electrons are
obtained by extracting them, by means of an anode, from the surface of a
diamond, which had been modified to contain conduction electrons.
Fermions, bosons and coherence
According to Quantum Mechanics, there are two types of particles in nature
termed fermions and bosons:
Fermions have half-integral spins, and by the Pauli Exclusion Principle, no two
identical particles can have exactly the same set of parameters, like energy,
position, spin orientation etc., which describe a single quantum state.
Electrons, with spin ½, are the best-known fermions, and therefore only two of
them, spin-up and spin-down, can occupy the same atomic orbital. This behaviour
explains the different properties of the chemical elements in the Periodic
Table.
Bosons have integral spins and are, in contrast, able to all occupy the same
quantum state. When they are in such a state, a single "coherent" wave
function is formed, within which the constituent particles act in total unison.
Light particles with spin unity, are the best-known bosons, and their ability to
go into the same coherent energy state is responsible for laser action. The
formation of a coherent state is also possible for particles with mass. Last
year the Nobel Prize was awarded to three scientists who succeeded in cooling
alkali gas atoms, with zero spin, to a temperature of 1/10 of a millionth of a
degree Kelvin (K), above absolute zero, in order to generate such a coherent
state for these atoms.
Superconduction: electrons forming a coherent state
It has been realised that superconduction can only be explained by
postulating that the charge carriers must have formed a coherent state. The
carriers then move in unison and cannot be scattered. The problem is that the
charge carriers are electrons (fermions). However, if two electrons could pair
and move together, as a single entity, the resultant charge carrier, consisting
of such a pair, will have integral (zero) spin and should thus act like a boson.
Cooper proposed a mechanism for the formation of such pairs, which involves a
resonant interaction between two electrons and the vibrating atoms. This has led
to the Bardeen-Cooper-Schrieffer (BCS) theory of superconduction [3], which
satisfactorily explains the original low temperature superconductors, but seems
to fail, in some aspects, when it comes to the new copper-oxide ones.
Where does diamond feature?
The experiments on diamond were not intended to generate a superconducting
material, but were done in an attempt to exploit another property of this
extraordinary material. Results obtained in 1979 [4], suggested that modified
diamond surfaces might act as an ideal "cold cathode" from which
copious amounts of electrons could be extracted at room temperature. Such a
device could revolutionise vacuum electronic devices, like TV tubes. At present
the required electrons are generated by heating suitable metal wires, typically
tungsten, to high temperatures.
The carbon atoms, constituting a pure diamond crystal, are very well bonded by
the interaction of their valence electrons; this explains the exceptional
hardness of diamonds. None of these electrons are able to act as charge
carriers: i.e. diamond is an insulator. However, the diamond crystal structure
is similar to those of semiconductor materials like silicon and germanium, and,
just as is done for the latter materials, it should be possible to insert
foreign atoms, or other crystal defects, which are able to release, or donate,
extra electrons that can act as charge carriers. Such a material is said to be a
doped n-type semiconductor. These electrons are expected to be at a higher
energy within a diamond than they would be outside and should be able to flow
easily out into the vacuum. To achieve n-type doping, with a large enough
density of electrons needed for cold cathode action, has proved to be a
formidable task and this problem has been studied worldwide for several decades.
Owing to diamond's extreme properties, the methods used to modify a standard
semiconductor material like silicon, cannot be applied. The method used in
Professor Prins' studies has been to inject suitable ions into the diamond
surface by means of an accelerator (generally called an ion implanter). This
method also introduces unwanted defects, which have to be removed, and again
this is more difficult in diamond than in any standard semiconductor material.
About three years ago Prins obtained successful n-type doping o diamond, by
either oxygen- or nitrogen-ion implantation. The question then was, would the
material be suitable for cold cathode action?
In the very first experiment, it was found that electrons could be extracted,
but not in the way anticipated. The anode was a gold-plated steel ball with a
diameter of 1 mm, which could be held at various distances, in the micrometer
range, above the surface of the diamond. For a chosen distance, the applied
potential to the anode ball, first had to reach a critical value, at which the
current then suddenly increased to a high value. Exhaustive experimentation, has
led to the conclusion that a stable, new material-phase has formed between the
diamond surface and the anode. This phase consists entirely of electrons, and
seems to be superconducting.
Dipole formation at the diamond's surface
According to the accepted principles of band theory, electrons, which find
themselves at a higher energy level than the vacuum level, must experience a
field extracting them into the vacuum, even when there is no electric field
being applied by an anode. Such electrons leave positive charges behind, which
accumulate within a layer (called the depletion layer) just below the surface.
These charges attract the electrons back towards the diamond. However, such
electrons cannot re-enter the diamond, because they are now at a lower energy,
and, therefore, they have to accumulate outside the positively charged surface.
These electrons also become bound within quantum states, which now lie within an
"electron-charge" layer adjacent to the surface. The two oppositely
charged layers, on both sides of the surface, form a dipole that screens the
field at the surface, and thus stops the net outflow of electrons. Accordingly,
equilibrium is achieved.
If an additional field is now applied to extract the electrons, the dipole
layers will increase in width and thus, the electron-charge layer will expand
towards the anode. At a high enough applied potential, it will fill the whole
gap between the diamond's surface and the anode, and establish an electrical
contact between them. It is only at this point that a current will start to
flow. The applied field will now drive this current by accelerating electrons
out of the diamond, through the gap, and into the anode. This implies that the
field is not zero at the diamond's surface. Accordingly, the depletion layer
will grow further in width and add more electrons to the gap. However, the layer
of electrons, filling the gap, cannot expand any further, and therefore the
electron density within the gap will increase. Because the electron density
increases, the resistance of the gap has to decrease. Steady-state current flow
can only be achieved when the density of the electrons reaches a constant value.
In turn, this situation can only be reached when the field, within the gap,
becomes zero. For the latter field to go to zero, the resistance has to become
zero: i.e. the electrons have to form a superconducting state. To do this, they
have to find a mechanism to form boson-like pairs.
The Heisenberg Uncertainty Relationship and
electron pair formation
According to the Heisenberg Uncertainty Relationship, the mathematical
product of the uncertainty in position and the momentum of a particle cannot be
smaller than a certain minimum amount: h/4p where h is Planck's constant. As the
electron density within the gap increases, owing to the presence of the field,
the distances between the electrons decrease. Furthermore, because the field
within the gap decreases, as the electron density increases, the difference in
momentum between the electrons that are exiting the diamond surface and those
that are entering the anode surface, also decreases. Thus, the uncertainties in
the position and momentum of each electron, within the electrode gap,
continuously decrease. Eventually, any further increase in electron density
would violate the Heisenberg Uncertainty Relationship. At this point, all the
electrons within the gap are restricted by the Heisenberg Uncertainty
Relationship to be within adjacent minimum uncertainty volumes. According to the
Pauli Exclusion Principle, each uncertainty volume can contain only two
electrons; and these must have opposite spins. An individual electron can now
not move out of its uncertainty volume and leave its partner behind, because
then it will have to enter an adjacent uncertainty volume, which is already
occupied by two electrons. Thus, the electrons can now only migrate as pairs.
The uncertainty volumes act as charge carriers with zero spin; i.e. they are
boson-like, and able to form the required superconducting phase.
From both experimental results, as well as theoretical analysis, using
accepted concepts from thermodynamics, band theory, and quantum mechanics, Prins
has produced compelling evidence that electrons, extracted from an n-type doped
diamond, form, and must form, a superconducting phase between the diamond
surface and the anode; and this happens at room, and higher, temperatures.
References:
1. H. K. Onnes, Leiden Comm. 120b, 122b, 124c (1911).
2. J. G. Bednorz and K. A. Müller, Z. Phys. B64 (1986) 189.
3. J. Bardeen, L. N. Cooper and J. R. Schrieffer, Phys. Rev. 108 (1957) 1175.
4. F. J. Himpsel, J. A. Knapp, J. A. van Vechten and D. E. Eastman, Phys. Rev.
B. 20 (1979) 624.
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