[MUSIC PLAYING]
PRESENTER: Second
of two Killian award
lectures to be presented by
Institute Professor Mildred
Spiewak Dresselhaus.
Let me simply remind you
that the Killian award
is the highest honor the faculty
can bestow on a colleague
as a recognition of, quote,
extraordinary professional
accomplishments and to
provide through the lectures
a means for the communication
of these accomplishments
to the faculty, students, other
members of the MIT community,
and to the general
public, thus also--
and I quote-- "honoring
the contributions
made by Dr. Killian to
the intellectual life
of the Institute."
To give Millie as
much time as possible,
I'll say no more about her
extraordinary accomplishments--
don't get up yet, Millie--
and her multifaceted
contributions to MIT.
But we'll simply introduce her
second lecture in a series.
The overall series is
called "New Materials
Through Science and New
Science through Materials."
The first lecture was
entitled "Adventures
in Carbon Research,"
and it was an adventure.
And I, knowing
nothing about physics,
enjoyed the adventure very much.
I'm looking forward to today's
lecture as I'm sure you are.
It's entitled "New Materials and
New Science by Intercalation."
Millie Dresselhaus.
[APPLAUSE]
MILDRED DRESSELHAUS:
It looks as if I
frightened quite a few people
since the last lecture.
I hope those that are here
will enjoy what I have to say.
Since the last lecture, I
had the nicest little note
from Dr. Killian, whom you
know is in the infirmary.
And thank God that
we have high tech.
High tech brought the lecture
to him in the infirmary
and he was able to watch
all of this on his screen.
He particularly
appreciated the remarks
I made about Mrs.
Killian and her interest
in women in science
and our women students
and faculty and
their activities.
And as I mentioned
in the first lecture,
I was dedicating the lecture
series to Mrs. Killian
and to her memory.
In the morning news today,
I heard about this week
being National Science
and Engineering week.
And it seemed that this
was a very fitting week
to have the Killian lectures
as a contribution of MIT
to the public
understanding of science.
As many of you know,
I have strong feelings
that scientific and
technological organizations
should give effort to the
public understanding of science
and also that the faculty
should participate
in these kinds of things.
So I'm very happy
that this happens
to be the week for
science and technology.
Well, in my first lecture,
I spoke about adventures.
And I guess that the
view graphs are not
so visible with the large
amount of lighting in the front.
Maybe-- that's
very much improved.
The first I devoted myself
to talking about some
of my adventures
in carbon research.
And today I'd like to
talk about something
different-- new
materials and new science
through intercalation.
This is really a talk--
maybe the first one
was, also but this
is a talk in materials science.
My own introduction to this
notion of new materials
through science and new
science through materials
came through Professor
Arthur von Hippel.
Professor Von Hippel
was the dominant figure
in the Department of
Electrical Engineering
and Computer Science for a
very, very long time in the area
of materials research.
And he had some notions
that I support 100%.
What he thought was
that through science,
we can learn about the
properties of materials
and generate materials that
would have properties that we
would like them to have.
So this is the kind of
materials engineering.
And it could be said
that Professor Von
Hippel is the father of our
modern notion of materials
engineering.
Now to make all of this
happen, to have people working
on science of new
materials, you have
to have a collaborative
effort between physicists
who are doing physics
on new materials,
chemists who make
the new materials,
and materials
scientists who worry
about the processing
of materials.
And in his laboratory at MIT, we
had a very nice blend, mixture,
of people working on all
aspects of the same materials
to make things as a whole
beyond what individuals
could do by themselves.
Now that was my inspiration.
The implementation came
through a different route--
through Ted Geballe, who at
the time that he inspired
me was a member of the technical
staff at the Bell Laboratories.
And in 1965, he and
a group of co-workers
discovered superconductivity
in intercalation compounds.
And I was at that
time already known
as an expert on the electronic
properties of graphite.
And he contacted me very
soon after this discovery
to say, gee.
We found the most
interesting thing.
We take carbon in the
form of graphite and we
add to the carbon
alkali metals--
potassium, rubidium,
and cesium--
and lo and behold,
we get something
that superconducting
when we know darn well
that none of the constituents
that we put in there
are superconducting
by themselves. cells.
This is a very interesting
phenomenon perhaps
you could do some of these
high field magneto-reflection
experiments and learn
something about what
happens to the
modification of the band's
structure of graphite that
promotes superconductivity.
Well, the boundary conditions
for doing those experiments
were very unfavorable
at that time.
And we thought about this
experiment for a few years,
but nothing much happened
in the way of implementation
until a paper was
published in 1971.
European Journal.
It was a very, very short paper.
But what it showed is that you
could see quantum oscillations
in intercalation compounds.
What Ted Geballe
did for me is he
alerted me to the field of
intercalation compounds.
And I started sort of
following the literature,
and sort of on the back burner.
We weren't really doing
anything actively.
But when this
paper came along, I
knew we should get
into the business
and do something serious.
And it was in this
way that I got
into the study of new materials.
So what is an intercalation
compound, anyway,
and why do we care about them?
Well, an intercalation
compound is
formed by the introduction of
entire layers of guest species
into a host material.
Suppose the host material is
graphite, as in our case here,
and we introduce layers
of some foreign species
like an alkaline metal.
What are the requirements
for this to happen?
For one thing, it's necessary
for the host material
to have extremely
strong bonding so
when the guest
species is introduced
it doesn't mess up the
layers of the host material.
And of course, that's
satisfy in graphite.
And the interplanar
binding is very weak
so that it's easy to
introduce whole layers
of a foreign species
into the host material.
Now there are, in fact, a number
of different host materials
that will work.
Graphite is only one of them.
Transition metal
dichalcogenides.
You can have there
semiconductors.
You can have metals.
That whole class of materials
will accommodate intercalation
as many other materials.
So there are a whole
bunch of host materials
and there are also a whole
bunch of intercalants that
could be put into each of them.
Now the connection between
this and materials engineering
is clear because by
selectively selecting
what you put into
the host material,
you can change, say, the
transport properties.
You can take a material--
let's say the transition metal
dichalcogenides--
semiconduct or
introduce materials
into between the layers and make
the whole system conducting.
Or in the case of graphite, you
could increase the conductivity
by more than an
order of magnitude.
Or you can put in
a magnetic species,
taking the host material
that was not magnetic at all
and make it magnetic,
or introduce
in the layers
superconducting elements
and make material
superconducting, et cetera.
So there is a whole variety
of things that can be put in.
And that gives
you an opportunity
to tailor the graphite
to make layered materials
and isotropic materials
with diverse properties.
Now why is graphite
so interesting?
Why graphite rather than
the other host materials?
Well, for a physicist, the
main attraction of graphite
is symmetry.
That when you introduce the
intercalant into graphite,
it goes into the graphite
in an ordered way
in the following sense
that the separation
between sequential layers
of the intercalant,
shown here as the
red, dashed lines--
the spacing between these is a
fixed number of graphite layers
and that fixed number
of graphite layers
is called the stage.
With all the other
host materials,
the staging phenomenon is
not a very well-established
periodicity.
But in the case
of graphite, it is
a well-established
periodicity, and it
allows us to do some rather
sophisticated experiments.
So that's the reason we got
into the study of the graphite
interpolation compounds.
Now, one way of
thinking about this--
and for the next
few minutes, I will
focus on the simple
property that you
could think of the intercalation
compound in a superposition
principle.
We often think-- in
electromagnetic theory,
we use the superposition
principle for many things.
But also in material
science, we often
have occasions where
we have superposition.
Here we have a superposition
of a package consisting
of two graphite
layers between which
the intercalant is contained.
That's one element.
That element, when it
occurs in a stage 1 compound
or in a stage n compound,
has the same separation.
It's always the same package.
The graphite layers
themselves retain
the same in-plane spacing
and the same spacing
between layers.
So we could think of that
as the second element.
And all we're doing is
playing building blocks.
We take this package
and the graphite layers
and we stack different
amounts of each
together depending on the stage.
So in that sense, the
intercalation compounds
are very simple.
AUDIENCE: How are these
materials prepared?
MILDRED DRESSELHAUS:
Oh, I don't have a view
graph how they're prepared.
I should have taken
the one along.
Obviously, I should've.
Well, there are many
ways to do this.
The simplest way
is vapor transport,
where you take the intercalant
in one part of an ampoule
and you have the graphite in
another part of the ampoule,
and then you maintain a
small temperature difference
with the graphite at a
slightly higher temperature.
And voila-- the gas phase
seeps into the region where
the graphite is, penetrates
between the layers,
and then you have an
intercalation compound.
AUDIENCE: Control
the number of stages?
MILDRED DRESSELHAUS:
By the vapor pressure.
And the temperature
difference, which
is another way of
saying the same thing.
Well, our inroad into the
intercalation business
was, as I said, through the
magneto reflection experiment.
That's what we
really started out
to do back with Ted
Geballe back in 1965.
That's what we had
been talking about.
But it took a long
time to do this.
Well, the magneto reflection
experiment-- yes, I really
should say, I said
before that I had
become well-known for
my work in graphite.
But it wasn't only that
I became known about it
for scientific reasons.
I also became known about
this for sociological reasons.
And this makes too good
a story not to tell
on an occasion like
this, so let me just
tell how I caused
the blackout of 1965.
I think that's how Ted
Geballe really found out
about me more than the science.
It so happened then on
November 9th at about 5 PM,
I was running an experiment
at the Magnet Lab
and I was the only one there.
It was me and the operators.
And just about the time
that we were at peak field,
we had two generators
running at the time.
We were at peak
field and watching
these oscillations coming.
And we were working on graphite.
And the oscillations
were coming in.
And all of a sudden,
the oscillations
started going wild.
And our recorder pan that
showed the magnetic field
started going wild from
0 all the way to-- well,
we must have been at that
time about 12, 13 tesla.
Going wildly back up and down
between 0 and 12 or 13 tesla.
Well, we had been instructed
at the Magnet Lab--
I'd been working
at the Magnet Lab
already about five
years at that time.
And I knew when a disaster
like that came, the thing to do
was throw the panic button.
I had never done this
before, but this seemed
like the right time to do it.
So I threw the panic
button and I left the room.
You know, I didn't want
this thing to blow up on me.
I went outside and the lights
were dimming and dimming.
First they were flickering,
and then all of a sudden they
went out.
By the time I got to the
control room at the Magnet Lab,
the whole place was
in utter darkness.
I said, gee, what I had done.
I was the only one
running the experiment.
It could only have been me.
And I looked outside
after the whole Magnet
Lab was in darkness.
I looked out on the street and
there wasn't a street light on,
either.
And I said, look what
I've done to Cambridge.
I had been using 10% of
the power of Cambridge
and I put Cambridge in
the state of darkness.
Well as you know, we had a
very big blackout in 1965
on November 9th.
We remained without
power for about a day.
And I'm told that the birth
rate went up noticeably
nine months later.
[LAUGHTER]
Anyhow, this was a
sort of a precursor
of the magneto
reflection experiments
on the intercalation compounds.
And the first set of
experiments that we ran
gave us results that
we couldn't believe.
And we tried these
experiments many times,
always getting the same results.
Basically that an
interpretation compound
had essentially the same
magnetic energy level
spectrum as graphite.
You can see here
the identification
of the various
magnetic transitions.
They're hardly changed.
And here is a plot of
the resonance's photon
energy versus magnetic field.
We did these experiments
many materials, many ways.
We always got the same results.
And we didn't believe
it and the public
didn't believe it, either.
And it was for
this reason that we
started looking at other
aspects of intercalation.
And the first thing that
we looked after the magneto
reflection was the phonon modes.
Phonon modes are significantly
different from electron modes.
So if we see nothing
in the electrons,
maybe we should see
something in the phonons.
So this seemed like a
totally unrelated experiment.
Let's see if we see anything
in this case that tells us
that we've intercalated.
Well here again, we got
results that were kind of also
slightly unbelievable.
The effect of intercalation
on the graphitic modes
was very, very small.
Here we're at about
1,600 wave numbers.
Here's the graphite line.
This mode, E2 G2 mode.
And it hardly changed
as we added intercalant.
It went up in stage.
However, there was some
evidence that intercalation
did something and that
we had a second Raman
line that was upshifted
by about 20 wave numbers.
And this line
increased in intensity
as the stage index
decreased-- that is,
the concentration increased.
And finally at stage
2 this first line
disappeared, and
also for stage 1.
Well, the explanation of this--
we came up with an explanation
for this pretty soon.
And the explanation is we had a
bunch of graphite layers here.
That they were very
strongly screened
from the intercalant layer
by a graphite bounding layer.
And when you get to stage
2, all the graphite layers
are graphite-bounding layers.
There are no
graphite-interior layers
that are like the graphite.
The layers that are
just like the graphite
give a resonance at
the same frequency
that the graphite does.
But the ones that are
on the bounding layer--
they have a lot
more charge and they
will give an upshifted mode.
So that whole story made
sense, even though the effects
were minuscule.
We learned some
science from this.
But we also learned from
this something about ethics.
From this experience,
I got a commitment
that we have to
teach the students,
our graduate students,
about the ethics of--
it's like teaching your own
children about the ethics
of how to live.
Teaching your students
about the ethics of science.
When we did these experiments
and presented them
at an APS meeting
right shortly after we
had done the experiments,
there was another group
that turned out that were
doing similar experiments.
And when we presented
our results,
they said our results
were totally wrong;
that they had done
the same experiments
and found large changes
in the Raman effect.
And, well, the large
changes were well-accepted
by the community because it had
been known that intercalation
changes the electronic
properties by orders
of magnitude.
So everybody thought that they
were right and we were wrong.
So we sat on our
results for a long time.
We presented the explanation of
our results also at that time.
And it was quite surprising
when six months later, we still
were sitting on our results
that we thought were wrong.
We found our
competition in print
with our proposed
explanation of the effect.
What that tells you is that
there's ethics in science,
and we have to have
everybody practice ethics.
Now we went on and studied the
stage dependence of the phonon
spectrum and the acceptors
is what I showed you
in the previous view graph.
That the frequency goes up
as you increase the amount
intercalant concentration,
decrease the stage.
And for donor compounds,
the opposite occurs.
That is, when you increase
the concentration,
the frequency goes down.
Now this is the one
that we could understand
from what had been done in
the literature on measurement
of lattice constants.
You have a donor compound
charged from the alkali metal.
Goes into the graphite layers.
It expands the
lattice constants.
Makes the mode soft.
And therefore you have a
decrease in phonon frequency.
Very, very understandable.
If that's the case,
then the opposite
should happen in the
acceptor compounds--
that the addition of intercalant
should sap up electrons.
Should take electrons
away the graphite layers.
Therefore, the lattice
constants should contract,
and therefore you should have
a stiffening of the mode,
and that's what's observed.
But the experiments
hadn't been done.
So a master's
thesis was generated
and we show that the lattice
constant for acceptors
and donors behaved
in an opposite way
and the different
magnitudes have
to do with the different
amounts of charge transfer.
This phononic problem
was a lot of fun, really,
because it provided some nice
problem sets for solid state
physics.
And I notice a number of
people in the audience
from my solid state
physics class.
And you're familiar
with these equations
for a couple phonons in a
solid, 1-dimensional line,
the top being for
similar species
and the second set of equations
for two species on a line.
Well, an intercalation
compound is really
two different species--
and intercalant and
graphite layers, each layer
represented by one ball.
And when you use that model, you
get very nice phonon dispersion
relations.
And just fitting these, you get
the constants for the forces.
So phonon dispersion relations
provided a nice set of examples
for the solid state course.
Now how do we understand the
experiments in transport?
I said that intercalation
changes the transport
properties by an
order of magnitude.
And then I said also that the
electronic structure didn't
change and the lattice mode
structure didn't change.
So what changed?
Well, what changed was the
position of the Fermi energy.
I guess the students in
the solid state class
have heard me say this very
often that you can change
the properties of
a solid greatly
by changing the
Fermi level, keeping
the electronic
structure the same.
Well, how does that work?
Well, here we have
the intercalant,
that's the brown,
positively-charged atom sitting
on the intercalant layer.
And if it's an alkali
metal, each potassium atom
will donate one electron.
The electron will go into
the graphite layers here.
But most of the
charge will go into
the graphite-bounding
layers, which are
right next to the intercalant.
There'll be an electrostatic
attraction between the two,
so we'll develop a very
large carrier concentration
in the graphite-bounding
layers--
very large compared to
the carrier concentration
in graphite itself, which
has almost no carriers.
10 to the minus 4
electrons per atom at room
temperature and less than
that at low temperature.
So intercalation
allows the change
in the carrier concentration
by approximately three orders
of magnitude.
So in that way, it's
possible to greatly change
the transport properties
of intercalation compounds.
Now the reason why
this whole thing works
so well is that the carriers
are taken from regions
where they have low mobility.
For example, a metal
like copper normally
has a very low mobility--
35 centimeters squared
per vole/second
is a small number
compared to 13,000 which
is characteristic of graphite.
So if you can take the carriers
from a low-mobility region
and promote them into
a high-mobility region,
you can really benefit
from that high mobility.
And that's why an intercalation
compound does have a high--
doesn't have a whole
lot of carriers,
but it has a high conductivity.
In fact, it is said,
some people claim
they could make compounds that
have conductivities higher
than copper at room temperature.
So now the reason
that works, again,
with small carrier
concentrations
is that each carrier
has a high mobility.
And exactly the
same concept is used
in modulation doping of
semiconductors superlattices
where you have a wide gap
semiconductor which you dope,
and the electrons will be
transferred into these lower
energy states into the low-gap
semiconductor gallium arsenide
where there are no impurities.
There are no scattering centers.
So that you have the carriers
and you have the high mobility
and you get high
conductivity in that way.
Well, we could also
understand from this model--
not much intelligence needed--
that the dominant, important
layers in graphite as far
as transport are concerned are
the graphite-bounding layers.
And to maximize
the conductivity,
you want the largest
fraction of this sandwich
to be graphite-bounding layers.
That tells you you want to
make a low-stage compound.
But don't make it too
small, because if you
make it too small then a
large fraction of the sample
will be taken up with
intercalant layers which
don't contribute very
much to conductivity.
And just to remind
you, what I said
here is the graphitic layer
is zone-folded to make
the different Stages.
And all that we're
doing here is we're
changing the position of the
Fermi level as we go along.
And that makes the difference
between stage 1, stage 2,
et cetera, for the
different conductivities.
Well, by the time we
had done this work,
I think that our
view was finally
accepted by the community.
It took several years.
The early days, it was almost
impossible to get any funding
to work on
intercalation compounds
And I must say that my earliest
work on the intercalation
compounds came about through
the courtesy of the Rockefeller
family.
In 1973, I was appointed for the
first time the Abby Rockefeller
Mauzé professor, and that
carried with it as a small
stipend.
I tried hard to get some money
to do intercalation compounds
from the federal agencies.
But having a physics
background, the referees
all always rejected
my proposals.
This work was good for chemists
but not for physicists.
So it took a little while
before any funding came about.
About four years later, when
people in other laboratories
showed that you could get
enhancements in conductivity
of, like, a graphite
by intercalation,
the funding agencies all of a
sudden became quite excited.
And it was about
that time that I
worked about the explanation
for transport in a system
where the graphitic layers
were essentially unchanged
from the host material.
Everything seemed to
come together at one time
and we got going.
And at that point, we started
developing a broader view
of the intercalation compounds.
A layered structure
is a 1-dimensional
and a 2-dimensional thing.
Perpendicular to the layers--
which is all that I
said up to this point--
we have 1-dimensional physics.
And in the layer planes, we
have 2-dimensional physics.
The 2-dimensional
physics is seeing
and study the intercalants.
So after a few years
of playing around
with the graphite, what
the intercalant did
to the graphite, we started
looking at what graphite
does to the intercalant.
Turn the problem around.
And there are rather
interesting things that happen,
and I'll be pretty brief
with what I have to say.
The first thing is
that you can have
different kinds of structures.
Three basic different
kinds of structures.
One is commensurate.
I'll mention what that is.
Another incommensurate.
And thirdly, discommensurate.
All of these things occur in
the intercalation compounds
as they do also in interfaces
that are grown stable
by molecular beam epitaxy.
And I'll have something to say
about how these different kinds
of materials interrelate.
Well, here is a
commensurate structure
of potassium on graphite.
It forms a 2-by-2 superlattice
in plane, 2-by-2 meaning
that the unit cell
this way is twice
the length of the unit cell
of the graphite itself.
Now why does it form a
commensurate lattice?
There were two forces
that are important.
One is that the potassiums
or the intercalant in general
likes to assume a
near-neighbor distance that's
almost the same as in
its native material.
It's a 2-dimensional material
and the potential minimum
for the attraction of the atoms
is almost unchanged from what
it is in the pristine material.
However, we have one
modulation feature,
and that's the graphite, which
makes the intercalant want
to sit over the hollow
of the honeycomb
structure of the
graphite lattice.
Because by doing so, we can
pack the 3-dimensional crystal
and make a closer packing
than any other way.
So just by these
geometrical arguments,
you could see that there
were two things acting.
One is the inter-atomic
forces and the other
is the attraction to the
graphite honeycomb lattice.
And in this case,
with very small space
from the closest packing
of the intercalant,
we can achieve registry
with the graphite.
So it likes to do that, and
in fact does so in the stage
1 compound.
And to prove that
we have that, you
do what diffraction pattern.
And that's a pretty
diffraction pattern--
I had to draw it for you to
show you the diffraction pattern
for a 2-by-2 superlattice.
Now what we see here also
occurs in molecular beam epitaxy
when you make a strain
layer superlattice.
This is exactly
the same concept.
We have a material
with one lattice
constant, another material
with another lattice constant.
And when you put very
thin layers together
the material with the small
lattice constant will stretch
and the material with the
large lattice constant
will contract so that
they form together
a system with some kind
of compromise lattice
constant, which you can
control with the grated layer
that you put as the substrate.
Now this strain layer
superlattice, of course,
has a big impact on
semiconductor technology.
Because in the early days
of molecular beam epitaxy,
it was thought that
you had to have
lattice matching
between the wide-gap
and narrow-gap semiconductors
that you put together.
With a straight
layer superlattice,
of course, you have a
great deal of flexibility
because there'll be some
stretching and stretch and give
and you can make
almost any material go
with almost any other material.
And that's greatly
expanded our horizons
in the superlattice field.
Now getting back to the
intercalation compounds,
another possibility--
when the nearest
neighbor distances
are totally unmatched to
the graphite substrate,
then they don't
pay any attention
to the graphite substrate
at all and they just
do their own thing
and they form layers
that are just like layers
in the host material.
Here we see cobalt
chloride and graphite.
So here's a graphite layer.
Here's a cobalt chloride
tri-layer sandwich
where the lattice constants
of the cobalt chloride
are exactly the same as they
are in native cobalt chloride.
But there are several
differences between this
and what happens in
molecular beam epitaxy.
For one thing, there
is an orientation,
a locking of the internal and
with respect to the graphite.
That is, the intercalant
doesn't go in any which way,
but it has some
preferred directions
where it can stay
away from the graphite
layer in a maximal sense.
The other thing
that happens that's
different from
molecular beam epitaxy
is you have atomically
sharp interfaces.
Just to show you how things
are in molecular beam epitaxy,
here is the case for
the semiconductors.
So here's silicon and
here's nickel filicide.
Now that forms a very nice
commensurate structure
very much like
potassium on graphite.
But here's a situation of
an incommensurate structure
of germanium with
barium fluoride
where the lattice consonants are
sufficiently different that you
don't form a nice
interface, and the interface
is mushy and corresponds to
several atomic distances.
However, for intermediate cases
where the lattice constants
don't differ by
that much, you can
form something that's called
a discommensurate phase where
locally you get phase-matching.
You have good registry
between the two materials.
But because the lattice constant
here is significantly smaller
than the lattice
constant up in here,
you have to have extra planes.
And the extra planes are what's
called discommensurations.
Now sometimes these
disconnects discommensurations
form periodic structures
all in themselves.
And when that happens, it's
called the stripe domain phase.
And I'll say
something about that.
We stumbled on this
whole thing, whole field,
in a very unexpected way.
Shortly after we got into while
looking at phonon spectrum
and intercalation
compounds, we found out
that in the bromine
system you could
look both at the graphite motes
and at the intercalation modes
separately.
And what that meant was
that we could monitor, say,
phase transitions,
looking at what happened
and the bromine layer,
and at the same time,
we could look
independently at what
happened in the host layer.
And then by changing the
temperature of the ideas,
that we could see something
associated with melting.
We were really looking for
melting of the bromine layers.
So here's a Raman spectrum
of the bromine stretch
mode in a bromine
intercalation compound.
And it's a nice lorentzian
line for a while.
And then the line
shape seems to change
and we go, all of a
sudden, it disappears.
And the disappearance
of the line
corresponds to a
melting transition.
But before it melted,
we had a line change.
Why should there
be a line change?
A shape change in line shape?
At room temperature,
the line shape
is lorentzian, which
indicates that you have
a homogeneously broadened line.
But then when the temperature
went up just by a few degrees--
345 degrees, for example--
no more lorentzian line shape
but Gaussian line shape,
which indicates
inhomogeneous broadening.
Now what could have caused that?
Well, from the Raman
spectrum, there
would be no way
to decipher that.
So we went to our good
friend Bob Birgeneau
and asked if he would like
to do some structural work
and see if there was
something funny that
happened in the bromine system.
And in so doing, we came upon
the striped domain phase.
I'd say a little bit about this.
What the striped
domain phase allows
us to do is to take
the bromine layer
and allow its lattice
constant to increase
above that which would be--
it would have it was
commensurate with the graphite
on a macroscopic scale.
While at the same
time, locally, there
would be a commensurate registry
between the bromine layers
and the graphite.
and now that's done
by having a domain.
Within this domain, we
have a commensurate phase.
And then there's a
wall where the bromine
is more dilute, less dense,
so that the average lattice
constant then will be larger.
And by having the
number of unit cells--
capital N-- decrease as the
temperature is increased,
then the average lattice
constant with bromine
could grow.
So that was the idea of this--
that's the idea of the
striped domain phase.
The physical basis of
why it occurs in graphite
is that graphite has
essentially no lattice expansion
whatsoever.
And anything that you
put in any intercalant
will have a normal lattice
expansion, so it will grow.
And as it grows, it wants
to accommodate itself
to a larger lattice constant.
It stays commensurate
for a while.
But afterwards, it decides
that this is no fun
and it goes into this
striped domain phase.
Now, the evidence for this comes
from some very beautiful work
that was done in
Birgeneau's group.
Here's the crystal structure.
Here's the bromine molecules
registered on graphite.
This is the room
temperature commensurate
phase for bromine
molecules per unit cell.
In the direction of this
large part of the unit cell,
seven times the graphite
lattice constant.
That's the direction in
which the striped domain
phase gets established.
Here are the diffraction
spots shown schematically.
The triangles correspond
to the graphite spots.
So between these
two graphite spots,
we divide the unit into seven
equal parts and we put spots--
these spots here are the
spots of the superlattice.
And they're equally spaced for
the case of the commensurate
unit cell.
But after we go above this
temperature, 342 degrees where
we started seeing
this Gaussian line
shape and the Raman
spectrum, these spots
are no longer divisible
by this distance.
But they move in the
direction of the arrows
firstly by little
amounts, but always
in this ratio
given by this code.
Now, the fact that they move
in this prescribed manner
and the incommensurability which
is related to the reciprocal
of the domain size, the fact
that this incommensurability
has a unique temperature
dependence which depends
on the reduced temperature
to the one-half power--
this and the motion of the
spots are an indication
or a signature for the onset
of the stripe domain phase.
Since initiating this
work, Bob has gone on
to do some very elegant work
using synchrotron radiation
where he's followed these
transitions in great detail
and has studied the
melting transition.
And I've always
felt happy that I've
introduced him to
intercalation compounds
because he's done some very
nice work in fundamental science
of 2-dimensional systems as a
result of this introduction.
The magnetic phase transitions
in intercalation compounds
are also extremely interesting.
And the main thing that we'd
wanted to do in this system
is a goal that still
is unaccomplished.
But I'm still hoping
that in a short time
this goal will be realized.
And this is the
observation, some kind
of direct observation
of magnetic vortices
in 2-dimensional systems.
Now we haven't done
that yet but we
have managed to see some very
elegant, very interesting
magnetic phase diagrams.
Many, many magnetic
phases, and making use
of the high magnetic fields.
And I thought that this
would be a good time
to give a plug for the
National Magnet Lab
without whose facilities
this work would not
have been possible.
The system I'd like to talk
about is europium in graphite.
And europium is a 4f
metal that's magnetic.
And the spins in the europium
will couple rather strongly
to the graphite where you have
a lot of conduction electrons.
And depending on the ordering
of the spins in the europium
layers, you get different kind
of scattering in the graphite
layers.
So we could see
magnetic phase changes
by monitoring either the,
say, the magnetization--
that's one thing.
But what I'm going
to show you here
are results on
conductivity measurements.
And this turned out to be a
very sensitive and kind of a fun
experiment.
It's kind of nice for
this kind of presentation
to hopefully some non-experts
in the audience, as well
as experts.
Here, we have
transport properties.
Resistivity as a function
of magnetic field.
And you see glitches.
You see big changes occurring
as a function of magnetic field.
Each of these glitches--
I put a little red dot--
shows a magnetic
phase transition.
This one here in
particular, this
is the z-axis magnetic field
and current directions.
You could see that what
we're plotting here
is different temperatures.
Each scan is taken at
a different temperature
and you can almost see the
magnetic phase diagram pop out
at you just by looking
at the transport data.
Well, taking each
of these temperature
sweeps, and this is
temperature here,
and going up in
temperatures make
dots for all of these
things, and you develop
a magnetic phase diagram.
That's simple enough.
But what is the explanation
of what the spins are doing?
That's a little
bit more difficult.
And there were two things
that were done here.
There was a model calculation
done by Date in Japan.
And we did a Monte
Carlo simulation
which identified what these
various spin states are.
And there was a little
bit of new physics
that came from this.
A little bit unexpected.
In magnetism course and
basic solid state course,
we always teach that the
Heisenberg Hamiltonian that
controls the
magnetic interaction
in a magnetic system
is a quadratic form
between adjacent spins.
And then we go second neighbors,
third neighbors, et cetera.
But with a
Hamiltonian like that,
you get nowhere with
this phase diagram.
You need to have some
higher-order terms,
these bi-quadratic terms and
4-ring spin terms that show up
in a triangular lattice.
These things had been
predicted but there
had been no real observations.
But for this system, in order
to get the ferrimagnetic phase,
the one with the two spins
up and one spin down, it's
necessary to have the terms
in the blue and the red.
And that was a new
piece of physics.
The reason they're so big is
because we have spin a half.
Europium you know is
a half-filled shell,
and that makes it
kind of interesting.
I should point out and
remarked that these experiments
were made use of the
highest fields available.
We went up to the maximum.
We use the hybrid magnet
for these experiments.
And that was kind
of a new experience.
We enjoyed that very much.
Well, superconductivity
is where we
started with the
intercalation compounds,
and that's where I'm going to
finish with the intercalation
compounds so we
have a full circuit.
And the bottom line
about superconductivity
and intercalation compounds
is that intercalation,
or the layering process,
enhances superconductivity.
Now we have two
pieces of evidence
for this that are
compelling to me.
One is that you
can make something
superconducting where the
constituents are neither are
superconducting.
And that's the case of
the alkaline metals.
And here's an example of C a K
stage 1 potassium in graphite.
And that has, oh, pretty
low transition temperature.
But still, in all, potassium
isn't superconducting,
nor is carbon.
Now when you take something
that is superconducting
like potassium mercury,
that's an interesting system.
It has a transition temperature
of about 1 degree Kelvin.
You put it in an intercalation
compound, instead of 1 degree,
you get 1 and 1/2 degrees.
So for systems that are
not superconducting,
you can make them
superconducting.
And systems that have a
low transition temperature,
you can enhance it.
So that's an interesting point.
So superconductivity in
intercalation compounds
is something that is worth
trying to understand.
However, there are things
about it that are unexpected.
This is an anisotropic system.
And you're quite prepared
that the critical field
will be anisotropic.
It should be.
And there's a theory for
anisotropy in critical field,
mean field theory.
And it gives a
prediction for what
the angular dependence
of the critical field
should be in terms of a
parameter HC2 parallel
for theta equals 0 when the
field is along the z-axis,
and HC2 perpendicular
when it's at right angles.
And the problem is that in
the intercalation compounds,
the ratio of these quantities
hC2 perpendicular to parallel
is not independent
of temperature.
It's supposed to be independent
of temperature, but it isn't.
Experimentally, it's
found not to be.
Also, the temperature
dependents of HC2
doesn't follow this
mean field value.
This mean field curve predicts
that HC2 should go in linearly.
Should have a linear
dependence on 1 minus t. t
is the reduced temperature close
to the critical temperature.
But by the time you're
about 6/10 of / little t
equal to 6/10, you should start
seeing a significant deviations
from the linear.
So in these two respects,
the intercalation compounds
show significant departures.
And here's some evidence.
This is Allison
Chaikin's PhD thesis,
or the beginnings of it.
And here is a plot now of HC2
versus angle for a temperature
close to the
critical temperature,
and this ratio here is 8.
Then when you get
closer to the t
equals 0 of reduced
temperature 0.3,
then this ratio turns out
to be significantly larger.
Unexpected from the
mean field model
for anisotopic superconductors.
The temperature dependence
of the critical field
is supposed to go like this.
It's supposed to have
saturated at low temperatures.
But as low in temperatures
we've gone so far,
it seems to be linear.
Data's kind of noisy,
but we're still
going way lower
than what you would
expect for a linear dependence.
So right now we're
headed in two directions.
Going to higher stages to look
for more anisotropy effects.
And hopefully to
get into the region
where the coherence
distance will become
less than a lattice constant.
In that regime, we should
have something approaching
2-dimensional
superconductivity where
we expect rather different
behavior from what
we have in the region here where
the coherence distance is large
compared to the repeat
distance in the unit cell.
Now this field has become
of course, very interesting
very recently because
of the discovery
of the high-temperature
superconductors which,
of course, are layered compounds
and have some similarity
to intercalation
compounds because we
have copper oxide layers here
and copper oxide layers here.
And then we have spacer layers.
The spacer layers are different
things for different compounds.
I've shown here a picture
of a lanthanum barium series
because I know what the crystal
structure is for that one.
For the 98 degree one, I've seen
different crystal structures.
And I'm not exactly sure.
People don't exactly
agree on what
the crystal structures are.
But what they all agree on is
that there are spacer layers.
Well, the spacer
layers are a lot
like intercalation compounds.
Because here we have
conduction electrons.
That's the graphite.
And here we have something else.
And what is the connection?
I don't know.
But there are other people
that think that there's
some connection here.
Because when Alison was talking
about this at the APS meeting
at the intercalation session
just before her talk started.
They had a room that
was about like this.
A lot of empty seats.
And then when her talk
was about to start,
people came in through the door.
And when her talk was going
on, the place was halfway--
was quite filled.
And as soon as
her talk was over,
half the audience got
up and left again.
So there are people
out there that
think that intercalation
compounds might have something
to do with the layering business
in the high-TC superconductors.
Well, from a pedagogical
point of view,
I think that the advent of
high-TC superconductivity
is a great boon for people
in materials research.
So many of the big discoveries
in the last decade or so
have been inaccessible
to students.
When MBE came in, everybody
knew this was very important
but almost nobody
had the finances,
either the money
to buy the systems
or the technical
support staff to do it.
When the scanning
tunneling microscope came,
it seemed like that also would
need some very, very special
engineering to make that go.
Of course, in a short
time, we found out
that that was
accessible, and now we
have scanning tunneling
microscopes all over the place.
But many fields of
science are quite
inaccessible to university
people because of the costs.
But this is different
because most anybody seems
to be able to make samples.
There are samples
all over the place
and there are all kinds
of measurements that
are interesting and exciting.
So this seems like a good
field to get students excited
about solid state physics.
Of course, you know that there
are a hundred other people
out there doing the same
experiment so you better
be doing something else
for your thesis work.
But you can participate in
the excitement for a while,
and that's a lot of fun.
Now here's a change of area.
But to me, it's
just a continuum.
There are very many things
that we would like to make.
Many kinds of
intercalation compounds
that we'd like to
make, but we don't
know how to synthesize them.
If we could just put in
any old, any arbitrary
metal in graphite, or, say, with
these high-TC superconductors,
we can just stuff in
a copper oxide layer.
If we knew how to do
any of these things
and put them between
graphite layers,
we could really do some
very exciting science.
And some years
ago I had the idea
that if we implanted
graphite with something
and then subsequently
intercalated,
that we might have some chance
of getting things in that
wouldn't go by themselves.
Why would that be?
Well, if you implant
something, you
know that with an
eye and a planner,
you can plant any element
in the periodic table.
So once you get these elements
between graphite layers,
it opens up the graphite layers.
And then once the graphite
layers are opened up,
then subsequent addition
of an intercalant species
should be easy.
Well, we tried this idea
and it sort of works.
But I think this technology
will take a lot of doing
and we haven't pursued it to
the extent that we should.
Now the idea is so we
intercalate graphite
with some species that
either doesn't interpolate
at all or intercalates poorly.
So here's a test sample
that we implanted here.
And this is a
protection aluminum foil
so that there's no
ions going over here.
And so we have the
reference control specimen.
And here is the sample,
implanted sample.
And what we do then
is we take the sample
and put it into our ampoule
and we start an intercalation.
And we see if there is
an enhanced intercalation
in the region that's
been implanted.
So we have a control sample.
Well, I guess is
the whole thing.
So we have the
control sample here
that was prepared at the same
time, the same intercalation
as this one, that half
of it wasn't planted.
And then we looked with Raman
spectroscopy on this side.
We have a microprobe
so we could look very
close to the implanted region.
And we do get an enhancement.
This is the
graphite-bounding layer.
So it shows that we've made
it a lower-stage compound
in this case, for the implanted
case, than for the other case.
Well, we've tried
this a little further,
but the results have not been
as encouraging as I would like.
However, I think
it's a good idea.
I would like to encourage
other people to work on it.
If we had more time, I guess we
would do more on it ourselves.
Last time, I talked about
the use of ion implantation
as a way to study regrowth
in anisotopic materials.
And I won't bore
you with this again
except to say that in so
doing, we used annealing
as a method of recovery.
So we put it in the ions.
We make this awful mess.
We introduce a lot of damage.
And then we anneal the sample
and we anneal out the mess
and we try to regrow the
system that we started with.
And in so doing, we measure
both the temperature dependents
and the time dependents.
The temperature dependents
gives us the activation energy
and the time dependents gives
us information on the kinetics.
Well while we were doing this,
we had the idea, gee whiz.
Instead of using
furnaces, why don't we
try using laser annealing?
And so we tried a
graphite sample.
And part of the laser
pulse irradiated the sample
with laser pulse
radiation with the idea
that we would affect
some annealing.
Well of course, the
opposite happened.
It didn't make annealing at
all but made more disorder.
But when we thought
about the disorder,
we found out that we stumbled
on a very good problem,
and that is the laser
melting of graphite.
And that's my last topic.
And I'll just say a
few things about that
because I think that's been
one of the most exciting things
that we've been doing
with graphite recently.
And it ends up with a big
controversy in the field.
And it's always nice to end the
talk with a big controversy.
Well, the idea of
the laser melting--
I mentioned last
time that graphite
is the highest melting
temperature material in nature.
It melts at about 4,450 Kelvin--
a very high temperature indeed.
So you can't put the
graphite in a crucible
to melt it because the crucible
will melt before the graphite.
So you have to melt the
graphite in its own crucible.
So here's graphite.
And we put a laser
beam in and we're
going to make a little
puddle of graphite
inside a graphite wafer.
So here's our laser pulse.
What we use--
30 nanosecond pulses.
We have a ruby laser.
And the energies--
I don't have even have
the photon energy there.
The skin depth falls off
with the usual skin depth
of a few hundred angstroms.
And once we melt
the graphite then
we have the melt temperature
here for a little distance.
And here's the melt front.
And as we increase
the laser energy,
the melt front moves
into the sample
and the laser pulse
is turned off.
The melt front recedes,
goes back to the surface,
and we get regrowth.
So that's basically what the
experiment is that we did.
And where do we sit on the
graphite phase diagram?
Here's the graphite
phase diagram.
A very historical phase diagram.
The phase diagram of
Bundy that has to do
with the formation of diamonds.
And so here is the
diamond phase that
was what the phase
diagram was concocted for.
But we're over here
at a very low pressure
but high temperature, and
we're going to melt carbon.
At least we think we're
going to melt carbon.
And we did four different
experiments initially
to show that you could
make liquid carbon.
One of them is
Rutherford backscattering
in the channeling mode.
Then we put in ion-implanted
markers, Raman scattering,
and transmission
electron microscopy.
None of these
experiments by themselves
were especially convincing that
liquid carbon had been formed.
But taken altogether,
there was no way
that we could explain all the
results without the formation
of liquid carbon.
The collaborators in this work--
we had a bunch of MIT people
and three people from Bell Labs.
[INAUDIBLE],,
Jacobson, and Gibson.
This experiment would
not have been possible
without the collaboration
of Bell Labs
and their wonderful facilities.
And I wanted to make one
philosophical comment
about where I think materials
research in the future
is going-- that we'll have
more and more of these kinds
of collaborations because
the cost of equipment
is going up and up and becoming
less and less accessible
for university-type people.
On the other side, from
the industrial standpoint,
people in industry are
getting more and more
pushed for fast results and
relevance of their research.
So they are getting more
interested in having university
people do far-out things.
So I see this as a future
trend in our field.
A future trend, and I think
maybe a welcome trend.
I don't think it's
a bad trend at all.
I think it's maybe a good trip.
Well, here are the
four experiments
about melting carbon.
This is the thesis
of John Steinbeck.
The first experiment
is Rutherford
backscattering channeling.
If the helium ions that we're
using to probe the carbon--
they have a high energy; 2 MeV--
If they're directed
along the z-axis,
then they don't see the
ions very effectively
and they'll miss the
ions to some degree.
And the scattering intensity,
the backscattering,
will be at this low level
here that I've indicated here
by the word channeling.
However, when the beam is
tipped away from the z-axis
by even a fraction of
a degree, then there'll
be copious scattering
of the carbon atoms
and we'll be up in this
range-- the random range.
Now if we introduce
disorder in the crystal--
that is, we don't have a
very well-defined z-axis
and it's off by a bit--
then we'll go from
this channeling
level to the random level.
Now there's also
a depth dependence
that Rutherford backscattering
is sensitive to.
Why is that?
The helium atoms that are
scattered from the surface
atoms will come back
with the maximum energy,
but those that get
scattered from a carbon
atoms at an anterior position
will come back with less energy
because they lose energy
coming to the surface.
So that the energy
of the helium atoms,
once they come out
of the graphite,
is an indication of the depth
from which they were scattered.
So we have a depth scale
that we show on the top.
Okay.
That's all the
background that you need
to understand the experiment.
Now we zap the graphite with
a laser at different levels.
And here are the
different levels.
And it's key-coded
on the curves.
Initially we stay at the
channeling direction.
But as the laser energy
increases this level,
we get some disorder scattering.
And the disorder
scattering increases
as the laser intensity
increases and it increases
and it increases.
Here, by the time we're 1.2
joules per square centimeter,
we have a sharp interface and
we get almost a random situation
at the surface.
That sharp interface moves
as we go inside the crystal.
The sharp interface
suggested that we
had melting but we
didn't really believe it,
so we tried the next
experiment which is a marker.
Now here's a marker of arsenic.
We're still going to do
Rutherford backscattering.
But now we'll do backscattering
from the arsenic atoms.
Arsenic, being
heavier than carbon,
will deliver
backscattered helium atoms
at a much higher energy.
So that's separated
from the energy
that we get from the
scattering of the carbon atoms.
When the laser energy
is 0 or very small,
we get this profile from the
Rutherford backscattering.
As a laser energy
increases, you could
think of the meltfront
moving in from the surface
inside the carbon region.
Still far from the
arsenic implant.
So when we're over
here, we still
retain essentially
the same arsenic line.
But as the meltfront
approaches that of the arsenic,
the arsenic joins in the puddle
and segregates to the surface,
and we see that effect here.
Much of the arsenic
leaves as it melts,
and the rest begins to
segregate to the surface.
So that experiment is
highly suggestive of having
molten carbon.
The Raman experiment
similarly can be explained
in terms of melting carbon.
And so here is 0 before we
start the laser radiation.
And after some laser
irradiation, in addition to
the Raman line, we get
a disorder-induced line.
And the line widths
get kind of broad,
indicating we have a
rather disordered system.
But as the laser energy
density increases,
instead of getting more
disorder, we get less disorder.
The lines sharpen up and
we get a spectrum that's
closer to that of the graphite.
Well, that's explained here
in this little picture.
Where initially we
make little, tiny--
the material is molten
for very, very small time,
the rapid solidification forms
very small little crystallites.
And the little crystallites
are in all random directions.
So you get a rather
disordered system.
But if the molten
region is allowed
to stay molten for a
longer period of time,
the little crystallites grow.
And in fact, some fraction
of these crystallites
might be oriented normal to
the original crystallites,
so we get some interesting
results from that.
So the Raman scattering
experiment also favors--
not compelling, but it's hard
to explain it by any other means
than we formed molten carbon.
The transmission electron
microscopy experiments
likewise can be explained in
terms of the melting of carbon.
Because for the high
laser energy densities,
we form a ring at
the 002 position,
indicating that we form
crystallites at right angles
to the direction of
the original carbon.
How could that form if we
didn't have molten carbon?
So that's fairly
conclusive evidence
that we formed molten carbon.
Now to explain the depth
of disorder quantitatively
as a function of the
amount of laser energy put
into the system, John
Steinbeck has formed a model.
He's solved the diffusion
equation, the heat equation,
with a driving term having to
do with the laser energy that's
put in.
And you have to take a lot
of effects into account.
It's not only
melting, but it's also
vaporization of the surface
because the vaporization
point of carbon is very
close to the melting point.
And when all of this is
done, we took two limits
into consideration.
One is that the liquid
carbon is a Ziman liquid,
having 4 electrons per atom.
So that's a metallic
form of carbon.
And another form was a
semi-insulating forum
where the constants
of the liquid form
were just like that of the
solid extrapolated continuously
across the phase transition.
Well, it's clear that
the metallic carbon
fits the results a whole lot
better than the solid extension
model, which is that
of an insulator.
However, we're not
the only people
that are working in this field.
When we came up
with these results,
some of our competitors up the
river who are doing experiments
like this in the
picosecond range
believe firmly that liquid
carbon is insulating.
In the interim, we've both
done a series of experiments.
Our experiments seem to say
that liquid carbon is metallic.
Their experiments seem to
say that liquid carbon is
insulating.
Well, two groups with different
results is interesting.
And that, of course, attracts
a few people into the foray,
so now there are six groups
that are working on this.
And the score is
pretty much even.
We have ourselves
with the Bell people--
these are the metallic types.
Bundy from GE, the man that
made diamonds to begin with,
he made liquid carbon also
and he thinks it's conducting.
And the GM people have recently
done experiments and wires--
they think it's conducting also.
But the Harvard people
have been joined
by a group at Los Alamos
and a group at Maryland.
And so we have
three against three.
Half think that it's
conducting, half
think that it's insulating.
Well, life is always interesting
when there's a controversy.
And this is a
friendly controversy.
We all talk to each other.
We all help each other.
But we don't really
know what the answer is.
Now, the coward's way out is
to look at the phase diagrams.
And here are a whole bunch
of phase diagrams for carbon.
And this gives you an idea of
what a mess the field is in.
These are all pretty recent.
Most of them are pretty recent.
And they're all very different.
And if you look at them, you see
that there are some people that
predict metallic liquid and
there's some people that
have insulating liquid.
And then they have even
two kinds of liquids.
So a cowardly way out is to
say that there are really
two liquid carbons, a
conducting and insulating one.
But I think that none of the
six groups working in the field
really believe this.
I think down deep we
feel that there's only
one kind of liquid
carbon in the range
that we're all looking at,
and we're not really sure
whether it's conducting
or insulating.
Well, these are some adventures
in the field of carbon science.
And you see that not
the whole story is told.
There's still many
untold stories
that will keep generations
of students busy.
When a student turns
in a PhD thesis at MIT,
there are two things that
we look for, I think,
from a philosophical sense.
That the student has
broken new ground.
That this is something,
a new field, a new area,
something that's totally
different from what
went before.
And if it's so, it usually leads
to more work than we started.
And that's what keeps
science going forward.
[APPLAUSE]
Questions.
Yes.
AUDIENCE: Is it possible
to laser anneal diamond?
MILDRED DRESSELHAUS: Is it
possible to laser anneal
diamond?
Yes.
AUDIENCE: --convert
it to graphite.
MILDRED DRESSELHAUS: Yes.
It does.
It converts.
It actually converts
it into a mixed phase.
That experiment was
done a few months
after we did our experiment.
But it was published in
a geological journal,
so it took us a while to
find out about each other.
And it was done by accident.
It was done under high pressure
in a diamond anvil cell.
And they zapped a sample
inside the diamond anvil cell
and they melted the carbon
and it became graphite.
And it was an unhappy a
thing for the diamond cell,
but a good thing for science.
And they thought that they
had discovered liquid carbon
for the first time.
So they were very
excited about it.
AUDIENCE: Usually, we
think that light doesn't
get absorbed by diamonds.
MILDRED DRESSELHAUS:
Well, diamond
has some small impurities.
Diamond-- it's very
difficult to make diamond
without any impurities.
Now actually, diamond doesn't
have enough impurities
for our taste,
because we're trying
to do these experiments
by implanting impurities
into diamond to make it
absorbing enough to do
some melting experiments.
So for our likes, we would
like it to be more absorbing.
But diamond does have
native impurities
that do absorb some light.
And that's what did it.
AUDIENCE: [INAUDIBLE]
MILDRED DRESSELHAUS:
Well, you would
think that the diamonds
they use for anvil cells
would be pretty good, actually.
Because you want
them to be strong.
But they still will
absorb to some degree.
I mean, these are pretty
intense laser beams.
Yes.
AUDIENCE: Yeah.
In that presentation, you
said that the [INAUDIBLE]
for lattices because they
tend to contract, they expand.
So they make it more technical.
So they have different layers.
But what I think
is in this process,
there is some changes
in the pervasive force
inside those layers.
Is there any more that has been
done for [INAUDIBLE] forces.
Changes in the
[INAUDIBLE] forces
to match these two layers?
Because as we know
that Raman [INAUDIBLE]
is sensitive to the
changes in a core constant.
MILDRED DRESSELHAUS: Yes.
Obviously the layers are
going to be straight.
And if the layers
get thick enough,
that you can't make the
straight layers superlattice.
People have-- I don't know
that anybody has done a Raman
experiment because I guess
the straight isn't totally
uniform through
the entire layer.
Although the layer has
to-- the lattice constant
has to be preserved.
Otherwise the whole
thing would crack.
So I guess the
whole thing would be
under some kind
of intense strain
the thicker the layer gets.
I don't know that
anybody has done
Raman scattering from the
strain layer superlattice.
But I would imagine that that's
certainly a doable experiment.
The experiment
probably has been done
and one can get a measure
of the force constant.
But as long as the
thing stays together,
then you're just
going to measure
the force constant for
the equilibrium situation.
So that's hardly what
you're looking for. .
AUDIENCE: But what about
the interface layer?
There will be some tension or--
MILDRED DRESSELHAUS:
That's right.
There is a tension at
the interface layer,
but it does stay together.
However, I'm told
that in time if you
keep these strain layer
superlattices around
for a year, or 10 years,
for some length of time,
a long time, then
they come apart.
They don't have a
long-term stability.
But maybe the ones that have
been made in more recent times
do stay together.
You know that the strain layer
superlattice has an old history
that the first few years of
the strain layer superlattice,
it didn't work at all.
This is a fellow at
IBM was doing it yet
and nobody paid much
attention to him.
But he not only did it, but he
worked out the theory for it.
And the credit has
gone to other people
who made it work more
effectively later on.
AUDIENCE: Probably
all of this work
that's done starting with
a solid and melting it.
MILDRED DRESSELHAUS: Yes.
AUDIENCE: The opposite would be
to start with paper and pencil
to see what you might get
[INAUDIBLE] structure.
MILDRED DRESSELHAUS: Yes,
that's an interesting thing
and a little difficult
to do on the Earth.
But maybe in an astrophysical
entities that's a possibility.
You know that liquid
carbon is something,
since we'd gotten
into the field,
we found out that this is
a topic of great interest
to astrophysicists
and to geologists
and of all kinds of people
that are not solid state types.
AUDIENCE: Do you know the
boiling point of graphite?
MILDRED DRESSELHAUS:
Yeah, it's not
much above the melting point.
It's about 4,700k.
So we're very close.
It's very different from the
other group 4 semiconductors--
well, carbon, because I'm
thinking of it as a group 4
material--
where the difference between the
boiling point and the melting
point is a very large
temperature difference.
But in this case,
it's very close.
It makes the properties
of liquid carbon
kind of interesting.
Well, I think we're finished.
And we'll--
[APPLAUSE]
