Good evening everybody. Welcome to this
evening. I'm Andrew Boothroyd, I'm a professor of physics here and I'm going
to tell you a little bit about some rather unremarkable looking materials
but which when when you cool them down to low temperatures possess some very
remarkable properties including complete loss of electrical resistance and the
ability to levitate indefinitely and I'm talking of course about superconductors
and what I hope to do this evening is to say a little bit about what electrical
resistance is and I'll give a little bit of history of superconductivity of
which Oxford - this department - has played a vital role. I'll say a little bit about
what superconductors do in magnetic fields, which is important for
applications, and I'll talk about quantum coherence which is something you've
heard about elsewhere and say something about the applications of
superconductivity. So that's the the recipe. Let me start off by talking a
bit about electrical resistance. So what is electrical resistance? Well, we know
that electrical current is carried by electrons, which flow through a metal and
if you were paying attention in your GCSE physics you will know that the flow
of current is controlled by Ohm's law, which tells you that the current is
proportional to the voltage and the constant of proportionality is
resistance. Our understanding of resistance, at least at some level, is
that you imagine that the electrons are just passing through the metal and every
time they encounter some object they they scatter they scatter randomly
and they form this kind of pinball path through the material. So I like to think
of it a bit - this is a bit like if you're at a party and you're trying to get from
one end of the party to the bar and there's all these people dancing away
and you've got to keep on bumping into people and working your way around
people - this is a bit like what electric electrons do in solids -
in metals. Alright, now resistance is something which changes with temperature
and this graph on the right here shows the resistance of copper, a metal that's
used in many wires and conductors, as a function of temperature. And you can see
that the resistance just drops smoothly as you reduce the temperature. This is in
units of - this an absolute units of Kelvin, so in these units room
temperature is actually somewhere near to this end of the graph, and this is the
absolute zero of temperature. People were actually very interested in the
early part of the 20th century in what actually really happens as you approach
the lowest possible temperatures down here. Notice that - so actually what
I'm plotting here is not resistance but resistivity, which is a measure of the
intrinsic resistance of a material and is independent of the shape of the
sample that you make, so it's always the same for raw materials. The resistivity
is just a measure of the intrinsic resistance. And notice - you probably can't
read it, but this is in units of 10 to the minus 9 I think, so just park
that number in your mind for later - 10 to the minus 9 Ohm meters is the size of
resistance in copper. So people are very interested in what happens as you cool
metals down to low temperatures, and some people said oh well what happens is that
the the atoms stop vibrating because everything's very cold and so that will
let the resistance - let the metal have a low resistance because the electrons can
just travel without colliding so much. And other people said: oh but at very low
temperatures the electrons won't have any energy so in fact what will happen
is they'll stop moving and the resistance will just rise up at low
temperatures. So there was just no understanding, and at that time the
person who was pioneering refrigeration technology was this man here: Heike
Kamerlingh Onnes in Leiden. This is a picture of his laboratory. It looks a
little bit like what our laboratories looked like today - full of wires and
pipes and tubes and things like that, but this was his refrigeration apparatus to
cool down to very low temperatures. And his achievement actually was to be able
to cool down to the temperature at which helium liquefies and for that he was
awarded the Nobel Prize in 1913. So it's a very impressive
achievement. Now one of the first things he did with his liquefier was to measure
the resistance of mercury as a function of temperature and these are
his original measurements that he made. So you can see these are the points, and as
he cooled down - I don't know whether you can see this scale, but this
says 4.4, 4.3, 4.2 - these are in units of Kelvin. As he cooled down his mercury
sample in his liquefier eventually all of a sudden he found this of a sudden
drop from this value to essentially something which was close to zero on his
scale. Then at lower temperatures he could not measure the
resistance and you can see here is written by hand 10 to the minus 5 Ohms.
That was the lowest he could measure the resistance of mercury with his
apparatus. So it was like an upper limit. And actually, when he first saw this he
thought there must be some problem with the apparatus because actually 4.2, which
is this axis temperature here, this is exactly the temperature at which helium
liquefies. So he assumed that this was just a spurious effect due to the
liquefying of helium. However he repeated this measurement and got the
same result and then he tried with different metals and found that they
also had this very sharp drop. Tin occurred at a bit low temperature. Lead
occurred at a bit higher temperature. So this seemed to be a common property of
the metals that he was measuring. It wasn't just an artifact of the apparatus.
So in fact it was just really bad luck in a way that his first measurement
produced a material that had this sharp drop at exactly the same temperature as
the liquefying point of helium. And he didn't know what was causing this, but he
coined the term superconductivity. obviously the resistance is very low
that means that conduction is very high and so in this in the case of mercury he
said that the transition temperature to this whatever was happening here was at
4.2 Kelvin Oh this is a picture of honest high key
conic accounting honors and his wife Maria this is one of their house parties
and he's got some of his physicist friends here and you can see that in
those days what they used to do at parties is they used to talk about
physics as a blackboard here whether they've been discussing physics and I
don't know can anybody recognize any famous people on this on this picture
yes of course we have Einstein here so so honest was he moved in you know in
high circles in physics he is very well known chap right so honest realized or
measured that the resistance of mercury and other metals went to very very low
values but nobody really knew how low it was and there were many experiments
where people tried to measure how know the resistance really was after the
metal went through this sharp superconducting transition and one of
the most celebrated experiments that was done I was in it was done in 60s by fire
and mills and what they did was they made a ring out of a superconducting
material they cooled it down to very low temperature so that became
superconducting so it had a very low resistance and they passed a magnet
through this ring and this will induce a current in the ring just just like the
way a dynamo works you pass a magnet through a coil you get a current flowing
in it so after the current had been induced the current has associated with
it a magnetic field and what filer Mills did was they measured this magnetic
field strength as a function of time and in fact they continued the experiment
for several months and continued measuring this magnetic field strength
as it decayed over time and in fact decayed extremely slowly
solet's so slowly in fact that they couldn't really detect a reduction in
the current over of timescale of several months and the conclusion from their
work was in fact that the current in this loop would persist for 100,000
years if they let it to continue going for that long which of course not very
practical so from that from that value fact they they could calculate that the
resistance of their loop here was or the resistivity I should say was not greater
than 10 to the minus 23 Oh meters
remember I said before that the resistivity of copper is 10 to the minus
9 ohm meters around room temperature so this is 14 orders of magnitude lower
resistance than the resistance of copper and we now believe in fact that the in a
superconducting state there really is zero resistance so that this current
here is the nearest thing we have on earth to perpetual motion very good so
the next development which occurred in this field was by these chaps my snow
and oxygen fell this is Meisner he was a professor in Leipzig and this is his PhD
student at the time oxen felt and what they did was they did
an experiment where they cooled a metal in a magnetic field and if it's just an
ordinary metal like copper then as you cool it down nothing really very much
happens the magnetic field is not disturbed by the the copper sample at
all and it's all very uneventful but if you cool a superconductor down in a
magnetic field at high temperatures it's still just behaving like a metal but at
low temperatures when it goes superconducting remarkably what happens
is that the superconducting material just expels excludes the flux and expels
it actively from the interior of the metal so literally the flux is pushed
out of the metal like this it's as if the superconductor develops an opposite
pointing magnetic field that just cancels out within the volume of the
superconductor this is very much remarkable and unexpected and it's a
behavior that cannot be described by classical physics at the time so it's a
quantum mechanical phenomenon and the story goes that when this when this
happened oxygen valve was in the laboratory making the measurements
rushed into my sinners office and said hey Meisner I have just discovered that
the superconductor is excluding the flux and my sir said wunderbar you've just
discovered the Meissner effect so I like to call it the Meisner ox and
felt effect but in textbooks it's usually called the Meisner effect which
i think is a bit uncharitable and at this point though there was no there was
no theories of superconductivity that described superconductors in any
adequate way and many people had tried and this this is one man who tried felix
bloch he was a PhD student and then a postdoc working with Heisenberg and he
tried for a whole year did nothing else but tried to develop a theory of
superconductivity in the early sort of 1930s he was working late 1920s and he
made one good theorem of superconductivity which is correct and
and is still known today but he actually made a second sort of panicked
tongue-in-cheek theorem of superconductivity which became known as
blocks second theorem of superconductivity and he made this as a
way of as a statement to describe his conclusions from this one year's futile
attempts to make a theory what he said was the only theorem about
superconductors that can be proved is that any theory of superconductivity can
is refutable so essentially nothing works so this this was his this was
frustrated mr. block all right this is a bit of an Oxford now so you can pay
attention here this these are two brothers called the
London brothers and this is Fritz and this is Heinz and these they were German
Jewish physicists in the early 1930s who who had to escape Germany to escape the
persecution of the Nazis and like many Jewish scientists at that time they
moved around them they moved to other parts of the world quite a few came to
the UK quite a few came to Oxford brought by Linda Linderman who was a
director of the clown dinner poetry at the time and the London brothers so they
were so Heinz was a PhD student here and Fritz was a postdoc here in the
department and while they were here for a relatively short time they worked in a
upstairs room in headington on the theory of that they tried to explain
this this phenomena called the Meissner and felt effect they tried to understand
and they worked on it very hard and eventually they got the theory that
worked they basically the theory is a sort of a
quantum mechanical equipment of Ohm's law that applies to superconductors in
the magnetic field and they realized from their theory that for it to
correctly describe this flux exclusion effect what must happen is that all the
electrons in the superconductor must be in exactly the same state of motion yes
this is a consequence this is the only consequence that can lead to this
phenomena here so this was this was a real breakthrough although other time it
wasn't particularly recognized as a breakthrough it has been particularly
important and as is the way the money ran out and so both of them had to be
let go by the department and hindsight she went to Bristol University had a
successful career in Bristol Fritz went to Paris and then went to North Carolina
Duke University in North Carolina and the story goes that Fritz because of his
German passport he wasn't allowed to travel on the the ship that he planned
to travel on he had to delay by by one sailing and as they went on the next
sailing and then it turned out that the previous sailing was torpedoed by the
Germans with great loss of life so he was actually a very lucky man sometimes
you have to be lucky in physics so I'm still charting the historical
development of superconductivity and a really really important breakthrough was
made by these three gentlemen bardeen cooper and schrieffer in the late 1950s
Cooper in fact realized that electrons in in Medan metals in fact attract one
another which sounds a bit counterintuitive because they're both
because electrons are negatively charged but actually they exist in a a
background of positive charge which so essentially the whole metal is neutral
and it turns out that Cooper showed Cooper was able to show that any small
interaction between the electrons would cause the electrons to want to pair up
to form a paired state which would have a lower energy and if the electrons
remained unpaired and just randomly moving around so these are called Cooper
pairs now and I like to think of it this rather like
this the lattice in which the electrons move it's a little bit like a mattress
and if two people sleep on a mattress close together then what happens is the
mattress deforms and attracts the people together on the other hand if the two
people lie on the mattress too far apart then the deformation of the mattress
does not tend to attract attract the people so this is a bit like this is a
bit like what causes the attraction between electrons in the superconductor
there's a defamation of the lattice which is actually a dynamic defamation
which causes an effect of attraction between the electrons and makes them
want to pair up like this and if you want a sort of more real space picture
what's going on here's the crystal lattice with the positive ions the
electrons are moving through this lattice is one particular electron
because it's negatively charged it attracts the atoms towards it like this
then it moves on and then another atom seeing a positively charged region will
actually attracted to that region and so in effect these two electrons have kind
of talked to each other and in a set in effect what happens is as the electron
moves through the metal it leaves behind a wake of positive charge which is like
a transient charging of the metal and then another electron will also leave a
wake like that and so you can see that as the electrons are attracted towards
the positive charge you can see that there's a sort of net tendency for that
funds to be bound together like that so this is a sort of hand waving picture
for why you get these so-called Cooper pairs superconductors and in essence the
BCS theory is founded upon the notion that electrons pair up in this way and a
second a second thing that they do is they form what's called the macroscopic
quantum coherence state so I want to just try to explain in very simple terms
what a macroscopic quantum coherence state is and for this we have to
appreciate that all particles according to quantum mechanics behave in some
circumstances like as if they have waves like as if they're waves and we all know
what waves are if you have a collection of waves like this which have got no
particular relationship to one then you sort of add them all together
what you end up with is kind of bunches of waves which I'd rather which
oscillate for a bit and then they decay away so you have little sort of packets
of waves but but with no particular regularity to the the whole so this is a
bit like the picture we would have for waves on water if you look at the
surface of water you can sort of see waves but after a little time they
eventually decay away and become rather jumbled up again and there's no kind of
regular behavior you know you know in the surface like this another another
example is the light that comes from ordinary lamps like this actually
consists of packets wave packets which are very very short in duration but but
have no special relationship to one another so it looks a bit like this so
this is situation that you this is that where we not how we normally encounter
waves in real life but there's another type of situation that can arise which
is where you have what's called the coherent wave and this occurs if all if
you start with a whole bundle of ways all of which have the same wave length
and where the maxima and the minima of the waves are all lined up like this
then if you add these together they reinforce one another constructive
interference and give you one big way with a large amplitude and which will
extend you know over a large distance so this is this is a coherent wave and this
is the kind of this is that this is a coherent wave is the kind of wave that
you get in laser light so this pointer here consists of a long stream of
photons or light waves which is coherent over very very long distances and times
now in a superconductor we have electron waves and when the super conductor is
just acting as a normal metal or it's just a normal metal then then we get
this situation where we have just short waves in this so-called incoherent
mixture like this whereas when the super conductor becomes superconducting when
it loses its resistance then what happens is the electron waves all can
organize themselves so that the phases and the wavelengths are all match to
match together and and form what's known as this cuckoo this coherent
microscopically coherent state like this so like the laser light and
it's the reason why they do this is subtle the reason why it's favorable
favorable for them to to lock all their phases together like that is rather
subtle but I think by analogy in the same as my kind of party analogy where
normal resistance is a bit like trying to get to the bar in a party with lots
of people that are in the way the other other thing that you could do is you
could all agree in the party in the room that everybody is going to step sideways
together and keep on doing that and if you do that and you can see that you can
get from one end of the room to the other without bumping into anybody so it
requires a degree of kind of corporation organization and that's that's
essentially what the electrons can do when they're going to superconducting
phase so I want to talk a little bit about magnetic levitation which is the
other property that superconductors have and for this I want to to demonstrate a
little bit this property this is a piece of well it has a piece of black ceramic
which is actually a superconducting material when we cool it down so I want
to I want to just demonstrate that so I'm going to cool this superconductor
down with a little bit of liquid nitrogen my super spoon and I've got another one here which is
the same but it's wrapped up in arm like a foam that's just to insulate it a bit going to cool that one down as well it
just takes a little time to just take all the heat out of the superconductor
and you can you can tell once it's cooled down because it stops it stops
bubbling vigorously so that one's nearly there this one is not okay so with this
one what I want to do is I've got a little steel plate here which contains
which has got some strong magnets which are just are laid on the surface like
this and they create a magnetic field and what I want to do is to just show
you what happens when we when we put the superconductor on on the magnet so you
see that that's floating and then after rather a few seconds if it loses all of
its super conductivity because it warms up above the transition temperature so I try that again
oops so this rather unstable as you can see you can see it's it's floating it
then it just warms up and then it goes it goes normal like that so this is an
example where the superconductor is actually behaving a bit like a magnet
and it's just repelling it's as if the superconductor has a magnetic pole and
it's just repelling the magnetic poles here light poles repel it's just it's
just floating above so this one here is a bit easier to play with because of the
insulation it lasts a bit longer so what I've got here is a number of
these magnets laid on a track like this and I've just cooled it down with a
little bit of space between the track and the superconductor itself and you
can see that now the superconductor is actually trapped onto the magnet so the
magnets on this there's a run a row of three magnets north south and north like
that and that the superconductor actually trapped on that magnet because
the magnetic field is stopped it from moving and it's actually quite it's
quite well trapped for example you can turn it sideways like this you should
even be able to times upside down we're just warmed up too far then I'll just
try that again so that's unusual right do you think
that's unusual why is it doing this because it's behaving as if it's
behaving almost as if it's both repelling the magnet and also when it's
underneath it's attracting the magnet so no conventional magnet can can do that
so what's going on here well what's happening is that when the when the
superconductor is a normal metal just behaving like an ordinary metal the
magnetic field lines just go all the way through the metal as if nothing happens
and it would drop and that's what happened the first time because it
wasn't sufficiently cold it just dropped but if we place the superconductor on
top of the magnetic the magnet as shown here you can see the magnetic field line
sort of distort and they get squashed like this and this provides an upward
force which holds the superconductor up and it's doing this because the magnetic
field cannot get inside the superconductor these waves are supposed
to represent those coherent electron waves these these lines yet there's no
magnetic field inside the superconductor now you can see that actually some of
these magnetic field lines actually go above the superconductor that's because
I cool as I cooled it on here some of the magnetic field lines will actually
go over the top of the superconductor like that so when you turn the
superconductor upside down what will happen is that these magnetic field
lines will still be going around the end of the superconductor
but the superconductor cannot drop because these lines cannot pass into the
superconductor they're forced to stay underneath and this can provide enough
force if you do it properly to hold the superconductor in place so this is why
it can behave both as a like as a attractive and as a repulsive magnet and
you can imagine that this this phenomenon could have applications in in
real life because you could you could make a track
a train track out of these magnets and you could put blocks of superconductors
on the underside of the train and levitate it and because there's no
friction or sound for that matter are the only resistance is the air
resistance this would be a very efficient way to travel along very fast
and with little energy and in fact these so-called mad left's have been in
existence for a long time but the technologies up till now has been based
on non superconducting magnets so it's really it requires a lot of energy then
to make them work but there are demonstration mag lifts which are made
precisely on this principle there's one in China and you know maybe this will be
one of the technologies of the future I have to finish shortly but I just wants
to say something about how this field is moving of course for this technology to
be practical we don't want to have to keep on cooling things down with the
liquid nitrogen we want things to work at room temperature and the evolution of
superconducting materials as a function of time has had a rather erratic and
unpredictable history so this was the first superconductor discovered by
honors in in 1908 and this had a transition temperature of 4.2 Kelvin if
you remember and then he discovered lead which is 7 Kelvin and other people
discovered niobium and so on and so forth and gradually people discovered
metals or alloys with higher and higher superconducting transition temperatures
a big breakthrough was made in 1986 when Ed Norton Mulla discovered this material
which I've been showing you today which is a ceramic based on copper and oxygen
with various other elements and this actually shot the superconducting
transition temperature up to well above 100 Kelvin in fact that the highest is
about 150 Kelvin but you have to pressurize it to get to that that kind
of temperature and there's also been another family discovered in the last 10
years based on iron which is which has reached quite high temperatures around
about 50 or 60 Kelvin and even actually three years ago the
this field is moving on and even three years ago a group in Germany discovered
and you won't believe this they discovered a superconductor based on
hydrogen sulfide yeah stink bomb where if you press pressurize
it to unimaginably high pressures these are so geological pressures 150 Giga
Pascal's that's a hundred and that's at that time like a million more than a
million atmospheric pressures I think then what they found was they got a
superconducting material which which worked at just below 200 Kelvin so that
that's that's the highest that's a world record for the highest temperature that
we have at the moment and on this graph there's actually two important
temperatures one of them is the temperature of liquid nitrogen which is
this stuff here and that is at 77 Kelvin so you can see that anything that works
at higher temperatures than that can be demonstrated on that you know in a
laboratory like this and this is quite cheap this this costs about the same as
milk so this is actually reasonably inexpensive this technology and the
other relevant temperature is this one here which is the lowest recorded
temperature on earth at the Vostok Antarctic the base - 89 Celsius and in
fact hydrogen sulfide is slightly above that so in fact one can argue that this
actually is a room-temperature superconductor if you go to Vostok on a
cold day one of the big applications of superconductors apart from
transportation is and carrying current is in magnets and the advantage over
ordinary magnets that have resistance of course is that you can put much higher
current in them because they don't dissipate energy and so you can generate
much higher magnetic fields and they also cost a lot less to run because
there's no there's very little energy lost in the power this that's used to
generate the currents and I'm sure you you're all aware that MRI magnets are in
fact these days are entirely made of superconductors if you're ever an
unfortunate enough to have to go into one of these things this is a big
superconducting magnet carrying about a thousand amps and so you roll into here
and yeah it's very interesting because a thousand amps is a big current and if
that was to fail and you were inside it hmm unfortunately because it remains
because it remains superconducting you know you just don't notice there's any
current there at all these are magnets in the CERN particle accelerator
superconducting magnets all the way around a 27 kilometer diameter ring and
these carry current of about 12,000 amps so huge huge currents but no power
dissipation so that they don't lead up at all so these are two of the
applications of superconducting magnets and just to finish those of us who work
ins in superconductors and there's quite a big group here in the department that
do we all have the vision of a future which looks something like this
now we have we have soup we have superconducting wires that carry our
power that's generated by some sort of renewable source with superconducting
generators superconducting storage devices that holds the current until you
need it superconducting motors you know
basically everything is made of superconductors even possibly quantum
computers made of squid technology so there's a huge number of potential
applications and the only limitation at moment is in fact that the materials
have to be cooled down to low temperatures which which just has
practical and cost considerations but if if somebody can find a room temperature
superconductivity then this this vision will become a reality so wishes luck
thank you you
