Superconductors & the BCS Theory Video Lecture - Civil Engineering (CE)

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no one
talking about three scientists one Nobel
Prize in 1971 for the important theory
that explained superconductivity
so we have John Bardeen which is this
fella here we've got Leon Koopa this
fella here and robot schrieffer and they
won the Nobel Prize in 1972 for the B C
s
theory which was meant to explain why
exactly there are some elements that
lose their resistance when they go past
the critical temperature I so obviously
didn't see the bcs theory and their last
names bardeen cooper schrieffer the BSC
theory is just named after them after
their three last names the scientists
that came up that theory and the idea
behind that theory was that we observed
not we but some scientists first
observed at the idea of
superconductivity in 1912 as 1912 was
when we found this found out that the
super conductivity exists
superconductivity and what was super
conductivity again that was when we have
a critical temperature which we call T C
same thing as critical temperature and
this will be a TC right here and what
this means that as soon as forgets his
temperature which let's say it's 1.2
Kelvin for example 1.2 Kelvin which will
be the critical temperature for
aluminium as soon as we get lowest
temperature for aluminium what happens
is there's virtually no resistance so
anything below it is there no resistance
which means the electrons can flow
unimpeded and we have perfect conduction
alright and this was not what we
expected we expected it to behave like
this but we have resistance temperature
I mean resistance being high and then
moving to a point where we have no more
temperatures as your Kelvin we still
have a slight bit of resistance because
we were always expected to be some
possible collisions but we found that
some elements and some alloys and some
compounds actually acted like
superconductors which meant they had
this critical temperature after which so
below which
there are no more is no more resistance
and Albert Einstein and some of the
other famous plunk and they all lived at
the time so when this super continuity
superconductivity was actually kind of
found out that it exists a behind Shack
she lived and he was obviously really
important it comes to quantum theory and
how it actually works has to with
quantum theory but he didn't he didn't
know the explanation so he didn't know
why superconductivity exists or how that
works but in 1972 is three gentlemen
came up with one theory that is there to
help explain how this is possible right
so you Albert Einstein hadn't figured it
out but these three gentlemen gave a
theory which tried to explain the
observed phenomenon here which we call
the critical temperature certain
elements making them superconductors
lowest temperature so we have to
actually discuss the dopant self says
discuss and this verb in this case
discuss is really important because that
means not just describe but give the
positive negative parts of this theory
discussed BCS theory of
superconductivity so I'll do first such
cook the outline of how it actually
works I'll give the steps of how this
actually works and remember this is a
quantum and this is a here I uses
quantum mechanics quantum mechanics are
really complicated so this is actual
what I'd go over now is couple steps is
very simplified but it's not actually
exactly how it is
but it's just simplified to allow you to
be able to understand it and appreciate
it but if you know this these five steps
you're not expected to know the exact
details of the mechanics but if you know
these five steps and you know how it
roughly works
that's good enough you don't need to
understand the whole theory it's too
complicated so we'll take up too much
time to be able to and I don't even
understand it myself so I couldn't even
explain every one or two but the idea is
just you have a simplified version which
will be good enough to get you to the
point where you're actually going to be
able to answer your questions in UHC
we've got the five steps so first step
is we want to be below the critical
temperature all right so below the
critical temperature there will still be
some vibrations so there
temperature vibrations are minimal now
we call the norm of operations we call
them phonons but below the quicker by
temperature we call them the virtual
phonons because they are so small that
they're almost not there they're so
small they're almost not there and
that's gonna be important in the second
I'll just talk about that in a second at
the first step we want to be below this
temperature but you all should know that
lowest temperature is still some
vibrations actually do happen they're
not called phonons anymore they called
virtual phone mount the second step or
second part if you know this theory is
that the electrons traveling in front
disturb the lattice what I mean by this
is these electrons right that you travel
in a straight path here and as I move
past we have the positive nuclei this is
a negative electron being shown by
negative - these will be attracted to
the negative so the positive were coming
because it's a negative its retracted
and that's what I mean by disturbed and
there's two in the third second or third
dot they come they come together so the
electron traveling in front disturb
status but the latter is moving it's
late so it means is now here it's right
here it's gonna pass through these two
and it's not gonna those two won't come
down instantly they'll actually come
down a bit delayed so once they pass
that's when these guys will come down
and they'll come down now because it's
delayed its remembered late it's gonna
take them a bit of time so enough time
for the electron to pass food which is
good because if it happen the same time
that would mean that this electron would
be going into the actual positive nuclei
which means it would lose energy because
it happened because it's delayed it
basically doesn't happen it doesn't they
don't collide but what does happen is in
this area we're gonna have a very
positively charged area between these
two nice this area is super positive
charge so I go through step one two
three we at three at the moment we have
is delayed lattice which I've drawn
again here so we have these this
electron had either a past right so it
used to be here then passed through and
when it passed through we have this is
phonons this is what we call the virtual
phone on
tiny vibration bridge the phone on and
it moved closer because of the
attraction to negative and now let's
left a really positive area here and
that's what is meant by step for
positive region created behind the first
electron right so as the first electron
moves past this positive region being
created now what happens is what is in
the negative electron behind it the
second electron
what is it trap what is it what will it
be attracted to well it will be
attracted to the positive region here
it's gonna be attracted positive and
this is really positive right there so
it's gonna be counted to it's gonna move
towards it and a good thing is if it
were to if these guys were to stay there
these two nuclei were to just remain
there what would happen would this
negative electron would passioned it
will collide with it will lose energy
but because these are virtual phonons
that means that have a high recoil so
they recall easily what I mean by recall
easily is a bounce back into their
original shape quite easily they
recalled quite quickly so what will
happen is once they have created
positive field they're going to bounce
back into the original shape leaving
that that positive field there for a
very short period time it's not going to
be there for long that's gonna be there
for long enough for this electron to
move into it
and then our electron will have moved as
well and will move in and then move past
and I'm not meant to grab that blue
stuff it's the idea is that there's no
collision here at all these virtual
photons make sure that it's gonna be
going there and then going back fast
enough to make sure there's a personal
collision with the actual Ektron but it
will actually propelled electrons these
positive holes make these electrons move
really fast but they make it move
straight at the goal is to move in a
straight line and these two electrons
that do this we call a electron pair or
this case a cooper pair so a cooper pair
you're named after a scientist Cooper
but there's not going to be these two
it's going to be more it's gonna be more
here as well and now these two will act
also like a Cooper pair and then these
two will eventually act like a Cooper
pair so you can imagine it'd be like a
chain reaction one after the other and
this is how we can make sure that
there's absolutely no resistance
so these Cooper pairs they have no
resistance which means there's no
collisions at all with the actual
lattice which means we have a perfect
conductor or in other words a super
conductor because there's no resistance
and these guys can just move unimpeded
now I'll quickly cut recapped again all
right so first we need to be below the
critical temperature and blowers
critical temperature there's still some
vibrations we call them the virtual
photons these vibrations are tiny and
they recall quite easily so we have the
electron the first electron the electron
moving past your actual positive nuclei
and the last movement slaves that means
these guys will take some time there
won't be colliding with the negative
electron they'll be delayed which means
they'll move past in this area once the
electrons are passed but when they do
move here that means that they create a
positive region or positive not a whole
region which means that the next
electron will be quite attracted that
positive region which means it will move
straight towards it and because it has
such a high recoil so step four was
positive region created behind first
electron step two was second electron
attracted to positive region and lattice
rebounds back into original shape so
it's attracted to it but
same time either his paws nuclei will
bounce back before the electron gets
there
which means no collisions remember
that's important no collisions and that
means it Elektra has not moved there in
a straight line has not collide of any
of the actual and phonons and these two
this combination of how this worked is
what we call the Cooper pair and there's
gonna be lots of Cooper pairs
you're gonna form reform constantly to
make sure we have a constant stream of
electrons going straight without any
resistance and that's what we call the
superconductor and that was there for
your theory that bcs theory and that
holds true for all the Middle's so for
example and the aluminium which has a
critical temperature of 1.2 kelvin or
mercury which has a critical temperature
of 3.4 kelvin i my battery's running out
that's good to know those two it will
work for right so it says discuss the
bcs theory of superconductivity this is
the theory and it works perfectly fine
for iminium and mercury but for your
cuprates which are the ones which have
of copper
and oxygen
it's actual its actual cops spelled
copper 2p copper and oxygen in its
actual formulas this would be one
example the one we mentioned earlier
we've got copper here and oxygen here
these guys this doesn't implies theory
doesn't work for these guys because they
don't actually have this kind of
structure didn't have this kind of
middle structure they have a structure
where you can see we've got these sheets
of copper oxides and there are really
good superconductors but it doesn't work
through these Cooper pairs and Cooper
pairs was for your middles but for your
actual kook rates we are guessing we
don't actually know exactly how it works
but we're guessing the actual way that
they become superconductors is for these
anti Ferro magnetism properties that
these Cooper oxides produce don't need
to know details of it but why you should
know for this part of the top point is
when you discuss it you say okay this is
a theory it works perfectly fine for the
minima myrrh cream for example but it
doesn't work for your cuprates which are
the ones which have a higher T critical
temperature and you'd say okay they
don't work for it and we believe there's
other mechanisms such as the anti
ferromagnetism that makes these
superconductors not new Cooper pairs but
yeah that's a research area of research
that we're trying to figure out more
about but at the moment we're knowing
that this theory the BCS theory doesn't
apply to your actual cuprates if that
was a part of that dot point so this cos
means if the pros and the cons are the
good and the bad sides of the actual
theory it's good part would be that it
explains why your aluminum and Mercury
are
superconductors but the sort of negative
aspect is it doesn't explain why
cuprates are superconductors because
they don't work these Cooper pairs they
have a different kind of structure I
hope that was useful
thank you for watching