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Superconductivity 2012
Demonstration
What did we see?
High-Tc materials
(How to make superconductors)
Some applications and important
properties
Department of Physics, Umeå University, Sweden
How do we show superconductivity?
Superconductors
1. have an electrical resistivity that is exactly zero,
2. refuse magnetic fields to enter the superconducting
volume.
(Lab experiment)
Let's try!
Department of Physics, Umeå University, Sweden
Meissner-Ochsenfeld effect
“Perfect“ metal
Superconductor
Room temperature
Room temperature,
with magnetic field
At low temperature
(T<Tc), after cooling
in a constant
magnetic field
Department of Physics, Umeå University, Sweden
"Perfect conductor" effect
“Perfect“ metal
Superconductor
Room temperature
Low temperature
(T<Tc)without
magnetic field
After applying a
magnetic field at low
temperature (T<Tc)
Department of Physics, Umeå University, Sweden
Why is the levitation stable?
When you balance things on soft springs the situation is usually
unstable. So why doesn't the magnet simply fall off ?
Because the field can penetrate! Take a ceramic:
Department of Physics, Umeå University, Sweden
Why is the levitation stable?
Although the grains are superconducting, the boundaries are
effectively thin "normal" films. Some field lines can find ways
to penetrate the ceramic, but then get "locked" in place - they
cannot move without crossing grains!
Department of Physics, Umeå University, Sweden
Two types of superconductors: Types I and II
Type I
Type II
Different behaviours in
magnetic fields (red):
Weak B-fields are always
repelled, by both types;
strong fields destroy the
superconductivity in type
I, but penetrate type II in
"vortex tubes" containing
one flux quantum each!
Department of Physics, Umeå University, Sweden
Superconducting materials
"Classical" superconductors: Metals and alloys!
Hg
Pb
4.2 K Discovered by Heike Kammerling Onnes in
1911 (Nobel Prize 1913)
7.2 K
Nb
9.2 K (0.2 T - type II element!)
NbTi 9.8 K 14 T (The "standard" superconductor)
NbN 16.1 K 16 T (used in thin film applications)
Nb3Sn 18 K 24 T (expensive and difficult to use)
Department of Physics, Umeå University, Sweden
High Transition Temperature Superconductors (HiTc:s)
MgB2
Department of Physics, Umeå University, Sweden
High Transition Temperature Superconductors (HiTc:s)
Some representative ”heads of families” of HiTcs:
La2-xSrxCuO4
38 K (Bednorz & Müller,
1986)
YBa2Cu3O7-d
92 K (Wu & Chu, 1987)
Bi2Ca2Sr2Cu3O10
110 K
Tl2Ba2Ca2Cu3O10
125 K
HgBa2Ca2Cu3O8
135 K
Department of Physics, Umeå University, Sweden
High Transition Temperature Superconductors (HiTc:s)
Quite complicated structures! One of the simplest is
YBa2Cu3Ox, "Y-1-2-3":
The basic structure is tetragonal,
with copper and oxygen forming a framework
into which we insert Ba and Y.
The formula is now YBa2Cu3O6, and this
material is NOT superconducting!
Department of Physics, Umeå University, Sweden
High Transition Temperature Superconductors (HiTc:s)
Quite complicated structures! One of the simplest is
YBa2Cu3Ox, "Y-1-2-3":
The basic structure is tetragonal,
with copper and oxygen forming a framework
into which we insert Ba and Y.
To get a superconducting material we must
add more oxygen, to obtain YBa2Cu3O7!
Department of Physics, Umeå University, Sweden
High Transition Temperature Superconductors (HiTc:s)
Quite complicated structures! One of the simplest is
YBa2Cu3Ox, "Y-1-2-3":
CuO chain
These are the
metallic,
superconducting
parts!
Ba spacer
To some extent, more
CuO planes mean
higher Tc!
Ba spacer
CuO plane
Y spacer
CuO plane
CuO chain
Department of Physics, Umeå University, Sweden
High Transition Temperature Superconductors (HiTc:s)
How to make YBa2Cu3Ox, "Y-1-2-3":
1. Mix and grind Y2O3, BaCO3 and CuO for a long time.
2. Heat in an oven at 900-925 oC for at least 1 hour.
3. Crush, re-grind, and repeat 2. a few times.
4. Press into a cake, then heat in pure oxygen gas at 450 oC
for at least 24 hours.
5. Time to test for superconductivity!
Department of Physics, Umeå University, Sweden
High Transition Temperature Superconductors (HiTc:s)
Higher values for Tc can be found for other materials, based
on Bi, Hg or Tl.
These are also layered, often with many parallel internal
layers of CuO:
Tl2Ba2CuO6
Tl-2201
(single CuO)
Tl2Ba2CaCu2O8
Tl-2212
2 layers 105 K
Tl2Ba2Ca2Cu3O10
Tl-2223
3 layers 125 K
(Bi-2223  110 K, Hg-1223  135 K)
Department of Physics, Umeå University, Sweden
85 K
A new star: MgB2
Superconductivity in MgB2 was discovered in 2001 with Tc
= 39 K, the highest for any "classical" superconductor.
The material is cheap,
easy to handle, nonpoisonous, and easily
formed into wires or
films/tapes. Problem:
The practical critical
field seems to be
limited to 3.5 T.
Department of Physics, Umeå University, Sweden
An even newer star: iron arsenides
In 2008, another type of layered, exotic superconductors,
based on iron and arsenic, was discovered.
Takahashi et al.,
Nature 453,
376 (2008)
Department of Physics, Umeå University, Sweden
An even newer star: iron arsenides
In 2008, another type of layered, exotic superconductors,
based on iron and arsenic, was discovered. Another family
is BaxKyFe2As2. Critical
temperatures up to
above 55 K have been
reported when
changing the La to
heavier rare earths.
Again, the material is
cheap and fairly easy to
handle, but As is clearly
poisonous!
Department of Physics, Umeå University, Sweden
Applications for superconductors
There are basically two types of applications:
Power circuits and electronics/measurements. Most
practical applications use type II superconductors.
Existing and future commercial devices:
Power transmission components, power storage devices,
electric motors and generators, frictionless bearings,
permanent magnets and electromagnets, voltage standards,
fast computers and electronics, microwave filters, .........
Department of Physics, Umeå University, Sweden
Applications for superconductors
In electronics, one possible application is in fast computers.
Clock pulses must be synchronized in a computer, but at 3
GHz light travels only 10 cm
during one clock pulse!
Shrinking a computer
means more concentrated
heating, killing the CPU.
The obvious solution is a
cool superconducting
computer!
Department of Physics, Umeå University, Sweden
Electronics and measurements: tunnelling
Tunneling between two
superconductors (”SIS”) can
be used as the basis for many
devices.
In principle, both electrons
and pairs can tunnel through a
Josephson junction, so the real
behaviour can be either
bistable (logic 1/0!) or
continuous.
Department of Physics, Umeå University, Sweden
Electronics and measurements: the SQUID
A particularly useful device is the SQUID:
Superconducting QUantum Interference Device
or
With a SQUID it is
possible to routinely
measure magnetic fields
down to well below
10-16 T!
Department of Physics, Umeå University, Sweden
Electronics and measurements: the SQUID
The SQUID can be used for
measurements (as a sensor).
Superconducting loop
Josephson junctions, called
”weak links”
External connections
Department of Physics, Umeå University, Sweden
Electronics and measurements: the SQUID
Each Josephson junction has
a maximum supercurrent
I = I0 sin g, so the maximum
current that can run through
the device is 2I0.
2I0
Department of Physics, Umeå University, Sweden
Electronics and measurements: the SQUID
If we apply a very weak
external magnetic field, a
circulating shielding current
will appear and no field will
exist inside the loop!
The external current must
decrease to avoid exceeding
the maximum supercurrents in
the junctions.
Department of Physics, Umeå University, Sweden
Electronics and measurements: the SQUID
When the magnetic field
corresponds to exactly ½
magnetic flux quantum inside
the ring, the circulating current
has its maximum and the
external current its minimum
value.
Department of Physics, Umeå University, Sweden
Electronics and measurements: the SQUID
If the field increases further,
one flux quantum is admitted
through a weak link, and the
circulating current reverses!
It can easily be shown that the
external current is a periodic
function
Imax = 2I0cos(pF/F0)
Department of Physics, Umeå University, Sweden
But how do you make ceramic "wires"?
There are two ways:
1. Thin films on a metal or ceramic substrate
2. "Powder-in-tube" technology
Stainless
steel band
Deposition of
ceramic film
Oxygen treatment
in hot oven
Department of Physics, Umeå University, Sweden
Storage
But how do you make ceramic "wires"?
There are two ways:
1. Thin films on a metal or ceramic substrate
2. "Powder-in-tube" technology
Fill a silver tube with
superconductor
powder, then draw to
desired shape, then
heat treat ("anneal").
Department of Physics, Umeå University, Sweden
But how do you make ceramic "wires"?
The "powder-in-tube" method is simlar to what you do to
"classical" superconductors:
Basic procedure:
- Make a Cu cylinder,
- make a lot of holes along axis,
- fill the holes with superconducting
rods,
- draw the whole cylinder to wire, as if it
were massive Cu!
This procedure works well with Nb-Ti,
which is soft and ductile like copper!
Department of Physics, Umeå University, Sweden
But how do you make ceramic "wires"?
All superconductor wires have similar internal "multistrand" structures!
NbTi wire
High-Tc (BiSSC) wires
Department of Physics, Umeå University, Sweden
Using type II superconductors
An obvious application for a superconductor is to transport
electric current.
What happens to electrons in a B-field ?
B-field
Current
Let us remember two laws:
Fm = qv  B
("Maxwell")
F=0
("Newton")
There will be a force on the magnetic field lines!
Department of Physics, Umeå University, Sweden
Using type II superconductors
Is this a problem ?
A moving field ↔ changing flux;
B-field
but - dF/dt = E !
This gives two problems:
1. A voltage appears along the
current flow; "resistance"!
2. This causes dissipation of
heat, since P = UI
Current
Department of Physics, Umeå University, Sweden
Using type II superconductors
Is this a problem ?
A moving field ↔ changing flux;
but - dF/dt = E !
This gives two problems:
1. A voltage appears along the
current flow; "resistance"!
2. This causes dissipation of
heat, since P = UI
Department of Physics, Umeå University, Sweden
Using type II superconductors
Or, if we measure voltage as a function of applied current
at constant temperature:
Department of Physics, Umeå University, Sweden
Using type II superconductors
Conclusion: We want to keep the flux lattice fixed in space!
How do we do this?
Flux lines prefer to go through non-superconducting
regions, because it requires energy to create a vortex tube!
So, we should insert impurity particles into the
superconductor!
This method is
called
flux pinning.
Department of Physics, Umeå University, Sweden
Using type II superconductors
You have already seen a magnet fly !
You can also make a really good magnetic bearing,
or ”freeze in” a field to make a permanent magnet – with a
field which you can shape exactly as you want it!
Department of Physics, Umeå University, Sweden
Using type II superconductors
BUT: Flux pinning also gives problems:
There is a”friction force” that keeps them in place, and
because J   X B, dBz/dx  Jc everywhere inside a type
II superconductor! Increasing external field:
Department of Physics, Umeå University, Sweden
Using type II superconductors
BUT: Flux pinning also gives problems:
There is a”friction force” that keeps them in place, and
because J   X B, dBz/dx  Jc everywhere inside a type
II superconductor! Increasing external field:
Department of Physics, Umeå University, Sweden
Using type II superconductors
BUT: Flux pinning also gives problems:
There is a”friction force” that keeps them in place, and
because J   X B, dBz/dx  Jc everywhere inside a type
II superconductor! Decreasing external field:
Department of Physics, Umeå University, Sweden
Using type II superconductors
BUT: Flux pinning also gives problems:
There is a”friction force” that keeps them in place, and
because J   X B, dBz/dx  Jc everywhere inside a type
II superconductor! :
This leads to a magnetic
hysteresis, and to energy loss
(= heating!). It can be shown
that the loss is proportional to
the thickness a of the
superconductor!
Department of Physics, Umeå University, Sweden
A possible novel application
The first practical application for high-Tc materials in
power circuits is likely to be something that cannot be made
without superconductivity. One such example is the
superconducting current limiter:
Consider a standard transformer (which
you can find in any electronic device, at
home or here):
U1/U2 = N1/N2 = I2/I1,
where 1 means "input" side, 2 "output"
side, and N is the number of wire turns!
http://www.yourdictionary.com
Department of Physics, Umeå University, Sweden
A possible novel application
The first practical application for high-Tc materials in
power circuits is likely to be something that cannot be made
without superconductivity. One such example is the
superconducting current limiter:
Suppose we make a transformer with
N2 = 1 (a single turn).
If we short-circuit the output, U2=0,
then U1 = NU2 = 0, for all currents!
Usually this is just stupid, but what if
we make the secondary one turn of
superconducting wire?
Department of Physics, Umeå University, Sweden
A possible novel application
Superconducting current limiter:
Primary current I1
I2 = N I1;
if the coil superconducts
U1 = U2 = 0, and P = UI = 0 !
However, whenever I2 > Ic the
secondary turns normal and
R1 = U1/I1 = N2U2/I2 = N2R2 !
Because N can be made large and
high-Tc materials have very large
normal resistivities, this works as a
"fuse"!
Department of Physics, Umeå University, Sweden
A possible novel application
Superconducting current limiter:
N1 = 500
N2 = 1
Ic ≈ 85 A at 77
K (measured!)
Tc ≈ 110 K
(Bi-2223)
Department of Physics, Umeå University, Sweden