Joint Research Activity 1

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Joint Research Activity 1 Opening the
Microkelvin Regime to Nanoscience
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This specific joint research activity is central
to the whole project Opening the microkelvin
regime to nanoscience It is this activity
which is going to make the whole thing
happen. Let us consider the tasks one by one.
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Task 1 Developing the new technology needed to
cool nanosamples and circuits to around or below
1 mK To integrate nanoscale experiments into
sub-millikelvin cryostats will require new
technology. The difficulties are largely those
of making thermal contact to the electron gases
in the nanostructures. This is especially true
with semiconductor nanostructures. At ultralow
temperatures such substrates become effective
thermal vacua and thermal contact is often
restricted to the pathways via the metallic leads
to the circuits.
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The only quantity which matters in cooling such
circuits is the ratio of the heat leak into the
circuit material to the thermal contact to the
refrigerant. Both quantities have to be attacked
in parallel. First we can make great efforts to
reduce the external heat leak. With the best
current refrigerators we can create enclosures
which are so well insulated that the heat leak
into an isolated non-conductor is already at the
level set by the background radioactive heating
(largely from nearby constructional concrete).
Metallic samples experience additional heating
from eddy currents generated by motion in stray
magnetic fields. However, these can also be
reduced to a level below 4 pW per mole which
translates to 10-24 watts into a micron cube
sample.
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The real difficulties come when we attach
leads, as this immediately connects the outside
world. We have to take this problem very
seriously and start with the best electrical
filtering possible, which fortunately is being
pursued with in JRA2. Secondly we must enhance
the thermal contact to the refrigerator. In a
semiconductor 2DEG, for example, the substrate
makes virtually no contribution to thermal
contact at the lowest temperatures which runs
entirely via the leads. Using ideas from BASEL
and ULANC we can thermally anchor each lead
directly in the mixing chamber liquid with
sintered silver pads and then furnish each lead
with its own mini nuclear stage to absorb any
final incoming energy in the nuclear bath.
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Just look for a moment at some of our best
technical setups for cooling helium (and then
copper). This is simply to give an idea of what
we can do now. This is what the ult community
brings to the table. I.e. largely the input from
TKK, CNRS and ULANC.
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Lets start with cooling superfluid 3He.
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Since we only need a small volume of copper to
cool liquid 3He, lets get it as close to the
specimen as possible, that is immerse it in the
liquid. So we start with a thin-walled
paper-epoxy box (to put our liquid 3He and
refrigerant in).
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We have added a sapphire tube (as in this
experiment we want to make NMR measurements in
the tower so-produced).
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We add the refrigerant, a stack of Cu plates
coated on one side with a 1mm layer of sintered
silver powder to make thermal contact with the
liquid.
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We add the refrigerant, a stack of Cu plates
coated on one side with a 1mm layer of sintered
silver powder to make thermal contact with the
liquid.
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We add a silver sinter pad to make contact for
precooling and a filling tube.
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To cut down the heat leak we add a second stage,
also furnished with a precooling link, and
filling tube. We put the inner cell inside.
This allows the inner cell to
have a very thin wall (und thus low slow-release
heat leak) because the pressure is supported by
the outer cell wall.
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The outer-cell copper refrigerant pads are
connected by high conductivity Ag wires (rr103)
to a single crystal Al s/conducting heat switch.
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Further silver wires lead to precooling pads
which will sit in the mixing chamber of the
dilution refrigerator (at 2 mK).
The heat switch is connected to the Ag cooling
sinters to sit in the mixing chamber. (The cone
is the mixing chamber base.)
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The whole structure is supported by a thin-walled
epoxy cylinder.
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We insert the cell into the mixing chamber.
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We push the cone joint together and screw it up.
Done!
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This system will cool superfluid helium-3 to
around or below 80 mK.
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Now let us use a similar system just to cool the
copper refrigerant. To do this we put a multiple
coper stage in the inner volume as in the
previous setup..
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We start with an epoxy box which we will immerse
in the outer cell (the box being filled with
vacuum).
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We attach three high-purity Ag-wire supports, two
at the bottom and one which also acts as thermal
link to the heat switch. (Remember the epoxy is
acting at these temperatures almost as a thermal
vacuum.)
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We attach three high-purity Ag-wire supports, two
at the bottom and one which also acts as thermal
link to the heat switch. (Remember the epoxy is
acting at these temperatures almost as a thermal
vacuum.)
Ag thermal link
Ag Mechanical supports
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The supports are spot-welded to the first Cu
refrigerant plate.
First Cu plate
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A superconducting heat switch (Al or Sn) is
melted/spot-welded to the copper plate.
S/c heat switch
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A second Cu refrigerant plate is added.
Cu plate No 2
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Then a second heat switch
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Finally the third and final Cu plate is added.
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A Pt NMR thermometer is added this measured the
susceptibility of the Pt nuclei and gives us the
temperature simply from Curies law The
thermometer is a bundle of fine uninsulated Pt
wires soldered with pure silver and on which we
will do NMR with a set of coils immersed in the
outer cell not touching the inner parts).
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We glue on to the final plate a pure Ag wire
heater to calibrate the thermometer (using a
microscopic amount of epoxy to stick it to the
plate.
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Finally we connect the heater with pure tin leads
to a thermal anchor on the outer plate, (and put
it all back in the box). The box, is put in turn
inside a Lancaster outer stage. (Only contact to
final Cu plate is via the heat switch and the Sn
leads.)
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(Ignore the jumps. Thats a problem of the heat
switch)!
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Note temperature 5 mK, thats 8 orders of
magnitude colder than room temperature (centre of
Sun only 5 orders warmer).
(Ignore the jumps. Thats a problem of the heat
switch)!
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Thus from our experience with working with
quantum fluids we can cool superfluid 3He to 80
mK and the electron system in copper to 5 mK.
Thats state of the art. How do we translate
that into a system for cooling nanoscience
samples?
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The following setup was used for a nano Andreev
interferometer experiment in the mixing chamber
of one of our machines using the simplest
possible methods. Just to get us started.
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We start with a bundle of high-conductivity
silver lead wires f1 mm each with a sintered Ag
pad to act as a thermal ground.
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The silicon substrate was glued with black
Stycast directly on to another similar thermal
ground wire
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The silicon substrate was glued with black
Stycast directly on to another similar thermal
ground wire and the circuit connections were
bonded straight on to the silver lead wires.
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That circuit immersed fully in the helium in the
mixing chamber cooled to at least 4 mK.
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The nano community think that 30 mK is about the
limit for dilution refrigerators so do not think
in these terms.
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Now this was without any particular clever
filtering on the leads. We of course would do
that but that is the job for JRA2 which we will
be hearing about. To enhance the thermal
contact to the refrigerator we use ideas from
BASEL and ULANC. We thermally anchor each lead
directly in the mixing chamber liquid with
sintered silver pads as above and then furnish
each lead with its own mini nuclear stage to
absorb any final incoming energy in the nuclear
bath.
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Finally we can envisage completely new
tailor-made nanoscale structures independent of
conventional semiconductors. Ideal candidates
for microkelvin cooling are carbon-nanotubes and
graphene structures which can be directly
immersed in superfluid 3He where there is a dense
3He quasiparticle gas making orders of magnitude
better contact directly to the structures.
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Finally we can envisage completely new
tailor-made nanoscale structures independent of
conventional semiconductors. Ideal candidates
for microkelvin cooling are carbon-nanotubes and
graphene structures which can be directly
immersed in superfluid 3He where there is a dense
3He quasiparticle gas making orders of magnitude
better contact directly to the structures.
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Task 2 Building with our SME partner BlueFors a
self-standing dilution refrigerator plus nuclear
cooling stage with nanoscample capability which
can be used in any lab in the world without the
need for refrigerants. This builds on task 1
and is our direct contribution to European and
other workers outside the consortium who have no
access to refrigerant technology. (Coord CNRS
TKK) This opens up nanoscience to everybody.
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Task 3 The next-generation microkelvin facility
(ULANC, SAS, TKK, CNRS, BASEL, RHUL) Using the
combined knowledge and expertise of the
applicants we are also planning an entirely new
advanced microkelvin refrigerator facility
intended exclusively for condensed-matter and
nanoscale experiments at milli- and microkelvin
temperatures. This will be sited at ULANC in a
purpose-built 90m2 laboratory hall with 7 m
clearance and a 3 m dewar pit dedicated to this
project, which is supported by k400 from the UK
Science Research Investment Fund. The
access-giving laboratories in this consortium
have a very large fraction of the world expertise
and capability in carrying out experiments at
sub-millikelvin temperatures. We propose to
build on this unique European resource by pooling
our existing knowledge along with the technology
developed in tasks 1 and 2 above to make this the
most advanced sub-microkelvin facility for
nanokelvin studies that current knowledge will
allow. (coord ULANC).
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