J oint R esearch A ctivity 1 Opening the Microkelvin Regime to Nanoscience J oint R esearch A ctivity 1 Opening the Microkelvin Regime to Nanoscience

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.

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.

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 ~ watts into a micron cube sample.

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.

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.

Lets start with cooling superfluid 3 He.

Since we only need a small volume of copper to cool liquid 3 He, 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 3 He and refrigerant in).

We have added a sapphire tube (as in this experiment we want to make NMR measurements in the tower so-produced).

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.

We add a silver sinter pad to make contact for precooling and a filling tube.

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.

The outer-cell copper refrigerant pads are connected by high conductivity Ag wires (rr~10 3 ) to a single crystal Al s/conducting heat switch.

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.)

The whole structure is supported by a thin-walled epoxy cylinder.

We insert the cell into the mixing chamber.

We push the cone joint together and screw it up. Done!

This system will cool superfluid helium-3 to around or below 80 K.

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..

We start with an epoxy box which we will immerse in the outer cell (the box being filled with vacuum).

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.)

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 Mechanical supports Ag thermal link

The supports are spot- welded to the first Cu refrigerant plate. First Cu plate

A superconducting heat switch (Al or Sn) is melted/spot-welded to the copper plate. S/c heat switch

A second Cu refrigerant plate is added. Cu plate No 2

Then a second heat switch

Finally the third and final Cu plate is added.

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).

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.

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.)

(Ignore the jumps. Thats a problem of the heat switch)!

Note temperature 5 mK, thats 8 orders of magnitude colder than room temperature (centre of Sun only 5 orders warmer).

Thus from our experience with working with quantum fluids we can cool superfluid 3He to ~ 80 K and the electron system in copper to ~5 K. Thats state of the art. How do we translate that into a system for cooling nanoscience samples?

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.

We start with a bundle of high-conductivity silver lead wires mm each with a sintered Ag pad to act as a thermal ground.

The silicon substrate was glued with black Stycast directly on to another similar thermal ground wire

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.

That circuit immersed fully in the helium in the mixing chamber cooled to at least 4 mK.

The nano community think that 30 mK is about the limit for dilution refrigerators so do not think in these terms.

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.

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 3 He where there is a dense 3 He quasiparticle gas making orders of magnitude better contact directly to the structures.

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.

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 90+m2 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).