Profile

I live in Bristol, but quite far out! I’m quite sporty so I cycle in to work every day. I haven’t made the transition to lycra yet, hopefully I never will. My main sport is fencing and I’m on the uni team . I REALLY love good food, particularly Indian and Italian. I’m very curious and love learning about new things, I think almost anything can become interesting if you look closely enough or from the right perspective. As a result, I spend quite a lot of my spare time reading or listening to podcasts (Stuff To Blow Your Mind is my favourite). Otherwise I like playing computer games when I’d rather relax and unwind.

Superconductors (SCs) have some truly remarkable properties! This means they’re used in everything from MRI machines to quantum computers. However, they only work at very low temperatures, often a few degrees above absolute zero (-273.15*C). The temperature at which they start working is known as the critical temperature.

There are several types of SC, but they can be broadly split into type 1 and type 2. We understand how type 1 SCs work, but they have very low critical temperatures and require liquid helium to get cold enough. We don’t understand how type 2 SCs work, but some of them have much higher critical temperatures! By much higher we mean above the boiling point of liquid nitrogen (-195.8*C).

The problem of how high temperature SCs (HTCs) work has been confounding us for 30 years. Lots of the ‘easy’ experiments have already been done. In order to get a better picture of how these materials behave, so that we can better formulate a theory for how they behave, I am investigating high temperature SCs under extreme conditions. That means very high pressures, very high magnetic fields, and very low temperatures. By high pressure I mean 10 Giga-Pascals (GPa) which is about 100 times the pressure at the bottom of the Mariana trench, the deepest part of the ocean. The magnetic fields we use can be as high as 45 Teslas (T). The magnets that produce these sorts of fields have the power consumption of a small city. Higher fields of up to 100T can be achieved by only switching the magnet on for a very short time so it doesn’t melt. Sometimes we use special refrigerators to investigate temperatures a few hundredths of degrees above absolute zero. Needless to say working with these kinds of conditions is quite cool.

However, these conditions come with a few problems! Chiefly achieving these sorts of pressures requires using something called a diamond anvil cell.

We flatten the points of two diamonds so that the flat bit is a circle just under a millimetre in diameter. This is because to get high pressures we need a big force and a small area. We squash a metal disk with a small hole in it called a gasket between these two diamonds, which leaves a circle of roughly half a millimetre diameter to put the sample in.

The samples we use therefore have to be 200 micron (a fifth of a millimetre) squares which are very fiddly to work with! The samples are about the size of large grains of dust. As if that wasn’t bad enough

we also have to put contacts on them so we can actually measure electrical properties. This requires using silver paint to paint on contacts by hand, and these contacts need to be about 30 micron squares. You need very steady hands and a lot of patience.

Lots of things I do take place over several days, and sometimes they can be left alone so I often have several activities going on at once. Here I’ll describe the process of setting up a typical experiment, but I also perform computational calculations to complement my experiments

Firstly, I need to prepare my samples! These are grown in a sealed crucible which is smashed once the growth is completed. The resulting debris is composed of very flat and very thin crystals, along with a whole load of junk/bits of smashed crucible. The first task is picking out the best looking (shiniest and flattest) crystals under a microscope using a bit of plastic on the end of a cocktail stick.

These samples will be around a millimetre in size, and so need to be cut into the 200 micron squares I require. To do this I use a wire saw. This is just a very thin bit of tungsten wire coated in an abrasive paste that is hooked up to an electric motor. It takes about 15 minutes to make a single cut.

I should now have many lovely square samples. I need so many because they’re very easy to break, and the survivors can vary a lot in quality! Only the best will be used in the final experiment. Before that though I need to paint on contacts to attach wires to in the final experiment. Under a microscope, using a 10 micron wire taped to a cocktail stick, I have to apply silver paint to the corners of my square samples. This requires a steady hand and a lot of patience!

Next I ‘anneal’ my samples. This is a feature of the class of superconductors that I study, known as the cuprates. Their properties, including the critical temperature below which they superconduct, change dramatically depending on how much oxygen they have absorbed, and this is controlled by the annealing process. How long the annealing takes depends on the precise material you’re working with. It can take anywhere from a few hours to 3 weeks! The annealing furnaces are typically heated to 300-600*C depending on what properties you want your SC to have.

The samples are now ready. I carefully place them in the anvil cell with a small amount of liquid (usually glycerol) and seal it shut by applying pressure. I full the sample space with liquid to make sure the pressure is evenly distributed.

I’m now ready to attach the cell to a probe and stick it in a big cryo-cooled magnet to conduct an experiment!

sdr