A couple of pictures from a photo session organized by Wilson architects back in the summer. I am posting them here mostly as a reference for my students for how clean the lab should be, and for how happy they are when the lab is so clean.
© Wilson Architects Inc., Anton Grassl / Esto
One problem with buying two identical fridges is how do you tell them apart. Well, one simple way, a long tradition in cryogenic labs, is to name your cryostats, you know, like ships. This process is not going too well so far in our group, and I take full responsibility.
Well, it did not help that fridges came from Leiden Cryogenics with creative names like Frolov1 and Frolov2. It is of course not an unreasonable idea to name fridges after famous physicists, for example Sheldon and Leonard, but who is going to want to work in a theorist-fridge? (Keep in mind the terrible Pauli effect)
Neither did it help that the fridge labeled “2” arrived first. That created a lot of confusion as in -Which fridge are we talking about? The first one, you know – that arrived the second… For a while they were called Left and Right, but of course that depends on the point of view… They were also called ‘Vector’ and ‘Solenoid’ after the kind of magnet that they had, until the magnets got swapped. And it became a nightmare. The second fridge, you know the vector, I mean solenoid, the one that used to be a vector and that arrived the first and so it is called the second.
To make things absolutely clear, people started saying ‘The fridge that is closer to the pink box’ as opposed to ‘the one in the center of the lab’. But the pink box has wheels, and this is not an elegant naming system at all. One of my students proposed ‘Ying’ and ‘Yang’ as in the opposites that somehow attract and coexist, but it gave me the wrong feeling to have metaphysical forces acting on delicate scientific equipment.
So. I have to make a decision. I will call one of them Narwhal, because of the top-loading probes that stick out like a tusk of a narwhal.
And the other one shall be called… Narwhal2? Oh boy, here we go again.
The way we do research is reminiscent of the timeless show “Fraggle Rock”. In the world of fraggles, there are fraggles, which mostly go about their silly business. And there are also doozers. Those are creatures that are focused, motivated and constantly building beautiful scaffolding of some transparent substance. They fill space with this scaffolding in no time. The twist is, that this scaffolding is very tasty, and fraggles love it – so they just grab it and eat it. In one episode fraggles start feeling bad about destroying the magnificent scaffolding castles built by doozers, so they stop doing it. This makes doozers very upset, because as it turns out, they enjoy making fraggles happy, and want their structures to be eaten.
In our research we work and fully depend on material scientists who grow magnificent structures, such as semiconductor nanowires. Growing nanowires is science with a twist of magic, because a lot of it is about methodical yet creative tuning of growth parameters that produces qualitatively new classes of materials. In a recent example, I was fortunate to collaborate with material scientists from Eindhoven on the characterization of a new type of nanostructure they produced – a semiconductor nanocross.
These crystals come about when two nanowires grow into each other. The Eindhoven group have achieved a high degree of control and reproducibility in producing these nanocrosses out of InSb. Our experimental team in Delft then contacted all four legs of a cross with metals and superconductors and found them to be of high electronic mobility and in the quasi-ballistic transport regime. Nanocrosses can find applications in research on Majorana fermions and topological quantum computing. Qubits based on Majorana fermions consist of several Majoranas, and to change the qubit state one has to switch Majorana positions. A recent theory found that this cannot be done in a single nanowire, and at least a nanowire T-junction, or a cross, is required.
Inspired by a recent conference on quantum computation
Can you guess what is being measured? (Hint: not Majorana, though these days any funny-looking graph is presented as such)
A couple of weeks ago we took advantage of Indian summer to walk around in a nearby Raccoon Creek park. Most of my group and friends/families joined.
And barbeque after the hike was timely, though there was way too much food:
Pennsylvania is very picturesque in the Fall!
This graph shows the flow of electrical current without developing a voltage (supercurrent) through a semiconductor coupled to two superconductors. The size of the maximum supercurrent (called critical current) can be controlled by a voltage on a nearby electrode (gate).
Seeing this effect is important for building up to Majorana fermion experiments. It has been reported first in this paper from 2005. So we are steadily catching up with state-of-the-art (Coulomb blockade from a couple of posts ago was first demonstrated in the 1980-ies).
Here are a few snapshots from the Condensed Matter floor of Allen Hall, freshly renovated home of the Physics Department at Pitt.
Lounge Center. The kitchen counter is where our espresso machine is going as soon as we get 220 V power for it:
Hallway, graduate student hall is at the far end.
One of the fridges in my lab got a Twitter account:
The other fridges don’t dig it yet.
This sequence of sharp peaks is a hallmark of single electron transport: in between the peaks current is shut down because electrons cannot overcome the capacitive energy associated with placing a single electron on a conducting island. At the peak the current flows again because the cost of having N and N+1 electrons on the island is equal.
In this case this so-called Coulomb blockade was obtained in a nanowire device contacted by two electrodes and tuned by an electrostatic gate, for which purpose the silicon substrate was used. This is one of the first low temperature measurements performed in our new lab in Pittsburgh.
These pictures show gold patterns defined by exposing photoresist with light, a step similar to photography, and then evaporating gold. They will be used for making contacts to nanowires.
In Pittsburgh we are so fancy, we use róse to cool our equipment. As can be seen in the below vine:
Due to a chilled water system leak the hallway outside the lab filled with magnificent pink water last week, it has a chemical added to prevent equipment corrosion. It made me appreciate the ramp that leads to my lab which stopped this water from getting inside. Now every time I have to push something heavy up the ramp I will think about this pink lake.
Inspired by the 1964 James Bond classic, we have built the Coldfinger. It is a crafted piece of copper that mounts at the coldest point of a dilution fridge. This is where the sample lives. It has an elongated shape because it needs to enter into the bore of a superconducting magnet, which is typically 2 or 3 inches in diameter.
The Coldfinger has PCB-based filters to cool down electrons that come from room temperature in order to explore our devices.
The Coldfinger was produced by our excellent machine shop and electronic shop at Pitt.
We had to drive to Ohio to get this espresso machine that needed a new home. Still not sure if it will go into our newly renovated Condensed Matter discussion room in Allen Hall, or stay in the lab. In the latter case a theorist infestation is a hazard, though it provides for nice conversations.
Following the method developed by Morgenstern group in Aachen, we are not positioning nanowires by picking them up from the chip where they group and depositing them on a chip where we build devices.
video by Peng Yu. Recommended to watch in highest quality.
Discovered over 100 years ago, it still works. Certain materials loose resistance when cooled below their critical temperature. One of these, a niobium-titanium-nitride alloy, is now available in Pittsburgh. It is an important material for Majorana fermion research, but also for single photon detectors, superconducting qubits… Almost on the first try we got the critical superconducting temperature of 11K, which is not the highest for this material, but will work for us in the short term. The film was produced in the AJA sputtering machine from a couple of posts below.
(Resistance vs. temperature plot in the ‘as is’ form)
Nine months later, the lab is full, and everything seems to work, apart from minor issues:
In this panorama you see a sputtering machine, three dilution fridges and of course the pink box.
Finally I understand what my collaborators from Neel Institute are working on:
Btw, isn’t he the guy from the Daily Show?..
This is a sputtering machine. It is a high vacuum system inside of which argon plasma kicks atoms out of a target, after which the target atoms land onto a sample creating high quality thin films of metals or dielectrics.
This is the plasma:
The machine was made for us by AJA International in Massachusetts. It is a Pitt facility, but it is temporarily located in my lab since it is demanding as a flower (needs water and power), and there was no other place for it on campus.
The button has been pushed, and our two dilution fridges from Leiden Cryogenics are cooling down. Today they both reach 3 K and we start the condensation of He3/He4 mixture which is the first phase of the final cooldown to the base temperature of ~7 mK.
You can also here the pulse tubes humming, though from where I sit in the meeting room I almost don’t hear them, just see the fridges through the glass window.