Introduction to Solid State Physics

Last semester I recorded 23 lectures on Solid State Physics for Undergraduate students. The first 15 lectures are following the wonderful texbook by Steven Simon “Oxford Solid State Basics”, and the last lectures cover superconductivity and advanced topics from Quantum Transport. This class can be a prequel to my Quantum Transport course and I hope it will be useful for students thinking of going into condensed matter research.

Because this is so great for binge watching, I am releasing the whole season at once.

Back to the Future

So there is this great movie that you should totally go see, it is about an eccentric scientist who befriends a simple-minded neighborhood teenager and sends him backwards in time in a car converted into a time machine…

Something like this just happened in our lab. We started out a couple of years ago as a cryogen-free operation. Instead of relying of liquid helium, we used closed-cycle cryostats where helium circulates between a compressor and a cryostat and extracts heat from a vacuum-shielded volume. There are two reasons why cryogen-free systems are great – they save money on (pricey) liquid helium and they are easy to operate at a push of a button.

But in the meantime our generous university set up a helium liquefier, which supplies nearly-free liquid helium to us. And so now we built our first liquid helium setup – a dunker stick! It derives its name from a concept of dunking your specimen into liquid helium in order to reach a low temperature of 4.2 Kelvin, at which helium is stored in liquid form.

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From the technological point of view, it is a step back compared to our cryofree setups. But we can use it for many quick measurements and tests, which is great.

Rashba Effect at 35

Last year was the 35th anniversary of the seminal paper by Bychkov and Rashba which introduced a spin-orbit coupling which is now known as “the Rashba effect“. It is a beautiful piece of physics that brings a bit of relativity into condensed matter world. Turns out, in an electric field (or under a broken symmetry) electron spins in a solid can behave as if they are in a magnetic field.

Anyhow, 35 years later the Rashba effect lives on and find itself at the frontiers of some of the hottest research of the 21-st century, as you can convince yourself by arxiving it (it is like googling but with arxiv.org).

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So a few months ago Aurelien Manchon approached me and several other folks to be part of a review of what is the face of Rashba effect 35 years later. As with all manuscripts, preparation took forever, so we are celebrating the 35th birthday of Rashba effect one year later. But finally the paper it out there and we are hoping it can serve those who want to know how Rashba spin-orbit physics beats along with spintronics, quantum computing, topological physics and cold atomic research.

I learned a lot about spin-orbit interaction myself from working on this review, I also learned the difference between stirring and steering, as in ‘steering wheel’ not ‘stirring wheel’ (thank you, anonymous referee!).

Helium Liquefier

Our University has sacrificed a parking lot to realize this project:

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It is an expansion to the machine shop which among other things now hosts this helium liquefier:

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Our lab is now connected to the helium recovery system, and we are building our first “wet” cryostat in which liquefied helium is used to cool a sample to 4.2 degrees Kelvin (-269 degrees centigrade). We are also now officially shopping for a used wet dilution refrigerator.

Quantum Triangles

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A friend once told me that all that physicists ever measure is either a sine wave or a straight line. Nothing proves him wrong better than this piece of data we obtained by measuring on a semiconductor nanowire device (Ge/Si nanowires). These double-triangles represent a charge stability diagram of a double quantum dot. Charge stability actually happens in between the triangles, where current is zero (blue), and the number of charges on each of the two quantum dots is fixed. We don’t know how many charges are there on each dot but it is few tens. In the triangular regions, charges move because they have enough energy to jump from one quantum dot into the other and out into the lead that takes them to our ammeter. Inside the triangles, we get a glimpse of quantum energy levels, orbital and spin, of the particles trapped in quantum dots – those energy levels are the stripes of the triangles. The axes of the graphs are voltages on remote electrodes located 10 nanometers away from the nanowire, voltages on these electrodes (gates) are used to change the capacitive energy of the quantum dots.

Pittsburgh Nano Jobs

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Our university’s Nanoscience Institute (PINSE) is looking for a Technical Director to run the nanofabrication and characterization facility. The job is part of a major reinvestment effort, and the new Director will play a central role in how the facility will look and operate going forward. We want to attract a truly outstanding candidate, so we are trying to spread the word as far and wide as we can.

The job ad is here: PINSE Technical Director

http://nano.pitt.edu/

Universal glory here we come

Finally the ultimate geeks of the internet caught up with topological quantum computing. (You would think they would be the first to figure it out)

PhD Comics interviews Caltech theorists Gil Refael and Jason Alicea on the subject of anyons. Our Delft device makes a cameo.

And here is a recent article from MIT Technology Review talking about the hardware aspects of this:

http://www.technologyreview.com/featuredstory/531606/microsofts-quantum-mechanics/

Graduate prayer

Long-long time ago when I was a graduate student chasing, as most of us, a wild vision of my great advisor, I found myself trying again and again to fabricate devices in the cleanroom, cool them down, measure them – with no luck! My aspiration was to observe amazing never before seen physics, yet I could not even see the things from many decades ago like the supercurrent. At that point, as an act of Universal discourse, I printed a small request on a sheet of paper and posted it on the wall in front of my measurement computer. It said ‘DEAR SOMETHING, PLEASE WORK!’. Time passed, and through sweat and pain the god of Something bestowed her benevolence on me. I saw the supercurrent – I was so happy I called my advisor late in the evening. (I haven’t been so excited since I have spotted a guy named ‘Larry Cooper’ at my first March Meeting and took him for Leon Cooper, the discoverer of Cooper pairs and of mini-Cooper cars).

Anyhow, when I visited my old lab in Urbana last year my prayer was still attached to the wall. Only it was the second one. To the God of Everything.

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For an experiment to be successful, EVERYTHING has to work. And for me supercurrent was only the beginning. Once you got something working, your appetite and your paranoia grow exponentially. We are at this point right now at Pitt. As soon as everything works, something will come out.

Helium Liquefier

The construction of a facility to liquefy helium has started in the back of the Physics building at Pitt. Liquid helium is the blood of most cryogenic experiments, it becomes liquid at 4.2 degrees Kelvin (which is cold) and simply immersing your experiment into this liquid gives you this low temperature.

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So far the project literally only scratched the surface, in a short while a new space will be added to host this helium equipment. Pitt is very much forward-looking with this new construction. The prices of helium gas have been going up, and combined with liquefaction costs they more and more often make low temperature experiments simply unaffordable. What makes our liquefier cost-effective is that it comes with a recovery system: helium that boils off in the labs around campus will be pumped back to the liquefier rather than lost into atmosphere. Our lab was already constructed with recovery plumbing, so we will be able to use local organic liquid helium from a friendly liquefactor down the street.

How to cook a quantum nanowire device

The last few months our group has dedicated to nanofabrication. It is a lot like cooking in that we talk a lot about recipes, and we get to wear white overall suits which make you feel like a chef. It is however much more strict than cooking as every recipe has to be perfectly tweaked for the final result to work. I have decided to explain how nanowire devices come about cookbook-style.

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Ingredients: 1 chip nanowires, 1 2” wafer silicon substrates, PMMA and MIBK, titanium, gold, hafnium oxide, acetone and isopropanol to taste.

The main ingredient is a nanowire. It is of course highly recommended to get the best and fresh nanowires, here we shall use indium antimonide wires grown in Eindhoven, but we also have other great recipes for example with silicon-germanium grown in Los Alamos. Great, so put nanowires aside, we will need them a bit later.

We start by preparing the substrates. Here again our recipe is a little extravagant as we shall be using substrates from France, but you can use substrates from your local market. We prepare a whole 2” wafer and then cut it into smaller pieces for final devices. The fist step is to fabricate markers, they are needed as coordinates and reference points for other layers of nanofabrication. They are produced by electron beam lithography which can define the cross-shaped patterns shown below. Make sure you spin PMMA before doing lithography, and then develop it in MIBK for about a minute.

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The final pattern is obtained by evaporating titanium and gold and liftoff of the rest of PMMA. You need to evaporate these metals in vacuum, so make sure you have an electron beam or a thermal evaporator. While these crosses are relatively large, the meaning of nanofabrication really comes to life when we form the layer of gates, also by electron beam lithography. These are parallel lines that can be as thin as 20 nanometers. In the picture below you can see an image of gates in a standard microscope in pink. The pink rectangle in the middle is so dense that individual gates cannot be resolved. You also see yellow gold markers in the corners, they were used to position the gates in the center. A cyan rhombus is a layer of hafnium oxide dielectric which will eventually separates gates from nanowires to help avoid shorts. This layer is produced by atomic layer deposition, in which organic molecules carrying hafnium atoms coat the subtrate and then disintegrate leaving behind a single layer of oxide.

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After these steps our substrates are cut into 10 mm squares and get shipped from Grenoble to Pittsburgh. Here we take the Dutch nanowires and place them one by one onto the gates, as shown in this video.

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The nanowire is a bright white object in the center. You can also see better the gates in the picture above because this picture is taken with a scanning electron microscope. Although these pictures are always in grayscale reflecting the color of electrons.

All this is done in the cleanroom which looks like this:

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The last step is to contact the nanowire with electrodes. We take a picture of exactly how the nanowire lies on the gates, and then design contacts (in a computer) to cover the tips of the wire and its sides. We will later pass current and measure voltage across the nanowires through these contacts, that is the experiment part of our work.

We use the electron beam lithography machine shown in the picture above to write the pattern of contacts on top of the nanowire. The substrate and the wire are coated with PMMA which is a polymer that gets broken down by a beam of electrons during lithography. Where the molecules of PMMA were broken they come off in MIBK leaving openings. Metals like titanium and gold can be deposited in those openings.

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Finally, rinse the chip in acetone and isopropanol and you are ready to measure! Enjoy your experiments.

Pittsburgh Quantum Institute

Last year we have started PQI, originally known as ‘Pitt Quantum Initiative’. This year, with support from the University this initiative has began transforming into ‘Pittsburgh Quantum Institute’ that will embrace quantum research at Pitt and Carnegie Mellon. In line with the tradition started last year, we are going to have a conference that will bring together some of the leading researchers in the field of Quantum Technologies.

From http://pqi.pitt.edu/ :

Pitt Quantum Initiative event on “Quantum Technologies”, 9-11 April 2014

Our second annual PQI event will be held from Wednesday April 9th to Friday April 11th this year. The first two days will be held in the University Club, and a final morning session will take place in the Cathedral of Learning.

As with last year’s event, we will again cover a broad range of areas, including
Quantum Chemistry
Quantum Materials
Quantum Phases and Phase transitions
Quantum Transport
Quantum Gases
Quantum Devices
The program will present a broad range of research being performed in these fields, and will feature prominent invited keynote lecturers, as well as highlighting current research in the PQI. All of the talks will be colloquium-style, and accessible to a broad range of people in quantum-related fields.

Invited Lecturers

Roi Baer (Weizmann)
Garnet Chan (Princeton)
Michel Devoret (Yale)
David DiVincenzo (Aachen)
Greg Engel (Chicago)
Leticia Gonzalez (Vienna)
Jack Harris (Yale)
Evelyn Hu (Harvard)
Randy Hulet (Rice)
Alfred Leitenstorfer (Konstanz)
Ana Maria Rey (JILA)
Seigo Tarucha (Tokyo)

Probe holder

Fridge is not a luxury, but a means of refrigeration. Yet it costs like a luxury car, in the ballpark of those fancy italian Maserati’s and Lamborghini’s. In our case the fridge was even built by a company with italian roots. However, what would you say if your Maserati came without a cup holder? Even a basic one like this…

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Well, our fridges came without probe holders. Probes are very useful for quick sample turnaround. They are long sticks which can be used to load samples into a cold fridge thereby shaving off days from the sample exchange cycle. The probes are long and awkward to handle. So we talked to our machine shop in Physics at Pitt to add a probe holder to the system. And they came up with a nifty design:

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And here is how it looks with the probe being held by it:

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