Introduction and motivation

In an effort to get more content on Tildes, I want to try and give an introduction on several 'hot topics' in semiconductor physics at a level understandable to laypeople (high school level physics background). Making physics accessible to laypeople is a much discussed topic at universities. It can be very hard to translate the professional terms into a language understandable by people outside the field. So I will take this opportunity to challenge myself to (hopefully) create an understandable introduction to interesting topics in modern physics. To this end, I will take liberties in explaining things, and not always go for full scientific accuracy, while hopefully still getting the core concepts across. If a more in-depth explanation is wanted, please ask in the comments and I will do my best to answer.

Today's topic

I will start this series with an introduction to spintronics and spin transistors.

What is spintronics?

Spintronics is named in analogy to electronics. In electronics, the flow of current (consisting of electrons) is studied. Each electron has an electric charge, and by pulling at this charge we can move electrons through wires, transistors, creating modern electronics. Spintronics also studies the flow of electrons, but it uses another property of the electrons, spin, to create new kinds of transistors.

What are transistors?

Transistors are small electronic devices that act as an on-off switch for current. We can flip this on-off switch by sending a signal to the transistor, so that the current will flow. Transistors are the basis for all computers and as such are used very widely in modern life.

What is spin?

Spin arises from quantum mechanics. However, for the purpose of explaining spin transistors we can think of an electron's spin as a bar magnet. Each electron can be thought of as a bar magnet that will align itself to a nearby magnetic field. Think of it as a compass (the electron) aligning itself to a fridge magnet when it's held near the compass.

What are spin transistors and how do they work?

Spin transistors are a type of transistor whose on-off switch is created by magnets. We take two bar magnets, whose north poles are pointed in the same way, and put them next to each other, leaving a small gap between them. This gap is filled with a material through which the electrons can move. Now we connect wires to the big bar magnets and let current (electrons!) flow through both magnets, via the gap. When the electrons go through the first magnet, their internal magnets will align themselves to the big bar magnet. However, once they are in the gap the electrons' internal magnets will start rotating and no longer point in the same direction as the big bar magnets. So that when the electrons arrive at the second magnet, they will be repelled just like when you try to push the north poles of two magnets together. This means the current will not flow, and the device is off! So, how do we get it to turn on?

By exposing the gap to an electric field, we can control the amount of rotation the electrons experience (this is called the Rashba effect). If we change the strength of this electric field so that the electrons will make exactly one full rotation while crossing the gap, then by the time they reach the second big bar magnet they will once again be pointing in the right direction. Now the electrons are able to move through the second big bar magnet, and out its other end. So by turning this electric field on, the spin transistor will let current flow, and if we turn the electric field off, no current will flow. We have created an on-off switch using magnets and spin!

That's cool, but why go through the effort of doing this when we have perfectly fine electronics already?

The process of switching between the on and off states of these spin transistors is a lot more energy efficient than with regular transistors. These types of transistors leak a lot less too. Normal transistors will leak, meaning that a small amount of current will go through even when the transistor is off. With spin transistors, this leak is a lot smaller. This once again improves the energy efficiency of these devices. So in short, spin transistors will make your computer more energy efficient. This type of transistor can also be made smaller than normal transistors, which leads to more powerful computers.

Feedback and interest

As I mentioned, I wrote this post as a challenge to myself to explain modern physics to laypeople. Please let me know where I succeeded and where I failed. Also let me know if you like this type of content and if I should continue posting other similar topics in the same format.

A press release about the University of Queensland's review of the old Stanhope "prison" experiment: New interpretation of one of psychology’s most famous experiments

The review itself: Rethinking the ‘nature’ of brutality: Uncovering the role of identity leadership in the Stanford Prison Experiment

Here's a news article about an HIV vaccine being tested on humans "in the field": Scientists hopeful as HIV vaccine candidate passes key test

Here's the scientific report: Evaluation of a mosaic HIV-1 vaccine in a multicentre, randomised, double-blind, placebo-controlled, phase 1/2a clinical trial (APPROACH) and in rhesus monkeys (NHP 13-19)

Most spaces flying the flag of science are often unfortunately exclusive in their focus on STEM sciences. In order to combat such a monopoly and until such time as Tildes opens up groups for the social sciences and humanities, I'd like to open this place up to discussion around some of the disciplines which have always personally interested me more than, say, astronomy or biology. Is anyone else here interested in sociology, archaeology, anthropology, linguistics..? Has anyone pursued work in those fields? Any interesting perspectives to offer or news of recent breakthroughs in any of those areas? All discussion is welcome.

As for myself, I'm particularly interested in sociocultural anthropology and archaeology--in the latter case, specifically as relates to the Neolithic and Bronze Age Near East. I'll soon be pursuing a degree in anthropology with an archaeological orientation at the University of Buenos Aires and hope to be working in the field soon after the end of my studies. I'm also incidentally interested in sociology, philosophy, and literature studies, but don't have any plans at the moment to pursue academic study thereof. Any questions? Feel free to ask.

I watched a documentary about Stonehenge tonight, and it proposed the theory that the location for Stonehenge was chosen because of a natural geological feature in the area.

There's a man-made path that proceeds south-west towards Stonehenge: "The Avenue". This path was built around the same era as Stonehenge itself. If you walk westward along The Avenue on the winter solstice, you'll be facing the point on the horizon where the sun sets. However, under The Avenue, there's an old natural geological formation from the time of the Ice Age: a series of ridges in the rock which just coincidentally align with the sunset on the winter solstice (an "axis mundi"). Before Stonehenge was built, there was a chalk knoll on that location. That meant that you could walk along a natural geological path towards the sunset on the shortest day of the year, and there was a local geological landmark in front of you.

The theory is that these natural geological formations coincidentally aligning with an astronomical phenomenon made the site a special one for early Britons. That's why there was a burial site there, and later Stonehenge was built there.

Here's the article by the archaeologist who discovered the Ice Age ridges: Researching Stonehenge: Theories Past and Present