It’s not much of an exaggeration to say that electrons form the backbone of the modern world. They are our workhorses—they bring energy from power plants to our homes and factories. They are our couriers—they carry information through circuits in our computers. Some day, they will be our providers—as the central elements in photovoltaic solar cells, they will capture the sun’s energy for our use. These current and future electronic devices rely on very precise answers to the question What do electrons do inside materials? Out of technological need and intellectual curiosity, condensed matter physicists have spent over a century discovering better and better answers—and they continue to do so today. In this post, I will try to give you a little glimpse into the form, beauty, and utility of such an answer.
As some of you know, I am currently a graduate student in an experimental condensed matter physics research group. I’m sure that physics graduate school is quite mysterious to people who aren’t in it, and one of my goals for this blog is to explain what I do there and why I choose to do it. I hope my thoughts can be useful to both prospective physics students and to my non-physicist friends and family.
This certainly is a big task, and there’s no chance of me completing it in a single post. Instead, I’d like to start by telling you about the “a-ha moment” when I suddenly realized that condensed matter physics was interesting. It happened on Friday, March 6, 2009, sometime between 3:20 PM and 4:40 PM. It was the spring semester of my sophomore year of college; I was sitting in my thermal physics class, and we were learning about phase diagrams. In particular, my professor showed us the phase diagram for H2O, which I will now show you:
There is a clear scientific meaning to this picture. Each point on the diagram represents a pressure value and a temperature value; the diagram itself tells us whether H2O will be a solid (ice), liquid (water), or gas (steam) at that particular combination of temperature and pressure. The thick black lines show the locations of phase transitions where H2O changes from one phase to another.
Until this point in my life, I had always felt a sort of impersonal interest in physics. But something suddenly “clicked” when I saw this phase diagram. I had a big realization—that scientific facts really only come alive when interpreted by a human and related back to human experience. I don’t know why this happened. It may have been that I had finally matured enough to realize this, or I may have just needed something special to come along and shake me out of my “scientific stupor.” At any rate, I had an epiphany, which I would now like to share with you.
The first thing to notice is that the phase diagram contains all of our common experiences with H2O. Atmospheric pressure at the surface of the Earth is around 1 bar, which you can find as a horizontal line on the graph. As we travel along this line from low temperature to high temperature, we see that H2O changes from ice to water at 273 K (equivalent to 0°C or 32°F) and from water to steam at 373 K (equivalent to 100°C or 212°F).
The second thing to notice is that the phase diagram contains so much more than all of our common experiences with H2O! People live at pressures near 1 bar and only deal with temperatures between 0-100°C, but H2O exists in a much wider range, and it does many other things besides melt and boil. Let’s discuss three of them:
- At low pressures—say, 0.001 bar—water does not exist at any temperature. H2O goes straight from ice to water vapor in a process called sublimation.
- There is a single point on the diagram where ice, water, and steam can all coexist. (I marked it with a red dot.) It is located at a pressure of 0.006 bar and a temperature of 0.01°C. Imagine visiting a planet whose atmospheric pressure was 0.006 bar. Water would only exist if the temperature was exactly 0.01°C. If you tried to melt ice, you would find that it simultaneously turned into both water and steam! (The picture below shows solid argon doing this at atmospheric pressure.)
- There is another special point where the dividing line between liquid and gas simply disappears. (I marked it with a blue dot.) It is located at a pressure of 221 bar and a temperature of 374°C. At greater temperatures and pressures, there just isn’t a difference between liquid and gas. An interesting consequence of this is that you could turn water into steam without boiling it if you could first raise its pressure to 230 bar, then increase its temperature to 380°C, and finally decrease its pressure back down to 1 bar.
In one sense, H2O’s phase diagram vividly demonstrates how narrow the human experience is. (Well, it demonstrates one aspect of a much larger narrowness.) We think that ice melts into water, but sometimes it sublimates into steam, and sometimes it does both. We think of water and steam as totally different, but under some circumstances, they aren’t. Living at a single pressure is like always looking in the same direction. Learning about this phase diagram is like suddenly turning your head to the left and right. A whole new vista becomes visible.
In another sense, the diagram shows that even boring everyday substances can become interesting when they’re subjected to unusual conditions. People like to travel because new and different surroundings are fun and exciting; physics teaches us that changing spatial coordinates is not the only way to achieve this effect. Moving to exotic temperatures and pressures is a whole new way to travel.
We can ask a lot of questions about phase diagrams. For example:
- Do all substances have similar phase diagrams?
- If not, what changes between substances? Are there certain groups that have similar phase diagrams?
- Why are different phases stable in different regions?
- Can we predict the phase diagram for a particular substance?
- Are there other kinds of phases besides solid, liquid, and gas?
- Are there other parameters besides temperature and pressure that can change a substance’s phase?
- Are there different kinds of phase transitions?
Answers to these questions can satisfy out natural curiosity about the physical world and lead to new technological applications. One goal of condensed matter physics is to answer them as fully as possible.
Phase diagrams didn’t point me unambiguously towards condensed matter physics, but they did open my mind to it. They taught me a new way of thinking, and they opened my internal eyes to viewless vistas. Ever since that day in March 2009, I have had a soft spot for them in my “physics heart,” and I hope you can at least partially appreciate why.
- The data for the melting and sublimation lines of water are from “International Equations for the Pressure along the Melting and along the Sublimation Curve for Ordinary Water Substance”, J. Phys. Chem. Ref. Data, Vol 23, No 3, 1994.
- The data for the boiling line of water are from IAPWS formulation for Industrial Use, 1997.
- The picture of argon ice is from http://en.wikipedia.org/wiki/File:Argon_ice_1.jpg.