First author

Some semiconductors and mobile-phone amplifiers include components known as two-dimensional electron systems (2DES). To design such devices, the first, fundamental piece of information you need is the energy spectrum of the system. This gives an indication of how much current flows for a given applied voltage. But the energy spectrum of a 2DES also contains much more subtle information and can reveal unusual quantum effects. Previously, such detailed information could be obtained only from theoretically calculated spectra, as the available experimental techniques generated heat and blurred the results. Now, Oliver Dial at the Massachusetts Institute of Technology and his colleagues have developed a method that can directly measure this spectrum at high resolution (see page 176).

How is this different from previous work?

The ocean makes a good analogy. Until now, researchers looked only at how ripples — system disturbances — affected the ocean's surface, or the lowest energy states available. Now, we can look at how many energy states are available at any place in the ocean — we can see the mountains and valleys on the ocean floor. In other words, the high energy states as well as the low. We get an information-rich spectrum, with peaks that tell us at what energy a particle can be put into the system and how long it can stay at that energy.

How does your method work?

It's difficult to look at the high-energy spectrum of a 2DES without it heating up. Among the challenges, we had to figure out how to inject electrons at energies of 100 kelvin, while keeping the experiment temperature at 100 millikelvin. Most researchers in this field want to do in-plane electron-transport measurements in 2DES. If you imagine an electron system as a flat sheet of paper, this involves adding electrons at one end and taking them out of the other, but this generates heat. Instead, we're pushing electrons into the middle of the sheet of the paper from above or below, with electrodes above and below to drive the current.

What was your most surprising finding?

One of the basic questions to ask in 2DES is 'How do the electrons get along together?'. One way to learn about this is to know what happens to an added electron. We might think that an extra electron would be bumped by the ones already there and wouldn't stay at the energy at which it was injected for very long. Remarkably, such electrons are quite stable. As a result, even though this is a complicated system, we can think about the extra electron as if it were alone.