By shooting a semiconductor with ultra-fast laser pulses, scientists have discovered a new quasiparticle that behaves like a drop of liquid. They describe it as a quantum droplet, and named it "dropleton."

These things were not predicted under any theory and surprised scientists when they appeared unexpectedly in extremely low temperature semiconductor experiments. They have properties unlike anything seen before.

“At first we scratched our heads,” said physicist Steven Cundiff of the University of Colorado and National Institute of Standards and Technology in Boulder, one of the authors of a paper appearing today in Nature. “But then we came up with this idea that what we were seeing was this new thing we’re calling a quantum droplet.”

Now before you start asking questions like, “What?” and “Huh?” we probably need to break things down a little here.

There are materials, such as metals, that are good conductors of electricity. Inside of a conductor like copper wire are countless copper atoms arranged in a lattice. The electrons of the copper atoms become unbound from their nuclei and are free to flow, allowing them to easily carry a current. The opposite of this is an insulator, like rubber, in which electrons stay put.

Sitting between these two extremes are materials like silicon semiconductors, in which some of the electrons can freely move and conduct electricity, while others are stuck. Pure silicon is actually not a good semiconductor because all of its atoms are covalently bonded to their neighbors. The electrons spend all their time stuck being shared between the atomic nuclei and can’t flow. But introducing impurities that take the place of some of the silicon atoms can free up some of the electrons, creating a semiconductor.

image: Brad Baxley

Describing how particles inside a solid semiconductor move is pretty complicated. There are countless atomic nuclei and electrons, all pushing and pulling on one another via electromagnetic forces. Writing out equations to explain how each particle acts when interacting with another particle (and then the quadrillions of other particles in the material) quickly gets out of hand. Instead, scientists describe the properties of quasiparticles, a simplified way of looking at the group dynamics of all the particles together.

When a photon comes into a silicon semiconductor, it hits one of the atomic nuclei, kicking free an electron. Left behind is one type of quasiparticle known as an electron hole. The electron hole is sort of like an empty bubble situated within all the other electrons of all the other atomic nuclei in the silicon lattice. In the same way that an air bubble in a cup of water will rise while all the other water drops tend to fall, the electron hole behaves the opposite of an electron. When describing it using the equations of quantum mechanics, the hole even has a positive charge, compared to the electron’s negative charge.

If there’s anything you probably already know about charges, it’s that opposites attract. One electron and one hole can come together and create a quasiparticle known as an exciton. From a quantum mechanical point of view, the hole has properties similar to a proton. In this way, the exciton behaves like a neutral hydrogen atom, in which an electron and proton are bound together.

Hydrogen atoms are very stable, one of the reasons you see them everywhere in the universe. The binding energy that holds the electrons and protons together is relatively strong. But the binding energy keeping an electron and a hole together is quite weak, about 1,000 times smaller than that of a hydrogen atom. Any little energetic jostle will break the bond between electron and hole and so excitons only form at very low temperatures, around -263 degrees Celsius.

What Cundiff's team did was cool a gallium-arsenide semiconductor down to that temperature and shoot it with a laser. The laser photons generated free electrons, holes, and eventually excitons inside their semiconductor (all on extremely short timescales of a few trillionths of a second). As the researchers increased the intensity of the laser, it created more and more excitons. But so many excitons start to interfere with one another, and this weakens the bonds between their electrons and holes. At a certain laser intensity, excitons can no longer form.

Image: Martin Mootz

So far, all standard stuff (or at least, standard for condensed matter physics researchers). Next, though, the team shifted the wavelength of the laser down a little and then shot it at the gallium arsenide. Now, the laser pulses created electrons, holes, and excitons. But the excitons could also come together into quasiparticles called a biexcitons, made of two excitons. In the same way that excitons are analogous to a hydrogen atom, a biexciton is like a hydrogen molecule, H 2 . The researchers also expected that the binds between these biexcitons would weaken as they increased their laser’s intensity.

“Then we noticed something very peculiar,” Cundiff said.

The biexcitons actually became more strongly bound, seeming to form a completely new configuration of four electrons and holes. The experiments also created quasiparticles of five electrons and holes, and six electrons and holes. “We were puzzled,” said Cundiff. “Hydrogen atoms don’t do this.”

After checking with some mathematical models, they realized they had discovered something completely new. In the exciton, the electrons and holes were forming something like hydrogen atoms. And then in biexciton, the excitons were regularly spaced apart from one another, just like atoms in a molecule. But in this new quasiparticle, the electrons and holes no longer had a fixed position relative to one another. Instead, they jostled like a small drop of liquid, hence the name dropleton.

By performing different control experiments, the team was able to eliminate the possibility that what they were seeing corresponded to any other known quasiparticles, says chemist Daniel Turner of New York University, who was not involved with this work. “Out of this complicated goo of electrons and holes, they’re able to distinguish a new phenomenon,” said Turner.

The finding doesn’t have any immediate applications – “I don’t think somebody’s going to build a device based on a quantum droplet,” said Cundiff – but is important for basic understanding of complex phenomena inside solid-state devices such as semiconductors and superconductors.