Electrons take on a new shape inside an unconventional metal

One of the great achievements of quantum physics has been to reformulate our view of the atom. The outside was the early 20th century model of a solar system in miniature, with electrons orbiting a solid core. Instead, quantum physics has shown that electrons live more interesting lives, zigzagging around the nucleus in clouds that look like little balloons. Known as atomic orbitals, these balloons come in all sorts of different shapes — perfectly circular, double-lobed, shaped like clover leaves. The number of lobes in the balloon indicates how much the electron orbits around the nucleus.

All of this is well and good for the individual atomsBut when atoms come together to form something solid—like a piece of metal, for example—the outermost electrons in the atoms can link arms and lose sight of the nucleus whence they came, forming many huge balloons that stretch out over the entire piece of metal. Stop spinning around cores and flow through the metal to carry electric currents, throwing off the multi-lobed balloon variety.

Now, researchers at the Quantum Materials Center (QMC) at the University of Maryland (UMD), in collaboration with theorists at the Center for Condensed Matter Theory (CMTC) and the Joint Quantum Institute (JQI), have produced the first experimental evidence for the existence of a single metal–and it likely has Others in their class are electrons that are able to maintain a more interesting multi-lobed structure as they move through a solid. The team experimentally studied the shape of these balloons and found that it was not a uniform surface, but rather a complex structure. This unusual metal is not only fundamentally interesting, but may also be useful for building noise-resistant quantum computers.

The researchers recently published their findings in the journal Physical review research.

“When I first discovered this, I was really excited,” says Hyunsoo Kim, a former postdoctoral researcher at QMC and lead author of the work. “But it took years to fully study, because it is not a traditional concept and also empirically difficult to put together High quality data. ”

In 2011, the team discovered for the first time that the metal in question — yttrium platinum bismuth, or YPtBi — can become a superconductor. Some materials become superconductors at low enough temperatures, and lose all resistance to electric current. YPtBi was an unlikely candidate for superconductivity because it has fewer mobile current-carrying electrons than most superconductors. But, to the researchers’ surprise, the technology became superconducting anyway. Moreover, the way it behaved when exposed to a magnetic field provided evidence that it was not an ordinary superconductor.

At the time, the researchers suspected that the shape of the electron orbitals was to blame and concluded that electrons that orbit themselves and trace more circles into space — that is, electrons with higher angular momentum — constitute an unprecedented case of superconductivity.

“We had what I would call circumstantial evidence that superconductivity consists of electron pairs with high angular momentum,” says Junpier Baglione, professor of physics at UMD, director of QMC, and lead of the experimental group in this collaboration. . “But there was no direct evidence for these high angular momentum electrons.”

To collect more direct evidence in the new experiments, the team turned up the temperature and studied matter in its natural, non-superconducting state. Next, they performed a classic measurement that plots something like a collective atomic orbital for all of the electrons floating in the metal.

When looking inside a mineral, one sees the atoms arranged in ordered repeating lattices called a crystal lattice. In a crystal, the atomic orbitals of the outermost electrons shift to each other. This allows the electrons to travel away from their original nuclei and carry current through the metal. In this rigid setup, a version of balloons orbiting still exists, but it is more common to visualize them not in space—where there are many bulky, unwieldy orbitals—but as a function of the speed and direction of traveling electrons. The fastest moving electrons in a crystal form their own balloon, a collective analog of the atomic orbitals known as the Fermi surface.

The shape of the Fermi surface reflects the basic crystal structure, which is not usually similar to the orbital structure of single atoms. But for materials like YPtBi that have very few mobile electrons, the Fermi surface is not very large. Because of this, they retain some of the properties of the electrons that hardly move at all, which are located in the center of the Fermi surface.

“The fact that nature determines the non-intuitive atomic arrangements that allow Fermi surfaces to retain signatures of atomic orbitals is fascinating and somewhat complex,” says Jai Dip Sao, co-director at JQI and colleague Jai Dip Sao, associate professor of physics at UMD and professor of theory. Collaborator on the new paper.

To reveal the fascinating and counterintuitive Fermi surface, the researchers placed a YPtBi crystal inside a magnetic field and measured the current flowing through the crystal as the field was adjusted. by rotating direction magnetic field, they were able to determine the speed of the fastest electrons in each direction. They found that, similar to a higher angular atomic orbit, the Fermi surface has a complex shape to it, with peaks and troughs along certain directions. The higher symmetry of the same crystal usually leads to a more uniform and ball-like Fermi surface, so it was surprising to find a more complex structure. This indicates the possibility that the collective electrons were exhibiting some higher angular momentum nature atomic orbitals.

In fact, theoretical calculations by the CMTC team showed that the experimental results matched the high-angular momentum model, leading the team to claim the first experimental observation of a high-angular momentum metal. The team cautions that even this empirical evidence may still be incomplete. What they measured depended not only on the Fermi surface but also on other properties of the electrons, such as their effective mass and the distribution of their velocities. In their work, the team systematically studied the angular dependence of these other quantities and showed that it was extremely unlikely for them to cause the observed peaks and troughs.

In addition to being fundamentally new, this metal with higher angular momentum has potential applications for quantum computing. There are predictions that some exotic superconducting state could give rise to properties that are not affected by the noise that occurs at any given time. These properties may be able to encode quantum bits, potentially allowing for the creation of more powerful quantum computers. It remains to be seen if YPtBi is bizarre in the right way for this to happen, but the new work is an important step toward finding out.

“There are many pieces to the puzzle of understanding what type of superconductor you have and whether you can exploit it to perform quantum computations,” says Baglioni. “There are some experimental challenges to getting the rest of the puzzle through. But I think we’re a big part of the way there.”