For the first time, physicists have captured electrons in 3D crystals

Physicists at the Massachusetts Institute of Technology (MIT) have trapped electrons in a pure crystal, marking the first time that an electron flat band has been achieved in a three-dimensional material. This rare state of electrons is due to the special cubic arrangement of the atoms (pictured), similar to the Japanese art "kagome". This result provides a new way for scientists to explore rare electronic states in 3D materials. Source: Joseph Checkelsky, Riccardo Comin, et al

Electrons move through conductive materials, like commuters during rush hour in Manhattan. Charged particles may squeeze and collide with each other, but for the most part, they don't care about the other electrons because they each rush forward with their own energy.

But when the electrons of a material are trapped together, they can enter the same energy state and behave like one. In physics, this collective, zombie-like state is called an electron "flat band". Scientists predict that when electrons are in this state, they can begin to feel the quantum effects of other electrons and act in a coordinated quantum manner. Then, bizarre behaviors such as superconductivity and unique forms of magnetism may emerge.

Now, physicists at the Massachusetts Institute of Technology have succeeded in trapping electrons in pure crystals. This is the first time that scientists have realized electron planar bands in three-dimensional materials. Through some chemical manipulation, the researchers also showed that they could convert crystals into superconductors – a material that conducts electricity with zero resistance.

The atomic geometry of the crystal makes it possible to capture electrons. This crystal, synthesized by physicists, has an atomic arrangement similar to the weaving pattern in the Japanese basket weaving art "kagome". In this particular geometry, the researchers found that electrons were "locked in a cage" instead of jumping between atoms and fixed in the same energy band.

The researchers say this flat-band state can be achieved by almost any combination of atoms — as long as they follow this three-dimensional geometry inspired by kagome. The findings, published in the journal Nature, provide a new way for scientists to explore rare electronic states in three-dimensional materials. These materials may one day be optimized to enable ultra-efficient power lines, supercomputing qubits, and faster, smarter electronic devices.

"Now that we know that we can make flat bands out of this geometry, we have a lot of incentive to study other structures that may have other new physical properties that could become platforms for new technologies," said study author Joseph Chekelski, associate professor of physics.

Set 3D traps

In recent years, physicists have succeeded in capturing electrons in two-dimensional materials and confirming their electron flat-band states. However, scientists have found that electrons trapped in the second dimension can easily escape from the third dimension, which makes it difficult to maintain the planar band state in the second dimension.

In their new study, Checkelsky, Comin, and their colleagues hope to implement planar bands in 3D materials so that electrons can be captured in all three dimensions and any foreign electron states can be more stably maintained. They think the kagome model may have worked.

In previous work, the team observed trapped electrons in the two-dimensional atomic lattice, similar to some kagome designs. When atoms are arranged in interconnected triangles, the electrons are confined to the hexagonal space between the triangles instead of jumping on the crystal lattice. But, like others, the researchers found that electrons can escape from the crystal lattice through the third dimension.

The team wondered: Could a lattice-like three-dimensional structure wrap electrons together? They searched for answers in material structure databases and discovered a specific atomic geometry that is often classified as pyrochlorite – a mineral with a highly symmetrical atomic geometry. The atomic three-dimensional structure of pychlore forms a repeating pattern of cubes, each with a surface similar to a kagom-like lattice. They found that, in theory, this geometry could effectively capture electrons within each cube.

Rock falls

To test this hypothesis, the researchers synthesized a pyrochlorite crystal in the lab.

"It's no different from the way nature makes crystals," Chekelski explained. "We put certain elements together – in this case, calcium and nickel – at very high temperatures to melt them, cool them and the atoms themselves will be arranged into this crystal, similar to the structure of a kagome."

They then measured the energy of the individual electrons in the crystal to see if they fell into the same flat energy band. To do this, researchers typically perform photoemission experiments in which they irradiate a single photon onto a sample, releasing a single electron. The detector can accurately measure the energy of individual electrons.

Scientists have used photoemission to confirm the flat-band state of various two-dimensional materials. Due to their physical, two-dimensional nature, these materials are relatively simple to measure using standard lasers. But for 3D materials, this task is even more challenging.

"For this experiment, you usually need a very flat surface," Comin explains. "But if you look at the surface of these 3D materials, they're like the Rocky Mountains, with undulating landscapes. Experiments with these materials are very challenging, which is part of the reason why no one has proven that they contain trapped electrons. ”

The team cleared this barrier using angle-resolved optical emission spectroscopy (ARPES), a superfocused beam capable of targeting specific locations on an inhomogeneous 3D surface and measuring the energy of individual electrons at those locations.

"It's like a helicopter landing on a very small platform, all of which is on this rocky landscape," Comin said.

Using ARPES, the team measured the energy of thousands of electrons in a synthetic crystal sample in about half an hour. They found that the electrons of the vast majority of crystals exhibit the same energy, confirming the flat-band state of the 3D material.

To see if they could manipulate these coordinated electrons into some bizarre electronic state, the researchers synthesized the same crystal geometry, this time replacing nickel atoms with rhodium and ruthenium atoms. On paper, the researchers calculated that this chemical exchange should transfer the flat band of electrons to zero energy - a state that automatically leads to superconductivity.

In fact, they found that when they synthesized a new crystal with a slightly different combination of elements, the crystal's electrons took on a flat band, this time in a superconducting state.

"This provides a new paradigm for thinking about how to find new and interesting quantum materials," Comyn said. "We showed that this special composition of the atomic arrangement can trap electrons, and we can always find these flat bands. It wasn't just a lucky hit. From this point of view, the challenge is how to optimize the flat-band material to realize its potential to maintain superconductivity at higher temperatures. ”

More information: Joseph Checkelsky et al., Three-dimensional planar bands in pyrogreen metal CaNi2, Nature (2023). DOI: 10.1038 / s41586 - 023 - 06640 - 1。 www.nature.com/articles/s41586 - 023 - 06640 - 1

Journal Information: Nature