Scientists find twisting 3-D raceway for electrons in nanoscale crystal slices

The possibility of developing so-called “topological matter” that can carry electrical current on its surface without loss at room temperature has attracted significant interest in the research community. The ultimate goal is to approach the lossless conduction of another class of materials, known as superconductors, but without the need for the extreme, freezing temperatures that superconductors require.


“Microchips lose so much energy through heat dissipation that it’s a limiting factor,”


“The smaller they become, the more they heat up.”

The studied material, an inorganic semimetal called cadmium arsenide (Cd3As2), exhibits quantum properties—which are not explained by the classical laws of physics—that offer a new approach to reducing waste energy in microchips. In 2014, scientists discovered that cadmium arsenide shares some electronic properties with graphene, a single-atom-thick material also eyed for next-generation computer components, but in a 3-D form.






“What’s exciting about these phenomena is that, in theory, they are not affected by temperature, and the fact they exist in three dimensions possibly makes fabrication of new devices easier,”


The cadmium arsenide samples displayed a quantum property known as “chirality” that couples an electron’s fundamental property of spin to its momentum, essentially giving it left- or right-handed traits. The experiment provided a first step toward the goal of using chirality for transporting charge and energy through a material without loss.


couple A to B = AとBをつなぐ、AとBが対になる



右手と左手は同じ形をしているけど、お椀を重ねるようには重ねられないですよね? キラリティーとは「構成要素が同じなのに立体構造(右手系)がその鏡像(左手系)と空間的重ならない」という性質のことを言います。右手と左手の役割(例えば右利きとか左利きとか)が違うように右手系の物質と左手系の物質では性質が異なります。なのでキラリティーのある物質の右手系(あるいは左手系)だけを完全に創り分けたいというのが、物質科学の夢です。
人や生物を創る構成物質(アミノ酸や糖類)にもキラリティーがあります。でも不思議なことに 身体の中のアミノ酸は左手系だけで右手系はありませんし、糖類は右手系しかありません。ではなぜ生命体の構成物質には右手系か左手系のどちらか一つしかな いのでしょうか?この問題は「ホモキラリティー問題」として知られる未だベールに包まれた生命科学の大きな謎です。このように「キラリティー」は常に科学者の好奇心を掻き立てる魅力あふれるテーマなのです。




In the experiment, researchers manufactured and studied how electric current travels in slices of a cadmium arsenic crystal just 150 nanometers thick, or about 600 times smaller than the width of a human hair, when subjected to a high magnetic field.


The crystal samples were crafted at Berkeley Lab’s Molecular Foundry, which has a focus in building and studying nanoscale materials, and their 3-D structure was detailed using X-rays at Berkeley Lab’s Advanced Light Source.


Many mysteries remain about the exotic properties of the studied material, and as a next step researchers are seeking other fabrication techniques to build a similar material with built-in magnetic properties, so no external magnetic field is required.

“This isn’t the right material for an application, but it tells us we’re on the right track,”


If researchers are successful in their modifications, such a material could conceivably be used for constructing interconnects between multiple computer chips, for example, for next-generation computers that rely on an electron’s spin to process data (known as “spintronics”), and for building thermoelectric devices that convert waste heat to electric current.


It wasn’t clear at first whether the research team would even be able to manufacture a pure enough sample at the tiny scale required to carry out the experiment,


“We wanted to measure the surface states of electrons in the material. But this 3-D material also conducts electricity in the bulk—it’s central region—as well as at the surface,” he said. As a result, when you measure the electric current, the signal is swamped by what is going on in the bulk so you never see the surface contribution.”



So they shrunk the sample from millionths of a meter to the nanoscale to give them more surface area and ensure that the surface signal would be the dominant one in an experiment.


“We decided to do this by shaping samples into smaller structures using a focused beam of charged particles,” he said. “But this ion beam is known to be a rough way to treat the material—it is typically intrinsically damaging to surfaces, and we thought it was never going to work.”


But Philip J.W. Moll, now at the Max Planck Institute for Chemical Physics of Solids in Germany, found a way to minimize this damage and provide finely polished surfaces in the tiny slices using tools at the Molecular Foundry. “Cutting something and at the same time not damaging it are natural opposites. Our team had to push the ion beam fabrication to its limits of low energy and tight beam focus to make this possible.”

「しかし、現在、Philip J.W. Mollが、ドイツのマックス・プランク固体化学物理研究所でこのダメージを最小化する方法を発見し、分子工場でツールを使ってその微小薄片表面を精巧に磨けるようにしています。”何かを切ると同時にそれを傷付けないようにすることは本来正反対です。我々のチームは、この事を可能にするために、イオンビーム加工を低エネルギーと光束密度の限界までプッシュする必要がありました。”」

natural opposites, natural oppositeは、natural enemy(天敵)と考え方は一緒で、本質的に正反対(真逆)と言った意味合いです。tight beam focusは、beam focusは直訳すればビーム焦点ですが、この場合、ビーム集束度、ビーム収束度でもいいかと。焦点を絞るは考えを絞るという意味合いもあるのでややこしいし、要は、ビームを限界まで細くするという意味になれば全く問題ありません。



When researchers applied an electric current to the samples, they found that electrons race around in circles similar to how they orbit around an atom’s nucleus, but their path passes through both the surface and the bulk of the material.


The applied magnetic field pushes the electrons around the surface. When they reach the same energy and momentum of the bulk electrons, they get pulled by the chirality of the bulk and pushed through to the other surface, repeating this oddly twisting path until they are scattered by material defects.



The experiment represents a successful marriage of theoretical approaches with the right materials and techniques,


“This had been theorized by Andrew Potter on our team and his co-workers, and our experiment marks the first time it was observed,” Analytis said. “It is very unusual—there is no analogous phenomena in any other system. The two surfaces of the material ‘talk’ to each other over large distances due to their chiral nature.”

「”これは、我々のチームのAndrew Potterと彼の同僚達によって理論化され、我々の実験がそれを初めて観測しました”とAnalytisは語った。”それはとても珍しく、他のどのような系においても類似した現象は存在しません。材料の2つの表面が、それらのキラル特性のおかげで長距離をお互いに応答し合っています。”」

“We had predicted this behavior as a way to measure the unusual properties expected in these materials, and it was very exciting to see these ideas come to life in real experimental systems,”


“Philip and collaborators made some great innovations to produce extremely thin and high-quality samples, which really made these observations possible for the first time.”

Researchers also learned that disorder in the patterning of the material’s crystal surface doesn’t seem to affect the behavior of electrons there, though disorder in the central material does have an impact on whether the electrons move across the material from one surface to the other.

「”Philipと協力者達が、実際にこれらの観測を初めて可能にした極薄で高品質なサンプルを作り出すためのいくつかの偉大な革新を起こしました” 研究者達はまた、材料中心における障害が電子が1つの表面から他の表面へとその材料を横切って移動するかどうかに影響を与えるのにもかかわらず、材料の結晶表面のパターニングでの障害が、そこでの電子の動きに影響を与えるようには見えないことを学びました。」

The motion of the electrons exhibits a dual handedness, with some electrons traveling around the material in one direction and others looping around in an opposite direction.


dual handednessで両利き手、両利き、dual roleなら一人二役、二役、二重の役割という意味になります。dual memoryなんてのもあります。

Researchers are now building on this work in designing new materials for ongoing studies, Analytis said. “We are using techniques normally restricted to the semiconductor industry to make prototype devices from quantum materials.”