{"id":3114,"date":"2021-10-27T10:00:00","date_gmt":"2021-10-27T10:00:00","guid":{"rendered":"https:\/\/modernsciences.org\/staging\/4414\/?p=3114"},"modified":"2021-10-13T07:20:47","modified_gmt":"2021-10-13T07:20:47","slug":"scientists-crafted-a-crystal-made-out-of-electrons-and-they-look-like-honeycombs","status":"publish","type":"post","link":"https:\/\/modernsciences.org\/staging\/4414\/scientists-crafted-a-crystal-made-out-of-electrons-and-they-look-like-honeycombs\/","title":{"rendered":"Scientists Crafted a \u201cCrystal\u201d Made Out of Electrons\u2014And They Look Like Honeycombs"},"content":{"rendered":"\n

When imagining a crystal, people often think of a network of atoms fixed into a specific spot, forming a repeating pattern of atoms called a lattice<\/em>. These ordered atoms create a regularity in properties for crystals, leading into the common crystalline characteristics that we know today, like face<\/em> and cleavage<\/em>.<\/p>\n\n\n\n

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The properties we usually associate with crystals, like the way they have \u201cfaces\u201d or the way they shine when shined on with light at specific angles, is more often than not a product of the regularity of the atoms within them. These properties can be clearly seen in crystals, like the diamond in this engagement ring. (Estate Diamond Jewelry\/Wikimedia Commons, 2019) <\/figcaption><\/figure><\/div>\n\n\n\n

Scientists have used this tendency of crystals to impart upon them several properties, like color. Truth be told, it\u2019s the difference in atomic components inside crystal lattices themselves that make these crystals different from one another. For diamonds, the atomic units are made of carbon (C) atoms; for rubies, it\u2019s aluminum oxide (Al2<\/sub>O3<\/sub>) with a sprinkle of chromium (Cr; the chromium is what gives rubies their signature red color).<\/p>\n\n\n\n

Some scientists over at the University of California, however, decided to address a question that was first asked almost 90 years ago: what if we could make a \u201ccrystal\u201d out of electrons<\/em>?<\/p>\n\n\n\n

That question was the brainchild of Hungarian physicist Eugene Wigner back in 1934. In essence, electrons are always zipping about from place to place, as is the case when you look at the surfaces of pure metals. However, once you cool down the said metal, these electrons start to slow; once you slow them enough, the repulsive interactions these electrons have with each other start to dominate (remember, electrons are negatively charged).<\/p>\n\n\n\n

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Eugene Wigner theorized that, if cooled down enough such that kinetic energy becomes overpowered by potential energy, the electrons\u2019 repulsive interactions will take over. And as these electrons are equally repulsing one another, they will soon\u00a0 arrange themselves and form honeycomb-like structures within a confined space, such as this flat representation of about 600 electrons. The triangles and squares represent \u201ccrystal defects\u201d of interruptions in the crystal structure, much like real crystals. (Arunas\/Wikimedia Commons, 2011) <\/figcaption><\/figure><\/div>\n\n\n\n

Now, assuming that they\u2019re confined to a space just an electron thick and that these electrons are equally repulsive to every other electron, they will tend to arrange themselves in a pattern wherein their repulsive interactions with each other electron surrounding them are equal. This leads them to form a structure reminiscent of honeycombs in bee hives. These electron \u201ccrystals\u201d have been coined Wigner crystals<\/em> after the person who first theorized their existence.<\/p>\n\n\n\n

And now, that\u2019s precisely what scientists just found, some 87 years after Wigner first pondered the question. A team of scientists over at the University of California (UC), Berkeley, just captured the first confirmed images of Wigner crystals, made purely of electrons. It took some fine-tuning to get these hard-earned images, though.<\/p>\n\n\n\n

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The image obtained by the team from the University of California, Berkeley, reveal the \u201choneycomb\u201d lattice of two-dimensional Wigner crystals\u2014just as how Wigner theorized it some 87 years ago. (Li et al, 2021) <\/figcaption><\/figure><\/div>\n\n\n\n

The team recognized that this isn\u2019t the first time these Wigner crystals have been made, though; what remains unique to their study is that the images they managed to take are our clearest proof yet of this bizarre material\u2019s existence.<\/p>\n\n\n\n

“If you say you have an electron crystal, show me the crystal,” study co-author and UC Berkeley physicist Feng Wang said in an interview with Nature<\/em>. Wang and team also published their findings in the said journal.<\/p>\n\n\n\n

The team managed to \u201ctrap\u201d these electrons and keep them a layer thick by essentially forming a sandwich of electrons. They formed two atom-thick sheets of material to serve as the \u201cbread\u201d of the sandwich: one made out of tungsten disulfide (WS2<\/sub>) and the other made out of tungsten diselenide (WSe2<\/sub>). Since the two are semiconductors, electrons can flow along their surfaces if their properties are tuned just right.<\/p>\n\n\n\n

In order to keep the electron \u201cfilling\u201d between these two sheets from overflowing, Wang and team applied an electric field across the sandwich. Afterwards, they cooled the entire thing to just five (5) degrees above absolute zero (-268.15 \u00b0C; -450.67 \u00b0F). In doing so, they stopped most of the electron movement, making their repulsive interactions with each other dominant.<\/p>\n\n\n\n

To achieve the honeycomb image of the Wigner crystal, Wang and team proceeded to use a scanning tunneling microscope (STM) to take these images. STMs are capable of taking \u201cimages\u201d of very small objects\u2014smaller than the wavelengths of visible light\u2014by measuring the current the STM tip<\/em> measures at each point on the surface of the said item.<\/p>\n\n\n\n

They had to place a single layer of graphene above the electron \u201csandwich\u2019 in order to keep the detector tip of the STM from heating the sandwich and causing the electrons to move again. Once they were able to assemble a \u201ccontour map\u201d of the electron sandwich based on the current measurement, they obtained the honeycomb image of the Wigner crystal\u2014just as Wigner himself predicted.<\/p>\n\n\n\n

Other scientists laud the graphene layer \u201ctrick\u201d Wang and team employed on their study, and hope that this method can be further developed to image other physical phenomena similar to Wigner crystals.<\/p>\n\n\n\n

(For more news on crystals, check out our piece on the future of perovskite crystals as nuclear radiation detectors<\/a>. Similarly, read further on the odd discovery of a \u201cquasicrystal\u201d under a nuclear test site<\/a>. Finally, if you\u2019re curious about graphene instead, check out our piece on how water behaves oddly when freezed atop it<\/a>.)<\/p>\n\n\n\n

References<\/h2>\n\n\n\n