Physicists flip pencil lead into metaphorical ‘gold’

MIT physicists and colleagues have metaphorically turned graphite, or pencil lead, into gold by isolating 5 ultrathin flakes stacked in a particular order. The ensuing materials can then be tuned to exhibit three vital properties by no means earlier than seen in pure graphite.

“It’s form of like one-stop buying,” says Lengthy Ju, an assistant professor within the MIT Division of Physics and chief of the work, which is reported within the Nature Nanotechnology. “On this case, we by no means realized that every one of those attention-grabbing issues are embedded in graphite.”

Additional, he says, “It is vitally uncommon to search out supplies that may host this many properties.”

Graphite consists of graphene, which is a single layer of carbon atoms organized in hexagons resembling a honeycomb construction. Graphene, in flip, has been the main target of intense analysis because it was first remoted about 20 years in the past. Then about 5 years in the past researchers together with a staff at MIT found that stacking particular person sheets of graphene, and twisting them at a slight angle to one another, can impart new properties to the fabric, from superconductivity to magnetism. The sphere of “twistronics” was born.

Within the present work, “we found attention-grabbing properties with no twisting in any respect,” says Ju, who can also be affiliated with the Supplies Analysis Laboratory.

He and colleagues found that 5 layers of graphene organized in a sure order enable the electrons transferring round inside the fabric to speak with one another. That phenomenon, referred to as electron correlation, “is the magic that makes all of those new properties potential,” Ju says.

Bulk graphite—and even single sheets of graphene—are good electrical conductors, however that is it. The fabric Ju and colleagues remoted, which they name pentalayer rhombohedral stacked graphene, turns into way more than the sum of its elements.

Novel microscope

Key to isolating the fabric was a novel microscope Ju constructed at MIT in 2021 that may rapidly and comparatively inexpensively decide a wide range of vital traits of a fabric on the nanoscale. Pentalayer rhombohedral stacked graphene is only some billionths of a meter thick.

Scientists together with Ju had been in search of multilayer graphene that was stacked in a really exact order, referred to as rhombohedral stacking. Says Ju, “there are greater than 10 potential stacking orders whenever you go to 5 layers. Rhombohedral is only one of them.” The microscope Ju constructed, referred to as Scattering-type Scanning Nearfield Optical Microscopy, or s-SNOM, allowed the scientists to establish and isolate solely the pentalayers within the rhombohedral stacking order they had been all for.

Three in a single

From there, the staff connected electrodes to a tiny sandwich composed of boron nitride “bread” that protects the fragile “meat” of pentalayer rhombohedral stacked graphene. The electrodes allowed them to tune the system with totally different voltages, or quantities of electrical energy. The consequence: they found the emergence of three totally different phenomena relying on the variety of electrons flooding the system.

“We discovered that the fabric may very well be insulating, magnetic, or topological,” Ju says. The latter is considerably associated to each conductors and insulators. Basically, Ju explains, a topological materials permits the unimpeded motion of electrons across the edges of a fabric, however not by the center. The electrons are touring in a single path alongside a “freeway” on the fringe of the fabric separated by a median that makes up the middle of the fabric. So the sting of a topological materials is an ideal conductor, whereas the middle is an insulator.

“Our work establishes rhombohedral stacked multilayer graphene as a extremely tunable platform to review these new prospects of strongly correlated and topological physics,” Ju and his co-authors conclude.

Along with Ju, authors of the paper are Tonghang Han and Zhengguang Lu. Han is a graduate pupil within the Division of Physics; Lu is a postdoctoral affiliate within the Supplies Analysis Laboratory. The 2 are co-first authors of the paper.