Physics Duo Finds Magic in Two Dimensions

The trained eye can see that Molybdenite is almost exactly the same color as graphite. It does the same thing as well, making for a good pencil filling. The two grids of atoms are not the same. The scientific record began to record the distinction in the late 19th century. The Swedish chemist known for his discovery of oxygen plunged each mineral into various acids and watched the fumes billowed forth. The man who paid for this approach with his life, died of a suspected heavy metal poisoning at the age of 43. He wrote a letter to the Royal Swedish Academy of Science in 1778 and said that he did not refer to it as commonly known. The transition metal is not known.

Molybdenite was a popular lubricant in the 20th century. Skis were able to glide farther through the snow and the bullet exit from rifle barrels was smoothed.

The same flakiness is driving a physics revolution today.

The first breakthrough was made with Scotch tape. In 2004, researchers discovered that they could use tape to peel off a single atom of graphite. The properties of these sheets were vastly different from those of the three-dimensional crystal they came from. Graphene was a new type of substance that was 2D. Condensed matter physics is a branch of physics that seeks to understand the many forms and behaviors of matter. Condensed matter physicists are the ones who brought us computer chips, lasers, LEDs, and solar panels. Thousands of physicists began studying the new material after it was discovered.

The discoverers of Graphene received a prestigious prize in physics. The signs that molybdenite might be even more magical than Graphene were seen by two young physicists. The lesser-known mineral has properties that make it difficult to study, but it caught the attention of Mak. The duo spent nearly a decade wrangling 2D molybdenite and a family of 2D crystals.

Their efforts are starting to pay off. 2D crystals of Molybdenum disulfide and its relatives can give rise to an enormous variety of exotic quantum phenomena, thanks to the work of a married couple. James Hone is a researcher at Columbia who supplies the Cornell lab with high-quality crystals. All of the modern physics can be done in a single material system.

The electrons behaved in unprecedented ways in these crystals. The particles are merged into a quantum fluid and frozen into ice. They have learned to assemble grids of artificial atoms that are now being used as test beds. The master electron tamers have published at least eight papers in Nature since opening their Cornell lab. The couple is expanding their understanding of what electrons can do.

Philip Kim is a prominent Condensed Matter physicist at Harvard University. I think it's sensational.

Rise of 2D Materials

electrons are reflected in a material's attributes electrons sail between atoms with ease in conductors Wood and glass have electrons in them. Silicon is ideal for switch currents on and off because of its electrons being forced to move with an influx of energy. The lightweight charged particles have behaved in many more exotic ways over the last 50 years.

In 1986 two IBM researchers detected a current of electrons moving through a copper oxide crystal without resistance. The ability of electricity to flow with perfect efficiency had been seen before, but only for certain reasons. Bednorz and Mller observed a strange phenomenon that persisted at a record-breaking 35 kelvins. Scientists found other cuprates that conduct above 100 kelvins. The number one goal of Condensed Matter physics is to find a substance that can superconduct electricity in our hot, roughly 300-kelvin world.

The key to superconductivity is to get the electrons to pair up. A quantum fluid can be melded into a group of bois. It is possible to overcome the repulsion of electrons with attractive forces. Superconductivity can't be found in everyday devices because of the extreme conditions. The discovery of cuprates raised hopes that the atomic lattice could glue the electrons together so they wouldn't get stuck.

40 years after Bednorz and Mller found cuprates, theorists still don't know how to strengthen it. There is a lot of research going on in the field of Condensed Matter Physics. Kim said thatCondensed Matter is a branch of physics. The discovery of 2D materials took place in 2004.

The University of Manchester in the United Kingdom discovered a shocking consequence of the material. Carbon atoms are arranged into sheets of hexagonal shapes in a crystal. Without the stabilizing influence of the stack, heat-generated vibrations would break up a one-layer sheet, according to theorists. They were able to peel off atomically thin sheets with little more than Scotch tape and persistence. The first truly flat material was graphene.

The world's lightest material is also the strongest, according to Hone. It was a huge disappointment for theorists that the material wouldn't hang together.

Physicists were interested in how the carbon flatland changed electrons. electrons act as if they have infinite mass when tripped up by the lattice of atoms through which they move electrons whizz around at a million meters per second, only a few hundred times slower than the speed of light. The electrons flew as if they didn't have a mass at all.

There was a field surrounding the wonder material. Researchers started to think in a broader way. 2D flakes of other substances could have powers of their own. Hone was one of them. He gave the crystal to two optical specialists in the Columbia lab of Tony Heinz. Everyone would change their careers as a result of the move.

A visiting professor and a graduate student received the sample. They were studying how light interacts with Graphene, but they were already thinking about other materials. They wanted a material with a flow of electrons that could be turned on and off, and which could be used as a transistor.

It was known thatMolybdenum disulfide was a material for electronics. It gained additional powers in 2D. The crystal stayed dark when they pointed a laser at it. They saw the 2D sheets shining brightly when they examined them under a microscope after ripping off layers with Scotch tape.

Every last photon that hits them is reflected by well-made sheets of a closely related material. Mak said that it was kind of mind-blowing when he met him at Cornell. The light can be reflected by a single sheet of atoms. The property could lead to amazing optical devices.

The same discovery was made by a physicist at the University of Berkeley. The community was interested in a 2D material that was reflective. The papers have received more than 16,000 citations for their work. Hone said that everybody with lasers started looking at 2D materials.

Two groups made landfall on a whole continent of 2D materials when they identified moly disulfide as a second wonder material. Moly disulfide is part of a family of substances known as transition metal dichalcogenides, in which atoms from the middle region of the periodic table are linked up with pairs of chemical compounds. Moly disulfide is the only naturally occurring one, but there are many more that can be made in labs. Weakly bound sheets make them vulnerable to a piece of tape.

The initial wave of excitement was short-lived as researchers were unable to get more out of them. Wang's group fell back on Graphene after they realized they couldn't attach metal to moly disulfide. He said that the stumbling block has been for a long time. We aren't very good at making contact right now. To study a material's electronic properties, researchers need to push electrons into it and measure the resistance of the resulting current. It is difficult to get electrons in or out of Semiconductors.

They initially felt ambivalence. Mak said it was unclear if we should work on the new material or work on the old one. We continued to do a few more experiments after we found it.

The two researchers were enchanted by each other as they worked. Their contact was limited to research focused emails. ai was asking where that piece of equipment was. What place did you put that? The woman said, "Shana said." Their relationship wascatalyzed by success and eventually turned romantic. Mak said that they worked on the same project in the same lab. The project worked well, making us happy.

All Physics All the Time

Academics were easy to find in the area. She earned a spot at the University of Science and Technology of China in Hefei when she was a star student and excelled in math, science and language as a child. She got the chance to study Russian and physics at Moscow State University because she qualified for a cultural exchange between China and the Soviet Union. She said that teens are eager to explore the world. I didn't think twice.

She saw more of the world than she thought she'd see. She lost her seat in the language program due to visa troubles. After landing in Moscow, she boarded a train that took her 5000 kilometers east. She arrived in the city of Irkutsk three days later. She was told to never touch anything without gloves.

She kept her gloves on and learned Russian in a single semester. She arrived in Moscow in the spring of 1990 after returning to the capital to finish her physics degree after the snow melted.

There were many chaotic years. Tanks rolled through the streets as the Communists tried to regain control of the government. There was fighting after a final exam. She said that they were told to turn off the lights in the dorm. There was a coupon system that rationed everything. Despite the turmoil, her professors continued with their research. Many of the scientists had this kind of attitude. She said that they really love what they do.

As the world order collapsed, she published a paper that caught the attention of Columbia. She moved to New York to help other international students get their feet wet in a foreign country. She recruited Wang to work in the lab and shared her experience with him. He said that she taught him how to not get upset with the laser.

After earning their PhD, most researchers take a post-doc position, but Shan joined Case Western Reserve University as an associate professor. She returned to the lab several years after taking a sabbatical. Her timing was perfect. She began working with a graduate student in the group.

Mak went to New York City in a different way. He struggled in school in Hong Kong because he didn't know much about physics. He picked physics because he was good at it.

His undergraduate research at Hong Kong University stood out and he was recruited to join Columbia. He spent most of his waking hours in the lab except for a game of soccer. An assistant professor at the University of California, Santa Barbara shared an apartment with another graduate student. If I could catch him at 2 o'clock in the morning, I would cook some pasta and discuss physics. It was all about physics.

The good times ended. Mak fell ill after he and Young went to the Amazon. He got sicker because his doctors weren't sure what to think of his test results. His life was saved by happenstance. Young described the situation to his father, who immediately recognized the signs of aplastic anemia, an unusual blood condition that was the subject of his own research. Mak said that it is rare to get this disease. It is rare to get a disease that your roommate's father is experts in.

Mak was helped by Young's father. He was in the hospital for much of his last year of graduate school. Mak's ardor for physics kept him going. Young said that the man was writing from his hospital bed. He was one of the most productive students in the history of the school. It was a miracle.

Mak recovered from further treatments. Young said that his interventions were his greatest contribution to physics.

Into the 2D Wilderness

Mak went to Cornell as a researcher in 2012 and then returned to Case Western a year later. They pursued individual projects with Graphene and other materials, but they also continued to uncover more secrets.

Mak learned the art of measuring electron transport at Cornell. In a field where researchers usually specialize in one type or the other, he and Shan were double threats. Kim said that he complains to Fai and Jie whenever he meets them. I don't know what to do.

The more they learned, the more fascinating they became. One of the properties of electrons is their charge and spin. Modern electronics are based on controlling the flow of charge. The spin of electrons could lead to smaller space filled devices. Mak discovered that electrons in 2D moly disulfide can acquire a third property, known as the "valley", which is controllable by the amount of momentum.

Another striking feature was identified by Mak andShan that year. Physicists watch for holes when electrons move through a crystal. There are holes in a material that can be positively charged. An exciton is formed when the positive hole attracts a negative electron and plugs the hole. The attraction between electrons and holes in 2D tungsten diselenide was so strong that it was hundreds of times stronger than a typical 3D Semiconductor. The finding suggested that electrons were more likely to do weird things than excitons.

The couple started a lab at the university. The materials were the focus of the new group after they were convinced that they were worthbetting their careers on. They also tied the knot.

Hone and his team at Columbia noticed that the properties of graphene got even more extreme when they put it on top of a high quality insulator. It was an example of a new aspect of 2D materials.

If you put one 2D material on top of another, the layers will be a fraction of a nanometer apart. The sheets merging into one substance is accomplished by stacking them. Wang said that it was not just two materials. A new material is created by you.

More elements are added to the stacking game with the diverse family of TMD lattices. Each TMD has its own strengths and weaknesses. Some are both magnetic and superconducting. They were going to mix and match them to fashion materials.

The properties of the stack were lackluster compared to what they had seen in Graphene. They didn't check the quality of the crystals. They were shocked when they saw the moly disulfide under the microscope. Some atoms were in the wrong place. One in 100 lattice sites had some problem that made it hard for the lattice to direct electrons. One defect per million atoms was the image of graphene. Hone said that the stuff they'd been buying was garbage.

He decided to go into the business of growing research- grade. Daniel Rhodes has experience growing crystals by melting powders of raw materials at high temperatures and then cooling them at a slow pace. It is similar to growing rock candy from sugar in water. Commercial methods take a few days, while the new process takes a month. The crystals it produced were hundreds to thousands of times better than those sold in chemical catalogs.

The unglamorous task of figuring out how to work with tiny particles that don't accept electrons was faced by Mak and Shan. To pump in electrons, Mak had picked up on the transport technique and the couple obsessed over a lot of details. It took Mak a long time to try out the many ways of setting up electrodes.

They spent a lot of time figuring out how to stack the tiny particles. Everything came together with this ability and Hone's crystal. After moving to Ithaca, New York, a cascade of innovative results came out of their lab.

Breakthroughs at Cornell

The graduate student said that everything is hard to pick up for some reason. Paper might cling to the crackling surface of a recently rubbed balloon, but the sheet of paper was shaped like Saudi Arabia. The plastic stuck to the glass slide was sticky to theGraphite. The motorized stand was directed by a computer. She gingerly lifted the stack into the air at a rate of one-fifth of a millionth of a meter per mouse click, staring intently at the computer monitor to see if she had succeeded.

She'd done it. With a few more clicks, the two-layer stack came free, and Xia moved quickly to deposit the flakes onto a third material. With a few more clicks she heated the surface, melting the slide's plastic glue before we could sneeze.

She said that she has a nightmare that disappears.

It took her more than an hour to assemble the bottom half of a simple device. One of dozens of sandwiches she has constructed and studied over the last year took several hours to assemble and had a whopping 10 layers.

The realization of a long-held dream of Condensed Matter Physicists is represented by the stacking of 2D materials in labs at Columbia, the Massachusetts Institute of Technology, Berkeley, Harvard and other institutions. Researchers can use materials found in the ground or grow them slowly in a lab. They can use the atomic equivalent of Lego bricks to build their own structures. Few have gone as far as the Cornell group.

The first major discovery at Cornell was the excitons, strongly bound electron-hole pairs they had seen in the previous year. Physicists are interested in quasiparticles because they may offer a way to achieve room temperature superconductivity.

The rules for excitons are similar to those for electron-electron pairs, which allow them to become Bose-Einstein condensates. The ability to flow with no resistance is one of the qualities of this coherent horde of quasi particles. Superconducts when it carries electric current.

electrons and holes are attracted to each other According to researchers, this could make their glue stronger. Keeping the electron from filling the hole is one of the challenges to exciton-based superconductivity. Mak and Shan have a plan to solve the second problem.

Clouds of atoms can be cooled by using powerful lasers. Physicists believe that excitons can form at higher temperatures. The Cornell group was able to make this happen. Extra electrons were put in the top layer and removed from the bottom of the sandwich. The excitons are long-lived because the electrons can't jump to the opposite layer to counteract their partners. There were signs of an exciton at a warm 100 kelvins. The excitons persisted for a lifetime in this setup. Mak described an improved apparatus where excitons seem to last for a long time.

The team is trying to create an exciton current by faking it. An electric field oriented in a way that encourages both electrons and holes to move in the same direction was proposed by Allan MacDonald and Jung-Jung Su. The Cornell group must once again wrestle with electrical contacts to pull it off. They have to make excitons and other things to move them in this case.

The excitons are on their way to 100 kelvins. It is a huge leap from the nanokelvin conditions that most bosonic condensates require.

Mak said that it would be nice to warm up the temperature by a billion times.

Magical Moiré Materials

The second 2D materials revolution was launched while the Cornell lab was ramping up their research. The new 2D material was created by twisting one layer of Graphene with respect to the layer below. The trick was to drop the upper layer so that the hexagons landed on top of each other. There is an offset between atoms that grows and shrinks as you move across a material. At the "magic angle" of 1.1 degrees, the unique crystal structure of the superlattice would cause graphene's electrons to slow and sense the repulsion of their neighbors.

Weird things happen when electrons know each other. electrons are thought to only interact with the lattice of atoms in normal conductors, and they race around too quickly to notice each other Slowed to a crawl, electrons can move each other and assume exotic quantum states. Experiments done by Jarillo-Herrero showed that a strong form of superconductivity can be found in twisted, magic-angle Graphene.

Researchers were introduced to a new method of controlling electrons. In the superlattice, electrons are able to see the supercells as if they were giant atoms. It's easy to fill the supercells with enough electrons. Using an electric field to dial up or down the average number of electrons per supercell, Jarillo-Herrero's group was able to make their twisted bilayer graphene device serve as a superconductor.

The field of "twistronics" was rushed into by physicists. Many people have found that twisting is difficult. There is no reason for atoms to fall neatly into the "magic" 1.1-degree alignment. A group of friends at other universities are working with twisted devices. It takes them a long time to create a device. Specific experiments are almost impossible to repeat because the devices behave differently.

A simpler way to create superlattices is presented by tmds. The way angle misalignment works is if you stack a lattice of slightly larger hexagons over a smaller one. The stack is more likely to stay still because there is no rotation between the layers. She said that she usually succeeds four times out of five.

It is possible to explore electron interactions with materials made of tmd moiré. The electrons in the materials are heavier than in the Graphene. Supercells sit 100 times further apart than the atoms inside them, which slows them down even more. The electrons get a chance to know their neighbors when they are far away.

One of the first to recognize the potential of the superlattices was Feng Wang. One of the simplest ways electrons can organize is through a state known as a Wigner crystal. The first picture of electrons holding each other at arm's length was published in Nature in 2021. The 2D physics community was already aware of Wang's activities and the Cornell factory was making their own devices. They discovered within months that electrons in their devices could be converted to Wigner crystal patterns.

The Cornell group was making materials for a power tool. The combination of technical features that these devices have made them perfectly represent one of the most important toy models in Condensed Matter physics was predicted by MacDonald and his team. The Hubbard model is a theory used to understand many electron behaviors. The model was proposed by Martin Gutzwiller, Junjiro Kanamori and John Hubbard in 1963. Take a picture of a grid of atoms. The Hubbard model assumes that each electron feels two competing forces: it wants to move by tunneling to neighboring atoms, but it also repulses its neighbors, which makes it want to stay where it is. Depending on which desire is strongest, different behaviors occur. The Hubbard model is unsolvable in all but the simplest of cases.

Some of the field's deepest mysteries, such as the nature of the glue that binding electrons into cuprates, could be solved with the help of the Hubbard model. Researchers could experiment with electrons in a sandwich and see what they would do. MacDonald said that it was hard to answer many important questions. By doing an experiment, we can do it. That is very innovative.

The Hubbard model simulator was built by stacking layers of diselenide and sulfide to create a superlattice and attaching electrodes to dial up or down an electric field. The amount of electrons in each supercell was determined by the electric field. Going from one electron to two electrons per supercell was like turning a lattice of hydrogen atoms into a lattice of helium atoms. In their initial Hubbard model publication in Nature, they reported simulating atoms with up to two electrons. They realized that the goal was to turn lead into gold. Mak said it was like going through the periodic table. They can make up a grid of atoms with the same number of electrons.

They looked at the artificial atoms. Adding positive protons to the centers of synthetic atoms could be used to control the supercells. The higher the charge of a nucleus, the harder it is for electrons to get away.

The control of the giant atoms was complete. They can summon a grid of ersatz atoms, even ones that aren't in nature, and transform them as they please. It is a power that borders on magic. Kim said that they had the most exciting and impressive effort.

The Cornell group used their designer atoms to settle a 70 year old debate. What if you could change the atoms of an insulator to make a metal? Is the changeover gradual or abrupt?

The thought experiment was carried out in the lab by the two people. They used heavy atoms to trap electrons and make the superlattice act like an insulator. They weakened the trap and let the electrons hop to freedom. They showed that the transition is not sudden by observing the falling electrical resistance as the superlattice acted like a metal. This finding, which they announced in Nature last year, opens up the possibility that the superlattice's electrons may be able to achieve a long-sought quantum spin liquid. Mak thinks that the most interesting problem one can tackle is that.

The couple lucked into something that some physicists consider their most significant discovery to date. Mak said it was a complete accident. They didn't expect it.

The researchers used a sandwich of transition metals and chalcogenide on top of the hexagons on the two layers. They discovered the transition between metal and insulator. They did an experiment with devices in which the top layer had been stacked backwards.

The resistance began to fall as the electrons began to hop. The researchers wondered if the moiré had begun to superconduct after it plummeted so low. They found a rare pattern of resistance known as the quantum anomalous Hall effect. The effect showed that the electrons along the edge of the material were different from those in the center. The electrons were trapped in the middle of the device They explained the low resistance around the perimeter. The researchers accidentally created a type of matter called a Chern insulator.

If the temperature rises above a few hundredths of a kelvin, the hall effect will fall apart. The group in Santa Barbara saw it in a twisted graphene sandwich. The effect was almost the same at nearly the same temperature, but in a device that anyone can recreate. They can do it 10 times in a row and I will take theirs any day. You can use it to do something.

Mak and Shan believe that with some fiddling, they can make Chern insulators that can survive to 50 or 100 kelvin. They may be able to switch on and off at specific places within a device if they succeed in their work.

Exploration in Flatland

The couple shows no signs of slowing down. Mak looked on as students worked on a project that would allow them to chill their devices to temperatures a thousand times colder than what they have worked with so far. The group hasn't had a chance to look for signs of superconductivity because of the amount of physics they've discovered. The answer to the question of whether a form of intrinsic magnetism to cuprates is an essential ingredient of the electron-binding glue is provided by the super fridge. Mak said that killing one of the components that theorists wanted to kill for a long time was similar.

He and his group haven't begun to experiment with some of the more controversial ones. After spending years inventing the equipment needed to move around the continent of 2D materials, they are finally going to venture beyond the moly disulfide beachhead they landed on.

Two researchers attribute their success to a culture of cooperation. The initial collaboration with Hone that introduced them to moly disulfide was just one of the many opportunities they enjoyed. The head of their lab didn't need to discuss their plans with us. People from other groups were talking to us. The experiments were done by us. Things were wrapped up.

They have fostered a relaxed environment at Cornell, where they oversee a few dozen postdocs, visiting researchers and students, all of whom are free to do their own thing. Students have good ideas. Sometimes, you don't want to be involved.

Their lab is unique because of their married life. They have learned how to use their strengths. As the three of us talked, she was a good manager because of her careful discipline that made her a good experimentalist. Mak enjoys working in the lab with the early career researchers. He is a member of the group. Young said that it seemed like their lab was their family. The two of them said they achieve more together than they could alone. Mak stated that one plus one is more than two.

They may stack up to be more than the sum of their parts. Researchers are speculating about how the new ways of domesticating electrons might improve technology. Bose-Einstein condensates could lead to ultra-sensitive quantum sensors and better control of Chern-like insulators could enable powerful quantum computers. Those are just some of the ideas. Radical applications are often added to by Incremental Improvements in Materials Science. It would have been difficult for the researchers who developed the transistor to predict the future of the device. The scientists who tried to fashion glass fibers that could carry light across their lab bench didn't know that 10,000 kilometer undersea optical fibers would one day link continents. Two-dimensional materials can change in similar ways. He said that a new materials platform generated its own applications.

While driving me to the bus stop, Shan and Mak told me about a recent vacation they took to Canada, where they once again displayed their knack for stumbling onto surprises. They had spent a lot of time trying to find a bear. On their way to the airport, they stopped at a botanical reserve and encountered a black bear.

Their approach is to wander around with each other in a new landscape to see what happens. Mak said that they don't have a lot of theoretical guidance. It can fail, but it can also fail unexpectedly.