News & Highlights

Non-technical description: In earlier work, we showed that electrons can freeze into an orderly pattern known as a Wigner crystal (see the 'Electrons crystallize!' post). A natural next question is how such a crystal actually behaves: how stiff it is, how much it trembles from quantum and thermal motion, and how it eventually melts. This has been hard to answer, because a crystal made entirely of electrons leaves almost no fingerprint that ordinary instruments can pick up.

In this work, we use light to create excitons (pairs of an electron and a 'hole') inside a semiconductor sheet only a few atoms thick. Surrounded by the electron crystal, each exciton slightly distorts the arrangement of electrons around it. As the exciton moves through the material, this distortion travels along with it, and the two together form a composite particle that we call a 'Wigner polaron'. Wigner polarons act as tiny built-in probes: using them, we can measure the electron crystal's stiffness and watch how it vibrates and melts, opening a new optical window into a state of matter made entirely of electrons.

The paper was published in Nature Physics and was carried out in collaboration with ETH Zurich, BNL, and the University of Heidelberg.

Non-technical description: When particles such as electrons are confined in a very small space, remarkable quantum effects emerge. This is, for example, how quantum dots (the tiny light emitters in QLED televisions) get their brilliant colors. Electrons are relatively easy to confine: they carry electric charge, so electric fields can trap and steer them. Excitons are much harder. An exciton is an electron bound to a 'hole' (the empty spot left behind by a missing electron), and the pair as a whole carries no net charge, so electric fields have almost nothing to grab onto. Trapping excitons in spaces just a few nanometers wide is a long-standing goal, because tightly confined excitons interact strongly with one another and could serve as sources of quantum light, where light is emitted one (photon) at a time.

In this work, we show that excitons can be trapped by electric fields that arise naturally in a 'moiré' superlattice. When two layers of an insulating crystal called hexagonal boron nitride are stacked with a small twist, they form a repeating pattern of patches whose built-in electric fields point up or down, out of the layers. While this out-of-plane field is well known, it is naturally accompanied by strong electric fields lying within the plane, concentrated right at the boundaries between patches, the so-called domain walls. By placing a semiconductor sheet just a few atoms thick right above this surface, we find that excitons become trapped in narrow one-dimensional channels running along these walls. These results establish an electrical route to trapping and arranging excitons at the nanometer scale.

The paper was published in Physical Review Letters.

Non-technical description: When two light beams cross, they interfere and create a standing wave, a regular pattern of bright and dark stripes. These standing waves are routinely used to trap atoms (known as an optical lattice), but can they also trap other particles, such as excitons (electron-hole pairs) in a semiconductor? To trap excitons in a useful way, the stripes would need to be only tens of nanometers apart. But the spacing of a conventional optical lattice, created by interference, cannot be much smaller than the wavelength of light, hundreds of nanometers, which is too coarse to strongly modify exciton behavior.

In this work, we create an optical lattice far below this so-called 'diffraction limit'. A metal surface carved with a fine pattern (a metasurface) converts laser light into plasmon polaritons, light waves that sit on the metal surface but are squeezed to much shorter length scales. These squeezed waves allow us to imprint a fine, repeating energy landscape onto excitons in a single-layer semiconductor placed nearby. Interestingly, this scheme uses about 100 times less laser power than shining light directly on the material, offering a power-efficient route to artificial crystals for excitons and light-driven quantum matter.

The paper was published in Science Advances and was carried out in collaboration with the Hafezi group at UMD.

Non-technical description: An optical cavity traps light by bouncing it back and forth between mirrors, so that light and matter interact far more strongly than they otherwise would. This trick is at the heart of lasers, sensors, and quantum optics. In earlier work, we showed that a single semiconductor monolayer, just a few atoms thick, can act as a near-perfect mirror (see the 'An atomically thin mirror' post).

In this work, we use these atomically thin mirrors to build a complete optical cavity. Besides being extraordinarily compact, the cavity behaves in unusual ways: the trapped light forms a 'flat band,' meaning the photons act as though they are very heavy. With a magnetic field, the cavity can also be made 'chiral,' where it responds differently to light that corkscrews to the left versus to the right (circularly polarized light). Together, these features offer new ways to control light and its interaction with matter, useful for future photonic and quantum-optical devices.

The paper was published in Science Advances and was carried out in collaboration with the Hafezi group at UMD. 

Non-technical description: When light shines on a semiconductor, it kicks electrons into higher energy states, leaving behind positively charged 'holes' that pair up with electrons to form excitons. While these excitons 'live' inside the material, they can alter its optical response, allowing a light beam's shape, direction, or color to change in a 'nonlinear' way, meaning the response is not simply proportional to how much light you put in.

In this work, we demonstrate a strong and tunable nonlinear response in a device made of three atom-thin layers of the semiconductor tungsten diselenide. Remarkably, even a small amount of laser light can dramatically change the material's behavior, switching it from absorbing light to letting light through. The effect can also be turned on and off with a voltage. This discovery opens a powerful new platform for advanced optical and quantum technologies.

The study was published in Nature Photonics and carried out in collaboration with the Hafezi group at UMD and QPress at BNL.

Non-technical description: When certain quantum particles (so-called bosons) are cooled and crowded together, they can lock into a single collective state, a Bose-Einstein condensate. This has been achieved with atoms, and theory predicts that excitons, bound electron-hole pairs in a semiconductor, could condense at far higher temperatures. The catch is that an exciton usually vanishes quickly, as its electron and hole recombine and emit light, before enough of them can gather to condense.

'Interlayer' excitons, with the electron in one atom-thin layer and the hole in a neighboring one, live much longer, but they are usually also much dimmer. Here we sandwich a single sheet of insulating hexagonal boron nitride between two layers of the semiconductor MoSe2: thick enough to keep the charges apart, yet thin enough that they remain tightly bound and bright, even at higher temperatures and densities. A voltage across the stack tunes the excitons' energy over a wide range and can push them across the device, giving us an electrical handle on a dense cloud of interlayer excitons and a promising platform for an exciton condensate.

The paper was published in Physical Review Letters and was carried out in collaboration with QPress at BNL.

Non-technical description: Crystals are made of atoms arranged in a regular pattern. In the 1930s, the physicist Eugene Wigner predicted that electrons themselves could also form such patterns, now called Wigner crystals. These crystals are very difficult to create because electrons are extremely light, and without chemical bonds to hold them in place, they tend to move around a lot—both from quantum effects (known as the uncertainty principle) and from heat.

In our work, we show that stacking two atomically thin semiconductor layers close together creates electric forces strong enough to hold Wigner crystals in place. Electrons in the two layers interlock to form a 'bilayer' Wigner crystal that is remarkably stable and has an intriguing stacking pattern. This system is an excellent playground for exploring how these electron crystals change, or 'melt,' when shaken by heat and by quantum fluctuations.

The study was published in Nature and carried out in collaboration with the Park, Kim, Lukin, and Demler groups at Harvard.

Non-technical description: An object (emitter) in its high energy state can release the stored energy by emitting light. This process, called spontaneous emission, happens almost everywhere, such as in light bulbs. Though one may think how fast an emitter radiates light is an intrinsic property of the emitter, scientists realized that it in fact also depends on the environment of the emitter. Controlling the radiative properties of an emitter by embedding it into complex optical structures (called cavities and photonic crystals) has since become a cornerstone of modern optics.

In this work, we control the light emission of excitons (bound electron-hole pairs) in an atomically thin semiconductor using nothing more than a simple mirror. By moving the semiconductor toward or away from the mirror, we change the excitons' surroundings and substantially modify how much light they emit (left figure). We show that this control can even happen in real time, which could lead to new applications in both classical and quantum optical information processing.

The paper was published in Physical Review Letters and was done in collaboration with the Kim and Lukin groups at Harvard.

Non-technical description: All matter emits thermal radiation above absolute zero temperature (also known as 'blackbody radiation'). Normally the hotter an object gets, the more thermal power it radiates, which is the principle behind infrared thermal imaging.

In this work, we show that an oxide called SmNiO3 breaks this basic rule: its thermal radiation barely changes as it heats up (left image). The trick is a metal-insulator transition, a switch between conducting and non-conducting states. As the temperature rises, the material becomes more metal-like in just the right way to make it look cooler than it actually is, canceling out the usual increase in glow. This effect could enable new technologies for thermal camouflage (hiding from infrared cameras) and heat management.

Non-technical description: Metals reflect light but become transparent if thinned down to less than tens of atoms thick. So ultimately, how thin can a mirror be? Can we make mirrors made of just a few layers of atoms?

One way to do this is to use a 'resonance' effect. Light of just the right color can make a material buzz with optical excitations. If those excitations release all their stored energy back as light, then all the incoming light is reflected, because the re-emitted light and the incoming light add up to send the beam straight back. In practice, though, some of that energy usually leaks away as heat, so the light ends up absorbed instead of reflected.

In this work, we show that in high-quality sheets of the semiconductor MoSe2, the excitons get rid of their energy almost entirely by emitting light (rather than losing it as heat) when the material is very cold. As a result, these sheets, though only three atoms thick, can act almost as a perfect mirror, and their reflection can be switched on and off with a voltage. This is about as thin as a mirror can get, and it opens up unique applications, from nonlinear quantum optics to metasurfaces (flat, patterned surfaces that steer light).

This paper was in Physical Review Letters and was done in collaboration with the Kim and Lukin groups at Harvard. News coverage by Physics, Nature, Chemistry World.

Non-technical description: Excitons — bound electron and hole pairs — are an elementary optical excitation of semiconductors. Inside an exciton, the spins of the electron and hole can be either parallel or antiparallel. Excitons with antiparallel spins (bright excitons) can quickly decay by emitting light, while those with parallel spins do not usually emit light. These so-called 'dark excitons' are attractive for information processing because they live longer than bright excitons, but are also harder to study experimentally.

In this work, we introduce a way to detect dark excitons in an atom-thin sheet of the semiconductor WSe2 by placing it right next to a metal surface. Near the metal, the dark excitons couple strongly to ripples of electrons and light that ride along the metal's surface (called surface plasmon polaritons). This coupling boosts the dark excitons' light emission enough that we can finally measure it. The method gives scientists a much better handle on studying and controlling excitons, opening new possibilities for metasurfaces and optoelectronics (electronics that work with light).

This work appeared in Nature Nanotechnology, and was done in collaboration with the Kim and Lukin groups at Harvard. Additional news coverage by Nature Nanotechnology.

Non-technical description:  Fuel cells can generate electricity by oxidizing fuels in an environmentally friendly fashion. Electrolyte — a component of fuel cells which separates the fuel from the oxidizer— needs high ionic conductivity but low electronic conductivity. Typically ionic conduction in the solid electrolyte is created by vacancies (missing atoms) in the lattice. These missing atoms, however, also often create electronic conduction, leading to efficiency loss or even catastrophic failure of fuel cells.

In this work, we report a completely new way to design the electrolyte, using a quantum material called SmNiO3. Unlike most materials, SmNiO3 becomes far better at blocking electrons once it soaks up hydrogen from the fuel, thanks to a metal-insulator transition (a switch from conducting to non-conducting). At the same time, it stays very good at carrying ions, on par with the best electrolytes at similar temperatures. That combination, blocking electrons while passing ions, is exactly what a fuel cell needs, so this result points to better low-temperature fuel cells and new rules for designing energy materials.

The paper appeared in Nature, with news coverage by Nature Energy, Harvard, Argonne National Lab, IEEE Spectrum, and National Science Review. This work was done in collaboration with the groups of H. Zhou and D. Fong at Argonne.

Non-technical description: Recently, 'brain-inspired' computing, using artificial neural networks, has driven huge progress in artificial intelligence, including image and speech recognition. Run on ordinary digital computers, though, these programs use far more power than a real brain, which is a big problem for battery-powered devices like phones. Building computer chips that imitate the brain directly in the hardware could make this kind of computing much more energy-efficient.

Here we review recent progress towards building these hardware — neuromorphic devices and circuits — using correlated materials and their phase transitions. We placed particular emphasis on materials synthesis and device physics, and critically evaluate their prospects. The paper was published in Proceedings of the IEEE, and you can read the full text here.

Non-technical description: Being able to dramatically change a material's properties by switching its 'electronic phase' (see the previous post for details) could open the door to many new electronic and optical devices.

Here we report the discovery of a new type of metal-to-insulator transition in an oxide material, SmNiO3, driven by electron doping. The oxide turns from a shiny metal to a transparent insulator when doped with lithium-ions or protons. Its electronic conductivity changes by more than eight orders of magnitude, among the highest ratio achieved in phase change materials at room temperature. We attribute such a phase transition to a Mott transition driven by a quantum mechanical effect called electron correlation. Such a drastic phase transition forms an emerging platform for studying many-body physics and for realizing oxide-based reconfigurable electronic/optical devices.

The paper was in Nature Communications with news coverage from Harvard, Phys.org.

Non-technical description: Electrons can move freely in a metal but not so in an insulator. Intriguingly, certain materials can switch between being a metal and an insulator depending on external conditions, just as water can change its phase between solid, liquid, and gas. Perhaps the most famous material hosting such so-called metal-insulator transitions is vanadium dioxide, whose electrical and optical properties can dramatically change when heated above 68 ºC. This phase change, if triggered electrically, may enable a range of new electronic and optical devices.

Here, by optimizing materials growth and device structures, we are able to trigger the phase change in VO2 by a voltage within a few millionths of a second—the fastest ever achieved electrically. Such a study on the dynamics of the phase transition not only forms the foundation for new electronics and optics, but can also help to resolve the everlasting debate over VO2's phase transition mechanism initiated by J.B. Goodenough and N.F. Mott.

The paper appeared in IEEE Electron Device Letters and was done in collaboration with Micron Technology, Inc. See related discussion in Nature.