Nature doesn't like having interfaces—this is why bubbles like to be round, and the surface of a pond settles to flat as long as it's not disturbed. These trends minimize the total amount of interface (or surface) that is present. As an exception to this behavior, certain materials are known to have a property, called negative stiffness, where the interface prefers to become distorted, or wavy, even without any external stimulation. Interfaces with negative stiffness have been considered in crystals before, but the characteristic has now also been found in modern magnetism.
In a joint experimental and theoretical study, Di Xiao and collaborators from several groups across the country and from China observed out-of-plane magnetism in a monolayer of chromium triiodide (CrI3). The study, entitled further described the dependence of the magnetic ordering on the number of layers in the material—bilayer CrI3 displays suppressed magnetization, whereas in trilayer CrI3 the interlayer ferromagnetism is restored. This thickness-dependent behavior is typical of van der Waals crystals. The findings are reported in an article entitled “Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit” that was published in this month’s issue of Nature.
Kevin Chen's article Experimental observation of optical Weyl points and Fermi arc-like surface states, published in Nature Physics, was the subject of a "News and Views" article entitled Topological Photonics: Come to Light. The physics idea leading to this paper originated from Penn State collaborator Mikael Rechtsman.
Topological states of matter can exhibit a range of unique quantum phenomena that are of interest to various fields of classical physics, such as acoustics, mechanics or photonics. Although a number of these topological states have been successfully emulated in photonics, their application has been restricted to certain frequencies. Most topological properties have been demonstrated in two-dimensional (2D) systems; however, a variety of new topological properties have been predicted for three-dimensional (3D) systems. The study published in Nature Physics marks an important step by emulating Weyl points, which constitute the simplest possible topologically nontrivial band structure, in three dimensions.
Research from Giannis Mpourmpakis' group was recently featured on the inside front cover of the Royal Society of Chemistry journal, Catalysis Science & Technology. The team’s investigations into a more energy-efficient catalytic process to produce olefins--the building blocks for polymer production--could impact potential applications in diverse technology areas from green energy and sustainable chemistry to materials engineering and catalysis.
The interface between nano-sized precious metal clusters such as Silver (Ag) and a semiconductor such as graphite (Gr) is called a heterojunction (Ag/Gr). Heterojunctions have great promise in enhance solar energy conversion due to their unique and enhanced optical, electronic, and chemical properties. When excited with laser pulses, electrons in the system acquire a mean energy higher than its thermal equilibrium value and are referred to as "hot electrons". In fact, graphitic materials are model systems for the study of hot electron dynamics. An ineffective screening within the layers of graphite allows the hot electrons to reach temperatures comparable to that in the solar photosphere!
In a study supported by the Center for Chemistry at the Space-Time Limit recently published in the Journal of the American Chemical Society, Hrvoje Petek and his group modified the Gr surface with Ag nanoclusters (NC)s to investigate how the excitation of the plasmonic resonance of the Ag/Gr heterojunction affects the generation, spatial distributions, and relaxation processes of hot electrons. Plasmonic resonance is a prominent feature of precious-metal nanoparticles; it is a sharp and intense absorption band in the visible range that arise from a collective resonant oscillation of the free electrons of the conduction band of the metal called plasmon.
Hydrogen powered fuel cell cars, developed by almost every major car manufacturer, are ideal zero-emissions vehicles because they produce only water as exhaust. However, their reliability is limited because the fuel cell relies upon a membrane that only functions in when enough water is present, limiting the vehicle’s operating conditions.
Karl Johnson and his group have found that the unusual properties of graphane – a two-dimensional polymer of carbon and hydrogen – could form a type of anhydrous “bucket brigade” that transports protons without the need for water, potentially leading to the development of more efficient hydrogen fuel cells for vehicles and other energy systems. Graduate research assistant Abhishek Bagusetty is the lead author on their paper “Facile Anhydrous Proton Transport on Hydroxyl Functionalized Graphane”, recently published in Physical Review Letters. Computational modeling techniques coupled with the high performance computational infrastructure at the University’s Center for Research Computing enabled them to design this potentially groundbreaking material.
In an article in the Proceedings of the National Academy of Science (doi: 10.1073/pnas.1611467114), David Waldeck and colleagues in Israel propose a mechanism for the enantioselectivity (chiral specificity) of non-covalent interactions between chiral molecules. Their study examines how the non-covalent interactions between molecules give rise to enantioselective interaction energies. Non-covalent interactions do not involve the formation of a bond; rather they include electrostatic interactions between permanent or induced dipoles as the electron clouds of the molecules rearrange and the purely quantum exchange interaction as the wavefunctions of the molecules overlap. Their two part study shows experimentally that charge redistribution in chiral molecules is accompanied by spin polarization and it shows theoretically that the exchange interactions for homochiral (both molecules have the same handedness) interactions differ from heterochiral ones.
In a letter published in the February 2017 issue of Nature Nanotechnology, Ben Hunt and his collaborators at the Massachusetts Institute of Technology, the University of California Santa Barbara, and the National Institute for Materials Science in Tsukuba, Japan describe how they engineered a graphene electron–hole bilayer device into a helical 1-dimensional (1D) conductor and characterized its transport properties. In a helical 1D conductor, electrons moving in opposite directions also have opposite spin polarizations, and such helical states can be obtained by combining two quantum Hall (QH) edge states with opposite spins and opposite momenta relative to the magnetic field (i.e. opposite chiralities).
“My colleagues at MIT came up with this ingenious way of producing helical edge states from two decoupled graphene layers, and then they proved their idea worked with a series of powerful transport experiments,” says Hunt. “I was thrilled to be able to make a contribution to the experiment by using capacitance measurements to help prove that the unique helical states they observe really are edge states.”
Photovoltaics in action: Electron motion in a type-II InSe/GaAs semiconductor heterostructure has been recorded in a movie immediately after photoexcitation with high spatial and temporal resolution.
Electrons are the lifeblood of semiconductor devices, from transistors that power computers and smart phones, and semiconductor diodes that light up the night, to photovoltaic cells that harvest solar energy to power it all. Under the influence of applied voltages or light stimulations, electrons flow through nanoscale channels and plummet potential gradients at interfaces of disparate materials. In a semiconductor device this ebb and flow occurs several times every nanosecond within billions of transistors on a single microchip, unseen by human eye, but creating text, images and movies in strings of 0s and 1s. But the true time and spatial scales on which electrons are energized and transported span a range of hundreds of femtoseconds and tens of nanometres. Thus, to capture in a movie the physical phenomena of electrons in a device requires a truly extraordinary camera. Writing in Nature Nanotechnology, Man et al. report an experiment that performs just that. Specifically, they record a movie of electron flow in energy, space and time within a semiconductor heterojunction composed of GaAs in physical contact with InSe by imaging electrons emitted into vacuum through the joint action of femtosecond duration IR generation and UV electron emission laser pulses.
Matter-Light Condensates Reach Thermal Equilibrium
Making use of improved microcavities, hybrid condensates of matter and light can be tuned to reach a thermal equilibrium state, despite their finite lifetime.
In a laser, coherent light is created by stimulated emission of photons from an “inverted” state of matter that is significantly out of thermal equilibrium. “Inverted” means that excited states of the matter are more occupied than lower energy states, so that emission is more likely than absorption. The coherence of laser light is closely related to a quite different, and less commonly encountered, state of matter—a Bose-Einstein condensate (BEC). In the textbook description of a BEC, at low enough temperatures or high enough densities, a large number of particles occupy the same state, producing a coherent state of matter. In contrast to laser light, the textbook BEC is in thermal equilibrium. Condensates of polaritons—half-light, half-matter quasiparticles—have so far been found in conditions halfway between those of an equilibrium BEC and those of a laser. Work by David Snoke and colleagues now shows that such polariton condensates can be tuned to reach a thermal equilibrium state. With this tunability between an equilibrium and nonequilibrium state, researchers can explore how the character of phase transitions evolves between the two limits.