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.

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.”

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.

Research focused on developing a new catalyst that would lead to large-scale implementation of capture and conversion of carbon dioxide (CO₂) was recently published in the Royal Society of Chemistry journal Catalysis Science & Technology. Principal investigator is Karl Johnson, and postdoctoral associate Jingyun Ye is lead author. The article “Catalytic Hydrogenation of CO₂ to Methanol in a Lewis Pair Functionalized MOF” is featured on the cover of Catalysis Science & Technology vol. 6, no. 24 and builds upon Johnson’s previous research that identified the two main factors for determining the optimal catalyst for turning atmospheric CO₂ into liquid fuel. The research was conducted using computational resources at the University’s Center for Simulation and Modeling.