Sustainable energy sources surround us, but we must harness them. The periodic, self-sustained deformation of a piezoelectric flag as it flaps in a fluid flow, for example, could be used to convert mechanical energy from wind, or a river or ocean current, into electrical power. The authors show that the interaction between fluid forcing, flapping dynamics, and the resonant properties of the output circuit can lead to a lock-in mechanism that significantly enhances flapping, energy transfer, and efficiency.

Future solid-state energy applications rely on high-performance thermoelectrics, and as in many fields progress requires intensive materials-design analysis. Here the authors compare calculations for bismuth telluride and its selenium-doped cousins, and find that highly efficient thermoelectric materials have complex nonparabolic band structures–a property shared with topological insulators. Recognizing this connection between high thermoelectric efficiency and topological insulators may benefit future searches for technologically promising materials in both areas.

Although both superconducting and magnetic devices have the potential to overcome limitations of current CMOS technology, traditionally they have remained separate areas of research. Here the authors develop nanopillar Josephson junctions with pseudo-spin-valve barriers that provide a unique, hybrid superconducting-magnetic capability. The states of these devices are written using spin-transfer torque and read using Josephson coupling, and yield 10-fold changes in superconducting critical current. This approach holds the promise for nanoscale, nonvolatile, cryogenic memory in future ultralow-power computing systems.

Thermally activated delayed-fluorescence (TADF) emitters have attracted increasing attention as third-generation electroluminescent materials for organic light-emitting diodes. This study presents a design strategy for highly efficient TADF emitters, guided by quantum-chemistry calculations of radiative and nonradiative decay rates. By optimizing HOMO-LUMO overlap density and suppressing nonradiative decay, the authors develop a purely organic emitter with very high quantum yields from photoluminescence and electroluminescence.

A simple means to convert waste heat into electrical energy is thermionic emission, in which the thermal energy of a hot cathode exceeds the work function for electrons to evaporate from its surface, for subsequent collection at a cold anode. Here thermionic emission from a monolayer of suspended graphene has been reconsidered, to account for the electrons in this material behaving like massless fermionic quasiparticles. The authors predict a scaling of current density as $T^3$, as opposed to the $T^2$ scaling predicted for metals by the traditional Richardson-Dushman equation. The efficiency of the authors’ proposed thermionic energy converter is about 45% at relatively low temperatures, with high current capability.

Charge screening on a polar surface under ambient conditions is difficult to investigate, because the surface is immediately affected by adsorbed polar molecules and ions. The authors control the degree of screening on a ferroelectric surface using charge gradient microscopy, and find that the kinetics is governed by an exponential recovery with a universal time constant, independent of the degree of screening. This study demonstrates that the degree of screening at a ferroelectric surface can be controlled mechanically, without disturbing the polarization beneath.

Future solid-state energy applications rely on high-performance thermoelectrics, and as in many fields progress requires intensive materials-design analysis. Here the authors compare calculations for bismuth telluride and its selenium-doped cousins, and find that highly efficient thermoelectric materials have complex nonparabolic band structures–a property shared with topological insulators. Recognizing this connection between high thermoelectric efficiency and topological insulators may benefit future searches for technologically promising materials in both areas.

High-resolution x-ray imaging in applications ranging from biomedical engineering to paleontology can benefit from inline holography: reconstructing a complex-valued image from x-rays diffracted by a sample. This approach is problematic in strongly absorbing and refracting samples, and when strong illumination inhomogeneities are present. The authors demonstrate, by simultaneously tracking both the complex-valued illumination and transmission functions (so-called ptychographic phase retrieval), that accurate quantitative images of even a highly absorbing uranium-molybdenum sample can be obtained.

Improving photoluminescence or nonlinear wavelength conversion is an important goal in engineering a wide variety of systems in optics, photonics, and nanoscience. To this end, it is challenging to design doubly resonant nanocavities that match a specific pair of wavelengths, as cavity modes are usually not independently tunable. The authors demonstrate flexible tuning of double resonances, and utilize guided-mode resonances localized at defects in photonic crystals to increase photoluminescence intensity by a factor of 2400.

Monitoring the temperature of a mesoscopic system in real time is a prerequisite for a number of fundamental experiments in classical and quantum thermodynamics. The authors demonstrate fast thermometry on a micrometer-sized metallic island at temperatures below 100 mK. Their device can be immediately integrated into superconducting circuits to perform calorimetric measurements of dissipation, with the potential to resolve the energy of a single microwave photon.

Cavity polaritons are elementary half-matter, half-light excitations trapped within a microcavity that exhibit unique properties useful both for probing fundamental physics and for producing novel applications. The authors present an all-optical confinement method for tuning the energy and lifetime of cavity polaritons based on a Fabry-Perot fiber cavity plus (In,Ga)As quantum wells. This system should enable not only the study of physical phenomena, such as polariton lasing and strongly correlated behavior, but also applications in optoelectronics and quantum photonics.

Sustainable energy sources surround us, but we must harness them. The periodic, self-sustained deformation of a piezoelectric flag as it flaps in a fluid flow, for example, could be used to convert mechanical energy from wind, or a river or ocean current, into electrical power. The authors show that the interaction between fluid forcing, flapping dynamics, and the resonant properties of the output circuit can lead to a lock-in mechanism that significantly enhances flapping, energy transfer, and efficiency.

Graphene is a promising material for next-generation flexible electronic devices, but its behavior under real-world stresses is poorly understood. What if the graphene were to tear? Using in situ scanning probe microscopy and electrical transport measurements, the authors show that the deterioration of graphene’s mechanical and electrical properties due to strain-induced rips is in fact partially reversible. This limited self-healing presents exciting implications for device applications.