Cables carry strong electric currents in long-distance power transmission, and in coils generating strong magnetic fields for MRI scanners, maglev trains, and particle accelerators. Low temperatures are helpful for everyday resistive cables, and necessary for superconducting ones. The author shows how a coating of thermomagnetic material could handily extract heat via a power cable’s own field, to assist or even replace cooling by cryogenic fluids.

Cables carry strong electric currents in long-distance power transmission, and in coils generating strong magnetic fields for MRI scanners, maglev trains, and particle accelerators. Low temperatures are helpful for everyday resistive cables, and necessary for superconducting ones. The author shows how a coating of thermomagnetic material could handily extract heat via a power cable’s own field, to assist or even replace cooling by cryogenic fluids.

Superconducting quantum interference devices (SQUIDs) are used across physics and engineering, from amplification to detecting tiny magnetic fields to quantum computing. In many applications the SQUID’s steady-state response is of interest, but here the authors measure and model the $transient$ response to short bursts of radiation. The resulting ringdown oscillations in voltage decay over time, with a rich structure that may be particularly useful in discriminating weak magnetic fluxes—in qubits, for example.

One path to quantum information processing involves light-matter interaction, such as light in a photonic-crystal waveguide coupled to a cloud of ultracold alkali atoms. However, to actually implement such a system, it is crucial to increase the coupling strength beyond what is presently achievable. The authors explain the design requirements to achieve large coupling by decreasing the group velocity of an edge mode in a hybrid-clad waveguide, allowing photons of “slow light” to interact with trapped cold atoms in a robust, chip-integrated setting.

Kesterite semiconductors have been successfully employed in thin-film solar cells, with potential for high conversion efficiency solely from abundant materials. Although the best devices have come from mixed sulfo-selenide compounds, the underlying defect physics in this family has remained mysterious. The authors present a comprehensive study of electronic defects as a function of chalcogenide composition, providing a much needed, consistent view of shallow and deep levels in Cu${}_2$ZnSn(S,Se)${}_4$ and their consequences for device performance.

Electricity can be obtained by either heating or illuminating a semiconductor—or both. Exploiting the photo-Seebeck effect, the authors devise a purely optical method for measuring at the microscale the transport properties of a semiconductor, cleanly, without having to deal with contacts, $p-n$ junctions, or removing electrical artifacts from the data. This technique allows fundamental knowledge of thermopower and carrier populations in semiconductors, to help in developing thermoelectric and photovoltaic applications.

Understanding the physics behind thermoelectric performance is key to materials engineering for efficient devices. The authors’ calculations show that the thermoelectric performance of Mg${}_2$(Ge,Sn), an effective and environmentally friendly material, is affected by multiple isosurface sections with complex shapes near the conduction-band minimum. Significant improvements in the figure of merit are still possible, for both $n$-type and $p$-type samples, and the results of this study provide guidance as to the best doping levels for a desired operating temperature.

One vein of spintronics research seeks to harness propagating magnetic domain walls for information processing. The authors engineer a potential landscape via local variations in a ring geometry, and image the motion of domain walls in rotating magnetic fields to quantify the contribution of the spatially varying potential to wall dynamics. Domain-wall velocity depends on ring width, being highest where the ring is widest, and such a potential thus could be selected to synchronize velocities and enable applications.

Scintillator materials continue to attract wide interest for radiation detection in high-energy physics and medical diagnostics. Elpasolite halides are among the most promising scintillators, due to their high structural symmetry and excellent performance, and here the authors reveal the unique emission mechanism of two mixed-anion members of this class. This study guides the design of materials with the exceptional energy resolution and light yield needed to improve applications.

Semiconductor technology can hinge on the physics of polycrystalline materials, not the nearly ideal single crystals of the research lab. Scattering of charge carriers at grain boundaries can spoil conductivity, and thus device performance. This study presents an analytical description of electron transport in highly doped polycrystals, focusing on electron tunneling through potential barriers at grain boundaries, while also including two intragrain scattering mechanisms. Although the authors begin with transparent conducting oxides in mind (for, say, flat-panel displays, or solar cells), their findings should extend to all semiconductors.

Sources of light in the terahertz (THz) wavelength range are extensively researched, and tunable planar Josephson junctions seem promising.They can be used $e.g.$ to calibrate sensitive detectors, but not all of the energy poured into the junctions is converted to THz light. Knowing how many photons come out is not simply a matter of knowing how much energy went in. To address this problem, the authors employ a detector that discriminates between coherent THz radiation and thermally induced blackbody radiation, with single-photon sensitivity.

Ultrasensitive detection of nanoscale particles has applications in important fields ranging from environmental monitoring to analysis of viral structures. The authors show that the dissipative interaction in an optical microcavity of high quality factor allows the detection of single nanoparticles, even when the real part of an analyte’s polarizability approaches zero. This innovative approach presents a significant step towards practical optical sensors for use in physics, analytical chemistry, environmental science, and molecular biology.

Supercomputers are weak at tasks of cognition and perception that humans perform handily, so $neuromorphic$ platforms, built of nanoelectronic components yet showing a real brain’s efficiency, are of intense research interest. Hardware mimicking biological synapses, specifically their plasticity (learning ability), is crucial. The authors use spin polarization of electrons in a magnetic tunnel junction in analogy to neurotransmitter release in synapses to demonstrate both short-term plasticity and long-term potentiation with spintronic devices.

Solid-state magnetic refrigeration could disrupt conventional technology based on gas compressors, but affordable materials with large magnetocaloric response are needed. Using direct measurements of adiabatic temperature change in three different regimes of magnetic-field sweep rate, the authors investigate the dynamics of the martensitic transition in a class of Heusler alloys. Here phase-boundary motion is not an issue, but the speed of nucleation can be a limiting factor at high sweep rates. For everyday devices working at around 10 Hz, though, these compounds are promising.

Long-distance quantum communication requires transmitting, buffering, and processing quantum information via nodes called quantum repeaters. Unfortunately, current technology is suited for either communication or storage, but not both simultaneously. To address this challenge, the authors propose an all-semiconductor hybrid quantum device that capacitively couples an optically active quantum-dot molecule to a gated-dot array. This optoelectronic device, which could be orders of magnitude faster than today’s repeaters, emits a photon whose polarization is entangled with the electron spins in the array.