Electrical control of spin transport is promising for achieving new device functionalities. Here we calculate the propagation of spin currents in a graphene-based spin-current demultiplexer under the effect of drift currents. We show that, using spin- and charge-transport parameters already obtained in experiments, the spin currents can be guided in a controlled way. In particular, spin-current selectivities up to ${10}^2$ can be achieved for measurements over a distance of $10\mu \mathrm{m}$ under a moderate drift current density of $20\mu \mathrm{A}/\mu \mathrm{m}$, meaning that the spin current in the arm that is off is only 1% of the current in the arm that is on. To illustrate the versatility of this approach, we show similar efficiencies in a device with four outputs and the possibility of multiplexer operation using spin drift. Finally, we explain how the effect can be optimized in graphene and two-dimensional semiconductors.

The realization of optically detected magnetic resonance via nitrogen vacancy centers in diamond faces challenges at high magnetic fields, which include growing energy consumption of control pulses as well as decreasing sensitivities. Here, we address these challenges with the design of shaped pulses in microwave control sequences that achieve orders of magnitude reductions in energy consumption and concomitant increases in sensitivity when compared to standard top-hat microwave pulses. The method proposed here is general and can be applied to any quantum sensor subjected to pulsed control sequences.

We introduce a fundamental concept and a pragmatic approach to effectively realize negative refraction for elastic waves using a parity-time ($\mathcal{P}\mathcal{T}$)-symmetric plate. In contrast to existing strategies that can achieve this property by making use of complex configurations of phononic crystals or metamaterials, the proposed $\mathcal{P}\mathcal{T}$-symmetric system enables tunable elastic loss and gain, by which the negative refraction can be directly realized and tuned. We demonstrate that, for elastic waves, this tunability can be achieved by piezoelectric units when they are shunted by electric circuits with serially-connected inductance and positive/negative electric resistance. Because the phase and energy flow in the space between the loss and gain units of the system should both be negative, the condition for the $\mathcal{P}\mathcal{T}$-symmetric-based negative refraction is searched by the scattering matrix method. This fundamental concept and its engineering implementation may pave the way toward tunable negative refraction based on the $\mathcal{P}\mathcal{T}$-symmetry approach.

We experimentally investigate the propagation of pulsed magnetostatic surface spin-wave (magnon) signals in an yttrium iron garnet (YIG) waveguide at millikelvin temperatures. Our measurements are performed in a dilution refrigerator at microwave frequencies. The excellent signal-to-noise ratio afforded by the low-temperature environment allows the propagation of the pulses to be observed in detail. The evolution of the envelope shape as the spin wave travels is consistent with calculations based on the known dispersion relation for YIG. We observe a temperature-dependent shift of the ferromagnetic resonance frequency below 4 K, which we suggest is due to the low-temperature properties of the substrate below the film, gallium gadolinium garnet. Our measurement and the accompanying calculations give insight into both low-temperature magnon dynamics in YIG and the feasibility of the use of propagating magnons in solid-state-quantum information processing.

The ideal solar-cell conversion efficiency limit known as the Shockley-Queisser (SQ) limit, which is based on a detailed balance between absorption and radiation, has long been a target for solar-cell researchers. While the theory for this limit uses several assumptions, the requirements in real devices have not been discussed fully. Given the current situation in which research-level cell efficiencies are approaching the SQ limit, a quantitative argument with regard to these requirements is worthwhile in terms of the understanding of the remaining loss mechanisms in current devices and the device characteristics of solar cells that are operating outside the detailed balance conditions. Here we examine two basic assumptions: (1) that the photogenerated carriers lose their kinetic energy via phonon emission in a moment (fast thermalization), and (2) that the photogenerated carriers are extracted into carrier reservoirs in a moment (fast extraction). Using a model that accounts for the carrier relaxation and extraction dynamics, we reformulate the nonequilibrium theory for solar cells in a manner that covers both the equilibrium and nonequilibrium regimes. Using a simple planar solar cell as an example, we address the parameter regime in terms of the carrier extraction time and then consider where the conventional SQ theory applies and what could happen outside the applicable range.

We demonstrate a new paradigm in nanoscale thermometry exploiting well-known switching measurements of a superconducting weak link. Such a weak link probed with nanosecond current pulses serves as a temperature-sensing element and, because of the fast inherent dynamics, is capable of delivering unprecedented temporal resolution. We use the thermometer to measure the dynamic temperature of electrons in a long superconducting wire relaxing to the bath temperature after application of a heating pulse, involving evaluation of the retrapping time. Our measurement delivers resolution better than 10 ns, with potential for further improvement. It extends the temporal resolution of existing experiments and introduces new possibilities for investigating thermodynamics at the nanoscale.

The characters of spin torque ferromagnetic resonance (ST FMR) are systematically studied in the nanoscale ring-shape confined magnetic tunnel junctions (MTJs). Different from the continuous magnetic films, the confined structure shows two resonance states with different resonance frequencies at a given external magnetic field. The two resonance states both originate from the free layer (FL) of MTJs and their resonance frequencies show a blueshift behavior with an increase of the magnetic field. The two states have been recognized by micromagnetic simulations and supported by theoretical analysis as the acoustic-like and optical-like fundamental resonance modes, respectively. They correspond to the in-phase and antiphase localized magnetization oscillation within the FL itself. In addition to the fundamental modes, high-order harmonic signals are also theoretically predicted. This study provides deep understanding of ferromagnetic resonance states in the geometrically confined MTJ structures, which may be applicable to develop spin-torque-based dual-mode microwave detectors.

In standard electron-beam-induced-current (EBIC) imaging, the scanning electron beam creates electron-hole pairs that are separated by an in-sample electric field, producing a current in the sample. In standard scanning electron microscopy (SEM), the scanning electron beam ejects secondary electrons (SEs), which are detected away from the sample. While a beam electron in a scanning transmission electron microscope (STEM) can produce many electron-hole pairs, the yield of SEs is only a few percent for beam energies in the range 60–300 keV, making the latter signal much more difficult to detect on sample as an EBIC. Here we show that the on-sample EBIC in a STEM registers both SE emission and SE capture as holes and electrons, respectively. Detecting both charge carriers produces differential image contrast not accessible with standard, off-sample SE imaging. In a double EBIC-imaging configuration incorporating two current amplifiers, both charge carriers can even be captured simultaneously. Compared with the current produced in standard EBIC imaging, which highlights only the regions in a sample that contain electric fields, the EBIC produced by SE emission, or SEEBIC, is small (picoampere scale). But SEEBIC imaging can produce contrast anywhere in a sample, exposing the texture of buried interfaces, connectivity, and other electronic properties of interest in nanoelectronic devices, even in metals and other structures without internal electric fields.

We investigate the laser-induced ultrafast injection of spin from a ferromagnetic (FM) material into a nonmagnetic (NM) one, for a diverse set of FM-NM interfaces. For all systems, we find the early time ($t<20$ fs) spin dynamics to be driven by spin currents, with majority spin transferred to the NM and minority spin to the FM. At later times, spin-orbit-induced spin flips further demagnetize the FM, but also deplete the NM of the substantial spin moment injected during the sub-20-fs spin dynamics. We identify the density of unoccupied states in the NM material to be the key physical property that underpins the physics of ultrafast demagnetization of FM layers and a complex interplay of this density of states with the spin-orbit (SO) coupling strength of the NM substrate to be responsible for spin-injection efficiency.

Oscillate boiling offers excellent heat transfer at temperatures above the Leidenfrost temperature. Here we realize an electrical microheater with an integrated thermal probe and resolve the thermal cycle during the high-frequency bubble oscillations. A thermal rate of ${10}^8$ K/s is found, indicating its applicability for compact and rapid heat transfer from microelectrical devices. We also report another oscillate-boiling regime occurring at lower heating power where the oscillation becomes unstable and halts after several milliseconds.

We propose a device concept, based on monolayer stanene, able to provide highly polarized spin currents (up to $98\mathrm{\%}$) with voltage-controlled spin polarization operating at room temperature and with small operating voltage ($0.3$ V). The concept exploits the presence of spin-polarized edge states in a stanene nanoribbon. The spin polarization of the total current can be modulated by a differential tuning of the transmission properties, and of the occupation of edge states of different spin, via the application of an in-plane electric field. We demonstrate device operation using ab initio and quantum-transport simulations.

We theoretically propose a phase-coherent thermal circulator based on ballistic multiterminal Josephson junctions. The breaking of time-reversal symmetry by either a magnetic flux or a superconducting phase bias allows heat to flow preferentially in one direction from one terminal to the next while heat flow in the opposite direction is suppressed. We find that our device can achieve a high circulation efficiency over a wide range of parameters and that its performance is robust with respect to the presence of disorder. We provide estimates for the expected heat currents for realistic samples.

High-irradiance lasers incident on metal surfaces create a complex, dynamic process through which the metal can rapidly change from highly reflective to strongly absorbing. Absolute knowledge of this process underpins important industrial laser processes such as laser welding, cutting, and metal additive manufacturing. Determining the time-dependent absorptance of the laser light by a material is important, not only for gaining a fundamental understanding of the light-matter interaction but also for improving process design in manufacturing. Measurements of the dynamic optical absorptance are notoriously difficult due to the rapidly changing nature of the absorbing medium. These data are also of vital importance to process modelers, whose complex simulations need reliable, accurate input data; yet, there are very few available. In this work, we measure the time-dependent, reflected light during a 10-ms laser spot weld using an integrating-sphere apparatus. From this, we calculate the dynamic absorptance for 1070-nm-wavelength light incident on 316L stainless steel. The time resolution of our experiment (less than 1 µs) allows the determination of the precise conditions under which several important physical phenomena occur, such as melt and keyhole formation. The average absorptances determined optically are compared with calorimetrically determined values, and it is found that the calorimeter severely underestimates the absorbed energy due to mass lost during the spot weld. Weld-nugget cross sections are also presented to verify our interpretation of the optical results, as well as to provide experimental data for weld-model validation.

We study the generation of propagating spin waves in $\mathrm{Ta}/(\mathrm{Co},\mathrm{Fe})\mathrm{B}$ bilayer waveguides by spin-orbit-torque antennas and compare them with spin waves generated by conventional inductive antennas. The spin-orbit torque is generated by a transverse microwave current across the waveguide. The detected spin-wave signals for a transverse in-plane magnetization (Damon-Eshbach configuration) exhibit the expected phase rotation and amplitude decay on propagation when current spreading is taken into account. Wavevectors up to about 6 $\mathrm{rad}/\mu \mathrm{m}$ can be excited by the spin-orbit-torque antennas despite current spreading, presumably due to the nonuniformity of the microwave current. The relative magnitude of the generated antidamping spin Hall and Oersted fields is calculated with an analytic model and it is found that they contribute approximately equally to the total effective field generated by the spin-orbit-torque antenna. The prospects for obtaining a pure spin-orbit-torque response are discussed, as are the scaling properties of spin-orbit-torque and inductive antennas.

Maxwell invented a refractive-index profile where light goes in circles and every point is focused. A device with such a profile is called the Maxwell fisheye and it is an absolute optical instrument: it has the ability to collect all rays from any emitting source and recombine them in phase at the corresponding focal point inside the device. Absolute optical instruments may find diverse applications if they can be made in integrated optics on a silicon chip for infrared light. We have fabricated the Maxwell fisheye in silicon photonics and have demonstrated its focusing properties. Our fabrication technique can also be applied to the manufacturing of other devices where smooth and sharp structures need to be made in one lithography step.

We demonstrate a scalable device architecture that facilitates indirect exchange between singlet-triplet spin qubits, mediated by an intermediate quantum state. The device comprises five quantum dots, which can be independently loaded and unloaded via tunneling to adjacent reservoirs, avoiding charge latch-up common in linear dot arrays. In a step toward realizing two-qubit entanglement based on indirect exchange, the architecture permits precise control over tunnel rates between the singlet-triplet qubits and the intermediate state. We show that by our separating qubits by approximately 1 $\mu \mathrm{m}$, the residual capacitive coupling between them is reduced to approximately 7 $\mu \mathrm{eV}$.

$\mathrm{Hf}/{\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}/\mathrm{Mg}\mathrm{O}$ is an attractive alternative free-layer structure for the realization of high-thermal-stability magnetoresistance random access memory (MRAM) because of its enhanced perpendicular magnetic anisotropy (PMA). Writing energy, which is equally crucial for MRAM, is determined not just by its PMA, but also by Gilbert damping and is so far unknown for this system. These parameters are particularly important in ultrathin ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ $(t_{\mathrm{CFB}}<1.3\mathrm{nm})$ for implementation in MRAM. Within this short thickness range, we find that the measured damping varies dramatically from approximately 0.026 to approximately 0.012. This strong thickness-dependent damping is attributed to extrinsic spin-pumping effects as revealed by the large spin-mixing conductance of approximately ${10}^{20}{\mathrm{m}}^{-2}$. It is also found through first-principles calculations that the intrinsic damping in this system will be larger in comparison to bulk ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$. We also establish that a previously excluded higher-order anisotropy term is crucial in the accurate determination of interfacial PMA in $\mathrm{Hf}/{\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}/\mathrm{Mg}\mathrm{O}$. Unlike similar perpendicular heterostructures, this system provides wide tunability in damping, opening up opportunities for niche applications in spintronics.

Incorporation of ions into battery electrodes can lead to phase transformations. When multiparticle phase-transforming electrodes charge or discharge, two processes must occur in each particle: the new phase must nucleate, and then grow until the particle is fully charged or discharged. A fundamental question is which of these two processes is rate limiting. Here we construct a simple stochastic model that shows how the relative rate of nucleation compared with growth determines the particle state-of-charge distributions in the electrode. We find that the number of particles that are partially charged at any time increases as the relative nucleation rate increases. The maximum number of particles that are actively charging occurs just before the time when the first particles are becoming completely charged. By comparing measured state-of-charge distributions with the model, we determine the relative rate of nucleation. We apply this procedure to measurements of the evolution of particles in ${\mathrm{Li}\mathrm{Fe}\mathrm{P}\mathrm{O}}_4$ cathodes and show we can account for the particle state-of-charge distribution as a function of the electrode state of charge.

We study the thermal-piezoresistive pumping mechanism within silicon micromechanical resonators. We develop a multiphysics finite-element model to predict the effective quality factor tuning from pumping for arbitrary geometries and dopants. Our model reproduces the effective quality factor tuning direction and magnitude of our fabricated devices versus direct current as a function of dopant type, concentration, thermal actuator width, and device orientation. We show that, in contrast to degenerate parametric amplification, thermal pumping steepens the phase slope near resonance with an increasing effective quality factor.

We compute spatiotemporal profiles of the electric field coming from a one-dimensional proton beam as it moves through five different animal tissues: muscle, lung, fat, gray matter, and liver. The complex electric permittivity as a function of frequency is assumed to follow the Cole-Cole model, containing experimentally determined parameters for each tissue. The shape of the tissues is taken to be a cylinder with a radius of 10 cm. The initial proton energies are chosen so as to yield the same range (i.e., depth at which the protons stop) inside all tissues. We consider two beams, both of them a square pulse, lasting 10 and 100 ns. Our results show that for both beams the maximal radial electric field at the outer boundary of a tissue is on the order of 150 mV/cm for all five tissues. Furthermore, soon after the last proton in the beam comes to rest, the spatial profile of the radial electric field along the beam axis acquires a maximum that coincides with the proton range. The measurable magnitude of the electric field and its spatial profile lend themselves to an application in proton radiotherapy, where the proton range is an important quantity of interest.