We observe photon-assisted Cooper-pair tunneling in an atomic-scale Josephson junction formed between a superconducting Nb tip and a superconducting Nb sample in a scanning tunneling microscope (STM) at 30 mK. High-resolution tunneling spectroscopy data show a zero-bias conduction peak and other sharp subgap peaks from coupling of the STM junction to resonances in the electromagnetic environment. The subgap peaks respond to incident microwave radiation by splitting into multiple peaks with the position and height depending on the frequency and amplitude of the microwaves. The interpeak spacing shows that the charge carriers are Cooper pairs rather than quasiparticles, and the power dependence reveals that the current originates from photon-assisted phase-incoherent tunneling of pairs rather than the more conventional phase-coherent tunneling of pairs that yields Shapiro steps.

We propose a lateral spin-blockade device that uses the interband Rashba effect in a symmetric double quantum well (QW), where the Rashba effect in the conventional sense vanishes because of its inversion symmetry. The interband Rashba effect manifests itself in the off-diagonal term (represented by the parameter $\eta$) in the QW space using the bonding and antibonding basis [Esmerindo Bernardes, John Schliemann, Minchul Lee, J. Carlos Egues, and Daniel Loss, Phys. Rev. Lett. 99, 076603 (2007)]. In such a system, spin selection is possible by tuning the device length, gate electric field and in-plane magnetic field. We particularly show illustrative mechanisms using a one-dimensional model with $\mathbf{k}=(k_F,0)$, where the selected spin can be blocked completely in the presence of the in-plane magnetic field. While the inclusion of the finite $k_y$ and/or the gate electric field deteriorates the spin polarization $P$, finite values remain for $P$ ($P>11\%$). Our proposal can also be regarded as an effective way of enhancing a variation of the Rashba-Edelstein effect, the generation of bulk spin polarization by electric current, based on semiconductor band engineering technology.

Phononic crystals (PCs) consisting of periodic materials with different acoustic properties have potential applications in functional devices. To realize more smart functions, it is desirable to actively control the properties of PCs on demand, ideally within the same fabricated system. Here, we report a tunable PC made of ${\mathrm{Ba}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Ti}{\mathrm{O}}_3$ (BST) ceramics, wherein a 20-K temperature change near room temperature results in a 20% frequency shift in the transmission spectra induced by a ferroelectric phase transition. The tunability phenomenon is attributed to the structure-induced resonant excitation of $A_0$ and $A_1$ Lamb modes that exist intrinsically in the uniform BST plate, while these Lamb modes are sensitive to the elastic properties of the plate and can be modulated by temperature in a BST plate around the Curie temperature. The study finds opportunities for creating tunable PCs and enables smart temperature-tuned devices such as the Lamb wave filter or sensor.

We report highly linearly polarized remote luminescence that emerges at the cleaved edges of nanoscale gallium selenide slabs tens of micrometers away from the optical excitation spot. The remote-edge luminescence (REL) measured in the reflection geometry has a degree of linear polarization above 0.90, with polarization orientation pointing toward the photoexcitation spot. The REL is dominated by an index-guided optical mode that is linearly polarized along the crystalline $c$ axis. This luminescence is from out-of-plane dipoles that are converted from in-plane dipoles through a spin-flip process at the excitation spot.

Because of its extremely high energy efficiency, adiabatic superconductor logic is one of the most promising candidates for the realization of a practical reversible computer. In a previous study, we proposed a logically and physically reversible logic gate using adiabatic superconductor logic, and numerically demonstrated reversible computing. In the numerical calculation, we assumed that the average energy dissipation at finite temperature corresponds to that at zero temperature. However, how the phase difference of a Josephson junction in adiabatic superconductor logic behaves at finite temperature is not yet well understood, and whether thermal noise can induce a nonadiabatic state change remains unclear. In the present study, we investigate energy dissipation in adiabatic superconductor logic at finite temperature through numerical analyses using the Monte Carlo method. We investigate the average and standard deviation of the energy dissipation through both numerical calculation and analytical estimation. Finally, we discuss the minimum energy dissipation required for adiabatic switching operations.

We probe the effects of structural disorder on the performance of organic photovoltaic (OPV) devices via drift-diffusion modeling. We utilize ensembles of spatially disordered one-dimensional mobility profiles to approximate the three-dimensional structural disorder present in actual devices. Each replica in our ensemble approximates one high-conductivity pathway through the three-dimensional network(s) present in a polymer-based bulk heterojunction solar cell, so that the ensemble-averaged behavior provides a good approximation to a full three-dimensional structurally disordered device. Our calculations show that the short-circuit current, fill factor, and power conversion efficiency of simulated devices are all negatively impacted by the inclusion of structural disorder, but that the open-circuit voltage is nearly impervious to structural defects. This is in contrast to energetic disorder, where previous studies found that spatial variation in the energy in OPV active layers causes a decrease in the open-circuit voltage. We also show that structural disorder causes the greatest detriment to device performance for feature sizes between 2 and 10 nm. Since this is on the same length scale as the fullerene crystallites in experimental devices, it suggests both that controlling structural disorder is critical to the performance of OPV devices and that the effects of structural disorder should be included in future drift-diffusion modeling studies of organic solar cells.

A high reversible capacity is a key feature for any rechargeable battery. In lithium-ion battery technology, tin-oxide anodes do fulfill this requirement, but a fast loss of capacity hinders a full commercialization. Using first-principles calculations, we propose a microscopic model that sheds light on the reversible lithiation-delithiation of SnO and reveals that a sintering of Sn causes a strong degradation of SnO-based anodes. When the initial irreversible transformation ends, active anode grains consist of Li-oxide layers separated by Sn bilayers. During the following reversible lithiation, the Li oxide undergoes two phase transformations that give rise to a Li enrichment of the oxide and the formation of a layered SnLi composite. We find that the model-predicted anode volume expansion and voltage profile agree well with experiments, and a layered anode grain is highly conductive and has a theoretical reversible capacity of 4.5 Li atoms per a SnO host unit. The model suggests that the grain structure has to remain layered to sustain its reversible capacity and a thin-film design of battery anodes could be a remedy for the capacity loss.

From radio-electronics signal analysis to biological sample actuation, surface acoustic waves (SAWs) are involved in a multitude of modern devices. However, only the most simple standing or progressive waves such as plane and focused waves have been explored so far. In this paper, we expand the SAW toolbox with a wave family named "swirling surface acoustic waves" which are the 2D anisotropic analogue of bulk acoustic vortices. Similarly to their 3D counterpart, they appear as concentric structures of bright rings with a phase singularity in their center resulting in a central dark spot. After the rigorous mathematical definition of these waves, we synthesize them experimentally through the inverse filtering technique revisited for surface waves. For this purpose, we design a setup combining arrays of interdigitated transducers and a multichannel electronic that enables one to synthesize any prescribed wave field compatible with the anisotropy of the substrate in a region called the “acoustic scene.” This work opens prospects for the design of integrated acoustic vortex generators for on-chip selective acoustic tweezing.

Spatial gradients in a refractive index are used extensively in acoustic metamaterial applications to control wave propagation through phase delay. This study reports the design and experimental realization of an acoustic gradient-index lens using a sonic crystal lattice that is impedance matched to water over a broad bandwidth. In contrast to previous designs, the underlying lattice features refractive indices that are lower than the water background, which facilitates propagation control based on a phase advance as opposed to a delay. The index gradient is achieved by varying the filling fraction of hollow, air-filled aluminum tubes that individually exhibit a higher sound speed than water and matched impedance. Acoustic focusing is observed over a broad bandwidth of frequencies in the homogenization limit of the lattice, with intensity magnifications in excess of 7 dB. An anisotropic lattice design facilitates a flat-faceted geometry with low backscattering at 18 dB below the incident sound-pressure level. A three-dimensional Rayleigh-Sommerfeld integration that accounts for the anisotropic refraction is used to accurately predict the experimentally measured focal patterns.

We analyze a design for a microwave circulator which could replace many of the commercial ferrite circulators that are ubiquitous in contemporary quantum superconducting microwave experiments. The lossless, lumped-element design is capable of being integrated on chip with other superconducting microwave devices, thus circumventing the many performance-limiting aspects of ferrite circulators. The design is based on the dynamic modulation of dc superconducting microwave quantum interference devices that function as nearly linear, tunable inductors. The connection to familiar ferrite-based circulators is a simple frame boost in the internal dynamics’ equation of motion. In addition to the general, schematic analysis, we also give an overview of many considerations necessary to achieve a practical design with a tunable center frequency in the 4–8-GHz frequency band, a bandwidth of 240 MHz, reflections at the $-20\text{−}\mathrm{dB}$ level, and a maximum signal power of approximately 100 microwave photons per inverse bandwidth.

We present a simple on-chip electronic thermometer with the potential to operate down to 1 mK. It is based on transport through a single normal-metal–superconductor tunnel junction with rapidly widening leads. The current through the junction is determined by the temperature of the normal electrode that is efficiently thermalized to the phonon bath, and it is virtually insensitive to the temperature of the superconductor, even when the latter is relatively far from equilibrium. We demonstrate here the operation of the device down to 7 mK and present a systematic thermal analysis.

The development of metamaterials, i.e., artificially structured materials that interact with waves in unconventional ways, has revolutionized our ability to manipulate the propagation of electromagnetic waves and their interaction with matter. One of the most exciting applications of metamaterial science is related to the possibility of totally suppressing the scattering of an object using an invisibility cloak. Here, we review the available methods to make an object undetectable to electromagnetic waves, and we highlight the outstanding challenges that need to be addressed in order to obtain a fully functional coating capable of suppressing the total scattering of an object. Our outlook discusses how, while passive linear cloaks are fundamentally limited in terms of bandwidth of operation and overall scattering suppression, active and/or nonlinear cloaks hold the promise to overcome, at least partially, some of these limitations.

We report on experiments with a microfabricated surface trap designed for confining a chain of ions in a ring. Uniform ion separation over most of the ring is achieved with a rotationally symmetric design and by measuring and suppressing undesired electric fields. After reducing stray fields, the ions are confined primarily by a radio-frequency pseudopotential and their mutual Coulomb repulsion. Approximately 400 ${{}^{40}\mathrm{Ca}}^{+}$ ions with an average separation of $9\mu \mathrm{m}$ comprise the ion crystal.

In field-effect transistors, the inherent randomness of dopants and other charges is a major cause of device-to-device variability. For a quasi-one-dimensional device such as carbon nanotube transistors, even a single charge can drastically change the performance, making this a critical issue for their adoption as a practical technology. Here we calculate the effect of the random charges at the gate-oxide surface in ballistic carbon nanotube transistors, finding good agreement with the variability statistics in recent experiments. A combination of experimental and simulation results further reveals that these random charges are also a major factor limiting the subthreshold swing for nanotube transistors fabricated on thin gate dielectrics. We then establish that the scaling of the nanotube device uniformity with the gate dielectric, fixed-charge density, and device dimension is qualitatively different from conventional silicon transistors, reflecting the very different device physics of a ballistic transistor with a quasi-one-dimensional channel. The combination of gate-oxide scaling and improved control of fixed-charge density should provide the uniformity needed for large-scale integration of such novel one-dimensional transistors even at extremely scaled device dimensions.

It is a challenge to fabricate bridge-type nano-superconducting quantum interference devices (bridge-type nano-SQUIDs) that operate without hysteresis over a broad temperature range. Hysteresis—defined by the difference between switching and retrapping current—is one of the foremost constraints to operating nano-SQUIDs with low noise. The quantum behavior of the switching current has been explored in bridge-type nano-SQUIDs, but studies exploring the parameters ruling the retrapping current are rare. Here, we study the temperature and magnetic-field dependence of the retrapping current in two different kinds of bridge-type nano-SQUID: trilayer aluminum-niobium-tungsten bridge-type nano-SQUIDs and suspended-bridge nano-SQUIDs. Our study confirms previous works showing that the retrapping current decreases as the bath temperature increases and is insensitive to the magnetic field. Using a thermal model originally proposed by Skocpol, Beasley, and Tinkham [J. Appl. Phys. 45, 4054 (1974)], we account for, and suggest a simple formula which describes, the temperature dependence of the retrapping current. Our calculations show that the magnitude of the retrapping current is mainly dependent on the superconducting transition temperature and the effective resistance of the weak link and that the temperature dependence of the retrapping current is ruled by the temperature dependence of the thermal conductivity in the normal and superconducting state. Finally, we apply our calculation to newly fabricated shunted bridge-type nano-SQUIDs, which show nonhysteretic current-voltage characteristics down to at least 250 mK and display systematic voltage modulations as a function of externally applied magnetic fields.

The choice of electrode for organic photovoltaics is known to be of importance to both device stability and performance, especially regarding the open-circuit voltage ($V_{\mathrm{OC}}$). Here we show that the work function of a nickel oxide anode, varied using an ${\mathrm{O}}_2$ plasma treatment, has a considerable influence on the open-circuit voltage $V_{\mathrm{OC}}$ of an organic solar cell. We probe recombination in the devices using transient photovoltage and charge extraction to determine the lifetime as a function of charge-carrier concentration and compare the experimental results with numerical drift-diffusion simulations. This combination of experiment and simulations allows us to conclude that the variations in $V_{\mathrm{OC}}$ are due to a change in surface recombination, localized at the NiO anode, although only a small change in carrier lifetime is observed.

We design, fabricate, and characterize a metasurface, whose multipole optical response depends significantly on the illumination direction. The metasurface is composed of gold-nanodisc dimers embedded in glass. In spite of their nanoscale size, the dimers exhibit a dominating electric-current-quadrupole response in a wide range of wavelengths around 700 nm when illuminated from one side, and a primarily electric-dipole response when illuminated from the opposite side. This leads to two consequences. First, the reflection coefficient of the metasurface considerably differs for the two sides of illumination. Second, quadrupole excitation results in a significant local enhancement of both electric and magnetic fields around the dimers. Our experimental spectroscopic data are in good agreement with simulations obtained using a multipole expansion model.

The storage properties of methane gas in Vycor porous glass (5.7 nm) are characterized in a wide pressure range from 0.7 to 89.7 MPa using nuclear magnetic resonance. We demonstrate the capability of high-field nuclear-magnetic-resonance relaxometry for the determination of the methane-gas storage capacity and the measurement of the hydrogen index, to a high degree of accuracy. This helps determine the excess gas in the pore space which can be identified to exhibit Langmuir properties in the low pressure regime of 0.7 to 39.6 MPa. The Langmuir model enables us to determine the equilibrium density of the monolayer of adsorbed gas to be 8.5% lower than that of liquid methane. We also identify the signatures of multilayer adsorption at the high pressure regime from 39.6 to 89.7 MPa and use the Brunauer-Emmet-Teller theory to determine the number of adsorbed layers of methane gas. We show how these measurements help us differentiate the gas stored in the Vycor pore space into free and adsorbed fractions for the entire pressure range paving way for similar applications such as studying natural-gas storage in gas shale rock or hydrogen storage in carbon nanotubes.

Photoinduced absorption signals measured by transient absorption spectroscopy are typically proportional to the product of absorption cross section ($\sigma$) and excited-state density ($N$). We show that this approximation does not hold at high probe-pulse intensities, and introduce the use of probe-intensity-dependent spectroscopy to decouple the two parameters. The singlet excited-state ($S_1\to S_2$) absorption cross section of the conjugated polymer F8BT is measured to be $1.6\times {10}^{-16}{\mathrm{cm}}^2\pm 40\%$ at 800 nm and ${3.7\times 10}^{-16}{\mathrm{cm}}^2\pm 30\%$ at 900 nm, with no variation over the time window surveyed. The robustness of these parameters is established by observing that only $N$ scales with excitation fluence and time delay, and conversely only $\sigma$ is dependent on probe wavelength. The technique may be useful for quantifying salient parameters in many systems, such as branching yields in systems exhibiting singlet fission or triplet production, or cross sections required for photophysical models.

At strong pump powers, a semiconductor optical cavity passes through a Hopf bifurcation and undergoes self-oscillation. We simulate this device using semiclassical Langevin equations and assess the effect of quantum fluctuations on the dynamics. Below threshold, the cavity acts as a phase-insensitive linear amplifier, with noise approximately $5\times \text{larger}$ than the Caves bound. Above threshold, the limit cycle acts as an analog memory, and the phase diffusion is approximately $10\times \text{larger}$ than the bound set by the standard quantum limit. We also simulate entrainment of this oscillator and propose an optical Ising machine and classical controlled not (cnot) gate based on the effect.