Low-temperature research laboratories with typical liquid-helium consumption of the order of tens of liters per day have greatly benefited from the recent development of small-scale liquefiers. In general, these liquefiers are based on Gifford-McMahon or pulse-tube closed-cycle refrigerators with a nominal cooling power ranging from 1 to 1.5 W at 4.2 K. The liquefaction rate for these cryocooler-based liquefiers depends on the pressure at which the helium is liquefied, although the final user conditions of the produced liquid helium are always atmospheric pressure and boiling temperature (e.g., 4.2 K at 100 kPa). Here, we show a systematic study on this effect, in which an enhancement in excess of 70% in liquefaction rate is found experimentally for pressures near and above the critical point of helium (220 kPa). We propose that the underlying mechanism for the liquefaction enhancement is based on the increase in cryocooler cooling power with temperature and the decrease of the helium enthalpy with pressure.

The electronic structural and phonon properties of ${\mathrm{Na}}_{2-\alpha }{\mathit{M}}_{\alpha }\mathrm{Zr}{\mathrm{O}}_3$ ($M=\mathrm{Li}$,K, $\alpha =0.0$,0.5,1.0,1.5,2.0) are investigated by first-principles density-functional theory and phonon dynamics. The thermodynamic properties of ${\mathrm{CO}}_2$ absorption and desorption in these materials are also analyzed. With increasing doping level $\alpha$, the binding energies of ${\mathrm{Na}}_{2-\alpha }{\mathrm{Li}}_{\alpha }\mathrm{Zr}{\mathrm{O}}_3$ are increased while the binding energies of ${\mathrm{Na}}_{2-\alpha }{\mathrm{K}}_{\alpha }{\mathrm{ZrO}}_3$ are decreased to destabilize the structures. The calculated band structures and density of states also show that, at the same doping level, the doping sites play a significant role in the electronic properties. The phonon dispersion results show that few soft modes are found in several doped configurations, which indicates that these structures are less stable than other configurations with different doping levels. From the calculated relationships among the chemical-potential change, the ${\mathrm{CO}}_2$ pressure, and the temperature of the ${\mathrm{CO}}_2$ capture reactions by ${\mathrm{Na}}_{2-\alpha }{\mathit{M}}_{\alpha }\mathrm{Zr}{\mathrm{O}}_3$, and from thermogravimetric-analysis experimental measurements, the Li- and K-doped mixtures ${\mathrm{Na}}_{2-\alpha }{\mathit{M}}_{\alpha }\mathrm{Zr}{\mathrm{O}}_3$ have lower turnover temperatures ($T_t$) and higher ${\mathrm{CO}}_2$ capture capacities, compared to pure ${\mathrm{Na}}_2\mathrm{Zr}{\mathrm{O}}_3$. The Li-doped systems have a larger $T_t$ decrease than the K-doped systems. When increasing the Li-doping level $\alpha$, the $T_t$ of the corresponding mixture ${\mathrm{Na}}_{2-\alpha }{\mathrm{Li}}_{\alpha }\mathrm{Zr}{\mathrm{O}}_3$ decreases further to a low-temperature range. However, in the case of K-doped systems ${\mathrm{Na}}_{2-\alpha }{\mathrm{K}}_{\alpha }\mathrm{Zr}{\mathrm{O}}_3$, although doping K into ${\mathrm{Na}}_2\mathrm{Zr}{\mathrm{O}}_3$ initially shifts its $T_t$ to lower temperatures, further increases of the K-doping level $\alpha$ causes $T_t$ to increase. Therefore, doping Li into ${\mathrm{Na}}_2\mathrm{Zr}{\mathrm{O}}_3$ has a larger influence on its ${\mathrm{CO}}_2$ capture performance than the K-doped ${\mathrm{Na}}_2\mathrm{Zr}{\mathrm{O}}_3$. Compared with pure solids ${\mathit{M}}_2\mathrm{Zr}{\mathrm{O}}_3$, after doping with other elements, these doped systems’ ${\mathrm{CO}}_2$ capture performances are improved.

We show that optical forces acting on carbon nanotubes are substantially enhanced when the optical wavelength is tuned to resonances in the electronic band structure. Using a tunable laser source, we experimentally demonstrate a resonant optical scattering force on single-walled carbon nanotubes and, by tuning the wavelength, exploit this force to achieve chirality enrichment of four chiralities of nanotubes. Our results represent a significant step towards optical manipulation and sorting of carbon nanotubes based on their chiral vector.

It is hypothesized that the decreased treatment efficiency in contemporary shock-wave lithotripters is related to tensile wave attenuation due to cavitation in the prefocal beam path. Utilizing high-speed imaging of the beam path and focal pressure waveform measurements, tensile attenuation is associated with bubble proliferation. By systematically testing different combinations of pulse-repetition frequency and gas concentration, we modulate the bubble-dissolution time to identify which conditions lead to bubble proliferation and show that reducing bubble proliferation in the beam path significantly improves acoustic transmission and stone comminution efficiency in vitro. In addition to experiments, a bubble-proliferation model is developed that takes gas diffusion across the bubble wall and bubble fragmentation into account. By aligning the model with experimental observations, the number of daughter bubbles produced after a single lithotripter bubble collapse is estimated to be in the range of $253\sim 510$. This finding is on the same order of magnitude with previous measurements of an isolated bubble collapse in a lithotripter field by Pishchalnikov, McAteer, and Williams [BJU Int. 102, 1681 (2008)], and this estimate improves the general understanding of lithotripsy bubble dynamics in the beam path.

Solid-state qubits have recently advanced to the level that enables them, in principle, to be scaled up into fault-tolerant quantum computers. As these physical qubits continue to advance, meeting the challenge of realizing a quantum machine will also require the development of new supporting devices and control architectures with complexity far beyond the systems used in today’s few-qubit experiments. Here, we report a microarchitecture for controlling and reading out qubits during the execution of a quantum algorithm such as an error-correcting code. We demonstrate the basic principles of this architecture using a cryogenic switch matrix implemented via high-electron-mobility transistors and a new kind of semiconductor device based on gate-switchable capacitance. The switch matrix is used to route microwave waveforms to qubits under the control of a field-programmable gate array, also operating at cryogenic temperatures. Taken together, these results suggest a viable approach for controlling large-scale quantum systems using semiconductor technology.

The spontaneous flapping of a flag in a steady flow can be used to power an output circuit using piezoelectric elements positioned at its surface. Here, we study numerically the effect of inductive circuits on the dynamics of this fluid-solid-electric system and on its energy-harvesting efficiency. In particular, a destabilization of the system is identified, leading to energy harvesting at lower flow velocities. Also, a frequency lock-in between the flag and the circuit is shown to significantly enhance the system’s harvesting efficiency. These results suggest promising efficiency enhancements of such flow-energy harvesters through the output circuit optimization.

Thermoelectric performance is of interest for numerous applications such as waste-heat recovery and solid-state energy conversion and will be seen to be closely connected to topological-insulator behavior. In this context, we here report first-principles transport and defect calculations for ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$ in relation to ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. The two compounds are found to contain remarkably different electronic structures in spite of being isostructural and isoelectronic. We discuss these results in terms of the topological-insulator characteristics of these compounds.

Strongly correlated electron systems such as the rare-earth nickelates ($R{\mathrm{NiO}}_3$, $R$ denotes a rare-earth element) can exhibit synapselike continuous long-term potentiation and depression when gated with ionic liquids; exploiting the extreme sensitivity of coupled charge, spin, orbital, and lattice degrees of freedom to stoichiometry. We present experimental real-time, device-level classical conditioning and unlearning using nickelate-based synaptic devices in an electronic circuit compatible with both excitatory and inhibitory neurons. We establish a physical model for the device behavior based on electric-field-driven coupled ionic-electronic diffusion that can be utilized for design of more complex systems. We use the model to simulate a variety of associate and nonassociative learning mechanisms, as well as a feedforward recurrent network for storing memory. Our circuit intuitively parallels biological neural architectures, and it can be readily generalized to other forms of cellular learning and extinction. The simulation of neural function with electronic device analogs may provide insight into biological processes such as decision making, learning, and adaptation, while facilitating advanced parallel information processing in hardware.

Several technologies and biotechnologies employing small-sized particles in microfluidics and nanofluidics rely on the ability of hampering thermal motion for progress. We experimentally demonstrate that nanoparticles suspended in a dilute polymer solution in Poiseuille flow can be trapped in the central region of a microtube, with a trapping efficiency that depends on the squared flow rate. The trap force is caused by the viscoelasticity of the suspending fluid, and can be modulated by selecting liquids with specific rheology. We also propose a simple theoretical argument that supports the experimental evidence, and links the trapping force to a dimensionless parameter comparing viscoelastic normal forces and Brownian forces. The theoretical argument distills into a simple equation, which could be used to downscale flow cytometers, or to design microfluidic devices for counting, coding, or separating nanoparticles.

The practicality of a newly proposed infrared-terahertz (IR-THz) double-resonance (DR) spectroscopic technique for remote trace-gas identification is explored. The strength of the DR signatures depends on known molecular parameters from which a combination of pump-probe transitions may be identified to recognize a specific analyte. Atmospheric pressure broadening of the IR and THz trace-gas spectra relaxes the stringent pump coincidence requirement, allowing many DR signatures to be excited, some of which occur in the favorable atmospheric transmission windows below 500 GHz. By designing the DR spectrometer and performing a detailed signal analysis, the pump-probe power requirements for detecting trace amounts of methyl fluoride, methyl chloride, or methyl bromide may be estimated for distances up to 1 km. The strength of the DR signature increases linearly with pump intensity but only as the square root of the probe power because the received signal is in the Townes noise limit. The concept of a specificity matrix is introduced and used to quantify the recognition specificity and calculate the probability of false positive detection of an interferent.

Pt-Al and Pt-Y-Al thin-film electrodes on yttria-stabilized zirconia electrolytes were prepared by dealloying of cosputtered Pt-Al or Pt-Y-Al films. The selective dissolution of Al from the Pt-alloy compound causes the formation of a highly porous nanowire network with a mean branch thickness below 25 nm and a pore intercept length below 35 nm. The oxygen-reduction capability of the resulting electrodes is analyzed in a micro-solid-oxide fuel-cell setup at elevated temperatures (598–873 K). Here, we demonstrate that these nanoporous thin films excel “state-of-the-art” fuel-cell electrodes in terms of catalytic activity and thermal stability. The nanoporous Pt electrodes exhibit exchange-current densities that are up to 13 times higher than conventional Pt electrodes, measured at 648 K. It is shown that the enhanced intrinsic electrocatalytic activity of these Pt electrodes is achieved by the addition of yttrium as a ternary constituent, which allows for an engineering of the material’s band structure.

The effects of a driven fast-oscillating spinneret on the bending instability of electrified jets, leading to the formation of spiral structures in electrospinning experiments with charged polymers, are explored by means of extensive computer simulations. It is found that the morphology of the spirals can be placed in direct correspondence with the oscillation frequency and amplitude. In particular, by increasing the oscillation amplitude and frequency, thinner fibers can be extracted by the same polymer material, thereby opening design scenarios in electrospinning experiments.

Typical logic elements utilizing the field effect rely on the change in carrier concentration due to the field in the channel region of the device. Ferroelectric-field-effect devices provide a nonvolatile version of this effect due to the stable polarization order parameter in the ferroelectric. In this work, we describe an oxide/oxide ferroelectric heterostructure device based on (001)-oriented ${\mathrm{PbZr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3\text{−}{\mathrm{LaNiO}}_3$ where the dominant change in conductivity is a result of a significant mobility change in the interfacial channel region. The effect is confined to a few atomic layers at the interface and is reversible by switching the ferroelectric polarization. More interestingly, in one polarization state, the field effect induces a 1.7-eV shift of the interfacial bands to create a new conducting channel in the interfacial PbO layer of the ferroelectric.

In electronic cooling with superconducting tunnel junctions, the cooling power is counterbalanced by the interaction with phonons and by the heat flow from the overheated leads. We study aluminum-based coolers that are equipped with a suspended normal metal and an efficient quasiparticle drain. At intermediate temperatures, the phonon bath of the suspended normal metal is cooled. By adjusting the junction transparency, we control the injection current and, thus, the temperature of the superconducting lead at the optimum cooling point. The best device shows remarkable cooling from 150 mK down to about 30 mK, a factor of 5 in temperature at a power of 40 pW. We discuss heat transport in our device and the reasons for cooling saturation at the low-temperature end.

We visualize the formation of fingered flow in dry model sandy soils under different rain conditions using a quasi-2D experimental setup and systematically determine the impact of the soil grain diameter and surface wetting properties on the water channeling phenomenon. The model sandy soils we use are random closely packed glass beads with varied diameters and surface treatments. For hydrophilic sandy soils, our experiments show that rain water infiltrates a shallow top layer of soil and creates a horizontal water wetting front that grows downward homogeneously until instabilities occur to form fingered flows. For hydrophobic sandy soils, in contrast, we observe that rain water ponds on the top of the soil surface until the hydraulic pressure is strong enough to overcome the capillary repellency of soil and create narrow water channels that penetrate the soil packing. Varying the raindrop impinging speed has little influence on water channel formation. However, varying the rain rate causes significant changes in the water infiltration depth, water channel width, and water channel separation. At a fixed rain condition, we combine the effects of the grain diameter and surface hydrophobicity into a single parameter and determine its influence on the water infiltration depth, water channel width, and water channel separation. We also demonstrate the efficiency of several soil water improvement methods that relate to the rain water channeling phenomenon, including prewetting sandy soils at different levels before rainfall, modifying soil surface flatness, and applying superabsorbent hydrogel particles as soil modifiers.

We explore the nonlinear optical response of three-dimensional gyroidal metamaterials, which show greater than tenfold enhancements compared to all other metallic nanomaterials as well as bulk gold. A simple analytical model for this metamaterial response shows how the reflectivity spectrum scales with the metal fill fraction and the refractive index of the material that the metallic nanostructure is embedded in. The ultrafast response arising from the interconnected three-dimensional nanostructure can be separated into electronic and lattice contributions with strong spectral dependences on the dielectric filling of the gyroids, which invert the sign of the nonlinear transient reflectivity changes. These metamaterials thus provide a wide variety of tunable nonlinear optical properties, which can be utilized for frequency mixing, optical switching, phase modulators, novel emitters, and enhanced sensing.

Mass spectrometry is used in a wide range of scientific disciplines including proteomics, pharmaceutics, forensics, and fundamental physics and chemistry. Given this ubiquity, there is a worldwide effort to improve the efficiency and resolution of mass spectrometers. However, the performance of all techniques is ultimately limited by the initial phase-space distribution of the molecules being analyzed. Here, we dramatically reduce the width of this initial phase-space distribution by sympathetically cooling the input molecules with laser-cooled, cotrapped atomic ions, improving both the mass resolution and detection efficiency of a time-of-flight mass spectrometer by over an order of magnitude. Detailed molecular-dynamics simulations verify the technique and aid with evaluating its effectiveness. This technique appears to be applicable to other types of mass spectrometers.

Controlling the interaction of an ultrafast laser pulse with a thin film remains a difficult task, especially when aiming to confine material modifications to subwavelength scales. We introduce a method to achieve reproducible submicron ablation of thin films from a dielectric surface, in a back-irradiation geometry. First, the pulse of 45-fs duration and 800-nm central wavelength nonlinearly interacts with the dielectric and undergoes strong but reproducible modifications of its intensity profile. Then, the pulse ablates a thin polymer film [four bilayers of poly(allylamine hydrochloride) and poly(sodium 4-styrene-sulfonate), 8 nm thick] from the back surface. We measure the hole with atomic force microscopy and study the influence of laser energy and focal plane position. The radius of the resulting hole is determined by a threshold intensity for ablation. Therefore, we also demonstrate how measuring the radius as a function of focal plane position provides a new approach to profiling a tightly focused laser beam under nonlinear propagation conditions. We compare the beam profile with that predicted by a widely used propagation model and show that the latter can semiquantitatively be applied to estimate the size of achievable holes.

We study the controlled generation of single microbubbles (approximately $100\mu \mathrm{m}$) at solid surfaces in water. By focusing a nanosecond pressure pulse ($<100\mathrm{ns}$), the single microbubble is deterministically generated over the focal area, which is rather immune to surface heterogeneity that is typically responsible for uncontrolled bubble nucleation. Such controlled bubble nucleation is realized by strong negative pressure generated by pulsed optical excitation of a carbon-nanotube–polymer composite and its tight focusing (approximately $100\mu \mathrm{m}$). The negative-pressure amplitude ($>50\mathrm{MPa}$), which is further increased by acoustic interference at an impedance-mismatched boundary, even exceeds the cavitation threshold of distilled water in a free-field (boundary-free) condition. This strong pressure can activate ubiquitous submicron nuclei in both water and surfaces, making the nucleation process controllable but irrelevant to a pit array ($<10\mu \mathrm{m}$ in diameter). The ability to control bubble formation can lead to new applications in cell-level therapy, e.g., ablation of individual cancerous cells (microhistotripsy).

Pushing the limits in temporal resolution for transmission electron microscopy (TEM) requires a revolutionary change in the electron source technology. In this paper, we study the possibility of employing a radio-frequency photoinjector as the electron source for a time-resolved TEM. By raising the beam energy to the relativistic regime, we minimize the space-charge effects which otherwise limit the spatiotemporal resolution of the instrument. Analysis and optimization of the system taking into account the achievable beam brightness, electron flux on the sample, chromatic and spherical aberration of the electron optic system, and space-charge effects in image formation are presented and supported by detailed numerical modeling. The results demonstrate the feasibility of 10-nm–10-ps spatiotemporal resolution single-shot MeV TEM.