We investigate the temperature dependence of the magnetic damping in exchange-biased $\mathrm{Pt}/{\mathrm{Fe}}_{50}{\mathrm{Mn}}_{50}/{\mathrm{Fe}}_{20}{\mathrm{Ni}}_{80}/\mathrm{Si}{\mathrm{O}}_x$ multilayers. In samples having a strong exchange bias, we observed a drastic decrease of the magnetic damping of $\mathrm{Fe}\text{−}\mathrm{Ni}$ with increasing temperature up to the blocking temperature. The results essentially indicate that the nonlocal enhancement of the magnetic damping can be choked by the adjacent antiferromagnet and its temperature-dependent exchange bias. We also point out that such a strong temperature-dependent damping may be very beneficial for spintronic applications.

We investigate thermal properties of a $\mathrm{NbN}$ single-photon detector capable of unit internal detection efficiency. Using an independent calibration of the coupling losses, we determine the absolute optical power absorbed by the $\mathrm{NbN}$ film and, via resistive superconductor thermometry, the temperature dependence of the thermal resistance $Z(T)$ of the $\mathrm{NbN}$ film. In principle, this approach permits simultaneous measurement of the electron-phonon and phonon-escape contributions to the energy relaxation, which in our case is ambiguous because of the similar temperature dependencies. We analyze $Z(T)$ with a two-temperature model and impose an upper bound on the ratio of electron and phonon heat capacities in $\mathrm{NbN}$, which is surprisingly close to a recent theoretical lower bound for the same quantity in similar devices.

Avalanche-photodiode-based single-photon detectors (SPDs), as crucial and practical components, are widely used in quantum key distribution (QKD) systems. For effective detection, most of these SPDs are operated in the gated mode, in which the gate is added to obtain high avalanche gain and is removed to quench the avalanche. The avalanche-transition region (ATR) is certain to exist in the process of adding and removing the gate. We first experimentally investigate the characteristic of the ATR, including in a commercial SPD and a high-speed SPD, and then propose an ATR attack to control the detector. In the experiment on hacking the plug-and-play QKD system, Eve introduces less than 0.5% quantum bit error rate and almost leaves no traces of her presence, including the photocurrent and afterpulse probability. We finally give possible countermeasures against this attack.

Defects in crystals are leading candidates for photon-based quantum technologies, but progress in developing practical devices critically depends on improving defect optical and spin properties. Motivated by this need, we study a new defect qubit candidate, the shallow donor in $\mathrm{Zn}\mathrm{O}$. We demonstrate all-optical control of the electron spin state of the donor qubits and measure the spin coherence properties. We find a longitudinal relaxation time $T_1$ exceeding 100 ms, an inhomogeneous dephasing time $T_2^{\ast }$ of $17\pm 2$ ns, and a Hahn-spin-echo time $T_2$ of $50\pm 13\mu \mathrm{s}$. The magnitude of $T_2^{\ast }$ is consistent with the inhomogeneity of the nuclear hyperfine field in natural $\mathrm{Zn}\mathrm{O}$. Possible mechanisms limiting $T_2$ include instantaneous diffusion and nuclear spin diffusion (spectral diffusion). These dephasing mechanisms suggest that with isotope and chemical purification qubit coherence times can be extended. This work should motivate further research on high-purity material growth, quantum-device fabrication, and high-fidelity control of the donor-$\mathrm{Zn}\mathrm{O}$ system for quantum technologies.

Finite-element simulations are conducted to investigate large elastoplastic deformations of rhenium under multimegabar pressures in a diamond anvil cell (DAC), with an emphasis on the effects of geometric and material properties. The following published experimental phenomena are reproduced: (1) the pressure distribution at the sample or diamond contact surface and the final sample thickness at pressure up to 300 GPa; (2) the cupping (i.e., appearance of a cuplike concave shape of the contact diamond-sample surface) and double cupping phenomena at megabar pressures; (3) three stages at the curve of the maximum pressure versus compressive force; (4) stages of material flow with increasing load; (5) pressure drop at the periphery after cupping in that region; and (6) change in direction of material flow to the center without change in the sign of the pressure gradient. The effects of the culet geometry, bevel angle, sample thickness, and sample or gasket system are analyzed in detail. The obtained results improve the understanding of the material mechanical response under extreme pressures and large elastoplastic deformations and are beneficial for the optimum design of a DAC with the goal of reaching the high and record high pressures once or multiple times.

Photoelectrochemical (PEC) water splitting to generate hydrogen is one of the most studied methods for converting solar energy into clean fuel because of its simplicity and potentially low cost. Despite over 40 years of intensive research, PEC water splitting remains in its early stages with stable efficiencies far less than 10%, a benchmark for commercial applications. We reveal that the desired photocorrosion stability sets a limit of 2.48 V [relative to the normal hydrogen electrode (NHE)] for the highest possible potential of the valence band (VB) edge of a photocorrosion-resistant semiconducting photocatalyst. This limitation excludes semiconducting photocatalysts with band gap less than 2.48 eV for PEC water splitting. We further demonstrate that such a limitation has deep roots in the underlying physics after deducing the relation between the energy position of the VB edge and free energy for a semiconductor. The disparity between the stability-limited VB potential at 2.48 V and the oxygen evolution reaction (OER) potential at 1.23 V vs NHE reduces the maximum solar-to-hydrogen (STH) conversion efficiency from common thought 30.7% to approximately 8% for long-term stable single-band-gap PEC water-splitting cells. Based on this understanding, we suggest that the most promising strategy to overcome this 8% efficiency limit is to decouple the requirements of efficient light harvesting and chemical stability by protecting the active semiconductor photocatalyst surface with a photocorrosion-resistant oxide coating layer.

Strings of equally spaced particles (particle train) are tremendously important in a variety of microfluidic applications. By using inertial microfluidics, particle trains can be formed near the channel walls. However, the high particle rotation and large local shear gradient near the microchannel walls can lead to blurred images and cell damage, thus negatively affecting applications related to flow cytometry. To address this challenge, we demonstrate that adding a tiny amount of hyaluronic acid biopolymer to an aqueous suspension drives self-assembly of a particle train on the centerline of a square-shaped straight microchannel, with a throughput up to approximately 2400 particles/s. The fraction of equally spaced particles increases by increasing the volumetric flow rate and the distance from the channel inlet. Numerical simulations corroborate the experimental observations and, together with a simple qualitative argument on the particle train stability, shed insights on the underlying mechanism leading to particle ordering.

Curvilinear nanomagnets can support magnetic skyrmions stabilized at a local curvature without any intrinsic chiral interactions. Here, we propose an alternative mechanism to stabilize chiral Néel skyrmion states relying on the gradient of curvature. We illustrate our approach with an example of a magnetic thin film with perpendicular magnetic anisotropy shaped as a circular indentation. We show that in addition to the topologically trivial ground state, there are two skyrmion states with winding numbers $\pm 1$ and a skyrmionium state with a winding number $0$. These chiral states are formed due to the pinning of a chiral magnetic domain wall at a bend of the nanoindentation due to spatial inhomogeneity of the curvature-induced Dzyaloshinskii-Moriya interaction. The latter emerges due to the gradient of the local curvature at the bend. While the chirality of the skyrmion is determined by the sign of the local curvature, its radius can be varied in a broad range by engineering the position of the bend with respect to the center of the nanoindentation. We propose a general method, which enables us to reduce the magnetic problem for any surface of revolution to the common planar problem by means of proper modification of constants of anisotropy and Dzyaloshinskii-Moriya interaction.

Efficient imaging of biomolecules, two-dimensional materials, and electromagnetic fields depends on retrieval of the phase of transmitted electrons. We demonstrate a method to measure phase in a scanning transmission electron microscope (STEM) using a nanofabricated diffraction grating to produce multiple probe beams. The measured phase is more interpretable than phase-contrast scanning transmission electron microscopy techniques without an off-axis reference wave, and the resolution could surpass that of off-axis electron holography. We apply this technique, called STEM holography, to image nanoparticles, carbon substrates, and electric fields. The contrast observed in experiments agrees well with contrast predicted in simulations.

Surface noise is a detrimental issue for sensing devices based on shallow nitrogen-vacancy (N-$V$) color-center diamonds. A recent experiment indicates that electric-field noise is significant compared to magnetic-field noise. They also found that the electric-field noise can be reduced with a protective surface layer, although the mechanism of noise reduction is not well understood. We examine the effect of a protective surface layer on the noise spectrum, which is caused by surface-charge fluctuations. We use the fluctuation-dissipation theorem to calculate and analyze the noise spectrum for six different surface-layer materials typically used for N-$V$-center diamond devices. We find that four parameters largely affect the noise spectrum: the effective relaxation time, the effective loss tangent, the power-law exponent of the noise spectrum, and the layer thickness. Our results suggest that a surface-covering layer is indeed useful for decreasing surface noise, but which material is most suitable depends on the device operational frequency range.

$\mathrm{Ge}$ with a quasi-direct band gap can be realized by strain engineering, alloying with $\mathrm{Sn}$, or ultrahigh n-type doping. In this work, we use all three approaches together to fabricate direct-band-gap $\mathrm{Ge}\text{−}\mathrm{Sn}$ alloys. The heavily doped n-type $\mathrm{Ge}\text{−}\mathrm{Sn}$ is realized with CMOS-compatible nonequilibrium material processing. $\mathrm{P}$ is used to form highly doped n-type $\mathrm{Ge}\text{−}\mathrm{Sn}$ layers and to modify the lattice parameter of $\mathrm{P}$-doped $\mathrm{Ge}\text{−}\mathrm{Sn}$ alloys. The strain engineering in heavily-$\mathrm{P}$-doped $\mathrm{Ge}\text{−}\mathrm{Sn}$ films is confirmed by x-ray diffraction and micro Raman spectroscopy. The change of the band gap in $\mathrm{P}$-doped $\mathrm{Ge}\text{−}\mathrm{Sn}$ alloy as a function of $\mathrm{P}$ concentration is theoretically predicted by density functional theory and experimentally verified by near-infrared spectroscopic ellipsometry. According to the shift of the absorption edge, it is shown that for an electron concentration greater than 1 × 10 cm the band-gap renormalization is partially compensated by the Burstein-Moss effect. These results indicate that $\mathrm{Ge}$-based materials have high potential for use in near-infrared optoelectronic devices, fully compatible with CMOS technology.

A method for reconstructing joint photon-number distributions of twin beams from the experimental photocount histograms is suggested and experimentally implemented. Contrary to the standard reconstruction methods, it incorporates spatial noise reduction based on spatial pairing of photons. Superior performance of the method above the usual one for the maximum-likelihood approach is demonstrated.

Control of magnetic domain-wall motion in nanowires has attracted great interest due to the possibility to develop nonvolatile memory and logic circuits. We show that efficient domain-wall pinning can be engineered by growing $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{Mg}\mathrm{O}$ ultra-thin magnetic films with perpendicular anisotropy on a patterned substrate exhibiting subnanometer steps modulation. The ratio of domain-wall velocity along and across the steps is found to be as high as 70, which corresponds to a variation of the depinning field up to 7 mT demonstrating a very efficient storing pinning scheme. In addition, we demonstrate very efficient domain-wall motion along the 70 nm conducts separating the steps. Our approach is compatible with nanoscale devices and large-scale mass production, opening new opportunities for domain-wall storage applications.

We investigate the charge-trapping behavior in nitrogen-deficient amorphous silicon nitride ($a$-${\mathrm{Si}}_3{\mathrm{N}}_{4-x}$) using first-principles calculations. The amorphous ensembles with one nitrogen atom missing are generated through melt-quench procedures. The nitrogen deficiency mainly produces one $\mathrm{Si}$—$\mathrm{Si}$ bond and one $K$ center ($\mathrm{Si}$ dangling bond). The energy level of defect states indicates that the $K$ centers act as possible trap sites. The transition levels of $K$ centers are estimated, and it is found that the Hubbard $U$ energy ranges from $-1.14$ to 1.11 eV. Even though most $K$ centers show positive $U$, the charge states of most centers in the ensemble are either positive or negative under the charge-neutrality condition, resulting in “seemingly negative-$U$” behavior. This is consistent with the diamagnetic signal in experiments. The charge-injection energy of $K$ centers is evaluated on the basis of the Franck-Condon approximation, and the average trap depths for electrons (1.33 eV below the conduction edge) and holes (1.54 eV above the valence edge) are in good agreement with experimental data.

Using spatial light interference of ultrafast laser pulses, we generate a lateral modulation in the magnetization profile of an otherwise uniformly magnetized film, whose magnetic excitation spectrum is monitored via the coherent and resonant interaction with elastic waves. We find an unusual dependence of the magnetoelastic coupling as the externally applied magnetic field is angle- and field-tuned relative to the wave vector of the magnetization modulation, which can be explained by the emergence of spatially inhomogeneous spin-wave modes. In this regard, the spatial light interference methodology can be seen as a user-configurable, temporally windowed, on-demand magnonic crystal, potentially of arbitrary two-dimensional shape, which allows control and selectivity of the spatial distribution of spin waves. Calculations of spin waves using a variety of methods, demonstrated here using the plane-wave method and micromagnetic simulation, can identify the spatial distribution and associated energy scales of each excitation, which opens the door to a number of excitation methodologies beyond our chosen elastic wave excitation.

The conversion of spin information into electrical signals is indispensable for spintronic technologies. Spin-to-charge conversion in ferromagnetic tunnel contacts is well described using linear (spin-)transport equations, provided that there is no applied bias, as in nonlocal spin detection. It is shown here that in a biased ferromagnetic tunnel contact, spin detection is strongly nonlinear. As a result, the spin-detection efficiency is not equal to the tunnel spin polarization. In silicon-based four-terminal spin-transport devices, even a small bias (tens of mV) across the $\mathrm{Fe}$/$\mathrm{Mg}\mathrm{O}$ detector contact enhances the spin-detection efficiency to values of up to 140% (spin extraction bias) or, for spin injection bias, reduces it to almost zero, while, parenthetically, the charge current remains highly spin polarized. Calculations reveal that the nonlinearity originates from the energy dispersion of the tunnel transmission and the resulting nonuniform energy distribution of the tunnel current, offering a route to engineer spin conversion. Taking nonlinear spin detection into account is also shown to explain a multitude of peculiar and puzzling spin signals in structures with a biased detector, including two- and three-terminal devices, and provides a unified, consistent, and quantitative description of spin signals in devices with a biased and unbiased detector.

Micropumps are microfluidic components that are widely used in applications such as chemical analysis, biological sensing, and microrobots. However, one obstacle in developing micropumps is the extremely low efficiency relative to their macroscale counterparts. This paper presents a dynamic sealing method for external gear pumps to reduce the volumetric losses through the clearance between the tips of gears and the housing by use of magnetorheological fluids. By mitigating these losses, we are able to achieve high efficiency and high volumetric accuracy with current mechanical architectures and manufacturing tolerances. Static and dynamic sealing using magnetorheological fluids are investigated theoretically and experimentally. The Mason numbers $\mathrm{Mn}(p)$ and $\mathrm{Mn}(\mathrm{\Omega })$, which are defined in terms of the pressure gradient of the flow and the velocity of the moving boundary, respectively, are used to characterize and evaluate the sealing performance. A range of magnetic field intensities is explored to determine optimal sealing effectiveness, where effectiveness is evaluated with the ratio of volumetric loss and the friction factor. Finally, we quantify the effectiveness of this dynamic sealing method under different working conditions for gear pumps.

Manipulating the topological property of iso-frequency contour (IFC) will provide a unique control for the interaction between light and matter, such as increased rates of spontaneous emission and negative refraction. Different from the aforementioned topological transition that is realized by controlling the in-plane anisotropic electromagnetic parameters in a two-dimensional case, we theoretically propose and experimentally demonstrate another topological transition by tuning the vertical electromagnetic parameters of the anisotropic medium. Specially, the critical IFC in this transition corresponds to two intersecting lines and can be called linear-crossing metamaterial (LCMM). Based on the directional propagation and negative refraction in LCMM, we demonstrate the focusing and super-resolution with partial cloaking in transmission-line-based LCMMs. The realization of LCMM will open up another avenue for controlling light propagation and verifying unusual phenomena involving zero-index media and hyperbolic metamaterials.

The signal and noise in lateral $\mathsf{T}$-shaped spin-valve structures is investigated from a theoretical point of view in the diffusive regime. Due to the $\mathsf{T}$ shape of the investigated device it is found that the nonlocal signal is strongly influenced by the width $D$ of the spin-conducting channel, varying substantially on a lateral length scale of the order of the spin-diffusion length ${\ell }_{\mathrm{sf}}$. This effect demonstrates the influence of two-dimensional dimensionality on spin transport in the spin channel and, in particular, the strong impact of not only the device size but also of the device shape on the nonlocal signal. The nonlocal signals in the longitudinal and transversal directions of the spin channel are calculated as a function of the angle between the magnetization of the various injecting and collecting magnetic electrodes. In addition, the various sources of noise induced in the lateral spin valve are discussed with a particular attention on the noise due to the thermally induced fluctuations of the spin-accumulation vector in the paramagnetic spin-conducting channel. An explicit expression of the dependence of the signal-to-noise ratio on ${\ell }_{\mathrm{sf}}$ in the paramagnetic spacer and on its dimension is obtained. Implications for the use of such lateral spin valves as sensors in read heads for hard-disk drives are discussed.

An SU(1,1) interferometer can be realized by replacement of the beam splitters in the Mach-Zehnder interferometer with parametric amplifiers. This interferometer scheme was proposed more than 30 years ago [Yurke et al., Phys. Rev. A 33, 4033 (1986)] and the enhancement of its signal-to-noise ratio was experimentally demonstrated in our recent work [Hudelist et al., Nat. Commun. 5, 3049 (2014)]. The sensitivity of any interferometer increases with increasing internal photon number. Therefore, the bright-seeded SU(1,1) interferometer has an advantage of boosted sensitivity. However, the phase sensitivity used to characterize the bright-seeded SU(1,1) interferometer has not been shown yet. In this article, with direct intensity detection, we experimentally demonstrate the phase-sensitivity enhancement of such a bright-seeded SU(1,1) interferometer compared with the phase-sensitivity scaling given by $1/\sqrt{N_s}$, known as the “shot-noise limit,” where $N_s$ is the average photon number inside the interferometer. It is this direct intensity detection that brings about the major advance of quantum enhancement for the bright-seeded SU(1,1) interferometer in real time, which is substantially different from our previous work. Our results may find applications in quantum metrology.