Non-Hermitian Hamiltonians can give rise to exceptional points (EPs), which have been extensively explored with nominally identical coupled resonators. Here, a non-Hermitian electromechanical system is developed, which hosts vibration modes that differ massively in their spatial profile, frequency, and even the sign of their dissipation. An optomechanical-like modulation is employed to parametrically couple two of these disparate modes whose dynamics are then probed via frequency spectroscopy. The resultant experimental responses are numerically simulated, to enable the underlying eigenvalues of the electromechanical system to be extracted, which reveal the nondegenerate modes real components undergo parametric normal mode splitting, while their imaginary parts coalesce. These results suggest that virtual EPs can be accessed in systems composed of highly mismatched resonators while still maintaining access to the nontrivial phenomena in their proximity, as recently demonstrated for topological operations, thus enabling non-Hermitian singularities to be more widely exploited with realistic physical systems.

We predict strong interface multiferroicity at $\mathrm{bcc}\text{−}\mathrm{Fe}/\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3(001)$ heterointerfaces by using first-principles calculations. For the identified stable atomic structure, we quantify the changes in the orientation of the magnetic moments of bcc $\mathrm{Fe}$ due to the reversal of the electric polarization of $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$. By determining the magnetoelectric tensor, we find that the magnetoelectric coupling of the present ferromagnetic-multiferroic interface is an order of magnitude larger than that of a ferromagnetic-ferroelectric interface, $\mathrm{bcc}\text{−}\mathrm{Fe}/\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3(001)$.

Magnetic sensors are important for detecting nuclear magnetization signals in nuclear magnetic resonance (NMR). As a complementary analysis tool to conventional high-field NMR, zero- to ultralow-field (ZULF) NMR detects nuclear magnetization signals in the submicrotesla regime. Current ZULF NMR systems are always equipped with high-quality magnetic shields to ensure that ambient magnetic-field noise does not dwarf the magnetization signal. An alternative approach is to separate the magnetization signal from the noise based on their differing spatial profiles, as can be achieved using a magnetic gradiometer. Here, we present a gradiometric ZULF NMR spectrometer with a magnetic-field-gradient noise of $17\mathrm{fT}/\mathrm{cm}{\mathrm{Hz}}^{1/2}$ in the frequency ranging from 100 to 400 Hz, based on a single vapor cell ($0.7\times 0.7\times 1.0{\mathrm{cm}}^3$). With applied white magnetic-field noise, we show that the gradiometric spectrometer achieves 13-fold enhancement in the signal-to-noise ratio (SNR) compared to the single-channel configuration. By reducing the influence of the common-mode magnetic-field noise, this work enables the use of compact and low-cost magnetic shields. Gradiometric detection also proves to be beneficial for eliminating systematic errors in ZULF-NMR experiments searching for exotic spin-dependent interactions and molecular parity violation.

We investigate the dynamical control of the heat flux exchanged in the near-field regime between a membrane made with a phase-change material and a substrate when the temperature of the membrane is tuned around its critical value. We show that in interaction with an external source of thermal radiation, this system is multistable and behaves as a thermal transistor, being able to dynamically modulate and even amplify super-Planckian heat fluxes. This behavior could be used to dynamically control heat fluxes exchanged at the nanoscale in systems out of thermal equilibrium and to process thermal information employing suspended membranes.

Superconducting circuits are a versatile platform to implement a multitude of Hamiltonians that perform quantum computation, simulation, and sensing tasks. A key ingredient for realizing a desired Hamiltonian is the irradiation of the circuit by a strong drive. These strong drives provide an in situ control of couplings, which cannot be obtained by near-equilibrium Hamiltonians. However, as shown in this paper, out-of-equilibrium systems are easily plagued by complex dynamics, leading to instabilities. The prediction and prevention of these instabilities is crucial, both from a fundamental and application perspective. We propose an inductively shunted transmon as the elementary circuit optimized for strong parametric drives. Developing a numerical approach that avoids the built-in limitations of perturbative analysis, we demonstrate that adding the inductive shunt significantly extends the range of pump powers over which the circuit behaves in a stable manner.

We theoretically investigate the critical current of a thermally biased superconductor-insulator-superconductor (S-I-S) Josephson junction formed by electrodes made from different Bardeen-Cooper-Schrieffer (BCS) superconductors. The response of the device is analyzed as a function of the asymmetry parameter, $r=T_{c_1}/T_{c_2}$. We highlight the appearance of jumps in the critical current of an asymmetric junction, namely, when $r\ne 1$. In fact, in such a case, at temperatures at which the BCS superconducting gaps coincide, the critical current suddenly increases or decreases. In particular, we thoroughly discuss the counterintuitive behavior of the critical current, which increases by enhancing the temperature of one lead, instead of monotonically reducing. In this case, we find that the largest jump of the critical current is obtained for moderate asymmetries, $r\simeq 3$. In view of these results, the discussed behavior can be speculatively proposed as a temperature-based threshold single-photon detector with photon-counting capabilities, which operates nonlinearly in the nondissipative channel.

Oocyte activation deficiency is considered to be the principal factor underlying fertilization failure after intracytoplasmic sperm injection (ICSI), and consequently exhibits a lack of cytosolic calcium (${\mathrm{Ca}}^{2+}$) oscillations in oocytes. Traditional artificial activation methods are usually proposed to induce an intracellular ${\mathrm{Ca}}^{2+}$ transient in oocytes. However, these current methods either cannot mimic the precise physiological processes of mammalian oocyte activation or there still exists controversy concerning their activation mechanisms. Here, we present a report on the potential of using nanosecond pulsed electric fields (nsPEFs) with a 10-ns duration not only to achieve a high activation rate of mouse oocytes, but also to significantly improve the developmental ability of parthenogenetic embryos. More importantly, in the exploration of activation mechanisms, we find that nsPEFs can regulate three different patterns of ${\mathrm{Ca}}^{2+}$ oscillation via mobilizing both ${\mathrm{Ca}}^{2+}$ influx from the outside and efflux from the endoplasmic reticulum (ER), and can even induce spontaneous ${\mathrm{Ca}}^{2+}$ oscillations. Furthermore, a nsPEFs-mediated ${\mathrm{Ca}}^{2+}$-channel kinetic model is introduced, and the simulation results agree well with our experiments. Our model predicts that nsPEFs could regulate ${\mathrm{Ca}}^{2+}$ oscillations via the direct effects on the openness of ${\mathrm{Ca}}^{2+}$ channels by simultaneously mobilizing extracellular and intracellular ${\mathrm{Ca}}^{2+}$. Therefore, as an effective and controllable method, nsPEFs provide a valuable tool for potential artificial activation treatment in the future.

We argue that a high-purity $\mathrm{Yb}$-doped silica glass can potentially be cooled via anti-Stokes-fluorescence optical refrigeration. In order to achieve net solid-state optical refrigeration, it is necessary for the cooling efficiency to be positive. This requires the pump wavelength to be greater than the mean fluorescence wavelength and the internal quantum efficiency as well as the absorption efficiency to be near unity. Our conclusion is reached by showing, using reasonable assumptions for the host-material properties, that the nonradiative decay rate of $\mathrm{Yb}$ ions can be made substantially smaller than the radiative decay rate. Therefore, an internal quantum efficiency of near unity can be obtained. Moreover, the background absorption coefficient in high-quality silica glass lies in an acceptable range to guarantee a near-unity absorption efficiency at room temperature. Using spectral measurements of the fluorescence from a $\mathrm{Yb}$-doped silica optical fiber at a range of temperatures, we estimate the minimum achievable temperature in $\mathrm{Yb}$-doped silica glass for different values of the internal quantum efficiency.

Nematodes have evolved to swim in highly viscous environments. Artificial mechanisms that mimic the locomotory functions of nematodes can be efficient viscous pumps. We experimentally simulate the motion of the head segment of Caenorhabditis elegans by introducing a reciprocating and rocking blade. We show that the bioinspired motion of the blade not only induces a flow structure similar to that of the worm, but also mixes the surrounding fluid by generating a circulatory flow. When confined between two parallel walls, the blade causes a steady Poiseuille flow through closed circuits. The pumping efficiency is comparable with the swimming efficiency of the worm. If implanted in a sealed chamber and actuated remotely, the blade can provide pumping and mixing functions for microprocessors cooled by polymeric flows and microfluidic devices.

Understanding the physical origin of noise affecting quantum systems is important for nearly every quantum application. Quantum-noise spectroscopy has been used in various quantum systems, such as superconducting qubits, nitrogen-vacancy centers, and trapped ions. Traditional spectroscopy methods are usually efficient in measuring noise spectra with mostly monotonically decaying contributions. However, there are important scenarios in which the noise spectrum is broadband and nonmonotonous, thus posing a challenge to existing noise-spectroscopy schemes. Here we compare several methods for noise spectroscopy: spectral decomposition based on the Carr-Purcell-Meiboom-Gill sequence, the recently presented dynamic sensitivity control (DYSCO) sequence, and a modified DYSCO sequence with a Gaussian envelope (gDYSCO). The performance of the sequences is quantified by analytic and numeric determination of the frequency resolution, bandwidth, and sensitivity, revealing a supremacy of gDYSCO to reconstruct nontrivial features. Using an ensemble of nitrogen-vacancy centers in diamond coupled to a high-density ${}^{13}\mathrm{C}$-nuclear-spin environment, we experimentally confirm our findings. The combination of the schemes presented offers potential to record high-quality noise spectra as a prerequisite to generate quantum systems unlimited by their spin-bath environment.

Spiking neural networks (SNNs) offer an event-driven and more-biologically-realistic alternative to standard artificial neural networks based on analog information processing. This can potentially enable energy-efficient hardware implementations of neuromorphic systems that emulate the functional units of the brain; namely, neurons and synapses. Recent demonstrations of ultrafast photonic computing devices based on phase-change materials (PCMs) show promise for addressing limitations of electrically driven neuromorphic systems. However, scaling these stand-alone computing devices to a parallel in-memory computing primitive is a challenge. In this work, we use the optical properties of the PCM ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ to propose a photonic SNN computing primitive, comprising a nonvolatile synaptic array integrated seamlessly with previously explored “integrate-and-fire” neurons. The proposed design realizes an “in-memory” computing platform that leverages the inherent parallelism of wavelength-division multiplexing. We show that the proposed computing platform can be used to emulate a SNN inferencing engine for image-classification tasks. The proposed design not only bridges the gap between isolated computing devices and parallel large-scale implementation but also paves the way for ultrafast computing and localized on-chip learning.

We propose a magnetic scanning-probe sensor based on a single-planar Josephson junction with a magnetic barrier. The planar geometry together with the high magnetic permeability of the barrier facilitates a double flux-focusing effect, which helps to guide magnetic flux into the junction and thus enhances field sensitivity of the sensor. We fabricate and analyze experimentally sensor prototypes with a superparamagnetic $\mathrm{Cu}\text{−}\mathrm{Ni}$ and a ferromagnetic $\mathrm{Ni}$ barrier. We demonstrate that the planar geometry allows easy miniaturization to nanometer scale and facilitates an effective utilization of the self-field phenomenon for amplification of sensitivity and a simple implementation of a control line for feedback operation over a broad dynamic range. We argue that the proposed sensor can outperform equally sized superconducting quantum-interference devices (SQUIDs) both in terms of magnetic-field sensitivity and spatial resolution, which makes it advantageous for scanning-probe microscopy.

We investigate the influence of Meissner screening and trapped magnetic flux on magnetization dynamics for a ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ film sandwiched between two thick $\mathrm{Nb}$ layers (100 nm) using broadband (5–20 GHz) ferromagnetic resonance (FMR) spectroscopy. Below the superconducting transition T_c of $\mathrm{Nb}$, significant zero-frequency line broadening (5–6 mT) and dc-resonance field shift (50 mT) to a low field are both observed if the $\mathrm{Nb}$ thickness is comparable to the London penetration depth of $\mathrm{Nb}$ films (≥100 nm). We attribute the observed peculiar behaviors to the increased incoherent precession near the ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}/\mathrm{Nb}$ interfaces and the effectively focused magnetic flux in the middle ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ caused by strong Meissner screening and (defect-)trapped flux of the thick adjacent $\mathrm{Nb}$ layers. This explanation is supported by static magnetic properties of the samples and comparison with FMR data on thick $\mathrm{Nb}/{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ bilayers. Great care should, therefore, be taken in the analysis of FMR response in ferromagnetic Josephson structures with thick superconductors, a fundamental property for high-frequency device applications of spin-polarized supercurrents.

We demonstrate microwave-free eddy-current imaging using nitrogen-vacancy centers in diamond. By detecting the eddy-current+induced magnetic field of conductive samples, we can distinguish between different materials and shapes and identify structural defects. Our technique allows the discrimination of different materials according to their conductivity. The sensitivity of the measurements is calculated as $8\times {10}^5\text{S}/\text{m}\sqrt{\text{Hz}}$ at 3.5 MHz for a cylindrical sample with radius $r_0=1\text{mm}$ and height $h=0.1\text{mm}$ (volume approximately $0.3{\text{mm}}^3$) at a distance of 0.5 mm. In comparison with existing technologies, the diamond-based device exhibits a greater bandwidth and spatial resolution. In particular, we demonstrate a flat frequency response from 0 Hz to 3.5 MHz and a spatial resolution of $348\pm 2\mu \text{m}$.

Despite extensive studies on the insulator-to-metal transition (IMT) in strongly correlated ${\mathrm{VO}}_2$, the fundamental mechanism underlying the current-driven IMT in ${\mathrm{VO}}_2$ is still not well understood. Although it is generally believed that the mechanism is Joule heating leading to a rise in temperature to above the normal transition temperature, there is ample experimental evidence demonstrating that the transition could be driven by nonthermal electronic processes. Here we formulate a phase-field model to demonstrate that the electric current may drive the IMT isothermally via the current-induced electron-correlation weakening. We discover that a current with a large density (on the order of $10{\mathrm{nA}/\mathrm{nm}}^2$) induces ultrafast resistive switching on the order of a few nanoseconds, consistent with experimental measurements. We also construct the temperature-current phase diagram and investigate the influence of the current on domain walls. This work is expected to provide guidance for understanding the current-driven IMT in ${\mathrm{VO}}_2$ and for designing ${\mathrm{VO}}_2$-based electric switching devices.

With first-principles calculations based on density-functional theory, we predict the potential for the unprecedentedly high thermoelectric figure of merit $zT=5$ at 800 K in an $n$-type ${\mathrm{Ba}}_2\mathrm{Bi}\mathrm{Au}$ full-Heusler compound. Such a high efficiency arises from an intrinsically ultralow lattice thermal conductivity coupled with a very high power factor reaching $7\mathrm{mW}{\mathrm{m}}^{-1}{\mathrm{K}}^{-2}$ at 500 K. The high $n$-type power factor originates from a light, sixfold degenerate conduction-band pocket along the $\mathrm{\Gamma }$-$X$ direction. Weak acoustic phonon scattering and sixfold multiplicity combine to yield both high mobility and a high Seebeck coefficient. In contrast, the flat-and-dispersive (or low-dimensional) valence band of ${\mathrm{Ba}}_2\mathrm{Bi}\mathrm{Au}$ fails to generate such high power factors due to strong acoustic phonon scattering. The Lorenz numbers at optimal doping are smaller than the Wiedemann-Franz value, an integral feature for $zT$ enhancement as electrons are the majority heat carriers.

Flexible electronic systems such as roll-up displays and wearable devices promise exciting possibilities that could change the way humans interact with the environment. However, they suffer from poor reliability of interconnects and devices. Interconnects on flex are prone to open-circuit failures due to mechanical stress, electrostatic discharge, and environmental degradation. Passive approaches such as the use of stretchable conductors and novel geometries improve their response to mechanical stress but cannot salvage the interconnect if a fault were to occur. Active approaches using self-healing techniques can repair a fault and have been demonstrated with use of methods that use relatively rare materials, change conventional interconnect-fabrication processes, address only faults due to mechanical stress, or do not permit stretching. In this work we discuss a self-healing technique that overcomes these limitations and demonstrate heals having metallic conductivity and nearly plastic stretchability. This is achieved by dispersion of conductive particles in an insulating fluid encapsulated over the interconnect. Healing is automatically triggered by the electric field appearing in the open gap of a failed interconnect, irrespective of the cause of failure. The field polarizes the conductive particles, causing them aggregate and chain up to bridge the gap and repair the fault. Using dispersions of copper microspheres in silicone oil, we show self-healing interconnects with the stretchable heal having conductivity of about $5\times {10}^5$ S/m and allowing strains from 12 to 60. Previously, stretchable interconnects used materials other than copper. Here we effectively show self-healing, stretchable copper. This work promises high-speed, self-healing, and stretchable interconnects on flex, thereby improving system reliability.

Label-free biosensors enable the monitoring of biomolecular interactions in real time, which is key to the analysis of the binding characteristics of biomolecules. While refractometric optical biosensors such as surface plasmon resonance (SPR) are sensitive and well-established, they are susceptible to any change of the refractive index in the sensing volume caused by minute variations in composition of the sample buffer, temperature drifts, and most importantly nonspecific binding to the sensor surface in complex fluids such as blood. The limitations arise because refractometric sensors measure the refractive index of the entire sensing volume. Conversely, diffractometric biosensors–for example, focal molography–only detect the diffracted light from a coherent assembly of analyte molecules. Thus any refractive index distribution that is noncoherent with respect to this molecular assembly does not add to the coherent signal. This makes diffractometric biosensors inherently robust and enables sensitive measurements without reference channels or temperature stabilization. The coherent assembly is generated by selective binding of the analyte molecules to a synthetic binding pattern–the mologram. Focal molography has been introduced theoretically [C. Fattinger, Phys. Rev. X 4, 031024 (2014)] and verified experimentally [V. Gatterdam, A. Frutiger, K.-P. Stengele, D. Heindl, T. Lübbes, J. Vörös, and C. Fattinger, Nat. Nanotechnol. 12, 1089 (2017)] in previous papers. However, further understanding of the underlying physics and a diffraction-limited readout is needed to unveil its full potential. This paper introduces refined theoretical models, which can accurately quantify the amount of biological matter bound to the mologram from the diffracted intensity. In addition, it presents measurements of diffraction-limited molographic foci, i.e., Airy discs. These improvements enable us to demonstrate a resolution in real-time binding experiments comparable to the best SPR sensors without the need for temperature stabilization or drift correction and to detect low-molecular-weight compounds label free in an endpoint format. The presented experiments exemplify the robustness and sensitivity of the diffractometric sensor principle.

We seek stable tractor beams that not only axially pull objects but also transversely trap the object. Transverse force acting on a spherical particle centered on the axis of acoustic Bessel beams is examined using the Gor’kov potential and partial wave expansion. The results suggest flexibility of trapping both relatively dense and stiff particles and relatively light and soft particles at the pressure maximum of the axisymmetric Bessel beam in the Rayleigh regime. This trapping is more flexible than trapping by standing waves and focused beams used in acoustic tweezers. The flexibility is interpreted by the contribution of axial velocity to the force potential and by momentum projection. We then reveal parameters (paraxiality, density ratio, and bulk modulus ratio) for simultaneous trapping and pulling of an elastic object and droplet in and beyond the Rayleigh regime. The results reveal the capability and conditions to simultaneously trap and pull a Rayleigh particle even in the situation when losses are accounted for. Our study guides designs of stable tractor beams and drives development of acoustic tweezers.

Strain engineering is a powerful technology that exploits the stationary external or internal stress of specific spatial distribution for controlling the fundamental properties of condensed materials and nanostructures. This advanced technique modulates in space the carrier density and mobility, the optical absorption, and in strongly correlated systems, the phase, e.g., insulator-metal or ferromagnetic-paramagnetic. However, while successfully accessing nanometer-length scales, strain engineering is yet to be brought down to ultrafast time scales allowing strain-assisted control of the state of matter at THz frequencies. We demonstrate control of an optically-driven insulator-to-metal phase transition by a picosecond strain pulse, which paves the way to ultrafast strain engineering in nanostructures with phase transitions. This is realized by simultaneous excitation of ${\mathrm{VO}}_2$ nanohillocks by a 170-fs laser and picosecond strain pulses finely timed with each other. By monitoring the transient optical reflectivity of the ${\mathrm{VO}}_2$, we show that strain pulses, depending on the sign of the strain at the moment of optical excitation, increase or decrease the fraction of ${\mathrm{VO}}_2$ that undergoes an ultrafast phase transition. A transient strain of moderate amplitude of approximately 0.1% applied during ultrafast photo-induced nonthermal transition changes the fraction of ${\mathrm{VO}}_2$ in the laser-induced phase by approximately 1%. In contrast, if applied after the photoexcitation when the phase transformations of the material are governed by thermal processes, a transient strain of the same amplitude produces no measurable effect on the phase state.