Antiferromagnets have considerable potential as spintronic materials. Their dynamic properties include resonant modes at frequencies higher than can be observed in conventional ferromagnetic materials. An alternative to single-phase antiferromagnets are synthetic antiferromagnets (SAFs), engineered structures of exchange-coupled ferromagnet/nonmagnet/ferromagnet trilayers. SAFs have significant advantages due to the wide-ranging tunability of their magnetic properties and inherent compatibility with current device technologies, such as those used for Spin-transfer-torque magnetic random-access memory production. Here we report the dynamic properties of fully compensated SAFs using broadband ferromagnetic resonance and demonstrate resonant optic modes in addition to the conventional acoustic (Kittel) mode. These optic modes possess the highest zero-field frequencies observed in SAFs to date with resonances of 18 and 21 GHz at the first and second peaks in antiferromagnetic Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling, respectively. In contrast to previous SAF reports that focus only on the first RKKY antiferromagnetic coupling peak, we show that a higher optic mode frequency is obtained for the second antiferromagnetic coupling peak. We ascribe this to the smoother interfaces associated with a thicker nonmagnetic layer. This demonstrates the importance of interface quality to achieving high-frequency optic mode dynamics entering the subterahertz range.

Materials with large perpendicular magnetic anisotropy (PMA) are candidates for spintronic devices, such as magnetic random-access memory, etc., due to their stable magnetic reference states. Because of shape anisotropy, the magnetic easy axis of oxide thin films favors in-plane orientation. In this Paper, we demonstrate a convenient means to control the magnetic anisotropy of ${\mathrm{Sr}\mathrm{Ru}\mathrm{O}}_3$ (SRO) ultrathin layers from in-plane to out-of-plane by capping with nonmagnetic materials. Tuning the anisotropy is achieved by imposition of symmetry mismatch at the interface-induced structural transition of SRO with suppressed octahedral tilt. These results suggest a potential direction for engineering magnetic oxide thin films with flexible tunable PMA using capping-layer-induced dissimilar symmetry.

Very-high-quality-factor superconducting radio-frequency cavities developed for accelerators can enable fundamental physics searches with orders of magnitude higher sensitivity, and they can also offer a path to a 1000-fold increase in the achievable coherence times for cavity-stored quantum states in three-dimensional circuit QED architecture. Here we report measurements of multiple accelerator cavities of resonant frequencies of $f_0=1.3$, 2.6, 5 GHz down to temperatures of about 10 mK and field levels down to a few photons, which reveal very long photon lifetimes up to 2 s, while also further exposing the role of the two-level systems (TLS) in niobium oxide. We also demonstrate how the TLS contribution can be greatly suppressed by vacuum heat treatments at 340–450 ${}^{\circ }\mathrm{C}$.

Vertical packaging of multiple giant magnetoresistance (multi-GMR) stacks is a very interesting noise reduction strategy for local magnetic sensor measurements, which has not been reported experimentally so far. Here, we fabricate multi-GMR sensors (up to 12 repetitions) that maintain a good GMR ratio, linearity, and low roughness. From magnetotransport measurements, two different resistance responses are observed with a crossover at around five GMR repetitions: steplike (N < 5) and linear (N ≥ 5) behavior, respectively. With the help of micromagnetic simulations, we analyze, in detail, the two main magnetic mechanisms: the Néel coupling distribution induced by the roughness propagation and the additive dipolar coupling between the N free layers. Furthermore, we correlate the dipolar coupling mechanism, which is controlled by the number of GMRs (N) and lateral dimensions (width), to the sensor performance (sensitivity, noise, and detectivity); this is in good agreement with analytical theory. The noise roughly decreases in multi-GMRs as $1/\sqrt{N}$ in both regimes (low-frequency $1/f$ and thermal noise). The sensitivity is even more strongly reduced, scaling as $N^{-1}$, in the strong dipolar regime (narrow devices), while converging to a constant value in the weak dipolar regime (wide devices). Interestingly, they are more robust against undesirable random telegraphic noise than single GMRs at high voltages, and the linearity can be extended towards a much larger magnetic field range, without dealing with the size and reduction of the GMR ratio. Finally, we identify the optimal conditions for which multi-GMRs exhibit lower magnetic field detectivity than that of single GMRs: wide devices operating in the thermal regime, where much higher voltage can be applied without generating remarkable magnetic noise. These results open the path towards spintronics sensors connected and coupled in three dimensions with reduced noise, compact footprint, and mainly tuned by the dipolar coupling.

At high temperature, thermal decoherence dominates so that the entanglement of quantum states is difficult to preserve. Realizing high-temperature entanglement is, therefore, a challenge to the current quantum technologies. Here, we demonstrate that considerable degrees of continuous-variable entanglement between two macroscopic objects placed in an environment of high temperature can be created through the medium of properly prepared light fields coupled to them. There are two steps to make such entanglement. First, by pumping an optical cavity field pressuring on a mechanical resonator as a macroscopic object with a blue-detuned drive field, the competition between the induced squeezing effect due to the blue-detuned drive and the existing thermal decoherence leads to a stable entanglement between the cavity field and mechanical resonator. A condition for realizing field-resonator entanglement is determined at any temperature and for any given optomechanical system. The second step is to entangle two distant mechanical resonators through a procedure of entanglement swapping. A detailed example of illustrating this entanglement swapping shows that a considerable degree of entanglement between the two mechanical resonators can be created. The current study proposes a route toward high-temperature entanglement in a realistic physical system.

Entanglement offers substantial advantages in quantum information processing, but loss and noise hinder its application in practical scenarios. Although it has been well known for decades that the classical communication capacity of lossy and noisy bosonic channels can be significantly enhanced by entanglement, no practical encoding and decoding schemes are available to realize any entanglement-enabled advantage. Here, we report structured encoding and decoding schemes for such an entanglement-assisted communication scenario. Specifically, we show that phase encoding on an entangled two-mode squeezed vacuum state saturates the entanglement-assisted classical communication capacity of a very noisy channel and overcomes the fundamental limit on covert communication that exists without the assistance of entanglement. We then construct receivers for optimum hypothesis-testing protocols with discrete phase modulation and for optimum noisy phase-estimation protocols with continuous phase modulation. Our results pave the way for entanglement-assisted communication and sensing in the radio-frequency and microwave spectral ranges.

We deploy a small 80 kg antineutrino detector, called MiniCHANDLER, based on a solid plastic scintillator, for nearly three months at a distance of 25 m from a 2.9 GW thermal-power-reactor core at the North Anna Nuclear Generating Station. We report the detection of an antineutrino signal resulting from inverse beta decay at $5.5\sigma$ significance with no overburden and minimal shielding. This result also demonstrates that three-dimensional segmentation can be used to significantly improve the signal-to-noise ratio, in this case by a factor of 4. In addition, this measurement represents an observation of a positron spectrum in a small surface-deployed detector; this observation of reactor antineutrinos is achieved with a mobile neutrino detector mounted in an ordinary small trailer.

In this study, by means of classical molecular dynamics simulations, we investigate the thermal-transport properties of hexagonal single-layer, zinc-blend, and wurtzite phases of $\mathrm{BN}$, $\mathrm{Al}\mathrm{N}$, and $\mathrm{Ga}\mathrm{N}$ crystals, which are very promising for the application and design of high-quality electronic devices. With this in mind, we generate fully transferable Tersoff-type empirical interatomic potential parameter sets by utilizing an optimization procedure based on particle-swarm optimization. The predicted thermal properties as well as the structural, mechanical, and vibrational properties of all materials are in very good agreement with existing experimental and first-principles data. The impact of isotopes on thermal transport is also investigated and between approximately $10$ and 50% reduction in phonon thermal transport with random isotope distribution is observed in $\mathrm{BN}$ and $\mathrm{Ga}\mathrm{N}$ crystals. Our investigation distinctly shows that the generated parameter sets are fully transferable and very useful in exploring the thermal properties of systems containing these nitrides.

Resonant soft x-ray scattering (RSXS) is a leading probe of valence-band order in materials best known for detecting charge-density-wave order in copper-oxide superconductors. One of the biggest limitations on the RSXS technique is the presence of a severe fluorescence background, which, like the RSXS cross section itself, is enhanced under resonant conditions. This background prevents the study of weak signals such as diffuse scattering from glassy or fluctuating order that is spread widely over momentum space. Recent advances in superconducting transition-edge-sensor (TES) detectors have led to major improvements in energy resolution and detection efficiency in the soft x-ray range. Here, we perform a RSXS study of stripe-ordered ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{Cu}\mathrm{O}}_4$ at the $\mathrm{Cu}$ $L_{3/2}$ edge (932.2 eV) using a TES detector with 1.5-eV resolution, to evaluate its utility for mitigating the fluorescence-background problem. We find that, for suitable degree of detuning from the resonance, the TES rejects the fluorescence background, leading to a five to ten times improvement in the statistical quality of the data compared to an equivalent, energy-integrated measurement. We conclude that a TES presents a promising approach to reducing background in RSXS studies and may lead to discoveries in materials exhibiting valence-band order that is fluctuating or glassy.

We use Bayesian optimization algorithms in combination with a nonequilibrium Green’s function transport model to increase the maximum operating temperature of terahertz quantum cascade lasers. This procedure lead to the recent temperature record of 210 K in terahertz quantum cascade lasers, and here we provide even-further-improved structures. The Bayesian optimization algorithm, which takes into account all the available history of the optimization, converges much faster and more securely than the commonly used genetic algorithm. Designs based on two and three wells per period are considered, and using the large amount of data generated, we systematically evaluate the studied schemes in terms of optimal extraction energy and relevance of electron-electron correlations. This analysis shows that the two-well scheme is superior for reaching high operating temperatures, while the three-well scheme is more robust to variations in layer thicknesses. Furthermore, we study the sensitivity to flux-rate fluctuations during growth and simulation-model inaccuracies, showing the period thickness needs to be controlled to within a few percent, which is challenging but achievable with present-day molecular-beam epitaxy. These limits to the growth accuracy can be a guiding principle for experimentalists, along with the suggestion to fabricate devices across the wafer radius so as to find the optimal period thickness.

The weak-value amplification technique has been proved to be useful for precision metrology in both theory and experiment. To explore the ultimate performance of weak-value amplification for multiparameter estimation, we investigate a general weak-measurement formalism with the assistance of the high-order Hermite-Gaussian (H-G) pointer and the quantum Fisher information matrix. Theoretical analysis shows that the ultimate precision of our scheme is improved by a factor of the square root of $2n+1$, where $n$ is the order of the Hermite-Gaussian mode. Moreover, the parameters’ estimation precision can approach the precision limit with the maximum-likelihood estimation method and the homodyne method. We also present a proof-of-principle experimental setup to validate the H-G–pointer theory and explore its potential applications in precision metrology.

Although the main loss channel of planar microwave superconducting resonators has been identified to be related to an external coupling to a two-level system (TLS) bath, the behavior of such resonators in the presence of an off-resonant pump has yet to be fully understood. Alongside the well-known power-dependent damping, we observe a frequency shift with a conspicuous maximum for intermediate pump power that is attributed to a spectrally asymmetric saturation of the TLSs. We derive a semiclassical model that describes both of these effects quantitatively. The model is validated experimentally by performing a two-tone spectroscopy of several resonators fabricated on various substrates. Together with the provided analytic formulas, the technique proposed here is a simple yet powerful tool to unambiguously identify the presence of a limiting TLS bath, and to characterize various properties thereof, such as its average dephasing rate.

A thermal diode based on the asymmetric radiative heat transfer between nanoparticles assisted by nonreciprocal graphene plasmon waveguides is proposed in this work. The thermal diode system consists of two particles and a drift-biased suspended graphene sheet in close proximity of them. Nonreciprocal graphene plasmons are induced by the drift currents in the graphene sheet, and then couple to the waves emitted by the particles in near-field regime. Based on the asymmetry with respect to their propagation direction of graphene plasmons, the thermal rectification between the two particles is observed. The performance of the radiative thermal diode can be actively adjusted through tuning the chemical potential or changing the drift currents in the graphene sheet. With a large drift velocity and a small chemical potential, a perfect radiative thermal diode with a rectification coefficient extremely approaching 1 can be achieved within a wide range of interparticle distance from near to far field. The dispersion relations of the graphene plasmons are adopted to analyze the underlying physics of the rectification effect. In addition, due to the wide-band characteristic of nonreciprocal graphene plasmons, drift-biased graphene can act as a universal platform for the thermal rectification between particles. The particles with a larger particle resonance frequency are preferable to achieve a better thermal diode. This technology could find broad applications in the field of thermal management at nanoscale.

We propose a method for ultrasensitive infrared (IR) vibrational spectroscopy of molecules with nanoscale footprints by combining the tip enhancement of a scattering-type scanning near-field optical microscope (s-SNOM) and the plasmon enhancement of breathing-mode (BM) plasmon resonances of graphene nanodisks (GNDs). To demonstrate this, we develop a quantitative model that is capable of computing accurately the s-SNOM signals of nanoscale samples. With our modeling, we show that the s-SNOM tip can effectively excite gate-tunable BM plasmonic resonances in GNDs with strong field enhancement and sensitive dependence on the size of GND. Moreover, we demonstrate that the intense electric field of tip-excited plasmonic BMs can strongly enhance the IR vibrational modes of molecules. As a result, IR vibrational signatures of individual molecular particles with sizes down to 1–2 nm are readily observable by s-SNOM. Our study sheds light on future ultrasensitive IR biosensing that takes advantage of both the tip and plasmon enhancement.

We report phonon renormalization induced by an external electric field E in ferroelectric poly(vinylidene fluoride–trifluoroethylene) [P(VDF-TrFE)] nanofibers through measuring the E-dependent thermal conductivity. Our experimental results are in excellent agreement with the theoretical ones derived from the lattice dynamics. The renormalization is attributed to the anharmonicity that modifies the phonon spectrum when the atoms are pulled away from their equilibrium positions by the electric field. Our finding provides an efficient way to manipulate the thermal conductivity by tuning external fields in ferroelectric materials.

One of the key challenges for the future development of efficient and stable metal-halide perovskite solar cells is related to the migration of ions in these materials. Mobile ions have been linked to the observation of hysteresis in the current-voltage characteristics, shown to reduce device stability against degradation and act as recombination centers within the band gap of the active layer. In the literature, one finds a broad spread of reported ionic defect parameters (e.g., activation energies) for seemingly similar perovskite materials, rendering the identification of the nature of these species difficult. In this work, we perform temperature-dependent deep-level transient spectroscopy (DLTS) measurements on methylammonium-lead-iodide perovskite solar cells and develop a extended regularization algorithm for inverting the Laplace transform. Our results indicate that mobile ions form a distribution of emission rates (i.e., a distribution of diffusion constants) for each observed ionic species, which may be responsible for the differences in the previously reported defect parameters. Importantly, different DLTS modes such as optical and current DLTS yield the same defect distributions. Finally, the comparison of our results with conventional boxcar DLTS and impedance spectroscopy verifies our evaluation algorithm.

Many quantum key distribution systems employ a laser followed by an optical attenuator to prepare weak coherent states in the source. Their mean photon number must be precalibrated to guarantee the security of key distribution. Here we experimentally show that this calibration can be broken with a high-power laser attack. We test four fiber-optic attenuator types used in quantum key distribution systems, and find that two of them exhibit a permanent decrease in attenuation after laser damage. This results in higher mean photon numbers in the prepared states and may allow an eavesdropper to compromise the key.