Asymmetric transmission devices that allow acoustic waves to propagate only along one direction receive significant attention because of the rich physics and many potential applications. However, most proposed designs, such as acoustic superlattices and nonlinear acoustic diodes, are much larger than the wavelength, which are impractical for aqueous applications that require unidirectionality at low frequencies. In this work, subwavelength asymmetric acoustic transmission (SAAT) is realized based on low-frequency forbidden transmission of solid-fluid acoustic superlattices. Our design is comprised of a 1D superlattice serving as a low-frequency forbidden transmitter and a periodic rectangular grating serving as the wave-front converter. The SAAT structure and superlattice constant are only $0.6\lambda$ and $0.128\lambda$, respectively, where $\lambda$ is the wavelength, while a very high rectifying efficiency of over ${10}^8$ is achieved. This SAAT design breaks the size-wavelength limitation to allow unidirectional low-frequency acoustic wave transmissions, which is desirable for the miniaturization of integrated acoustic devices in unidirectional signal-transmission applications.

Bulk hexagonal boron nitride (hBN) is a highly nonlinear natural hyperbolic material that attracts major attention in modern nanophotonics applications. However, studies of its optical properties in the visible part of the spectrum and quantum emitters hosted by bulk hBN have not been reported to date. In this work, we study the emission properties of hBN crystals in the red spectral range using sub-band-gap optical excitation. Quantum emission from defects is observed at room temperature and characterized in detail. Our results advance the use of hBN in quantum nanophotonics technologies and enhance our fundamental understanding of its optical properties.

Quantum levitation enabled by repulsive Casimir force has been desirable due to the potential exciting applications in passive-suspension devices and frictionless bearings. In this paper, dynamically tunable stable levitation is theoretically demonstrated based on the configuration of dissimilar gratings separated by an intervening fluid using exact scattering theory. The levitation position is insensitive to temperature variations and can be actively tuned by adjusting the lateral displacement between the two gratings. This work investigates the possibility of applying quantum Casimir interactions into macroscopic mechanical devices working in a noncontact and low-friction environment for controlling the position or transducing lateral movement into vertical displacement at the nanoscale.

We present a spectral analysis of Rydberg atoms in strong microwave fields using electromagnetically induced transparency (EIT) as an all-optical readout. The measured spectroscopic response enables optical, atom-based electric-field measurements of high-power microwaves. In our experiments, microwaves are irradiated into a room-temperature rubidium vapor cell. The microwaves are tuned near the two-photon $65D–66D$ Rydberg transition and reach an electric-field strength of $230\mathrm{V}/\mathrm{m}$, about 20% of the microwave-ionization threshold of these atoms. A Floquet treatment is used to model the Rydberg-level energies and their excitation rates. We arrive at an empirical model for the field-strength distribution inside the spectroscopic cell that yields excellent overall agreement between the measured and calculated Rydberg EIT-Floquet spectra. Using spectral features in the Floquet maps, we achieve an absolute strong-field measurement precision of 6%.

In this work, we determine the intrinsic mechanical energy dissipation limit for single-crystal resonators due to anharmonic phonon-phonon scattering in the Akhiezer ($\mathrm{\Omega }\tau \ll 1$) regime. The energy loss is derived using perturbation theory and the linearized Boltzmann transport equation for phonons, and includes the direction- and polarization-dependent mode-Grüneisen parameters in order to capture the strain-induced anharmonicity among phonon branches. This expression reveals the fundamental differences among the internal friction limits for different types of bulk-mode elastic waves. For cubic crystals, 2D-extensional modes have increased dissipation compared to width-extensional modes because the biaxial deformation opposes the natural Poisson contraction of the solid. Additionally, we show that shear-mode vibrations, which preserve volume, have significantly reduced energy loss because dissipative phonon-phonon scattering is restricted to pure-shear phonon branches, indicating that Lamé- or wineglass-mode resonators will have the highest upper limit on mechanical efficiency. Finally, we employ key simplifications to evaluate the quality factor limits for common mode shapes in single-crystal silicon devices, explicitly including the correct effective elastic storage moduli for different vibration modes and crystal orientations. Our expression satisfies the pressing need for a reliable analytical model that can predict the phonon-phonon dissipation limits for modern resonant microelectromechanical systems, where precise manufacturing techniques and accurate finite-element methods can be used to select particular vibrational mode shapes and crystal orientations.

Memory effects result from the history-dependent behavior of a system, are abundant in our daily life, and have broad applications. Here, we explore the possibilities of generating memory effects in simple isolated quantum systems. By utilizing geometrical effects from a class of lattices supporting flatbands consisting of localized states, memory effects could be observed in ultracold atoms in optical lattices. As the optical lattice continuously transforms from a triangular lattice into a kagome lattice with a flatband, history-dependent density distributions manifest quantum memory effects even in noninteracting systems, including fermionic as well as bosonic systems, in the proper ranges of temperatures. Rapid growth of ultracold technology predicts a bright future for quantum memory-effect systems, and here two prototypical applications of geometry-induced quantum memory effects are proposed: A cold-atom-based accelerometer using an atomic differentiator to record the mechanical change rate of a coupled probe, and an atomic quantum memory cell for storing information with write-in and readout schemes.

We propose and analyze an optically loaded quantum memory that exploits capacitive coupling between self-assembled quantum-dot molecules and electrically gated quantum-dot molecules. The self-assembled dots are used for spin-photon entanglement, which is transferred to the gated dots for long-term storage or processing via a teleportation process heralded by single-photon detection. We illustrate a device architecture enabling this interaction and outline both its operation and fabrication. We provide self-consistent Poisson-Schrödinger simulations to establish the design viability, to refine the design, and to estimate the physical coupling parameters and their sensitivities to dot placement. The device we propose generates heralded copies of an entangled state between a photonic qubit and a solid-state qubit with a rapid reset time upon failure. The resulting fast rate of entanglement generation is of high utility for heralded quantum networking scenarios involving lossy optical channels.

Large magnetocaloric effects can be obtained in Ni-Mn-based Heusler alloys due to the magnetostructural transition between martensite and austenite. This phase transformation proceeds via nucleation and growth. By direct measurements of the adiabatic temperature change $\mathrm{\Delta }{T}_{\mathrm{ad}}$ using different magnetic-field-sweeping rates from 0.01 up to $1500\mathrm{T}{\mathrm{s}}^{-1}$, we study the dynamic behavior of the two Heusler compounds ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{35}{\mathrm{In}}_{15}$ and ${\mathrm{Ni}}_{45}{\mathrm{Mn}}_{37}{\mathrm{In}}_{13}{\mathrm{Co}}_5$ transforming near room temperature. From these experiments, we conclude that the nucleation process is rather slow in contrast to the relatively fast movement of the phase boundary between martensite and austenite. This is a limiting factor for cooling concepts operating at frequencies beyond 100 Hz. However, the dynamic effects of the transition are negligible in field rates typically used in magnetic refrigeration. These findings are essential considering the suitability of Heusler compounds for energy-efficient solid-state cooling.

Synaptic memory is considered to be the main element responsible for learning and cognition in humans. Although traditionally nonvolatile long-term plasticity changes are implemented in nanoelectronic synapses for neuromorphic applications, recent studies in neuroscience reveal that biological synapses undergo metastable volatile strengthening followed by a long-term strengthening provided that the frequency of the input stimulus is sufficiently high. Such “memory strengthening” and “memory decay” functionalities can potentially lead to adaptive neuromorphic architectures. In this paper, we demonstrate the close resemblance of the magnetization dynamics of a magnetic tunnel junction (MTJ) to short-term plasticity and long-term potentiation observed in biological synapses. We illustrate that, in addition to the magnitude and duration of the input stimulus, the frequency of the stimulus plays a critical role in determining long-term potentiation of the MTJ. Such MTJ synaptic memory arrays can be utilized to create compact, ultrafast, and low-power intelligent neural systems.

Ultrasensitive optical detection of nanometer-scaled particles is highly desirable for applications in early-stage diagnosis of human diseases, environmental monitoring, and homeland security, but remains extremely difficult due to ultralow polarizabilities of small-sized, low-index particles. Optical whispering-gallery-mode microcavities, which can enhance significantly the light-matter interaction, have emerged as promising platforms for label-free detection of nanoscale objects. Different from the conventional whispering-gallery-mode sensing relying on the reactive (i.e., dispersive) interaction, here we propose and demonstrate to detect single lossy nanoparticles using the dissipative interaction in a high-$Q$ toroidal microcavity. In the experiment, detection of single gold nanorods in an aqueous environment is realized by monitoring simultaneously the linewidth change and shift of the cavity mode. The experimental result falls within the theoretical prediction. Remarkably, the reactive and dissipative sensing methods are evaluated by setting the probe wavelength on and off the surface plasmon resonance to tune the absorption of nanorods, which demonstrates clearly the great potential of the dissipative sensing method to detect lossy nanoparticles. Future applications could also combine the dissipative and reactive sensing methods, which may provide better characterizations of nanoparticles.

We examine the application of Josephson radiation emitters to spectral calibration of single-photon-resolving detectors. Josephson junctions are patterned in a YBCO film on a bicrystal sapphire substrate and are voltage controlled to generate radiation in the frequency range of 0.05–1 THz. The detector used in this work consists of a gate-defined quantum-dot photon-to-charge transducer coupled to a single-electron transistor. Both the emitter and the detector are equipped with a matching on-chip wide-band antenna. The combination of a tuneable emitter and detector allows us to determine the efficacy of the YBCO emitter and also to analyze the elementary processes involved in the detection.

In this contribution, we elaborate a conductivity model for highly doped polycrystalline semiconductors. The prominent feature of the model is the description of grain-boundary scattering by field emission, i.e., quantum-mechanical tunneling of electrons through potential barriers at grain boundaries. For this purpose, we adapt a theory of Stratton [Theory of field emission from semiconductors, Phys. Rev. 125, 67 (1962)] to double Schottky barriers at grain boundaries. We provide strong evidence that field emission rather than the predominantly applied thermionic emission is the dominant transport path across grain boundaries in semiconductors with carrier concentrations exceeding approximately ${10}^{19}{\mathrm{cm}}^{-3}$. We obtain a comprehensive conductivity model for highly doped polycrystalline semiconductors by combining field emission with two intragrain scattering mechanisms, that are ionized-impurity and electron-phonon scattering. The model is applied to a wide range of literature data in order to show its applicability and explanatory power. The literature data comprise, in particular, transparent conductive oxides with a special emphasis on aluminum-doped ZnO.

Scintillators attract wide research interest for their distinct applications in radiation detection. Elpasolite halides are among the most promising scintillators due to their high structural symmetry and good scintillation performance. A better understanding of their underlying scintillation mechanism opens up possibilities in scintillator development. In this work, we employ a variety of experimental techniques to study the two mixed-anion elpasolites ${\mathrm{Cs}}_2\mathrm{Na}R{\mathrm{Br}}_3{\mathrm{I}}_3$ ($R=\mathrm{La}$, Y). The emission of intrinsic ${\mathrm{Cs}}_2\mathrm{Na}R{\mathrm{Br}}_3{\mathrm{I}}_3$ with a light yield ranging from 20 000 to $40000\mathrm{ph}/\mathrm{MeV}$ is dominant by self-trapped exciton emission. Partial substitution of $R$ with Ce introduces a competing emission, the ${\mathrm{Ce}}^{3+}$ $5d$-to-$4f$ radiative transition. Ab initio calculations are performed to investigate the electronic structures as well as the binding energies of polarons in ${\mathrm{Cs}}_2\mathrm{Na}R{\mathrm{Br}}_6$. The calculated large self-trapped exciton binding energies are consistent with the observed high light yield due to self-trapped exciton (STE) emission. The unique electronic structure of halide elpasolites as calculated enhances the STE stability and the STE emission. The highly tunable scintillation properties of mixed-anion elpasolites underscore the role of their complex scintillation mechanism. Our study provides guidance for the design of elpasolite scintillators with exceptional energy resolution and light yield desirable for applications.

We report the local control of the domain-wall velocity by tailoring the domain-wall potential landscape via local variations of a curved ring geometry. Employing time-resolved scanning-transmission x-ray microscopy, we dynamically image the motion of domain walls in rotating magnetic fields and quantify the contribution of the spatially varying potential to the domain-wall dynamics. We explain our experimentally obtained angular dependences of domain-wall velocities by the interplay between long-range forces arising from the Zeeman interaction of domain walls with the external magnetic field with local forces arising from variations of domain-wall energy due to a varying ring width. The interplay of these forces leads to distortion-free wall motion, and we use the engineered domain-wall potential landscape for spatial synchronization of domain-wall velocities in ferromagnetic rings, which are both a key prerequisite for the implementation of domain-wall-based devices.

We report an investigation of the thermoelectric properties of ${\mathrm{Mg}}_2{\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ solid solution with $x=0.5$ using models based on first-principles calculations and experimental data. The model gives transport properties including the figure of merit $ZT$ as functions of carrier concentration and temperature. The model for the $n$-type predicts high $ZT$ at optimized doping, and suggests that the $ZT$ value can exceed 2 at $T=1000\mathrm{K}$.

The traditional measurement of the thermoelectric Seebeck coefficient gives a global value for a given material. This method requires heating and electrical contacts. Here, we report a local optical measurement of carrier populations which are not in thermal equilibrium with the lattice of the material. This contactless method enables access to the local gradients of the two fundamental thermodynamical properties, namely the temperature and the electrochemical potential. Therefore, we can determine the Seebeck coefficient related to the light-induced thermoelectric properties of the material. Moreover, we demonstrate the linear relationship between voltage and temperature gradients at a micrometer scale.

Electronic defects in ${\mathrm{Cu}}_2\mathrm{ZnSn}{({\mathrm{Se}}_x{\mathrm{S}}_{1-x})}_4$ ($x=0$, 0.4, 0.6, and 1) kesterite solar cells are studied by temperature-dependent admittance spectroscopy and photoluminescence studies. Using admittance spectroscopy, we find that the substitution of sulfur by selenium decreases the depth of a dominant acceptor level from 0.29 to 0.12 eV. In addition, a deep-acceptor defect at about 0.5 eV above the valence band is found for the $x=0.4–1$ devices. A shallow transition level with an energy of 0.14–0.09 eV is deduced from the thermal quenching of the photoluminescence yield, which we attribute to a donor level.

The emerging field of on-chip integration of nanophotonic devices and cold atoms offers extremely strong and pure light-matter interaction schemes, which may have a profound impact on quantum information science. In this context, a long-standing obstacle is to achieve a strong interaction between single atoms and single photons and at the same time trap atoms in a vacuum at large separation distances from dielectric surfaces. In this work, we study waveguide geometries that challenge these conflicting objectives. The designed photonic-crystal waveguide is expected to offer a good compromise, which additionally allows for easy manipulation of atomic clouds around the structure, while being tolerant to fabrication imperfections.

We investigate the transient dynamics of a lumped-element oscillator based on a dc superconducting quantum interference device (SQUID). The SQUID is shunted with a capacitor, forming a nonlinear oscillator with a resonance frequency in the range of several gigahertz. The resonance frequency is varied by tuning the Josephson inductance of the SQUID with on-chip flux lines. We report measurements of decaying oscillations in the time domain following a brief excitation with a microwave pulse. The nonlinearity of the SQUID oscillator is probed by observing the ringdown response for different excitation amplitudes while the SQUID potential is varied by adjusting the flux bias. Simulations are performed on a model circuit by numerically solving the corresponding Langevin equations incorporating the SQUID potential at the experimental temperature and using parameters obtained from separate measurements characterizing the SQUID oscillator. Simulations are in good agreement with the experimental observations of the ringdowns as a function of applied magnetic flux and pulse amplitude. We observe a crossover between the occurrence of ringdowns close to resonance and adiabatic following at a larger detuning from the resonance. We also discuss the occurrence of phase jumps at a large amplitude drive. Finally, we briefly outline prospects for a readout scheme for superconducting flux qubits based on the discrimination between ringdown signals for different levels of magnetic flux coupled to the SQUID.

A solid-state mechanism for cooling high-current cables is proposed based on the Ettingshausen effect, i.e., the transverse-thermoelectric cooling generated in magnetic fields. The intense current running in the cable generates a strong magnetic field around it that can be exploited by a small current running in a coating layer made out of a strong “thermomagnetic” material to induce a temperature difference between the cable core and the environment. Both analytical calculations and realistic numerical simulations for the steady state of bismuth coatings in typical magnetic fields are presented. The latter yield temperature drops $\simeq 60\mathrm{K}$ and $>100\mathrm{K}$ for a single- and double-layer coating, respectively. These encouraging results should stimulate the search for better thermomagnetic materials in view of applications such as self-cooled superconducting cables working at room temperature.