Low-energy muon-spin-rotation spectroscopy (LE-$\mu \mathrm{SR}$) is employed to study silicon and carbon vacancies in proton-irradiated $4H$-$\mathrm{Si}\mathrm{C}$. We show that the implanted muon is quickly attracted to the negative $\mathrm{Si}$ vacancy ($V_{\mathrm{Si}}$), where it forms a paramagnetic muonium (${\mathrm{Mu}}^0$) state, resulting in a reduction of the diamagnetic fraction. In samples with predominantly $\mathrm{C}$ vacancies ($V_{\mathrm{C}}$), on the other hand, the formation of ${\mathrm{Mu}}^0$ is very short lived and the muon quickly captures a second electron to form a diamagnetic ${\mathrm{Mu}}^{-}$ state. The results are corroborated by density-functional calculations, where significant differences in the relaxation mechanism of the nearest-neighbor dangling bonds of the vacancies are discussed. We propose that the LE-$\mu \mathrm{SR}$ technique is capable of differentiating between high-spin and negative-$U$ behavior in semiconducting materials. Finally, our findings emphasize the large potential of LE-$\mu \mathrm{SR}$ to probe near-surface semiconductor defects, a capability that is crucial for further development of many electronic and quantum technology applications.

Nonclassical optomechanical correlations enable optical control of mechanical motion beyond the limitations of classical driving. Here we investigate the feasibility of using pulsed cavity optomechanics to create and verify nonclassical phase-sensitive correlations between light and the motion of a levitated nanoparticle in a realistic scenario. We show that optomechanical two-mode squeezing can persist even at the elevated temperatures of state-of-the-art experimental setups. We introduce a detection scheme based on optical homodyning that allows the revealing of nonclassical correlations without full optomechanical state tomography. We provide an analytical treatment using the rotating-wave approximation (RWA) in the resolved-sideband regime and prove its validity with a full numerical solution of the Lyapunov equation beyond the RWA. We build on parameters of current experiments for our analysis and conclude that the observation of nonclassical correlations, which are essential for quantum sensing, quantum engines, and quantum simulations with levitated nanoparticles, is possible with state-of-the-art capabilities. The general treatment can be applied to other optomechanical platforms.

Subcooled flow boiling heat transfer of carbon dioxide (${\mathrm{CO}}_2$) at the microscale near the critical pressure is experimentally studied. The onset of nucleate boiling and the local heat-transfer coefficient are obtained under 26 experimental conditions, consisting of 17 sample points each, yielding 442 data points. Excess temperatures of about 1 K at the onset of nucleate boiling and heat-transfer coefficients of up to $200\mathrm{kW}/{\mathrm{m}}^2\mathrm{K}$ are recorded. The heat-transfer coefficient and the onset of nucleate boiling models and correlations developed for low reduced pressures are compared to current experimental results. Modifications to these models are introduced to account for conditions pertinent to reduced pressures around unity. Overall, the correlations do not fully capture current measurements, suggesting that the physics of flow boiling heat transfer near the critical conditions exhibits some unique characteristics. Furthermore, it is observed that the experimentally obtained heat-transfer coefficient is better predicted by a model developed for flow boiling of ${\mathrm{CO}}_2$. However, with increasing pressure, deviations between experiments and predictions increase.

We explore the periodically driven dynamics of two weakly coupled flux qubits and propose a scheme to generate a maximally entangled state based on Floquet engineering. The periodic driving modifies the quasienergy spectrum of the system and induces crossing of quasienergies, and the weak coupling between them lifts the degeneracy of quasienergies. It is found that the phenomena of entanglement resonance of Floquet state appear at certain values of the driving parameters, which could be demonstrated by virtue of the Rabi-frequency spectrum and average concurrence during one driving period. Our scheme relaxes the experimental difficulty in adjusting the static parameters of the system to establish entanglement, and may open interesting perspectives for devising active decoherence-immune quantum-information devices.

A comprehensive understanding of numerous electrical current-induced magnetic texture transformations is necessary to ensure the reliability of skyrmionic devices during operation. Here, we present an experimental study of unipolar current-induced skyrmion-stripe transformation in a $\mathrm{Pt}/\mathrm{Co}/\mathrm{Fe}/\mathrm{Ir}$ magnetic bilayer. High current density pulses induce a densely packed skyrmion state, as commonly reported in many other studies, and skyrmion nucleation is expected to lessen with diminishing current density. However, at a lower current density where pinning effects become significant, a regime where current-induced skyrmion annihilation and skyrmion-to-stripe transformation is observed. Kerr imaging reveals that, under a low current pulse, the rapidly expanding stripes crowd out and annihilate the skyrmions before quickly decaying and leaving behind a sparse skyrmion population. Our findings establish an additional requirement of a minimum operating current density in the design of skyrmionic devices to avoid unintended skyrmion deletion. On the other hand, this skyrmion annihilation can also be strategically employed as a technique for skyrmion density control using solely current modulation in future skyrmionic devices.

In contrast to the commonly used spherical Janus particles, here we use engineered Janus particles that are fabricated by means of the photolithography technique for precise control over their geometry and coated regions. Specifically, we study a “lollipop”-shaped complex particle, the head of which is coated with gold while its tail is left bare. Due to their distinct electrical properties (i.e., electrical polarizability), the particle exhibits force equilibrium, where opposite dielectrophoretic forces acting on its head and tail exactly cancel each other to yield a stable-equilibrium position. This is realized in a quadrupolar electrode array, where the equilibrium position of the engineered particle can be tuned by the frequency. This stands in contrast to the standard dielectrophoretic behavior, where the particle shifts position from either the center of the quad to the very edge of the electrodes when shifting from a negative to positive dielectrophoretic response, respectively. This opens up opportunities for positioning control of such complex particles for self-assembly, biosensing, biomimetic spermatozoa, and more.

Deeper understanding of electrical and thermal transport is critical for further development of thermoelectric materials. Here we describe the thermoelectric performance of a group of rare-earth-bearing half-Heusler phases determined in a wide temperature range. Polycrystalline samples of $\mathrm{Sc}\mathrm{Ni}\mathrm{Sb}$, $\mathrm{Dy}\mathrm{Ni}\mathrm{Sb}$, $\mathrm{Er}\mathrm{Ni}\mathrm{Sb}$, $\mathrm{Tm}\mathrm{Ni}\mathrm{Sb}$, and $\mathrm{Lu}\mathrm{Ni}\mathrm{Sb}$ are synthesized by arc melting and densified by spark plasma sintering. They are characterized by powder x-ray diffraction and scanning electron microscopy. The physical properties are studied by means of heat-capacity and Hall-effect measurements performed in the temperature range from 2 to 300 K, as well as electrical-resistivity, Seebeck-coefficient, and thermal-conductivity measurements performed in the temperature range from 2 to 950 K. All the materials except $\mathrm{Tm}\mathrm{Ni}\mathrm{Sb}$ are found to be narrow-gap intrinsic $p$-type semiconductors with rather light charge carriers. In $\mathrm{Tm}\mathrm{Ni}\mathrm{Sb}$, the presence of heavy holes with large weighted mobility is evidenced by the highest power factor among the series ($17\mu \mathrm{W}{\mathrm{K}}^{-2}{\mathrm{cm}}^{-1}$ at 700 K). The experimental electronic relaxation time calculated with the parabolic band formalism is found to range from $0.8\times {10}^{-14}$ to $2.8\times {10}^{-14}$ s. In all the materials studied, the thermal conductivity is between 3 and $6{\mathrm{W\; m}}^{-1}{\mathrm{K}}^{-1}$ near room temperature (i.e., smaller than in other pristine $d$-electron half-Heusler phases reported in the literature). The experimental observation of the reduced thermal conductivity appears fully consistent with the estimated low sound velocity as well as strong point-defect scattering revealed by Debye-Callaway modeling. Furthermore, analysis of the bipolar contribution to the measured thermal conductivity yields abnormally large differences between the mobilities of $n$-type and $p$-type carriers. The latter feature makes the compounds examined excellent candidates for further optimization of their thermoelectric performance via electron doping.

Compact chip-scale comb sources are of significant interest for many practical applications. Here, we experimentally study the generation of supercontinuum (SC) in an axially varying integrated waveguide. We show that the local tuning of the dispersion enables the continuous blue shift of dispersive waves by more than 200 nm due to their trapping by the strongly compressed pump pulse. This mechanism provides an insight into supercontinuum generation in varying-dispersion integrated waveguides. Pumped close to 2.2 $\mu \mathrm{m}$ in the femtosecond regime and at a pulse energy of approximately 4 pJ, the output spectrum extends from 1.1 $\mu \mathrm{m}$ up to 2.76 $\mu \mathrm{m}$ and shows good coherence properties. Octave-spanning SC is also observed at an input energy as low as approximately 0.9 pJ. We show that the supercontinuum is more robust against variations of the input-pulse parameters and is also spectrally flatter in waveguides with an optimized dispersion profile than in dispersion-engineered ones. This research demonstrates the potential of varying-dispersion waveguides for coherent SC generation and paves the way for integrated low-power applications, such as chip-scale frequency-comb generation, precision spectroscopy, optical-frequency metrology, and wide-band wavelength division multiplexing in the near infrared.

Charge-current-induced spin-polarized current, via spin-momentum-locked states, is very attractive for spintronics. The orientation of spin polarization caused by an electric field depends on the helicity of the spin texture, which is normally opposite for typical topological surface states and Rashba states. Here, we report the manipulation of the two opposite spin textures in Dirac semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$ nanoplates. Spin-polarized transport signals originated from topological surface states are detected through Hall-bar-like $\mathrm{Co}$ electrodes, showing a maximum at the Dirac point and robustness up to room temperature. By comparison, opposite helicity of the spin texture is measured by applying $\mathrm{Co}$ electrodes across the whole nanoplate. Furthermore, it is found that the opposite spin texture is only localized underneath the electrodes, and the spin texture of topological surface states can be singled out by a nonlocal device geometry. The results show that the clockwise and counterclockwise spin textures can be achieved simultaneously in a single Dirac semimetal device by patterning metal electrodes with different configurations, which are promising for future spintronic applications.

The diffusion coefficient of holes can provide knowledge about carrier localization in ($\mathrm{In}$,$\mathrm{Ga}$)$\mathrm{N}$, where the carrier dynamics are altered by randomly fluctuating potential landscape. In group-III nitrides, the diffusivity of holes is difficult to measure by electrical methods but it can be studied using optical techniques. Here, we investigate the dependence of the hole diffusion coefficient on direction and carrier density in $c$-plane and $m$-plane ($\mathrm{In}$,$\mathrm{Ga}$)$\mathrm{N}$ structures by employing the light-induced transient-grating technique. We show that the hole diffusion coefficient is anisotropic in the $m$-plane structure, where it is several times larger along the $a$ crystallographic direction than along the $c$ direction. Such anisotropy is observed within the broad range of carrier densities from ${10}^{18}$ to ${10}^{20}{\mathrm{cm}}^{-3}$. The diffusivity changes nonmonotonously with increasing photoexcitation, this dependence being different in thick and thin layers. We argue that an unexpectedly high diffusion coefficient at low carrier densities in thick quantum wells can be a signature of efficient hole transport via percolative paths occurring due to compositional disorder. In turn, a decrease of diffusivity with the excitation can reflect the effect of Coulomb blockade of these paths. Finally, we demonstrate that disorder impacts carrier diffusivity even at carrier densities above ${10}^{19}{\mathrm{cm}}^{-3}$, where the overflow of localized states must be included to explain the observed increase of the diffusion coefficient with the carrier density.

The application of voltage noise with the same statistical properties as fundamental thermal noise controllably amplified the Brownian motion of $\lambda$ DNA molecules suspended in solution inside a nanoslit. We analyze the trajectories of single molecules and find that their self-diffusivity in the direction of the applied electric field increases in proportion with the variance of the voltage noise. The highest effective diffusivity achieved corresponds to an effective temperature of 5300 K. However, unlike thermal noise, the voltage noise causes correlated fluctuations of different molecules and their segments. This technique unlocks a previously inaccessible effective temperature regime for studies and applications of noise-dependent phenomena.

Exceptional points (EPs) in photonics are associated with non-Hermitian Hamiltonians with energy gain or loss, a notable example is EPs in parity-time symmetric Hamiltonians. We show that, counterintuitively, actual energy gain or loss is not required to generate this type of non-Hermitian degeneracy, since the eigenvalue of the optical Hamiltonian is the mode's propagation constant and not the energy. This is demonstrated by a simple three-layer insulator-metal-insulator (I-M-I) plasmonic waveguide, the eigenmodes of which are known to experience degeneracy at a certain point in the parametric space suggested to be used for rainbow trapping. We identify this point to be, in fact, an EP with the coalescence of the eigenmodes, despite the system having neither parity nor time symmetry. Furthermore, we demonstrate the manifestation of a third-order EP, which is generated by merging two separate EPs in the parametric space of the I-M-I waveguide. The presented results reveal unconventional properties of the Hamiltonian of a simple plasmonic waveguide and provide an insight into the nature of EPs in non-Hermitian plasmonic systems in general, suggesting the possibility of accessing even higher-order EP regimes in simple photonic structures without the need for optical gain or loss.

For phase-only spatial light modulators (SLMs), it is necessary to calibrate phase-response curves in various applications with light wavefront shaping. Here we develop a method to calibrate the global phase-response curve, based on the generalized optical spatial differentiation. We show that by only setting two specific profiles in SLM and analyzing the light reflected by a planar interface under cross-polarization conditions, the phase delay can be evaluated from two shots of intensity images. In comparison with conventional interferometric or diffractive approaches, we experimentally demonstrate that the present method enables in situ calibration and takes advantages of simple process and high stability. Besides, the method can also be used to calibrate the phase-response curve of each single pixel.

Both the charge- and spin-transport properties of a topological Dirac semimetal (TDSM) nanowire with two lateral gates and a perpendicular magnetic field are investigated by using the Green function method in combination with the Landauer-Büttiker formula. A fully spin-polarized surface current can be generated in the considered system and it can be turned on or off by varying the electron energy and magnetic field strength. The underlying physics of this transport phenomenon originates from the quantum anomalous Hall-like insulator state of the TDSM nanowire caused by the magnetic field. Further studies indicate the spin-polarized current with a strong robustness against disorder, displaying the feasibility of designing a topological spin filtering and switch device based on the considered system.

An optical system combining phase-shifting interferometry (PSI) and particle image velocimetry (PIV) is built and verified with simultaneous two-dimensional temperature- and velocity-field measurements of a convective flow. The well-known Rayleigh-Bénard convection in laminar regime in a cubical cavity filled with water is chosen as the experimental validation case. Three-, four-, and six-bucket temporal phase-shifting equations using a rotating polarizer method are tuned under different light-source power conditions, first without PIV, to produce high-resolution phase-shifted data. The results showed that the three-bucket phase-shifting equation is the most robust method over a wide range of laser powers, while the PIV tracers decreased the PSI precision from 1.5% in the case without tracers to 3.0% when seeded at 0.02 wt%. The temporal and spatial resolution of the PSI measurement is 0.1 s and 6.47 $\mu \mathrm{m}$, respectively. Owing to the combined PSI and PIV technique, both temperature and velocity characteristics are obtained, unveiling the existence of several flow bifurcations as the Rayleigh number is increased up to $1.06\times {10}^5$. This optical setup is a potential paradigm shift in heat- and fluid-flow visualization, while having a great potential in biosensor development for concurrent velocity-, concentration-, and temperature-field measurements of aerosols and flows with multicomponent species.

${\mathrm{Mn}}_3\mathrm{Sn}$ is an antiferromagnet with a noncollinear spin order, and the spin structures can be controlled by an external magnetic field. When a magnetic field is applied, ${\mathrm{Mn}}_3\mathrm{Sn}$ with the inverse triangular spin structure exhibits the coexistence of antiferromagnetism and weak ferromagnetism. Here we observe that both the magnetization hysteresis loops and anomalous Hall curves of ${\mathrm{Mn}}_3\mathrm{Sn}$ single-layer films present exchange bias after field cooling, which are generally observed in ferromagnet/antiferromagnet bilayers. A part of the antiferromagnetic spin structure of ${\mathrm{Mn}}_3\mathrm{Sn}$ with negligible net moments is frozen by the field-cooling process, while for the other part of ${\mathrm{Mn}}_3\mathrm{Sn}$, the switching of the inverse triangular spin structure with weak moments by external magnetic fields results in sizable anomalous Hall curves. The stable frozen antiferromagnetic part pins the weak moments and the inverse triangular spin structures can explain the exchange bias in magnetization hysteresis loops and anomalous Hall curves. The finding of exchange bias in antiferromagnetic ${\mathrm{Mn}}_3\mathrm{Sn}$ single-layer films with noncollinear spin order adds a different dimension to antiferromagnetic spintronics.