We investigate Janus and alloy structures of $\mathrm{Pt}$$X_n{Y}_{2-n}$ ($X$, $Y$ = $\mathrm{S}$, $\mathrm{Se}$, $\mathrm{Te}$; $0\le n\le 2$) materials on the basis of first-principles plane-wave simulations. Using cluster-expansion theory to study alloys of $\mathrm{Pt}$$X_n{Y}_{2-n}$ monolayers at various concentrations, for half coverage ($n=1$), our results indicate that Janus-type structures are not energetically the most favorable for $\mathrm{Pt}$$XY$ monolayers; however, they are dynamically and thermally stable. To distinguish Janus $\mathrm{Pt}$$XY$ structures, we report the Raman-active modes and compared them with those of bare $\mathrm{Pt}$$X_2$ monolayers. The electronic band gaps calculated with use of hybrid functionals are on par with available experimental data. Spin-orbit coupling significantly modifies the electronic band structure of $\mathrm{Pt}$$XY$ monolayers. Because of the electronegativity differences of different chalcogen atoms on each surface of Janus $\mathrm{Pt}$$XY$ structures, the arising dipole moment significantly modifies the band alignments on both surfaces. We find that hydrogen-evolution and oxygen-evolution reactions occur on different surfaces and that applied strain enhances the catalytic activity. We also investigate the monovacancy and stacking effects on the electronic properties of $\mathrm{Pt}$$X_2$ and $\mathrm{Pt}$$XY$ structures. Our results indicate that due to their intrinsic dipole moments and band gaps, Janus $\mathrm{Pt}$$XY$ monolayers are perfect candidates for water-splitting reactions.

Understanding the dynamics of quantum systems to time-varying perturbations is not only a question of basic physics, it is a question that can have life-critical relevance. Specifically, as GPS signals take on greater roles in autonomous navigation, means must be found to ensure the moment-by-moment integrity of those signals, and thereby the integrity of GPS atomic clock frequencies. The frequency of an electromagnetic field, like the microwave field in a GPS atomic clock, is stabilized to a quantum system’s energy structure by modulating the field’s frequency and then demodulating the quantum system’s response. The first harmonic (i.e., fundamental) demodulated signal provides the correction for field frequency, while the second harmonic is often taken as an instantaneous status-of-health indicator for the correction signal. The interpretative problem for the second harmonic is that the modulation frequency, the quantum system’s dephasing rate, and the Rabi frequency are all approximately equal in these frequency stabilization systems, violating the textbook assumptions of quantum mechanics: the quasistatic approximation (QSA), the adiabatic approximation, and the sudden approximation. Here, we take a careful experimental and theoretical look at the QSA under realistic conditions for field stabilization (i.e., the approximation most often employed for the engineering interpretation of atomic dynamics behavior). Our results demonstrate that the second harmonic can be employed as a rapid-response herald of correction signal degradation, and that while the QSA is useful as a means for diagnosing causal effects through the second harmonic, a small-signal approximation (SSA) is better.

We report the epitaxial growth of the $\mathrm{Cr}$-doped ${\mathrm{In}}_{2-x}{\mathrm{Cr}}_x{\mathrm{O}}_3$ (x = 0.05) ($\mathrm{Cr}$:${\mathrm{In}}_2{\mathrm{O}}_3$) semiconducting thin films on perovskite-type (111)-oriented $0.7\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_3$−$0.3{\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3$ (PMN-PT) ferroelectric single-crystal substrates in the form of ferroelectric field-effect devices that allow us to obtain an in situ tuning of the electron carrier density and magnetoresistance (MR) as well as the resistance in a reversible and nonvolatile manner, thereby stringently disclosing the relationship between the MR and the electron carrier density. Specifically, for the thinnest 25-nm $\mathrm{Cr}$:${\mathrm{In}}_2{\mathrm{O}}_3$ film the polarization switching of the PMN-PT from the positively polarized $P_r^{+}$ state to the negatively polarized $P_r^{-}$ state results in a large increase in the resistance and MR. Particularly, at T = 10 K, the polarization switching induces reversible and nonvolatile changes in the magnitude and sign of MR, demonstrating strong coupling between the MR and the electron carrier density. Moreover, regardless of the polarization states of PMN-PT, MR for films with different thicknesses can be quite well described by a combination of the two-band model and the semiempirical model proposed by Khosla and Fischer based on which the positive MR (PMR) and negative MR (NMR) could be disentangled into positive component [MR(+)] and negative component [MR(−)], respectively. We find that the polarization-switching-induced large decrease in the PMR and the change in the sign of MR from positive to negative is mainly due to the rapid decrease in the MR(+), demonstrating that the coupling between MR(+) and electron carrier density plays a dominant role in controlling the magnitude and sign of MR.

The synthesis of ultrathin photonic structures in order to effectively redirect and mold optical wave fronts into arbitrary shapes is of crucial importance in modern beam steering, imaging, and sensing technologies. To this end, planar subwavelength systems such as optical metasurfaces have been intensely investigated in recent years. Such arrangements rely on abrupt, yet controllable, phase shifts imparted on the incident beam, by means of judiciously designed anisotropic nanoantennas. Here, we propose and demonstrate an altogether different methodology in order to manipulate the flow of light, by adopting a diatomic parity-time ($\mathcal{P}\mathcal{T}$)-symmetric Bravais-lattice topology, the unit cell of which involves only a transparent and a lossy optical component. In this respect, a honeycomblike configuration is employed, the principal symmetries of which are progressively broken through specific geometric transformations. The complex near-field coupling interactions between neighboring diffractive elements give rise to a discerning enhancement or attenuation along specific directions in the far field, over a broad range of wavelengths in the visible domain. In this work, we report the realization of an all-passive $\mathcal{P}\mathcal{T}$-symmetric optical metasurface on a flexible polyimide substrate, capable of demonstrating selective directional scattering. Our study draws a clear connection between surface topology and radiation directivity, which can be systematically utilized toward observing unconventional transport effects in flat- and curved-space $\mathcal{P}\mathcal{T}$ lattices.

The nanoscale behavior of liquid molecules and solutes along the interface with solids controls many processes such as molecular exchanges, wetting, electrochemistry, nanofluidics, biomolecular function, and lubrication. Experimentally, several techniques can explore the equilibrium molecular arrangement of liquids near the surface of immersed solids but quantifying the nanoscale flow patterns naturally adopted by this interfacial liquid remains a considerable challenge. Here we describe an approach based on atomic force microscopy, and able to quantify the flow direction preferentially adopted by liquids along interfaces with nanoscale precision. The approach, called vortex dissipation microscopy (VDM), uses high-frequency directional oscillations to derive local flow information around each location of the interface probed. VDM effectively derives nanoscale flow charts of the interfacial liquid parallel to a solid and can operate over a broad range of soft and hard interfaces. To illustrate its capabilities, we quantify the dynamics of aqueous solutions containing $\mathrm{K}\mathrm{Cl}$ or ${\mathrm{Mg}\mathrm{Cl}}_2$ along the surface of a same graphene oxide flake. We show that dissolved ${\mathrm{K}}^{+}$ ions can move evenly in all directions along the interface whereas ${\mathrm{Mg}}^{2+}$ ions tend to move in registry with the underlying lattice due to enthalpic effects. The results provide in situ nanoscale insights into the ion-specific sieving properties of graphene oxide membranes.

The excitation power and temperature dependences of photoluminescence (PL) emission in 6H polytype of silicon carbide are investigated extensively. A sublinear power-function dependence of the donor-acceptor pair emission intensity on the excitation power is explained by a rate-equation model. A very low power exponent $(k=0.3)$ in a power-law dependence of PL intensity on the excitation power of $\mathrm{B}$-$\mathrm{N}$ codoped 6H-$\mathrm{Si}\mathrm{C}$ at room temperature indicates that three-particle Auger recombination is the main carrier recombination pathway. Auger recombination in an $\mathrm{Al}$-$\mathrm{N}$ codoped $\mathrm{Si}\mathrm{C}$ does not manifest as strongly as that of the $\mathrm{B}$-$\mathrm{N}$ codoped sample due to high density of ionized impurities which capture the nonequilibrium carriers faster. The ionized impurities form localized potential wells due to their random distribution, which is revealed by a blue shift of the PL band with increasing excitation intensity (approximately 16 meV per decade) and with decreasing temperature. Low-temperature Hall-effect measurement of the $\mathrm{Al}$-$\mathrm{N}$ codoped sample confirms the high density of ionized impurities and significantly reduced ionization energy of dopant states by approximately 53 meV due to the high dopant densities. These results could lead to realization of tunable $\mathrm{Si}\mathrm{C}$-based light sources in severe operating conditions.

A large fraction of quantum science and technology requires low-temperature environments such as those afforded by dilution refrigerators. In these cryogenic environments, accurate thermometry can be difficult to implement, expensive, and often requires calibration to an external reference. Here, we theoretically propose a primary thermometer based on measurement of a hybrid system consisting of phonons coupled via a magnetostrictive interaction to magnons. Thermometry is based on a cross-correlation measurement in which the spectrum of back-action driven motion is used to scale the thermomechanical motion, providing a direct measurement of the phonon temperature independent of experimental parameters. Combined with a simple low-temperature compatible microwave cavity readout, this primary thermometer is expected to become a promising alternative for thermometry below 1 K.

We report the realization of a semimetal state in a phononic crystal, which hosts nodal-chain degeneracy in momentum space. The nodal-chain dispersion consists of interconnecting degenerate lines, and is protected by the nonsymmorphic and time-reversal symmetries together. The nontrivial Zak phases, induced by the nodal-chain configuration, give rise to the nonleaky drumhead surface modes on the surfaces. When the nonsymmorphic symmetry is broken, the nodal-chain semimetal transits to valley insulating state. The valley interface states present on the interface of two distinct valley phases, by which a topological focusing, which is free from reflection, is demonstrated. These findings enrich the knowledge of nodal-line materials and may pave the way to innovative acoustic devices.

The dominant source of decoherence in contemporary frequency-tunable superconducting qubits is $1/f$ flux noise. To understand its origin and find ways to minimize its impact, we systematically study flux noise amplitudes in more than 50 flux qubits with varied superconducting quantum interference device (SQUID) geometry parameters and compare our results to a microscopic model of magnetic spin defects located at the interfaces surrounding the SQUID loops. Our data are in agreement with an extension of the previously proposed model, based on numerical simulations of the current distribution in the investigated SQUIDs. Our results and detailed model provide a guide for minimizing the flux noise susceptibility in future circuits.

$\mathcal{P}\mathcal{T}$-symmetric systems can be used as ultrasensitive sensors by exploiting the non-Hermitian degeneracy at the exceptional point. However, operating and controlling these systems at the exceptional point are usually difficult. Here, we study the effect of rotation on threshold gain of circular Bragg lasers with radial $\mathcal{P}\mathcal{T}$ symmetry. We find that, even deviating away from the exceptional point, these lasers can provide a two-orders-of-magnitude enhancement in the rotation sensitivity compared with the conventional circular Bragg lasers of similar geometry and threshold gain. Additionally, these lasers are more robust against perturbation in operating condition than sensors that must operate at the exceptional point.

The existing techniques for measuring high-dimensional pure states of light in the orbital angular momentum (OAM) basis either involve a large number of single-pixel data acquisitions and substantial postselection errors that increase with dimensionality, involve substantial loss, or require interference with a reference beam of known phase. Here, we propose an interferometric technique that can measure an unknown pure state using only nine multipixel image acquisitions without involving postselection, loss, or a separate reference beam. The technique essentially measures two complex correlation functions of the input field and then employs a recursive postprocessing algorithm to infer the state. We experimentally demonstrate the technique for pure states up to dimensionality of 25, reporting a mean fidelity greater than $90\mathrm{\%}$ up to 11 dimensions. Our technique can significantly improve the performance of OAM-based information processing applications.

Enhancing energy product and thermal stability at the same time while retaining the concentration of rare-earth (RE) elements in RE-$3d$ compounds is desirable for permanent magnetic applications. Herein, all-electron electronic structure calculations are used to reveal possibilities of enhancing saturation magnetization (${\mu }_0{M}_s$) and uniaxial magnetic anisotropy ($K_u$) in ${\mathrm{Th}\mathrm{Mn}}_{12}$-type ${\mathrm{Sm}\mathrm{Fe}}_{12}$ by carefully exploring substitutional and interstitial impurities. More specifically, an addition of only one N atom, among $2p$-electron elements, per formula unit cell of ${\mathrm{Sm}\mathrm{Fe}}_{12}$ not only improves its structural stability but also enhances $K_u$ by more than 2 times and ${\mu }_0{M}_s$ up to 5%, which are superior to those of the currently known permanent magnetic materials. Our calculations further demonstrate that these permanent magnetic properties of ${\mathrm{Sm}\mathrm{Fe}}_{12}$ can also be enhanced by replacing $\mathrm{Fe}$ with $\mathrm{Co}$, in agreement with an experiment [Y. Hirayama et al., Scr. Mater. 138, 62 (2017)].

The arrangement of organic molecules at the donor-acceptor interface in an organic photovoltaic (OPV) cell can have a strong effect on the generation of charge carriers and thereby cell performance. In this paper, we report the molecular-level exploration of the ensemble of interfacial donor-acceptor pair geometries and the charge-transfer (CT) rates to which they give rise. Our approach combines molecular-dynamics simulations, electronic structure calculations, machine learning, and rate theory. This approach is applied to the boron subphthalocyanine chloride (donor) and ${\mathrm{C}}_{60}$ (acceptor) OPV system. We find that the interface is dominated by a previously unreported donor-acceptor pair edge geometry, which contributes significantly to device performance in a manner that depends on the initial conditions. Quantitative relations between the morphology and CT rates are established, which can be used to advance the design of more efficient OPV devices.

Intrinsic polarization has been demonstrated in layered structures to reduce the energy gap. Here we demonstrate that strain-induced polarization can increase the energy gap and induce a metallic-to-semiconducting phase transition in zigzag nanoribbons of single-layered transition-metal dichalcogenides, such as ${\mathrm{Mo}\mathrm{S}}_2$, ${\mathrm{Mo}\mathrm{Se}}_2$, ${\mathrm{WS}}_2$, and ${\mathrm{W}\mathrm{Se}}_2$. This study provides guidance for designing quantum piezotronic devices.

Charge-carrier-transport phenomena on nanoscopic length and ultrashort timescales are of great interest for a multitude of ultrafast dynamic processes occurring in chemical reactions as well as modern devices such as solar cells or transistors. However, the investigation of such ultrashort current pulses is very challenging and mostly based on indirect methods such as spectroscopic changes induced by the charge transport. Here we monitor the short current pulse generated by the dissipative tunneling of charge carriers through a barrier between two adjacent quantum wells via its radiated legacy in the terahertz frequency range. By examining quantum structures with intermediate barriers of different thickness, we demonstrate that the spectrum of the emitted radiation is a direct measurement of the charge-transfer dynamics associated with the tunneling process. Our findings indicate that these incoherent tunnel currents do not start instantaneously but build up on a timescale of approximately 170 fs.

We make use of a custom-designed cryo-CMOS multiplexer to enable multiple quantum devices to be characterized in a single cooldown of a dilution refrigerator. Combined with a packaging approach that integrates cryo-CMOS chips and a hot-swappable parallel-device test platform, we describe how this setup takes the standard wiring configuration of a dilution refrigerator as input and expands the capability for batch characterization of quantum devices at milli-Kelvin temperatures and high magnetic fields. The architecture of the cryo-CMOS multiplexer is discussed, and its performance is benchmarked using few-electron quantum dots and Hall mobility mapping measurements.

Drop-on-demand (DOD) inkjet printing is well characterized and a well-studied problem, but nonaxisymmetric effects are typically ignored, while these effects can severely reduce the print-head performance and its stability. In this paper we first review nonaxisymmetric droplet formation originating from geometrical effects. We then focus on the possibility that observed nonaxisymmetry arises from surface instabilities of the meniscus by a Rayleigh-Taylor-like (RT) mechanism. It is shown theoretically that the meniscus can become RT unstable beyond a critical acceleration. A comparison with data extracted from high-speed recordings of the meniscus oscillations show that the critical accelerations are exceeded. Using the time duration that the critical acceleration is exceeded and the maximal growth rate, the extent of growth of the unstable wave is estimated.

Parametric generation of subharmonics in a composite multiferroic resonator is observed and investigated. The resonator has the form of a disk and contains two mechanically coupled layers, one of which is amorphous ferromagnet $\mathrm{Fe}\text{−}\mathrm{B}\text{−}\mathrm{Si}\text{−}\mathrm{C}$ and the other piezoelectric lead zirconate titanate. The resonator is placed inside two planar electromagnetic coils with orthogonal axes. A static magnetic field of 0–100 Oe is applied parallel to the plane of the resonator. The resonator is excited in the frequency range f = 9–10 kHz by either a harmonic magnetic field with an amplitude of up to 5 Oe generated by one of the coils, or a harmonic electric field with an amplitude of up to 500 V/cm applied to the piezoelectric layer. When the pump field is above a certain threshold, generation of a subharmonic of half-frequency (f/2) is observed for three different excitation methods. The first two employed either the direct magnetoelectric effect or the converse magnetoelectric effect, while in the third a transformer system is utilized. The subharmonic is generated in a limited range of pump frequencies and its amplitude is a nonlinear function of both the pump-field amplitude and the strength of static magnetic field. A theory of parametric generation of the subharmonic in a multiferroic resonator is developed, taking into account the magnetoacoustic nonlinearity of the ferromagnetic layer of the structure and excitation of acoustic resonances near the pump and subharmonic frequencies. The theory qualitatively describes the main characteristics of the subharmonic generation.

Polydimethylsiloxane (PDMS) is widely used in acoustofluidic devices among other lab-on-a-chip systems, but a systematic study of its acoustic parameters is lacking, making it difficult to study the acoustic-PDMS interactions accurately during device designs and evaluations. Here the phase velocities and attenuation coefficients of longitudinal and shear acoustic waves propagating in Sylgard 184 samples are measured throughout the frequency band from 3.0 to 7.0 MHz. Five sample groups are prepared, corresponding to different base-agent mixing ratios $\sigma$ and curing temperatures $T$, $\left(\sigma ,T\right)=(10:1,85{}^{\circ }\mathrm{C})$, (7:1, 85 ${}^{\circ }\mathrm{C}$), (5:1, 85 ${}^{\circ }\mathrm{C}$), (10:1, 65 ${}^{\circ }\mathrm{C}$), and (10:1, 100 ${}^{\circ }\mathrm{C}$). The shear wave measurements involved examining the change of the reflection coefficient at a solid surface caused by the attachment of the samples. For longitudinal waves, the parameters are obtained by analyzing their round-trip transmissions inside double-layered solid-PDMS structures. Apart from these, the influence of sample aging on the concerned parameters are also examined. With the measured results, the parameters lead to fitted functions based on the Kramers-Kronig principle, which should benefit users working outside the studied frequency range. Finally, the two independent parameters in the elastic matrix of PDMS are provided.