In the last decade, there has been remarkable progress on the practical integration of on-chip quantum photonic devices, yet quantum-state generators remain an outstanding challenge. Simultaneously, the quantum-dot photonic-crystal-resonator platform has demonstrated a versatility for creating nonclassical light with tunable quantum statistics thanks to a newly discovered self-homodyning interferometric effect that preferentially selects the quantum light over the classical light when using an optimally tuned Fano resonance. In this work, we propose a general structure for the cavity quantum electrodynamical generation of quantum states from a waveguide-integrated version of the quantum-dot photonic-crystal-resonator platform, which is specifically tailored for preferential quantum-state transmission. We support our results with rigorous finite-difference time-domain and quantum-optical simulations and show how our proposed device can serve as a robust generator of highly pure single- and even multiphoton states.

The classical Hall effect in ordinary isotropic conducting materials describes the occurrence of a voltage perpendicular to the direction of the electric-current flow and perpendicular to the imposed magnetic-field vector. The Hall effect is routinely used in magnetic-field sensors. Here, we fabricate and characterize microstructured anisotropic metamaterials composed of a single semiconducting constituent ($n$-type ZnO) for which the direction and the sign of the Hall electric field can be tailored by microstructure. This class of metamaterials includes the possibility of a Hall voltage parallel—rather than perpendicular—to the magnetic-field vector. One possible future application arising from this far-reaching control of the effective electric-conductivity tensor is a sensor measuring the circulation of a magnetic field.

Glass-transition temperatures ($T_g$) and liquid fragilities are measured along a line of constant Ge content in the system Ge-As-Te, and contrasted with the lack of glass-forming ability in the twin system Ge-Sb-Te at the same Ge content. The one composition established as free of crystal contamination in the latter system shows a behavior opposite to that of a more covalent system. The comparison of $T_g$ vs bond density in the three systems Ge-As-chalcogen differing in chalcogen, i.e., S, Se, or Te, shows that as the chalcogen becomes more metallic, i.e., in the order $\mathrm{S}<\mathrm{Se}<\mathrm{Te}$, the bond-density effect on $T_g$ becomes systematically weaker, with a crossover at $⟨r⟩=2.3$. When the more metallic Sb replaces As at $⟨r⟩$ greater than 2.3, incipient metallicity rather than directional bond covalency apparently gains control of the physics. This observation leads us to an examination of the electronic conductivity and then semiconductor-to-metal (SC-$M$) transitions, with their associated thermodynamic manifestations in relevant liquid alloys. The thermodynamic components, as seen previously, control liquid fragility and cause fragile-to-strong transitions during cooling. We tentatively conclude that liquid-state behavior in phase-change materials is controlled by liquid-liquid (LL) and SC-$M$ transitions that have become submerged below the liquidus surface. In the case of the Ge-Te binary, a crude extrapolation to GeTe stoichiometry indicates that the SC-$M$ transition lies about 20% below the melting point, suggesting a parallel with the intensely researched “hidden liquid-liquid transition” in supercooled water. In the water case, superfast crystallization initiates in the high-fragility domain some 4% above the LL transition temperature (${\mathrm{T}}_{\mathrm{LL}}$) which is located at approximately 15% below the (ambient pressure) melting point.

Transition-metal dichalcogenide field-effect transistors (FETs) have been actively explored for low-power electronics, light detection, and sensing. Albeit promising, their performance is strongly limited by low-frequency noise (LFN). Here, we report on the study of LFN in ${\mathrm{MoS}}_2$ FETs on ${\mathrm{SiO}}_2$ substrates in ambient conditions using photodoping. Using this external excitation source allows us to access different nonequilibrium steady states and cross over different noise regimes. We observe a dependence of the noise power spectrum on the transient decay time window, approaching $1/f$ type when the system is closer to equilibrium, and identify a dependence of the LFN on channel thickness. Monolayer and bilayer devices exhibit random telegraph noise for insulating regimes and $1/f$-type Hooge mobility fluctuations (HMFs) for conductive regimes. Thicker devices exhibit mainly $1/f$-type carrier-number fluctuations (CNFs). In the latter, we observe a photodoping-induced change from a near-parabolic to a near-linear dependence of the inverse $1/f$ noise amplitude above the threshold gate voltage. This change indicates a crossover in the LFN mechanism from CNFs to HMFs. We demonstrate that the study of conductance and noise under photodoping is an effective tool to identify dominating carrier noise mechanisms in few-atomic-layer FETs for a wide range of doping regimes.

Liquid-crystal electrolytes enable nonlinear electrophoresis of colloidal particles with velocities proportional to the square of the applied field. We demonstrate that the magnitude and even the polarity of electrophoretic mobility can be controlled by the anisotropic electric conductivity and dielectric permittivity of the liquid crystal. In particular, the reversal of electrophoretic mobility can be triggered either by temperature or composition changes that alter the signs of the conductivity and permittivity anisotropies. Controllable reversal of mobility adds to the list of advantages of anisotropic electrolytes over their isotropic counterparts.

In this paper, we discuss the intriguing possibility of tailoring the scattering response of an electrically small object for camouflage and illusion applications using metasurfaces. As a significant example, we focus our attention on the cylindrical geometry and derive the analytical conditions needed to camouflage the geometrical and electrical characteristics of dielectric and metallic cylinders coated with ideal metasurfaces. A closed-form expression of the camouflaging metasurface depending on the cylinder’s characteristics is derived. Furthermore, the frequency behavior and the limitations of this technique are discussed with the aid of relevant examples. In order to overcome these limitations, a solution based on the use of lossy metasurfaces is proposed.

We present a generic system of three bosonic modes coupled parametrically with a time-varying coupling modulated by a combination of two pump harmonics, and we show how this system provides the minimal platform for realizing nonreciprocal couplings that can lead to gainless photon circulation, and phase-preserving or phase-sensitive directional amplification. Explicit frequency-dependent calculations within this minimal paradigm highlight the separation of amplification and directionality bandwidths, a feature generic to such schemes. We also study the influence of counterrotating interactions that can adversely affect directionality and the associated bandwidth; we find that these effects can be mitigated by suitably designing the properties of the auxiliary mode that plays the role of an engineered reservoir to the amplification mode space.

A detailed computational analysis of thermal transport in supported graphene reveals the possibility of tuning its thermal conductivity by targeted chemical modifications of the substrate’s surface. Based on classical molecular dynamics with an accurate charge-optimized bond-order force field and a time-domain normal-mode analysis, our approach allows us to distinguish collective from single-phonon excitations. The computations reveal a disproportional reduction of the thermal conductivity, due to the two different excitations, when graphene interacts with a substrate. Deposition of graphene on a bare silica surface leads to a dramatic reduction of the thermal conductivity and a change in the heat transport mechanism. Remarkably, partial hydroxylation of the silica surface almost doubles the thermal conductivity of the collective excitations. Thus, specific surface terminations allow for control of the thermal conductivity of graphene.

Measurement of multiple Andreev Reflection (MAR) in a Josephson junction made from an InAs quantum well heterostructure with epitaxial aluminum is used to quantify a highly transparent effective semiconductor-superconductor interface with near-unity transmission. The observed temperature dependence of MAR does not follow a conventional BCS form but instead agrees with a model in which the density of states in the quantum well acquires an effective induced gap, in our case, $180\mu \mathrm{eV}$, close to that of the epitaxial superconductor, indicating an intimate contact between Al and the InAs heterostructure. The carrier density dependence of MAR is investigated using a depletion gate revealing the subband structure of the semiconductor quantum well, consistent with magnetotransport experiments of the bare InAs performed on the same wafer.

We present a model to describe the spatiotemporal evolution of guided modes in semiconductor nanowires based on a coupled-mode formalism. Light-matter interaction is modeled based on semiconductor Bloch equations, including many-particle effects in the screened Hartree-Fock approximation. Appropriate boundary conditions are used to incorporate reflections at waveguide end facets, thus allowing for the simulation of nanowire lasing. We compute the emission characteristics and temporal dynamics of CdS and ZnO nanowire lasers and compare our results both to finite-difference time-domain simulations and to experimental data. Finally, we explore the dependence of the lasing emission on the nanowire cavity and on the material relaxation time.

Spin waves are of great interest as an emerging solution for computing beyond the limitations of scaled transistor technology. In such applications, the frequency of the spin waves is important as it affects the overall frequency performance of the resulting devices. In conventional ferromagnetic thin films, the magnetization dynamics in ferromagnetic resonance and spin waves are limited by the saturation magnetization of the ferromagnetic (FM) material and the external bias field. High-frequency applications would require high external magnetic fields which limit the practicality in a realistic device. One solution is to couple microwave excitations to perpendicular standing spin waves (PSSWs) which can enable higher oscillation frequencies. However, efficient coupling to these modes remains a challenge since it requires an excitation that is nonuniform across the FM material thickness and current methods have proven to be inefficient, resulting in weak excitations. Here, we show that by creating periodic undulations in a 100-nm-thick ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ layer, high-frequency PSSWs ($>20\mathrm{GHz}$) can be efficiently excited using micrometer-sized transducers at bias fields below 100 Oe which absorb nearly 10% of the input rf power. Efficient excitation of such spin waves at low fields may enable high-frequency spintronic applications using exchange-dominated magnetic oscillations using very low external magnetic fields and, with design optimizations, can bring about alternative possibilities in the field.

An understanding of polarization-heat interactions in pyroelectric and electrocaloric thin-film materials requires that the electrothermal response is reliably characterized. While most work, particularly in electrocalorics, has relied on indirect measurement protocols, here we report a direct technique for measuring both pyroelectric and electrocaloric effects in epitaxial ferroelectric thin films. We demonstrate an electrothermal test platform where localized high-frequency (approximately 1 kHz) periodic heating and highly sensitive thin-film resistance thermometry allow the direct measurement of pyrocurrents ($<10\mathrm{pA}$) and electrocaloric temperature changes ($<2\mathrm{mK}$) using the “2-omega” and an adapted “3-omega” technique, respectively. Frequency-domain, phase-sensitive detection permits the extraction of the pyrocurrent from the total current, which is often convoluted by thermally-stimulated currents. The wide-frequency-range measurements employed in this study further show the effect of secondary contributions to pyroelectricity due to the mechanical constraints of the substrate. Similarly, measurement of the electrocaloric effect on the same device in the frequency domain (at approximately 100 kHz) allows for the decoupling of Joule heating from the electrocaloric effect. Using one-dimensional, analytical heat-transport models, the transient temperature profile of the heterostructure is characterized to extract pyroelectric and electrocaloric coefficients.

Microscopic imaging of local magnetic fields provides a window into the organizing principles of complex and technologically relevant condensed-matter materials. However, a wide variety of intriguing strongly correlated and topologically nontrivial materials exhibit poorly understood phenomena outside the detection capability of state-of-the-art high-sensitivity high-resolution scanning probe magnetometers. We introduce a quantum-noise-limited scanning probe magnetometer that can operate from room-to-cryogenic temperatures with unprecedented dc-field sensitivity and micron-scale resolution. The Scanning Quantum Cryogenic Atom Microscope (SQCRAMscope) employs a magnetically levitated atomic Bose-Einstein condensate (BEC), thereby providing immunity to conductive and blackbody radiative heating. The SQCRAMscope has a field sensitivity of 1.4 nT per resolution-limited point (approximately $2\mu \mathrm{m}$) or $6\mathrm{nT}/\sqrt{\mathrm{Hz}}$ per point at its duty cycle. Compared to point-by-point sensors, the long length of the BEC provides a naturally parallel measurement, allowing one to measure nearly 100 points with an effective field sensitivity of $600\mathrm{pT}/\sqrt{\mathrm{Hz}}$ for each point during the same time as a point-by-point scanner measures these points sequentially. Moreover, it has a noise floor of 300 pT and provides nearly 2 orders of magnitude improvement in magnetic flux sensitivity (down to ${10}^{-6}{\mathrm{\Phi }}_0/\sqrt{\mathrm{Hz}}$) over previous atomic probe magnetometers capable of scanning near samples. These capabilities are carefully benchmarked by imaging magnetic fields arising from microfabricated wire patterns in a system where samples may be scanned, cryogenically cooled, and easily exchanged. We anticipate the SQCRAMscope will provide charge-transport images at temperatures from room temperature to 4 K in unconventional superconductors and topologically nontrivial materials.

We present an experimental comparison between two x-ray phase-contrast imaging techniques currently under development, speckle tracking and beam tracking. The comparison is centered on the absorption and ultrasmall-angle scattering signals retrieved with polychromatic radiation from homogeneous and inhomogeneous samples of different thicknesses. Our analysis shows that the ultrasmall-angle scattering signal retrieved with speckle tracking does not increase linearly with the thickness for the inhomogeneous sample, and is different from zero for the homogeneous sample. The results obtained from beam tracking, instead, are in good agreement with the expectation.

Hysteresis-loop measurements using an extraordinary Hall effect are carried out at different temperatures (4.2–300 K) in $\mathrm{Pt}/\mathrm{Co}/{\mathrm{AlO}}_x$ trilayers. The ${\mathrm{AlO}}_x$ layer is prepared by plasma oxidation of the Al capping layer during various oxidation times. The samples are subsequently annealed at different temperatures $(300°\mathrm{C}–450°\mathrm{C})$. Most of the samples exhibit perpendicular anisotropy of increasingly high amplitude as the measurement temperature decreases. In addition, some samples exhibit unusual phenomena such as a temperature-induced reorientation of anisotropy from in plane to out of plane or a large increase of anisotropy at low temperature associated with the onset of exchange bias. The perpendicular magnetic anisotropy, coercivity, thermally induced anisotropy reorientation, and exchange-bias effects are explained by the influence of different chemical bonds, namely, $\mathrm{Co}─\mathrm{Al}$, $\mathrm{Co}─\mathrm{O}$, and $\mathrm{Co}─\mathrm{Pt}$, which appear and/or evolve as a function of oxidation time and annealing temperature. They are linked to the formation of new phases such as CoO or CoPt alloy and to the evolution of the $\mathrm{Co}/{\mathrm{AlO}}_x$, $\mathrm{Co}/\mathrm{Pt}$, and $\mathrm{Co}/\mathrm{CoO}$ interfaces modified by the oxidation time and the interdiffusion between species taking place during the anneals.

We describe the general mechanism of tetragonal distortion in Heusler compounds $X_{2}YZ$. From 286 compounds studied using density-functional theory, 62% are found to be tetragonal at zero temperature. Such a large share of compounds with tetragonal distortions can be explained by the peak-and-valley character of the density of states (DOS) of these compounds in the cubic phase (arising from localized $d$ bands and van Hove singularities) in conjunction with a smooth shift of peaky DOS structure relative to the Fermi energy, $E_F$, when valence electrons are added to the system. A shift of the DOS in the $Y$ or $Z$ series leads to an alternation of stable and nonstable cubic phases that depends on the value of the DOS at $E_F$ in the cubic phase. Groups of compounds with a large share of tetragonal distortions are identified and explained.

As shown in the early 1960s, the gain in stimulated Raman scattering (SRS) is drastically suppressed when the rate of creation of phonons (via a pump-to-Stokes conversion) is exactly balanced by the rate of phonon annihilation (via a pump-to-anti-Stokes conversion). This occurs when the phonon coherence waves—synchronized vibrations of a large population of molecules—have identical propagation constants for both processes; i.e., they are phase-velocity matched. As recently demonstrated, hydrogen-filled photonic crystal fiber pumped in the vicinity of its zero-dispersion wavelength provides an ideal system for observing this effect. Here we report that Raman gain suppression is actually a universal feature of SRS in gas-filled hollow-core fibers and that it can strongly impair SRS even when the phase mismatch is high, particularly at high pump powers when it is normally assumed that nonlinear processes become more (not less) efficient. This counterintuitive result means that intermodal stimulated Raman scattering (for example, between ${\mathrm{LP}}_{01}$ and ${\mathrm{LP}}_{11}$ core modes) begins to dominate at high power levels. The results reported have important implications for fiber-based Raman shifters, amplifiers, or frequency combs, especially for operation in the ultraviolet, where the Raman gain is much higher.

Doping is an increasingly popular technique for improving the characteristics of cutting-edge ${\mathrm{HfO}}_x$ nonvolatile memory devices, but relatively few dopant species have been investigated. In this work, the properties of 50 different cation and anion dopants in ${\mathrm{HfO}}_x$ are explored using density-functional theory and are corroborated with experimental data. Depending on the atomic species, dopants are found to preferentially form on either substitutional or interstitial lattice sites and to reduce the formation energy of oxygen vacancies in the surrounding oxide. The behavior of cation dopants in ${\mathrm{HfO}}_x$ is also found to be well predicted by six properties: dopant valence, atomic radius, native-oxide enthalpy of formation, coordination number, magnetization, and charge transfer with the ${\mathrm{HfO}}_x$ lattice. These results can be used to optimize dopant selection for tuning of the switching characteristics of ${\mathrm{HfO}}_x$-based resistance-change random-access-memory and conductive-bridge random-access-memory devices.

We report on III-nitride waveguides with $c$-plane $\mathrm{GaN}/(\mathrm{Al},\mathrm{Ga})\mathrm{N}$ quantum wells in the strong light-matter coupling regime supporting propagating polaritons. They feature a normal-mode splitting as large as 60 meV at low temperatures thanks to the large overlap between the optical mode and the active region, a polariton decay length up to $100\mu \mathrm{m}$ for photonlike polaritons and a lifetime of 1 to 2 ps, with the latter values being essentially limited by residual absorption occurring in the waveguide. The fully lattice-matched nature of the structure allows for very low disorder and high in-plane homogeneity, which is an important asset for the realization of polaritonic integrated circuits that could support nonlinear polariton wave packets up to room temperature thanks to the large exciton binding energy of 40 meV.

The Mott-Schottky analysis in the dark is a frequently used method to determine the doping concentration of semiconductors from capacitance-voltage measurements, even for such complex systems as polymer:fullerene blends used for organic solar cells. While the analysis of capacitance-voltage measurements in the dark is relatively well established, the analysis of data taken under illumination is currently not fully understood. Here, we present experiments and simulations to show which physical mechanisms affect the Mott-Schottky analysis under illumination. We show that the mobility of the blend has a major influence on the shape of the capacitance-voltage curve and can be obtained from data taken under reverse bias. In addition, we show that the apparent shift of the built-in voltage observed previously can be explained by a shift of the onset of space-charge-limited collection with illumination intensity.