Low-defect two-dimensional electron systems (2DESs) are essential for studies of fragile many-body interactions that only emerge in nearly-ideal systems. As a result, numerous efforts have been made to improve the quality of modulation-doped ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$/GaAs quantum wells (QWs), with an emphasis on purifying the source material of the QW itself or achieving better vacuum in the deposition chamber. However, this approach overlooks another crucial component that comprises such QWs, the ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ barrier. Here we show that having a clean Al source and hence a clean barrier is instrumental to obtain a high-quality GaAs 2DES in a QW. We observe that the mobility of the 2DES in GaAs QWs declines as the thickness or Al content of the ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ barrier beneath the QW is increased, which we attribute to the surface segregation of oxygen atoms that originate from the Al source. This conjecture is supported by the improved mobility in the GaAs QWs as the Al cell is cleaned out by baking.

Picosecond ultrasonics was used to study the photoelastic properties of zinc-blende (cubic) $c{\text{-Al}}_x{\text{Ga}}_{1-x}\text{N}$ with $x$ around 0.5. The velocities for longitudinal sound in the alloys were measured using ultrafast UV pump-probe experiments with (AlGa)N membranes. Strong Brillouin oscillations were observed in (AlGa)N films attached to GaAs substrates. These oscillations are due to the dynamical interference of the probe beams reflected from the sample surface and interfaces and a picosecond-duration strain pulse propagating in the alloy layer. Optical and elasto-optical parameters including the complex refractive index and the fundamental band gap of the cubic nitride alloys are determined and compared with the values obtained by ellipsometry.

Electrostatic charging of insulating fine particles can be responsible for numerous phenomena ranging from lightning in volcanic plumes to dust explosions. However, even basic aspects of how fine particles become charged are still unclear. Studying particle charging is challenging because it usually involves the complexities associated with many-particle collisions. To address these issues, we introduce a method based on acoustic levitation, which makes it possible to initiate sequences of repeated collisions of a single submillimeter particle with a flat plate, and to precisely measure the particle charge in situ after each collision. We show that collisional charge transfer between insulators is dependent on the hydrophobicity of the contacting surfaces. We use glass, which we modify by attaching nonpolar molecules to the particle, the plate, or both. We find that hydrophilic surfaces develop significant positive charges after contacting hydrophobic surfaces. Moreover, we demonstrate that charging between a hydrophilic and a hydrophobic surface is suppressed in an acidic environment and enhanced in a basic one. Application of an electric field during each collision is found to modify the charge transfer, again depending on surface hydrophobicity. We discuss these results within the context of contact charging due to ion transfer, and we show that they lend strong support to $\mathrm{O}{\mathrm{H}}^{-}$ ions as the charge carriers.

Vapor deposited organic glasses are currently in use in many optoelectronic devices. Their operation temperature is limited by the glass transition temperature of the organic layers and thermal management strategies become increasingly important to improve the lifetime of the device. Here we report the unusual finding that molecular orientation heavily influences heat flow propagation in glassy films of small molecule organic semiconductors. The thermal conductivity of vapor deposited thin-film semiconductor glasses is anisotropic and controlled by the deposition temperature. We compare our data with extensive molecular dynamics simulations to disentangle the role of density and molecular orientation on heat propagation. Simulations do support the view that thermal transport along the backbone of the organic molecule is strongly preferred with respect to the perpendicular direction. This is due to the anisotropy of the molecular interaction strength that limits the transport of atomic vibrations. This approach could be used in future developments to implement small molecule glassy films in thermoelectric or other organic electronic devices.

We demonstrate a simple self-assembly method based on solid state dewetting of ultrathin silicon films and germanium deposition for the fabrication of efficient antireflection coatings on silicon for light trapping. We fabricate SiGe islands with a high surface density, randomly positioned and broadly varied in size. This allows one to reduce the reflectance to low values in a broad spectral range (from 500 nm to 2500 nm) and a broad angle (up to ${55}^{\circ })$ and to trap within the wafer a large portion of the impinging light $\left(\sim 40\%\right)$ also below the band gap, where the Si substrate is nonabsorbing. Theoretical simulations agree with the experimental results, showing that the efficient light coupling into the substrate is mediated by Mie resonances formed within the SiGe islands. This lithography-free method can be implemented on arbitrarily thick or thin ${\mathrm{SiO}}_2$ layers and its duration only depends on the sample thickness and on the annealing temperature.

Vanadium dioxide $\left({\mathrm{VO}}_2\right)$ undergoes a structural transformation from monoclinic (insulator) to tetragonal (metallic) upon heating above 340 K, accompanied by abrupt changes to its electronic, optical, and mechanical properties. Not only is this transition scientifically intriguing, but there are also numerous applications in sensing, memory, and optoelectronics. Here we investigate the effect different substrates and the processing conditions have on the characteristics metal-insulator transition (MIT), and how the properties can be tuned for specific applications. ${\mathrm{VO}}_2$ thin films were grown on $c$-plane sapphire (0001) and p-type silicon $\langle 100\rangle$ by pulsed laser deposition. High-resolution x-ray diffraction along with transmission electron microscopy reveals textured epitaxial growth on sapphire by domain-matching epitaxy, while the presence of a native oxide layer on silicon prevented any preferential growth resulting in a polycrystalline film. An orientation relationship of $\langle 001\rangle$(010)${}_{{\mathrm{VO}}_2}$||$\langle 1\overline{1}00\rangle$(0001)${}_{{\mathrm{Al}}_2{\mathrm{O}}_3}$ was established for ${\mathrm{VO}}_2$ grown on sapphire, while no such relationship was found for ${\mathrm{VO}}_2$ grown on silicon. Surface-energy minimization is the driving force behind grain growth, as the lowest energy ${\mathrm{VO}}_2$ plane grew on silicon, while on sapphire the desire for epitaxial growth was dominant. Polycrystallinity of films grown on silicon caused a weaker and less prominent MIT than observed on sapphire, whose MIT was higher in magnitude and steeper in slope. The position of the MIT was shown to depend on the competing effects of misfit strain and grain growth. Higher deposition temperatures caused an increase in the MIT, while compressive strain resulted in a decreased MIT.

Motivated by recent experiments, we investigate the pressure-dependent electronic structure and electron-phonon (e-ph) coupling for simple cubic phosphorus by performing first-principles calculations within the full potential linearized augmented plane-wave method. As a function of increasing pressure, our calculations show a valley feature in $T_c$, followed by an eventual decrease for higher pressures. We demonstrate that this $T_c$ valley at low pressures is due to two nearby Lifshitz transitions, as we analyze the band-resolved contributions to the e-ph coupling. Below the first Lifshitz transition, the phonon hardening and shrinking of the $\gamma$ Fermi surface with $s$-orbital character results in a decreased $T_c$ with increasing pressure. After the second Lifshitz transition, the appearance of $\delta$ Fermi surfaces with $3d$-orbital character generate strong e-ph interband couplings in $\alpha \delta$ and $\beta \delta$ channels, and hence lead to an increase of $T_c$. For higher pressures, the phonon hardening finally dominates, and $T_c$ decreases again. Our study reveals that the intriguing $T_c$ valley discovered in experiment can be attributed to Lifshitz transitions, while the plateau of $T_c$ detected at intermediate pressures appears to be beyond the scope of our analysis. This strongly suggests that aside from e-ph coupling, electronic correlations along with plasmonic contributions may be relevant for simple cubic phosphorus. Our findings hint at the notion that increasing pressure can shift the low-energy orbital weight towards $d$ character, and as such even trigger an enhanced importance of orbital-selective electronic correlations despite an increase of the overall bandwidth.

Complex oxide interfaces are a promising platform for studying a wide array of correlated electron phenomena in low dimensions, including magnetism and superconductivity. The microscopic origin of these phenomena in complex oxide interfaces remains an open question. Here we investigate the magnetic properties of semi-insulating $\mathrm{NdTi}{\mathrm{O}}_3\text{/}\mathrm{SrTi}{\mathrm{O}}_3$ (NTO/STO) interfaces and present the first millikelvin study of NTO/STO. The magnetoresistance (MR) reveals signatures of local ferromagnetic order and of spin-dependent thermally activated transport, which are described quantitatively by a simple phenomenological model. We discuss possible origins of the interfacial ferromagnetism. In addition, the MR also shows transient hysteretic features on a time scale of $\sim 10-100\mathrm{s}$. We demonstrate that these are consistent with an extrinsic magnetothermal origin, which may have been misinterpreted in previous reports of magnetism in STO-based oxide interfaces. The existence of these two MR regimes (steady-state and transient) highlights the importance of time-dependent measurements for distinguishing signatures of ferromagnetism from other effects that can produce hysteresis at low temperatures.

To extend materials design and discovery into the space of metastable polymorphs, rapid and reliable assessment of transformation kinetics to lower energy structures is essential. Herein we focus on diffusionless polymorphic transformations and investigate routes to assess their kinetics using solely crystallographic arguments. As part of this investigation we developed a general algorithm to map crystal structures onto each other, and ascertain the low-energy (fast-kinetics) transformation pathways between them. Pathways with minimal dissociation of chemical bonds, along which the number of bonds (in ionic systems the first-shell coordination) does not decrease below that in the end structures, are shown to always be the fast-kinetics pathways. These findings enable the rapid assessment of the kinetics of polymorphic transformation and the identification of long-lived metastable structures. The utility is demonstrated on a number of transformations including those between high-pressure ${\mathrm{SnO}}_2$ phases, which lack a detailed atomic-level understanding.

It is known that solid-state reaction in high-pressure oxygen can stabilize high-oxidation phases of Y-Ba-Cu-O superconductors in powder form. We extend this superoxygenation concept of synthesis to thin films which, due to their large surface-to-volume ratio, are more reactive thermodynamically. Epitaxial thin films of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ grown by pulsed laser deposition are annealed at up to 700 atm ${\mathrm{O}}_2$ and $900{}^{\circ }\mathrm{C}$, in conjunction with Cu enrichment by solid-state diffusion. The films show the clear formation of ${\mathrm{Y}}_2{\mathrm{Ba}}_4{\mathrm{Cu}}_7{\mathrm{O}}_{15-\delta }$ and ${\mathrm{Y}}_2{\mathrm{Ba}}_4{\mathrm{Cu}}_8{\mathrm{O}}_{16}$ as well as regions of ${\mathrm{YBa}}_2{\mathrm{Cu}}_5{\mathrm{O}}_{9-\delta }$ and ${\mathrm{YBa}}_2{\mathrm{Cu}}_6{\mathrm{O}}_{10-\delta }$ phases, according to scanning transmission electron microscopy, x-ray diffraction, and x-ray absorption spectroscopy. Similarly annealed ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ powders show no phase conversion. Our results demonstrate a route of synthesis towards discovering more complex phases of cuprates and other superconducting oxides.

We report the critical role of La doping in the topological Hall effect observed in $\mathrm{L}{\mathrm{a}}_x\mathrm{E}{\mathrm{u}}_{1-x}\mathrm{O}$ thin films $\left(\sim 50\mathrm{nm}\right)$ grown by molecular beam epitaxy. When the La doping exceeds 0.036, topological Hall effect emerges, which we attribute to the formation of magnetic skyrmions. Besides, the La doping is found to play a critical role in determining the phases, densities, and sizes of the skyrmions in the $\mathrm{L}{\mathrm{a}}_x\mathrm{E}{\mathrm{u}}_{1-x}\mathrm{O}$ thin films. The maximum region of the skyrmion phase diagram is observed on the $\mathrm{L}{\mathrm{a}}_{0.1}\mathrm{E}{\mathrm{u}}_{0.9}\mathrm{O}$ thin film. As the La doping increases, the skyrmion density increases while the skyrmion size decreases. Our findings demonstrate the important role of La doping for the skyrmions in EuO films, which could be important for future studies of magnetic skyrmions in Heisenberg ferromagnets.

$\mathrm{AlF}{\mathrm{e}}_2{\mathrm{B}}_2$ is a ferromagnet with the Curie temperature around 300 K and has the potential to be an outstanding rare-earth free candidate for magnetocaloric applications. However, samples prepared from the melt contain additional phases which affect the functional response of the $\mathrm{AlF}{\mathrm{e}}_2{\mathrm{B}}_2$ phase. We report on the effects of Al content in samples with the initial (nominal) composition of $\mathrm{A}{\mathrm{l}}_x\mathrm{F}{\mathrm{e}}_2{\mathrm{B}}_2$, where $x=1.0$, 1.1, and 1.2 prepared by arc-melting followed by suction casting and annealing. The as-cast $\mathrm{A}{\mathrm{l}}_x\mathrm{F}{\mathrm{e}}_2{\mathrm{B}}_2$ alloys contain $\mathrm{AlF}{\mathrm{e}}_2{\mathrm{B}}_2$ as well as additional phases, including the primary solidifying FeB and $\mathrm{A}{\mathrm{l}}_{13}\mathrm{F}{\mathrm{e}}_4$ compounds, which are ferromagnetic and paramagnetic, respectively, at 300 K. The presence of these phases makes it difficult to extract the intrinsic magnetic properties of $\mathrm{AlF}{\mathrm{e}}_2{\mathrm{B}}_2$ phase. Annealing of $\mathrm{A}{\mathrm{l}}_x\mathrm{F}{\mathrm{e}}_2{\mathrm{B}}_2$ alloys at 1040 °C for 3 days allows for reaction of the FeB with $\mathrm{A}{\mathrm{l}}_{13}\mathrm{F}{\mathrm{e}}_4$ to form the $\mathrm{AlF}{\mathrm{e}}_2{\mathrm{B}}_2$ phase, significantly reduces the amount of additional phases, and results in nearly pure $\mathrm{AlF}{\mathrm{e}}_2{\mathrm{B}}_2$ phase as confirmed with XRD, magnetization, scanning electron microscopy, and electronic transport. The values of the magnetization, effective magnetic moment per Fe atom, specific heat capacity, electrical resistivity, and Seebeck coefficient for the $\mathrm{AlF}{\mathrm{e}}_2{\mathrm{B}}_2$ compound have been established.

We report the properties of the reported transparent conductor CuI, including the effect of heavy $p$-type doping. The results, based on first-principles calculations, include an analysis of the electronic structure and calculations of optical and dielectric properties. We find that the origin of the favorable transparent conducting behavior lies in the absence in the visible of strong interband transitions between deeper valence bands and states at the valence-band maximum that become empty with $p$-type doping. Instead, strong interband transitions to the valence-band maximum are concentrated in the infrared with energies below 1.3 eV. This is contrast to the valence bands of many wide-band-gap materials. Turning to the mobility, we find that the states at the valence-band maximum are relatively dispersive. This originates from their antibonding Cu $d$–I $p$ character. We find a modest enhancement of the Born effective charges relative to nominal values, leading to a dielectric constant $\varepsilon \left(0\right)=6.3$. This is sufficiently large to reduce ionized impurity scattering, leading to the expectation that the properties of CuI can still be significantly improved through sample quality.

Functional oxides are considered as promising materials for photonic applications due to their extraordinary and various optical properties. Especially, yttria-stabilized zirconia (YSZ) has a high refractive index (∼2.15), leading to a good confinement of the optical mode in waveguides. Furthermore, YSZ can also be used as a buffer layer to expand toward a large family of oxides-based thin-films heterostructures. In this paper, we report a complete study of the structural properties of YSZ for the development of integrated optical devices on sapphire in telecom wavelength range. The substrate preparation and the epitaxial growth using pulsed-laser deposition technique have been studied and optimized. High-quality YSZ thin films with remarkably sharp x-ray diffraction rocking curve peaks in ${{10}^{-3}}^{\circ }$ range have then been grown on sapphire (0001). It was demonstrated that a thermal annealing of sapphire substrate before the YSZ growth allowed controlling the out-of-plane orientation of the YSZ thin film. Single-mode waveguides were finally designed, fabricated, and characterized for two different main orientations of high-quality YSZ (001) and (111). Propagation loss as low as 2 dB/cm at a wavelength of 1380 nm has been demonstrated for both orientations. These results pave the way for the development of a functional oxides-based photonics platform for numerous applications including on-chip optical communications and sensing.

Backscattering from nanostructured surfaces greatly diminishes the efficacy of light trapping solar cells. While the analytical design of broadband, angle-independent antireflection coatings on nanostructured surfaces proved inefficient, numerical optimization proves a viable alternative. Here, we numerically design and experimentally verify the performance of single and bilayer antireflection coatings on a 2D hexagonal diffractive light trapping pattern on crystalline silicon substrates. Three well-known antireflection coatings, aluminum oxide, silicon nitride, and silicon oxide, which also double as high-quality surface passivation materials, are studied in the 400–1000 nm band. By varying thickness and conformity, the optimal parameters that minimize the broadband total reflectance (specular and scattering) from the nanostructured surface are obtained. The design results in a single-layer antireflection coating with normal-angle wavelength-integrated reflectance below 4% and a bilayer antireflection coating demonstrating reflection down to 1.5%. We show experimentally an angle-averaged reflectance of $\sim 5.2\%$ up to 60° incident angle from the optimized bilayer antireflection-coated nanostructured surface, paving the path toward practical implementation of the light trapping solar cells.

Using methods of statistical kinetic theory parametrized with first-principles interatomic interactions that include chemical and strain contributions, we investigated the kinetics of decomposition and microstructure formation in Al-Cu alloys as a function of temperature and alloy concentration. We show that the decomposition of the solid solution forming platelets of copper, known as Guinier-Preston (GP) zones, includes several stages and that the transition from GP1 to GP2 zones is determined mainly by kinetic factors. With increasing temperature, the model predicts a gradual transition from plateletlike precipitates to equiaxial ones and at intermediate temperatures both precipitate morphologies may coexist.

High-entropy alloys (HEAs) are a new class of materials constructed from multiple principal elements statistically arranged on simple crystallographic lattices. Due to the large amount of disorder present, they are excellent model systems for investigating the properties of materials intermediate between crystalline and amorphous states. Here we report the effects of systematic isoelectronic replacements, using Mo-Y, Mo-Sc, and Cr-Sc mixtures, for the valence electron count 4 and 5 elements in the body-centered cubic (BCC) Ta-Nb-Zr-Hf-Ti high-entropy alloy (HEA) superconductor. We find that the superconducting transition temperature $T_{\mathrm{c}}$ strongly depends on the elemental makeup of the alloy, and not exclusively its electron count. The replacement of niobium or tantalum by an isoelectronic mixture lowers the transition temperature by more than 60%, while the isoelectronic replacement of hafnium, zirconium, or titanium has a limited impact on $T_{\mathrm{c}}$. We further explore the alloying of aluminium into the nearly optimal electron count ${\left[\mathrm{TaNb}\right]}_{0.67}{\left(\mathrm{ZrHfTi}\right)}_{0.33}$ HEA superconductor. The electron count dependence of the superconducting $T_{\mathrm{c}}$ for ${\left(\mathrm{HEA}\right)\mathrm{Al}}_x$ is found to be more crystallinelike than for the ${\left[\mathrm{TaNb}\right]}_{1-x}{\left(\mathrm{ZrHfTi}\right)}_x$ HEA solid solution. For an aluminum content of $x=0.4$ the high-entropy stabilization of the simple BCC lattice breaks down. This material crystallizes in the tetragonal $\beta$-uranium structure type and superconductivity is not observed above 1.8 K.