Si/SiGe heterostructures are of high interest for high-mobility transistor and qubit applications, specifically for operations below $4.2\mathrm{K}$. In order to optimize parameters such as charge mobility, built-in strain, electrostatic disorder, charge noise, and valley splitting, these heterostructures require Ge concentration profiles close to monolayer precision. Ohmic contacts to undoped heterostructures are usually facilitated by a global annealing step activating implanted dopants, but compromising the carefully engineered layer stack due to atom diffusion and strain relaxation in the active device region. We demonstrate a local laser-based annealing process for recrystallization of ion-implanted contacts in SiGe, greatly reducing the thermal load on the active device area. To quickly adapt this process to the constantly evolving heterostructures, we deploy a calibration procedure based exclusively on optical inspection at room temperature. We measure the electron mobility and contact resistance of laser-annealed Hall bars at temperatures below $4.2\mathrm{K}$ and obtain values similar or superior to that of a globally annealed reference sample. This highlights the usefulness of laser-based annealing to take full advantage of high-performance Si/SiGe heterostructures.

Diamond shows great promise for opening up new paradigms in the semiconductor industry and quantum electronics. Here, we investigate the influence of thermal annealing on the structural and electrical transport properties of heavily boron-doped polycrystalline diamond (BPD) thin films. Our structural analyses show that annealing beyond $600{}^{\circ }\mathrm{C}$ can induce severe local amorphization in a BPD thin film and transform it into a binary mixture of spatially separate domains of amorphous carbon (a-C) and diamond grains. Due to this annealing-induced morphology and phase segregation, the BPD thin films demonstrate a significant decrease of the electron localization radius and a distinct increase of the Ginzburg-Landau coherence length. Our research provides physical insight into the conversion of diamond to a-C and aids in defining the application scope of BPD by revealing its heat tolerance.

Since charge-carrier radiative recombination is a key process in semiconductors that enables light emission, it is critical to understand its chemical trend and microscopic mechanisms. Using III-nitride semiconductors as prototypical examples, we rigorously study the radiative recombination mechanisms employing first principles. We show that the radiative recombination coefficient is competitively impacted by the band gap, effective masses, and dipole matrix elements, but the band gap plays a dominant role. The radiative recombination coefficient is actually stronger in AlN, as compared to the ones in InN and GaN, despite the fact that AlN-based light-emitting diodes (LEDs) currently exhibit low efficiencies. However, the polarization direction of light emission in AlN is qualitatively different from those in InN and GaN due to valence-band reordering, which may lead to additional complication in light extraction. Our insights pave the way to optimize the optoelectronic performance of wide-band-gap LEDs.

Metallic crystalline nanoparticles (NPs) have been shown to display intriguing size-dependent properties. However, the properties of metallic glass (MG) NPs remain largely unexplored. Using molecular dynamics simulations, we produce $\mathrm{C}{\mathrm{u}}_{64}\mathrm{Z}{\mathrm{r}}_{36}$ MG NPs ranging in size from 1 to 20 nm by cooling molten systems at a relatively slow rate of ${10}^9$ K/s. Results indicate that NPs are coated with a Cu-rich layer with a thickness that increases with NP size. We employ various simulated rates of heating, from ${10}^{11}$ to ${10}^{13}$ K/s, to determine the melting points of these NPs. The results show a significant decrease in the solidus temperature for NPs smaller than 10 nm. Furthermore, as NP size decreases, the fraction of Cu full icosahedra within the NPs increases, suggesting that smaller NPs are stronger and harder. These findings provide insights for designing heterogeneous metallic nanoglass materials that leverage the size-dependent properties of MG NPs.

Josephson junctions are the key components used in superconducting qubits for quantum computing. The advancement of quantum computing is limited by a lack of stability and reproducibility of qubits, which ultimately originates in the amorphous tunnel barrier of the Josephson junctions and other material imperfections. Pinholes in the junction have been suggested as one of the possible contributors to these instabilities, but evidence of their existence and the effect they might have on transport is unclear. We use molecular dynamics to create three-dimensional atomistic models to describe $\mathrm{Al}\text{−}{\mathrm{AlO}}_x\text{−}\mathrm{Al}$ tunnel junctions, showing that pinholes form when oxidation of the barrier is incomplete. Following this, we use the atomistic model and simulate the electronic transport properties for tunnel junctions with different barrier thicknesses using the nonequilibrium Green's function formalism. We observe that pinholes may contribute to excess quasiparticle current flow in $\mathrm{Al}\text{−}{\mathrm{AlO}}_x\text{−}\mathrm{Al}$ tunnel junctions with thinner barriers, and in thicker barriers we observe weak-points that facilitate leakage currents even when the oxide is continuous. We find that the disordered nature of the amorphous barrier results in significant variations in the transport properties. Additionally, we determine the current-phase relationship for our atomistic structures, confirming that devices with pinholes and weak-points cause a deviation from the ideal sinusoidal Josephson relationship.

Antiferromagnets can host ultrafast spin dynamics in the terahertz (THz) frequency range, but an energy-efficient generation of THz magnons necessary for high-speed information processing devices is challenging. Fortunately, some antiferromagnetic compounds possess substantial magnetoelastic coupling between spins and strains, which opens the way for their excitation by mechanical stimuli. Here, we report a theoretical study of the spin dynamics excited in single-crystalline NiO by picosecond acoustic pulses, which can be created by optomechanical transducers driven by femtosecond laser pulses. To describe the interrelated spin and strain dynamics in this antiferromagnet distinguished by a strong magnetoelastic coupling, we carry out micromagnetoelastic simulations based on the numerical solution of the Landau-Lifshitz-Gilbert equation for sublattice magnetizations and the elastodynamic equation for mechanical displacements. The simulations show that the propagating “bipolar” pulse of the longitudinal strain generates correlated clockwise and counterclockwise precessions of the sublattice magnetizations, which have a complex spatial distribution in the region behind the pulse front. The spatiotemporal analysis of the simulation data reveals that the spin dynamics excited by the pulses with durations $\tau$ smaller than about 7 ps comprises a monochromatic spin wave with the frequency $\nu \approx 450$ GHz. Moreover, a second monochromatic spin wave having the frequency $\nu \approx 2$ THz emerges at $\tau \le 3$ ps. By comparing the calculated dispersion of spin and elastic waves in NiO we demonstrate that the revealed monochromatic magnons originate from two magnetoacoustic resonances existing in this antiferromagnetic compound. Importantly, the acoustic pulses with durations $\tau \le 3$ ps appear to be capable of creating antiferromagnetic magnons with THz and sub-THz frequencies in the absence of external magnetic fields. Our findings shed light on the magnetoacoustic phenomena in antiferromagnets and indicate that the single-crystalline NiO is a promising material for the development of ultrafast magnonic devices with a low power consumption.

The bilayer perovskite ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ has recently been found to enter a superconducting state under hydrostatic pressure at temperatures as high as 80 K. The onset of superconductivity is observed concurrent with a structural transition which suggests that superconductivity is inherently related to this specific structure. Here we perform density functional theory based structural relaxation calculations and identify several promising routes to stabilize the crystal structure which hosts the superconducting state at ambient pressure. We find that the structural transition is controlled almost entirely by a reduction of the $b$-axis lattice constant, which suggests that uniaxial compression along the [010] direction or in-plane biaxial compression are sufficient as tuning parameters to control this transition. Furthermore, we show that increasing the size of the $A$-site cations can also induce the structural transitions via chemical pressure and identify ${\mathrm{Ac}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ and Ba-doped ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ as potential candidates for a high temperature superconducting nickelate at ambient pressure.

The pyroresistive response of conductive polymer composites (CPCs) has attracted much interest because of its potential applications in many electronic devices requiring a significant responsiveness to changes in external physical parameters such as temperature or electric fields. Although extensive research has been conducted to study how the properties of the polymeric matrix and conductive fillers affect the positive temperature coefficient pyroresistive effect, the understanding of the microscopic mechanism governing such a phenomenon is still incomplete. In particular, to date, there is little body of theoretical research devoted to investigating the effect of the polymer thermal expansion on the electrical connectivity of the conductive phase. Here, we present the results of simulations of model CPCs in which rigid conductive fillers are dispersed in an insulating amorphous matrix. By employing a meshless algorithm to analyze the thermoelastic response of the system, we couple the computed strain field to the electrical connectedness of the percolating conductive particles. We show that the electrical conductivity responds to the local strains that are generated by the mismatch between the thermal expansion of the polymeric and conductive phases and that the conductor-insulator transition is caused by a sudden and global disconnection of the electrical contacts forming the percolating network.

Understanding the interplay between micromechanics and macroscopic plastic deformation in amorphous solids like granular materials is vital. In our study, we directly measured T1 events and associated microscopic stresses in densely packed assemblies of photoelastic disks with varying interparticle friction under pure shear. Our findings demonstrate the existence of T1 events in granular materials under shear, where the statistics of T1 events show a strong correlation with the global shear stress to pressure ratio, and these behaviors are independent of interparticle friction. By dividing T1 events into five categories, we can demonstrate how changes in friction affect their local structure and stability. This provides valuable information about the unique features of T1 events in granular materials, which differ from those in metallic glasses and other frictionless amorphous solids. Additionally, our exploration reveals unique characteristics in the local stresses of T1 events, distinct from frictionless systems like emulsions. Collectively, these findings highlight how interparticle friction modulates the micromechanical properties of T1 events and how their statistical behavior intricately links with the macromechanical responses of granular materials under shear forces.

Yttrium iron garnet (YIG) is a prototypical material in spintronics due to its exceptional magnetic properties. To exploit these properties, high quality thin films need to be manufactured. Deposition techniques like sputter deposition or pulsed laser deposition at ambient temperature produce amorphous films, which need a postannealing step to induce crystallization. However, not much is known about the exact dynamics of the formation of crystalline YIG out of the amorphous phase. Here, we conduct extensive time and temperature series to study the crystallization behavior of YIG on various substrates and extract the crystallization velocities as well as the activation energies needed to promote crystallization. We find that the type of crystallization as well as the crystallization velocity depend on the lattice mismatch to the substrate. We compare the crystallization parameters found in literature with our results and find excellent agreement with our model. Our results allow us to determine the time needed for the formation of a fully crystalline film of arbitrary thickness for any temperature.

Reports on spin Hall magnetoresistance, magnonic spin currents from thermal gradients, and spin transfer-torque magnetic random-access memory using compensated ferrimagnets largely discuss bulk magnetization but lack consideration of depth profiles or interfacial characteristics. Here, magnetic and structural characterization of profiles and interfaces was performed for nearly compensated gadolinium iron garnet (GdIG) thin films. X-ray diffraction and reciprocal space maps show that sputter deposited GdIG on Si is polycrystalline with the desired cubic garnet phase, and GdIG on gadolinium gallium garnet (GGG) is epitaxial with <0.06% compressive strain. Temperature-dependent magnetometry confirms the compensation temperatures of GGG/GdIG and Si/GdIG to be 285 and 260 K, respectively, both near room temperature. Interestingly, these measurements suggest the presence of unsaturated rare-earth moments, which result in a characteristic hysteresis between heating and cooling sequences in the magnetization-temperature curves at zero field. Depth-profile measurements from polarized neutron reflectometry (PNR) indicate up to 91% volume fraction in GdIG on Si. At the interface, PNR reveals a region containing magnetized Fe-doped GGG, a low-density GdIG at the GGG/GdIG interface, and a thin magnetically dead layer at the Si/GdIG interface. Cross-sectional transmission electron microscopy and energy dispersive x-ray spectroscopy confirm the assessment of PNR. The magnetic characteristics of interfacial regions are attributed to intermixing of Fe-Ga at the GGG/GdIG interface and the presence of amorphous Fe-Si at the Si/GdIG interface.

Via large-scale molecular dynamics simulations, we observe the melting of a topological glass of stiff ring polymers by incorporating flexible ring polymers, along an isobaric path. As more flexible ring polymers are introduced, stacked glassy structures emerge in the stiffer ring polymers with reduced orthogonal threading. This eventually evolves to a stacked columnar structure at an increased fraction of flexible ring polymers. Depletion interactions between the stiff and flexible rings drive the stacking, contingent on the disparity in flexibility in the ring polymer mixture.

Using nudged elastic band calculations and first-principles molecular dynamics with enhanced sampling, we study the formation and dynamics of the divacancy ${\mathrm{V}}_{\mathrm{C}}{\mathrm{V}}_{\mathrm{Si}}$ (VV) in cubic silicon carbide, including VV rotation and migration. We show that for all processes studied here the energy barriers and preferred pathway depend on the charge state of the defects. Our results indicate that the influence of multiple charge states and entropic effects should be considered for a quantitative description of the physical and dynamical properties of point defects at finite temperatures. In addition, we demonstrate that molecular dynamics simulations using machine-learning potentials can efficiently and reliably capture entropic effects and yield accurate free-energy barriers.

The electronic properties of two-dimensional (2D) material systems strongly depend on their atomic arrangement, so validating the structure of an experimentally fabricated 2D system is an important task for researchers. First-principles density-functional theory (DFT) is used to analyze about 100 configurations of Pb intercalated under buffer-layer graphene on SiC(0001). These configurations are constructed by using seven different types of supercells corresponding to varying strain. By comparing the chemical potentials of the intercalated Pb structures, we find that the thermodynamically most preferred intercalated Pb configurations appear under the low-strained buffer-layer graphene. We also find that plentiful structures with the lowest chemical potentials are amorphous-like Pb monolayers. These are almost energetically degenerate with various Pb(111)-like monolayers having slightly higher chemical potentials. Our DFT results also indicate that intercalated plumbene-like monolayers under the high-strained buffer-layer graphene generally have higher chemical potentials. However, intercalated plumbene-like monolayers under a low-strained buffer-layer graphene do also exist with significantly lower chemical potentials relative to the structures under the high-strained buffer-layer graphene, despite some distortions in their structures. These results are consistent with previously available experimental results and suggest further studies on this unique system to promote the synthesis of phases with the anticipated properties, which are attractive for applications.

The spin Hall effect (SHE) maintains a distinct, pivotal role within spintronics. In this study, we systematically investigate the intrinsic SHE of topological semimetals with a single Dirac nodal ring (DNR) in the half-Heusler ABC family of compounds (HfCoAs, HfCoP, HfNiAs, ZrCoP, TaCoGe, TiSiNi, NbCoSi). Taking HfCoAs as an example, we show that an ideal single DNR lies within the ${\mathcal{M}}_y$ mirror plane adjacent to the Fermi surface when spin-orbit coupling (SOC) is absent, as protected by combined space-time inversion ($\mathcal{PT}$) symmetry and ${\mathcal{M}}_y$ mirror-reflection symmetry. With the inclusion of the SOC, the DNR is gapped out resulting in plenty of minimal band gap near the Fermi level, which yields a large spin Hall conductivity owing to the large spin Berry curvature hotspots around the nodal ring. Our research provides a comprehensive understanding of spin transport properties inherent in the simplest topological DNR semimetals, providing a solid foundation for exploring and engineering more complicated nodal line semimetals. This work may also offer practical applications in advancing spintronics device development with suitable SHE materials.

Recent experimental and theoretical work has focused new interest on magnon contributions to transport properties of metallic ferromagnets. Here we present the Seebeck coefficient, or thermopower, measured from $78\text{to}325\mathrm{K}$ for ${\mathrm{Co}}_x{\mathrm{Fe}}_{1-x}$ thin films and discuss the role of the Gilbert damping parameter, ${\alpha }_{\mathrm{GD}}$, on electron-magnon interactions. These measurements are made with micromachined Si-N thermal isolation platforms that allow excellent control of the thermal gradient applied to a thin film. We first present zero-field measurements of the absolute Seebeck coefficient, ${\alpha }_{\text{abs}}$, for two films with $x=0.25$, the alloy composition previously shown to have low damping and long-lived spin excitations, and a third film with $x=50$ where ${\alpha }_{\mathrm{GD}}$ is higher, and more typical of other 3D alloy ferromagnets. We compare these to pure Co and Fe films, and to simple expectations from the electronic DOS. This indicates that a large additional thermopower appears where spin excitations are long-lived. This additional zero-field thermopower can be explained by magnon drag, where momentum is transferred from thermal, exchange-dominated magnons to the electron system. We then present the dependence of the thermopower on the direction of an applied in-plane magnetic field. Comparison of this magnetothermopower (MTP) to anisotropic magnetoresistance (AMR) measured via four-wire electrical resistance measured on the same films shows, as in previous measurements of thermal conductivity in the same samples, a field-direction dependent contribution to thermopower. The MTP also allows improved estimation of the electronic diffusion thermopower, which then allows us to construct a plausible model for the zero-field thermopower of the low-damping films that adds the theoretically predicted magnon drag, and matches measured ${\alpha }_{\text{abs}}$ well at low $T$. The field-direction dependent contribution, not previously observed, also suggests interaction of electrons with the lower wave-vector dipole or dipole-exchange dominated magnetostatic spin waves, which is an alternate manifestation of magnon drag.