Condensed Matter

Machine learning is applied to a large number of modern devices that are essential in building energy efficient smart society. Audio and face recognition are among the most well-known technologies that make use of such artificial intelligence. In materials research, machine learning is adapted to predict materials with certain functionalities, an approach often referred to as materials informatics. Here we show that machine learning can be used to extract material parameters from a single image obtained in experiments. The Dzyaloshinskii-Moriya (DM) interaction and the magnetic anisotropy distribution of thin film heterostructures, parameters that are critical in developing next generation storage class magnetic memory technologies, are estimated from a magnetic domain image. Micromagnetic simulation is used to generate thousands of random images for training and model validation. A convolutional neural network system is employed as the learning tool. The DM exchange constant of typical Co-based thin film heterostructures is studied using the trained system: the estimated values are in good agreement with experiments. Moreover, we show that the system can independently determine the magnetic anisotropy distribution, demonstrating the potential of pattern recognition. This approach can considerably simplify experimental processes and broaden the scope of materials research.

We report the magnitude of the induced magnetic moment in CVD-grown epitaxial and rotated-domain graphene as a result of the proximity effect in the vicinity of the ferromagnetic substrates Co and Ni, using polarised neutron reflectivity (PNR). Although rotated-domain graphene is known to interact weakly with the ferromagnetic underlayer in comparison with the epitaxial graphene, the PNR results indicate an induced magnetic moment of ??0.57 μ B /C atom at 10 K for both structures. The origin of the induced magnetic moment is found to be due to the opening of the graphene's Dirac cone as a result of the strong C p z ??d hybridisation, which was confirmed by additional PNR measurements using a non-magnetic Ni 9 Mo 1 and Cu substrates. We validated our PNR fitting models using the Bayesian uncertainty analysis and corroborated the results by X-ray magnetic circular dichroism measurements.

The attributes of group-V-donor spins implanted in an isotopically purified 28 Si crystal make them attractive qubits for large-scale quantum computer devices. Important features include long nuclear and electron spin lifetimes of 31 P, hyperfine clock transitions in 209 Bi and electrically controllable 123 Sb nuclear spins. However, architectures for scalable quantum devices require the ability to fabricate deterministic arrays of individual donor atoms, placed with sufficient precision to enable high-fidelity quantum operations. Here we employ on-chip electrodes with charge-sensitive electronics to demonstrate the implantation of single low-energy (14 keV) P + ions with an unprecedented 99.87±0.02 % confidence, while operating close to room-temperature. This permits integration with an atomic force microscope equipped with a scanning-probe ion aperture to address the critical issue of directing the implanted ions to precise locations. These results show that deterministic single-ion implantation can be a viable pathway for manufacturing large-scale donor arrays for quantum computation and other applications.

Nanowires (NWs) with their quasi-one-dimensionality often present different structural and opto-electronic properties than their thin-film counterparts. The thinner they are the larger these differences are, in particular in the carrier-phonon scattering and thermal conductivity. In this work, we present femtosecond transient absorbance measurements on GaAs0.8P0.2 NWs of two different diameters, 36 and 51 nm. The results show that thinner NWs sustain the hot-carriers at a higher temperature for longer times than thicker NWs. We explain the observation suggesting that in thinner NWs, the build-up of a hot-phonon bottleneck is easier than in thicker NWs because of the increased phonon scattering at the NW sidewalls which facilitates the build-up of a large phonon density. The large number of optical phonons emitted during the carrier relaxation processes generate a non-equilibrium population of acoustic phonons that propagates less efficiently in thin NWs. This makes the possible acoustic-to-optical phonon up-conversion process easier, which prolongs the LO phonon lifetime resulting in the slowdown of the carrier cooling. The important observation that the carrier temperature in thin NWs is higher than in thick NWs already at the beginning of the hot carrier regime suggests that the phonon-mediated scattering processes in the non-thermal regime play a major role at least for the carrier densities investigated here (8x1018-4x1019 cm-3). Our results also suggest that the boundary scattering of phonons at crystal defects is negligible compared to the surface scattering at the NW sidewalls.

Strong light-matter interactions facilitate not only emerging applications in quantum and non-linear optics but also modifications of materials properties. In particular the latter possibility has spurred the development of advanced theoretical techniques that can accurately capture both quantum optical and quantum chemical degrees of freedom. These methods are, however, computationally very demanding, which limits their application range. Here, we demonstrate that the optical spectra of nanoparticle-molecule assemblies, including strong coupling effects, can be predicted with good accuracy using a subsystem approach, in which the response functions of the different units are coupled only at the dipolar level. We demonstrate this approach by comparison with previous time-dependent density functional theory calculations for fully coupled systems of Al nanoparticles and benzene molecules. While the present study only considers few-particle systems, the approach can be readily extended to much larger systems and to include explicit optical-cavity modes.

Divacancy spins in silicon carbide implement qubits with outstanding characteristics and capabilities, including but not limited to, 64 ms coherence time, spin-to-photon interfacing, and sizable, 10%??0% room-temperature read-out contrast, all of these in an industrial semiconductor host. Despite these great demonstrations, there are still numerous open questions on the physics of divacancy point defects. In particular, spin relaxation, which sets the fundamental limit for the spin coherence time, has not been thoroughly studied yet. Here, we carry out theoretical simulations of environmental spin induced spin relaxation processes of different divacancy configurations in 4H-SiC. We reveal magnetic field values where the longitudinal spin relaxation time T 1 drops resonantly due to the coupling to either 29 Si and 13 C nuclear spins or electron spins associated with other defects and dopants. We quantitatively analyze the dependence of the T 1 time on the concentration of the defect spins and the applied magnetic field in the most relevant cases and provide a simple analytical expression allowing either for estimation of the T 1 time in samples with known spin defect concentration or for estimation of the local spin defect concentration of an ensemble or a single divacancy qubit from the measured relaxation rates.

We develop a dipole model describing the thermal far-field radiation of a nanoparticle in close vicinity to a substrate. By including in our description the contribution of eddy currents and the possibility to choose different temperatures for the nanoparticle, the substrate, and the background, we generalize the existing models. We discuss the impact of the different temperatures, particle size, emission angle, and the distance dependence for all four combinations of gold and SiC nanoparticles or substrates.

The semiconductor-metal junction is one of the most critical factors for high performance electronic devices. In two-dimensional (2D) semiconductor devices, minimizing the voltage drop at this junction is particularly challenging and important. Despite numerous studies concerning contact resistance in 2D semiconductors, the exact nature of the buried interface under a three-dimensional (3D) metal remains unclear. Herein, we report the direct measurement of electrical and optical responses of 2D semiconductor-metal buried interfaces using a recently developed metal-assisted transfer technique to expose the buried interface which is then directly investigated using scanning probe techniques. We characterize the spatially varying electronic and optical properties of this buried interface with < 20 nm resolution. To be specific, potential, conductance and photoluminescence at the buried metal/MoS2 interface are correlated as a function of a variety of metal deposition conditions as well as the type of metal contacts. We observe that direct evaporation of Au on MoS2 induces a large strain of ~5% in the MoS2 which, coupled with charge transfer, leads to degenerate doping of the MoS2 underneath the contact. These factors lead to improvement of contact resistance to record values of 138 kohm-um, as measured using local conductance probes. This approach was adopted to characterize MoS2-In/Au alloy interfaces, demonstrating contact resistance as low as 63 kohm-um. Our results highlight that the MoS2/Metal interface is sensitive to device fabrication methods, and provides a universal strategy to characterize buried contact interfaces involving 2D semiconductors.

Aharonov-Bohm (AB) effect, the well-known archetype of electron-wave interference phenomena, has been explored extensively through transport measurements. However, these techniques lack spatial resolution that would be indispensable for studying the magnetic and electrostatic AB oscillations at the nanometer scale. Here, we demonstrated that scanning tunneling microscopy (STM) can be used as an AB interferometer operating on nanometer length scales and the magneto-electric Aharonov-Bohm effect in minimally twisted bilayer graphene (TBG) was directly measured by using STM. In the minimally TBG, there is a triangular network of chiral one-dimensional states hosted by domain boundaries due to structural reconstruction. Taking advantage of the high spatial resolution of the STM, both the magnetic and electrostatic AB oscillations arising from electron interference along moiré-scale triangular quantum paths in the minimally TBG were measured. Our work enables measure and control of the AB effect and other electron-wave interference at the nanoscale.

Thin films of topological insulators (TI) attract large attention because of expected topological effects from the inter-surface hybridization of Dirac points. However, these effects may be depleted by unexpectedly large energy smearing ? of surface Dirac points by the random potential of abundant Coulomb impurities. We show that in a typical TI film with large dielectric constant ??0 sandwiched between two low dielectric constant layers, the Rytova-Chaplik-Entin-Keldysh modification of the Coulomb potential of a charge impurity allows a larger number of the film impurities to contribute to ? . As a result, ? is large and independent of the TI film thickness d for d>5 nm. In thinner films ? grows with decreasing d due to reduction of screening by the hybridization gap. We study the surface conductivity away from the neutrality point and at the neutrality point. In the latter case, we find the maximum TI film thickness at which the hybridization gap is still able to make a TI film insulating and allow observation of the quantum spin Hall effect, d max ?? nm.