Session D02: Scattering and Microscopy

Small-angle Neutron Scattering, from Angstroms to almost Microns and Back Again.

Neutron scattering is powerful technique for investigating material characteristics. Small-angle Neutron Scattering (SANS) is used to study large scale structures varying from 1 to 500 nanometers. I will present a few examples on how SANS can be used to investigate different properties of materials ranging from alloys to magnetic materials.  I will also give a quick overview of other neutron instruments available for users at the High Flux Isotope Reactor (HFIR) and Spallation Neutron Source (SNS) at Oak Ridge National Laboratory.

    

Origin and Structure of the First Sharp Diffraction Peak of Amorphous Silicon

Based on a systematic study of the structure of the First Sharp Diffraction Peak (FSDP) on the structure factor at wave vector space of the continuous random network (CRN) models of amorphous silicon, we found a significant contribution to the intensity and position of the FSDP appears from the atomic correlations from radial distances up to 15 Å. Analysis of the radial shell-by-shell contribution to the FSDP revels the FSDP primarily depends on the atomic pair correlations associated with the second and fourth radial shells, with some background corrections from the first radial shell and minor perturbative corrections from the rest of the radial shells. Likewise, the numerical calculations suggest an approximate functional relation between the position (Q₀) of the FSDP and the average radial distance of the Si atoms in the second radial shell, as Q₀ is inversely proportional to the cubic power of the average radial distance of the atoms at the second radial shell of the amorphous network.

    

Witnessing Light-Driven Entanglement using Time-Resolved Resonant Inelastic X-Ray Scattering

Quantum computing takes advantage of principles of quantum mechanics, specifically entanglement, to encode information, which in turn requires characterizing and controlling entanglement within these materials. However, defining a figure of merit for entanglement within a material is both theoretically and experimentally challenging. At equilibrium, extracting entanglement witnesses from spectroscopies is feasible, but this approach cannot be directly extended out of equilibrium and is incompatible with laser control of materials. Here, we propose a systematic approach to quantify the time-dependent entanglement of transient states of quantum materials through time-resolved resonant inelastic x-ray scattering(trRIXS). We demonstrate the efficiency of our approach using a quarter-filled extended Hubbard model(EHM), and predict light-enhanced quantum entanglement that we attribute to the proximity to a phase boundary. This work sets the stage for experimentally witnessing and controlling entanglement in light-driven quantum materials via solid-state accessible ultrafast spectroscopic measurements.   

Nanostructure variation in hydrogenated voids present amorphous silicon by small angle X-ray scattering: A computational study

The nanostructure variation of hydrogenated voids due to temperature and hydrogen mobility is studied using the Small-Angle X-ray Scattering (SAXS) simulation in a high-quality amorphous silicon model obtained from classical molecular dynamics simulations. Hydrogen mobility at different temperatures is examined based on first-principle density functional theory and nanostructure variation is estimated based on Guinier’s approximations in SAXS patterns, and convex hulls approximation in three-dimensional distribution of bonded and non-bonded hydrogen in silicon matrix. In this study, the nanovoids propagation due to non-bonded hydrogen is also discussed.

    

Engineering Niobium-Germanium Interfaces for Voltage-Tunable Quantum Devices

Voltage-tunable hybrid superconductor-semiconductor Josephson junctions have recently emerged as promising building blocks for low-loss frequency-tunable quantum devices such as qubits, couplers, and magnetic flux sensors. The realization of hybrid devices in group IV semiconductors is of particular interest due to higher scalability and low dielectric loss at microwave frequencies. However, inducing superconductivity in Si and Ge via proximity effect has been proven to be challenging because of large interfacial energy barriers and defect densities. Here, we utilize molecular beam epitaxy to engineer the energy bands at Nb-Ge interfaces. By creating a gradient in Nb:Ga ratio throughout the superconducting layers, we create smooth potential gradients at the interfaces. Various thermal cycling schemes under vacuum and in inert atmospheres are used for tuning the interface structures. Using transmission electron microscopy we determine the competing secondary phases that may form in the stacks. This is complemented by cryogenic magneto-transport measurements on the resulting Nb/Ge heterostructures where critical physical parameters including the induced gap size, the critical magnetic field, and the normal coherence length for the proximitized phases are determined.