Ab Initio Calculation of Li-Sn System: Unraveling New Phases of Superconducting Materials with Increasing Compression

Stoichiometry, crystal compound, electronic attributes and superconductivity of compressed lithium-tin composites have been thoroughly studied using quantum mechanical genetic algorithm approach and the first principles computations based on density functional theory. Our simulations at moderate pressure (5-20 GPa) predict a complex convex hull diagram, with the following stable Li‑rich phases: I4/mmm-Li6Sn2, -Li7Sn2, -Li5Sn2, Ama2-Li4Sn2, -Li5Sn2, -Li6Sn2, C2/m-Li4Sn1, P21/m-Li6Sn2, -Li7Sn2 and Cmcm-Li4Sn2. Careful examination at their independent elastic parameters reveals sufficient mechanical stability in them. These phases are metallic system, with reasonably high electron concentration near to Fermi level or N(EF) that ranges from 0.6 to 2.4 states/eV cell. It is also interesting for us to observe soft modes and steep-flat energy bands at Fermi levels of Li6Sn2 structures which are stable throughout the pressure range. These features are prerequisites for superconducting behavior. Linear response function with Gaussian and tetrahedron methods reveals satisfactory superconducting transition temperature Tc (3.1 ~ 6.6 K) and Tc (2.1 ~ 2.4 K), respectively. Structural transition results for based elements Li and Sn agree well with literature thus signifying reliable prediction of intermediate phases.

Material Limitations for the Development of High Performance SiC NWFETs

Back-gated field effect transistors (FETs) based on 3C-SiC nanowire (NW) were fabricated and the electrical characterization revealed devices with either ohmic or rectifying contacts leading to two different operation modes. The transistors with ohmic-like contacts manifest very weak gating effect and the device switching off is not achievable even for high negative gate voltages due to the high electron concentration along the nanowires. In contrast, the devices with Schottky contact barrier at Source / Drain regions demonstrate a well determined switching off and in general better performance thanks to the modulation of the drain current through the control of Schottky barriers transparency at the source and drain regions. Nevertheless, ohmic contact devices are expected to demonstrate even better performance if the NW material quality as well as the quality of the interface with the gate oxide is substantially improved.