Publications

Monoclinic NaNiO[subscript 2] is re-investigated as a positive electrode material for sodium ion batteries. We observe reversible Na extraction and insertion of about 120 mAh/g when NaNiO[subscript 2] is cycled between 1.25 V and 3.75 V, though discharge only proceeds to Na0.91NiO[subscript 2]. The voltage profile shows large and pronounced steps, which are fully reversible. Charging to 4.5 V leads to higher charge and discharge capacity, though with poor coulombic efficiency and rapid capacity fade. X-ray diffraction of charged samples indicates that high voltage charging leads to the formation of an inactive phase.

Nanobelts, nanoribbons and other quasi-one-dimensional nanostructures formed from layered, so-called, van der Waals semiconductors have garnered much attention due to their high-performance, tunable optoelectronic properties. For layered alloys made from the gallium monochalcogenides GaS, GaSe, and GaTe, near-continuous tuning of the energy bandgap across the full composition range has been achieved in GaSe1-xSx and GaSe1-xTex alloys. Gold-catalyzed vapor-liquid-solid (VLS) growth of these alloys yields predominantly nanobelts, nanoribbons and other nanostructures for which the fast crystal growth front consists of layer edges in contact with the catalyst. We demonstrate that in the S-rich, GaS1-xTex system, unlike GaSe1-xSx and GaSe1-xTex, the Au-catalyzed VLS process yields van der Waals nanowires for which the fast growth direction is normal to the layers. The high mismatch between S and Te leads to extraordinary bowing of the GaS1-xTex alloy's energy bandgap, decreasing by at least 0.6 eV for x as small as 0.03. Calculations using density functional theory confirm the significant decrease in bandgap in S-rich GaS1-xTex. The nanowires can exceed fifty micrometers in length, consisting of tens of thousands of van der Waals-bonded layers with triangular or hexagonal cross-sections of uniform dimensions along the length of the nanowire. We propose that the low solubility of Te in GaS results in an enhancement in Te coverage around the Au catalyst-nanowire interface, confining the catalyst to the chalcogen-terminated basal plane (rather than the edges) and thereby enabling layer-by-layer, c-axis growth.

Abstract Binary evolution theory predicts that the second common envelope (CE) ejection can produce low-mass (0.32-0.36 M⊙) subdwarf~B (sdB) stars inside ultrashort-orbital-period binary systems as their helium cores are ignited under nondegenerate conditions. With the orbital decay driven by radiation of gravitational waves, the minimum orbital period that an sdB binary formed in this channel can reach before the Roche-lobe overflow is ~20 minutes. However, only four sdB binaries with orbital periods below an hour have been reported so far, while none of them has an orbital period approaching the above theoretical limit. This leads to an unclear role of the second CE ejection in forming ultracompact sdB binaries and a blocked link between detached helium-burning-star binaries and gravitational-wave sources, AM CVn stars. Here we report the discovery of a 20.5-minute-orbital-period ellipsoidal binary, TMTS J052610.43+593445.1, in which the visible helium-burning sdB star is being tidally deformed by an invisible carbon-oxygen white dwarf (WD) companion. This sdB star is inferred to have a mass of ~0.33 M⊙, approaching that of the helium-ignition limit. Owing to the low-mass sdB star, WD companion, and extremely short orbital period, this system provides the first clear case to justify the formation of sdB binaries through the nondegenerate channel of second CE ejection. In particular, the radius of this low-mass sdB star is only 0.066 R⊙, about seven Earth radii, representing the most compact nondegenerate star ever known. As the shortest-orbital-period single-degenerate detached binary discovered up to date, TMTS J052610.43+593445.1 provides a crucial bridge to map the binary evolution scheme from the second CE ejection to the formation of AM CVn stars having a helium-star donor, and will serve as a crucial verification binary of space-borne gravitational-wave detectors in the future.

Fuzzy cellular neural networks (FCNN) are novel classes of cellular neural networks. In this paper, the basic theory of FCNN is presented. FCNN is a generalization of CNN by using fuzzy operations in the synaptic law calculation allowing us to combine the low level information processing capability of CNNs with the high level information processing capability, such as image understanding, of fuzzy systems. The FCNN structures are based on the uncertainties in human cognitive processes and in modeling neural systems, and provide an interface between the human expert and the classical CNN.