TUNING EXOTIC PROPERTIES IN LAYERED MATERIALS THROUGH DOPING AND INTERCALATION

Abstract

The layered transition metal dichalcogenides are a family of compounds rich in exotic properties. Research on some of these properties, such as large, positive magnetoresistance, Quantum Spin Hall effect, and superconductivity, is commonly driven by incomplete knowledge concerning them and materials in which they are found, as well as their unfulfilled technological potential. In this work, the three doped or intercalated LTMD systems, W\(_{1-􀀀x}\)M\(_{􀀀x}\)Te\(_{2}\) (M = Mo, Ta, Re; x < 0.03), WTe\(_{2-x}\)Se\(_{x}\) (0 < x< 2), and M\(_{x}\)TaS\(_{2}\) (M = Ga, In; x < 0.1), are synthesized and their properties of interest are characterized as a function of composition. In WTe\(_{2}\), isoelectronic doping with Mo is observed to reduce the magnetoresistance gradually with an apparent linear trend, whereas aliovalent doping with either Ta or Re results in a more rapid suppression with a seemingly higher order decay. This apparent sensitivity of the magnetoresistive effect to aliovalent dopants over the simple disorder caused by isoelectronic doping provides experimental evidence for a recently proposed hypothesis that large, positive magnetoresistance in WTe\(_{2}\) arises from interactions between near perfectly balanced hole and electron populations. In the WTe\(_{2-x}\)Se\(_{x}\) system the variation of structural properties and resistivity trends is investigated, and the discovery of an unreported structure type between (0.12 < x < 0.23) which varies from semimetal to small band-gap semiconductor over this composition range is reported. These results suggest that creating a semiconducting tungsten dichalcogenide that shares key features of the WTe\(_{2}\) structure may be achieved through doping. The implications of these findings for the possiblity of producing a Quantum Spin Hall insulator, based on the predictions of recent theoretical studies, in this and related systems are discussed. For M\(_{x}\)TaS\(_{2}\) the effects of intercalation on superconductivity in charge-density wave expressing polytypes are studied. In the 2H polytype, a sample exhibiting an anomalously high H\(_{c2}\) value relative to its T\(_{c}\) is observed with In as an intercalate but not with Ga. For the 1T polytype, progressive In intercalation produces samples in which a Mott state gives way to superconductivity coexisting with a large charge-density wave feature. Such behaviors potentially suggest that these materials can provide valuable information concerning either the mechanism of interaction between different correlated electronic states or unconventional superconductivity. Plausible alternative explanations are also considered and several avenues for future verification are suggested.