Extraordinary Electronic Properties in Uncommon Structure Types

by Mazhar Nawaz Ali

Institution: Princeton University
Department: Chemistry
Degree: PhD
Year: 2015
Keywords: centrosymmetric; Chalcogenide; Dirac; Magnetoresistance; Superconductivity; WTe2; Inorganic chemistry; Condensed matter physics; Chemistry
Record ID: 2058452
Full text PDF: http://arks.princeton.edu/ark:/88435/dsp01bv73c266h


In this thesis I present the results of explorations into several uncommon structure types. In Chapter 1 I go through the underlying idea of how we search for new compounds with exotic properties in solid state chemistry. The ideas of exploring uncommon structure types, building up from the simple to the complex, using chemical intuition and thinking by analogy are discussed. Also, the history and basic concepts of superconductivity, Dirac semimetals, and magnetoresistance are briefly reviewed. In chapter 2, the 1s-InTaS2 structural family is introduced along with the discovery of a new member of the family, Ag0:79VS2; the synthesis, structure, and physical properties of two different polymorphs of the material are detailed. Also in this chapter, we report the observation of superconductivity in another 1s structure, PbTaSe2. This material is especially interesting due to it being very heavy (resulting in very strong spin orbit coulping (SOC)), layered, and noncentrosymmetric. Electronic structure calculations reveal the presence of a bulk 3D Dirac cone (very similar to graphene) that is gapped by SOC originating from the hexagonal Pb layer. In Chapter 3 we show the re-investigation of the crystal structure of the 3D Dirac semimetal, Cd3As2. It is found to be centrosymmetric, rather than noncentrosymmetric, and as such all bands are spin degenerate and there is a 4-fold degenerate bulk Dirac point at the Fermi level, making Cd3As2 a 3D electronic analog to graphene. Also, for the first time, scanning tunneling microscopy experiments identify a 2x2 surface reconstruction in what we identify as the (112) cleavage plane of single crystals; needle crystals grow with a [110] long axis direction. Lastly, in chapter 4 we report the discovery of \titanic" (sadly dubbed \large, nonsaturating" by Nature editors and given the acronym XMR) magnetoresistance (MR) in the non-magnetic, noncentrosymmetric, layered transition metal dichalcogenide WTe2; over 13 million% at 0.53 K in a magnetic field of 60 Tesla (the largest positive magnetoresistance ever reported). Unlike what is seen in other known materials, there is no saturation of the magnetoresistance value even at very high applied fields. In semimetals, very high MR may attributed to a balanced hole-electron "resonance" condition; as described here, WTe2 appears to be the first known material where this resonance is nearly perfect.