Luke G. Marshall is a Ph.D. candidate under the guidance of supervisors John B. Goodenough and Jianshi Zhou. His dissertation research is focused on several families of rare-earth and transition-metal perovskite-related oxides.
These compounds can produce many interesting and unusual properties when they are near the transition from localized to itinerant electronic behavior (in other words, the transition from the electrons being bound to one atom—localized—to the electrons being unbound, and able to move between atoms along “energy bands”). The most basic result of this localized to itinerant electronic transition can be an insulator-metal transition in the material: when the electrons are localized they do not conduct when placed in a circuit so the material is insulating, but when they become itinerant and can move along the band, the material becomes conducting (metallic).
In transition-metal perovskite-related oxides this type of electronic transition is known to produce many interesting and unusual properties such as high-temperature superconductivity in the cuprates and colossal magnetoresistance in the manganites. However, understanding how these physical properties arise requires careful consideration of many different potential drivers such as mixed valencies, structural distortions and transitions, and magnetic interactions between the cation arrays. Because of this complexity, the localized to itinerant electronic crossover remains a challenging problem in solid-state physics.
One goal of Luke’s research is to study this transition in the nickelate RNiO3 (R=Rare-Earth) perovskites, single-valent compounds in which the σ-band is filled and the π-band is ¼-filled. In this system the electron bandwidth can be tuned by substituting different rare-earth cations; as a result, this system provides a unique opportunity to study this crossover more simply.
While the phase diagram for this compound is well known, magnetic rare earth ions (Sm, Nd, etc.) have prevented detailed study of the evolution from Pauli to Curie-Weiss paramagnetism that occurs with the reduction in size of the rare-earth cation. To account for this, Luke has used high-pressure synthesis to create a series of RNiO3 samples using different ratios of non-magnetic rare-earth cations, R=La1-xYx and Y1-yLuy, and studied their magnetic and transport properties. He has also shown that the localized to itinerant crossover can also be explored by substituting Ga3+ for Ni3+ in LaNi1-xGaxO3.