Browsing by Author "Choe, Jesse"

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    Chemical tuning of electrical and magnetic properties in the transition metal dichalcogenides
    (2019-04-19) Choe, Jesse; Morosan, Emilia; Kelly, Kevin; Natelson, Douglas
    Transition metal dichalcogenides are a diverse class of layered materials. Due to their quasi two-dimensional nature, they are a sandbox for investigating low dimensional physics, but can be doped in a variety of ways. Not only can substitutional doping occur on either the transition metal or chalcogen site, but intercalation between the layers can tune the system as well. Here I report on the results of three transition metal dichalcogenide systems with three drastically different results. In the Copper-Platinum-Selenium system, initial results suggest two new superconductors in the ternary phase diagram. Doping platinum into TiSe$_2$ results in an increase in the resistivity of several order of magnitude. Angle-resolved photo-emission spectroscopy shows that Platinum doping induces a pseudo gap in the system. Scanning tunneling microscopy measurements show domain wall formation. Power law fits to the resistivity suggests that the electrical transport is dominated by Luttinger liquid behavior. Finally, the intercalation of Iron into TiS$_2$ gives rise to several magnetic features. Large bowtie magnetoresistance arises showing an increase of up to 40\%. Hysteresis in magnetization shows sharp switching behavior coinciding with the bowtie in magnetoresistance. Ferromagnetic order is seen in conjunction with glassy behavior. These results are compared and contrasted to the results in Fe$_x$TiS$_2$. My results in these systems show that the depth and breadth of physical phenomena in the transition metal dichalcogenide system make it a fascinating system for investigating strongly correlated systems.
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    Magnetization and magnetoresistance in iron intercalated transition metal dichalcogenides
    (2016-04-26) Choe, Jesse; Morosan, Emilia; Kelly, Kevin
    The understanding of magnetism in strongly correlated electronic systems is a vital area of research. Not only is it linked to other phenomena like high temperature superconductivity in the cuprates and iron pnictides, but magnetic materials have been used in electronics since before the computer. As it becomes harder to prop up Moore's law by increasing the density of transistors, mankind must look towards new methods to improve technology or risk stagnation. Research into alternative materials for technology, such as transition metal dichalcogenides, is a promising direction of research to maintain the rate of technological improvement. Our work focuses on the effect of iron intercalation in TiS$_2$. Single crystals of Fe$_x$TiS$_2$ (0 $\le x \le$ 1) were grown using vapor transport. Anisotropic susceptibility and magnetization measurements of the samples were measured, showing ferromagnetism and sharp switching behavior in the magnetization. Finally electrical transport measurements were taken, both with and without field. Measurements of magnetoresistance for $x$ = 0.2 and 0.3 show large magnetoresistance (up to $\sim$ 60\%) and an atypical `bowtie' shape.
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    Metal-to-insulator transition in Pt-doped TiSe 2 driven by emergent network of narrow transport channels
    (Springer Nature, 2021) Lee, Kyungmin; Choe, Jesse; Iaia, Davide; Li, Juqiang; Zhao, Junjing; Shi, Ming; Ma, Junzhang; Yao, Mengyu; Wang, Zhenyu; Huang, Chien-Lung; Ochi, Masayuki; Arita, Ryotaro; Chatterjee, Utpal; Morosan, Emilia; Madhavan, Vidya; Trivedi, Nandini
    Metal-to-insulator transitions (MIT) can be driven by a number of different mechanisms, each resulting in a different type of insulator—Change in chemical potential can induce a transition from a metal to a band insulator; strong correlations can drive a metal into a Mott insulator with an energy gap; an Anderson transition, on the other hand, due to disorder leads to a localized insulator without a gap in the spectrum. Here, we report the discovery of an alternative route for MIT driven by the creation of a network of narrow channels. Transport data on Pt substituted for Ti in 1T-TiSe2 shows a dramatic increase of resistivity by five orders of magnitude for few % of Pt substitution, with a power-law dependence of the temperature-dependent resistivity ρ(T). Our scanning tunneling microscopy data show that Pt induces an irregular network of nanometer-thick domain walls (DWs) of charge density wave (CDW) order, which pull charge carriers out of the bulk and into the DWs. While the CDW domains are gapped, the charges confined to the narrow DWs interact strongly, with pseudogap-like suppression in the local density of states, even when they were weakly interacting in the bulk, and scatter at the DW network interconnects thereby generating the highly resistive state. Angle-resolved photoemission spectroscopy spectra exhibit pseudogap behavior corroborating the spatial coexistence of gapped domains and narrow domain walls with excess charge carriers.