Strongly correlated molecular magnetism in triangular-lattice cluster materials

Solid state chemistry;Condensed matter physics;Materials Science;Geometric magnetic frustration;Emergent behavior;Valence bond phases;Chemistry

Sheckelton, John PatrickBowen, Kit H. ;

Johns Hopkins University

关键词: Solid state chemistry;    Condensed matter physics;    Materials Science;    Geometric magnetic frustration;    Emergent behavior;    Valence bond phases;    Chemistry;   

Others  :  https://jscholarship.library.jhu.edu/bitstream/handle/1774.2/39523/SHECKELTON_DISSERTATION_FINAL.tex?sequence=3&isAllowed=y

瑞士|英语

来源: JOHNS HOPKINS DSpace Repository

【 摘 要 】

When valence shell electrons are unpaired, the electron’s intrinsic angular momentum, its spin, can give rise to interesting and useful magnetic states of matter. A large ensemble of unpaired electrons in a solid that strongly interact can yield emergent phenomena, or materials properties that appear greater than the sum of their parts. This dissertation describes the synthesis and thorough characterization of a novel class of magnetic materials at the intersection of molecular and non-molecular solids, whereby small transition metal clusters behave as molecules in extended solids and harbor one unpaired electron per cluster. Antiferromagnetic exchange interactions between cluster spins give rise to novel magnetic ground states, while the utilization of bonding within clusters affords energetic stability to minimize defects. The material LiZn2Mo3O8 is composed of triangular Mo3O13 clusters, each with one unpaired electron (S = 1/2) per cluster. The clusters themselves are arranged on a triangular lattice in two-dimensional layers separated by non-magnetic cation interlayers. Since all of the antiferromagnetic interactions between clusters cannot be simultaneously satisfied, a geometrically frustrated magnetic state results. In LiZn2Mo3O8, no long-range magnetic order is observed down to T = 0.01 K. Upon cooling below T ≈ 100 K, measurements indicate a condensed valence-bond state is formed, in which two-thirds of the high-temperature paramagnetic spins no longer contribute to the magnetic susceptibility and instead possibly form a resonating, itinerant spin-liquid state. This is similar to the resonating valence-bond state proposed by Phillip Anderson to be the parent magnetic ground state to high temperature superconductivity, which becomes superconducting upon charge doping. To test this, the doping of LiZn2Mo3O8 is achieved via chimie douce deintercalation of Zn2+ to preserve the parent structure of the Mo3O8 magnetic layers. No superconductivity is observed; however LiZn2−xMo3O8 is a rare 2D triangular lattice frustrated magnet amenable to the full range of charge doping. Lastly, the analogous material Nb3Cl8 is described. Here, a triangular lattice of S = 1/2 Nb3Cl13 clusters is held together in layers by van der Waals interactions. Upon cooling below T ≈ 90 K, the magnetic signal completely vanishes due to a structural distortion concomitant with the formation of a valence-bond solid between adjacent layers. This material, in contrast to LiZn2Mo3O8, avoids a frustrated magnetic state by multi-site interactions of magnetic clusters to form spin singlets. Nb3Cl8 thus demonstrates the instability of frustrated magnets toward charge order and chemical bond formation. In other words, this system forms static chemical bonds over, for example, resonating valence bonds. These cluster-magnet-based materials allow unparalleled structural and electronic control while offering a new strategy in the rational design of quantum states of matter.;;