Unravelling the electronic and magnetic properties of quantum materials: from metal-organic frameworks to frustrated magnets

Iliceto, Andrea (2024). Unravelling the electronic and magnetic properties of quantum materials: from metal-organic frameworks to frustrated magnets. University of Birmingham. Ph.D.

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Abstract

Quantum materials are expected to be the future of technology, with emerging IT relying on new materials capable of switchable and stimuli-responsive behaviour. Quantum mechanics governs the physical nature of all materials, able to describe how atoms and electrons behave and interact. At the macroscopic level, quantum effects are not normally visible and the system can be approximated by a classical description, however, in quantum materials, these effects remain noticeable and manifest over a large range of energy and length scales. This work presents a density-functional theory (DFT) study on two promising quantum materials: \(CrCl_2(pym) [pym = pyrimidine]\) and \(Cu_4(OH)_6FBr\), candidate structures to host the S=2 Haldane phase and the quantum spin liquid phase respectively. This research also aims at the discovery of new quantum materials via structure prediction of \(CrCl_2(pydz) [pydz = pyridazine]\).

DFT struggles to account for the strong electron correlations found in quantum materials and, therefore, a Hubbard U correction is needed. Such an approach has been first applied to study the low-dimensional metal-organic magnet \(CrCl_2(pym)\). DFT+U (U = 3 eV) geometry optimisation results found the presence of a Jahn-Teller distortion, confirming a \(Cr^{2+}\) oxidation state with crystal field splitting resulting in S = 2. Band-structure and density of states calculations showed a band gap of 1.2 eV. Despite \(CrCl_2(pym)\) approaching the quasi-one-dimensional antiferromagnet limit with a magnetic exchange ratio \(J_1\)/\(J_2\) = 8.4, the Haldane phase was not confirmed by experimental collaborators, most likely suppressed by superexchange interactions or single ion anisotropy.

Another focus of this work is the investigation of the quantum spin liquid candidate barlowite \([Cu_4(OH)_6FBr]\). Despite barlowite having an adequately small spin of S=\(\frac{1}{2}\) to be a quantum spin liquid, such a quantum phase was not found, although non-collinear DFT calculations confirmed the frustrated magnetic nature of the system. Structural disorder is present in the system; DFT+U (U = 5 eV) geometry optimisations relaxed the system in the \(Pnma\) and \(P6_3/mmc\) space groups. Band-structure calculations found a band gap of 0.56 eV, and a similar insulating nature was found for analogues containing different halides, namely claringbullite \([Cu_4(OH)_6FCl]\) and iodide \([Cu_4(OH)_6FI]\).

Building on the techniques fine tuned in modelling the systems presented above, a search for novel quantum materials was carried out starting from \(CrCl_2(pym)\) in which Cr was exchanged with a series of transition metals (V, Fe, Co, Ni) to create new systems, discover trends and test for their properties. Furthermore, the search was expanded to include a different organic ligand, pyridazine. The novel \(CrCl_2(pydz)\) structure was discovered by carrying out structure searching by means of the Wyckoff Alignment of Molecules (WAM) technique. Magnetic exchange results show that this material is quasi-one-dimensional, with \(J_1\)/\(J_2\) = 16.9. This is a promising result, suggesting that the DFT tested requirements to host the S = 2 Haldane phase are met and this structure is ready to be physically synthesised and experimentally proven.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):