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• W3165152024 abstract "Molecular crystals are an invaluable experimental platform for condensed matter physics because of their highly customisable crystal geometry and ease of synthesis. In particular, they are excellent candidates for the investigation of low-dimensional magnetism. Not only are low-dimensional magnets an active area of research, as they can exhibit exotic quantum phases of matter, but they may also have applications in rapidly emerging quantum technologies such as quantum computation. However, due to the large number of atoms in the unit cell, ab initio studies of these materials are difficult. For this reason, it is common to model molecular crystals using effective Hamiltonians. In these effective theories, a simple lattice model is used, with parameters that can be carefully calculated and benchmarked to ensure the salient features of the material are present. Broadly, this thesis presents several theoretical investigations of molecular crystals that exhibit novel magnetic properties. These analyses are carried out using density functional theory to parametrise effective models. The solutions to these models, given by quantum many-body theory and Tomonaga-Luttinger liquid theory lead us to make experimental predictions.In the first three chapters I introduce and explore the background theory of effective Hamiltonians and one-dimensional-physics (Chapter 1), quasi-one-dimensional theory and the chain random phase approximation (Chapter 2), and density functional theory (Chapter 3).In Chapter 4, we predict that the magnetic properties of [Cu(acac)2], an elastically flexible crystal, change dramatically when the crystal is bent. We find that unbent [Cu(acac)2] is an almost perfect Tomonaga-Luttinger liquid. Broken-symmetry density functional calculations reveal that the magnetic exchange interactions along the chains is an order of magnitude larger than the interchain exchange. The geometrically frustrated interchain interactions cannot magnetically order the material at any experimentally accessible temperature. The ordering temperature (TN), calculated from the chain random phase approximation, increases by 24 orders of magnitude when the material is bent. We demonstrate that geometric frustration both suppresses TN and enhances the sensitivity of TN to bending. In [Cu(acac)2], TN is extremely sensitive to bending, but remains too low for practical applications, even when bent. We show that partially frustrated materials could achieve the balance of high TN and good sensitivity to bending required for practical applications of mechanomagnetic elastic crystals.In Chapter 5, we parametrise the spin model of many charge transfer salts in the EtnMe4-nX[Pd(dmit)2]2 family. We include both the scalene Heisenberg and ring exchange interactions using broken-symmetry density functional theory calculations. In all materials, we find that the strongest exchange coupling is along one crystallographic axis – the dimer stacking direction. We solve our model via the chain random phase approximation (CRPA). In this approach, one starts from the exact form for the one-dimensional magnetic susceptibility of a Heisenberg spin-1/2 chain and treats interchain interactions via the RPA. On an isosceles triangular lattice, the interchain interactions are perfectly frustrated. Within the CRPA, this prevents ordering at any temperature. We find that the anisotropy in the interchain coupling leads to an effective unfrustrated interchain interaction, given by the difference of the interchain couplings.  Then, with the help of our collaborators, we parametrise an extended Hubbard model for EtMe3Sb[Pd(dmit)2]2, a spin liquid candidate. This gives a scalene triangular model where the largest net exchange interaction is three times larger than the mean interchain coupling. The chain random phase approximation  shows that the difference in the interchain couplings is equivalent to a bipartite interchain coupling, favouring long-range magnetic order. This competes with ring exchange, which favours quantum disorder. Ring exchange wins. We predict that the thermal conductivity, κ, along the chain direction is much larger than that along the crystallographic axes and that κ/T → 0 as T → 0 along the crystallographic axes, but that κ/T → a constant > 0 as T → 0 along the chain direction.In Chapter 6, we look at a group of materials with very similar geometries, differing mainly in the separation of molecules along one of the lattice directions. We use density functional theory and Wannier functions to theoretically investigate the mechanism of Heisenberg exchange between Cu2+ ions within the lattice via pyrazine ligands. We parametrise two complimentary tight-binding models and show that the superexchange between the Cu2+ ions is dominated by a through-space interaction between hybrid Cu-pyz orbitals centered on the copper atoms. In addition, we explain why the π-orbitals on the pyrazine ligand do not play a significant role in the superexchange interaction." @default.
• W3165152024 created "2021-06-07" @default.
• W3165152024 creator A5052560513 @default.
• W3165152024 date "2020-10-21" @default.
• W3165152024 modified "2023-09-23" @default.
• W3165152024 title "Derivation of effective low-energy Hamiltonians for chemically complex materials from first principles" @default.
• W3165152024 doi "https://doi.org/10.14264/7ca468a" @default.
• W3165152024 hasPublicationYear "2020" @default.
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