RESUMEN
The Cu(ii) ions in [(C2H5)3NH]2Cu2(C2O4)3 form a hyperhoneycomb lattice and show no indication of long-range magnetic order down to 60 mK. It has therefore been suggested that [(C2H5)3NH]2Cu2(C2O4)3 is a three dimensional quantum spin liquid. We construct a tight-binding model of [(C2H5)3NH]2Cu2(C2O4)3 from Wannier orbital overlaps. Including interactions within the Jahn-Teller distorted Cu-centered eg Wannier orbitals leads to a highly anisotropic effective Heisenberg model. We show that this anisotropy arrises from interference between different superexchange pathways. This demonstrates that when two (or more) orbitals contribute to the localised spin superexchange can be significantly richer than in the textbook single orbital case. The hyper-honeycomb lattice contains two symmetry distinct sublattices of Cu atoms arranged in coupled chains. We show that one sublattice is strongly dimerized, the other forms isotropic antiferromagnetic chains. Integrating out the strongest (intradimer) exchange interactions leaves extremely weakly coupled Heisenberg chains.
RESUMEN
We predict that the magnetic properties of [Cu(acac)2 ], an elastically flexible crystal, change drastically when the crystal is bent. It is found 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 are 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. Partially frustrated materials could achieve the balance of high TN and good sensitivity to bending required for practical applications of mechanomagnetic elastic crystals.
