Advanced quantum methods for the largest magnetic molecules
Prof. Dr. Jürgen Schnack (Department of Physics, Bielefeld University, Bielefeld)

We discuss the application of advanced quantum methods to magnetic molecules. These methods are the Density Matrix Renormalization Group method (DMRG), Quantum Monte Carlo (QMC) and the Finite Temperature Lanczos Method (FTLM). DMRG approximates the true quantum states of a spin system by matrix product states and allows for an accurate calculation of low-lying energies as well as spectral functions. This enabled us to deduce the spin Hamiltonian parameters and describe the INS spectra of an Fe₁₈ spin ring with Hilbert space dimension of about 10¹⁴. The magnetic susceptibility of this compound was evaluated using QMC. QMC is also a powerful approximation for large bipartite spin systems, one can for instance determine magnetic observables of spin chains, which we did in order to demonstrate that the spin tube system [(CuCl₂tachH)₃Cl]Cl₂ at low temperatures behaves as an effective spin-3/2 chain.
For spin systems with Hilbert space dimensions smaller than 10¹⁰ one can employ FTLM, which delivers very accurate approximations and is well suited to model e.g. gadolinium compounds used for magnetic cooling applications.