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Dipartimento di Fisica e Scienze della Terra

Theoretical approaches

Many-body models from ab-initio calculations

 A new ab-initio scheme for molecular nanomagnets has been developed and is now extensively used. The strong correlation effects which characterize the magnetic d-electrons are not included by the usual mean-field approaches, but are explicitly accounted for within a generalized Hubbard model, constructed using localized Boys orbitals. This method works very well for prototypical molecular nanomagnets, and is also succesfully applied to complex supramolecular systems of interest for quantum-information processing. For these such high level calculations are essential as we need understand terms as small as 10 micro-eV, which represents a typical qubit interaction strenght in these systems.

For details see A. Chiesa, S. Carretta, P. Santini, G. Amoretti, and E. Pavarini, Many-Body Models for Molecular Nanomagnets, Phys. Rev. Lett. 110, 157204 (2013)


Large-scale calculations for quantum Hamiltonians

We have developed state-of-the-art parallel codes for simulating the behavior of quantum systems described by large Hilbert spaces. The associated static and time-dependent behavior is obtained by numerical simulations of large matrices representing Hamiltonians, rate matrices and time-evolution operators. In particular, zero- and finite-temperature Lanczos codes are used to simulate the behavior of molecular nanomagnets with large number of magnetic ions (see, e.g., E. Garlatti, S. Carretta, J. Schnack, G. Amoretti, P. Santini, Theoretical design of molecular nanomagnets for magnetic refrigeration,i Applied Physics Letters 103, 202410 (2013).  

The relaxation of such systems in the phonon bath and the associated effects in NMR are obtained by solving large systems of master-equations (see, e.g., P. Santini, S. Carretta, E. Liviotti, G. Amoretti, P. Carretta, M. Filibian, A. Lascialfari, E. Micotti, NMR as a Probe of the Relaxation of the Magnetization in Magnetic Molecules, Phys. Rev. Lett. 94, 077203 (2005)..

High-performance codes have also been developed for simulating the time-evolution of qubit systems in quantum.computation setups based on molecular nanomagnets (see. e,g, P. Santini, S. Carretta, F. Troiani, and G. Amoretti, Molecular Nanomagnets as Quantum Simulators, Phys. Rev. Lett. 107, 230502 (2011)) or photons in circuits of superconducting resonators (see, e.g., S. Carretta, A. Chiesa, F. Troiani, D. Gerace, G. Amoretti, and P. Santini, Quantum Information Processing with Hybrid Spin-Photon Qubit Encoding, Phys. Rev. Lett. 111, 110501 (2013)).

DFT calculations for the ab-initio assignment of the muon implantation site

Muon Spin Rotation Spectroscopy (µSR) is a powerful technique to investigate superconductors and magnetic materirials. Muons are implanted in interstitial sites and, whenever there are sizeable internal magnetic fields at these sites, the µSR experiment yields their distribution. Such is typically the case of a type II  superconductors with a votex lattice, and of a magnetically ordered compound. In the second case the internal fields may be used to extract the magnetic moments  of the ions if the implantation site is known.

Recently we learned how to determine a-priori the implantation site from DFT calculations that include the muon, simulated as a light hydrogen isotope. For example in Phys. Rev. B  87 064401 we use the site stability for LaCoPO to understand the changes that this material, isostructural to a high Tc iron superconductor, undergoes with pressure. Left the grey volumes represent the isoenergy surfaces for a muon for three relative minima, A, B and C, in a unit cell of LaCoPO.  The energy of these isosurfaces increases from left to right an when it reaches the value appropriate for site C (right) a path opens to the A stabler site, demonstrating that C is unstable.