Remarkable deviations from the normal Fermi liquid behavior have been systematically detected in different d or f -electrons compounds. A widely accepted mechanism for such non-Fermi liquid (NFL) behavior is the existence of a coupling between itinerant electronic degrees of freedom and critical long- wavelength magnetic fluctuations, in proximity of a quantum phase transition . In order to test the local origin of “bad metallic” or NFL states, we investigated the coherence properties of the metallic state near the MIT in the periodic Anderson model, i.e. a generic model for HF compounds. We pointed out the existence of a NFL behaviour in this system associated to an extreme fragility with respect to temperature fluctuations.
In agreement with the quantum criticality scenario, this novel NFL state is located in the neighbourhood of a quantum phase transition, but unlike the standard scenario, the relevant quantum transition here is a MIT. This work demonstrates that the coupling to long-wavelength magnetic fluctuations is not a prerequisite for the realization of a NFL scenario but local temporal magnetic fluctuations alone can provide sufficient scattering to produce an incoherent metallic state. We iden- tified the origin of such large local magnetic fluctuations as due to the competition of different (FM-AFM) magnetic correlations at small and large doping.
In a more recent work we studied the formation of paramagnetic excitations (i.e. para-magnons) which characterize the screening process in doped charge-transfer insu- lators, e.g. Cuprates superconductors. In particular, we investigated at a model level the modifications in the spin excitation spectrum in such systems, linking them to the coupling to ligand Oxygen bands. We have shown that the coupling to conduction bands leads to a suppression of the paramagnons ex- citations by reduction of the electronic states available for screening. These results are revelant for the latest RIXS measurements in Cuprates.
Optically trapped ultra-cold fermions
The recent development of experimental methods for the optically trapped ultra-cold atoms attracted a great interest in condensed matter physics. The possibility of investigating artificially created systems of few atoms provides an essential insight to anomalous properties of real correlated materials, such as unconventional superconductors.
In this work Dr. A.Privitera, Prof.M.Capone and me studied the BCS to Bose-Einstein Condensate (BEC) crossover of a model of optically trapped two-component fermions with attractive interaction. This study revealed that a direct experimental observation of the BCS-BEC crossover can not be achieved in presence of the trapping potential. We demon- strated that the combined action of the harmonic trapping poten- tial and the large attraction leads to a collapse of the fermionic cloud, which prevents from reaching the BEC regime. We proposed and verified an alternative, experimentally feasible, protocol to disentangle the effects of the trapping potential and finally reveal the BCS-BEC crossover in this inhomogeneous system.
Correlation and magnetism in nano-systems
In collaboration with Dr.A.Valli and Prof.M.Capone, we studied the magnetic properties of zig-zag graphene nano-flakes (ZGNF) using in-homogeneous dynamical mean-field theory, a non-perturbative approach which can efficiently treat the effect of strong correlation in inhomogeneous systems. At half-filling and for realistic values of the local interaction, we show that the ZGNF is in a fully compensated antiferromagnetic (AF) state, which is found to be robust against temperature fluctuations. Introducing charge carriers in the AF back- ground drives the ZGNF metallic and stabilises a magnetic state with a net un- compensated moment at low temperature. We ascribe the change in magnetism to the delocalization of the doped holes in the proximity of the edges, which mediate ferromagnetic correlations between the localized magnetic moments. Depending on the hole concentration, the magnetic transition may display a pronounced hysteresis over a wide range of temperature, indicating the coexistence of magnetic states with different symmetry. This suggests the possibility of achieving the electrostatic control of the magnetic state of ZGNFs to realize a switchable spintronic device. The results obtained in this study have been published in Physical Review B and paved the way to a further study.
In collaboration with Dr.C.Weber we introduced a new methodology to solve the effective Anderson impurity model in the context of dynamical mean-field theory, based on the exact diagonalization method. We propose a strategy to effectively refine the exact diagonalization solver by combining a finite-temperature Lanczos algorithm with an adapted version of the cluster perturbation theory. We show that the augmented diagonalization yields an improved accuracy in the description of the spectral function of the single-band Hubbard model and is a reliable approach for a full d-orbital manifold calculation.