Research

Research interests

My research interests are centered around the problem of predicting the transport characteristics of complex interacting nano-junctions . I consider different aspects of this problem:

Interaction and interference Transport through systems in which the dwelling time in the junction is large enough or in other terms in which the coupling to the leads is not so strong are strongly affected by electron-electron interaction. In these conditions a mean field approach to the problem is not justified. A very reach physics belongs to this regime, from the Coulomb blockade to the Kondo effect, experimentally accessible also at the nano scale. On the other hand, interference characterizes fundamental discoveries in mesoscopic physics, like the solid state realization of the Ahronov-Bohm effect, or the weak localization. More recently intramolecular interference have also attracted increasing attention. In recent works on benzene and triple dot junctions I have already proven that Coulomb blockade and interference can coexist in what we named an interference single electron transistor (ISET). Under the necessary condition of the presence of quasi-degeneracies, Coulomb interaction even favours interference in ISETs.

Orbital complexity The transport characteristics of nano-junctions depend on their energy spectrum but also on the spatial distribution and spin arrangement of their many-body energy eigenstates i.e. on their orbital complexity. Particularly in the field of molecular electronics, the tendency is to cope with this complexity either by treating the system ab initio but at the price of relegating the interaction to more or less sophisticated effective single particle approaches, or to study the fully interacting dynamics, also non-perturbative in the coupling to the leads, but assuming over-simplified models that sometimes loose the specific character of the junction. In the study of the benzene interference SET we adopted an intermediate strategy: we described the system via a semi-empirical interacting Hamiltonian, proceeded to its exact diagonalization (Configuration Interaction method) and finally set up the transport calculation in the eigenbasis of the isolated molecule. Due to the relatively high complexity the problem (4096 states) we used a group theoretical approach that simplified the diagonalization procedure but above all allowed to classify the eigenstates according to their symmetry. Most of the transport features could indeed be related to the symmetry of the many-body states.

Current transfer The tunnelling processes in a nano-junction can be confined to a very narrow region, virtually a point. Good examples are the tunnelling between an STM tip and the substrate or an ideal break junction with a molecule or a atomic chain stretched between the leads. In other cases the tunnelling area is relatively extended compared to the system size. Examples are the substrate-molecule interface in coated STM molecular junctions or in carbon nanotubes transistors where typically a considerable fraction of the tube is embedded into the lead. To my knowledge little is known about the role played in the tunnelling process, and consequently in the current voltage characteristics, by momentum or angular momentum conservation. In contrast, the accent has been always put only on energy conservation and the consequent classification of transport as due to in elastic or inelastic processes.

Nano-electromechanics Progress in the fabrication of nano-junctions has opened in recent years the access to the mechanical degrees of freedom. Suspended carbon nanotubes, molecular junctions, as well as free standing silicon based devices are just some examples of systems in which the presence of movable parts change qualitatively the transport characteristics of the nano-junction. Since my Ph.D. thesis I am interested in nanoelectromechanical systems (NEMS). I investigated the various transport regimes in which a quantum shuttle (oscillating quantum dot) can operate and looked for signatures into the current and current noise. More recently I also studied the dynamical symmetry breaking realized by in the non-linear transport through a molecular junction with equivalent mechanical configurations. At present I am working on the effect of phonons in the transport through suspended carbon nanotubes.

Spintronics The spin degree of freedom plays an important role in nano-junctions due to the ferromagnetic leads and/or to the exchange interaction in the system itself. I studied spin effects in molecular and double or triple quantum dot junctions. The interplay of leads polarization with interference effects allows to achieve in these systems all electrical spin control of the nanojunction. At present, I am focusing on transport through metal phthalocyanines in the STM setup.

Interaction and interference	Transport through systems in which the dwelling time in the junction is large enough or in other terms in which the coupling to the leads is not so strong are strongly affected by electron-electron interaction. In these conditions a mean field approach to the problem is not justified. A very reach physics belongs to this regime, from the Coulomb blockade to the Kondo effect, experimentally accessible also at the nano scale. On the other hand, interference characterizes fundamental discoveries in mesoscopic physics, like the solid state realization of the Ahronov-Bohm effect, or the weak localization. More recently intramolecular interference have also attracted increasing attention. In recent works on benzene and triple dot junctions I have already proven that Coulomb blockade and interference can coexist in what we named an interference single electron transistor (ISET). Under the necessary condition of the presence of quasi-degeneracies, Coulomb interaction even favours interference in ISETs.
Orbital complexity	The transport characteristics of nano-junctions depend on their energy spectrum but also on the spatial distribution and spin arrangement of their many-body energy eigenstates i.e. on their orbital complexity. Particularly in the field of molecular electronics, the tendency is to cope with this complexity either by treating the system ab initio but at the price of relegating the interaction to more or less sophisticated effective single particle approaches, or to study the fully interacting dynamics, also non-perturbative in the coupling to the leads, but assuming over-simplified models that sometimes loose the specific character of the junction. In the study of the benzene interference SET we adopted an intermediate strategy: we described the system via a semi-empirical interacting Hamiltonian, proceeded to its exact diagonalization (Configuration Interaction method) and finally set up the transport calculation in the eigenbasis of the isolated molecule. Due to the relatively high complexity the problem (4096 states) we used a group theoretical approach that simplified the diagonalization procedure but above all allowed to classify the eigenstates according to their symmetry. Most of the transport features could indeed be related to the symmetry of the many-body states.
Current transfer	The tunnelling processes in a nano-junction can be confined to a very narrow region, virtually a point. Good examples are the tunnelling between an STM tip and the substrate or an ideal break junction with a molecule or a atomic chain stretched between the leads. In other cases the tunnelling area is relatively extended compared to the system size. Examples are the substrate-molecule interface in coated STM molecular junctions or in carbon nanotubes transistors where typically a considerable fraction of the tube is embedded into the lead. To my knowledge little is known about the role played in the tunnelling process, and consequently in the current voltage characteristics, by momentum or angular momentum conservation. In contrast, the accent has been always put only on energy conservation and the consequent classification of transport as due to in elastic or inelastic processes.
Nano-electromechanics	Progress in the fabrication of nano-junctions has opened in recent years the access to the mechanical degrees of freedom. Suspended carbon nanotubes, molecular junctions, as well as free standing silicon based devices are just some examples of systems in which the presence of movable parts change qualitatively the transport characteristics of the nano-junction. Since my Ph.D. thesis I am interested in nanoelectromechanical systems (NEMS). I investigated the various transport regimes in which a quantum shuttle (oscillating quantum dot) can operate and looked for signatures into the current and current noise. More recently I also studied the dynamical symmetry breaking realized by in the non-linear transport through a molecular junction with equivalent mechanical configurations. At present I am working on the effect of phonons in the transport through suspended carbon nanotubes.
Spintronics	The spin degree of freedom plays an important role in nano-junctions due to the ferromagnetic leads and/or to the exchange interaction in the system itself. I studied spin effects in molecular and double or triple quantum dot junctions. The interplay of leads polarization with interference effects allows to achieve in these systems all electrical spin control of the nanojunction. At present, I am focusing on transport through metal phthalocyanines in the STM setup.