Quantum pumping, in its different forms, is attracting attention from different fields, from fund... more Quantum pumping, in its different forms, is attracting attention from different fields, from fundamental quantum mechanics, to nanotechnology, to superconductivity. We investigate the crossover of quantum pumping from the adiabatic to the antiadiabatic regime in the presence of dissipation, and find general and explicit analytical expressions for the pumped current in a minimal model describing a system with the topology of a ring forced by a periodic modulation of frequency ω. The solution allows following in a transparent way the evolution of pumped dc current from much smaller to much larger ω values than the other relevant energy scale, the energy splitting introduced by the modulation. We find and characterize a temperature-dependent optimal value of the frequency for which the pumped current is maximal.
Ab-initio calculations within Density Functional Theory combined with experimental Raman spectra ... more Ab-initio calculations within Density Functional Theory combined with experimental Raman spectra on cluster-beam deposited pure carbon films provide a consistent picture of sp-carbon chains stabilized by sp 3 or sp 2 terminations, the latter being sensitive to torsional strain. This unexplored effect promises many exciting applications since it allows one to modify the conductive states near the Fermi level and to switch on and off the on-chain π-electron magnetism.
The calculation of self-energy corrections to the electron bands of a metal requires the evaluati... more The calculation of self-energy corrections to the electron bands of a metal requires the evaluation of the intraband contribution to the polarizability in the small- q limit. When neglected, as in standard GW codes for semiconductors and insulators, a spurious gap opens at the Fermi energy. Systematic methods to include intraband contributions to the polarizability exist, but require a computationally
Using a tight-binding atomistic simulation, we simulate the recent atomic-force microscopy experi... more Using a tight-binding atomistic simulation, we simulate the recent atomic-force microscopy experiments probing the slipperiness of graphene flakes made slide against a graphite surface. Compared to previous theoretical models, where the flake was assumed to be geometrically perfect and rigid, while the substrate is represented by a static periodic potential, our fully-atomistic model includes quantum mechanics with the chemistry of bond breaking and bond formation, and the flexibility of the flake. These realistic features, include in particular the crucial role of the flake rotation in determining the static friction, in qualitative agreement with experimental observations.
We investigate the adiabatic evolution of a set of non-degenerate eigenstates of a parameterized ... more We investigate the adiabatic evolution of a set of non-degenerate eigenstates of a parameterized Hamiltonian. Their relative phase change can be related to geometric measurable quantities that extend the familiar concept of Berry phase to the evolution of more than one state. We present several physical systems where these concepts can be applied, including an experiment on microwave cavities for which off-diagonal phases can be determined from published data.
We perform a systematic investigation of the resonance and vibrational properties of naphthyl-ter... more We perform a systematic investigation of the resonance and vibrational properties of naphthyl-terminated sp carbon chains (dinaphthylpolyynes) by combined multi-wavelength resonant Raman (MWRR) spectroscopy, ultraviolet-visible spectroscopy, and Fourier-transform infrared (FT-IR) spectroscopy, plus ab initio density functional theory (DFT) calculations. We show that the MWWR and FT-IR spectroscopies are particularly suited to identify chains of different lengths and different terminations, respectively. By DFT calculations, we further extend those findings to sp carbon chains end-capped by other organic structures. The present analysis shows that combined MWRR and FT-IR provide a powerful tool to draw a complete picture of chemically stabilized sp carbon chains.
The many-body dynamics of interacting electrons in condensed matter and quantum chemistry is ofte... more The many-body dynamics of interacting electrons in condensed matter and quantum chemistry is often studied at the quasiparticle level, where the perturbative diagrammatic series is partially resummed. Based on Hedin's equations for self-energy, polarization, propagator, effective potential, and vertex function, dressed (skeleton) Feynman diagrams are enumerated. Such diagram counts provide useful simple checks for extensions of the theory for future realistic simulations.
The many-body dynamics of interacting electrons in condensed matter and quantum chemistry is ofte... more The many-body dynamics of interacting electrons in condensed matter and quantum chemistry is often studied at the quasiparticle level, where the perturbative diagrammatic series is partially resummed. Based on Hedin's equations for self-energy, polarization, propagator, effective potential, and vertex function, dressed (skeleton) Feynman diagrams are enumerated. Such diagram counts provide useful simple checks for extensions of the theory for future realistic simulations.
We investigate the adiabatic evolution of a set of non-degenerate eigenstates of a parameterized ... more We investigate the adiabatic evolution of a set of non-degenerate eigenstates of a parameterized Hamiltonian. Their relative phase change can be related to geometric measurable quantities that extend the familiar concept of Berry phase to the evolution of more than one state. We present several physical systems where these concepts can be applied, including an experiment on microwave cavities for which off-diagonal phases can be determined from published data.
Polynomial approximations around the global minimum of the adiabatic potential energy surface of ... more Polynomial approximations around the global minimum of the adiabatic potential energy surface of several polyatomic molecules display an unphysical saddle point of comparatively small energy, leading to a region where the potential is negative and unbounded. This poses an energy upper limit for a reliable evaluation of the vibrational levels. We argue that the presence of such saddle points is
We report a surprising hysteretic behavior in the dynamics of a simple one-dimensional nonlinear ... more We report a surprising hysteretic behavior in the dynamics of a simple one-dimensional nonlinear model inspired by the tribological problem of two sliding surfaces with a thin solid lubricant layer in between. In particular, we consider the frictional dynamics of a harmonic chain confined between two rigid incommensurate substrates which slide with a fixed relative velocity. This system was previously found, by explicit solution of the equations of motion, to possess plateaus in parameter space exhibiting a remarkable quantization of the chain center-of-mass velocity (dynamic pinning) solely determined by the interface incommensurability. Starting now from this quantized sliding state, in the underdamped regime of motion and in analogy to what ordinarily happens for static friction, the dynamics exhibits a large hysteresis under the action of an additional external driving force F ext . A critical threshold value F c of the adiabatically applied force F ext is required in order to alter the robust dynamics of the plateau attractor. When the applied force is decreased and removed, the system can jump to intermediate sliding regimes (a sort of "dynamic" stick-slip motion) and eventually returns to the quantized sliding state at a much lower value of F ext . On the contrary no hysteretic behavior is observed as a function of the external driving velocity.
Neural networks simulations have always been a complex computational challenge because of the req... more Neural networks simulations have always been a complex computational challenge because of the requirements of large amount of computational and memory resources. Due to the nature of the problem, a high performance computing approach becomes vital, because the dynamics often involves the update of a large network for a large number of time steps. Moreover, the parameter space can be fairly large. An advanced optimization for the single time step is therefore necessary, as well as a strategy to explore the parameter space in an automatic fashion.
We study the frictional sliding of two ideally incommensurate surfaces with a third incommensurat... more We study the frictional sliding of two ideally incommensurate surfaces with a third incommensurate sheet -a sort of extended lubricant -in between. When the mutual ratios of the three periodicities in this sandwich geometry are chosen to be the golden mean φ = (1 + √ 5)/2, this system is believed to be statically pinned for any choice of system parameters. In the present study we overcome this pinning and force the two "substrates" to slide with a mutual velocity V ext , analyzing the resulting frictional dynamics. An unexpected feature is an asymmetry of the relative sliding velocity of the intermediate lubricating sheet relative to the two substrates. Strikingly, the velocity asymmetry takes an exactly quantized value which is uniquely determined by the incommensurability ratio, and absolutely insensitive to all other parameters. The reason for quantization of the velocity asymmetry will be addressed. This behavior is compared and contrasted to the corresponding one obtained for a representative cubic irrational, the spiral mean ω.
Strain-engineering in SiGe nanostructures is fundamental for the design of optoelectronic devices... more Strain-engineering in SiGe nanostructures is fundamental for the design of optoelectronic devices at the nanoscale. Here we explore a new strategy, where SiGe structures are laterally confined by the Si substrate, to obtain high tensile strain avoiding the use of external stressors, and thus improving the scalability. Spectro-microscopy techniques, finite element method simulations and ab initio
With the help of a simple two-dimensional model we simulate the tribological properties of a thin... more With the help of a simple two-dimensional model we simulate the tribological properties of a thin lubricant film consisting of linear (chain) molecules in the ordinary soft-lubricant regime. We find that friction generally increases with chain length, in agreement with their larger bulk viscosity. When comparing the tribological properties of molecules which stick bodily to the substrates with others carrying a single sticking termination, we find that the latter generally yield a larger friction than the former.
In the atomic force microscope, the nanoscale force topography of even complex surface superstruc... more In the atomic force microscope, the nanoscale force topography of even complex surface superstructures is extracted by the changing vibration frequency of a scanning tip. An alternative dissipation topography with similar or even better contrast has been demonstrated recently by mapping the (x, y)-dependent tip damping: but the detailed damping mechanism is still unknown. Here we identify two different tip dissipation mechanisms: local mechanical softness, and hysteresis. Motivated by recent data, we describe both of them in a one-dimensional model of Moiré superstructures of incommensurate overlayers. Local softness at "soliton" defects yields a dissipation contrast that can be much larger than the corresponding density or corrugation contrast. At realistically low vibration frequencies, however, a much stronger and more effective dissipation is caused by the tip-induced nonlinear jumping of the soliton, naturally developing bi-stability and hysteresis. Signatures of this mechanism are proposed for experimental identification.
Quantum pumping, in its different forms, is attracting attention from different fields, from fund... more Quantum pumping, in its different forms, is attracting attention from different fields, from fundamental quantum mechanics, to nanotechnology, to superconductivity. We investigate the crossover of quantum pumping from the adiabatic to the antiadiabatic regime in the presence of dissipation, and find general and explicit analytical expressions for the pumped current in a minimal model describing a system with the topology of a ring forced by a periodic modulation of frequency ω. The solution allows following in a transparent way the evolution of pumped dc current from much smaller to much larger ω values than the other relevant energy scale, the energy splitting introduced by the modulation. We find and characterize a temperature-dependent optimal value of the frequency for which the pumped current is maximal.
Ab-initio calculations within Density Functional Theory combined with experimental Raman spectra ... more Ab-initio calculations within Density Functional Theory combined with experimental Raman spectra on cluster-beam deposited pure carbon films provide a consistent picture of sp-carbon chains stabilized by sp 3 or sp 2 terminations, the latter being sensitive to torsional strain. This unexplored effect promises many exciting applications since it allows one to modify the conductive states near the Fermi level and to switch on and off the on-chain π-electron magnetism.
The calculation of self-energy corrections to the electron bands of a metal requires the evaluati... more The calculation of self-energy corrections to the electron bands of a metal requires the evaluation of the intraband contribution to the polarizability in the small- q limit. When neglected, as in standard GW codes for semiconductors and insulators, a spurious gap opens at the Fermi energy. Systematic methods to include intraband contributions to the polarizability exist, but require a computationally
Using a tight-binding atomistic simulation, we simulate the recent atomic-force microscopy experi... more Using a tight-binding atomistic simulation, we simulate the recent atomic-force microscopy experiments probing the slipperiness of graphene flakes made slide against a graphite surface. Compared to previous theoretical models, where the flake was assumed to be geometrically perfect and rigid, while the substrate is represented by a static periodic potential, our fully-atomistic model includes quantum mechanics with the chemistry of bond breaking and bond formation, and the flexibility of the flake. These realistic features, include in particular the crucial role of the flake rotation in determining the static friction, in qualitative agreement with experimental observations.
We investigate the adiabatic evolution of a set of non-degenerate eigenstates of a parameterized ... more We investigate the adiabatic evolution of a set of non-degenerate eigenstates of a parameterized Hamiltonian. Their relative phase change can be related to geometric measurable quantities that extend the familiar concept of Berry phase to the evolution of more than one state. We present several physical systems where these concepts can be applied, including an experiment on microwave cavities for which off-diagonal phases can be determined from published data.
We perform a systematic investigation of the resonance and vibrational properties of naphthyl-ter... more We perform a systematic investigation of the resonance and vibrational properties of naphthyl-terminated sp carbon chains (dinaphthylpolyynes) by combined multi-wavelength resonant Raman (MWRR) spectroscopy, ultraviolet-visible spectroscopy, and Fourier-transform infrared (FT-IR) spectroscopy, plus ab initio density functional theory (DFT) calculations. We show that the MWWR and FT-IR spectroscopies are particularly suited to identify chains of different lengths and different terminations, respectively. By DFT calculations, we further extend those findings to sp carbon chains end-capped by other organic structures. The present analysis shows that combined MWRR and FT-IR provide a powerful tool to draw a complete picture of chemically stabilized sp carbon chains.
The many-body dynamics of interacting electrons in condensed matter and quantum chemistry is ofte... more The many-body dynamics of interacting electrons in condensed matter and quantum chemistry is often studied at the quasiparticle level, where the perturbative diagrammatic series is partially resummed. Based on Hedin's equations for self-energy, polarization, propagator, effective potential, and vertex function, dressed (skeleton) Feynman diagrams are enumerated. Such diagram counts provide useful simple checks for extensions of the theory for future realistic simulations.
The many-body dynamics of interacting electrons in condensed matter and quantum chemistry is ofte... more The many-body dynamics of interacting electrons in condensed matter and quantum chemistry is often studied at the quasiparticle level, where the perturbative diagrammatic series is partially resummed. Based on Hedin's equations for self-energy, polarization, propagator, effective potential, and vertex function, dressed (skeleton) Feynman diagrams are enumerated. Such diagram counts provide useful simple checks for extensions of the theory for future realistic simulations.
We investigate the adiabatic evolution of a set of non-degenerate eigenstates of a parameterized ... more We investigate the adiabatic evolution of a set of non-degenerate eigenstates of a parameterized Hamiltonian. Their relative phase change can be related to geometric measurable quantities that extend the familiar concept of Berry phase to the evolution of more than one state. We present several physical systems where these concepts can be applied, including an experiment on microwave cavities for which off-diagonal phases can be determined from published data.
Polynomial approximations around the global minimum of the adiabatic potential energy surface of ... more Polynomial approximations around the global minimum of the adiabatic potential energy surface of several polyatomic molecules display an unphysical saddle point of comparatively small energy, leading to a region where the potential is negative and unbounded. This poses an energy upper limit for a reliable evaluation of the vibrational levels. We argue that the presence of such saddle points is
We report a surprising hysteretic behavior in the dynamics of a simple one-dimensional nonlinear ... more We report a surprising hysteretic behavior in the dynamics of a simple one-dimensional nonlinear model inspired by the tribological problem of two sliding surfaces with a thin solid lubricant layer in between. In particular, we consider the frictional dynamics of a harmonic chain confined between two rigid incommensurate substrates which slide with a fixed relative velocity. This system was previously found, by explicit solution of the equations of motion, to possess plateaus in parameter space exhibiting a remarkable quantization of the chain center-of-mass velocity (dynamic pinning) solely determined by the interface incommensurability. Starting now from this quantized sliding state, in the underdamped regime of motion and in analogy to what ordinarily happens for static friction, the dynamics exhibits a large hysteresis under the action of an additional external driving force F ext . A critical threshold value F c of the adiabatically applied force F ext is required in order to alter the robust dynamics of the plateau attractor. When the applied force is decreased and removed, the system can jump to intermediate sliding regimes (a sort of "dynamic" stick-slip motion) and eventually returns to the quantized sliding state at a much lower value of F ext . On the contrary no hysteretic behavior is observed as a function of the external driving velocity.
Neural networks simulations have always been a complex computational challenge because of the req... more Neural networks simulations have always been a complex computational challenge because of the requirements of large amount of computational and memory resources. Due to the nature of the problem, a high performance computing approach becomes vital, because the dynamics often involves the update of a large network for a large number of time steps. Moreover, the parameter space can be fairly large. An advanced optimization for the single time step is therefore necessary, as well as a strategy to explore the parameter space in an automatic fashion.
We study the frictional sliding of two ideally incommensurate surfaces with a third incommensurat... more We study the frictional sliding of two ideally incommensurate surfaces with a third incommensurate sheet -a sort of extended lubricant -in between. When the mutual ratios of the three periodicities in this sandwich geometry are chosen to be the golden mean φ = (1 + √ 5)/2, this system is believed to be statically pinned for any choice of system parameters. In the present study we overcome this pinning and force the two "substrates" to slide with a mutual velocity V ext , analyzing the resulting frictional dynamics. An unexpected feature is an asymmetry of the relative sliding velocity of the intermediate lubricating sheet relative to the two substrates. Strikingly, the velocity asymmetry takes an exactly quantized value which is uniquely determined by the incommensurability ratio, and absolutely insensitive to all other parameters. The reason for quantization of the velocity asymmetry will be addressed. This behavior is compared and contrasted to the corresponding one obtained for a representative cubic irrational, the spiral mean ω.
Strain-engineering in SiGe nanostructures is fundamental for the design of optoelectronic devices... more Strain-engineering in SiGe nanostructures is fundamental for the design of optoelectronic devices at the nanoscale. Here we explore a new strategy, where SiGe structures are laterally confined by the Si substrate, to obtain high tensile strain avoiding the use of external stressors, and thus improving the scalability. Spectro-microscopy techniques, finite element method simulations and ab initio
With the help of a simple two-dimensional model we simulate the tribological properties of a thin... more With the help of a simple two-dimensional model we simulate the tribological properties of a thin lubricant film consisting of linear (chain) molecules in the ordinary soft-lubricant regime. We find that friction generally increases with chain length, in agreement with their larger bulk viscosity. When comparing the tribological properties of molecules which stick bodily to the substrates with others carrying a single sticking termination, we find that the latter generally yield a larger friction than the former.
In the atomic force microscope, the nanoscale force topography of even complex surface superstruc... more In the atomic force microscope, the nanoscale force topography of even complex surface superstructures is extracted by the changing vibration frequency of a scanning tip. An alternative dissipation topography with similar or even better contrast has been demonstrated recently by mapping the (x, y)-dependent tip damping: but the detailed damping mechanism is still unknown. Here we identify two different tip dissipation mechanisms: local mechanical softness, and hysteresis. Motivated by recent data, we describe both of them in a one-dimensional model of Moiré superstructures of incommensurate overlayers. Local softness at "soliton" defects yields a dissipation contrast that can be much larger than the corresponding density or corrugation contrast. At realistically low vibration frequencies, however, a much stronger and more effective dissipation is caused by the tip-induced nonlinear jumping of the soliton, naturally developing bi-stability and hysteresis. Signatures of this mechanism are proposed for experimental identification.
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Papers by Nicola Manini