ABSTRACT We study an organic semiconductor microcavity operating in the strong-coupling regime us... more ABSTRACT We study an organic semiconductor microcavity operating in the strong-coupling regime using fs pump-probe spectroscopy. By probing the induced absorption associated with the one-exciton to two-exciton transition, we are able to characterize the time-dependent population densities of states in the upper-and lower-polariton branches following impulsive excitation. We model the time-dependent polariton dynamics and provide direct evidence of a scattering process that returns upper-branch cavity polaritons to states in the exciton reservoir having a rate of (150 fs)(-1). A slower process similarly populates lower-branch polaritons by return scattering from the exciton reservoir.
2009 9th IEEE Conference on Nanotechnology (IEEE-NANO), 2009
Nanotransistors typically operate in far-from-equilibrium (FFE) conditions, that cannot be descri... more Nanotransistors typically operate in far-from-equilibrium (FFE) conditions, that cannot be described neither by drift-diffusion, nor by purely ballistic models. An analytical model capable to describe the operation of transistors in FFE conditions would then be required in order to swiftly assess their performance and limitations. In addition, in carbon-based nanotransistors, source and drain contacts are often characterized by the formation of Schottky Barriers (SBs), with strong influence on transport. Here we present a model for one-dimensional field-effect transistors (FETs), taking into account on equal footing both SB contacts and FFE transport regime. Our model represents a significant advancement with respect to the currently available ideal or semi-ideal transport models. We show that the interplay of SB and ambipolar FFE transport gives rise to a number of features in device characteristics, often detected in experiments.
We investigate a bilayer system of critical HgTe quantum wells each featuring a spin-degenerate p... more We investigate a bilayer system of critical HgTe quantum wells each featuring a spin-degenerate pair of massless Dirac fermions. In the presence of an electrostatic inter-layer Coulomb coupling, we determine the exciton condensate order parameter of the system self-consistently. Calculating the bulk topological $\mathbb Z_2$ invariant of the resulting mean field Hamiltonian, we discover a novel time reversal symmetric topological exciton condensate state coined the helical topological exciton condensate. We argue that this phase can exist for experimentally relevant parameters. Interestingly, due to its multi-band nature, the present bilayer model exhibits a nontrivial interplay between spontaneous symmetry breaking and topology: Depending on which symmetry the condensate order parameter spontaneously picks in combined orbital and spin space, stable minima in the free energy corresponding to both trivial and nontrivial gapped states can be found.
The topological nature of Weyl semimetals (WSMs) is corroborated by the presence of chiral surfac... more The topological nature of Weyl semimetals (WSMs) is corroborated by the presence of chiral surface states, which connect the projections of the bulk Weyl points by Fermi arcs (FAs). We study a bilayer structure realized by introducing a thin insulating spacer into a bulk WSM. Employing a self-consistent mean-field description of the interlayer Coulomb interaction, we propose that this system can develop an interlayer electron-hole pair condensate. The formation of this excitonic condensate leads to partial gapping of the FA dispersion. We obtain the dependence of the energy gap and the critical temperature on the model parameters, finding, in particular, a linear scaling of these quantities with the separation between the Weyl points in momentum space. A detrimental role is played by the curvature of the FAs, although the pairing persists for moderately small curvature. A signature of the condensate is the modification of the quantum oscillations involving the surface FAs.
Under certain conditions, Hg(Cd)Te quantum wells (QWs) are known to realize a time-reversal symme... more Under certain conditions, Hg(Cd)Te quantum wells (QWs) are known to realize a time-reversal symmetric, two-dimensional topological insulator phase. Its low-energy excitations are welldescribed by the phenomenological Bernevig-Hughes-Zhang (BHZ) model that interpolates between Schrödinger and Dirac fermion physics. We study the polarization function of this model in random phase approximation (RPA) in the intrinsic limit and at finite doping. While the polarization properties in RPA of Dirac and Schrödinger particles are two comprehensively studied problems, our analysis of the BHZ model bridges the gap between these two limits, shedding light on systems with intermediate properties. We gain insight into the screening properties of the system and on its characteristic plasma oscillations. Interestingly, we discover two different kinds of plasmons that are related to the presence of intra-and interband excitations. Observable signatures of these plasmons are carefully analyzed in a variety of distinct parameter regimes, including the experimentally relevant ones for Hg(Cd)Te QWs. We conclude that the discovered plasmons are observable by Raman or electron loss spectroscopy.
The manipulation of the electron spin degree of freedom is at the core of the spintronics paradig... more The manipulation of the electron spin degree of freedom is at the core of the spintronics paradigm, which offers the perspective of reduced power consumption, enabled by the decoupling of information processing from net charge transfer. Spintronics also offers the possibility of devising hybrid devices able to perform logic, communication, and storage operations. Graphene, with its potentially long spin-coherence length, is a promising material for spin-encoded information transport. However, the small spin-orbit interaction is also a limitation for the design of conventional devices based on the canonical Datta-Das spin-FET. An alternative solution can be found in magnetic doping of graphene, or, as discussed in the present work, in exploiting the proximity effect between graphene and Ferromagnetic Oxides (FOs). Graphene in proximity to FO experiences an exchange proximity interaction (EPI), that acts as an effective Zeeman field for electrons in graphene, inducing a spin precessio...
Physical Review B Condensed Matter and Materials Physics, Apr 3, 2010
In this work, we present a performance analysis of field-effect transistors (FETs) based on recen... more In this work, we present a performance analysis of field-effect transistors (FETs) based on recently fabricated 100% hydrogenated graphene (the so-called graphane) and theoretically predicted semihydrogenated graphene (i.e., graphone). The approach is based on accurate calculations of the energy bands by means of GW approximation, subsequently fitted with a three-nearest neighbor sp3 tight-binding Hamiltonian, and finally used to compute ballistic transport in transistors based on functionalized graphene. Due to the large energy gap, the proposed devices have many of the advantages provided by one-dimensional graphene nanoribbon FETs, such as large Ion and Ion/Ioff ratios, reduced band-to-band tunneling, without the corresponding disadvantages in terms of prohibitive lithography and patterning requirements for circuit integration.
Nanotransistors typically operate in far-from-equilibrium (FFE) conditions, that cannot be descri... more Nanotransistors typically operate in far-from-equilibrium (FFE) conditions, that cannot be described neither by drift-diffusion, nor by purely ballistic models. In carbonbased nanotransistors, source and drain contacts are often characterized by the formation of Schottky Barriers (SBs), with strong influence on transport. Here we present a model for onedimensional field-effect transistors (FETs), taking into account on equal footing both SB contacts and FFE transport regime. Intermediate transport is introduced within the Buttiker probe approach to dissipative transport, in which a non-ballistic transistor is seen as a suitable series of individually ballistic channels. Our model permits the study of the interplay of SBs and ambipolar FFE transport, and in particular of the transition between SB-limited and dissipation-limited transport.
Nanotransistors typically operate in far-from-equilibrium (FFE) conditions, that cannot be descri... more Nanotransistors typically operate in far-from-equilibrium (FFE) conditions, that cannot be described neither by drift-diffusion, nor by purely ballistic models. An analytical model capable to describe the operation of transistors in FFE conditions would then be required in order to swiftly assess their performance and limitations. In addition, in carbon-based nanotransistors, source and drain contacts are often characterized by the formation
ABSTRACT We study an organic semiconductor microcavity operating in the strong-coupling regime us... more ABSTRACT We study an organic semiconductor microcavity operating in the strong-coupling regime using femtosecond pump-probe spectroscopy. By probing the photo-induced absorption bands, we characterize the time-dependent population densities of states in the two polariton branches. We found evidence of a scattering process from the upper-branch cavity polaritons to the exciton reservoir having a rate of (150 fs)-1. A slower process similarly populates lower-branch polaritons with a rate of around (3ps)-1
International Conference on Fibre Optics and Photonics, 2012
ABSTRACT Strongly coupled cavity-polaritons are characterized by their anti-crossing behavior in ... more ABSTRACT Strongly coupled cavity-polaritons are characterized by their anti-crossing behavior in the dispersion plot. 35-fs time period Rabi oscillations and polariton relaxation dynamics in a J-aggregate microcavity were observed using ultrafast spectroscopy.
where t is the in-plane hopping term, equal to 2.7 eV, m == 0.13 eV is an empirical parameter int... more where t is the in-plane hopping term, equal to 2.7 eV, m == 0.13 eV is an empirical parameter introduced to reproduce the energy band gap and f (k) is
... the following of this section, we will discuss the potential of these options, evaluated thro... more ... the following of this section, we will discuss the potential of these options, evaluated throughmodeling. ... Vdd can be hardly reduced because the ideal subthreshold swing at room temperature is 60 ... at least 0.24 V. The situation can be more favorable with a Tunnel FET, where the ...
Performance analysis of Field Effect Transistors based on hydrogenated graphene is performed thro... more Performance analysis of Field Effect Transistors based on hydrogenated graphene is performed through a multiscale approach based on calculations of the energy bands by means of GW approximation, a three-nearest neighbor (3NN) sp 3 tight-binding Hamiltonian, and a ballistic transport model. The considered device exhibits large Ion and Ion/I off ratios, due to the large bandgap, with reduced lithographic constrains as compared to one-dimensional channels.
ABSTRACT The two-dimensional topological insulating phase has been experimentally discovered in H... more ABSTRACT The two-dimensional topological insulating phase has been experimentally discovered in HgTe quantum wells (QWs). The low-energy physics of two-dimensional topological insulators (TIs) is described by the Bernevig-Hughes-Zhang (BHZ) model, where the realization of a topological or a normal insulating phase depends on the Dirac mass being negative or positive, respectively. We solve the BHZ model for a mass domain configuration, analyzing the effects on the edge modes of a finite Dirac mass in the normal insulating region (soft-wall boundary condition). We show that at a boundary between a TI and a normal insulator (NI), the Dirac point of the edge states appearing at the interface strongly depends on the ratio between the Dirac masses in the two regions. We also consider the case of multiple boundaries such as NI/TI/NI, TI/NI/TI and NI/TI/NI/TI.
ABSTRACT We study an organic semiconductor microcavity operating in the strong-coupling regime us... more ABSTRACT We study an organic semiconductor microcavity operating in the strong-coupling regime using fs pump-probe spectroscopy. By probing the induced absorption associated with the one-exciton to two-exciton transition, we are able to characterize the time-dependent population densities of states in the upper-and lower-polariton branches following impulsive excitation. We model the time-dependent polariton dynamics and provide direct evidence of a scattering process that returns upper-branch cavity polaritons to states in the exciton reservoir having a rate of (150 fs)(-1). A slower process similarly populates lower-branch polaritons by return scattering from the exciton reservoir.
2009 9th IEEE Conference on Nanotechnology (IEEE-NANO), 2009
Nanotransistors typically operate in far-from-equilibrium (FFE) conditions, that cannot be descri... more Nanotransistors typically operate in far-from-equilibrium (FFE) conditions, that cannot be described neither by drift-diffusion, nor by purely ballistic models. An analytical model capable to describe the operation of transistors in FFE conditions would then be required in order to swiftly assess their performance and limitations. In addition, in carbon-based nanotransistors, source and drain contacts are often characterized by the formation of Schottky Barriers (SBs), with strong influence on transport. Here we present a model for one-dimensional field-effect transistors (FETs), taking into account on equal footing both SB contacts and FFE transport regime. Our model represents a significant advancement with respect to the currently available ideal or semi-ideal transport models. We show that the interplay of SB and ambipolar FFE transport gives rise to a number of features in device characteristics, often detected in experiments.
We investigate a bilayer system of critical HgTe quantum wells each featuring a spin-degenerate p... more We investigate a bilayer system of critical HgTe quantum wells each featuring a spin-degenerate pair of massless Dirac fermions. In the presence of an electrostatic inter-layer Coulomb coupling, we determine the exciton condensate order parameter of the system self-consistently. Calculating the bulk topological $\mathbb Z_2$ invariant of the resulting mean field Hamiltonian, we discover a novel time reversal symmetric topological exciton condensate state coined the helical topological exciton condensate. We argue that this phase can exist for experimentally relevant parameters. Interestingly, due to its multi-band nature, the present bilayer model exhibits a nontrivial interplay between spontaneous symmetry breaking and topology: Depending on which symmetry the condensate order parameter spontaneously picks in combined orbital and spin space, stable minima in the free energy corresponding to both trivial and nontrivial gapped states can be found.
The topological nature of Weyl semimetals (WSMs) is corroborated by the presence of chiral surfac... more The topological nature of Weyl semimetals (WSMs) is corroborated by the presence of chiral surface states, which connect the projections of the bulk Weyl points by Fermi arcs (FAs). We study a bilayer structure realized by introducing a thin insulating spacer into a bulk WSM. Employing a self-consistent mean-field description of the interlayer Coulomb interaction, we propose that this system can develop an interlayer electron-hole pair condensate. The formation of this excitonic condensate leads to partial gapping of the FA dispersion. We obtain the dependence of the energy gap and the critical temperature on the model parameters, finding, in particular, a linear scaling of these quantities with the separation between the Weyl points in momentum space. A detrimental role is played by the curvature of the FAs, although the pairing persists for moderately small curvature. A signature of the condensate is the modification of the quantum oscillations involving the surface FAs.
Under certain conditions, Hg(Cd)Te quantum wells (QWs) are known to realize a time-reversal symme... more Under certain conditions, Hg(Cd)Te quantum wells (QWs) are known to realize a time-reversal symmetric, two-dimensional topological insulator phase. Its low-energy excitations are welldescribed by the phenomenological Bernevig-Hughes-Zhang (BHZ) model that interpolates between Schrödinger and Dirac fermion physics. We study the polarization function of this model in random phase approximation (RPA) in the intrinsic limit and at finite doping. While the polarization properties in RPA of Dirac and Schrödinger particles are two comprehensively studied problems, our analysis of the BHZ model bridges the gap between these two limits, shedding light on systems with intermediate properties. We gain insight into the screening properties of the system and on its characteristic plasma oscillations. Interestingly, we discover two different kinds of plasmons that are related to the presence of intra-and interband excitations. Observable signatures of these plasmons are carefully analyzed in a variety of distinct parameter regimes, including the experimentally relevant ones for Hg(Cd)Te QWs. We conclude that the discovered plasmons are observable by Raman or electron loss spectroscopy.
The manipulation of the electron spin degree of freedom is at the core of the spintronics paradig... more The manipulation of the electron spin degree of freedom is at the core of the spintronics paradigm, which offers the perspective of reduced power consumption, enabled by the decoupling of information processing from net charge transfer. Spintronics also offers the possibility of devising hybrid devices able to perform logic, communication, and storage operations. Graphene, with its potentially long spin-coherence length, is a promising material for spin-encoded information transport. However, the small spin-orbit interaction is also a limitation for the design of conventional devices based on the canonical Datta-Das spin-FET. An alternative solution can be found in magnetic doping of graphene, or, as discussed in the present work, in exploiting the proximity effect between graphene and Ferromagnetic Oxides (FOs). Graphene in proximity to FO experiences an exchange proximity interaction (EPI), that acts as an effective Zeeman field for electrons in graphene, inducing a spin precessio...
Physical Review B Condensed Matter and Materials Physics, Apr 3, 2010
In this work, we present a performance analysis of field-effect transistors (FETs) based on recen... more In this work, we present a performance analysis of field-effect transistors (FETs) based on recently fabricated 100% hydrogenated graphene (the so-called graphane) and theoretically predicted semihydrogenated graphene (i.e., graphone). The approach is based on accurate calculations of the energy bands by means of GW approximation, subsequently fitted with a three-nearest neighbor sp3 tight-binding Hamiltonian, and finally used to compute ballistic transport in transistors based on functionalized graphene. Due to the large energy gap, the proposed devices have many of the advantages provided by one-dimensional graphene nanoribbon FETs, such as large Ion and Ion/Ioff ratios, reduced band-to-band tunneling, without the corresponding disadvantages in terms of prohibitive lithography and patterning requirements for circuit integration.
Nanotransistors typically operate in far-from-equilibrium (FFE) conditions, that cannot be descri... more Nanotransistors typically operate in far-from-equilibrium (FFE) conditions, that cannot be described neither by drift-diffusion, nor by purely ballistic models. In carbonbased nanotransistors, source and drain contacts are often characterized by the formation of Schottky Barriers (SBs), with strong influence on transport. Here we present a model for onedimensional field-effect transistors (FETs), taking into account on equal footing both SB contacts and FFE transport regime. Intermediate transport is introduced within the Buttiker probe approach to dissipative transport, in which a non-ballistic transistor is seen as a suitable series of individually ballistic channels. Our model permits the study of the interplay of SBs and ambipolar FFE transport, and in particular of the transition between SB-limited and dissipation-limited transport.
Nanotransistors typically operate in far-from-equilibrium (FFE) conditions, that cannot be descri... more Nanotransistors typically operate in far-from-equilibrium (FFE) conditions, that cannot be described neither by drift-diffusion, nor by purely ballistic models. An analytical model capable to describe the operation of transistors in FFE conditions would then be required in order to swiftly assess their performance and limitations. In addition, in carbon-based nanotransistors, source and drain contacts are often characterized by the formation
ABSTRACT We study an organic semiconductor microcavity operating in the strong-coupling regime us... more ABSTRACT We study an organic semiconductor microcavity operating in the strong-coupling regime using femtosecond pump-probe spectroscopy. By probing the photo-induced absorption bands, we characterize the time-dependent population densities of states in the two polariton branches. We found evidence of a scattering process from the upper-branch cavity polaritons to the exciton reservoir having a rate of (150 fs)-1. A slower process similarly populates lower-branch polaritons with a rate of around (3ps)-1
International Conference on Fibre Optics and Photonics, 2012
ABSTRACT Strongly coupled cavity-polaritons are characterized by their anti-crossing behavior in ... more ABSTRACT Strongly coupled cavity-polaritons are characterized by their anti-crossing behavior in the dispersion plot. 35-fs time period Rabi oscillations and polariton relaxation dynamics in a J-aggregate microcavity were observed using ultrafast spectroscopy.
where t is the in-plane hopping term, equal to 2.7 eV, m == 0.13 eV is an empirical parameter int... more where t is the in-plane hopping term, equal to 2.7 eV, m == 0.13 eV is an empirical parameter introduced to reproduce the energy band gap and f (k) is
... the following of this section, we will discuss the potential of these options, evaluated thro... more ... the following of this section, we will discuss the potential of these options, evaluated throughmodeling. ... Vdd can be hardly reduced because the ideal subthreshold swing at room temperature is 60 ... at least 0.24 V. The situation can be more favorable with a Tunnel FET, where the ...
Performance analysis of Field Effect Transistors based on hydrogenated graphene is performed thro... more Performance analysis of Field Effect Transistors based on hydrogenated graphene is performed through a multiscale approach based on calculations of the energy bands by means of GW approximation, a three-nearest neighbor (3NN) sp 3 tight-binding Hamiltonian, and a ballistic transport model. The considered device exhibits large Ion and Ion/I off ratios, due to the large bandgap, with reduced lithographic constrains as compared to one-dimensional channels.
ABSTRACT The two-dimensional topological insulating phase has been experimentally discovered in H... more ABSTRACT The two-dimensional topological insulating phase has been experimentally discovered in HgTe quantum wells (QWs). The low-energy physics of two-dimensional topological insulators (TIs) is described by the Bernevig-Hughes-Zhang (BHZ) model, where the realization of a topological or a normal insulating phase depends on the Dirac mass being negative or positive, respectively. We solve the BHZ model for a mass domain configuration, analyzing the effects on the edge modes of a finite Dirac mass in the normal insulating region (soft-wall boundary condition). We show that at a boundary between a TI and a normal insulator (NI), the Dirac point of the edge states appearing at the interface strongly depends on the ratio between the Dirac masses in the two regions. We also consider the case of multiple boundaries such as NI/TI/NI, TI/NI/TI and NI/TI/NI/TI.
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Papers by Paolo Michetti