arXiv subjects

M. Pisarra

Publications and source records attributed to M. Pisarra.

4 recordsLinked to original sources

arXiv2023

Engineering a Spin-Orbit Bandgap in Graphene-Tellurium Heterostructures

Intensive research has focused on harnessing the potential of graphene for electronic, optoelectronic, and spintronic devices by generating a bandgap at the Dirac point and enhancing the spin-orbit interaction in the graphene layer. Proximity to heavy p elements is a promising approach; however, their interaction in graphene heterostructures has not been as intensively studied as that of ferromagnetic, noble, or heavy d metals, neither as interlayers nor as substrates. In this study, the effective intercalation of Te atoms in a graphene on Ir(111) heterostructure is achieved. Combining techniques such as low energy electron diffraction and scanning tunneling microscopy, the structural evolution of the system as a function of the Te coverage is elucidated, uncovering up to two distinct phases. The presented angle-resolved photoemission spectroscopy analysis reveals the emergence of a bandgap of about 240 meV in the Dirac cone at room temperature, which preserves its characteristic linear dispersion. Furthermore, a pronounced n-doping effect induced by Te in the heterostructure is also observed, and remarkably the possibility of tuning the Dirac point energy towards the Fermi level by reducing the Te coverage while maintaining the open bandgap is demonstrated. Spin-resolved measurements unveil a non-planar chiral spin texture with significant splitting values for both in-plane and out-of-plane spin components. These experimental findings are consistent with the development of a quantum spin Hall phase, where a Te-enhanced intrinsic spin orbit coupling in graphene surpasses the Rashba one and promotes the opening of the spin-orbit bandgap.

cond-mat.mes-hall↗

arXiv2018

Interband $π$-like plasmon in silicene grown on silver

Silicene, the two-dimensional allotrope of silicon, is predicted to exist in a low-buckled honeycomb lattice, characterized by semimetallic electronic bands with graphenelike energy-momentum dispersions around the Fermi level (represented by touching Dirac cones). Single layers of silicene are mostly synthesized by depositing silicon on top of silver, where, however, the different phases observed to date are so strongly hybridized with the substrate that not only the Dirac cones, but also the whole valence and conduction states of ideal silicene appear to be lost. Here, we provide evidence that at least part of this semimetallic behavior is preserved by the coexistence of more silicene phases, epitaxially grown on Ag(111). In particular, we combine electron energy loss spectroscopy and time-dependent density functional theory to characterize the low-energy plasmon of a multiphase-silicene/Ag(111) sample, prepared at controlled silicon coverage and growth temperature. We find that this mode survives the interaction with the substrate, being perfectly matched with the π-like plasmon of ideal silicene. We therefore suggest that the weakened interaction of multiphase silicene with the substrate may provide a unique platform with the potential to develop different applications based on two-dimensional silicon systems.

cond-mat.mtrl-sci↗

arXiv2017

Plasmon properties and hybridization effects in Silicene

The plasmonic character of monolayer silicene is investigated by time-dependent density functional theory in the random phase approximation. The energy-loss function of the system is analyzed, with particular reference to its induced charge-density fluctuations, i.e., plasmon resonances and corresponding dispersions, occurring in the investigated energy-momentum region. At energies larger than 1.5eV, two intrinsic interband modes are detected and characterized. The first one is a hybridized pi-like plasmon, which is assisted by competing one-electron processes involving sp2 and sp3 states. The second one is a more conventional pi-sigma plasmon, which is more intense than the pi-like plasmon and more affected by one-electron processes involving the sigma bands, with respect to the analogous collective oscillation in monolayer graphene. At energies below 1eV, two extrinsic intraband modes are predicted to occur, which are generated by distinct types of Dirac electrons (associated with different Fermi velocities at the so-called Dirac points). The most intense of them is a two-dimensional plasmon, having an energy-momentum dispersion that resembles that of a two-dimensional electron gas. The other is an acoustic plasmon that occurs for specific momentum directions and competes with the two-dimensional plasmon at mid infrared energies. The strong anisotropic character of this mode cannot be explained in terms of the widely used Dirac-cone approximation. As in mono-, bi-, and few-layer graphene, the extrinsic oscillations of silicene are highly sensitive to the concentration of injected or ejected charge carriers. More importantly, the two-dimensional and acoustic plasmons appear to be a signature of the honeycomb lattice, independently of the chemistry of the group-IV elements and the details of the unit-cell geometry.

cond-mat.str-el↗

arXiv2013

Acoustic plasmons in extrinsic free-standing graphene

An acoustic plasmon is predicted to occur, in addition to the conventional two-dimensional (2D) plasmon, as the collective motion of a system of two types of electronic carriers coexisting in the very same 2D band of extrinsic (doped or gated) graphene. The origin of this novel mode resides in the strong anisotropy that is present in the graphene band structure near the Dirac point. This anisotropy allows for the coexistence of carriers moving with two distinct Fermi velocities along the Gamma-K direction, which leads to two modes of collective oscillation: one mode in which the two types of electrons oscillate in phase with one another [this is the conventional 2D graphene plasmon, which at long wavelengths (q->0) has the same dispersion, q^1/2, as the conventional 2D plasmon of a 2D free electron gas], and the other mode found here corresponding to a low-frequency acoustic oscillation [whose energy exhibits at long wavelengths a linear dependence on the 2D wavenumber q] in which the two types of electrons oscillate out of phase. If this prediction is confirmed experimentally, it will represent the first realization of acoustic plasmons originated in the collective motion of a system of two types of carriers coexisting within the very same band.

cond-mat.mes-hall↗