Spin-Orbit Effects on the Dynamical Properties of Polarons in Graphene Nanoribbons (original) (raw)

Impact of the Electron-Phonon Interactions on the Polaron Dynamics in Graphene Nanoribbons

The journal of physical chemistry. A, 2016

The influence of the electron-phonon (e-ph) interactions on the filed-included polaron dynamics in armchair graphene nanoribbons (GNRs) is theoretically investigated in the scope of a two-dimensional tight-binding model. The results show that the localization of the polaronic charge increases when the strength of e-ph coupling also increases. Consequently, the polaron saturation velocity decreases for higher e-ph coupling strengths. Interestingly, the interplay between the e-ph coupling strength and the GNR width changes substantially the polaron dynamics properties.

Polaron Properties in Armchair Graphene Nanoribbons

The journal of physical chemistry. A, 2016

By means of a two dimensional tight-binding model with lattice relaxation in a first order expansion, we report different polaron properties depending on the armchair graphene nanoribbons width family as well as on its size. We obtain that representatives of 3p+2 family do not present a polaronic mediated charge transport. As for 3p and 3p+1 families, the polaron behavior was completely dependent on the system's width. Particularly, we observed a greater degree of delocalization for broader nanoribbons; narrower nanoribbons of both families, on the other hand, typically presented a more localized polaronic-type transport. Energy levels and occupation numbers analysis are performed in order to rigourously assess the nature of the charge carrier. Time evolution in the scope of the Ehrenfest Molecular Dynamics was also carried out to confirm the collective behavior and stability of the carrier as a function of time. We were able to determine that polarons in nanoribbons of 3p famil...

Influence of quasi-particle density over polaron mobility in armchair graphene nanoribbons

Physical chemistry chemical physics : PCCP, 2018

An important aspect concerning the performance of armchair graphene nanoribbons (AGNRs) as materials for conceiving electronic devices is related to the mobility of charge carriers in these systems. When several polarons are considered in the system, a quasi-particle wave function can be affected by that of its neighbor provided the two are close enough. As the overlap may affect the transport of the carrier, the question concerning how the density of polarons affect its mobility arises. In this work, we investigate such dependence for semiconducting AGNRs in the scope of nonadiabatic molecular dynamics. Our results unambiguously show an impact of the density on both the stability and average velocity of the quasi-particles. We have found a phase transition between regimes where increasing density stops inhibiting and starts promoting mobility; densities higher than 7 polarons per 45 Å present increasing mean velocity with increasing density. We have also established three different...

Rashba spin-orbit interaction in graphene armchair nanoribbons

The European Physical Journal B, 2013

We study graphene nanoribbons (GNRs) with armchair edges in the presence of Rashba spinorbit interactions (RSOI). We impose the boundary conditions on the tight binding Hamiltonians for bulk graphene with RSOI by means of a sine transform and study the influence of RSOI on the spectra and the spin polarization in detail. We derive the low energy approximation of the RSOI Hamiltonian for the zeroth and first order in momentum and test their ranges of validity. The choice of a basis appropriate for armchair boundaries is important in the case of mode-coupling effects and leads to results that are easy to work with.

Bipolaron Dynamics in Graphene Nanoribbons

Scientific Reports

Graphene nanoribbons (GNRs) are two-dimensional structures with a rich variety of electronic properties that derive from their semiconducting band gaps. In these materials, charge transport can occur via a hopping process mediated by carriers formed by self-interacting states between the excess charge and local lattice deformations. Here, we use a two-dimensional tight-binding approach to reveal the formation of bipolarons in GNRs. our results show that the formed bipolarons are dynamically stable even for high electric field strengths when it comes to GNRs. Remarkably, the bipolaron dynamics can occur in acoustic and optical regimes concerning its saturation velocity. the phase transition between these two regimes takes place for a critical field strength in which the bipolaron moves roughly with the speed of sound in the material.

Tight-binding theory of the spin-orbit coupling in graphene

Physical Review B, 2010

The spin-orbit coupling in graphene induces spectral gaps at the high-symmetry points. The relevant gap at the ⌫ point is similar to the splitting of the p orbitals in the carbon atom, being roughly 8.5 meV. The splitting at the K point is orders of magnitude smaller. Earlier tight-binding theories indicated the value of this intrinsic gap of 1 eV, based on thecoupling. All-electron first-principles calculations give much higher values, between 25 and 50 eV, due to the presence of the orbitals of the d symmetry in the Bloch states at K. A realistic multiband tight-binding model is presented to explain the effects the d orbitals play in the spin-orbit coupling at K. Thecoupling is found irrelevant to the value of the intrinsic spin-orbit-induced gap. On the other hand, the extrinsic spin-orbit coupling ͑of the Bychkov-Rashba type͒, appearing in the presence of a transverse electric field, is dominated by thehybridization, in agreement with previous theories. Tightbinding parameters are obtained by fitting to first-principles calculations, which also provide qualitative support for the model when considering the trends in the spin-orbit-induced gap in graphene under strain. Finally, an effective single-orbital next-nearest-neighbor hopping model accounting for the spin-orbit effects is derived.

Electron–Lattice Coupling in Armchair Graphene Nanoribbons

The Journal of Physical Chemistry Letters, 2012

We report the effects of electron−lattice coupling on the charge density distribution study of armchair graphene nanoribbons (GNRs). Here, we perform a theoretical investigation explaining the unexpected electronic density states observed experimentally. By means of a tight-binding approach with electron−lattice coupling, we obtained the same characteristic pattern of charge density along the C−C bonds suggested by both scanning tunneling and transmission electron microscopic measurements. Our results suggest electronic localized states whose sizes are dependent on the GNR width. We also show that our model rescues the quasi-particle charge-transport mechanism in GNRs. The remarkable agreement with experimental evidence allows us to conclude that our model could be, in many aspects, a fundamental tool when it comes to the phenomenological understanding of the charge behavior in this kind of system.

Stability conditions of armchair graphene nanoribbon bipolarons

Journal of Molecular Modeling

Graphene nanoribbons are 2D hexagonal lattices with semiconducting band gaps. Below a critical electric field strength, the charge transport in these materials is governed by the quasiparticle mechanism. The quasiparticles involved in the process, known as polarons and bipolarons, are self-interacting states between the system charges and local lattice distortions. To deeply understand the charge transport mechanism in graphene nanoribbons, the study of the stability conditions for quasiparticles in these materials is crucial and may guide new investigations to improve the efficiency for a next generation of graphene-based optoelectronic devices. Here, we use a two-dimensional version of the Su-Schrieffer-Heeger model to investigate the stability of bipolarons in armchair graphene nanoribbons (AGNRs). Our findings show how bipolaron stability is dependent on the strength of the electron-phonon interactions. Moreover, the results show that bipolarons are dynamically stable in AGNRs for electric field strengths lower than 3.0 mV/Å. Remarkably, the system's binding energy for a lattice containing a bipolaron is smaller than the formation energy of two isolated polarons, which suggests that bipolarons can be natural quasiparticle solutions in AGNRs.

Intrinsic spin–orbit interaction in carbon nanotubes and curved nanoribbons

Solid State Communications, 2012

We present a theoretical study of spin-orbit interaction effects on single wall carbon nanotubes and curved graphene nanoribbons by means of a realistic multiorbital tight-binding model, which takes into account the full symmetry of the honeycomb lattice. Several effects relevant to spin-orbit interaction, namely, the importance of chirality, curvature, and a family-dependent anisotropic conduction and valence band splitting are identified. We show that chiral nanotubes and nanoribbons exhibit spin-split states. Curvature-induced orbital hybridization is crucial to understand the experimentally observed anisotropic spin-orbit splittings in carbon nanotubes. In fact, spin-orbit interaction is important in curved graphene nanoribbons, since the induced spin-splitting on the edge states gives rise to spinfiltered states.

Interplay between symmetry and spin-orbit coupling on graphene nanoribbons

Physical Review B, 2013

We study the electronic structure of chiral and achiral graphene nanoribbons with symmetric edges, including curvature and spin-orbit effects. Curved ribbons show spin-split bands, whereas flat ribbons present spin-degenerate bands. We show that this effect is due to the breaking of spatial inversion symmetry in curved graphene nanoribbons, while flat ribbons with symmetric edges possess an inversion center, regardless of their having chiral or achiral edges. We find an enhanced edge-edge coupling and a substantial gap in narrow chiral nanoribbons, which is not present in zigzag ribbons of similar width. We attribute these size effects to the mixing of the sublattices imposed by the edge geometry, yielding a behavior of chiral ribbons that is distinct from those with pure zigzag edges.