Conductance signatures of electron confinement induced by strained nanobubbles in graphene

Abstract

We investigate the impact of strained nanobubbles on the conductance characteristics of graphene nanoribbons using a combined molecular dynamics -tight-binding simulation scheme. We describe in detail how the conductance, density of states, and current density of zigzag or armchair graphene nanoribbons are modified by the presence of a nanobubble. In particular, we establish that lowenergy electrons can be confined in the vicinity or within the nanobubbles by the delicate interplay between the pseudomagnetic field pattern created by the shape of the bubble, mode mixing, and substrate interaction. The coupling between confined evanescent states and propagating modes can be enhanced under different clamping conditions, which translates into Fano resonances in the conductance traces.The fine control over nanofabrication techniques has not only increased the performance of existing electronic devices 1 , but has also allowed the emergence of concept devices based on the strictly quantum-mechanical properties of electrons. One such proposal is the incorporation of patterned ferromagnetic or superconducting films on two dimensional electron gas (2DEG) structures. Under the right conditions and design parameters, these can be tailored to provide non-homogeneous magnetic fields able to interact strongly with the underlying electrons in the ballistic transport regime 2-5 . Ideally, the spatial profile of such fields should be extremely sharp along the transport direction and homogeneous in the transverse direction, so that the resulting magnetic barrier might behave as an effective momentum filter, which is necessary to achieve control of the ballistic transmission 4,5 . In addition, strong and sharp barriers generally beget richer transmission characteristics, including the stabilization of confined states within the barrier 6 . The same concept has been proposed following the advent of graphene as a versatile two-dimensional platform for nanoscale electronic devices, with local magnetic barriers being one of several proposed means to confine, guide, and control electron flow 7-11 . The need for robust and tunable confinement strategies is more fundamental in graphene electronic devices than in conventional semiconductors: on account of their massless Dirac character, charge carriers in graphene are vulnerable to the phenomenon of Klein tunneling, and cannot be adequately confined by standard electrostatic means, particularly in the ballistic regime. However, even though the search towards achieving control of the electron flow in graphene remains one of the most active research areas when it comes to applications of graphene in the electronics industry, little progress has been made towards this concept of magnetic confinement. This is partly because of the size requirements that call for magnetic barriers that are much smaller than the electronic mean free path, and also because of the need to limit the spatial extent of the magnetic field within regions equally small, since it might be desirable to have portions of t...

Graphene Nanobubbles as Valley Filters and Beam Splitters

“…Experimental methods for producing such controllable strain fields include direct applied pressure from STM tips [31], gas inflation [32][33][34][35][36], and substrate engineering [37][38][39][40][41][42][43][44][45]. Most of these approaches result in spatially localized strain fields taking the form of a pseudomagnetic dot.…”

Analytic approach to magneto-strain tuning of electronic transport through a graphene nanobubble: perspectives for a strain sensor

“…A number of attempts have been published in the literature to model such patterns by a gaussian distributed field, and the corresponding models for the associated Dirac single-particle eigenstates and energy eigenvalues can only be studied numerically [24]. More recently, Bahamon et al [32] studied the conductance induced by different strain nanobubles numerically using molecular dynamics and tight-binding simulations. However, STEM experiments reveal that the magnitude of the pseudomagnetic field due to local strain patterns is nearly uniform within a region with a characteristic radius on the order of 15 − 25 nm [18][19][20]26].…”

Quantum transport in graphene in presence of strain-induced pseudo-Landau levels

“…The connection between strain tensor and bond length deformation is only approximate 30 but sufficient for the present analysis of generic quantum transport fingerprints of strain-induced pLLs. Especially, we note that the effect of the PMF on the LDOS is qualitatively unchanged by relaxation using molecular dynamics methods 22,27,[31][32][33][34][35] and that second nearest neighbor terms only contribute with a scalar potential without generating pseudomagnetic effects 22,26 . Outside the central dot region, we apply a smoothing to the strain tensor to assure a soft transition to the strain-free pristine regions.…”