Anomalous ballistic transport in disordered bilayer graphene: A Dirac semimetal induced by dimer vacancies

Abstract

We report anomalous quantum transport features in bilayer graphene in the presence of a random distribution of structural vacancies. By using an efficient real-space Kubo-Greenwood transport methodology, the impact of a varying density of dimer versus nondimer vacancies is investigated in very large scale disordered models. While nondimer vacancies are shown to induce localization regimes, dimer vacancies result in an unexpected ballistic regime whose energy window surprisingly enlarges with increasing impurity density. Such counterintuitive phenomenon is explained by the formation of an effective linear dispersion in the bilayer band structure, which roots in the symmetry breaking effects driven by dimer vacancies, and provides a realization of Dirac semimetals in high dimension. DOI: 10.1103/PhysRevB.93.041403Introduction. Single layer graphene (SLG) has attracted great attention owing to its remarkable electrical, chemical, and mechanical properties providing an endless list of novel opportunities for practical applications [1]. SLG possesses a linear electronic spectrum with chiral (A-B sublattice) symmetry which leads to exotic low-energy transport such as Klein tunneling [2,3], weak antilocalization [4,5], and half-integer quantum Hall effect [6,7].Bilayer graphene (BLG) differs from SLG by the parabolic band dispersion which, however, retains the chiral nature of low-energy electronic excitations. One of the salient and unique properties of BLG is the possibility of creating an electronic band gap by applying an external gate voltage [8,9]. However surprisingly, the understanding of quantum transport in disordered BLG remains far less understood than the SLG case, because of the enhanced structural complexity. Experimental studies evidence critical differences in transport behaviors of SLG and BLG [10]. Some generalization of the localization theory in BLG has been derived [11], while a minimum conductivity σ min 4e 2 /h is predicted at the charge neutrality point (CNP) [12][13][14]. A recent scanning tunneling microscopy (STM) study shows that vacancies in graphite induce peculiar impurity states known as zero-energy modes (ZEMs) which are maximally localized at the defect position and then decay as the inverse of the distance from the vacancy [15]. The impact of ZEMs on BLG is so far poorly understood, especially the role played by dimer and nondimer vacancies which strongly differ in terms of symmetry breaking characteristics.Here, we start by analyzing the localization features of ZEMs in BLG for all types of vacancies, and found a highly inhomogeneous sublattice state population (pseudospin polarization) as reported in STM experiments [15]. The depletion of low-energy states in one sublattice is additionally further classified into two different classes depending on the vacancy position. Then by using efficient computational methods, we explore quantum transport in disordered BLG

Numerical analysis of electronic conductivity in graphene with resonant adsorbates: comparison of monolayer and Bernal bilayer

“…This is reminiscent of the midgap state produced by asymmetry between the number of atoms A and B in monolayer graphene [41,14]. In agreement with previous findings for MLG [4,14,16,20] and BLG [12,29,32], for large values of energies E the DOS is weakly affected by the presence of disorder. Finally near the Dirac energy there is an intermediate regime where the pseudo-gap is filled ("plateau").…”

“…[14,15,16,17,18,19,20,21,22,23,24,25] and Refs. therein), and for BLG [26,27,28,29,30,31,32,12]. Most of them consider a standard Hamil-tonian that takes only into account the hopping between the nearest neighbors orbitals.…”

“…Most of them consider a standard Hamil-tonian that takes only into account the hopping between the nearest neighbors orbitals. Yet some studies show the importance of hopping beyond nearest neighbors on electronic structure and transport properties [4,14,24,25,32]. In this paper, we present electronic properties of MLG and BLG obtained by two tight-binding (TB) models: the standard model with nearest neighbor only (TB1) and a TB model including the effect of the hopping beyond nearest neighbors (TB2).…”

“…For this reason, theses scatters are called resonant scatters. The adsorbate is simulated by a simple vacancy in the plane of p z orbital as usually done [4,12,20,23,32]. Indeed the covalent bonding between the adsorbate and the carbon atom of graphene to which it is linked, eliminates the p z orbital from the relevant energy window.…”

Two-particle excitations under coexisting electron interaction and disorder

“…We attribute this behavior to enhanced quantum correction induced by electron-hole interactions. PACS numbers: 71.35.-y, 64.70.Tg, 31.15.V-, 71.35.CcIntroduction.-Recent experiments have shown that most correlated materials contain a significant amount of defects, which appear to be intrinsic [1][2][3][4][5][6][7]. These inhomogeneities could significantly affect device performance.…”