pseudo-Landau Levels Research Articles

Stability of edge states in strained graphene

Spatially inhomogeneous strains in graphene can simulate the effects of valley-dependent magnetic fields. As demonstrated in recent experiments [N. Levy et al., Science 329, 544 (2010) and K. K. Gomes et al., Nature (London) 483, 306 (2012)], the realizable magnetic fields are large enough to give rise to well-defined flat pseudo-Landau levels, potentially having counterpropagating edge modes. In this work, we address the conditions under which such edge modes are visible. We find that, whereas armchair edges do not support counterpropagating edge modes, zigzag edges do so, through a selective-hybridization mechanism. We then discuss effects of interactions on the stability of counterpropagating edge modes and find that, for the experimentally relevant case of Coulomb interactions, interactions typically decrease the stability of the edge modes. Finally, we generalize our analysis to address the case of spontaneous valley polarization, which is expected to occur in charge-neutral strained graphene [P. Ghaemi et al., Phys. Rev. Lett. 108, 266801 (2012) and D. A. Abanin and D. A. Pesin, Phys. Rev. Lett. 109, 066802 (2012)] .

Pseudomagnetoexcitons in strained graphene bilayers without external magnetic fields

We propose a strained graphene double-layer (SGDL) system for detecting pseudomagnetoexcitons (PME) in the absence of external magnetic fields. The carriers in each graphene layer experience different strong pseudomagnetic fields (PMFs) due to strain engineering, which give rise to Landau quantization. The pseudo-Landau levels of electron-hole pairs under inhomogeneous PMFs in the SGDL are obtained analytically in the absence of Coulomb interactions. Based on the derived optical absorption selection rule for PMEs, we interpret the optical absorption spectra as indicating the formation of Dirac-type PMEs. We also predict that in the presence of inhomogeneous PMFs, the superfluidity-normal phase-transition temperature of PMEs is greater than that under homogeneous PMFs.

Interaction-Induced Topological Insulator States in Strained Graphene

The electronic properties of graphene can be manipulated via mechanical deformations, which opens prospects for both studying the Dirac fermions in new regimes and for new device applications. Certain natural configurations of strain generate large nearly uniform pseudomagnetic fields, which have opposite signs in the two valleys, and give rise to flat spin- and valley-degenerate pseudo-Landau levels (PLLs). Here we consider the effect of the Coulomb interactions in strained graphene with a uniform pseudomagnetic field. We show that the spin or valley degeneracies of the PLLs get lifted by the interactions, giving rise to topological insulator states. In particular, when a nonzero PLL is quarter or three-quarter filled, an anomalous quantum Hall state spontaneously breaking time-reversal symmetry emerges. At half-filled PLLs, a weak spin-orbital interaction stabilizes the time-reversal-symmetric quantum spin-Hall state. These many-body states are characterized by the quantized conductance and persist to a high temperature scale set by the Coulomb interactions, which we estimate to be a few hundreds Kelvin at moderate strain values. At fractional fillings, fractional quantum Hall states breaking valley symmetry emerge. These results suggest a new route to realizing robust topological states in mesoscopic graphene.

Pseudo-magnetic field distribution and pseudo-Landau levels in suspended graphene flakes

Combining the tight-binding approximation and linear elasticity theory for a planar membrane, we investigate stretching of a graphene flake assuming that two opposite edges of the sample are clamped by the contacts. We show that, depending on the aspect ratio of the flake and its orientation, gapped states may form in the membrane in the vicinity of the contacts. This gap in the pre-contact region should be the biggest for the armchair orientation of the flake and W/L≲1.