Study of edge states and conductivity in spin-orbit coupled bilayer graphene

We present an elaborate and systematic study of the conductance properties of a zigzag bilayer graphene nanoribbon modeled by a Kane-Mele (KM) Hamiltonian. The interplay of the Rashba and the intrinsic spin-orbit couplings with the edge states, electronic band structures, charge and spin transport are explored in details. We have analytically derived the conditions for the edge states for a bilayer KM nanoribbon and show how these modes decay for lattice sites inside the bulk. It is particularly interesting to note that for a finite-size ribbon an even number of zigzag ribbon hosts a finite energy gap at the Dirac points, while the odd ones do not. This asymmetry is present both in presence and absence of a bias voltage that may exist between the layers. The interlayer Rashba spin-orbit coupling, along with the intralayer intrinsic spin-orbit and intralayer Rashba spin-orbit couplings seem to destroy the quantum spin Hall (QSH) phase where the QSH phase is identified by the presence of a conductance plateau (of magnitude 4e/h) in the vicinity of zero Fermi energy. The plateau is sensitive to the values of the spin-orbit coupling parameters. Further, the spin polarized conductance data reveal that a bilayer KM ribbon is found to be more efficient for spintronic applications compared to a monolayer graphene. Finally, a quick check with experiments is done via computing the effective mass of electrons.Comment: 12 page

Quantum phases of a spin-1 ultracold Bose gas with three body interactions

We study the effects of both a repulsive and an attractive three body interaction potential on a spin-1 ultracold Bose gas using mean field approach (MFA). For an antiferromagnetic (AF) inter- action, we have found the existence of the odd-even asymmetry in the Mott insulating (MI) lobes in presence of both the repulsive two and three body interactions. In case of a purely three body repulsive interaction, the higher order MI lobes stabilize against the superfluid phase. However, the spin nematic (singlet) formation is restricted upto the first (second) MI lobes for the former one, while there is neither any asymmetry nor spin nematic (singlet) formation is observed for the later case. The results are confirmed after carefully scrutinizing the spin eigen value and spin nematic order parameter for both the cases. On the other hand, for an attractive three body interaction, the third MI lobe is predominantly affected, where it completely engulfs the second and the fourth MI lobes at large values of the interaction strength. Albeit no significant change is observed beyond the fourth MI lobe. In the ferromagnetic case, the phase diagram shows similar features as that of a scalar Bose gas. We have compared our results on the MFA phase diagrams for both types of the interaction potential via a perturbation expansion in both the cases.Comment: 8 pages, 7 figure

Investigating Dirty Crossover through Fidelity Susceptibility and Density of States

We investigate the BCS-BEC crossover in an ultracold atomic gas in the presence of disorder. The disorder is incorporated in the mean-field formalism through Gaussian fluctuations. We observe evolution to an asymmetric line-shape of fidelity susceptibility as a function of interaction coupling with increasing disorder strength which may point to an impending quantum phase transition. The asymmetric line-shape is further analyzed using the statistical tools of skewness and kurtosis. We extend our analysis to density of states (DOS) for a better understanding of the crossover in the disordered environment.Comment: 12 pages, 6 figures. To appear in Int. J. Mod. Phys.

Effect of Length Scales on Microstructure Evolution During Severe Plastic Deformation

Effect of length scales on microstructure evolution during Severe Plastic Deformation (SPD) was studied by machining commercial purity metals: Ni 200, Oxygen Free High Conductivity OFHC) Cu and Al 1100. By performing Orientation Imaging Microscopy (OIM) in the chips created, a switch over in microstructure evolution in small length scales was demonstrated. In this, microstructure refinement during SPD was replaced by an anomalous lack of refinement in small length scales. This switchover was found to be rampant in OFHC Cu, followed by Ni 200 but almost absent in Al 1100. It was hypothesized that the switchover is a consequence of a coupled effect of high strain gradients and small deformation volumes. In order to quantify the switchover, flow of material in the deformation zone of machining was characterized in-situ using SEM based Digital Image Correlation (DIC). For doing this, a deformation stage capable of machining within the chamber of a Scanning Electron Microscope (SEM) was designed and fabricated. It was seen that OFHC Cu develops a sharp deformation zone during machining followed by a significantly more diffuse deformation zone in Ni 200 and then Al 1100. It was hypothesized that the switchover kicks in when the dimensions of the deformation zone approach those associated with Geometrically Necessary Boundaries that form during SPD. Criteria for the aforementioned switchover based on this hypothesis were verified for Ni 200, OFHC Cu and Al 1100. Effect of pre-deformation was studied by rolling Ni 200 samples prior to machining. It was seen that pre-deformation instigates the aforementioned switchover in microstructure evolution, reasons for which were discussed. A phenomenological model for predicting microstructure statistics resulting from SPD on Ni 200 in small length scales was setup. Contrary to common perception, it was shown that larger strain gradients giving rise to larger crystallographic curvatures instigate the aforementioned switchover resulting in lack of microstructure refinement