Anisotropic minimal conductivity of graphene bilayers

Fermi line of bilayer graphene at zero energy is transformed into four separated points positioned trigonally at the corner of the hexagonal first Brillouin zone. We show that as a result of this trigonal splitting the minimal conductivity of an undoped bilayer graphene strip becomes anisotropic with respect to the orientation $\theta$ of the connected electrodes and finds a dependence on its length $L$ on the characteristic scale $\ell=\pi/\Delta k\simeq 50 nm$ determined by the inverse of k-space distance of two Dirac points. The minimum conductivity increases from a universal isotropic value $\sigma^{min}_{\bot}=(8/\pi)e^2/h$ for a short strip $L\ll \ell$ to a higher anisotropic value for longer strips, which in the limit of $L\gg \ell$ varies from $(7/3)\sigma^{min}_{\bot}$ at $\theta=0$ to $3\sigma^{min}_{\bot}$ over an angle range $\Delta \theta\sim \ell/L$.Comment: 4 pages, 2 figure

Gate-controlled supercurrent reversal in MoS2_2-based Josephson junctions

Motivated by recent experiments revealing superconductivity in MoS$_2$, we investigate the Josephson effect in the monolayer MoS$_2$ at the presence of an exchange splitting. We show that the supercurrent reversal known as $0-\pi$ transition can occur by varying the doping via gate voltages. This is in contrast to common superconductor/ferromagnet/superconductor junctions in which successive $0-\pi$ transition take place with the variation of junction length or temperature. In fact for the case of MoS$_2$ we find that both the amplitude and the period of oscillations show a dependence on the doping which explains the predicted doping induced supercurrent reversal. These effects comes from the dependence of density and Fermi velocity on the doping strength beside the intrinsic spin splitting in the valence band which originates from spin-orbit interaction.Comment: 5 pages, 3 figure

Enhanced Andreev reflection in gapped graphene

We theoretically demonstrate unusual features of superconducting proximity effect in gapped graphene which presents a pseudospin symmetry-broken ferromagnet with a net pseudomagnetization. We find that the presence of a band gap makes the Andreev conductance of graphene superconductor/pseudoferromagnet (S/PF) junction to behave similar to that of a graphene ferromagnet-superconductor junction. The energy gap $\Delta_N$ enhance the pseudospin inverted Andreev conductance of S/PF junction to reach a limiting maximum value for $\Delta_N\gg \mu$, which depending on the bias voltage can be larger than the value for the corresponding junction with no energy gap. We further demonstrate a damped-oscillatory behavior for the local density of states of the PF region of S/PF junction and a long-range crossed Andreev reflection process in PF/S/PF structure with antiparallel alignment of pseudomagnetizations of PFs, which confirm that, in this respect, the gapped normal graphene behaves like a ferromagnetic graphene.Comment: 7.2 pages, 5 figures, accepted for publication in Phys. Rev.

Josephson effect in mesoscopic graphene strips with finite width

We study Josephson effect in a ballistic graphene strip of length $L$ smaller than the superconducting coherence length and arbitrary width $W$. We find that the dependence of the critical supercurrent $I_{c}$ on $W$ is drastically different for different types of the edges. For \textit{smooth} and \textit{armchair} edges at low concentration of the carriers $I_{c}$ decreases monotonically with decreasing $W/L$ and tends to a constant minimum for a narrow strip $W/L\lesssim1$. The minimum supercurrent is zero for smooth edges but has a finite value $e\Delta_{0}/\hbar$ for the armchair edges. At higher concentration of the carriers, in addition to this overall monotonic variation, the critical current undergoes a series of peaks with varying $W$. On the other hand in a strip with \textit{zigzag} edges the supercurrent is half-integer quantized to $(n+1/2)4e\Delta_{0}/\hbar$, showing a step-wise variation with $W$.Comment: 4 pages, 3 figure

The Pearl of the Prairies: The History of the Winnipeg Filipino Community

Canadian historical and national narratives often prize the creation of “White Canada” through immigration from European nations. Significant movements of people from the Asia-Pacific region often get left out of these narratives, even though Asian populations have been in Canada as long as white settlers. Furthermore, the growing body of Asian Canadian literature itself has developed a tunnel vision for East and South Asian immigrants, neglecting myriad other groups from regions such as Southeast Asia. While Chinese, Japanese, and South Asian immigrants have dominated immigration from Asia until recently, other groups such as Filipinos have long been living and working within Canada. Today, the Philippines is the largest source country of immigrants to Canada and yet remains one of the least studied ethnic communities. This dissertation analyzes the history of the Filipino community in Winnipeg to the 1980s, as well as detailing their longer history in Canada. Today, Winnipeg has the third largest Filipino population in Canada, which is the largest in terms of per-capita population. The major research question, “Why Winnipeg?”, forms the heart of this dissertation. What factors in Canada and the Philippines have combined to create the historically vibrant Prairie community? This study first lays out the history of the community to fill a knowledge gap on the Filipino diaspora in Canada, particularly from a historical perspective, analyzing themes of post-Second World War international relations, labour history, and the history of under-development in the Philippines. This analysis argues that a serendipitous confluence of events led to the origins and growth of the Winnipeg Filipino community. After laying this historical foundation, the themes of identity and memory are explored. This dissertation adopts the term “Filipino Self” and the “Filipino Other” to describe how, through the medium of ethnic media, the Winnipeg community negotiated a Filipino identity in the diaspora that directly engaged with the Philippine national identity, demonstrating the tight connection maintained to their Southeast Asian homeland. The analysis then examines the dynamics of historical memory within the community, and the politics that come along with crafting such discussions, through the examination of a museum exhibit celebrating 50 years of Filipinos in Winnipeg

U-duality in three and four dimensions

Using generalised geometry we study the action of U-duality acting in three and four dimensions on the bosonic fields of eleven dimensional supergravity. We compare the U-duality symmetry with the T-duality symmetry of double field theory and see how the $SL(2)\otimes SL(3)$ and SL(5) U-duality groups reduce to the SO(2,2) and SO(3,3) T-duality symmetry groups of the type IIA theory. As examples we dualise M2-branes, both black and extreme. We find that uncharged black M2-branes become charged under U-duality, generalising the Harrison transformation, while extreme M2-branes will become new extreme M2-branes. The resulting tension and charges are quantised appropriately if we use the discrete U-duality group $E_d(Z)$.Comment: v1: 35 pages; v2: minor corrections in section 4.1.2, many references added; v3: further discussion added on the conformal factor of the generalised metric in section 2 and on the Wick-rotation used to construct examples in section