Using magnetic stripes to stabilize superfluidity in electron-hole double monolayer graphene

Experiments have confirmed that double monolayer graphene does not generate finite-temperature electron-hole superfluidity, because of very strong screening of the pairing attraction. The linear dispersing energy bands in monolayer graphene block any attempt to reduce the strength of the screening. We propose a hybrid device with two sheets of monolayer graphene in a modulated periodic perpendicular magnetic field. The field preserves the isotropic Dirac cones of the original monolayers but reduces the slope of the cones, making the monolayer Fermi velocity $v_F$ smaller. We demonstrate that with current experimental techniques, the reduction in $v_F$ can weaken the screening sufficiently to allow electron-hole superfluidity at measurable temperatures.

Figure 1

Possible realization of the device. An array of ferromagnetic stripes with periodicity $2d_B$, placed on top of two monolayer sheets of graphene, produces a periodic magnetic field $B_z\simeq \pm B$.

Figure 2

Interaction parameter $r_s$ for a magnetic field of periodicity $2d$ for dielectric constants $\kappa =3$ (monolayers embedded in a h-BN substrate) and $\kappa =2$ (free-standing monolayers, separated by h-BN). Superfluidity occurs for $r_x\ge r_s^{\mathrm{onset}}=2.35$ [2].

Figure 3

Maximum superfluid gap ${\mathrm{\Delta }}_{\max }$ for different values of the magnetic field $B$, as a function of sheet densities $n$. Sheet separation is $D=2$ nm. At arrow points: ${\mathrm{\Delta }}_{\max }$ in Kelvin.

Figure 4

(a) Periodic deformation profile $h$ of a graphene sheet. (b) Induced strain in the sheet. (c) Induced pseudomagnetic field in the sheet.

Figure 5

(a) $T_{\text{KT}}^{\max }$ as a function of $B$. Array spacings $d=3.1$ and $2.7$ give maximum transition temperatures for embedded and free-standing systems, respectively. (b) $T_{\text{KT}}^{\max }$ as a function of $d$.