Abstract
Recent measurements of the resistivity in magic-angle twisted bilayer graphene near the superconducting transition temperature show twofold anisotropy, or nematicity, when changing the direction of an in-plane magnetic field [Cao etal., Science 372, 264 (2021)SCIEAS0036-807510.1126/science.abc2836]. This was interpreted as strong evidence for exotic nematic superconductivity instead of the widely proposed chiral superconductivity. Counterintuitively, we demonstrate that in two-dimensional chiral superconductors the in-plane magnetic field can hybridize the two chiral superconducting order parameters to induce a phase that shows nematicity in the transport response. Its paraconductivity is modulated as cos(2θ_{B}), with θ_{B} being the direction of the in-plane magnetic field, consistent with experiment in twisted bilayer graphene. We therefore suggest that the nematic response reported by Cao etal. does not rule out a chiral superconducting ground state.
Highlights
Recent measurements of the resistivity in magic-angle twisted bilayer graphene near the superconducting transition temperature show twofold anisotropy, or nematicity, when changing the direction of an in-plane magnetic field [Cao et al, Science 372, 264 (2021)]
Its paraconductivity is modulated as cosð2θBÞ, with θB being the direction of the inplane magnetic field, consistent with experiment in twisted bilayer graphene
Nematic fluctuation in the correlated insulating phases was observed in magic-angle twisted bilayer graphene (MATBG) by scanning tunneling microscopy (STM) [43,44,45], in twisted double bilayer graphene [46]
Summary
In this Letter, we formulate the phase transition of a chiral (d Æ id)-wave superconductor driven by a critical inplane magnetic field in a prototype honeycomb lattice of MATBG and demonstrate that the new phase is nematic with twofold anisotropy in the transport response. When the magnetic field B is larger than a critical one Bc, the two chiral states are hybridized witph ffiffiequal contributions of the form 1⁄2ξ1 þ expð2iθBÞξ2= 2, as depicted by points at the equator of the Bloch sphere. The coefficient of this superposition is modulated by the direction of the magnetic field denoted by angle θB. To construct the GL Lagrangian, we build the symmetry-allowed quadratic, quartic, and gradient terms of order parameters
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