Mixed singlet-triplet superconducting state within the moir\'e $t$-$J$-$U$ model as applied to the description of twisted WSe$_2$ bilayer

Abstract

We analyze an analog of the $t$-$J$-$U$ model as applied to the description of a single moir\'e flat band of twisted WSe$_2$ bilayer. To take into account the correlation effects induced by a significant strength of the Coulomb repulsion, we use the Gutzwiller approach and compare it with the results obtained by the Hartree-Fock method. We discuss in detail the graduate appearance of a two dome structure of the superconducting state in the phase diagram by systematically increasing the Coulomb repulsion integral, $U$. The two superconducting domes residing on both sides of a Mott insulating state can be reproduced for a realistic parameter range in agreement with the available experimental data. According to our analysis the paired state has a highly unconventional character with a mixed $d+id$ (singlet) and $p-ip$ (triplet) symmetry. Both components of the mixed paired state are of comparable amplitudes. However, as shown here, a transition between pure singlet and pure triplet pairing should be possible in the considered system by tuning the gate voltage, which controls the magnitude of the valley-dependent spin-splitting in the system.