arXiv subjects

P. Sahebsara

Publications and source records attributed to P. Sahebsara.

4 recordsLinked to original sources

arXiv2021

Charge order and antiferromagnetism in twisted bilayer graphene from the variational cluster approximation

We study the possibility of charge order at quarter filling and antiferromagnetism at half-filling in a tight-binding model of magic angle twisted bilayer graphene. We build on the model proposed by Kang and Vafek [Physical Review X 8(3), 031088 (2018)], relevant to a twist angle of $1.30^\circ$, and add on-site and extended density-density interactions. Applying the variational cluster approximation with an exact-diagonalization impurity solver, we find that the system is indeed a correlated (Mott) insulator at fillings $\frac14$, $\frac12$ and $\frac34$. At quarter filling, we check that the most probable charge orders do not arise, for all values of the interaction tested. At half-filling, antiferromagnetism only arises if the local repulsion $U$ is sufficiently large compared to the extended interactions, beyond what is expected from the simplest model of extended interactions.

cond-mat.str-el↗

arXiv2021

Chiral $p$-wave superconductivity in twisted bilayer graphene from dynamical mean field theory

We apply cluster dynamical mean field theory with an exact-diagonalization impurity solver to a Hubbard model for magic-angle twisted bilayer graphene, built on the tight-binding model proposed by Kang and Vafek (2018), which applies to the magic angle $1.30^\circ$. We find that triplet superconductivity with $p+ip$ symmetry is stabilized by CDMFT, as well as a subdominant singlet $d+id$ state. A minimum of the order parameter exists close to quarter-filling and three-quarter filling, as observed in experiments.

cond-mat.str-el↗

arXiv2015

Chiral triplet superconductivity on the graphene lattice

Motivated by the possibility of superconductivity in doped graphene sheets, we investigate superconducting order in the extended Hubbard model on the two-dimensional graphene lattice using the variational cluster approximation (VCA) and the cellular dynamical mean-field theory (CDMFT) with an exact diagonalization solver at zero temperature. The nearest-neighbor interaction is treated using a mean-field decoupling between clusters. We compare different pairing symmetries, singlet and triplet, based on short-range pairing. VCA simulations show that the real (nonchiral), triplet $p$-wave symmetry is favored for small $V$, small on-site interaction $U$ or large doping, whereas the chiral combination $p+ip$ is favored for larger values of $V$, stronger on-site interaction $U$ or smaller doping. CDMFT simulations confirm the stability of the $p+ip$ solution, even at half-filling. Singlet superconductivity (extended $s$-wave or $d$-wave) is either absent or sub-dominant.

cond-mat.supr-con↗

arXiv2006

Antiferromagnetism and Superconductivity in layered organic conductors: Variational cluster approach

The $κ$-(ET)$_2$X layered conductors (where ET stands for BEDT-TTF) are studied within the dimer model as a function of the diagonal hopping $t^\prime$ and Hubbard repulsion $U$. Antiferromagnetism and d-wave superconductivity are investigated at zero temperature using variational cluster perturbation theory (V-CPT). For large $U$, Néel antiferromagnetism exists for $t' < t'_{c2}$, with $t'_{c2}\sim 0.9$. For fixed $t'$, as $U$ is decreased (or pressure increased), a $d_{x^2-y^2}$ superconducting phase appears. When $U$ is decreased further, the a $d_{xy}$ order takes over. There is a critical value of $t'_{c1}\sim 0.8$ of $t'$ beyond which the AF and dSC phases are separated by Mott disordered phase.

cond-mat.supr-con↗