arXiv subjects

Sina Moradian

Publications and source records attributed to Sina Moradian.

4 recordsLinked to original sources

arXiv2020

Superconducting Symmetry Phases and Dominant bands in (Ca-) Intercalated AA- Bilayer Graphene

Built on a realistic multiband tight-binding model, mirror symmetry is used to map a calcium-intercalated bilayer graphene Hamiltonian into two independent single layer graphene-like Hamiltonians with renormalized hopping. The quasiparticles exhibit two types of chirality. Here a quasi-particle consists of two electrons from opposing layers where possess an additional quantum number called "cone index" which can be regarded as the eigenvalue of mirror symmetry operations. To obtain tight-binding parameters, both effective monolayer Schrodinger equations are solved analytically and fitted to first-principles band structure results. Two quasi-particles (four electrons) can team up to build a Cooper pair with even or odd chirality. Treatment of the pairing Hamiltonian leads to two decoupled gap equations. The pairing of quasi-particles with different cone indexes is forbidden. The decoupled gap equations are solved analytically to obtain all the possible superconducting phases. Two nearly "flat bands" crossing the Fermi energy, each related to the graphene-like structures, are responsible for two distinct superconductivity gaps that emerge. Depending on how much these bands are affected by the intercalant and which is closer to the Fermi energy, distorted s-wave or d-wave superconductivity may become dominant. Numerical calculations reveal that d-wave superconductivity is dominant in both sectors. For these two dominant phases, within the range of 0-6 K which superconductivity has been observed, numerically the transition from single-gap to dual-gap superconductivity is possible. Adopting the two-gap viewpoint of superconductivity in C$_6$CaC$_6$, the dominant $d$-wave states should have the same critical temperature. Around $T_c=2K$ these two relations intersect, otherwise, superconductivity has been realized just in one of these two sectors and disappears in the other one.

cond-mat.supr-con↗

arXiv2020

Comment on: Locally self-consistent embedding approach for disordered electronic systems

We comment on article by Yi Zhang , Hanna Terletska, Ka-Ming Tam, Yang Wang, Markus Eisenbach, Liviu Chioncel, and Mark Jarrell [Phys. Rev. B {\bf 100}, 054205 (2019)]\cite{Zhang} in which to study substitution disordered systems, they presented an embedding scheme for the locally self-consistent method. Here we show that their methods is a truncated case of our super-cell approximation, achieved by neglecting super-cell wave vectors dependence on self-energy $Σ_{sc}({\bf K}_{n},E)$ and replacing them by a local on-site self-energy, $Σ_{sc}({\bf K}_{n},E)=Σ_{sc}(L,L,E)$ in our articles\cite{Moradian01, Moradian02, Moradian03}. Also their real and k-space self-energies in the limit of the number of super-cell sites, $N_{c}$, approaching the number of lattice sites, N, do not recover exact self-energies $Σ(l, l', E)$ and $Σ({\bf k}, E)$. For highlighting advantages of our methods with respect to other approximations such as dynamical cluster approximation (DCA)\cite{Jarrell} in capturing electron localization, we apply our real space super-cell approximation (SCA), and super-cell local self-energy approximation (SCLSA) to one and two dimensional substitution disorder alloy systems. Our electron localization probability calculations for these systems determine non zero values that indicate electrons localization.

cond-mat.mes-hall↗

arXiv2018

Beyond real space super cell approximation, corrections to the real space cluster approximation

Motion of a single electron in a disordered alloy and or interacting electrons systems such as magnetic materials, strongly correlated systems and superconductors is replaced by motion of that in an effective medium which is denoted by self-energy. The study of disordered alloy and interacting electrons systems based on single electron motion is an old challenge and an important problem in condensed matter physics. In this paper we introduce a real space approximation beyond super cell approximation for the study of these systems to capture multi-site effects. Average disordered alloy or interacting system is replaced by a self-energy, $Σ(i,j,E)$. We divided self-energy in q-space $Σ({\bf q}; E)=\frac{1}{N}\sum_{ij}e^{i{\bf q}.{\bf r}_{ij}}Σ(i,j; E)$ into two parts $Σ({\bf q}; E)=\frac{1}{N_{c}}\sum_{IJ\in\; \mbox{\tiny same cluster}}e^{i{\bf q}.{\bf r}_{IJ}}Σ(I,J; E)+\frac{1}{N}\sum_{ij\notin \:\mbox{\tiny same cluster}}e^{i{\bf q}.{\bf r}_{IJ}}Σ(I,J,E)$ where $\{Lc_{1}, Lc_{2},Lc_{3}\}$ are dimensions of the super cell. We show that neglecting the second term of q-space self-energy leads to super cell approximation $e^{iq_{j} Lc_{j}}=1$, hence $ q_{j}$ determined by $ q_{j} Lc_{j}=2πn_{j}$. Then we kept this correction in the second step to add self energies of sites in different super cells which leads to fully q-dependent self energy in the first Brillouin zone (FBZ). Our self-energy in FBZ is casual, fully q-dependent, and continuous with respect to ${\bf q}$. It recovers coherent potential approximation in the single site approximation and is exact when the number of sites in the super cell approaches to the total number of lattice sites. We illustrate that this approximation undertakes electrons localization for one and two dimensional alloy systems which isn't observed by previous multi site approximations.

cond-mat.dis-nn↗

arXiv2018

Superconducting Phases in Lithium Decorated Graphene LiC 6

A study of possible superconducting phases of graphene has been constructed in detail. A realistic tight binding model, fit to ab initio calculations, accounts for the Li-decoration of graphene with broken lattice symmetry, and includes $s$ and $d$ symmetry Bloch character that influences the gap symmetries that can arise. The resulting seven hybridized Li-C orbitals that support nine possible bond pairing amplitudes. The gap equation is solved for all possible gap symmetries. One band is weakly dispersive near the Fermi energy along $Γ\rightarrow M$ where its Bloch wave function has linear combination of $d_{x^2-y^2}$ and $d_{xy}$ character, and is responsible for $ d_{x^{2}-y^{2}}$ and $d_{xy}$ pairing with lowest pairing energy in our model. These symmetries almost preserve properties from a two band model of pristine graphene. Another part of this band, along $K\rightarrow Γ$, is nearly degenerate with upper $s$ band that favors extended $s$ wave pairing which is not found in two band model. Upon electron doping to a critical chemical potential $μ_1=0.22 eV$ the pairing potential decreases, then increases until a second critical value $μ_2$=1.3 eV at which a phase transition to a new phase which is not appear in two band model. This phase in the pristine graphene converts to usual extended s-wave pairing.

cond-mat.supr-con↗