[Paper Review] Superconducting dome in doped quasi-2d organic Mott insulators: a paradigm for strongly-correlated superconductivity

Charles-David Hébert, P. Sémon+1 more|arXiv (Cornell University)|Sep 23, 2015

Physics of Superconductivity and Magnetism3 citations

TL;DR

This study uses plaquette cellular dynamical mean-field theory and continuous-time quantum Monte Carlo to show that superconductivity in doped quasi-2D organic Mott insulators arises from a first-order transition between a pseudogap metal and a correlated metal, not from an antiferromagnetic quantum critical point. The superconducting dome is enhanced by ~25% at finite doping compared to half-filling, with a broadened U/t range, and the transition is tied to the Mott transition's continuation into the doped regime, mirroring cuprate physics.

ABSTRACT

Layered organic superconductors of the BEDT family are model systems for the interplay of the Mott transition with superconductivity, magnetic order and frustration. Recent experimental studies on a hole-doped version of BEDT compounds reveal an enhancement of superconductivity and a rapid crossover between two different conducting phases above the superconducting dome. One of these phases is a Fermi liquid, the other not. Using plaquette cellular dynamical mean field theory with state of the art continuous-time quantum Monte Carlo calculations, we study this problem with the two-dimensional Hubbard model on the anisotropic triangular lattice. Phase diagrams as a function of temperature $T$ and interaction strength $U/t$ are obtained for anisotropy parameters $t'=0.4t$, $t'=0.8t$ and various fillings. As for cuprates, we find, at finite doping, a first-order transition between two normal-state phases. For $T$ above the critical point of the first-order transition, there is a Widom line where crossovers occur. The results are in broad agreement with experiment. This suggests that for compounds with intermediate to high frustration, very light-doping should reveal the first-order transition and associated crossovers. These crossovers could leave traces in the superconducting phase. We also predict that destroying the superconducting phase by a magnetic field should reveal the first-order transition. Finally, we predict that electron-doping should also lead to an increased range of $U/t$ for superconductivity but with a reduced maximum $T_c$. This work also clearly shows that the superconducting dome here is tied to the Mott transition and its continuation as a transition separating pseudogap phase from correlated metal in doped compounds, as in the cuprates. Contrary to heavy fermions for example, the maximum $T_c$ is definitely not attached to an antiferromagnetic quantum critical point.

Motivation & Objective

• To understand the origin of the superconducting dome in doped quasi-2D organic Mott insulators, particularly in hole-doped BEDT-based compounds.
• To determine whether the enhanced superconductivity and normal-state crossover observed experimentally are linked to antiferromagnetic quantum criticality or to the Mott transition.
• To investigate the role of electronic frustration (via t′/t ratio) in suppressing magnetic order and modifying superconducting and normal-state phases.
• To predict signatures of the first-order transition in the superconducting state and under magnetic field suppression.
• To compare the behavior of hole-doped and electron-doped systems in the context of strongly correlated superconductivity.

Proposed method

• Plaquette cellular dynamical mean-field theory (CDMFT) is used to solve the two-dimensional Hubbard model on an anisotropic triangular lattice.
• Continuous-time quantum Monte Carlo (CTQMC) calculations are employed to compute self-energy and Green's functions with high accuracy.
• Phase diagrams are computed as functions of temperature T, interaction strength U/t, filling n, and frustration parameter t′/t (0.4t, 0.8t).
• The presence of a first-order transition between pseudogap and metallic phases is identified via thermodynamic and spectral analysis.
• The Widom line, marking crossover behavior above the critical point, is mapped in the T–U/t plane.
• The effects of electron-doping are compared to hole-doping to assess symmetry in superconducting behavior.

Experimental results

Research questions

• RQ1 Does the superconducting dome in doped organic Mott insulators originate from an antiferromagnetic quantum critical point or from the continuation of the Mott transition?
• RQ2 How does frustration (t′/t) influence the suppression of magnetic order and the stability of superconductivity?
• RQ3 Can the first-order transition between pseudogap and metallic phases in the normal state be probed by destroying superconductivity with a magnetic field?
• RQ4 Why is the superconducting dome broader and Tc higher in doped compounds compared to the half-filled case?
• RQ5 How does electron-doping affect the maximum Tc and the range of U/t for superconductivity?

Key findings

• The maximum superconducting critical temperature Tc,max is enhanced by approximately 25% at finite doping compared to its value at half-filling.
• The range of U/t over which superconductivity appears is significantly broadened in doped compounds, especially at higher frustration (t′/t = 0.8).
• A first-order transition between a pseudogap phase and a correlated metal occurs at finite doping and low temperature, continuously connected to the Mott transition at half-filling.
• The superconducting dome is not associated with an antiferromagnetic quantum critical point, as confirmed by the absence of long-range antiferromagnetic order near optimal doping.
• Applying a magnetic field to suppress superconductivity should reveal the first-order normal-state transition between pseudogap and metallic phases in lightly doped, highly frustrated compounds.
• Electron-doping is predicted to reduce the maximum Tc while still extending the range of U/t for superconductivity, indicating a different doping asymmetry than in cuprates.