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Register a new userDoping asymmetry of superconductivity coexisting with antiferromagnetism in spin fluctuation theory
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We generalize the theory of Cooper pairing by spin excitations in the metallic antiferromagnetic state to include situations with electron and/or hole pockets. We show that Cooper pairing arises from transverse spin waves and from gapped longitudinal spin fluctuations of comparable strength. However, each of these interactions, projected on a particular symmetry of the superconducting gap, acts primarily within one type of pocket. We find a nodeless $d_{x^2-y^2}$-wave state is supported primarily by the longitudinal fluctuations on the electron pockets, and both transverse and longitudinal fluctuations support nodeless odd-parity spin singlet $p-$wave symmetry on the hole pockets. Our results may be relevant to the asymmetry of the AF/SC coexistence state in the cuprate phase diagram, as well as for the nodal gap observed recently for strongly underdoped cuprates.
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A microscopic Hamiltonian reflecting the correct symmetry of $f$-orbitals is proposed to discuss superconductivity in heavy fermion systems. In the orbitally degenerate region in which not only spin fluctuations but also orbital fluctuations develop considerably, cancellation between spin and orbital fluctuations destabilizes $d_{x^{2}-y^{2}}$-wave superconductivity. Entering the non-degenerate region by increasing the crystalline electric field, $d_{x^{2}-y^{2}}$-wave superconductivity mediated by antiferromagnetic spin fluctuations emerges out of the suppression of orbital fluctuations. We argue that the present scenario can be applied to recently discovered superconductors CeTIn$_{5}$ (T=Ir, Rh, and Co).
In iron pnictides, high temperature superconductivity emerges after suppressing antiferromagnetism by doping. Here we show that antiferromagnetism in Ca$_{1-x}$La$_x$FeAs$_2$ is robust against and even enhanced by doping. Using $^{75}$As-nuclear magnetic resonance and nuclear quadrupole resonance techniques, we find that an antiferromagnetic order occurs below the Neel temperature $T_{rm N}$ = 62 K at a high doping concentration ($x$ = 0.15) where superconductivity sets in at the transition temperature $T_{rm c}$ = 35 K. Unexpectedly, $T_{rm N}$ is enhanced with increasing doping, rising up to $T_{rm N}$ = 70 K at $x$ = 0.24. The obtained phase diagram of this new system enriches the physics of iron-based high-$T_{rm c}$ superconductors.
Unconventional superconductivity in molecular conductors is observed at the border of metal-insulator transitions in correlated electrons under the influence of geometrical frustration. The symmetry as well as the mechanism of the superconductivity (SC) is highly controversial. To address this issue, we theoretically explore the electronic properties of carrier-doped molecular Mott system $kappa$-(BEDT-TTF)$_2$X. We find significant electron-hole doping asymmetry in the phase diagram where antiferromagnetic (AF) spin order, different patterns of charge order, and SC compete with each other. Hole-doping stabilizes AF phase and promotes SC with $d_{xy}$-wave symmetry, which has similarities with high-$T_c$ cuprates. In contrast, in the electron-doped side, geometrical frustration destabilizes the AF phase and the enhanced charge correlation induces another SC with extended-$s$+$d_{x^2-y^2}$-wave symmetry. Our results disclose the mechanism of each phase appearing in filling-control molecular Mott systems, and elucidate how physics of different strongly-correlated electrons are connected, namely, molecular conductors and high-$T_c$ cuprates.
A new mechanism for superconductivity in the newly discovered Co-based oxide is proposed by using charge fluctuation. A single-band extended Hubbard model on the triangular lattice is studied within random phase approximation. $f$-wave triplet superconductivity is stabilized in the vicinity of charge-density-wave instability, which is in sharp contrast with the square-lattice case. The physical origin of the realization of the $f$-wave triplet state as well as the relevance to experiments are discussed.
Interplay between antiferromagnetism and superconductivity is studied by using the 3-dimensional nearly half-filled Hubbard model with anisotropic transfer matrices $t_{rm z}$ and $t_{perp}$. The phase diagrams are calculated for varying values of the ratio $r_{rm z}=t_{rm z}/t_{perp}$ using the spin fluctuation theory within the fluctuation-exchange approximation. The antiferromagnetic phase around the half-filled electron density expands while the neighboring phase of the anisotropic $d_{x^{2}-y^{2}}$-wave superconductivity shrinks with increasing $r_{rm z}$. For small $r_{rm z}$ $T_{rm c}$ decreases slowly with increasing $r_{rm z}$. For moderate values of $r_{rm z}$ we find the second order transition, with lowering temperature, from the $d_{x^{2}-y^{2}}$-wave superconducting phase to a phase where incommensurate SDW coexists with $d_{x^{2}-y^{2}}$-wave superconductivity. Resonance peaks as were discussed previously for 2D superconductors are shown to survive in the $d_{x^{2}-y^{2}}$-wave superconducting phase of 3D systems. Soft components of the incommensurate SDW spin fluctuation mode grow as the coexistent phase is approached.
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