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Register a new userSuperconductivity from energy fluctuations in dilute quantum critical polar metals
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Added by
Pavel Volkov Andreevich
Publication date
2021
fields
Physics
and research's language is
English
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Superconductivity in low carrier density metals challenges the conventional electron-phonon theory due to the absence of retardation required to overcome Coulomb repulsion. In quantum critical polar metals, the Coulomb repulsion is heavily screened, while the critical transverse optic phonons decouple from the electron charge. In the resulting vacuum, the residual interactions between quasiparticles are carried by energy fluctuations of the polar medium, resembling the gravitational interactions of a dark matter universe. Here we demonstrate that pairing inevitably emerges from gravitational interactions with the energy fluctuations, leading to a dome-like dependence of the superconducting $T_c$ on carrier density. Our estimates show that this mechanism may explain the critical temperatures observed in doped SrTiO$_3$. We provide predictions for the enhancement of superconductivity near polar quantum criticality in two and three dimensional materials that can be used to test our theory.
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We consider an s-wave superconductor in the vicinity of a second-order ferromagnetic (FM) or spin-density-wave (SDW) quantum critical point (QCP), where the superconductivity and magnetism arise from separate mechanisms. The quantum critical spin fluctuations reduce the superconducting T_c. Near a FM QCP, we find that T_c falls to zero as 1/|ln kappa| in 3D and as kappa in 2D, where kappa ~ |J-J_c|^nu is the inverse correlation length of the spin fluctuations, and measures the distance |J-J_c| from the quantum critical point. SDW quantum critical fluctuations, on the other hand, suppress T_c to zero as sqrt(kappa) in 2D, and suppress T_c only to a finite value in 3D, producing a cusp of the form (const + |J-J_c|^nu).
We study the properties of $s$-wave superconductivity induced around a nematic quantum critical point in two-dimensional metals. The strong Landau damping and the Cooper pairing between incoherent fermions have dramatic mutual influence on each other, and hence should be treated on an equal footing. This problem is addressed by analyzing the self-consistent Dyson-Schwinger equations for the superconducting gap and Landau damping rate. We solve the equations at zero temperature without making any linearization, and show that the superconducting gap is maximized at the quantum critical point and decreases rapidly as the system departs from this point. The interplay between nematic fluctuation and an additional pairing interaction, caused by phonon or other boson mode, is also investigated. The total superconducting gap generated by such interplay can be several times larger than the direct sum of the gaps separately induced by these two pairing interactions. This provides a promising way to achieve remarkable enhancement of superconductivity.
The Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state near the antiferromagnetic quantum critical point (AFQCP) is investigated by analyzing the two dimensional Hubbard model on the basis of the fluctuation exchange (FLEX) approximation. The phase diagram against the magnetic field and temperature is compared with that obtained in the BCS theory. We discuss the influences of the antiferromagnetic spin fluctuation through the quasiparticle scattering, retardation effect, parity mixing and internal magnetic field. It is shown that the FFLO state is stable in the vicinity of AFQCP even though the quasiparticle scattering due to the spin fluctuation is destructive to the FFLO state. The large positive slope dH_{FFLO}/dT and the convex curvature (d^{2}H_{FFLO}/dT^{2} > 0) are obtained, where H_{FFLO} is the critical magnetic field for the second order phase transition from the uniform BCS state to the FFLO state. These results are consistent with the experimental results in CeCoIn_5. The possible magnetic transition in the FFLO state is examined.
75As-zero-field nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements are performed on CaFe2As2 under pressure. At P = 4.7 and 10.8 kbar, the temperature dependences of nuclear-spin-lattice relaxation rate (1/T1) measured in the tetragonal phase show no coherence peak just below Tc(P) and decrease with decreasing temperature. The superconductivity is gapless at P = 4.7 kbar but evolves to that with multiple gaps at P = 10.8 kbar. We find that the superconductivity appears near a quantum critical point under pressures in the range 4.7 kbar < P < 10.8 kbar. Both electron correlation and superconductivity disappear in the collapsed tetragonal phase. A systematic study under pressure indicates that electron correlations play a vital role in forming Cooper pairs in this compound.
We report a high-pressure single crystal study of the superconducting ferromagnet UCoGe. Ac-susceptibility and resistivity measurements under pressures up to 2.2 GPa show ferromagnetism is smoothly depressed and vanishes at a critical pressure $p_c = 1.4$ GPa. Near the ferromagnetic critical point superconductivity is enhanced. Upper-critical field measurements under pressure show $B_{c2}(0)$ attains remarkably large values, which provides solid evidence for spin-triplet superconductivity over the whole pressure range. The obtained $p-T$ phase diagram reveals superconductivity is closely connected to a ferromagnetic quantum critical point hidden under the superconducting `dome.
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