Possible superconductivity in nearly antiferromagnetic itinerant fermion systems

Abstract

Strong spin fluctuations arising in itinerant fermion systems close to a magnetic instability may induce or inhibit superconductivity depending on the nesting wave vector qo for which the instability occurs. If~q o~i s small but finite, triplet pairing is favored and singlet pairing is suppressed as efficiently as in nearly ferromagnetic systems (qo --0). If~q o~i s large, there is a repulsive contribution from backward scattering by which triplet as well as singlet pairings are strongly depressed.The cases of UPt3, CeP13, and some organic compounds are considered.It has been known for a long time that strong spin fiuctuations ("paramagnons'") in nearly ferromagnetic (NF) fermion systems, prevent BCS singlet pairing superconductivity, ' but favor triplet pairing. This result has been well illustrated in the case of the triplet superconductivity of liquid He. The present note considers the effects of a paramagnon-mediated interaction near to a magnetic instability at finite wave vector qo. It is found that if the system is nearly antiferromagnetic (NAF) with a large value of~q o~, then backward scattering from the paramagnon-mediated interaction is repulsive for both parallel and antiparallel spin states, so that both triplet and singlet pairings are suppressed. As the wave vector qo is varied (increasing from zero), the paramagnon effect changes continuously from a qualitatively ferromagnetic to a qualitatively antiferromagnetic character, so that the effective interaction between parallel spina changes continuously from a strong attraction to a strong repulsion.These effects appear relevant for the study of possible mechanisms responsible for the observed superconductivity in the heavy-fermion systems UPt3, CeP13, or in some organic compounds under pressure such as the Bechgaard ditetramethyltetraselenafulvalenium salts (TMTSF)qX.We first briefly recall some known results. In the paramagnon model' for a single parabolic band of itinerant fermions, a strong Hubbard contact repulsion I among opposite spins [ Fig. 1(a)] can induce a magnetic transition when it is strong enough for the Stoner criterion:to be fulfilled. Here I=Imax' (q, O)=IX (qo, O) and X (q, co) is the dynamic spin-correlation function for free fermions as a function of the momentum transfer q and the frequency co. When max X (q, O) occurs for q=qo --0, the system undergoes a ferromagnetic instabili-

Spin and charge excitations in artificial hole- and electron-doped infinite layer cuprate superconductors

“…Paramagnons have also been observed in the prototypical electron-doped (e-doped) cuprate compound Nd 2−x Ce x CuO 4 (NCCO) 11,12 . These findings have triggered several works 7,[13][14][15] , aiming to show that paramagnons are the driving excitations in Cooper pair formation. However there is no obvious correlation between the critical temperature (T c ) and the paramagnon energy, and this may indicate that also other low energy excitations play a role, such as phonons (via electron-phonon coupling) and charge modes 11,12,16 .…”

Midgap edge states and pairing symmetry of quasi-one-dimensional organic superconductors

“…For a 1D p xwave superconductor with only one species of spin, Eqs. (4) and (9) imply that the two edge states at the opposite ends are described by the Majorana operators of the opposite parity:Ψ † = ±Ψ [27]. There was a proposal to use such edge Majorana fermions for quantum computing [27].…”

“…Another signature of triplet pairing is that the Knight shift does not change between the normal and superconducting states [7]. However, the temperature dependence of the NMR relaxation rate [8] and analogy with the high-temperature superconductors led to an alternative proposal of the d-wave symmetry [9]. It was also proposed that a singlet Q1D superconductivity can overcome the Pauli paramagnetic limit by forming the spatially nonuniform Larkin-Ovchinnikov-FuldeFerrell state [10].…”

Basic considerations on broken-symmetry states of organic superconductors

“…This immediately follows from the expression of the vertex part in (3) close to T SDW . A large increase of the forward scattering between carriers that move in opposite directions yields quite unfavorable conditions for conventional BCS superconductivity [34]. These conditions become even worst when the influence of interchain exchange J ⊥ is included in the analysis [27].…”

“…As suggested by Emery [38,34], long-lived antiferromagnetic spin fluctuations of wavevector Q 0 give rise to an oscillating potential in real space at the corresponding periodicity, to which electrons are coupled. Electrons moving in opposite directions can thus avoid local repulsion and even be attracted to each other if they move on different chains.…”