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Register a new userTriplet Cooper Pair Formation by Anomalous Spin Fluctuations in Non-centrosymmetric Superconductors
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A microscopic theory for the spin triplet Cooper pairing in non-centrosymmetric superconductors like CePt_3Si and CeTSi_3 (T=Rh, Ir) is presented. The lack of inversion symmetry leads to new anomalous spin fluctuations which stabilize the triplet part in addition to the singlet part originating from the centrosymmetric spin fluctuations. It is shown that both parts have similar nontrivial momentum dependence of A_1 type. Therefore the mixed singlet-triplet gap function has accidental line nodes on both Fermi surface sheets which are stable as function of temperature. This gap function explains the salient features of CePt_3Si and CeTSi_3 superconductors.
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In this chapter we discuss the physical properties of a particular family of non-centrosymmetric superconductors belonging to the class heavy-fermion compounds. This group includes the ferromagnet UIr and the antiferromagnets CeRhSi3, CeIrSi3, CeCoGe3, CeIrGe3 and CePt3Si, of which all but CePt3Si become superconducting only under pressure. Each of these superconductors has intriguing and interesting properties. We first analyze CePt3Si, then review CeRhSi3, CeIrSi3, CeCoGe3 and CeIrGe3, which are very similar to each other in their magnetic and electrical properties, and finally discuss UIr. For each material we discuss the crystal structure, magnetic order, occurrence of superconductivity, phase diagram, characteristic parameters, superconducting properties and pairing states. We present an overview of the similarities and differences between all these six compounds at the end.
Pair density wave superconductivity constitutes a novel electronic condensate proposed to be realized in certain unconventional superconductors. Establishing its potential existence is important for our fundamental understanding of superconductivity in correlated materials. Here we compute the dynamical magnetic susceptibility in the presence of a pair density wave ordered state, and study its fingerprints on the spin-wave spectrum including the neutron resonance. In contrast to the standard case of d-wave superconductivity, we show that the pair density wave phase exhibits neither a spin-gap nor a magnetic resonance peak, in agreement with a recent neutron scattering experiment on underdoped La$_{1.905}$Ba$_{0.095}$CuO$_4$ [Z. Xu et al., Phys. Rev. Lett. 113, 177002 (2014)].
Identification of pairing mechanisms leading to the unconventional superconductivity realized in copper-oxide, heavy-fermions, and organic compounds is one of the most challenging issues in condensed-matter physics. Clear evidence for an electron-phonon mechanism in conventional superconductors is seen by the isotope effect on the superconducting transition temperatures $T_{rm SC}$, since isotopic substitution varies the phonon frequency without affecting the electronic states. In unconventional superconductors, magnetic fluctuations have been proposed to mediate superconductivity, and considerable efforts have been made to unravel relationships between normal-state magnetic fluctuations and superconductivity. Here, we show that characteristic experimental results on the ferromagnetic (FM) superconductor UCoGe ($T_{rm Curie} sim 2.5 $ K and $T_{rm SC} sim 0.6$ K) can be understood consistently within a scenario of the spin-triplet superconductivity induced by FM spin fluctuations. Temperature and angle dependencies of the upper critical magnetic field of the superconductivity ($H_{c2}$) are calculated on the basis of the above scenario by solving the Eliashberg equation. Calculated $H_{c2}$ well agrees with the characteristic experimental results observed in UCoGe. This is a first example that FM fluctuations are shown to be a pairing glue of superconductivity.
The paramagnetic properties in non-centrosymmetric superconductors with and without antiferromagnetic (AFM) order are investigated with focus on the heavy Fermion superconductors, CePt_3Si, CeRhSi_3 and CeIrSi_3. First, we investigate the spin susceptibility in the linear response regime and elucidate the role of AFM order. The spin susceptibility at T=0 is independent of the pairing symmetry and increases in the AFM state. Second, the non-linear response to the magnetic field are investigated on the basis of an effective model for CePt_3Si which may be also applicable to CeRhSi_3 and CeIrSi_3. The role of antisymmetric spin-orbit coupling (ASOC), helical superconductivity, anisotropic Fermi surfaces and AFM order are examined in the dominantly s-, p- and d-wave states. We emphasize the qualitatively important role of the mixing of superconducting (SC) order parameters in the p-wave state which enhances the spin susceptibility and suppresses paramagnetic depairing effect in a significant way. Therefore, the dominantly p-wave superconductivity admixed with the s-wave order parameter is consistent with the paramagnetic properties of CePt_3Si at ambient pressure. We propose some experiments which can elucidate the novel pairing states in CePt_3Si as well as CeRhSi_3 and CeIrSi_3.
At the interface between a ferromagnetic insulator and a superconductor there is a coupling between the spins of the two materials. We show that when a supercurrent carried by triplet Cooper pairs flows through the superconductor, the coupling induces a magnon spin current in the adjacent ferromagnetic insulator. The effect is dominated by Cooper pairs polarized in the same direction as the ferromagnetic insulator, so that charge and spin supercurrents produce similar results. Our findings demonstrate a way of converting Cooper pair supercurrents to magnon spin currents.
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