Spin-triplet paired phases inside ferromagnet induced by Hund's rule coupling and electronic correlations: Application to UGe2\mathrm{UGe}_2

We discuss a mechanism of real-space spin-triplet pairing, alternative to that due to quantum paramagnon excitations, and demonstrate its applicability to $\mathrm{UGe_2}$. Both the Hund's rule ferromagnetic exchange and inter-electronic correlations contribute to the same extent to the equal-spin pairing, particularly in the regime in which the weak-coupling solution does not provide any. The theoretical results, obtained within the orbitally-degenerate Anderson lattice model, match excellently the observed phase diagram for $\mathrm{UGe_2}$ with the coexistent ferromagnetic (FM1) and superconducting ($A_1$-type) phase. Additionally, weak $A_2$- and $A$-type paired phases appear in very narrow regions near the metamaganetic (FM2 $\rightarrow$ FM1) and FM1 $\rightarrow$ paramagnetic first-order phase-transition borders, respectively. The values of magnetic moments in the FM2 and FM1 states are also reproduced correctly in a semiquantitative manner. The Hund's metal regime is also singled out as appearing near FM1-FM2 boundary

Superconductivity in the three-band model of cuprates: Variational wave function study and relation to the single-band case

The $d$-$wave$ superconductivity is analyzed within the three-band $d$-$p$ model with the use of the diagrammatic expansion of the Guztwiller wave function method (DE-GWF). The determined stability regime of the superconducting state appears in the range of hole doping $\delta\lesssim 0.35$, with the optimal doping close to $\delta\approx 0.19$. The pairing amplitudes between the $d$-orbitals due to copper and $p_x/p_y$ orbitals due to oxygen are analyzed together with the hybrid $d$-$p$ pairing. The $d$-$d$ pairing between the nearest neighboring atomic sites leads to the dominant contribution to the SC phase. Moreover, it is shown that the decrease of both the Coulomb repulsion on the copper atomic sites ($U_d$) and the charge transfer energy between the oxygen and copper atomic sites ($\epsilon_{dp}$) increases the pairing strength as it moves the system from the strong to the intermediate-correlation regime, where the pairing is maximized. Such a result is consistent with our analysis of the ratio of changes in the hole content at the $d$ and $p$ orbitals due to doping, which, according to experimental study, increases with the increasing maximal critical temperature [cf. Nat. Commun. 7, 11413 (2016)]. Furthermore, the results for the three-band model are compared to those for the effective single-band picture and similarities between the two approaches are discussed. For the sake of completeness, the normal-state characteristics determined from the DE-GWF approach are compared with those resulting from the Variational Quantum Monte Carlo method with inter-site correlations included through the appropriate Jastrow factors

Coexistent spin-triplet superconducting and ferromagnetic phases induced by the Hund's rule coupling and electronic correlations II: Effect of applied magnetic field

Recently proposed local-correlation-driven pairing mechanism, describing ferromagnetic phases (FM1 and FM2) coexisting with spin-triplet superconductivity (SC) within a single orbitally degenerate Anderson lattice model, is extended to the situation with applied Zeeman field. The model provides and rationalizes in a semiquantitative manner the principal features of the phase diagram observed for $\mathrm{UGe_2}$ in the field absence [cf. Phys. Rev. B $\mathbf{97}$, 224519 (2018)]. As spin-dependent effects play a crucial role for both the ferromagnetic and SC states, the role of the Zeeman field is to single out different stable spin-triplet SC phases. This analysis should thus be helpful in testing the proposed real-space pairing mechanism, which may be regarded as complementary to spin-fluctuation theory suitable for $\mathrm{^3He}$. Specifically, we demonstrate that the presence of the two distinct phases, FM1 and FM2, and associated field-driven metamagnetic transition between them, induce respective metasuperconducting phase transformation. At the end, we discuss briefly how the spin fluctuations might be incorporated as a next step into the considered here renormalized quasiparticle picture

Photoemission signature of momentum-dependent hybridization in CeCoIn5

Hybridization between f electrons and conduction bands (c-f hybridization) is a driving force for many unusual phenomena. To provide insight into it, systematic studies of CeCoIn 5 heavy fermion superconductor have been performed by angle-resolved photoemission spectroscopy (ARPES) in a large angular range at temperature of T = 6 K. The used photon energy of 122 eV corresponds to Ce 4d-4f resonance. Calculations carried out with the relativistic multiple scattering Korringa-Kohn-Rostoker method and one-step model of photoemission yielded realistic simulation of the ARPES spectra, indicating that Ce-In surface termination prevails. Surface states, which have been identified in the calculations, contribute significantly to the spectra. Effects of the hybridization strongly depend on wave vector. They include a dispersion of heavy electrons and bands gaining f-electron character when approaching Fermi energy. We have also observed a considerable variation of f-electron spectral weight at EF , which is normally determined by both matrix element effects and wave vector dependent c-f hybridization. Fermi surface scans covering a few Brillouin zones revealed large matrix element effects. A symmetrization of experimental Fermi surface, which reduces matrix element contribution, yielded a specific variation of 4f-electron enhanced spectral intensity at EF around Gamma barre and M barre points. Tight-binding approximation calculations for Ce-In plane provided the same universal distribution of 4f-electron density for a range of values of the parameters used in the model

Photoemission signature of momentum-dependent hybridization in CeCoIn 5

International audienceHybridization between f electrons and conduction bands (c-f hybridization) is a driving force for many unusual phenomena. To provide insight into it, systematic studies of CeCoIn5 heavy fermion superconductor have been performed by angle-resolved photoemission spectroscopy (ARPES) in a large angular range at temperature of T = 6 K. The used photon energy of 122 eV corresponds to Ce 4d-4f resonance. Calculations carried out with relativistic multiple scattering Korringa-Kohn-Rostoker method and one-step model of photoemission yielded realistic simulation of the ARPES spectra indicating that Ce-In surface termination prevails. Surface states, which have been identified in the calculations, contribute significantly to the spectra. Effects of the hybridization strongly depend on wave vector. They include a dispersion of heavy electrons and bands gaining f-electron character when approaching Fermi energy. We have also observed a considerable variation of felectron spectral weight at EF , which is normally determined by both matrix element effects and wave vector dependent c-f hybridization. Fermi surface scans covering a few Brillouin zones revealed large matrix element effects. A symmetrization of experimental Fermi surface, which reduces matrix element contribution, yielded a specific variation of 4f-electron enhanced spectral intensity at EF aroundΓ andM points. Tight-binding approximation calculations for Ce-In plane provided the same universal distribution of 4f-electron density for a range of values of the parameters used in the model