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Register a new userCoexistence of spin-triplet superconductivity and ferromagnetism induced by the local Hunds rule exchange
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We characterize the coexistence of itinerant ferromagnetism and spin-triplet superconductivity within a single mechanism involving local (Hunds rule) exchange among $d$ electrons. The ratio of transition temperatures and the spin anisotropy of the superconducting gap is estimated for $ZrZn_2$. The $A$ phase is stable in very low applied and molecular fields, whereas the $A1$ phases persists in higher fields. A small residual magnetic moment is present below the Stoner threshold in the superconducting phase.
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We discuss general implications of the local spin-triplet pairing among correlated fermions that is induced by the Hunds rule coupling in orbitally degenerate systems. The quasiparticle energies, the magnetic moment, and the superconducting gap are determined for principal superconducting phases, in the situation with the exchange field induced by both the local Coulomb and the Hunds rule exchange interactions. The phase diagram, as well as the evolution in an applied magnetic field of the spin-triplet paired states near the Stoner threshold is provided for a model two-band system. The appearance of the spin-polarized superconducting phase makes the Stoner threshold a hidden critical point, since the pairing creates a small but detectable uniform magnetization. The stability of the superconducting state against the ferromagnetism with an alternant orbital ordering appearing in the strong-coupling limit is also discussed.
We discuss general implications of the local spin-triplet pairing among fermions induced by local ferromagnetic exchange, example of which is the Hunds rule coupling. The quasiparticle energy and their wave function are determined for the three principal phases with the gap, which is momentum independent. We utilize the Bogolyubov-Nambu-De Gennes approach, which in the case of triplet pairing in the two-band case leads to the four-components wave function. Both gapless modes and those with an isotropic gap appear in the quasiparticle spectrum. A striking analogy with the Dirac equation is briefly explored. This type of pairing is relevant to relativistic fermions as well, since it reflects the fundamental discrete symmetry-particle interchange. A comparison with the local interband spin-singlet pairing is also made.
Strongly correlated systems exhibit a rich phenomenology due to the antagonism of a diversity of ordered phases. The aftermath of this interplay can lead to a coexistence which takes place at a microscopic level, or, a phase separation in which non-overlapping single-order domains extend throughout the material. In most cases it appears experimentally challenging to disentangle the two scenarios, unless, there exist robust and measurable properties particular to only one of the two types of coexistence. This is for instance the case when the type of coexistence decides on the appearance of topologically protected excitations, such as, Majorana fermions. In this work, we explore a concrete example falling into this category of systems, and specifically, we investigate one-dimensional odd-parity spin-triplet superconductors in the presence of antiferromagnetism. We determine the symmetry conditions for the occurrence of Majorana edge states and explore their response to variations of the strength and orientation of the antiferromagnetic field $boldsymbol{M}$, as well as, the spin structure of the Cooper pairs controlled by the so-called $boldsymbol{d}$-vector.
High-quality KFe2As2 single crystals have been studied by transport, magnetization and low-T specific heat measurements. Their analysis shows that superconductivity occurs (in some cases coexists) in the vicinity of disordered magnetic phases (Griffiths and spin-glass type) depending of the amount of local magnetic moments (probably excess Fe derived)and sample inhomogeneity. The achieved phenomenological description of our data including also data from the literature provides a consistent explanation of the observed non-Fermi-liquid behavior and of the nominally large experimental Sommerfeld coefficient gamma_n about 94 mJ/mol K^2. We suggest that the intrinsic value (directly related to the itinerant quasi-particles) gamma_el about 60(10)mJ/mol K^2 is significantly reduced compared with gamma_n. Then an enhanced relative jump of the electronic specific heat Delta C_el/gamma_el T_c about 0.8 and a weak total electron-boson coupling constant lambda less or about 1 follow.
Superconducting characteristics such as the Meissner-Ochsenfeld state, screening supercurrents and hysteresis loops of type-II superconductors were observed from the temperature and magnetic field dependences of the magnetic moment, m(T, H), in graphite powders reacted with sulfur for temperatures below 9.0 K. The temperature dependence of the lower critical field Hc1(T) was determined and the zero-temperature penetration depth, lambda(0), was estimated (lambda (0) = 227 nm). The superconductivity was observed to be highly anisotropic and to coexist with a ferromagnetic state that has a Curie temperature well above room temperature. A continuous transition from the superconducting state to the ferromagnetic state could be achieved by simply increasing the applied magnetic field.
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