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Register a new userKFe2As2: coexistence of superconductivity and local moment derived spin-glass
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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.
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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.
The upper critical field Hc2(T) of the multiband superconductor KFe2As2 has been studied via low-temperature thermal expansion and magnetostriction measurements. We present compelling evidence for Pauli-limiting effects dominating Hc2(T) for H || a, as revealed by a crossover from second- to first-order phase transitions to the superconducting state in the magnetostriction measurements down to 50 mK. Corresponding features were absent for H || c. To our knowledge, this crossover constitutes the first confirmation of Pauli limiting of the Hc2(T) of a multiband superconductor. The results are supported by modeling Pauli limits for single-band and multiband cases.
The temperature, field, and field-orientation dependences of the electronic specific heat Ce of the ironpnictide superconductor KFe2As2 have been investigated. Thermodynamic evidence of the presence of line nodes is obtained from the T and $sqrt{H}$ linear dependences of Ce/T in the low-T and low-H region. Under a magnetic field rotated within the tetragonal ab plane, a fourfold oscillation is observed in Ce with a sign change at 0.08Tc. On the basis of the Doppler-shift analysis, the observed Ce minima in H // [100] at low T indicate the presence of line nodes somewhere on the Fermi surface where the Fermi velocity is parallel to the [100] direction; this is consistent with the octet-line-node scenario proposed recently by a photoemission experiment. In addition, the low-T Ce/T exhibits an unusual upturn on cooling at moderate fields only for H // ab, which is understood in terms of the strong Pauli paramagnetic effect on multiband superconductivity.
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 single crystals of K0.8Fe2Se1.4S0.4 are successfully synthesized by self-flux method with the superconducting transition temperatures Tconset = 32.8 K and Tczero = 31.2 K. In contrast to external pressure effect on superconductivity, the substitution of S for Se does not suppress Tc, which suggests that chemical doping may mainly modulate the anion height from Fe-layer rather than compressing interlayer distance. The investigation of the micromagnetism by electron spin resonance shows clear evidence for strong spin fluctuation at temperatures above Tc. Accompanied by the superconducting feature spectra, a novel resonance signal develops gradually upon cooling below Tc, indicating the coexistence of superconductivity and magnetism in K0.8Fe2Se1.4S0.4 crystal.
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