We have investigated the magnetic phase diagram near the upper critical field of KFe2As2 by magnetic torque and specific heat experiments, using a high-resolution piezo-rotary positioner to precisely control the parallel orientation of the magnetic field with respect to the FeAs layers. We observe a clear double transition when the field is oriented strictly in-plane, and a characteristic upturn of the upper critical field line well beyond the Pauli limit at 4.7 T. This provides firm evidence that an FFLO state is realized in this iron-based KFe2As2 superconductor.While the upper critical field (Hc2) of type-II superconductors is usually determined by the orbital limit for superconductivity [1], there are rare cases in which the Pauli paramagnetic limit, at which the Zeeman split Fermi surfaces suppress Cooper pairing, occurs instead [2,3]. When a superconductor approaches the latter, an exotic superconducting state was predicted by Fulde, Ferrell, Larkin and Ovchinnikov [4,5]. Under certain conditions, a superconductor could overcome the Pauli limit by forming the 'FFLO' state in which the Cooper pairs have a finite center-of-mass momentum. Cooper pairing between the Zeeman split Fermi surfaces is possible only with an oscillating component of the order parameter amplitude in real space. Only a few observations of the FFLO state have been reported to date despite its high technological relevance for high magnetic field applications. The Q-phase in the heavy fermion superconductor CeCoIn5 is regarded as a possible realization [6][7][8], although the superconductivity coexists with an incommensurate spin-density wave [8]. In some layered organic superconductors, evidence has been provided for an FFLO state without magnetic order [9][10][11][12][13][14][15][16]. The rarity of the FFLO state is due to the strict requirements on its occurrence. It needs a very large Ginzburg-Landau parameter ≡ / ≫ 1 ( : penetration depth, : coherence length), and a Maki parameter greater than 1.85 [17,18]. Only in this case does the Pauli limit occur below the orbital limit. In addition, the superconductor must be in the clean limit [19,20]. Low dimensionality and anisotropic Fermi surfaces can further stabilize the FFLO state [21]. The Hc2 line in the magnetic (H-T) phase diagram shows a characteristic saturation at the Pauli limit, where the second-order transition (SOT) changes into a discontinuous first-order transition (FOT). However, as soon an FFLO state is formed, a characteristic increase of Hc2 occurs to fields beyond the Pauli limi. KFe2As2 is the overdoped end member of the Ba1-xKxFe2As2 family of the multiband Fe-based superconductors with Tc of ~3.4 K. In recent magnetostriction experiments it was found that Hc2 is Pauli limited for fields applied parallel to the FeAs layers [22,23]. However,

We present measurements of the magnetic torque, specific heat and thermal expansion of the bulk transition metal dichalcogenide (TMD) superconductor NbS2 in high magnetic fields, with its layer structure aligned strictly parallel to the field using a piezo rotary positioner. The upper critical field of superconducting TMDs in the 2D form is known to be dramatically enhanced by a special form of Ising spin orbit coupling. This Ising superconductivity is very robust to the Pauli paramagnetic effect and can therefore exist beyond the Pauli limit for superconductivity. We find that superconductivity beyond the Pauli limit still exists in bulk single crystals of NbS2 for a precisely parallel field alignment. However, the comparison of our upper critical field transition line with numerical simulations rather points to the development of a Fulde-Ferrell-Larkin-Ovchinnikov state above the Pauli limit as a cause. This is also consistent with the observation of a magnetic field driven phase transition in the thermodynamic quantities within the superconducting state near the Pauli limit.

A combined experimental and theoretical investigation on the cerium(IV) oxo complex [(LOEt)2Ce(=O)(H2O)]⋅MeC(O)NH2 (1; LOEt−=[Co(η5‐C5H5){P(O)(OEt)2}3]−) demonstrates that the intermediate spin‐state nature of the ground state of the cerium complex is responsible for the versatility of its reactivity towards small molecules such as CO, CO2, SO2, and NO. CASSCF calculations together with magnetic susceptibility measurements indicate that the ground state of the cerium complex is of multiconfigurational character and comprised of 74 % of CeIV and 26 % of CeIII. The latter is found to be responsible for its reductive addition behavior towards CO, SO2, and NO. This is the first report to date on the influence of the multiconfigurational ground state on the reactivity of a metal–oxo complex.