The new alpha-Fe(Te,Se) superconductors share the common iron building block and ferminology with the LaFeAsO and BaFe(2)As(2) families of superconductors. In contrast with the predicted commensurate spin-density-wave order at the nesting wave vector (pi, 0), a completely different magnetic order with a composition tunable propagation vector (deltapi, deltapi) was determined for the parent compound Fe_{1+y}Te in this powder and single-crystal neutron diffraction study. The new antiferromagnetic order survives as a short-range one even in the highest T_{C} sample. An alternative to the prevailing nesting Fermi surface mechanism is required to understand the latest family of ferrous superconductors.

We report our study of the evolution of superconductivity and the phase diagram of the ternary Fe(Se 1-x Te x ) 0.82 (0 ≤ x ≤ 1.0) system. We discovered a new superconducting phase with T c , max = 14 K in the 0.3 < x < 1.0 range. This superconducting phase is suppressed when the sample composition approaches the end member FeTe 0.82 , which exhibits an incommensurate antiferromagnetic order. We discuss the relationship between the superconductivity and magnetism of this material system in terms of recent results from neutron scattering measurements. Our results and analyses suggest that superconductivity in this new class of Fe-based compounds is associated with magnetic fluctuations, and therefore may be unconventional in nature.

The iron chalcogenide Fe(1+y)(Te(1-x)Se(x)) is structurally the simplest of the Fe-based superconductors. Although the Fermi surface is similar to iron pnictides, the parent compound Fe(1+y)Te exhibits antiferromagnetic order with an in-plane magnetic wave vector (pi,0) (ref. 6). This contrasts the pnictide parent compounds where the magnetic order has an in-plane magnetic wave vector (pi,pi) that connects hole and electron parts of the Fermi surface. Despite these differences, both the pnictide and chalcogenide Fe superconductors exhibit a superconducting spin resonance around (pi,pi) (refs 9, 10, 11). A central question in this burgeoning field is therefore how (pi,pi) superconductivity can emerge from a (pi,0) magnetic instability. Here, we report that the magnetic soft mode evolving from the (pi,0)-type magnetic long-range order is associated with weak charge carrier localization. Bulk superconductivity occurs as magnetic correlations at (pi,0) are suppressed and the mode at (pi, pi) becomes dominant for x>0.29. Our results suggest a common magnetic origin for superconductivity in iron chalcogenide and pnictide superconductors.

Neutron scattering is used to probe magnetic excitations in FeSe_{0.4}Te_{0.6} (T_{c} = 14 K). Low energy spin fluctuations are found with a characteristic wave vector (1/21/2L) that corresponds to Fermi surface nesting and differs from Q_{m} = (delta01/2) for magnetic ordering in Fe_{1+y}Te. A spin resonance with variant Planck's over 2piOmega_{0} = 6.51(4) meV approximately 5.3k_{B}T_{c} and variant Planck's over 2piGamma = 1.25(5) meV develops in the superconducting state from a normal state continuum. We show that the resonance is consistent with a bound state associated with s_{+/-} superconductivity and imperfect quasi-2D Fermi surface nesting.

We have investigated the effect of Fe nonstoichiometry on properties of the Fe 1+y (Te, Se) superconductor system by means of resistivity, Hall coefficient, magnetic susceptibility, and specific heat measurements. We find that the excess Fe at interstitial sites of the (Te, Se) layers not only suppresses superconductivity, but also results in a weakly localized electronic state. We argue that these effects originate from the magnetic coupling between the excess Fe and the adjacent Fe square planar sheets, which favors a short-range magnetic order.

Type-II Dirac/Weyl semimetals are characterized by strongly tilted Dirac cones such that the Dirac/Weyl node emerges at the boundary of electron and hole pockets as a new state of quantum matter, distinct from the standard Dirac/Weyl points with a point-like Fermi surface which are referred to as type-I nodes. The type-II Dirac fermions were recently predicted by theory and have since been confirmed in experiments in the PtSe 2 -class of transition metal dichalcogenides. However, the Dirac nodes observed in PtSe 2 , PdTe 2 and PtTe 2 candidates are quite far away from the Fermi level, making the signature of topological fermions obscure as the physical properties are still dominated by the non-Dirac quasiparticles. Here we report the synthesis of a new type-II Dirac semimetal NiTe 2 in which a pair of type-II Dirac nodes are located very close to the Fermi level. The quantum oscillations in this material reveal a nontrivial Berry's phase associated with these Dirac fermions. Our first principles calculations further unveil a topological Dirac cone in its surface states. Therefore, NiTe 2 may not only represent an improved system to formulate the theoretical understanding of the exotic consequences of type-II Dirac fermions, it also facilitates possible applications based on these topological carriers.

We have determined the resistive upper critical field H c2 for single crystals of the superconductor Fe 1.11 Te 0.6 Se 0.4 using pulsed magnetic fields of up to 60 T. A rather high zero-temperature upper critical field of 0 H c2 ͑0͒Ϸ47 T is obtained in spite of the relatively low superconducting transition temperature ͑T c Ϸ 14 K͒. Moreover, H c2 follows an unusual temperature dependence, becoming almost independent of the magnetic field orientation as the temperature T → 0. We suggest that the isotropic superconductivity in Fe 1.11 Te 0.6 Se 0.4 is a consequence of its three-dimensional Fermi-surface topology. An analogous result was obtained for ͑Ba, K͒Fe 2 As 2 , indicating that all layered iron-based superconductors exhibit generic behavior that is significantly different from that of the "high-T c " cuprates.

The Al effect on the electrochemical properties of layered double hydroxides (LDHs) is not properly probed, although it is demonstrated to notably promote the capacitive behavior of LDHs. Herein, ternary NiCo 2 Al x layered double hydroxides with varying levels of Al stoichiometry are purposely developed, grown directly on mechanically flexible and electrically conducting carbon cloth (CC@NiCo 2 Al x -LDH). Al plays a significant role in determining the structure, morphology, and electrochemical behavior of NiCo 2 Al x -LDHs. At an increasing level of Al in NiCo 2 Al x -LDHs, there is a steady evolution from 1D nanowire to 2D nanosheets. The CC@NiCo 2 Al-LDH at an appropriate level of Al and with the nanowire-nanosheet mixed morphology exhibits both significantly enhanced electrochemical performance and excellent structural stability, with about a 2.3-fold capacitance of NiCo 2 -OH. When applied as the anode in a flexible asymmetric supercapacitor (ASC), the CC@NiCo 2 Al-LDH gives rise to a remarkable energy density of 44 Wh kg −1 at the power density of 462 W kg −1 , together with remarkable cyclic stability with 91.2% capacitance retention over 15 000 charge-discharge cycles. The present study demonstrates a new pathway to significantly improve the electrochemical performance and stability of transition metal LDHs, which are otherwise unstable in structure and poorly performing in both rate and cycling capability.