F. von Rohr

Synthesis of a new alkali metal–organic solvent intercalated iron selenide superconductor with Tc ≈ 45 K

We report on a new iron selenide superconductor with a T(c) onset of 45 K and the nominal composition Li(x)(C(5)H(5)N)(y)Fe(2-z)Se(2), synthesized via intercalation of dissolved alkaline metal in anhydrous pyridine at room temperature. This superconductor exhibits a broad transition, reaching zero resistance at 10 K. Magnetization measurements reveal a superconducting shielding fraction of approximately 30%. Analogous phases intercalated with Na, K and Rb were also synthesized and characterized. The superconducting transition temperature of Li(x)(C(5)H(5)N)(y)Fe(2-z)Se(2) is clearly enhanced in comparison to those of the known superconductors FeSe(0.98) (T(c) ~ 8 K) and A(x)Fe(2-y)Se(2) (T(c) ~ 27-32 K) and is in close agreement with critical temperatures recently reported for Li(x)(NH(3))(y)Fe(2-z)Se(2). Post-annealing of intercalated material (Li(x)(C(5)H(5)N)(y)Fe(2-z)Se(2)) at elevated temperatures drastically enlarges the c-parameter of the unit cell (~44%) and increases the superconducting shielding fraction to nearly 100%. Our findings indicate a new synthesis route leading to possibly even higher critical temperatures for materials in this class: by intercalation of organic compounds between Fe-Se layers.

Emergence of superconductivity in BaNi[subscript 2](Ge[subscript 1-x]P[subscript x])[subscript 2] at a structural instability

The physical properties and structural evolution of the 122-type solid solution BaNi2(Ge1-xPx)2 are reported. The in-plane X-X (X = Ge1-xPx) dimer formation present in the end member BaNi2Ge2, which results in a structural transition to orthorhombic symmetry, is completely suppressed to zero temperature on P substitution near x = 0.7, and a dome-shape superconducting phase with a maximum Tc = 2.9 K emerges. Clear indications of phonon softening and enhanced electron-phonon coupling are observed at the composition of the structural instability. Our findings show that dimer breaking offers new possibilities as a tuning parameter of superconductivity.

Direct evidence for a pressure-induced nodal superconducting gap in the Ba0.65Rb0.35Fe2As2 superconductor.

Identifying the superconducting (SC) gap structure of the iron-based high-temperature superconductors (Fe-HTS’s) remains a key issue for the understanding of superconductivity in these materials. In contrast to other unconventional superconductors, in the Fe-HTS’s both d-wave and extended s-wave pairing symmetries are close in energy, with the latter believed to be generally favored over the former. Probing the proximity between these very different SC states and identifying experimental parameters that can tune them, are of central interest. Here we report high-pressure muon spin rotation experiments on the temperature-dependent magnetic penetration depth λ (T ) in the optimally doped Fe-HTS Ba0.65Rb0.35Fe2As2. At ambient pressure this material is known to be a nodeless s-wave superconductor. Upon pressure a strong decrease of λ (0) is observed, while the SC transition temperature remains nearly constant. More importantly, the low-temperature behavior of 1/λ2 (T ) changes from exponential saturation at zero pressure to a power-law with increasing pressure, providing unambiguous evidence that hydrostatic pressure promotes nodal SC gaps. Comparison to microscopic models favors a d-wave over a nodal s+−-wave pairing as the origin of the nodes. Our results provide a new route of understanding the complex topology of the SC gap in Fe-HTS’s.The superconducting gap structure in iron-based high-temperature superconductors (Fe-HTSs) is non-universal. In contrast to other unconventional superconductors, in the Fe-HTSs both d-wave and extended s-wave pairing symmetries are close in energy. Probing the proximity between these very different superconducting states and identifying experimental parameters that can tune them is of central interest. Here we report high-pressure muon spin rotation experiments on the temperature-dependent magnetic penetration depth in the optimally doped nodeless s-wave Fe-HTS Ba0.65Rb0.35Fe2As2. Upon pressure, a strong decrease of the penetration depth in the zero-temperature limit is observed, while the superconducting transition temperature remains nearly constant. More importantly, the low-temperature behaviour of the inverse-squared magnetic penetration depth, which is a direct measure of the superfluid density, changes qualitatively from an exponential saturation at zero pressure to a linear-in-temperature behaviour at higher pressures, indicating that hydrostatic pressure promotes the appearance of nodes in the superconducting gap.

Low-temperature magnetic fluctuations in the Kondo insulator SmB6

We present the results of a systematic investigation of the magnetic properties of the three-dimensional Kondo topological insulator SmB6 using magnetization and muon-spin relaxation/rotation (μSR) measurements. The μSR measurements exhibit magnetic field fluctuations in SmB6 below ∼15 K due to electronic moments present in the system. However, no evidence for magnetic ordering is found down to 19 mK. The observed magnetism in SmB6 is homogeneous in nature throughout the full volume of the sample. Bulk magnetization measurements on the same sample show consistent behavior. The agreement between μSR, magnetization, and NMR results strongly indicate the appearance of intrinsic bulk magnetic in-gap states associated with fluctuating magnetic fields in SmB6 at low temperature.

Superconductivity and magnetism in RbxFe2−ySe2: Impact of thermal treatment on mesoscopic phase separation

An extended study of the superconducting and normal-state properties of various as-grown and post-annealed Rb

Evidence for strong lattice effects as revealed from huge unconventional oxygen isotope effects on the pseudogap temperature in La 2 − x Sr x CuO 4

{}_{x}

Probing the pairing symmetry in the over-doped Fe-based superconductorBa0.35Rb0.65Fe2As2as a function of hydrostatic pressure

Fe

Author Correction: Signatures of the topological s+− superconducting order parameter in the type-II Weyl semimetal Td-MoTe2

{}_{2\ensuremath{-}y}

Cobalt complexes of tetradentate, bipyridine-based macrocycles: their structures, properties and photocatalytic proton reduction

Se

Effect of pressure-driven local structural rearrangement on the superconducting properties ofFeSe0.5Te0.5

{}_{2}