A Spike Inside the Dome: The transition temperature T _c of iron-based superconductors has a dome-shaped dependence on chemical doping, and the superconductivity that develops underneath may obscure a potential quantum critical point (QCP) residing at absolute zero. With the aim of detecting signatures of this quantum criticality, Hashimoto et al. (p 1554 ; see the Perspective by Sachdev ) measured the penetration depth of the pnictide series BaFe ₂ (As _{1− x} P _x ) ₂ as a function of x. A sharp peak right around the point where T _c has a maximum ( x = 0.30) was observed, implying that the superfluid density diminishes sharply where one would expect it to be the most robust. This unusual finding is interpreted as a sign of a QCP at x = 0.30.

The parent compounds of iron-arsenide superconductors, AFe2As2 (A=Ca,$\hskip0.3em${}Sr,$\hskip0.3em${}Ba), undergo a tetragonal to orthorhombic structural transition at a temperature TTO in the range 135\char21{}205 K depending on the alkaline-earth element. Below TTO the free standing crystals split into equally populated structural domains, which mask intrinsic, in-plane, anisotropic properties of the materials. Here we demonstrate a way of mechanically detwinning CaFe2As2 and BaFe2As2. The detwinning is nearly complete, as demonstrated by polarized light imaging and synchrotron x-ray measurements, and reversible, with twin pattern restored after strain release. Electrical resistivity measurements in the twinned and detwinned states show that resistivity, $\rho${}, decreases along the orthorhombic ao axis but increases along the orthorhombic bo axis in both compounds. Immediately below TTO the ratio $\rho${}bo/$\rho${}ao=1.2 and 1.5 for Ca and Ba compounds, respectively. Contrary to CaFe2As2, BaFe2As2 reveals an anisotropy in the nominally tetragonal phase, suggesting that either fluctuations play a larger role above TTO in BaFe2As2 than in CaFe2As2 or that there is a higher temperature crossover or phase transition.

Effective demagnetizing factors that connect the sample magnetic moment with the applied magnetic field are calculated numerically for perfectly diamagnetic samples of various nonellipsoidal shapes. The procedure is based on calculating the total magnetic moment by integrating the magnetic induction obtained from a full three-dimensional (3D) solution of the Maxwell equations using an adaptive mesh. The results are relevant for superconductors (and conductors in ac fields) when the London penetration depth (or the skin depth) is much smaller than the sample size. Simple but reasonably accurate approximate formulas are given for practical shapes including rectangular cuboids, finite cylinders in axial and transverse fields, as well as infinite rectangular and elliptical cross-section strips.

In all known fermionic superfluids, Cooper pairs are composed of spin-1/2 quasi-particles that pair to form either spin-singlet or spin-triplet bound states. The “spin” of a Bloch electron, however, is fixed by the symmetries of the crystal and the atomic orbitals from which it is derived and, in some cases, can behave as if it were a spin-3/2 particle. The superconducting state of such a system allows pairing beyond spin-triplet, with higher spin quasi-particles combining to form quintet or septet pairs. We report evidence of unconventional superconductivity emerging from a spin-3/2 quasi-particle electronic structure in the half-Heusler semimetal YPtBi, a low-carrier density noncentrosymmetric cubic material with a high symmetry that preserves the p-like j = 3/2 manifold in the Bi-based Γ₈ band in the presence of strong spin-orbit coupling. With a striking linear temperature dependence of the London penetration depth, the existence of line nodes in the superconducting order parameter Δ is directly explained by a mixed-parity Cooper pairing model with high total angular momentum, consistent with a high-spin fermionic superfluid state. We propose a k ⋅ p model of the j = 3/2 fermions to explain how a dominant J = 3 septet pairing state is the simplest solution that naturally produces nodes in the mixed even-odd parity gap. Together with the underlying topologically nontrivial band structure, the unconventional pairing in this system represents a truly novel form of superfluidity that has strong potential for leading the development of a new series of topological superconductors.

Irreversible magnetic properties of $\text{Ba}{\left({\text{Fe}}_{0.93}{\text{Co}}_{0.07}\right)}_2{\text{As}}_2$ single crystals were studied by using several techniques including transport, magnetization, and magneto-optics. The mixed-state response was found to be very similar to high-$T_c$ cuprates, such as Y-Ba-Cu-O and Nd-Ce-Cu-O. In particular, we find a nonmonotonic “fishtail” shape of the magnetization loops $M\left(T=\text{const},H\right)$ and a corresponding feature in $M\left(T,H=\text{const}\right)$ scans, as well as a very large magnetic relaxation rate. The supercurrent density, $j$, in the critical state was evaluated from magnetization and from direct transport measurements close to $T_c$. At 5 K we estimate a moderate $j\approx 2.6\pm 0.2\times {10}^5\text{A}/{\text{cm}}^2$. Analysis of the temperature and field dependencies of the magnetic relaxation is consistent with the collective pinning and creep (weak pinning and fast creep) model and suggests a crossover from the collective to the plastic creep regime in fields exceeding the field at which the fishtail magnetization is maximum. While the zero-field anisotropy of the magnetic properties in the superconducting state of $\text{Ba}{\left({\text{Fe}}_{0.93}{\text{Co}}_{0.07}\right)}_2{\text{As}}_2$ is small, the vortex behavior changes for different orientations of magnetic field with respect to the crystal axes, implying field-dependent anisotropy of pinning properties.

Cr₂Ge₂Te₆ is proposed as an insulating ferromagnetic substrate for the growth of tetradymite-type topological insulators, based on a refined characterization of its transport, magnetic, optical, and calculated electronic properties. It is found to be a soft ferromagnet with no visible magnetic domains over relatively large length scales and to be highly insulating with an indirect band gap and low carrier concentration. Further we present the fabrication of Bi₂Te₃-Cr₂Ge₂Te₆ heterostructure samples by chemical vapor deposition and show that crystals of the two phases are oriented such that the hexagonal Te planes are aligned at their interfaces.