Sixfold enhancement of superconductivity in a tunable electronic nematic system.

The electronic nematic phase-in which electronic degrees of freedom lower the crystal rotational symmetry-is commonly observed in high-temperature superconductors. However, understanding the role of nematicity and nematic fluctuations in Cooper pairing is often made more complicated by the coexistence of other orders, particularly long-range magnetic order. Here we report the enhancement of superconductivity in a model electronic nematic system that is not magnetic, and show that the enhancement is directly born out of strong nematic fluctuations associated with a quantum phase transition. We present measurements of the resistance as a function of strain in Ba1-x Sr x Ni2As2 to show that strontium substitution promotes an electronically driven nematic order in this system. In addition, the complete suppression of that order to absolute zero temperature leads to an enhancement of the pairing strength, as evidenced by a sixfold increase in the superconducting transition temperature. The direct relation between enhanced pairing and nematic fluctuations in this model system, as well as the interplay with a unidirectional charge-density-wave order comparable to that found in the cuprates, offers a means to investigate the role of nematicity in strengthening superconductivity.

Law and Disorder: Special Stacking Units-Building the Intergrowth Ce6Co5Ge16.

A new structure type of composition Ce6Co5Ge16 was grown out of a molten Sn flux. Ce6Co5Ge16 crystallizes in the orthorhombic space group Cmcm, with highly anisotropic lattice parameters of a = 4.3293(5) Å, b = 55.438(8) Å, and c = 4.3104(4) Å. The resulting single crystals were characterized by X-ray diffraction, and the magnetic and transport properties are presented. The Sn-stabilized structure of Ce6Co5Ge16 is based on the stacking of disordered Ce cuboctahedra and is an intergrowth of existing structure types including AlB2, BaNiSn3, and AuCu3. The stacking of structural subunits has previously been shown to be significant in the fields of superconductivity, quantum materials, and optical materials. Herein, we present the synthesis, characterization, and complex magnetic behavior of Ce6Co5Ge16 at low temperature, including three distinct magnetic transitions.

Beyond triplet: Unconventional superconductivity in a spin-3/2 topological semimetal.

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.

Proton and ammonia intercalation into layered iron chalcogenides.

Structurally related to the iron-based superconductors, two new intercalated iron chalcogenides (H0.5NH3)Fe2Ch2 where Ch = S, Se have been prepared. By topochemical conversion, the protons were exchanged by lithium to form (Li0.5NH3)Fe2Ch2. Hydrogen bonding plays a significant role in the guest-host interactions of these intercalated phases.

Physical properties of CeGe2-x (x=0.24) single crystals.

We present data on the anisotropic magnetic properties, heat capacity and transport properties of CeGe2-x (x=0.24) single crystals. The electronic coefficient of the heat capacity, γ∼110 mJ mol(-1) K(-2), is enhanced; three magnetic transitions, with critical temperatures of ≈7, ≈5 and ≈4 K are observed in thermodynamic and transport measurements. The ground state has a small ferromagnetic component along the c-axis. Small applied field, below 10 kOe, is enough to bring the material to an apparent saturated paramagnetic state (with no further metamagnetic transitions up to 55 kOe) with a reduced, below 1.2 µB, saturated moment.