We show that the quasi-skutterudite superconductor Sr(3)Ir(4)Sn(13) undergoes a structural transition from a simple cubic parent structure, the I phase, to a superlattice variant, the I' phase, which has a lattice parameter twice that of the high temperature phase. We argue that the superlattice distortion is associated with a charge density wave transition of the conduction electron system and demonstrate that the superlattice transition temperature T(*) can be suppressed to zero by combining chemical and physical pressure. This enables the first comprehensive investigation of a superlattice quantum phase transition and its interplay with superconductivity in a cubic charge density wave system.

A fundamental issue concerning iron-based superconductivity is the roles of electronic nematicity and magnetism in realising high transition temperature (T c). To address this issue, FeSe is a key material, as it exhibits a unique pressure phase diagram involving non-magnetic nematic and pressure-induced antiferromagnetic ordered phases. However, as these two phases in FeSe have considerable overlap, how each order affects superconductivity remains perplexing. Here we construct the three-dimensional electronic phase diagram, temperature (T) against pressure (P) and isovalent S-substitution (x), for FeSe1−xSx. By simultaneously tuning chemical and physical pressures, against which the chalcogen height shows a contrasting variation, we achieve a complete separation of nematic and antiferromagnetic phases. In between, an extended non-magnetic tetragonal phase emerges, where T c shows a striking enhancement. The completed phase diagram uncovers that high-T c superconductivity lies near both ends of the dome-shaped antiferromagnetic phase, whereas T c remains low near the nematic critical point.

The family of the superconducting quasiskutterudites (Ca(x)Sr(1-x))(3)Rh(4)Sn(13) features a structural quantum critical point at x(c)=0.9, around which a dome-shaped variation of the superconducting transition temperature T(c) is found. Using specific heat, we probe the normal and the superconducting states of the entire series straddling the quantum critical point. Our analysis indicates a significant lowering of the effective Debye temperature on approaching x(c), which we interpret as a result of phonon softening accompanying the structural instability. Furthermore, a remarkably large enhancement of 2Δ/k(B)T(c) and ΔC/γT(c) beyond the Bardeen-Cooper-Schrieffer values is found in the vicinity of the structural quantum critical point. The phase diagram of (Ca(x)Sr(1-x))(3)Rh(4)Sn(13) thus provides a model system to study the interplay between structural quantum criticality and strong electron-phonon coupling superconductivity.

We report a highly unusual angular variation of the upper critical field (Hc2) in epitaxial superlattices CeCoIn5(n)/YbCoIn5(5), formed by alternating layers of n and a 5 unit-cell thick heavy-fermion superconductor CeCoIn5 with a strong Pauli effect and normal metal YbCoIn5, respectively. For the n = 3 superlattice, Hc2(θ) changes smoothly as a function of the field angle θ. However, close to the superconducting transition temperature, Hc2(θ) exhibits a cusp near the parallel field (θ = 0 • ). This cusp behavior disappears for n = 4 and 5 superlattices. This sudden disappearance suggests the relative dominance of the orbital depairing effect in the n = 3 superlattice, which may be due to the suppression of the Pauli effect in a system with local inversion symmetry breaking. Taking into account the temperature dependence of Hc2(θ) as well, our results suggest that some exotic superconducting states, including a helical superconducting state, might be realized at high magnetic fields.PACS numbers: 74.25. Op, 81.15.Hi In the absence of time reversal symmetry or space inversion symmetry, the Fermi surface (FS) can often be split into portions with different spin structures. To stabilize superconductivity under such conditions where spin degeneracy is lifted, unconventional pairing of quasiparticles is needed, leading to exotic superconducting states very different from the conventional BCS pairing state of (k ↑, -k ↓). Considering the situation of the broken time reversal symmetry alone, Fulde and Ferrell [1], and Larkin and Ovchinnikov [2] proposed the pairing state of (k ↑, -k+q ↓) on a Zeeman-split FS. This so-called FFLO pairing state leads to the modulation of the superconducting order parameter in real space with the modulation wavelength of the order of 1/|q|. On the other hand, in the lack of space inversion symmetry, a Rashba-type spin-orbit coupling splits the FS into branches with spins of opposite rotation sense [3]. When the magnetic field is applied to such a system, a pairing state with a finite center-of-mass momentum can also be realized, resulting in a helical superconducting state analogous to the FFLO phase.However, such exotic superconducting states have been poorly explored because of the lack of suitable materials. Recent advancement in heavy fermion thin film fabrication technology [4,5] has enabled the preparation of superlattices formed by alternate stacking of c-axis oriented CeCoIn 5 and YbCoIn 5 with atomic layer thicknesses. The large Fermi velocity mismatch across the interface between CeCoIn 5 and YbCoIn 5 significantly reduces the transmission probability of quasiparticles, thereby ensuring quasi-two-dimensional superconductivity confined within CeCoIn 5 layers [6,7]. This provides a unique opportunity to explore the physics discussed above. This is because bulk CeCoIn 5 with strong Pauli effect has been reported to host the FFLO phase at low temperatures and high magnetic field [8][9][10][11]. In the superlattice, the electronic structure becomes two-dimensional, which is expec...