Superconductivity-localization interplay and fluctuation magnetoresistance in epitaxial BaPb1−xBixO3 thin films

Abstract

BaPb 1−x Bi x O 3 is a superconductor, with transition temperature T c = 11 K, whose parent compound BaBiO 3 possess a charge ordering phase and perovskite crystal structure reminiscent of the cuprates. The lack of magnetism simplifies the BaPb 1−x Bi x O 3 phase diagram, making this system an ideal platform for contrasting high-T c systems with isotropic superconductors. Here we use high-quality epitaxial thin films and magnetotransport to demonstrate superconducting fluctuations that extend well beyond T c . For the thickest films (thickness above ∼ 100 nm) this region extends to ∼ 27 K, well above the bulk T c and remarkably close to the higher T c of Ba 1−x K x BiO 3 (T c = 31 K). We drive the system through a superconductor-insulator transition by decreasing thickness and find the observed T c correlates strongly with disorder. This material manifests strong fluctuations across a wide range of thicknesses, temperatures, and disorder presenting new opportunities for understanding the precursor of superconductivity near the 2D-3D dimensionality crossover.1 arXiv:1801.03864v2 [cond-mat.supr-con] In contrast to the layered cuprate superconductors, BaPb 1−x Bi x O 3 (BPBO, T c = 11 K) and Ba 1−x K x BiO 3 (BKBO, T c = 31 K) are isotropic and nonmagnetic, however, there are still interesting similarities [1,2]. The bismuthates are complex oxides with oxygen octahedra similar to the cuprates, and the parent insulating BaBiO 3 (BBO) possesses a competing phase, a charge density wave (CDW), which is suppressed for superconducting compositions.The study of the simpler, conventional bismuthate may lead to a deeper understanding of the role of CDW physics in the more complicated cuprates. The cuprate phase diagram is characterized by numerous electronic and magnetic phases and the properties are strongly influenced by disorder [3]. In thin conventional superconductors, disorder can lead to a pseudogap reminiscent of the high-T c cuprates, suggesting a possible connection between the layered cuprate structure and dimensionally confined conventional superconductors [4].In superconducting BPBO single crystals, Luna et al.[5] found a reduction in the density of states consistent with a disorder-driven metal-insulator transition and predicted a disorderfree T c of 17 K in the strong coupling limit and 52 K in the weak coupling limit for x = 0.25.Here we demonstrate an extended region of positive magnetoresistance in epitaxial thin films of BaPb 0.75 Bi 0.25 O 3 that is well described by superconducting fluctuations. This fluctuation regime persists for the thickest films that are well within the 3D regime, consistent with the high disorder found in our films. Restricting film thickness causes a superconductor-toinsulator transition (SIT) that correlates with disorder. Although our results are consistent with the disorder levels found in bulk single crystals [5], we find that the critical thickness for superconductivity depends on extrinsic factors related to the poor lattice matching of BPBO with common perovskite substrate...

Cooling a polaronic liquid: Phase mixture and pseudogap-like spectra in superconducting $Ba_{1-x}K_{x}BiO_{3}$

“…An obvious point is that phase separation sidesteps a global metal-insulator transition, as occurs in the nickelates, allowing for the survival of a metallic Fermi surface amid strong interactions and setting the stage for superconductivity. At a deeper level, recent studies have highlighted how disorder 45 and the lengthscale of structural phase separation 24 in the bismuth oxides appear to be relevant to T c . Additionally, the electronic phase mixture seen above T c implies that a direct superconductor-insulator transition (SIT) indeed occurs along the x axis of the phase diagram at T = 0, as has long been suspected.…”

Signatures of two-dimensional superconductivity emerging within a three-dimensional host superconductor

“…For example, the phase could be emergent due to increased interactions and localization near the SIT. Such unusual scaling behavior has also been connected to a partially disordered stripe-like nanoscale structural phase separation due to the fact that the stripe width matches the superconducting coherence length at the Bi composition which maximizes T c (the optimal doping point) (13,14). Additionally, the role played by the high-temperature dimorphic nature of BPBO (two coexisting crystal structures with different electronic properties) in the composition range x = [0.18−0.30] (Fig.…”

Feeble Metallicity and Robust Semiconducting Regime in Structurally Sensitive Ba(Pb, Sn)O$_3$ Alloys

“…The solid solution also demonstrates the metal to insulator transition at x=0. 35. In recent years, bilayers and interfaces made out of the two end-members BaBiO 3 (BBO) and BaPbO 3 (BPO) have gained attention to realize two-dimensional superconductivity [18][19][20] . Furthermore, these are also explored as promising topological materials under pressure and chemical doping 6,7 .…”