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

Abstract

Many complex electronic systems exhibit so-called pseudogaps, which are poorly-understood suppression of low-energy spectral intensity in the absence of an obvious gap-inducing symmetry. Here we investigate the superconductor Ba 1−x K x BiO 3 near optimal doping, where unconventional transport behavior and evidence of pseudogap(s) have been observed above the superconducting transition temperature T c , and near an insulating phase with longrange lattice distortions. Angle-resolved photoemission spectroscopy (ARPES) reveals a dispersive band with vanishing quasiparticle weight and "tails" of deep-energy intensity that strongly decay approaching the Fermi level. Upon cooling below a transition temperature T p > T c , which correlates with a change in the slope of the resistivity vs. temperature, a partial transfer of spectral weight near E F into the deep-binding energy tails is found to result from metal-insulator phase separation. Combined with simulations and Raman scattering, our results signal that insulating islands of ordered bipolarons precipitate out of a disordered polaronic liquid and provide evidence that this process is regulated by a crossover in the electronic mean free path.Pseudogaps represent a departure from the expectations of standard band theory and the Fermi liquid theory of electronic excitations, which together serve as a successful starting point for understanding many condensed matter systems. They could potentially originate from any ways in which the conventional theories might break down, e.g., due to disorder, fluctuations, strong interactions, and/or strong correlations. But it is also conceivable that some observed pseudogaps might be less mysterious than they first seem, in the sense that they are rooted a "hidden" order

Quantum Monte Carlo study of lattice polarons in the two-dimensional three-orbital Su–Schrieffer–Heeger model

“…At half filling, we find a bond disproportionated state that can be viewed as a lattice of localized bipolarons. Upon hole-doping, this state gives way to a polaron-liquid-like state with short-range correlations, consistent with proposals for nano-scale phase separation or strongly fluctuating lattice polarons in doped BKBO [52][53][54][55][56][57]. We also find s-wave superconducting tendencies, which primarily form on the oxygen sublattice.…”

“…With increasing hole concentrations, P (r) decreases at the larger distances, signalling an overall relaxation of the bond disproportionated on long length scales but the persistence of short-range correlations. Such behavior could reflect nanoscale phase separation [52]; however, studies on large clusters are likely needed to clarify this issue. Finally, in the high doping region, where the system is metallic (e.g.…”

Epitaxial growth of perovskite SrBiO3 film on SrTiO3 by oxide molecular beam epitaxy

“…Hole-doped perovskite bismuthates Ba 1−x K x BiO 3 (x = 0.4) and Sr 1−x K x BiO 3 (x = 0.6) become superconducting below ∼30 K and ∼12 K, respectively [1,2]. Their parent compounds BaBiO 3 (BBO) and SrBiO 3 (SBO) have drawn intense interest [3][4][5][6][7][8][9][10][11][12][13][14][15][16] owing to the hightemperature superconductivity and also because of their indirect bandgap semiconductor nature rather than metals as would be expected from the half-filled Bi 6s band assuming the formal 4+ valence of Bi. The most common explanation of the nonmetallic behavior invokes the concept of charge disproportionation of Bi 4+ into the more stable and closed-shell atomic configurations of Bi 3+ (6s 2 ) and Bi 5+ (6s 0 ) [3,4,17].…”

Polaron and bipolaron tendencies in a semiclassical model for hole-doped bismuthates

“…Several works 40,41,58,59 , including the previous DQMC study of a comparable model 27 , have proposed that the metallic phase of the bismuthates (and other negative CT systems like the Nickelates) can be viewed as a polaron liquid. With this idea in mind, Fig.…”

“…A later study on Ba 1−x K x BiO 3 (BKBO) with x = 0.49 observed an increased bandwidth 39 consistent with the theoretical prediction 36 that the long-range Coulomb interaction increases the bandwidth and enhances the e-ph coupling to generate a high T c . Another ARPES study on BKBO films interpreted the normal state spectra in terms of nanoscale phase separation and a polaron-liquid-like metallic phase 40 .…”