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A New Look at an Old Puzzle: ARPES on Ba1-xKxBiO3

日程 : 2019年3月11日(月) 2:00 pm - 3:00 pm 場所 : 物性研究所本館6階 第5セミナー室 (A615) 講師 : Nicholas C. Plumb 所属 : Swiss Light Source, Paul Scherrer Institut 世話人 : 近藤猛 (63370)

The fact that Tc in perovskite bismuth oxides can exceed 30 K has been known since right around the discovery of the high-Tc cuprates. Yet for various reasons, the bismuthates were never studied to nearly the extent of cuprates or, later, iron-based superconductors. This is pity, because their phenomenology and underlying physics connect with a wide array of contemporary interests: not only unconventional/high-Tc superconductivity, but also metal-insulator and insulator-superconductor transitions, (bi)polarons, DWs/charge-order, disordered systems, and so on. Recently we have succeeded in performing ARPES in situ on high-quality films of Ba1-xKxBiO3. We revealed that the band structure of the insulating parent compound (x = 0) can be reasonably captured by simple LDA [1]. The gap opens in a predominantly oxygen-derived band, signaling the role of a negative charge transfer energy and supporting the notion that the ground state involving ordered BiO6 breathing distortions is “bond disproportionated,” as opposed to classically charge-ordered among the bismuth cations. Our latest experiments [2] probe within the “under- to optimally-doped” region of the phase diagram. There we see a strongly dispersing metallic band forming a Fermi surface, despite an absence of peaks in the energy spectra that are the signatures of Landau-like quasiparticles. We observe, moreover, two types of pseudogap-like behaviors – i.e., gap-like suppressions of spectral intensity in the absence of an obvious gap-inducing symmetry. The first of these extends over a broad energy scale and persists above room temperature; the other is set in a narrow region around EF and opens in a well-defined temperature range above Tc. This latter “pseudogap” is revealed to be a signature of metal-insulator phase separation. I will discuss how our observations fit within a polaronic understanding of these materials. In particular, we view the transition to phase separation as the precipitation of ordered bipolaronic insulating regions out of a disordered polaronic liquid. Some possible implications for superconductivity will also be discussed. (For potential users of Swiss Light Source, this talk will also mention recent developments and future plans for SIS beamline and the SLS as a whole.)

参考文献:
[1] N.C.Plumb et al., Momentum-Resolved Electronic Structure of the High-Tc Superconductor Parent Compound BaBiO3, Phys. Rev. Lett. 117, 037002 (2016).
[2] M.Naamneh et al., Cooling a polaronic liquid: Phase mixture and pseudogap-like spectra in superconducting Ba1-xKxBiO3, arxiv:1808.06135.

 


(公開日: 2019年03月07日)