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

Muntaser Naamneh, M. Yao+18 more|arXiv (Cornell University)|Aug 18, 2018

Physics of Superconductivity and Magnetism39 references4 citations

TL;DR

This study reveals that in superconducting Ba₁₋ₓKₓBiO₃ near optimal doping, a transition at Tₚ > T_c induces metal-insulator phase separation, where insulating bipolaronic islands precipitate from a disordered polaronic liquid. ARPES and Raman data show a pseudogap-like suppression of spectral weight near the Fermi level, driven by a crossover in the electronic mean free path, offering a hidden order explanation for the pseudogap via polaronic phase separation.

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 long-range 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.

Motivation & Objective

• To understand the origin of pseudogap-like spectral features in superconducting Ba₁₋ₓKₓBiO₃ above T_c.
• To investigate the role of electron-lattice interactions and polaronic effects in driving electronic inhomogeneity.
• To determine whether the observed pseudogap behavior arises from a hidden order, such as phase-separated bipolaronic states.
• To correlate resistivity anomalies with electronic spectral changes and identify the transition temperature Tₚ.
• To test the hypothesis that a crossover in the electronic mean free path governs the onset of spectral weight transfer.

Proposed method

• Angle-resolved photoemission spectroscopy (ARPES) was used to map the electronic band structure and spectral weight distribution in thin films of Ba₁₋ₓKₓBiO₃ near optimal doping (x ≈ 0.39).
• Raman scattering measurements were performed using a low-power 514.5 nm laser on thin films and SrTiO₃ substrates to probe lattice and electronic excitations, with substrate contributions subtracted.
• A three-orbital Su-Schrieffer-Heeger model on a 2D Lieb lattice was employed to simulate electron-lattice coupling, with hopping integrals modulated by oxygen displacement variables x_r and y_r.
• Theoretical calculations solved for equilibrium atomic displacements by minimizing the total energy E, using the Fermi-Dirac distribution to compute occupied states and spectral weight functions.
• Spectral weight functions A(k,ω) were computed via the Green's function formalism, incorporating many-body effects and broadening with δ = 0.3 eV.
• Density functional theory-derived parameters were used, including t_sp = 2.08 eV, t_pp = 0.056 eV, and electron-phonon coupling α = 4a⁻¹, with K = 0.104 eV/a².

Experimental results

Research questions

• RQ1 What causes the pseudogap-like suppression of spectral weight near the Fermi level in Ba₁₋ₓKₓBiO₃ above T_c?
• RQ2 How is the resistivity anomaly at Tₚ > T_c related to electronic spectral changes and phase separation?
• RQ3 Can the observed spectral features be explained by the formation of insulating bipolaronic islands within a disordered polaronic liquid?
• RQ4 What role does the electronic mean free path play in mediating the transition at Tₚ?
• RQ5 Is the pseudogap in this system a signature of a hidden order, such as phase-separated bipolarons, rather than a fluctuating or preformed pair state?

Key findings

• A distinct transition at Tₚ ≈ 140 K, marked by a change in the slope of resistivity vs. temperature, correlates with a partial transfer of spectral weight from near the Fermi level into deep-binding energy tails in ARPES spectra.
• The spectral weight transfer is attributed to metal-insulator phase separation, where insulating islands of ordered bipolarons precipitate from a disordered polaronic liquid.
• ARPES reveals a dispersive band with vanishing quasiparticle weight and strong intensity decay toward the Fermi level, indicating strong many-body effects.
• Raman scattering confirms the presence of lattice distortions and local polaronic modes, supporting the formation of localized bipolaronic states.
• Theoretical simulations on a 2D Lieb lattice with electron-lattice coupling reproduce the observed spectral features, including the pseudogap-like suppression and spectral weight redistribution.
• The crossover in the electronic mean free path at Tₚ governs the onset of phase separation, suggesting a critical role of electron localization in driving the pseudogap behavior.