[Paper Review] The fate of quasiparticles at high-temperature

Andrew Hunter, S.B.M. Beck+18 more|PubMed|Aug 4, 2023

Material Science and Thermodynamics49 references4 citations

TL;DR

This study reveals that quasiparticles in the correlated metal Sr2RuO4 persist up to temperatures above 200 K, with their residue Z increasing rather than decreasing with temperature. Using angle-resolved photoemission spectroscopy and dynamical mean-field theory, the authors show quasiparticles vanish not by losing weight but via super-Planckian scattering and excessive broadening, challenging the conventional view of quasiparticle decay in bad metals.

ABSTRACT

We study the temperature evolution of quasiparticles in the correlated metal Sr_{2}RuO_{4}. Our angle resolved photoemission data show that quasiparticles persist up to temperatures above 200 K, far beyond the Fermi liquid regime. Extracting the quasiparticle self-energy, we demonstrate that the quasiparticle residue Z increases with increasing temperature. Quasiparticles eventually disappear on approaching the bad metal state of Sr_{2}RuO_{4} not by losing weight but via excessive broadening from super-Planckian scattering. We further show that the Fermi surface of Sr_{2}RuO_{4}-defined as the loci where the spectral function peaks-deflates with increasing temperature. These findings are in semiquantitative agreement with dynamical mean field theory calculations.

Motivation & Objective

• To resolve the fate of quasiparticles in correlated metals at high temperatures, particularly in the bad metal regime beyond the Mott-Ioffe-Regel limit.
• To challenge the prevailing assumption that quasiparticle residue Z decreases with temperature in bad metals.
• To determine whether quasiparticle weight or spectral broadening dominates the loss of quasiparticle character in Sr2RuO4.
• To test the consistency of ARPES data with dynamical mean-field theory predictions for temperature-dependent quasiparticle behavior.
• To clarify the role of spectral function line shape evolution and background subtraction in extracting reliable quasiparticle parameters.

Proposed method

• Angle-resolved photoemission spectroscopy (ARPES) was used to measure the momentum- and energy-resolved spectral function of Sr2RuO4 across a range of temperatures up to ~250 K.
• The quasiparticle residue Z was extracted from the dispersion relation by fitting the self-energy to a first-order expansion around the quasiparticle peak energy ω₀.
• A pragmatic background subtraction method was applied, using spectra at π/a momentum where no direct transitions occur, to isolate the intrinsic spectral function.
• The line shape evolution was modeled using a modified spectral function expression: A(k,δω) ≈ Z / π * [Z(Σ′′₀ + αδω)] / [δω² + Z(Σ′′₀ + αδω)²], allowing for temperature-dependent broadening and peak intensity analysis.
• The experimental results were compared with dynamical mean-field theory (DMFT) calculations, using a global, temperature-independent scaling factor to align spectra.
• The self-energy derivative ∂Σ′/∂ω was computed from DMFT data to validate the extracted Z values and confirm consistency with the ARPES-derived quasiparticle weight.

Figure 1: (a) Schematic of the Fermi surface of Sr 2 RuO 4 . (b) Low-temperature ARPES data showing the $\beta$ and $\gamma$ band dispersing symmetrically around the Brillouin zone boundary. Yellow and green lines are the bare dispersions of the $\beta$ and $\gamma$ bands obtained from the Hamiltoni

Experimental results

Research questions

• RQ1 Does the quasiparticle residue Z decrease with increasing temperature in Sr2RuO4, as traditionally assumed in bad metals?
• RQ2 What is the dominant mechanism responsible for the disappearance of quasiparticle peaks at high temperatures—loss of spectral weight or excessive broadening?
• RQ3 How does the Fermi surface, defined by the peak of the spectral function, evolve with temperature in Sr2RuO4?
• RQ4 To what extent do ARPES-measured spectral functions remain consistent with quasiparticle behavior when the peak becomes broad and less intense?
• RQ5 Is the observed increase in Z with temperature consistent with DMFT predictions for multiband correlated systems?

Key findings

• The quasiparticle residue Z increases with temperature in Sr2RuO4, contrary to the conventional expectation of Z → 0 at high T.
• Quasiparticles persist in the spectral function up to temperatures above 200 K, well beyond the Fermi liquid regime and the Mott-Ioffe-Regel limit.
• The spectral peak broadens significantly with increasing temperature, evolving from an asymmetric Fermi liquid shape to a Lorentzian, indicating super-Planckian scattering.
• The Fermi surface, defined as the loci of maximum spectral function intensity, deflates with increasing temperature, reflecting a reduction in coherence.
• The ARPES data, after background subtraction, show excellent qualitative and semi-quantitative agreement with DMFT calculations, supporting the increase in Z with temperature.
• The study demonstrates that quasiparticle weight can increase with temperature even as peak intensity diminishes, challenging the assumption that broadening always implies reduced Z.

Figure 2: (a) MDCs at $E_{F}$ extracted from ARPES data with Lorentzian fits to quantify peak positions of the $\beta$ and $\gamma$ band. Additional examples of fits are shown in Supplemental Material. (b) Temperature dependence of $k(T)-k(T=0)$ at $E=E_{F}$ and $E=-29$ meV. Different colors and sym