[Paper Review] Hybridization and superconducting gaps in heavy-fermion superconductor PuCoGa5 probed via the dynamics of photoinduced quasiparticles

Diyar Talbayev, S. A. Trugman+9 more|OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information)|Jun 21, 2010

Rare-earth and actinide compounds4 citations

TL;DR

This study probes the electronic structure of the heavy-fermion superconductor PuCoGa5 using ultrafast pump-probe spectroscopy to investigate quasiparticle relaxation dynamics. It finds weak electron-phonon coupling (λ = 0.2–0.26), ruling out phonon-mediated superconductivity, and provides direct spectroscopic evidence for a hybridization gap and heavy quasiparticles forming the superconducting condensate, supporting magnetically mediated d-wave pairing.

ABSTRACT

We have examined the relaxation of photoinduced quasiparticles in the heavy-fermion superconductor PuCoGa5. The deduced electron-phonon coupling constant is incompatible with the measured superconducting transition temperature Tc=18.5 K, which speaks against phonon-mediated superconducting pairing. Upon lowering the temperature, we observe an order-of-magnitude increase of the quasiparticle relaxation time in agreement with the phonon bottleneck scenario - evidence for the presence of a hybridization gap in the electronic density of states. The modification of photoinduced reflectance in the superconducting state is consistent with the heavy character of the quasiparticles that participate in Cooper pairing.

Motivation & Objective

• To determine the electron-phonon coupling strength in PuCoGa5 and assess its role in mediating superconductivity.
• To investigate the nature of quasiparticle relaxation dynamics in the normal and superconducting states.
• To provide spectroscopic evidence for a hybridization gap in the electronic density of states.
• To determine whether the same heavy quasiparticles participating in the normal state also form the Cooper pairs in the superconducting state.
• To clarify the pairing mechanism in Pu-based heavy-fermion superconductors, particularly whether it is phonon-mediated or unconventional.

Proposed method

• Ultrafast pump-probe optical spectroscopy using 45-femtosecond laser pulses to excite and monitor photoinduced reflectance changes in PuCoGa5.
• Time-resolved measurements of reflectance changes with sub-100 fs time resolution to track quasiparticle relaxation dynamics.
• Analysis of relaxation time (τ3) and amplitude (A3) of the reflectance signal to extract electron-phonon coupling and hybridization gap effects.
• Application of the relaxation time (RT) model to interpret quasiparticle recombination dynamics and identify deviations due to superconducting gap formation.
• Comparison of temperature-dependent reflectance spectra to identify phonon bottleneck effects and hybridization gap signatures in the normal state.
• Use of fixed relaxation time (τ3 = 22.9 ps) at low temperatures to fit data and infer suppression of amplitude A3 due to modified density of states near the Fermi level.

Experimental results

Research questions

• RQ1 Is the superconducting transition temperature in PuCoGa5 consistent with phonon-mediated pairing via electron-phonon coupling?
• RQ2 Does the observed quasiparticle relaxation dynamics in the normal state indicate the presence of a hybridization gap?
• RQ3 Are the same heavy quasiparticles responsible for both the hybridization gap and the superconducting condensate?
• RQ4 How does the modification of the density of states near the Fermi level below Tc affect the photoinduced reflectance response?
• RQ5 What is the nature of the pairing mechanism in PuCoGa5—conventional phonon-mediated or unconventional magnetic pairing?

Key findings

• The electron-phonon coupling constant in PuCoGa5 is measured to be λ = 0.2–0.26, which is too weak to explain the observed Tc = 18.5 K, ruling out phonon-mediated superconductivity.
• A pronounced order-of-magnitude increase in quasiparticle relaxation time at low temperatures (T < 10 K) is observed, consistent with a phonon bottleneck and providing direct evidence for a hybridization gap in the electronic density of states.
• The amplitude of the quasiparticle recombination signal (A3) is significantly suppressed below Tc, indicating a modification of the density of states near the Fermi level due to the opening of the superconducting gap.
• The long relaxation time (τ3 ≥ 22.9 ps) at 9.2 K, even below Tc, is consistent with the RT model and indicates suppressed quasiparticle relaxation due to the phonon bottleneck.
• The suppression of A3 at low temperatures cannot be explained by the RT model alone, pointing to a fundamental change in the electronic structure due to the superconducting gap.
• The data support a scenario in which the same heavy quasiparticles formed by Pu 5f–conduction band hybridization participate in Cooper pairing, indicating unconventional, magnetically mediated superconductivity.