Fulleride superconductivity tuned by elastic strain due to cation compositional disorder

Area of Science:

• Condensed Matter Physics
• Materials Science
• Superconductivity

Background:

• Dynamical elastic strain fluctuations impact metal-to-insulator and superconducting transitions in strongly correlated systems.
• Alkali-intercalated fullerene superconductors exhibit complex electronic behavior influenced by structural and electronic correlations.

Purpose of the Study:

• To investigate the effect of elastic strain, introduced via controlled structural disorder, on the properties of bandwidth-controlled alkali-intercalated fullerene superconductors.
• To establish a quantitative relationship between structural disorder, elastic strain fluctuations, and the superconducting critical temperature (Tc).

Main Methods:

• Systematic introduction of static local structural disorder in K_xCs_3-xC₆₀ using K+ and Cs+ co-dopants with differing ionic radii.
• Characterization using synchrotron X-ray powder diffraction (SXRPD) to monitor unit cell volume changes.
• ¹³³Cs Nuclear Magnetic Resonance (NMR) spectroscopy to probe electronic states and spin-lattice relaxation rates.

Main Results:

• Observed a crossover from Mott-Jahn-Teller insulating (MJTI) to strongly correlated Jahn-Teller metallic (JTM) states for x_K < 1.28, evidenced by SXRPD and ¹³³Cs NMR.
• Superconducting critical temperature (Tc) exhibits a dome-like dependence on unit-cell volume and the U/W ratio.
• Maximum Tc is reduced by ~12% compared to Cs₃C₆₀, with the reduction linearly dependent on the variance of cation size (σ_T²).

Conclusions:

• Structural disorder-induced elastic strain fluctuations attenuate critical fluctuations, leading to a reduction in superconducting Tc.
• The study establishes a clear link between structural disorder, elastic strain dynamics, and the electronic ground state in these fullerene superconductors.