Pressure evolution of f electron hybridized state in CeCoIn₅ studied by optical conductivity

Optical conductivity σ(ω) of CeCoIn₅ has been measured under high pressure to 8 GPa and at low temperatures to 6 K, to study the pressure evolution of the conduction (c)-f electron hybridized state near the Fermi level. At ambient pressure, CeCoIn₅ is a heavy fermion superconductor with T_c=2.3 K, and has moderately strong c-f hybridization. σ(ω) at ambient pressure shows a marked infrared peak due to optical excitations involving the c-f hybridized state. With increasing pressure, the infrared peak becomes broader, and its center shifts to higher energy. This result indicates that the large density of states of the c-f hybridized state becomes broader with increasing pressure, and that the peak of the density of states shifts away from the Fermi level. Althouth such pressure evolution of the c-f hybridized state had been widely assumed on the basis of theoretical consideration, the present optical result demonstrates it clearly and directly.