Charge relaxation resistance in the cotunneling regime of multichannel Coulomb blockade: Violation of Korringa-Shiba relation

We study the low-frequency admittance of a small metallic island coupled to a gate electrode and to a massive reservoir via a multichannel tunnel junction. The ac current is caused by a slowly oscillating gate voltage. We focus on the regime of inelastic cotunneling in which the dissipation of energy (the real part of the admittance) is determined by two-electron tunneling with creation of electron-hole pairs on the island. We demonstrate that at finite temperatures but low frequencies the energy dissipation is ohmic whereas at zero temperature it is superohmic. We find that (i) the charge relaxation resistance (extracted from the real part of the admittance) is strongly temperature dependent, and (ii) the imaginary and real parts of the admittance do not satisfy the Korringa-Shiba relation. At zero temperature the charge relaxation resistance vanishes in agreement with the recent zero-temperature analysis [M. Filippone and C. Mora, Phys. Rev. B 86, 125311 (2012); P. Dutt, T. L. Schmidt, C. Mora, and K. Le Hur, Phys. Rev. B 87, 155134 (2013)].