Note on the Coulomb blockade of a weak tunnel junction with Nyquist noise: Conductance formula for a broad temperature range

Abstract

We revisit the Coulomb blockade of the tunnel junction with conductance much smaller than $e^2/\hbar$. We study the junction with capacitance $C$, embedded in an Ohmic electromagnetic environment modelled by a series resistance $R$ which produces the Nyquist noise. In the semiclassical limit the Nyquist noise charges the junction by a random charge with a Gaussian distribution. Assuming the Gaussian distribution, we derive analytically the temperature-dependent junction conductance $G(T)$ valid for temperatures $k_BT \gtrsim (R_K/2\pi R)E_c$ and resistances $R \gtrsim R_K$, where $R_K = h/e^2$ and $E_c=e^2/2C \ \text{is}$ the single-electron charging energy. Our analytical result shows the leading dependence $G(T) \propto e^{-E_c/4k_BT}$, so far believed to exist only if $(R_K/\pi R)E_c \ll k_BT \ll E_c$ and $R \gg R_K$. The validity of our result for $k_BT \gtrsim (R_K/2\pi R)E_c$ and $R \gtrsim R_K$ is confirmed by a good agreement with the numerical studies which do not assume the semiclassical limit, and by a reasonable agreement with experimental data for $R$ as low as $R_K$. Our result also reproduces various asymptotic formulae derived in the past. The factor of $1/4$ in the activation energy $E_c/4$ is due to the semiclassical Nyquist noise.