Thermal conductivity for coupled charged harmonic oscillators with noise in a magnetic field.

We introduce a $d$-dimensional system of charged harmonic oscillators in a magnetic field perturbed by a stochastic dynamics which conserves energy but not momentum. We study the thermal conductivity via the Green-Kubo formula, focusing on the asymptotic behavior of the Green-Kubo integral up to time $t$ (i.e., the integral of the correlation function of the total energy current). We employ the microcanonical measure to calculate the Green-Kubo formula in general dimension $d$ for uniformly charged oscillators. We also develop a method to calculate the Green-Kubo formula with the canonical measure for uniformly and alternately charged oscillators in dimension $1$. We prove that the thermal conductivity diverges in dimension $1$ and $2$ while it remains finite in dimension $3$. The Green--Kubo integral calculated with the microcanonical ensemble diverges as $t^{1/4}$ for uniformly charged oscillators in dimension $1$, while it is known to diverge as $t^{1/2}$ without magnetic field. This is the first rigorous example of the new exponent $1/4$ in the asymptotic behavior for the Green-Kubo integral. We also demonstrate that our result provides the first rigorous example of a diverging thermal conductivity with vanishing sound speed. In addition, employing the canonical measure in the Green-Kubo formula, we prove that the Green-Kubo integral for uniformly and alternately charged oscillators respectively diverges as $t^{1/4}$ and $t^{1/2}$. This means that the exponent depends not only on a non-zero magnetic field but also on the charge structure of oscillators.