Phase and Charge reentrant phase transitions in two capacitively coupled   Josephson arrays with ultra-small junction
  We have studied the phase diagram of two capacitively coupled Josephson junction arrays with charging energy, $E_c$, and Josephson coupling energy, $E_J$. Our results are obtained using a path integral Quantum Monte Carlo algorithm. The parameter that quantifies the quantum fluctuations in the i-th array is defined by $\alpha_i\equiv \frac{E_{{c}_i}}{E_{J_i}}$. Depending on the value of $\alpha_i$, each independent array may be in the semiclassical or in the quantum regime: We find that thermal fluctuations are important when $\alpha \lesssim 1.5 $ and the quantum fluctuations dominate when $2.0 \lesssim \alpha $. We have extensively studied the interplay between vortex and charge dominated individual array phases. The two arrays are coupled via the capacitance $C_{{\rm inter}}$ at each site of the lattices. We find a {\it reentrant transition} in $\Upsilon(T,\alpha)$, at low temperatures, when one of the arrays is in the semiclassical limit (i.e. $\alpha_{1}=0.5 $) and the quantum array has $2.0 \leq\alpha_{2} \leq 2.5$, for the values considered for the interlayer capacitance. In addition, when $3.0 \leq \alpha_{2} < 4.0$, and for all the inter-layer couplings considered above, a {\it novel} reentrant phase transition occurs in the charge degrees of freedom, i.e. there is a reentrant insulating-conducting transition at low temperatures. We obtain the corresponding phase diagrams and found some features that resemble those seen in experiments with 2D JJA. 
Guillermo Ramirez-Santiago (Instituto de Fisica, UNAM MEXICO) and   Jorge V. Jose (Dept. Phys. Northeastern University, Bost...