Superfluidity of flexible chains of polar molecules

We study properties of quantum chains in a gas of polar bosonic molecules confined in a stack of N identical one- and two- dimensional optical lattice layers, with molecular dipole moments aligned perpendicularly to the layers. Quantum Monte Carlo simulations of a single chain (formed by a single molecule on each layer) reveal its quantum roughening transition. The case of finite in-layer density of molecules is studied within the framework of the J-current model approximation, and it is found that N-independent molecular superfluid phase can undergo a quantum phase transition to a rough chain superfluid. A theorem is proven that no superfluidity of chains with length shorter than N is possible. The scheme for detecting chain formation is proposed.