Intermediate quantum theory of crystalline solids

I. M. Lifshit︠s︡

Weiping Zhang, Daniel F. Walls

In this paper, a quantum-field theory is employed to study the diffraction of an ultracold atomic beam by a crystalline solid surface. We show that the crystalline solid surface acts as a nonlinear atomic grating for an ultracold atomic beam with a Bose condensate. The nonlinearity in such a mechanical atomic grating is due to the phonon exchange between the atoms in the ultracold atomic beam via the surface lattice vibration. Such a phonon exchange leads to an attractive interatom interaction in the ultracold atomic beam, which results in the generation of correlated atomic pairs in momentum space. Such correlated atomic pairs have similar properties to the conjugate photon pairs in nonclassical light.

Massimo Boninsegni

The bound state of a 3He atom at the interface between coexisting crystalline and superfluid phases of 4He is studied theoretically by means of first principle Quantum Monte Carlo simulations. We consider both the case of a solid/superfluid 4He interface, as well as that of a few solid layers of 4He forming on an attractive substrate. It is observed that the 3He bound state is sharply localized in a well-defined, quasi-2D layer of 4He in the region intermediate between the solid and the liquid. The layer in which the 3He atom resides displays an intriguing interplay of atomic localization and quantum-mechanical exchanges; is not necessarily the first “non-solid” layer, as suggested in previous studies, and it is affected by the attractive strength of the substrate.

Takahumi Suzuki, Naoki Kawashima

We investigate a supersolid state in hardcore boson models on the face-centered-cubic (fcc) lattice. The supersolid state is characterized by a coexistence of crystalline order and superfluidity. Using a quantum Monte Carlo method based on the directed-loop algorithm, we calculate static structure factors and superfluid density at finite temperature, from which we obtain the phase diagram. The supersolid phase exists at intermediate fillings between a three-quarter-filled solid phase and a half-filled solid phase. We also discuss the mechanism of the supersolid state on the fcc lattice.