Weakly-interacting Systems Research Articles

Communication: Compact orbitals enable low-cost linear-scaling ab initio molecular dynamics for weakly-interacting systems.

Today, ab initio molecular dynamics (AIMD) relies on the locality of one-electron density matrices to achieve linear growth of computation time with the system size, crucial in large-scale simulations. While Kohn-Sham orbitals strictly localized within predefined radii can offer substantial computational advantages over density matrices, such compact orbitals are not used in AIMD because a compact representation of the electronic ground state is difficult to find. Here, a robust method for maintaining compact orbitals close to the ground state is coupled with a modified Langevin integrator to produce stable nuclear dynamics for molecular and ionic systems. This eliminates a density matrix optimization and enables first orbital-only linear-scaling AIMD. An application to liquid water demonstrates that low computational overhead of the new method makes it ideal for routine medium-scale simulations, while its linear-scaling complexity allows us to extend first-principle studies of molecular systems to completely new physical phenomena on previously inaccessible length scales.

Various topological Mott insulators and topological bulk charge pumping in strongly-interacting boson system in one-dimensional superlattice

In this paper, we study a one-dimensional boson system in a superlattice potential. This system is experimentally feasible by using ultracold atomic gases, and attracts much attention these days. It is expected that the system has a topological phase called a topological Mott insulator (TMI). We show that in strongly-interacting cases, the competition between the superlattice potential and the on-site interaction leads to various TMIs with a non-vanishing integer Chern number. Compared to the hard-core case, the soft-core boson system exhibits rich phase diagrams including various non-trivial TMIs. By using the exact diagonalization, we obtain detailed bulk-global phase diagrams including the TMIs with high Chern numbers and also various non-topological phases. We also show that in adiabatic experimental setups, the strongly-interacting bosonic TMIs exhibit the topological particle transfer, i.e., the topological charge pumping phenomenon, similarly to weakly-interacting systems. The various TMIs are characterized by topological charge pumping as it is closely related to the Chern number, and therefore the Chern number is to be observed in feasible experiments.

Many positron and positronium interactions*1

The low-energy positronium–positronium (Ps–Ps) scattering parameters, s-wave scattering lengths and effective ranges for symmetric/antisymmetric channels, are evaluated on the basis of the semi-phenomenological Ps–Ps effective potentials and of the existence of the Ps molecule. The Ps–Ps scattering parameters obtained by recent more elaborate ab initio calculations are reviewed briefly, and shown to be consistent with those by effective potential approach. We also discuss the possible phases of Ps or electron–positron many-body states, and the effects of the Ps–Ps interactions for them. Especially, we estimate the interaction effects in Ps-Bose–Einstein condenstates (Ps-BEC) with the obtained Ps–Ps scattering lengths, and show that Ps-BEC is very weakly-interacting system.

Many positron and positronium interactions

The low-energy positronium–positronium (Ps–Ps) scattering parameters, s-wave scattering lengths and effective ranges for symmetric/antisymmetric channels, are evaluated on the basis of the semi-phenomenological Ps–Ps effective potentials and of the existence of the Ps molecule. The Ps–Ps scattering parameters obtained by recent more elaborate ab initio calculations are reviewed briefly, and shown to be consistent with those by effective potential approach. We also discuss the possible phases of Ps or electron–positron many-body states, and the effects of the Ps–Ps interactions for them. Especially, we estimate the interaction effects in Ps-Bose–Einstein condenstates (Ps-BEC) with the obtained Ps–Ps scattering lengths, and show that Ps-BEC is very weakly-interacting system.

Bosons in a random potential: condensation and screening in a dense limit

The authors demonstrate that an extended Bose condensate can be stable in a random potential for a suitable weak-repulsive limit of a dense Bose gas, even though the non-interacting case is pathological. The condensate exists primarily because the interactions allow screening of the random potential. This may happen even when the chemical potential is in the Lifshitz tails of the single-particle case. Indeed, the authors argue that there are no Lifshitz tail states in their dense but weakly-interacting system. Using a number-phase representation, they calculate the increase in the depletion of the condensate with increasing randomness (at fixed density) which indicates the eventual destruction of the condensed phase-perhaps to a localized phase. The physical picture discussed should be relevant to the understanding of helium thin films.