Interaction-induced chiral-transport inversion

Abstract

We investigate the chiral dynamics of locally interacting bosons in a two-leg flux ladder, where on-site interactions, despite being fully isotropic, counterintuitively reverse the flux-induced chiral transport of density distribution. For a Bose-Einstein condensate (in the mean-field regime), this reversal arises from an interactiondriven dynamical band-occupation inversion, which selectively populates single-particle states of the opposing chirality. Strikingly, the chiral-transport inversion has a few-body, hence beyond-mean-field, origin, as the formation of two-body bound states with reversed chirality dominates the few-body dynamics. This dual pathway, that is, occupation inversion and bound-state formation, underlies the chiral-transport inversion, which challenges the conventional wisdom that isotropic interactions cannot bias density transport. Our work reveals the interplay between interactions and chirality and highlights how correlations engineer exotic quantum transport.

Figures (3)

• Figure 1: Chiral transport of a BEC. (a) Illustration of the IIICC phenomenon in a square flux ladder. As illustrated in the lower panel, interactions induce the inversion of chiral transport. (b) Chiral displacement under different interactions: $U=0$ (purple solid), $U=5$ (laurel green dash-dotted), and $U=10$ (gray dashed). Inset: phase diagram where the color contour represents the averaged chiral displacement over the evolution time $t_{f}=20$. (c)-(e): the evolution of wave functions for $U=0$ (c), $U=5$ (d) and $U=10$ (e), respectively. The magenta solid and lake blue dashed curves represent the components A and B, respectively. For the calculations, we take $\phi=\pi/2$, and the length of the ladder $L=101$. The BEC is initially prepared in a superposition state $|\psi_{ini}\rangle$ [see Eq. (\ref{['eq:initial_state']})]. The units of length, energy and time in this paper are fixed by setting J=1 and the lattice constant as $1$.
• Figure 2: Stroboscopic projections $P_{k\pm}$ of the time-evolved wave function onto the Bloch states of the free Hamiltonian. (a)-(d) depict the stroboscopic projection probabilities for cases with $U=0, 2.5, 5$, and $10$, respectively. The inset in (a) displays the energy bands, with the background color representing the magnitudes of component A of the Bloch states. The red and cyan curves represent projections onto the lower ($P_{k+}$) and upper ($P_{k-}$) bands, respectively. In the case with $U=0$, the projection probabilities remain constant over time and are identical to those with finite interaction strengths and at $t=0$. Notably, in (c) where an intermediate interaction is present, a clear inversion of the stroboscopic projection probabilities over time is observed. All other parameters are consistent with those used in Fig. \ref{['fig:PD_MF']}.
• Figure 3: Chiral transport of a two-body initial state. Chiral displacement $D_{\text{c}}$ (a) and two-particle chiral displacement $D_{\text{c}}^{(2)}$ (f) are plotted for a fixed flux $\phi=0.9\pi$ and different interactions $U=0, 5$ and $10$. The line conventions for different interactions are the same with those in Fig. \ref{['fig:PD_MF']}(b). Subfigures (b) and (g) display the corresponding phase diagrams, where the background colors represent the average values of $D_{\text{c}}$ (b) and $D_{\text{c}}^{(2)}$ (g), respectively. Projection of the two-body initial state onto the eigenstates of the full Hamiltonian given by ED method: (c)-(e) correspond to $U=0,5$ and $10$, respectively. The background color indicates the projection probability. The three scattering bands defined in the main text, denoted by $|++\rangle$, $|+-\rangle$, and $|--\rangle$, are shown with the pink dash-dotted, dotted and dashed curves, respectively. The evolution of two-particle distribution for $U=0$ (h) $U=5$ (i) are also shown. The insets illustrate the opposite transports contributed by single-particle and two-particle bound states. The magenta solid and lake blue dashed curves correspond to the distributions on leg A and B, respectively. The length of the square flux ladder is $41$. Other parameters are the same as those in Fig. \ref{['fig:PD_MF']}.