Topological charge pumping in the interacting bosonic Rice-Mele model

A.L.C. Hayward, C. Schweizer, M. Lohse, M. Aidelsburger, F. Heidrich-Meisner, arXiv:1810.07043

We investigate topological charge pumping in a system of interacting bosons in the tight-binding limit, described by the Rice-Mele model. An appropriate topological invariant for the many-body case is the change of polarization per pump cycle, which we compute for various interaction strengths from infinite-size matrix-product-state simulations. We verify that the charge pumping remains quantized as long as the pump cycle avoids the superfluid phase. In the limit of hardcore bosons, the quantized pumped charge can be understood from single-particle properties such as the integrated Berry curvature constructed from Bloch states, while this picture breaks down at finite interaction strengths. These two properties -- robust quantized charge transport in an interacting system of bosons and the breakdown of a single-particle invariant -- could both be measured with ultracold quantum gases extending a previous experiment [Lohse et al., Nature Phys. 12, 350 (2016)]. Furthermore, we investigate the entanglement spectrum of the Rice-Mele model and argue that the quantized charge pumping is encoded in a winding of the spectral flow in the entanglement spectrum over a pump cycle.

Large magnetic thermal conductivity induced by frustration in low-dimensional quantum magnets

Jan Stolpp, Shang-Shun Zhang, Fabian Heidrich-Meisner, Cristian D. Batista, arXiv:1809.08429

We study the magnetic field-dependence of the thermal conductivity due to magnetic excitations in frustrated spin-1/2 Heisenberg chains. Near the saturation field, the system is described by a dilute gas of weakly-interacting fermions (free-fermion fixed point). We show that in this regime the thermal conductivity exhibits a non-monotonic behavior as a function of the ratio $\alpha = J_2/J_1$ between second and first nearest-neighbor antiferromagnetic exchange interactions. This result is a direct consequence of the splitting of the single-particle dispersion minimum into two minima that takes place at the Lifshitz point $\alpha = 1/4$. Upon increasing $\alpha$ from zero, the inverse mass vanishes at $\alpha = 1/4$ and it increases monotonically from zero for $\alpha \geq 1/4$. By deriving an effective low-energy theory of the dilute gas of fermions, we demonstrate that the Drude weight $K_{\rm th}$ of the thermal conductivity exhibits a similar dependence on $\alpha$ near the saturation field. Moreover, this theory predicts a transition between a two-component Tomonaga-Luttinger liquid and a vector-chiral phase at a critical value $\alpha = \alpha_c$ that agrees very well with previous density matrix renormalization group results. We also show that the resulting curve $K_{\rm th}(\alpha)$ is in excellent agreement with exact diagonalization (ED) results. Our ED results also show that $K_{\rm th}(\alpha)$ has a pronounced minimum at $\alpha \simeq 0.7$ and it decreases for sufficiently large $\alpha$ at lower magnetic field values. We also demonstrate that the thermal conductivity is significantly affected by the presence of magnetothermal coupling.

Non-Equilibrium Mass Transport in the 1D Fermi-Hubbard Model

S. Scherg, T. Kohlert, J. Herbrych, J. Stolpp, P. Bordia, U. Schneider, F. Heidrich-Meisner, I. Bloch, and M. Aidelsburger, Phys. Rev. Lett. 121, 130402 (2018), arXiv:1805.10990

We experimentally and numerically investigate the sudden expansion of fermions in a homogeneous
one-dimensional optical lattice. For initial states with an appreciable amount of doublons, we observe a dynamical phase separation between rapidly expanding singlons and slow doublons remaining in the trap center, realizing fermionic quantum distillation in the strongly-interacting limit. For initial states without doublons, we observe a suppressed interaction dependence of the asymptotic expansion speed compared to bosons, which is traced back to the interaction energy produced in the quench.

Quantum phases of strongly interacting bosons on a two-leg Haldane ladder

Sebastian Greschner and Fabian Heidrich-Meisner, Phys. Rev. A 97, 033619 (2018), arXiv:1710.08109

We study the ground-state physics of a single-component Haldane model on a hexagonal two-leg ladder geometry with a particular focus on strongly interacting bosonic particles. We concentrate our analysis on the regime of less than one particle per unit cell. As a main result, we observe several Meissner-like and vortex-fluid phases, both for a superfluid as well as a Mott-insulating background. Furthermore, we show that for strongly interacting bosonic particles, an unconventional vortex-lattice phase emerges, which is stable even in the regime of hard-core bosons. We discuss the mechanism for its stabilization for finite interactions by a means of an analytical approximation. We show how the different phases may be discerned by measuring the nearest- and next-nearest-neighbor chiral currents as well as their characteristic momentum distributions.

Many-body localization of spinless fermions with attractive interactions in one dimension

Sheng-Hsuan Lin, Björn Sbierski, Florian Dorfner, Christoph Karrasch, Fabian Heidrich-Meisner, SciPost Phys. 4, 002 (2018), arXiv:1707.06759

We study the finite-energy density phase diagram of spinless fermions with attractive interactions in one dimension in the presence of uncorrelated diagonal disorder. Unlike the case of repulsive interactions, a delocalized Luttinger-liquid phase persists at weak disorder in the ground state, which is a well-known result. We revisit the ground-state phase diagram and show that the recently introduced occupation-spectrum discontinuity computed from the eigenspectrum of one-particle density matrices is noticeably smaller in the Luttinger liquid compared to the localized regions. Moreover, we use the functional renormalization scheme to study the finite-size dependence of the conductance, which resolves the existence of the Luttinger liquid as well and is computationally cheap. Our main results concern the finite-energy density case. Using exact diagonalization and by computing various established measures of the many-body localization-delocalization transition, we argue that the zero-temperature Luttinger liquid smoothly evolves into a finite-energy density ergodic phase without any intermediate phase transition.