The transport properties of gapless edge modes at boundaries between topologically distinct domains are of fundamental and technological importance. We experimentally studied long-distance quantized Hall drifts in a harmonically confined topological pump of ultracold fermionic atoms. We found that quantized drifts halt and reverse their direction when the atoms reach a critical slope of the confining potential, revealing the presence of a topological boundary. The drift reversal corresponded to a band transfer between a band with Chern number C = +1 and another with C = –1 through a gapless edge mode, in agreement with the bulk-edge correspondence for noninteracting particles. Nonzero repulsive Hubbard interactions led to the emergence of an additional edge in the system through a mechanism in which pairs of fermions are split.
The transport properties of gapless edge modes at boundaries between topologically distinct domains are of fundamental and technological importance. We experimentally studied long-distance quantized Hall drifts in a harmonically confined topological pump of ultracold fermionic atoms. We found that quantized drifts halt and reverse their direction when the atoms reach a critical slope of the confining potential, revealing the presence of a topological boundary. The drift reversal corresponded to a band transfer between a band with Chern number C = +1 and another with C = –1 through a gapless edge mode, in agreement with the bulk-edge correspondence for noninteracting particles. Nonzero repulsive Hubbard interactions led to the emergence of an additional edge in the system through a mechanism in which pairs of fermions are split.
Two-dimensional topological insulators possess conducting edge states at their boundary while being insulating in the bulk. We investigate the edge state emergent at a smooth topological phase boundary of interacting fermions within a full real-space analysis of the time-reversal invariant Hofstadter-Hubbard model. We characterize the localization of the edge state and the topological phase boundary by means of the local compressibility, the spectral density, a generalized local spin Chern marker as well as the Hall response and find good agreement between all these quantities. Computing the edge state spectra at the interface we observe robustness of the edge state against fermionic two-body interactions and conclude that interactions only shift its position. Hence the bulk-boundary correspondence for the interacting system is confirmed. Since experimental probing of edge states remains a challenge in ultracold atom setups, we propose the detection of the local compressibility by measuring correlations with a quantum gas microscope.
Topological states of matter are peculiar quantum phases showing different edge and bulk transport properties connected by the bulk-boundary correspondence. While noninteracting fermionic topological insulators are well established by now and have been classified according to a tenfold scheme, the possible realization of topological states for bosons has not been explored much yet. Furthermore, the role of interactions is far from being understood. Here, we show that a topological state of matter exclusively driven by interactions may occur in the p band of a Lieb optical lattice filled with ultracold bosons. The single-particle spectrum of the system displays a remarkable parabolic band-touching point, with both bands exhibiting non-negative curvature. Although the system is neither topological at the single-particle level nor for the interacting ground state, on-site interactions induce an anomalous Hall effect for the excitations, carrying a nonzero Chern number. Our work introduces an experimentally realistic strategy for the formation of interaction-driven topological states of bosons.
Since the experimental realization of synthetic gauge fields for neutral atoms, the simulation of topologically nontrivial phases of matter with ultracold atoms has become a major focus of cold-atom experiments. However, several obvious differences exist between cold-atom and solid-state systems, for instance the small size of the atomic cloud and the smooth confining potential. In this article we show that sharp boundaries are not required to realize quantum Hall or quantum spin Hall physics in optical lattices and, on the contrary, that edge states in a smooth confinement exhibit additional interesting properties, such as spatially resolved splitting and merging of bulk bands and the emergence of robust auxiliary states in bulk gaps to preserve the topological quantum numbers. In addition, we numerically validate that these states are robust against disorder. Finally, we analyze possible detection methods, with a focus on Bragg spectroscopy, to demonstrate that the edge states can be detected and that Bragg spectroscopy can reveal how topological edge states are connected to the different bulk bands.
A topological “Thouless” pump represents the quantized motion of particles in response to a slow, cyclic modulation of external control parameters. The Thouless pump, like the quantum Hall effect, is of fundamental interest in physics, because it links physically measurable quantities, such as particle currents, to geometric properties of the experimental system, which can be robust against perturbations and, thus, technologically useful. So far, experiments probing the interplay between topology and interparticle interactions have remained relatively scarce. Here, we observe a Thouless-type charge pump in which the particle current and its directionality inherently rely on the presence of strong interactions. Experimentally, we utilize a two-component Fermi gas in a dynamical superlattice which does not exhibit a sliding motion and remains trivial in the single-particle regime. However, when tuning interparticle interactions from zero to positive values, the system undergoes a transition from being stationary to drifting in one direction, consistent with quantized pumping in the first cycle. Remarkably, the topology of the interacting pump trajectory cannot be adiabatically connected to a noninteracting limit, highlighted by the fact that only one atom is transferred per cycle. Our experiments suggest that Thouless charge pumps are promising platforms to gain insights into interaction-driven topological transitions and topological quantum matter. Published by the American Physical Society 2024
Large Chern-number topological phase is always an important topic in modern physics. Here we investigate the topological superfluids in a coupled-layer system, in which transitions between different topological superfluids can be realized by controlling the binding energy, interlayer tunneling, and layer asymmetry, etc. These topological transitions are characterized by energy gap closing and reopening at the critical points at zero momentum, where the Chern number and sign of Pfaffian undergo a discontinuous change. Topological protected edge modes at the boundaries are ensured by the bulk-edge correspondence. In a trapped potential the edge modes are spatially localized at the interfaces between distinct topological superfluids, where the number of edge modes is equal to the Chern-number difference between the left and right superfluids. These topological transitions can be detected by spin texture at or near zero momentum, which changes discretely across the critical points due to band inversion. The model can be generalized to a multilayer system in which the Chern number can be equal to any positive integer. These large Chern-number topological superfluids provide fertile grounds for exploring exotic quantum matters in the context of ultracold atoms.
Topological phases of matter are the center of much current interest, with promising potential applications in, e.g., topologically protected transport and quantum computing. Traditionally such states are prepared by tuning the system Hamiltonian while coupling it to a generic bath at very low temperatures. However, this approach is often ineffective, especially in cold-atom systems. It was recently shown that topological phases can emerge much more efficiently even in the absence of a Hamiltonian, by properly engineering the interaction of the system with its environment, to directly drive the system into the desired state. Here we concentrate on dissipatively induced two-dimensional Chern insulator (lattice quantum Hall) states. We employ open quantum systems tools to explore their transport properties, such as persistent currents and the conductance in the steady state, in the presence of various Hamiltonians. We find that, in contrast with equilibrium systems, the usual confinement of currents to the edge, as well as the usual relation between the Chern topological number and the Hall conductance, could be broken. We explore the intriguing edge behaviors and elucidate under which conditions the Hall conductance is quantized.