08:00  16:30

registration in house 4

09:15  09:30

Opening  Roderich Moessner, director of the MPIPKS & scientific coordinators


chair: Yang Zhang

09:30  10:30

Long Ju
(Massachusetts Institute of Technology)
Fractional quantum anomalous Hall effect in graphene
The fractional quantum anomalous Hall effect (FQAHE), the analog of the fractional quantum Hall effect at zero magnetic field, is predicted to exist in topological flat bands under spontaneous timereversalsymmetry breaking . The demonstration of FQAHE could lead to nonAbelian anyons which form the basis of topological quantum computation . In this talk, I will report the observation of integer and fractional QAH effects in a rhombohedral pentalayer graphene/hBN moiré superlattice. At zero magnetic field, we observed plateaus of quantized Hall resistance at filling factors v = 1, 2/3, 3/5, 4/7, 4/9, 3/7 and 2/5 of the moiré superlattice. These features are accompanied by clear dips in the longitudinal resistance Rxx. In addition, we observed signatures of the composite Fermi liquid (CFL) at halffilling. By tuning the gate displacement field D and v, we observed phase transitions from CFL and FQAH states to other correlated electron states. Our graphene system provides an ideal platform for exploring charge fractionalization and (nonAbelian) anyonic braiding at zero magnetic field, especially considering a lateral junction between FQAHE and superconducting regions in the same device.

10:30  11:00

Tingxin Li
(Shanghai Jiao Tong University)
Integer and fractional quantum anomalous Hall effects in 2D semiconductor moiré superlattices
The emergence of topological moiré flat bands provides exciting opportunities to realize the lattice analogs of both the integer and fractional quantum Hall effects without the need for an external magnetic field. These effects are known as the integer and fractional quantum anomalous Hall (IQAH and FQAH) effects. In this talk, I will mainly present electrical transport studies of moiré superlattices built on 2D transition metal dichalcogenide (TMDc) semiconductors. We have successfully achieved highly tunable topological phases in both TMDc heterobilayer and TMDc homobilayer moiré superlattices. Specifically, we have observed a robust IQAH effect and signatures of quantum spin Hall effect in ABstacked WSe2/MoTe2. Furthermore, both the IQAH effect and the longsought FQAH effect have been realized in twisted bilayer MoTe2. The band topology in TMDc moiré superlattices is highly tunable by external electric fields, which enable us to realize novel topological quantum phase transitions. Our studies pave the path for the investigation of fractionally charged excitations and anyonic statistics at zero magnetic field based on 2D moiré materials.
[1] F. Xu et al. PRX 13, 031037 (2023).
[2] T. Li et al. Nature 600, 641646 (2021).
[3] T. Li et al. Nature 597, 350354 (2021).

11:00  11:30

coffee break

11:30  12:00

Emil Bergholtz
(Stockholm University)
Fractional Chern insulators and their competitors
Fractional Chern insulators (FCIs), initially conceptualized as lattice counterparts of fractional quantum Hall (FQH) states in the absence of a magnetic field, stand as a captivating frontier in condensed matter physics, illustrating a complex interplay between robust electronelectron interactions, band topology, and quantum geometry. This presentation provides an overview of selected recent key developments in the realm of FCIs, synthesizing cuttingedge theoretical advancements and remarkable experimental findings.
After over a decade of theoretical exploration, engineered moiré superlattice materials have recently emerged as an experimental platform for FCI physics. Milestone experimental results include weakfield FQH states in magic angle twisted bilayer graphene on boron nitride, originating from lattice FCIs rather than continuum Landau level states. Even more spectacular recent observations have identified authentic FCIs exhibiting a (zerofield) fractional quantum anomalous Hall effect in both twisted transition metal dichalcogenides and multilayer graphene.
The primary focus of this presentation is to elucidate the distinctions between moiré Chern bands and Landau levels, arising due to symmetrybreaking competing states as a consequence of the underlying quantum geometry in the moiré lattice setting. This platform also opens intriguing avenues for numerous new phenomena beyond Landau level physics, including FCIs in bands with higher Chern numbers, hightemperature FCIs, fractional anomalous Hall crystals combining charge density wave (CDW) order with anyon excitations, nonAbelian genons accompanying topological lattice defects, spin singlet and multicomponent FCIs, as well as a plethora of competing ordered single and multiband states. This offers an exciting prospect where fundamental theory evolves through direct interaction with experiments, with first principles calculations serving as a bridge linking these two realms.

12:00  12:30

Trithep Devakul
(Stanford University)
Designer topological bands from an electrostatic superlattice
We show that an electrostatic superlattice, applied to an appropriate 2D material with a Berry curvature hotspot, can be used to engineer topological minibands with highly tunable properties. In particular, we give a general recipe for designing topological flat bands that are favorable for realizing fractional Chern insulators at partial filling.

12:30  13:30

lunch

13:30  14:00

discussion


chair: Jeroen van den Brink

14:00  14:30

Yonglong Xie
(Rice University)
Fractional Chern insulators and symmetry breaking in twisted graphene
In this talk, I will first review our observation of FCI states at low magnetic fields in MATBG aligned with hBN enabled by highresolution local compressibility measurements. Our findings highlight the interplay between symmetry, topology, and interactions leading to a competition between ordered electronic solid phases and fractionalized electronic liquid phases. Next, I will discuss the impact of the pervasive supermoiré pattern in multilayer moiré heterostructures, also present in MATBG aligned with hBN, on the landscape of manybody phases in twisted trilayer graphene.

14:30  15:00

Nicolas Regnault
(École Normale Supérieure de Paris)
Moiré fractional Chern insulators

15:00  15:30

Fengcheng Wu
(Wuhan University)
Interactiondriven topological phase diagram of twisted bilayer \(MoTe_2\)
Twisted bilayer MoTe$_2$ is a promising platform to investigate the interplay between band topology and manybody interaction. In this talk, I will present our theoretical study of its interactiondriven quantum phase diagrams based on a threeorbital model, which can be viewed as a generalization of the KaneMeleHubbard model with one additional orbital and longrange Coulomb repulsion. We predict a cascade of phase transitions tuned by the twist angle $\theta$. At the hole filling factor $\nu=1$ (one hole per moir\'e unit cell), the ground state can be in the multiferroic phase with coexisting spontaneous layer polarization and magnetism, the quantum anomalous Hall phase, and finally the topologically trivial magnetic phases, as $\theta$ increases from $1.5^{\circ}$ to $5^{\circ}$. At $\nu=2$, the ground state can have a secondorder phase transition between an antiferromagnetic phase and the quantum spin Hall phase as $\theta$ passes through a critical value. The dependence of the phase boundaries on model parameters such as the gatetosample distance, the dielectric constant, and the moir\'e potential amplitude is examined. The predicted phase diagrams can guide the search for topological phases in twisted transition metal dichalcogenide homobilayers.

15:30  16:00

Kai Sun
(University of Michigan)
Topological exact flat bands beyond Kvalley
From recent studies on flat bands in moiré structures, many fascinating novel quantum phenomena have been unearthed or proposed. In this talk, we delve into the expansive question of potential routes towards topological exact flat bands and their classifications. Our study reveals that there exist various potential pathways towards exact topological flat bands, not limited to Kvalley 2D materials and not necessarily requiring bilayer or twisting. For instance, we demonstrate that in materials featuring quadratic band touching points, an exact topological flat band can be achieved with only a singlelayer 2D material subjected to periodic strains. These exact flat bands showcase novel traits akin to those found in magicangle twistedbilayer graphene. Apart from this similarity, these flat bands attain a significantly more uniform Berry curvature distribution, positioning them as a promising platform for the pursuit of novel fractional topological states. Moreover, we will also show that this example is not an isolated occurrence and can be extended to various setups. We additionally present a classification for these topological flat bands, based on their topology, spacegroup symmetry, and degree of degeneracy.

16:00  16:30

coffee break

16:30  17:30

fqah24 colloquium (chair: Marin Bukov, MPIPKS)
Xiaodong Xu (University of Washington)
Observation of fractional quantum anomalous Hall effect
The interplay between spontaneous symmetry breaking and topology can result in exotic quantum states of matter. A celebrated example is the quantum anomalous Hall (QAH) effect, which exhibits an integer quantum Hall effect at zero magnetic field due to topologically nontrivial bands and intrinsic magnetism. In the presence of strong electronelectron interactions, fractionalQAH (FQAH) effect at zero magnetic field can emerge, which is a lattice analog of fractional quantum Hall effect without Landau level formation. In this talk, I will present experimental observation of FQAH effect in twisted MoTe2 bilayer, using combined magnetooptical and transport measurements. In addition, we find an anomalous Hall state near the filling factor 1/2, whose behavior resembles that of the composite Fermi liquid phase in the halffilled lowest Landau level of a twodimensional electron gas at high magnetic field. Direct observation of the FQAH and associated effects paves the way for researching charge fractionalization and anyonic statistics at zero magnetic field.

17:30  18:30

poster session (focus on odd poster numbers)

18:30  19:30

welcome dinner

19:30

discussion
