Session D41: Moire Materials Beyond The Hubbard Model

A Chiral Kondo Lattice in Doped Transition Metal Dichalcogenides Heterobilayers

Two-dimensional transition metal dichalcogenides have emerged as a novel platform to stack and twist bilayers to engineer novel strongly correlated phases. Here we present a theory to describe the recent realization of a heavy fermion state in stacked MoTe2/WSe2 bilayers. It is shown that the hybridization function realizes a p-wave structure, which endows the system with non-trivial properties not easily accessible in solid-state heavy fermion compounds such as a Kondo temperature that is directly tunable via the density. Last, recent experiments in the dilute doping regime and the effects of interfacial disorder will be discussed.   

The Landau level approach to twisted homobilayer transition metal dichalcogenides

Recent experiments on twisted MoTe₂ have reported the first observation of the fractional quantum anomalous Hall effect (FQAHE), demonstrating exotic gapped states with broken time-reversal symmetry and fractionally charged excitations. Low-energy continuum model descriptions of transition metal dichalcogenide (TMD) twisted homobilayers that have $K$-valley valence band maxima, such as MoTe₂ and WSe₂, indicate that their topmost moiré minibands carry non-zero Chern numbers and become almost flat at a magic angle. Additional studies showed that the quantum geometry of the band is nearly ideal at the magic angle, suggesting a connection to the physics of the lowest Landau level that is responsible for the FQAHE stabilization. In this talk, I will introduce an approximation to the continuum model that explicitly maps the system to an effective problem of Landau levels in a periodic potential. Our model explains why magic angles emerge and why the quantum geometry is almost ideal. I will discuss the regime of validity of the approach and how it is a powerful starting point for the study of interactions, promising to shed light on some of the questions raised by the experiments.   

The Possibility of Novel Superconductity with High-Order Excitonic Complexes

Two-dimensional monolayers and bilayers of transition metal dichalcogenides (TMDs) allow for numerous higher-order excitonic species, including not only single excitons (an electron-hole bound pair), but also trions (an exciton bound to an extra charge), biexcitons (two excitons bound together), indirect excitons (an electron and hole in adjacent layers of a bilayer), charged biexitons (a biexciton bound to an extra charge), and "quaternions" (an exciton bound to two extra charges). This last complex is a boson that carries net charge, and therefore a Bose-Einstein condensate of them would also be a superconductor. In this talk I will review this menagerie of excitonic states, when they are predicted to appear, and the experimental evidence we have for them, with a focus on the evidence for doubly charged excitons which could provide a non-BCS mechanism for superconductivity.   

Topological superconductivity in doped magnetic moiré semiconductors

Interests in excitonic mechanism for superconductivity are revived by recent advances in semiconducting heterostructures, where bound inter-layer excitons carrying twice the electron charge have been experimentally observed. In this talk, I will review general conditions for the emergence of superconductivity from these composite charged excitons and highglight why semiconducting multilayers are natural plateform for its existence. Then, using a realistic model for transition metal dichalcogenide heterobilayers, I will show that topological superconductivity can emerge upon doping the insulating magnetic state stabilized at integer filling of the topmost valence moiré band.

We shall observe that the effective attraction between charge carriers is generated by an electric p-wave Feshbach resonance arising from interlayer excitonic physics and has a tunable strength, which may be large compared to the bandwidth. Together with the low moiré carrier densities reachable by gating, this robust attraction enables access to the long-sought p-wave BEC-BCS transition. Finally, topological protection arises from an emergent time reversal symmetry emerging when the magnetic order and long wavelength magnetic fluctuations do not couple different valleys. The resulting topological superconductor features helical Majorana edge modes, leading to half-integer quantized spin-thermal Hall conductivity and to charge currents induced by circularly polarized light or other time-reversal symmetry-breaking fields.

Moire Materials Beyond The Hubbard ModelHeavy quasiparticles and cascades without symmetry breaking in twisted bilayer graphene

Among the variety of correlated states found in Twisted bilayer graphene (TBG), the cascades happen in a much larger energy, twist angle and temperature range than other effects, pointing to a hierarchy of phenomena [1]. The cascades manifest in different measurements, including the strong reorganization of the density of states, up to tenths of meV, and the oscillations of the remote bands energies in Scanning Tunneling Microscope experiments, the sawtooth peaks in the inverse compressibility and anomalies in the transport properties.  Previous proposals to explain the cascades involve in one way or another a symmetry breaking process. Using a combined Dynamical Mean Field Theory + Hartree approach, we have shown [2] that the cascades are a property of the normal state, associated to the formation of local moments and heavy quasiparticles, and not a symmetry breaking process. The phenomena reproduced by our calculations include the cascade flow of spectral weight, the oscillations of the remote band energies, the asymmetric jumps of the inverse compressibility and the signatures in the transport and has consequences in other experiments and in the stability of the symmetry breaking phases. Due to the fragile topology of TBG, we predict a strong momentum differentiation in the incoherent spectral weight.   In the talk I will also address other measurements which may help distinguishing the phenomenology of the cascades discussed here from proposals involving symmetry breaking. 

 

[1] Wong et al, Nature 582, 198 (2020), Zondiner et al, Nature 582, 203 (2020), Polski et al, arXiv:2205.05225 

[2] A. Datta, M.J. Calderón, A. Camjayi, and E. Bascones, Nature Communications 14, 5036 (2023)