B. Evrard | Searcharxiv

arXiv2024

AC-Stark Spectroscopy of Interactions between Moir\'e Excitons and Polarons

We use nonlinear pump-probe spectroscopy to study optical excitations in a charge-tunable MoSe$_2$\WS$_2$ moir\'e heterostructure. An intense red-detuned laser pulse creates a photonic dressing of the material by introducing a large virtual population of excitons or exciton polarons in a deep moir\'e potential. By measuring the resulting ac Stark effect with a weak resonant laser pulse, we gain access to the nature and mutual interactions of the elementary optical excitations. At charge neutrality, our measurements reveal that different exciton resonances, associated with confinement of their center-of-mass motion in the moir\'e potential, have a significant spatial overlap. The resulting short-range interactions manifest themselves as a density-dependent blueshift for same-valley excitons and bound biexciton states for opposite-valley excitons. The attractive polaron resonance that appears upon injection of electrons into the heterostructure shows a contrasting behavior: Here, we observe an electron-density-independent light shift and a clear pump-power-dependent saturation. These features are equivalent to that of an ensemble of independent two-level emitters and indicate a breakdown of the Fermi-polaron picture for optical excitations of electrons subject to a strong moir\'e potential. Our work establishes an experimental approach to elucidate the elementary optical excitations of semiconductor moir\'e heterostructures, providing a solid ground for the spectroscopy of correlated electronic and excitonic states in such materials.

cond-mat.str-el↗

arXiv2018

Spontaneous formation and relaxation of spin domains in antiferromagnetic spin-1 quasi-condensates

Quantum systems of many interacting particles at low temperatures generally organize themselves into ordered phases of matter, whose nature and symmetries are captured by an order parameter. In the simplest cases, this order parameter is spatially uniform. For example, a system of localized spins with ferromagnetic interactions align themselves to a common direction and build up a macroscopic magnetization on large distances. However, non-uniform situations also exist in nature, for instance in antiferromagnetism where the magnetization alternates in space. The situation becomes even richer when the spin-carrying particles are mobile, for instance in the so-called stripe phases emerging for itinerant electrons in strongly-correlated materials. Understanding such inhomogeneously ordered states is of central importance in many-body physics. In this work, we study experimentally the magnetic ordering of itinerant spin-1 bosons in inhomegeneous spin domains at nano-Kelvin temperatures. We demonstrate that spin domains form spontaneously after a phase separation transition, \textit{i.e.} in the absence of external magnetic force, purely because of the antiferromagnetic interactions between the atoms. Furthermore, we explore how the equilibrium domain configuration emerges from an initial state prepared far-from-equilibrium.

cond-mat.quant-gas↗