arXiv subjects

Jonas Göser

Publications and source records attributed to Jonas Göser.

8 recordsLinked to original sources

arXiv2024

Strongly Enhanced Electronic Bandstructure Renormalization by Light in Nanoscale Strained Regions of Monolayer MoS$_2$/Au(111) Heterostructures

Understanding and controlling the photoexcited quasiparticle (QP) dynamics in monolayer transition metal dichalcogenides lays the foundation for exploring the strongly interacting, non-equilibrium 2D quasiparticle and polaritonic states in these quantum materials and for harnessing the properties emerging from these states for optoelectronic applications. In this study, scanning tunneling microscopy/spectroscopy with light illumination at the tunneling junction is performed to investigate the QP dynamics in monolayer MoS$_2$ on an Au(111) substrate with nanoscale corrugations. The corrugations on the surface of the substrate induce nanoscale local strain in the overlaying monolayer MoS$_2$ single crystal, which result in energetically favorable spatial regions where photoexcited QPs, including excitons, trions, and electron-hole plasmas, accumulate. These strained regions exhibit pronounced electronic bandstructure renormalization as a function of the photoexcitation wavelength and intensity as well as the strain gradient, implying strong interplay among nanoscale structures, strain, and photoexcited QPs. In conjunction with the experimental work, we construct a theoretical framework that integrates non-uniform nanoscale strain into the electronic bandstructure of a monolayer MoS$_2$ lattice using a tight-binding approach combined with first-principle calculations. This methodology enables better understanding of the experimental observation of photoexcited QP localization in the nanoscale strain-modulated electronic bandstructure landscape. Our findings illustrate the feasibility of utilizing nanoscale architectures and optical excitations to manipulate the local electronic bandstructure of monolayer TMDs and to enhance the many-body interactions of excitons, which is promising for the development of nanoscale energy-adjustable optoelectronic and photonic technologies.

cond-mat.mtrl-sci↗

arXiv2024

Correlated magnetism of moiré exciton-polaritons on a triangular electron-spin lattice

We demonstrate evidence of correlated magnetism for exciton-polaritons in a MoSe$_{2}$/WS$_{2}$ moiré heterostructure with near-parallel alignment subject to electron doping. In our experiments, interactions between electrons and moiré excitons are controlled electrostatically by field-effect doping, and the polaritonic regime of strong light-matter coupling is established in an open cryogenic microcavity. Remarkably, at filling fractions around one electron per moiré cell, we observe drastic and nonlinear enhancement of the effective polariton Landé factor as a hallmark of correlated magnetism, which is cavity-controlled via resonance tuning of light and matter polariton constituents. Our work establishes moiré van der Waals heterostructures as an outstanding platform for studies of correlated phenomena in the presence of strong light-matter coupling and many-body phases of lattice-ordered excitons, charges and spins.

cond-mat.mes-hall↗

arXiv2024

Implementation of the bilayer Hubbard model in a moiré heterostructure

Moiré materials provide a unique platform for studies of correlated many-body physics of the Fermi-Hubbard model on triangular spin-charge lattices. Bilayer Hubbard models are of particular significance with regard to the physics of Mott insulating states and their relation to unconventional superconductivity, yet their experimental implementation in moiré systems has so far remained elusive. Here, we demonstrate the realization of a staggered bilayer triangular lattice of electrons in an antiparallel MoSe$_{2}$/WS$_{2}$ heterostructure. The bilayer lattice emerges due to strong electron confinement in the moiré potential minima and the near-resonant alignment of conduction band edges in MoSe$_{2}$ and WS$_{2}$. As a result, charge filling proceeds layer-by-layer, with the first and second electron per moiré cell consecutively occupying first the MoSe$_{2}$ and then the WS$_{2}$ layer. We describe the observed charging sequence by an electrostatic model and provide experimental evidence of spin correlations on the vertically offset and laterally staggered bilayer lattice, yielding absolute exciton Landé factors as high as $600$ at lowest temperatures. The bilayer character of the implemented spin-charge lattice allows for electrostatic tunability of Ruderman-Kittel-Kasuya-Yosida magnetism, and establishes antiparallel MoSe$_{2}$/WS$_{2}$ heterostructures as a viable platform for studies of bilayer Hubbard model physics with exotic magnetic phases on frustrated lattices.

cond-mat.str-el↗

arXiv2024

Field-induced hybridization of moiré excitons in MoSe$_2$/WS$_2$ heterobilayers

We study experimentally and theoretically the hybridization among intralayer and interlayer moiré excitons in a MoSe$_2$/WS$_2$ heterostructure with antiparallel alignment. Using a dual-gate device and cryogenic white light reflectance and narrow-band laser modulation spectroscopy, we subject the moiré excitons in the MoSe$_2$/WS$_2$ heterostack to a perpendicular electric field, monitor the field-induced dispersion and hybridization of intralayer and interlayer moiré exciton states, and induce a cross-over from type I to type II band alignment. Moreover, we employ perpendicular magnetic fields to map out the dependence of the corresponding exciton Landé $g$-factors on the electric field. Finally, we develop an effective theoretical model combining resonant and non-resonant contributions to moiré potentials to explain the observed phenomenology, and highlight the relevance of interlayer coupling for structures with close energetic band alignment as in MoSe$_2$/WS$_2$.

cond-mat.mes-hall↗

arXiv2023

Imaging lattice reconstruction in homobilayers and heterobilayers of transition metal dichalcogenides

Moiré interference effects have profound impact on the optoelectronic properties of vertical van der Waals structures. Here we establish secondary electron imaging in a scanning electron microscope as a powerful technique for visualizing registry-specific domains in vertical bilayers of transition metal dichalcogenides with common moiré phenomena. With optimal parameters for contrast-maximizing imaging of high-symmetry registries, we identify distinct crystal realizations of WSe$_2$ homobilayers and MoSe$_2$-WSe$_2$ heterobilayers synthesized by chemical vapor deposition, and demonstrate ubiquitous lattice reconstruction in stacking-assembled bilayers with near parallel and antiparallel alignment. Our results have immediate implications for the optical properties of registry-specific excitons in layered stacks of transition metal dichalcogenides, and demonstrate the general potential of secondary electron imaging for van der Waals twistronics.

cond-mat.mes-hall↗

arXiv2023

Excitons in mesoscopically reconstructed moiré heterostructures

Moiré effects in twisted or lattice-incommensurate vertical assemblies of two-dimensional crystals give rise to a new class of quantum materials with rich transport and optical phenomena, including correlated electron physics in flat bands of bilayer graphene and moiré excitons in semiconductor heterostructures. These phenomena arise from modulations of interlayer hybridization on the nanoscale of spatially varying atomic registries of moiré supercells. Due to finite elasticity, however, lattices of marginally-twisted homobilayers and heterostructures can transform from moiré to periodically reconstructed patterns with triangular or hexagonal tiling. Here, we expand the notion of nanoscale lattice reconstruction to the mesoscopic scale of extended samples and demonstrate rich consequences in optical studies of excitons in MoSe$_2$-WSe$_2$ heterostructures with parallel and antiparallel alignment. Our results provide a unified perspective on diverse and partly controversial signatures of moiré excitons in semiconductor heterostructures by identifying domains with exciton properties of distinct effective dimensionality and establish mesoscopic reconstruction as a compelling feature of real samples and devices with inherent finite-size effects and disorder. Generalized to stacks of other two-dimensional materials, this notion of mesoscale domain formation with emergent topological defects and percolation networks will instructively expand our understanding of fundamental electronic, optical, and magnetic properties of van der Waals heterostructures.

cond-mat.mes-hall↗

arXiv2022

Energy dispersive X-ray spectroscopy of atomically thin semiconductors

We report the implementation of energy dispersive X-ray spectroscopy for layered semiconductors in the form of atomically thin transition metal dichalcogenides. The technique is based on a scanning electron microscope equipped with a silicon drift detector for energy dispersive X-ray analysis. By optimizing operational parameters in numerical simulations and experiments, we achieve layer-resolving sensitivity for few-layer crystals down to the monolayer limit and demonstrate elemental composition profiling in vertical and lateral heterobilayers of transition metal dichalcogenides. The technique can be straight-forwardly applied to other layered two-dimensional materials and van der Waals heterostructures, thus expanding the experimental toolbox for quantitative characterization of layer number, atomic composition, or alloy gradients for atomically thin materials and devices.

cond-mat.mes-hall↗

arXiv2022

Coulomb-correlated states of moiré excitons and charges in a semiconductor moiré lattice

Semiconductor moiré heterostructures exhibit rich correlation-induced many-body phenomena with signatures of emergent magnetism, Mott insulating states or generalized Wigner crystals observed in optical spectroscopy by probing intralayer excitons. However, as the staggered band alignment in the underlying WSe$_2$/WS$_2$ heterobilayer system separates photoexcited electrons and holes to form lowest-energy interlayer excitons, direct access to interactions between correlated charge states and ground state moiré excitons remained elusive. Here, we use MoSe$_2$/WS$_2$ heterostacks with type-I band alignment to probe Coulomb interactions of elementary charges with the ground and excited states of moiré excitons. In addition to positive and negative moiré trions reminiscent of their canonical counterparts in monolayer MoSe$_2$, we reveal novel many-body states formed between moiré excitons and charges at fractional filling factors of the periodic moiré lattice. For both electrons and holes, these states exhibit doping-dependent Landé factors as a hallmark of correlation-induced magnetism, identifying the MoSe$_2$/WS$_2$ heterobilayer as a unique system for studies of correlated phenomena in ambipolar doping regimes.

cond-mat.mes-hall↗