Qiuyang Li

Two-dimensional (2D) magnetism is known to be unstable in the isotropic Heisenberg model but can be realized through the introduction of magnetic anisotropy. This anisotropy leads to rich, highly symmetry-dependent exchange interactions which can introduce strong quasiparticle correlations. In a 2D magnet, signatures of magnetic and electronic anisotropy as well as strong correlations between charge, spin, and lattice have been reported. However, the exact effects of these correlations on the electronic states remain unclear. Here, using angle-resolved photoemission spectroscopy (ARPES), we studied the electronic structure of this 2D magnet. We probed detailed band structure changes as a function of magnetic ordering and external parameters. The observance of such coupling can have potential impacts on the understanding of correlative behavior in 2D magnets and the development of spintronic devices.

The realization of the potential of hybrid inorganic organic systems requires an understanding of the coupling between the constituents: its nature and its strength. The observation of hybrid optical transitions in the monolayer WS2/terrylene hybrid is reported. The first‐principle calculations, linear optical, and transient absorption spectroscopy are employed to investigate the optical spectrum of the hybrid, which exhibits a new transition that does not appear in the constituents’ spectra. The calculations indicate type II level alignment, with the highest occupied level of terrylene in the gap of WS2. Exploiting state‐resolved transient absorption, the response of the hybrid interface to optical excitation is selectively probed. The dynamics reveal rapid hole transfer from WS2 to the terrylene layer, with a decay time of 88 ps. This hole transfer induces a bleach of the hybrid transition, which indicates that terrylene contributes to …

V00. 00104: Nematicity from strong quantum spin fluctuations in a two-dimensional XY-type van der Waals magnet*AbstractPresenter:Gaihua Ye(Texas Tech University)Authors:Gaihua Ye(Texas Tech University)Zhipeng Ye(Texas Tech University)Cynthia C Nnokwe(Texas Tech University)Rui He(Texas Tech University)Zeliang Sun(University of Michigan)Qiuyang Li(University of Michigan)Hui Deng(University of Michigan)Kai Sun(University of Michigan)Liuyan Zhao(University of Michigan)Nan Huang(University of Tennessee)David Mandrus(University of Tennessee)We study XY-type magnet NiPS 3 to probe spin-induced nematicity in 2D system. We identified three Raman signatures for the static, long-range antiferromagnetic (AFM) order in bulk NiPS 3: suppressed spin fluctuations (revealed by ceased quasi-elastic scattering, QES), broken translational symmetry (revealed by the presence of a folded phonon at …

The properties of colloidal quantum-confined semiconductor nanocrystals (NCs), including zero-dimensional (0D) quantum dots, 1D nanorods, 2D nanoplatelets, and their heterostructures, can be tuned through their size, dimensionality, and material composition. In their photovoltaic and photocatalytic applications, a key step is to generate spatially separated and long-lived electrons and holes by interfacial charge transfer. These charge transfer properties have been extensively studied recently, which is the subject of this Review. The Review starts with a summary of the electronic structure and optical properties of 0D–2D nanocrystals, followed by the advances in wave function engineering, a novel way to control the spatial distribution of electrons and holes, through their size, dimension, and composition. It discusses the dependence of NC charge transfer on various parameters and the development of the Auger …

Excitons, bound electron-hole pairs, influence the optical properties in strongly interacting solid state systems. Excitons and their associated many-body physics are typically most stable and pronounced in monolayer materials. Bulk systems with large exciton binding energies, on the other hand, are rare and the mechanisms driving their stability are still relatively unexplored. Here, we report an exceptionally large exciton binding energy in single crystals of the bulk van der Waals antiferromagnet CrSBr. Utilizing state-of-the-art angle-resolved photoemission spectroscopy and self-consistent ab-initio GW calculations, we present direct spectroscopic evidence that robust electronic and structural anisotropy can significantly amplify the exciton binding energy within bulk crystals. Furthermore, the application of a vertical electric field enables broad tunability of the optical and electronic properties. Our results indicate that CrSBr is a promising material for the study of the role of anisotropy in strongly interacting bulk systems and for the development of exciton-based optoelectronics.

We unravel the influence of quasiparticle screening in the nonequilibrium exciton dynamics of monolayer WS 2. We report pump photon energy-dependent exciton blue-and redshifts from time-resolved-reflectance contrast measurements. Based on a phenomenological model, we isolate the effective impact of excitons and free carriers on the renormalization of the quasifree particle band gap, exciton binding energy, and linewidth broadening. By this, our work not only provides a comprehensive picture of the competing phenomena governing the exciton dynamics in WS 2 upon photoexcitation, but also demonstrates that exciton and carrier contributions to dynamic screening of the Coulomb interaction differ significantly.

Transient absorption spectroscopy is a powerful tool to monitor the out-of-equilibrium optical response of photoexcited semiconductors. When this method is applied to two-dimensional semiconductors deposited on different substrates, the excited state optical properties are inferred from the pump-induced changes in the transmission/reflection of the probe, i.e., ΔT/T or ΔR/R. Transient optical spectra are often interpreted as the manifestation of the intrinsic optical response of the monolayer, including effects such as the reduction of the exciton oscillator strength, electron-phonon coupling or many-body interactions like bandgap renormalization, trion or biexciton formation. Here we scrutinize the assumption that one can determine the non-equilibrium optical response of the TMD without accounting for the substrate used in the experiment. We systematically investigate the effect of the substrate on the broadband …

Vertical heterostructures of transition metal dichalcogenides (TMDs) host interlayer excitons with electrons and holes residing in different layers. With respect to their intralayer counterparts, interlayer excitons feature longer lifetimes and diffusion lengths, paving the way for room temperature excitonic optoelectronic devices. The interlayer exciton formation process and its underlying physical mechanisms are largely unexplored. Here we use ultrafast transient absorption spectroscopy with a broadband white-light probe to simultaneously resolve interlayer charge transfer and interlayer exciton formation dynamics in a MoSe2/WSe2 heterostructure. We observe an interlayer exciton formation timescale nearly an order of magnitude (~1 ps) longer than the interlayer charge transfer time (~100 fs). Microscopic calculations attribute this relative delay to an interplay of a phonon-assisted interlayer exciton cascade and …

The newly emergent two-dimensional (2D) van der Waals (vdW) magnets provide an ideal platform to understand the interplay between quantum fluctuations and symmetry breaking orders. Among them, the antiferromagnet NiPS 3 has drawn tremendous attention due to the possible realization of 2D XY model in its 2D limit, as well as its exceptionally sharp exciton mode. In this talk, I will show our recent optical spectroscopy results on bulk and few-layer NiPS 3. First, I will use Raman signatures of spin fluctuations, magnetism-induced broken translational symmetry and broken rotational symmetry to show the dimensionality crossover of the magnetic state in NiPS 3 from the long-range antiferromagnetic order in bulk to the spin-induced nematic order in few-layers. Second, I will correlate the thickness, temperature, and spatial dependencies of magnetism to the evolution of the intriguing excitons in NiPS 3. Finally, I …

Fluctuations and disorder effects are substantially enhanced in reduced dimensionalities. While they are mostly considered as the foe for long-range orders, fluctuations and disorders can also stimulate the emergence of novel phases of matter, for example, vestigial orders. Taking 2D magnetism as a platform, existing efforts have been focused on maintaining 2D long-range magnetic orders by suppressing fluctuations, whereas the other side, exploiting fluctuations for realizing new 2D magnetic phases, remains as an uncharted territory. Here, using a combination of NV spin relaxometry, optical spectroscopy, and Monte Carlo simulations, we report, in an XY-type honeycomb magnet NiPS3, the phase transition from the zigzag AFM order in 3D bulk to a new Z3 vestigial Potts-nematicity in 2D few layers. Spin fluctuations are shown to significantly enhance over the GHz-THz range as the layer number of NiPS3 reduces, using the NV spin relaxometry and the optical Raman quasi-elastic scattering. As a result, the Raman signatures of the zigzag AFM for bulk NiPS3, a zone-folded phonon at ~30cm-1 from the broken translational symmetry (PBTS) and a degeneracy lift of two phonons at ~180cm-1 for the broken 3-fold rotational symmetry (PBRS), evolve into the disappearance of PBTS and the survival of PBRS in few-layer NiPS3, with a critical thickness of ~10nm. The optical linear dichroism microscopy images all three nematic domain states in a single few-layer NiPS3 flake. The large-scale Monte Carlo simulations for bilayer NiPS3 model confirms the absence of long-range zigzag AFM order but the formation of the Z3 vestigial Potts-nematic …

Anisotropy plays a key role in science and engineering. However, the interplay between the material and engineered photonic anisotropies has hardly been explored due to the vastly different length scales. Here we demonstrate a matter-light hybrid system, exciton-polaritons in a 2D antiferromagnet, CrSBr, coupled with an anisotropic photonic crystal (PC) cavity, where the spin, atomic lattice, and photonic lattices anisotropies are strongly correlated, giving rise to unusual properties of the hybrid system and new possibilities of tuning. We show exceptionally strong coupling between engineered anisotropic optical modes and anisotropic excitons in CrSBr, which is stable against excitation densities a few orders of magnitude higher than polaritons in isotropic materials. Moreover, the polaritons feature a highly anisotropic polarization tunable by tens of degrees by controlling the matter-light coupling via, for instance, spatial alignment between the material and photonic lattices, magnetic field, temperature, cavity detuning and cavity quality-factors. The demonstrated system provides a prototype where atomic- and photonic-scale orders strongly couple, opening opportunities of photonic engineering of quantum materials and novel photonic devices, such as compact, on-chip polarized light source and polariton laser.

The unique optical properties of transition metal dichalcogenide (TMD) monolayers have attracted significant attention for both photonics applications and fundamental studies of low-dimensional systems. TMD monolayers of high optical quality, however, have been limited to micron-sized flakes produced by low-throughput and labour-intensive processes, whereas large-area films are often affected by surface defects and large inhomogeneity. Here we report a rapid and reliable method to synthesize macroscopic-scale TMD monolayers of uniform, high optical quality. Using 1-dodecanol encapsulation combined with gold-tape-assisted exfoliation, we obtain monolayers with lateral size > 1 mm, exhibiting exciton energy, linewidth, and quantum yield uniform over the whole area and close to those of high-quality micron-sized flakes. We tentatively associate the role of the two molecular encapsulating layers as …

Here we present an interferometric wide field hyperspectral microscope based on a common-path birefringent interferometer with translating wedges, to measure photoluminescence emission from two-dimensional semiconductors. We show diffraction-limited hyperspectral photoluminescence microscopy from two-dimensional materials across millimeter areas, proving that our hyperspectral microscope is a compact, stable and fast tool to characterize the optical properties and the morphology of 2D materials across ultralarge areas.

The phonon dynamics and dielectrically screened electron-phonon coupling of atomically thin molybdenum disulfide are directly observed for the first time ever by UEDS diffuse intensity transients in excellent agreement with novel density functional theory calculations.

The optoelectronic properties of quantum confined semiconductor nanocrystals depend critically on the band edge electron and hole levels and their exciton fine structures. Transient absorption (TA) spectroscopy has been widely used to probe the dynamics of photogenerated electrons, holes, and excitons in these materials through their state filling induced bleach of the band edge exciton transition. Such effects, in principle, reflect the band edge fine structures and are well understood for the conduction band electrons. However, the valence band hole state filling signals remain poorly understood due to the complexity of the valence band level structure and the presence of fast hole trapping in many materials. Herein, we report a study of the valence band hole state filling effect by comparing the TA spectra of CdSe quantum dots (QDs) with different degrees of hole trapping and by selective removal of the …

MoSe 2/WSe 2 transition metal dichalcogenide heterostructures host spatially-indirect interlayer excitons (ILX) following interlayer charge transfer (ICT). We perform ultrafast transient absorption spectroscopy directly probing the ILX formation time and find it to be significantly longer than ICT.

In Transition Metal Dichalcogenide Heterostructures (TMD HS) with Type II band alignment, such as MoSe 2/WSe 2, photoexcited carriers can form interlayer excitons (ILX) with electrons/holes residing in different layers. Spatially-indirect ILXs exhibit longer recombination times than intralayer excitons (10-100x longer) making these HS a promising platform for observing possible Bose-Einstein condensation of excitons and for applications in spin/valley optoelectronics. Our understanding of the ILX kinetics is limited to recombination resolved via photoluminescence spectroscopy. We directly study the early formation process of the ILX in a MoSe 2/WSe 2 HS on a sub-ps timescale using transient absorption spectroscopy. Upon selective photoexcitation of the MoSe 2 layer, we resolve interlayer hole scattering to WSe 2 and the signature of the ILX around~ 1.4 eV, which grows in on a 400 fs timescale (ie slower than …

In single-layer (1L) transition metal dichalcogenides, the reduced Coulomb screening results in strongly bound excitons which dominate the linear and the nonlinear optical response. Despite the large number of studies, a clear understanding on how many-body and Coulomb correlation effects affect the excitonic resonances on a femtosecond time scale is still lacking. Here, we use ultrashort laser pulses to measure the transient optical response of 1L-WS2. In order to disentangle many-body effects, we perform exciton line-shape analysis, and we study its temporal dynamics as a function of the excitation photon energy and fluence. We find that resonant photoexcitation produces a blue shift of the A exciton, while for above-resonance photoexcitation the transient response at the optical bandgap is largely determined by a reduction of the exciton oscillator strength. Microscopic calculations based on excitonic …

Quantum confined semiconductor nanocrystals (NCs), in particular, zero-dimensional (0D) quantum dots, one-dimensional (1D) nanorods, and two-dimensional (2D) nanoplatelets, possess properties that differ significantly from their bulk counterparts and have been widely used as light harvesting and charge separation materials in photocatalytic applications, such as light-driven H2 generation. An efficient NC-based photocatalytic system requires both efficient exciton dissociation by charge transfer from NCs to adsorbed acceptors or catalysts and slow charge recombination. This chapter provides a review of recent studies of fundamental exciton and carrier dynamics in cadmium chalcogenide NCs and their effects on the light-driven H2 generation performances. The review starts with an introduction of the electronic structure of 0D-2D NCs, the band alignments of NC heterojunction or interfaces, and the exciton …

In this work, we observe plasmon-induced hot electron extraction in a heterojunction between indium tin oxide nanocrystals and monolayer molybdenum disulfide. We study the sample with ultrafast differential transmission, exciting the sample at 1750 nm where the intense localized plasmon surface resonance of the indium tin oxide nanocrystals is and where the monolayer molybdenum disulfide does not absorb light. With the excitation at 1750 nm, we observe the excitonic features of molybdenum disulfide in the visible range, close to the exciton of molybdenum disulfide. Such a phenomenon can be ascribed to a charge transfer between indium tin oxide nanocrystals and monolayer molybdenum disulfide upon plasmon excitation. These results are a first step toward the implementation of near-infrared plasmonic materials for photoconversion.