Session N21: Valley & Spin Polarization in 2D

Theory of Temperature-dependent Collective Excitations in Excitonic Insulators

Semimetals with small gaps are predicted to be unstable against long-range Coulomb interaction that leads to an excitonic insulator state. However, an interaction-induced excitonic insulator state is indistinguishable from a charge density wave state that forms due to electron-phonon interactions from the symmetry perspective. Both mechanisms lead to energy gaps and spatial inhomogeneity. Motivated by a recent momentum-resolved electron energy-loss spectroscopy study of soft collective modes in the semimetal 1T-TiSe₂, we develop a time-dependent mean-field theory to study temperature-dependent collective excitations. The model includes both intraband and interband excitations, and direct and exchange interactions, and allows for the formation of exciton condensates at low temperatures. When the system does not support an exciton insulator state,  thermally-excited plasmons are observed in intraband excitations while excitons are observed in interband excitations. When the condensates form, however, interband collective modes including Higgs modes and $2P$-like excitons are observed in both interband and intraband excitations even at low temperatures when plasmons are not formed. We then comment on the implications of our theory for the interpretation of the experimental observations.   

Tip enhanced photoluminesence imaging of waveguide-mode-coupled infrared quantum dot emission

Waveguide modes are a ubiquitous manifestation of total internal reflection in high index dielectrics. Scanning near field optical microscopy reveals their long propagation distance (low loss) particularly in the short wave infrared range (SWI, 1000-2000) nm, while shorter wavelengths become lossy due to band edge absorption. Although these sub-diffractional excitations offer enhanced emission, they are inaccessible in conventional diffraction-limited microscopies. Here, using tip-enhanced photoluminesence (nano-PL), we study infrared PbS quantum dots coupled to bulk MoS2 waveguide modes. The magnitude of the emission enhancement peaks around 900-1000 nm excitation wavelengths. This suggests that SWI-range nano-PL offers superior performance to vis-range emitters when coupled to TMD waveguides. This work highlights the promise of the infrared range for coupling emitters to confined light-matter excitations.   

Tunable optical non-linearity in mono-layer semiconductors

Excitons in monolayer TMDC semiconductors provide a unique platform for achieving strongly non-linear light-matter interactions. Starting from the linear response of 2D excitons, several approaches have successfully demonstrated enhanced non-linearities as the phase space available to excitons is reduced. Such approaches include local strain engineering, moiré potentials, and trion saturation. Recent studies have further demonstrated the ability to quantum confine neutral excitons in 2D TMDCs using electrostatic gating [1], allowing for the deterministic positioning of tunable quantum emitters.

Here we leverage this new technique to control the optical non-linearity of 2D TMDCs excitons across different regimes of confinement. This approach is a promising starting point for scalable devices with strong photon-photon interactions. In turn, such interacting states of photons could provide a route to realize quantum many-body phases of light, with potential applications in quantum light technologies and optical computing.

[1] Thureja, D., et al. Electrically tunable quantum confinement of neutral excitons. Nature 606, 298–304 (2022).   

Many-body interactions and exciton complexes in a layered semiconductor

Due to their heavy carrier mass and gate tunability, two-dimensional semiconductors are an exciting platform to study the interplay between many-body effects and exciton physics. In particular, the family of III-VI metal monochalcogenides stems out due to its attractive optical properties and flat valence band dispersion. The influence of such singularity in 2D semiconductors remains largely unexplored, mainly due to difficulties in the fabrication that usually yield non-reproducible results. Here, we employ photoluminescence spectroscopy of charge-tunable excitons in few-layer metal monochalcogenides to probe many-body interactions at the flat band. Furthermore, we provide a reliable marker for the flat band position, to corroborate our interpretation. Our results pave the way for a reproducible and potentially manufacturable device to study the effects of flat-band physics without the need for a twist-angle in the heterostack.

    

Optical absorption spectra and interlayer excitonic transitions in bulk transition metal dichalcogenides from first principles

Transition metal dichalcogenides (TMDs) comprise a class of layered materials highly attractive for optoelectronics due to their scalability and thickness-dependent electrical and optical properties. While significant attention has been given to single-layer TMDs, a rather limited number of works have accurately addressed the optoelectronic properties of the few-layer case, which still retains many of the features of the monolayer case. Here, we will present the electronic and optical properties of bulk 2H group VIB TMDs. We performed many-body perturbation theory (MBPT) simulations to compute the quasiparticle bandstructure within G0W0, and subsequently solve Bethe-Salpeter equation for the optical absorption including electron-hole correlations. Our results correlate extremely well with existing experimental results, particularly for the excitonic peaks. We show that, due to the symmetry of the bands, the first exciton present in all these systems is dark, occurring in the vicinity of K point, whereas the brighter excitons occur in H-K high-symmetry direction, with transitions between the VBM and CBM+1, and strong interlayer component. Using that information, we estimate the theoretical photovoltaic performance of these group VIB TMDs by calculating the short-circuit current, open-circuit voltage, and power conversion efficiency, presenting values higher than semiconductors of similar width.   

In-Plane and Out-of-Plane Excitonic Coupling in 2D Molecular Crystals

Understanding the nature of molecular excitons in low-dimensional molecular solids is of paramount importance in fundamental photophysics and various applications such as energy harvesting, switching electronics and display devices. Despite this, the spatial evolution of molecular excitons and their transition dipoles have not been captured in the precision of molecular length scales. Here we show in-plane and out-of-plane excitonic evolution in quasi-layered two-dimensional (2D) PTCDA crystals assembly-grown on hexagonal BN crystals. Complete lattice constants with orientations of two herringbone-configured basis molecules were determined with polarization-resolved spectroscopy and electron diffraction methods. In the truly 2D limit of single layers, two Frenkel emissions Davydov-split by Kasha-type intralayer coupling exhibited energy inversion with decreasing temperature, which enhances excitonic coherence. As increasing thickness, the transition dipole moments of newly emerging charge transfer excitons are reoriented because of mixing with the Frenkel states. The current spatial anatomy of 2D molecular excitons will inspire deeper understanding and thus ground-breaking applications of low-dimensional molecular systems.   

New optical features of substitutional defects in 2D materials

Defect engineering of 2D materials offers enormous opportunities to tune material properties. Compared to the most common vacancies in 2D materials, substitutional defects require unique and controllable approaches to generate. We have studied two types of substitutional defects in 2D materials, self-limited along the out-of-plane and in-plane directions, respectively. The first type is atomic substitution: a nitrogen atom substituting a chalcogen atom in 2D transition metal dichalcogenides (TMDs), which yields new distinct photoluminescence features well separated from the free excitons of 2D TMDs [1]. The second type is layer substitution: an entire layer of chalcogen atoms in 2D TMD substituted by another type of chalcogen atoms, namely, Janus TMDs. Due to the intrinsic vertical dipole, Janus TMDs form unconventional interaction with adjacent materials including other 2D material layers. These unconventional interactions were probed by optical signature changes such as ultra-low frequency Raman modes and photoluminescence yield change [2,3]. The engineering of such defects in 2D materials presents unique opportunities for optoelectronic devices and quantum information platforms.

    

Opening a new window into opto-twistronics

Structure-property interlinkage has been a long-lasting and intriguing paradigm in quantum materials studies. Recently, the great versatility of creating new quantum phases via manipulating the twist degree of freedom in van der waals materials have been demonstrated. One question is, how would the twist change the philosophy of light-matter interactions? In this talk, I will discuss our low temperature optically coupled microwave impedance microscopy setup and its implications on imaging excitonic excitations in twistronic systems, and how this novel technique can be employed to study the previously inaccessible quantum beauty lurking in electron excitation, transfer, and transport at the nanometer scale.  

    

Two-dimensional single-valley exciton qubit and optical spin magnetization generation

Creating and manipulating coherent qubit states are actively pursued in two-dimensional (2D) materials research. Significant efforts have been made towards the realization of two-valley exciton qubits in monolayer transition-metal dichalcogenides (TMDs), based on states from their two distinct valleys in k-space. Here, we propose a new scheme to create qubits in 2D materials utilizing a novel kind of degenerate exciton states in a single valley. Combining group theoretic analysis and ab initio GW plus Bethe-Salpeter equation (GW-BSE) calculations, we demonstrate such novel qubit states in substrate-supported monolayer bismuthene – which has been successfully grown using molecular beam epitaxy. In each of the two distinct valleys in the Brillouin zone, strong spin-orbit coupling along with the symmetry leads to a pair of degenerate 1s exciton states with opposite spin configurations. Specific coherent linear combinations of the two degenerate excitons in a single valley can be excited with light polarizations, enabling full manipulation of the exciton qubits and their spin configurations. In particular, a net spin magnetization can be generated. Our finding opens new routes to create and manipulate qubit systems in 2D materials.   

An ab initio study of electron-hole pairs in a correlated van der Waals antiferromagnet: NiPS3

The recently discovered two-dimensional (2D) van der Waals magnet NiPS₃ provides a new route to examine many-body quasiparticles under 2D confinement. Excitons are of particular interest due to their strong coupling to the magnetic ground state in this material. Here, by using a first principles-based approach, we find bright excitons between 1.4 eV to 1.7 eV, similar to the sharp coherent and band edge excitons experimentally observed. Our analysis shows that each exciton in NiPS₃ is composed of a combination of d-d and charge-transfer character, where the relative ratio of each pairing configuration changes with exciton energy. Finally, the wave function of the electrons and holes is revealed to be spatially separated, with electrons and holes residing on different magnetic sublattices. This suggests a microscopic origin of the observed strong magneto-exciton coupling.

    

Control moiré magnetic moment interactions by light polarizations

The ability of controlling many-body interactions between carriers is essential to explore correlated phenomena and access complex electronic phase diagram. Transition metal dichalcogenide (TMD) moiré superlattice emerges to be a powerful platform for studying correlated physics due to its highly tunable many-body Hamiltonian. Recently, a non-equilibrium, light induced ferromagnetic states was observed in WSe₂/WS₂ heterobilayers, pointing to dynamic optical control of electron correlations. Here we report light polarization controlled magnetic interactions between localized moments in WSe₂/WS₂ moiré superlattices. Using a continuous-wave pump-probe reflective magnetic circular dichroism (RMCD) measurements, we show that ferromagnetic states can be induced by linearly polarized pump, i.e., no optical orientation of carrier spins are needed. However, under circularly polarized pump, we find that the RMCD response is drastically enhanced at low excitation regime, and near completely suppressed when the photo-excited exciton-spin density is comparable to moiré cell density. Our results reveal a collaboration-competition relation between excitons mediated spin-spin interaction and valley-pseudo magnetic field effect in the formation of ferromagnetic states.   

Using polarized twisted light to tailor the superposition of finite-momentum valley exciton states in transition-metal dichalcogenide monolayers

A twisted light is a spatially structured light that carries a new degree of freedom of quantized orbital angular momenta (OAM), which, in addition to that of intrinsic spin angular momentum (SAM), i.e., polarization, is appealing for new quantum information technology. In this talk, we will present a theoretical study of the photo-excitation of valley excitons in transition-metal dichalcogenide monolayers (TMD-ML's) by using polarized Laguerre-Gaussian beams, one of the best-known twisted light's (TL's). Our studies show that the photoexcitation of polarized TL incident to a TMD-ML leads to the formation of the superposition of finite-momentum exciton (SFME) states, forming an exciton wave packet whose geometric pattern over the momentum and real spaces are encoded by the optical OAM. Furthermore, the momentum-dependent optical matrix element (MD-OME) of the SFME states for the exchange-split longitudinal and transverse exciton bands of TMD-ML's under TL excitation are shown to be highly directional and dependent on the polarization of the applied TL. The MD-OME's of the SFME states under the linearly polarized TL excitation mimic an exciton multiplexer allowing for selectively detecting the optical signatures of the meV-split longitudinal and transverse exciton bands of TMD-ML's.   

Spin-valley physics in strained transition metal dichalcogenides monolayers

Transition metal dichalcogenides (TMDCs) are ideal candidates to explore the manifestation of spin-valley physics under external stimuli. Here, we investigate the influence of strain on the spin, orbital angular momenta and g-factors of monolayer TMDCs within first principles[1]. Our calculations reveal the behavior of direct exciton g-factors under the isolated impact of strain: tensile (compressive) strain increases (decreases) the absolute value of g-factors. Strain variations of ~1% modify the bright (A and B) exciton g-factors by ~0.3 (0.2) for W (Mo) based compounds and the dark exciton g-factors by ~0.5 (0.3) for W (Mo) compounds, suggesting that strain can be responsible for g-factor fluctuations observed experimentally. We complete our analysis for the Γ and Q valleys, revealing that spin degree of freedom dominates. This fundamental microscopic insight into the role of strain in the spin-valley physics of TMDCs is crucial to understand recent experiments[2,3]. [1] Faria Junior et al., NJP 24, 083004 (2022). [2] Covre, Faria Junior et al., Nanoscale 14, 5758 (2022). [3] Blundo, Faria Junior et al., PRL 129, 067402 (2022).   

Valley mediated singlet- and triplet-polaron dynamics in a doped WSe2 monolayer

The valley degree of freedom, as an additional quantum index in the electronic bands of solids, has found an increasingly important role in understanding the electronic and optical properties of atomically thin materials. Here, we study the formation of new optical resonances in a WSe2 monolayer as the doping level gradually increases. Singlet and triplet polarons are new quasiparticles that arise when optically excited electron-hole pairs couple to the Fermi sea in the same or opposite valleys. Using two-dimensional electronic coherent spectroscopy, we reveal their distinct quantum dephasing, valley depolarization, and tunable valley coherence.