Session Y48: Excited State VII: 2D materials and quantum materials

First-principles calculations of excitons in 2D: photoemission spectra, relaxation dynamics, and magnetic effects

This talk will show our recent theoretical and computational studies of new exciton physics in van der Waals materials including monolayer transition metal dichalcogenides and 2D magnets. In the first part, we show that the photoelectrons from excitons hold unique energy dispersions and spectra weights, which unveil the fundamental physical properties of these excitons. First-principle calculations based on many-body perturbation theories unveil results that agree well with the measured pump-probe photoemission spectra in monolayer WSe2. We further demonstrate a valley- and spin-selective excitonic energy relaxation pathway, which leads to novel ultrafast dynamics and the discovery of selection rules for exciton-phonon couplings. In the second part, we present recent theoretical and first-principles studies of excitons in 2D magnetic semiconductors. We show that 2D magnetic semiconductors exhibit a rich set of excitons of Frenkel or Wannier types, which encode excited-state information dictated by the underlying crystal structure and magnetic order. We then present our recent study of the roles of interlayer couplings on the excitons and optical properties of these materials. We further connect our theoretical discoveries to experimental results and explore their potential applications.   

Substrate effects on the properties of few-layer MoS2 and the MoS2/WS2 heterostructure from first-principles GW

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are important materials in optoelectronic applications, in which these 2D materials are often placed on suitable substrates. As a result, the electronic and optical properties of TMDs can be modulated by the various substrates that they are deposited on. Here, we focus on the effect of two commonly used substrates in experiments – hexagonal boron nitride and gold – on three systems: monolayer MoS2, bilayer MoS2, and the MoS2/WS2 heterostructure. To reduce the computational cost, we employ the first-principles dielectric embedding GW to capture the dielectric effect of the substrate, which leads to a quantitative agreement with existing experiments. We further discuss the strengths and weaknesses of different approaches in describing the substrate effect.   

Bandstructure and excitons in WSe2 as a function of the inter-layer distance.

The band structure of monolayer WSe₂ is studied at various levels of approximation, from density functional theory (DFT) to quasi-particle self-consistent GW (QSGW) with and without vertex contributions (Ladder Diagrams) and with and without spin-orbit coupling (SOC) as a function of the distance between the layers. The dielectric function is also studied within random phase approximation (RPA) and the Bethe-Salpeter equation (BSE). Our results show that the QSGW gap scales linearly with the inter-layer distance. At infinite distance, we extrapolate the gap to be ∼ 3.7 (eV), which decreases to ∼ 3.5 (eV) after including ladder diagrams. We also find two excitons for the in-plane wavevector using the BSE method, where the first has a binding energy of ∼ 1 (eV). Unlike the QSGW gap, the position of the first exciton peak does not noticeably change with the inter-layer distance, suggesting that the long range effects on the screened Coulomb interaction in QSGW and BSE cancel out. Symmetry labeling of the bands near the gap is also carried out showing allowed transitions in an optical process presented along with the transition rates.   

Spectroscopic Signatures of Moiré-Confined Excitons in Bilayer TMDCs from First Principles

Few-layer transition metal dichalcogenides (TMDCs) have emerged as the ideal experimental platform to study many-body interactions in low dimensions due to their weak dielectric screening, which enhances many-electron interactions, and straightforward optical characterization. When two layers of TMDCs are stacked with a finite twist angle, a superlattice-periodic moiré potential emerges that modulates the localized quasiparticle and optical excitations.  Up to now, however, the experimental characterization of moiré-localized excitons has been indirect. Here, we derive the spectroscopic signatures of moiré-confined excitons in twisted 2D materials that can be measured in current time-resolved angle-resolved photoemission spectroscopy (TR-ARPES) setups. We show that, by simultaneously measuring the distribution of the holes and electrons bound to the photoexcited excitons, it is possible to directly extract both the center-of-mass and internal structure of an exciton. Our calculations are in good agreement with recent experiments, showing that moiré excitons are surprisingly localized even for relatively small moiré unit cells with a moiré lattice parameter of ~ 6 nm. Finally, we comment on a new method to extract accurate moiré potentials from first principles.   

Engineering of Strongly Bound Interlayer Excitons in Mg2TiO4 Thin Films

Two-dimensional transition metal oxides (2DTMOs) are a promising addition to the growing array of functional 2D materials, with potential applications related to their long-lived, strongly bound excitons. 2DTMOs are expected to be stable since they do not react readily with water or oxygen. However, unlike many other 2D materials, 2DTMOs do not natively form stackable van der Waals-bonded layers, so they present challenges for structural prediction and characterization. Recent experimental work on the MgO(001) surface has demonstrated the growth of thin films of Mg₂TiO₄, whose low energy electronic states are dominated by Ti and O orbitals. We review the structure of these thin films, computed at the DFT level, and present many-body calculations of their electronic excitations. Our DFT calculations demonstrate that the film has a band offset favorable for interlayer exciton formation. Motivated by that work, we present GW and GW-BSE calculations of quasiparticle energies, exciton binding energies, and optical absorption spectra. These calculations characterize the suite of intra- and interlayer excitons that exist in Mg₂TiO₄ and shed light on the importance of film thickness in controlling their relative binding energies.   

Quasiparticle energy level alignment of a PTCDA-MoS2 bilayer from first-principles GW calculations

Two-dimensional (2D) transition metal dichalcogenide (TMD) semiconductor crystals are highly sensitive to their environments, and molecular adsorbates decorating the 2D material surfaces constitute a route to tuning their electronic and optical properties. Here, we use a first-principles GW approach to compute the quasiparticle (QP) energies and energy level alignment at an interface formed by an atomically-flat molecular PTCDA monolayer and a MoS₂ monolayer. To reduce the computational cost, we use the substrate screening GW approach which reduces the expense associated with computing the interface polarizability by using smaller supercells. We show that the respective band gaps of the molecular PTCDA and MoS₂ substrate are both modified upon interface formation and we explore the QP energy level alignment at the interface. We also discuss how a molecular monolayer may controllably alter the photophysics of a TMD monolayer.   

Anisotropy of the quasiparticle electronic structure and excitons of pentacene:black phosphorus interfaces

Black phosphorus (BP) is a layered material that has emerged as a promising 2D semiconductor with anisotropic properties.  In this work, we study interfaces formed by a monolayer of pentacene adsorbed on monolayer BP by placing the pentacene monolayer along the zigzag and armchair directions of the BP substrate, respectively. We performed first-principles GW plus Bethe–Salpeter equation (GW-BSE) calculations to determine the quasiparticle band structure and excitons of the pentacene:BP interfaces in both orientations. We focus on the anisotropy of the interactions between the monolayer pentacene and BP, and discuss how the anisotropy of BP modulates the interfacial quasiparticle electronic structure and optical properties, as well as how the adsorption of the pentacene monolayer modulates the anisotropy of BP properties. Our results shine light on the understanding of the BP surface passivation via molecular adsorption and provide benchmark results for future experimental and computational studies.   

Electronic and optical properties of bent phosphorene: state-of-the-art approximations and optoelectronic response

Low-dimensional materials have gained attention due to their remarkable physical and chemical properties. Mechanical bending can significantly tune the electronic and optical properties of phosphorene nanoribbons. In this work, we investigate the electronic and optical properties of phosphorene nanoribbons from first-principles. Our work has two outcomes: (1) We can push the limits of meta-GGA density functionals [1] for the band gaps of phosphorene nanoribbons so that they compete in accuracy with the more expensive hybrid HSE06 method. (2) The empty in-gap state in armchair nanoribbons introduced by bending [2] can significantly affect the properties of low-energy excitons at high curvatures. 

[1] B. Neupane, H. Tang, N. K. Nepal, S. Adhikari, and A. Ruzsinszky, Physical Review Materials 5, 063803 (2021).

[2] L. Yu, A. Ruzsinszky, and J. P. Perdew, Nano letters 16, 2444 (2016).

    

Electronic and Optical Properties of monolayer boron nitride on a graphite substrate

In this work, we investigate the electronic and optical properties of monolayer boron nitride (mBN) grown on Highly Ordered Pyrolytic Graphite (HOPG) substrate based on density functional theory and many-body perturbation theory. Our results indicate that the screening of the semi-metallic graphite substrate results in a giant bandgap renormalization of mBN. Also, we find a huge reduction of exciton binding energy in the presence of HOPG substrate and a blue-shifting of the 1s-exciton energy to above 6 eV. Furthermore, we demonstrate that strong electron–phonon interactions also play an important role in the optical process of mBN and give rise to the multiple phonon sidebands as observed from photoluminescence measurements. As a result, we propose that both the strong dielectric screening from the substrate and the strong electron–phonon interaction must be explicitly included to the analysis of luminescence measurements.    

Excitonic exchange splitting and effects of point-defects on the optical properties in transition metal dichalcogenides (TMDs) nanoribbon

We assess the spin-orbit coupling (SOC) effects on the optical properties, especially the excitonic exchange splitting [1] of transition metal dichalcogenide (TMDs) nanoribbons [2] using the ab initio GW-Bethe-Salpeter (BSE) approximation. We also investigate how point-defect chalcogen vacancies [3] alter the optical response of flat and bent TMDs nanoribbons of various widths. We present some new exciton features as results of the transitions between defect states and edge states of the nanoribbons. The SOC effects in TMD nanoribbons are involved in the formation of bound excitons, which may open the door for possibilities of spintronics applications of TMD nanoribbons.

[1] M. Palummo et al., Phys. Rev. B 81, 121303(R), (2010) 

[2] H. Tang, et al. arXiv preprint arXiv:2109.04551 (2021) 

[3] S. Refaely-Abramson et al., Phys. Rev. Lett. 121, 167402 (2018)   

Defect polaritons from first principles

Control over the optical properties of defects in solid-state materials is necessary for their application in quantum technologies. In this study, we demonstrate, from first principles, how to tune these properties via the formation of defect polaritons in an optical cavity. We show that the polaritonic splitting that shifts the absorption energy of the lower polariton is much higher than can be expected from a Jaynes-Cummings interaction. We also find that the absorption intensity of the lower polariton increases by several orders of magnitude, suggesting a possible route toward overcoming phonon-limited single photon emission from defect centers. These findings are a result of an effective continuum of electronic transitions near the lowest-lying electronic transition that dramatically enhances the strength of the light-matter interaction. We expect our findings to spur experimental investigations of strong light-matter coupling between defect centers and cavity photons for applications in quantum technologies.   

Tunable exciton states and optical properties of magnetic nanoribbons

Tunable exciton states and optical properties of magnetic nanoribbons

Hong Tang, Bimal Neupane, Santosh Neupane, and Adrienn Ruzsinszky

Department of Physics, Temple University, Philadelphia, PA 19122

Layered transition metal halides (TMH), such as CrX3 (X=Cl, Br or I), have amazing magnetic properties even with the thickness reduced down to a monolayer [1]. It has been shown that the giant magneto-optical response is dominated by the excitonic effects in the CrI3 monolayer [2]. With nanoribbons cut from monolayer transition metal dichalcogenide, the reduced spin-orbit coupling influences the characteristic of formatted exciton states [3]. We use first-principles GW and Bethe-Salpeter equation calculations to investigate the excitonic states of magnetic nanoribbons. We show how the edge states re-shape the bound excitons and how this leads to dramatic change of the optical properties of these magnetic nanoribbons, which may have important applications in magneto-optical and optoelectronic devices.

Work is supported by the DOE-BES DE-SC0021263.

[1]. Bevin Huang, Genevieve Clark, Efrén Navarro-Moratalla, et al. Nature,546, 270, 2017.

[2]. Meng Wu 1,2, Zhenglu Li 1,2, Ting Cao 1,2 & Steven G. Louie, Nature communications, (2019) 10:2371.

[3]. Hong Tang, Bimal Neupane, Santosh Neupane, Shiqi Ruan, Niraj K. Nepal, Adrienn Ruzsinszky, arXiv:2109.04551