Session G65: Defects and Dopants in Low Dimensional Materials

Covalent Quantum Defects of Carbon Nanotubes: A New Material for Quantum Information Science

sp³ defects of single wall carbon nanotubes (SWCNTs), often referred to “organic color centers”,¹ are rapidly emerging as a new material system for chemistry, physics, materials science, engineering and quantum technologies. These defects are created via covalent bonding of organic functional groups to the sp² lattice of SWCNTs and display quantum mechanical properties in a way similar to color centers of solid state systems. In this talk, I will provide a brief over view on our recent accomplishments in understanding and controlling quantum optical properties of this new material system. Specifically, I will cover (1) unique molecularly tunable electronic structures of these covalent quantum defects^2,, (2) demonstration of room temperature single photon emission at 1.55 µm telecommunication wavelength with 0.99 single photon purity,^{3, 4} and (3) single defect magneto PL experiment revealing hidden spin sensitive electronic fine structure states.
¹ Nat. Photon 11, 535 2017; ²Nat. Chem. 10, 1089 2018; ³ Nat. Photon. 11, 577 2017; ⁴ Nat. Mater. 17, 663 2018;   

Carrier recombination mechanisms at quantum point defects in wide band gap two-dimensional materials

The identification and design of defects in two-dimensional (2D) materials as promising defect based qubits and single photon emitters requires a deep understanding of the underlying carrier recombination mechanisms. Yet, the dominant mechanism of carrier recombination at defects in 2D materials has not been well understood. In order to address these concerns, we developed first-principles methods to calculate the radiative and nonradiative recombination rates at defects in 2D materials, using h-BN as a prototypical example. We reveal the carrier recombination mechanism at defects in 2D materials being mostly dominated by defect-defect state recombination. In particular, we disentangle the nonradiative recombination mechanism into key physical quantities: the zero-phonon line and Huang-Rhys factor. At the end, we identified that strain can effectively tune the electron-phonon coupling at defect centers and drastically change the nonradiative recombination rates. This work serves as a general platform for understanding carrier recombination at defects in 2D materials, while providing pathways for engineering of quantum efficiency.
[1] Feng Wu, Tyler J. Smart, Junqing Xu, and Yuan Ping, Phys. Rev. B 100, 081407(R) (2019).   

Curvature-driven atomic localization and dipole alignment of quantum emitters in h-BN

Hexagonal boron nitride (h-BN) has recently emerged as a promising materials platform for developing various solid-state quantum technologies. In particular, a number of color centers in h-BN have been found to be stable and bright single-photon-emitters (SPEs) operating at room temperature, which are crucial elements for quantum optical applications. In this talk, we combine first-principles theory and experiment to investigate the intimate relation between curvature in h-BN and atomic localization of SPEs. We use density functional theory to calculate the energetically stable configuration of various defect models of the SPEs in a buckled h-BN plane with different curvatures. We show that the vacancy-derived point defects in h-BN prefer to form in the highest-curvature area of the buckle and we find that the high curvature induces a dimer reconstruction for the atoms surrounding vacancy. Our result provides not only a microscopic understanding on the recent experimental observations of the SPEs being located in h-BN buckles but also strongly suggests that the atomic origin of the SPEs may be vacancy-derived. We also discuss several key features of the SPEs formed on h-BN buckle such as dipole orientation, which would be helpful to design future experiments of the SPEs in h-BN.   

Excitons and Radiative Lifetimes at Point Defects in Hexagonal Boron Nitride from First Principles

Point defects in hexagonal boron nitride (hBN) have been recently investigated as promising single-photon emitters. Previous theoretical work has proposed candidate atomic structures for these localized emitters using density-functional theory (DFT) calculations, which focused on the defect formation energy, symmetry, and single-particle electronic transitions. Here, we use DFT plus the GW-Bethe-Salpeter equation method to compute the ground and excited states of a set of candidate hBN defect structures. Our calculations can predict the band gap, excitons and radiative lifetimes of the various defects, while including anisotropic dielectric screening and spin-orbit interaction effects. We show that the radiative lifetime can differ by orders of magnitude among different candidate structures, and it can be used as an effective physical quantity to rule out candidate defects. We analyze our results for the most promising structure, which exhibits energy and radiative lifetime in very good agreement with experiment. Through a statistical analysis, we comment on the likelihood that our calculations can successfully identify the structure behind the single-photon emitters observed experimentally.   

Ab initio theory of negatively charged boron vacancy qubit in hBN

Highly correlated orbitals coupled with phonons in two-dimension are identified for paramagnetic and optically active boron vacancy in hexagonal boron nitride by first principles methods which are responsible for recently observed optically detected magnetic resonance signal. We report on ab initio analysis of the correlated electronic structure of this center by density matrix renormalization group and Kohn-Sham density functional theory methods. By establishing the nature of the bright and dark states as well as the position of the energy levels, we provide a complete description of the magneto-optical properties and corresponding radiative and non-radiative routes which are responsible for the optical spin polarization and spin dependent luminescence of the defect. Our findings pave the way toward advancing the identification and characterization of room temperature quantum bits in two-dimensional solids.

Reference
[1] V. Ivády, G. Barcza, G. Thiering, S. Li, H. Hamdi, Ö. Legeza, J.-P. Chou, and A. Gali, arXiv:1910.07767 (2019).   

Boron Dangling Bonds as Single Photon Emitters in Hexagonal Boron Nitride

Defects in semiconductors are an attractive candidate to realize quantum information applications such as quantum computing and cryptography, as well as nanoscale sensing. Hexagonal boron nitride (h-BN) is a desirable host for these quantum defects due to its two-dimensional crystal structure, excellent stability, and wide band gap. Single-photon emission has been observed in h-BN from point defects in the lattice, but microscopic identification of the underlying defect has proved elusive. In this work, we employ hybrid density functional theory to demonstrate that the properties of boron dangling bonds are consistent with the experimental reports. Specifically, doubly occupied boron dangling bonds give rise to optical emission at 2.06 eV with a Huang-Rhys factor of 2.3. The emission is linearly polarized, with indirect excitation into the conduction band explaining the lack of dipole alignment seen in experiment. The boron dangling bond possesses a metastable triplet state, which can be used to realize spin-sensing applications.   

Influence of the environment on the coherence properties of spin-defects in low-dimensional solids and nanostructures: a computational study

Several recent studies have shown that in three-dimensional materials (e.g. diamond and SiC), at low temperature and in the presence of a large magnetic field, the central spin decoherence is mainly due to the fluctuating magnetic field induced by nuclear spin flip-flop transitions. Hence the interaction between electronic defects with the nuclear spin bath of the crystal is the dominant one in determining spin-defect decoherence times. However, in the case of two-dimensional (2D) and nanostructured semiconductors, the interaction with the environment, for example a supporting subtract, is expected to significantly affect spin-coherence times. We present a computational study aimed at understanding environmental effects on coherent lifetimes of spin-defects. We evaluated coherence functions using the Cluster Correlation Expansion method, and we computed the Hahn-echo T₂ time – an important metric for qubit performance – for spin defects in 2D transition metal di-chalcogenides [1] interacting with various substrates, and for nanodiamonds with different surface terminations.

[1] Meng Ye, H. Seo, G. Galli. NPJ Comp. Mat. 5, 44 (2019)   

Carbon dimers as the source of 4.1 eV luminescence in hexagonal boron nitride

Hexagonal boron nitride (h-BN) is an exciting material for electronics and optoelectronics, as well as for quantum information applications. h-BN samples typically display bright luminescence with a zero-phonon line (ZPL) at 4.1 eV, and single-photon emission associated with this line has been observed. The source of the luminescence has been intensely debated, though there seems to be broad agreement that carbon is involved. We propose that the carbon dimer, C_B–C_N, gives rise to this ubiquitous narrow luminescence band. Such carbon dimers have actually been observed in transmission electron microscopy. Our first-principles calculations, based on hybrid density functional theory, show that the neutral state of the dimer is stable over a wide range of electron chemical potentials. The calculated ZPL energy, Huang-Rhys factor, and radiative lifetime are all close to the experimental values. The optical transition occurs between two localized π-type defect states, with the lower-lying state localized on C_N, while the higher-lying state is localized on C_B. We find the transition to be dipole allowed with polarization in the h-BN plane.   

Gate-defined quantum dots in monolayer and bilayer WSe2: Part I, Fabrication

Gate-defined quantum dots in monolayer and bilayer WSe₂ are a platform for novel quantum transport and quantum optoelectronic investigations of WSe₂, as well as for potential applications in coherent valleytronics. In this presentation, we discuss design considerations and implementations for the fabrication of ~100 nm diameter gate-defined, p-type quantum dots in monolayer and bilayer WSe₂. The devices were controlled using a combination of bottom confining gates and a top accumulation gate, with hBN used for gate insulation and encapsulation of the WSe₂. Top hBN and WSe₂ were transferred onto Pt for reliable p-type bottom contact.   

Gate-defined quantum dots in monolayer and bilayer WSe2: Part II, Measurement

Gate-defined quantum dots in monolayer and bilayer WSe₂ are a platform for novel quantum transport and quantum optoelectronic investigations of WSe₂, as well as for potential applications in coherent valleytronics. In this presentation, we discuss low-temperature electronic transport measurements of p-type gate-defined quantum dots in monolayer and bilayer WSe₂. These devices are operated with gates above and below the WSe₂ layer so that carriers are accumulated in the WSe₂, then selectively depleted to define the dot. We report observations of Coulomb blockade as well as excited state spectroscopy of confined holes in WSe₂ dots.   

A carbon dimer defect as a spin qubit candidate in hexagonal boron nitride: an ab-initio study

Hexagonal boron nitride has been recently found to host a variety of quantum defects that are potentially useful for advancing solid-state quantum technologies. However, optically addressable spin quantum bits (qubits), such as the diamond NV center, have not yet been reported in h-BN. In this talk, we propose a carbon dimer(C₂) defect as a promising candidate of optically active spin qubit in h-BN. We use first-principles density functional theory to investigate its structural and electronic properties. We show that the C₂ defect features a non-zero spin ground state (S=1), arising from defect-induced deep levels in the band gap of the host material, and localized unpaired electrons. We calculate the defect formation energy of the C₂ defect and other C-related defects in h-BN and we show that the C₂ defect is energetically stable. To assist potential future experiments, we report the zero-phonon line, the zero-field splitting tensor, and the hyperfine tensor of the C₂ defect at various DFT levels of theory, including the HSE hybrid functional. Our study aimed at searching coherent spin qubits in h-BN can be potentially applied to a wide variety of other two-dimensional van der Waals materials systems.   

Ab initio design and control of quantum emitters in low-dimensional materials

The formation of atomic defects is unavoidable in 2D materials with currently available growth techniques¹. Nevertheless, a myriad of functionalities in modern optoelectronic and nanophotonic devices leverage quantum defects including the recent demonstration of single photon emitters in 2D materials². In parallel, advances in scanning probe techniques provide opportunities to directly create, manipulate and characterize defects down to the atomic scale in 2D materials³. We predict optically active quantum defects in 2D transition metal dichalcogenides from first-principles. We will discuss the excited state defect configuration(s) and the corresponding electron-phonon interactions⁴ to quantify the optical efficiency of emitters via the Huang-Rhys factor. Our work presents a pathway for maximizing the optical efficiency of designer quantum emitters in low-dimensional systems and provides a deterministic choice for defect creation at the atomic scale³.
[1] D. Edelberg, et al, Nano Lett. 19, 7, (2019)
[2] C. Chakraborty, et al Nat. Nano. 10, 507 (2015)
[3] O. Dyke et al, Nat. Mat., 4, 497, (2019)
[4] P. Narang et al, Adv. Func. Mat. /10.1002/adfm.201904557 (2019)   

Mie Resonant Dielectric Metastructure based Quantum Optical Circuits Integrated with Single Photon Source: A new paradigm for Quantum Information Processing

Realization of scalable optical quantum information processing systems requires optical circuits built around on-chip single photon sources (SPS) in spatially regular arrays[1] to provide the needed light manipulating functions of enhancement of SPS emission rate, control on emission directionality, guiding, splitting and recombining to enable on-chip controlled photon interference and entanglement. To this end we have introduced use of light manipulating units (LMU) based on metastructures of subwavelength dielectric building blocks in which a collective Mie mode provides all the above noted functions while eliminating mode mismatch between the components of the network, including the SPS[2]. In this talk we present FEM based simulation and design of such SPS-LMUs that result in two types of entanglement over large on-chip distances: (1) path-entanglement via interference of photons from distinct SPSs, and (2) emergence of coherence and entanglement via direct photon-mediated long-range coupling- resulting in super-radiance with ~2-fold decay rate enhancement of the coupled SPSs[3].
[1] J.Zhang et.al, J.Appl.Phys.120,243103(2016)
[2] S. Chattaraj et.al,arXiv1811.06652v1(2018)
[3] S.Chattaraj et.al,IEEEJ.Quant.Electron.2019,Accepted for Publication