Showing 1–11 of 11 results for author: Defo, R K

Ultracoherent self-assembled diamond nanomechanics reveals superfluid dynamics

Authors: Guanhao Huang, Chang Jin, Sophie Weiyi Ding, Chaoshen Zhang, Aaron M. Day, Tobias Elbs, Neil Sinclair, Sukhad Dnyanesh Joshi, Rodrick Kuate Defo, Bertrand I. Halperin, Evelyn Hu, Marko Lončar

Abstract: From gravitational-wave detection, protein force microscopy, to exploration of quantum-classical boundaries, many anticipated discoveries in fundamental science require improving measurement sensitivity limits. Through the fluctuation-dissipation theorem, mechanical dissipation sets the acoustic noise for this limit. Yet, even in high-purity crystals, the microscopic mechanisms responsible for the… ▽ More From gravitational-wave detection, protein force microscopy, to exploration of quantum-classical boundaries, many anticipated discoveries in fundamental science require improving measurement sensitivity limits. Through the fluctuation-dissipation theorem, mechanical dissipation sets the acoustic noise for this limit. Yet, even in high-purity crystals, the microscopic mechanisms responsible for the acoustic loss remain poorly understood. Tension-induced dissipation dilution offers a route to ultralow acoustic loss, but is challenging to implement in crystalline materials including single-crystal diamond. Here we realize a strain-engineered diamond nanomechanical platform using a liquid-assisted van der Waals self-assembly process that harnesses intrinsic surface forces to apply tensile stress exceeding 1 GPa. At cryogenic temperatures these resonators achieve quality factors beyond 10 billion (intrinsic material quality factors beyond 100 million). This exceptional coherence turns them into a sensitive probe for residual dissipation, elucidating three distinct two-level-system channels and one topological dissipation channel from a surface superfluid helium film. Our work shows how advancing mechanical coherence opens access to new regimes of physics in hybrid quantum systems, precision metrology, and condensed-matter physics. △ Less

Submitted 12 February, 2026; v1 submitted 1 July, 2025; originally announced July 2025.

Comments: 35 pages, 16 figures

Theoretical Investigation of Yield-Enhancing Equilibrium Negatively Ionized Tin-Vacancy Center Preparation Pathways in N-Doped Diamond

Authors: Aditya Bahulikar, Steven L. Richardson, Rodrick Kuate Defo

Abstract: The elucidation of the mechanism of Sn$V^-$ formation in diamond is especially important as the Sn$V^-$ color center has the potential to be a superior single-photon emitter when compared to the N$V$ and to other Group IV color centers. The typical formation of the Sn$V$ involves placing Sn in diamond by ion implantation, but the formation of a charged Sn$V$ species requires an additional complica… ▽ More The elucidation of the mechanism of Sn$V^-$ formation in diamond is especially important as the Sn$V^-$ color center has the potential to be a superior single-photon emitter when compared to the N$V$ and to other Group IV color centers. The typical formation of the Sn$V$ involves placing Sn in diamond by ion implantation, but the formation of a charged Sn$V$ species requires an additional complication. This complication is related to the energy cost associated with electronic transitions within the host diamond. Effectively, producing the Sn$V^-$ charge state using an electron obtained from a band edge of the host diamond is less energetically favorable than having the Sn$V^-$ receive an electron from a neighboring donor dopant. Among donor dopants, substitutional N (N$_\text{C}$) is always present in even the purest synthetic or natural diamond sample. The mechanism of electron donation by N$_\text{C}$ has been proposed by Collins for charging the N$V$ in diamond and it has been used to interpret many experimental results. Therefore, in this paper we use DFT to explore the pathways for the formation of the Sn$V^-$ charge state due to electron donation arising from the presence of N$_\text{C}$ in the host diamond. Explicitly, defect concentrations are calculated in equilibrium in each of the explored pathways to determine the yield of the Sn$V^-$ throughout each of the pathways. The importance of our work is to suggest experimental ways of enhancing the yield of charged states like the Sn$V^-$ in diamond for transformative applications in optoelectronics and quantum information. △ Less

Submitted 7 April, 2025; v1 submitted 30 December, 2024; originally announced January 2025.

Structure-Adaptive Topology Optimization Framework for Photonic Band Gaps with TM-Polarized Sources

Authors: Aditya Bahulikar, Feng Wang, Mustafa Cenk Gursoy, Rodrick Kuate Defo

Abstract: We present a structure-adaptive topology optimization framework for engineering photonic band gaps with TM-polarized sources based on computation of the photonic density of states with a uniform source substituting for the standard Dirac delta function sources in formalisms analogous to $Γ$-point integration and to integration over a full Brillouin zone. We generalize the limiting uniform and Dira… ▽ More We present a structure-adaptive topology optimization framework for engineering photonic band gaps with TM-polarized sources based on computation of the photonic density of states with a uniform source substituting for the standard Dirac delta function sources in formalisms analogous to $Γ$-point integration and to integration over a full Brillouin zone. We generalize the limiting uniform and Dirac delta function sources to more general collections of sources, such that the union of the sources in a given collection is hyperuniform. The uniform-source approach necessarily leads to the fastest computations. We also demonstrate how our approach can be generalized to the treatment of the frequency-dependent optical response of materials. Finally, we show that we can recover known two-dimensional photonic crystals for the TM polarization. A key advantage of our work is its ability to optimize for a specific midgap frequency and band gap in a structure-adaptive manner. Our work leverages the insight that the determination of the minimum supercell size and the minimum precision to which the frequencies within the photonic band gap must be sampled will lead to the observation of photonic-crystal structures when the $Γ$-point formalism for the uniform-source approach is employed. Additionally, our $Γ$-point and full Brillouin zone formalisms for the uniform-source approach inherently encourage binarized designs even in gradient descent. △ Less

Submitted 21 April, 2025; v1 submitted 13 November, 2024; originally announced November 2024.

Computing Using Shallow NV-Center Charges in Diamond

Authors: Rodrick Kuate Defo, Steven L. Richardson

Abstract: The static electric dipole-dipole coupling between donor-acceptor pairs (DAPs) in wide-bandgap semiconductors has recently emerged as a means of realizing a quantum science platform through optically controllable, long-range interactions between defects in the solid state. In this work, we generalize DAPs to consider arbitrary dopant populations and demonstrate that the charge of the NV center in… ▽ More The static electric dipole-dipole coupling between donor-acceptor pairs (DAPs) in wide-bandgap semiconductors has recently emerged as a means of realizing a quantum science platform through optically controllable, long-range interactions between defects in the solid state. In this work, we generalize DAPs to consider arbitrary dopant populations and demonstrate that the charge of the NV center in diamond is well suited for quantum science. Explicitly, we leverage experimental results [see Z. Yuan et al., PRR 2, 033263 (2020)] to show that shallow NV centers can be efficiently initialized to a given relative population of the negative and neutral charge states and that modulating the surface termination would allow for control of the timescale over which the initialization and subsequent computations would occur. Furthermore, we argue that the observation of electroluminescence from the neutral charge state of the NV center [see N. Mizuochi et al., Nat. Photon. 6, 299 (2012)], but not from the negative charge state, implies the ability to interface with the NV center's charge in a manner analogous to the spin interface enabled by the spin-state dependent fluorescence of the NV center. △ Less

Submitted 4 May, 2024; v1 submitted 26 February, 2024; originally announced February 2024.

Suppressing electromagnetic local density of states via slow light in lossy quasi-1d gratings

Authors: Benjamin Strekha, Pengning Chao, Rodrick Kuate Defo, Sean Molesky, Alejandro W. Rodriguez

Abstract: We propose a spectral-averaging procedure that enables computation of bandwidth-integrated local density of states (LDOS) from a single scattering calculation, and exploit it to investigate the minimum extinction achievable from dipolar sources over finite bandwidths in structured media. Structure-agnostic extinction bounds are derived, providing analytical insights into scaling laws and fundament… ▽ More We propose a spectral-averaging procedure that enables computation of bandwidth-integrated local density of states (LDOS) from a single scattering calculation, and exploit it to investigate the minimum extinction achievable from dipolar sources over finite bandwidths in structured media. Structure-agnostic extinction bounds are derived, providing analytical insights into scaling laws and fundamental design tradeoffs with implications to bandwidth and material selection. We find that perfect LDOS suppression over a finite bandwidth $Δω$ is impossible. Inspired by limits which predict nontrivial $\sqrt{Δω}$ scaling in systems with material dissipation, we show that pseudogap edge states of quasi-1d bullseye gratings can -- by simultaneously minimizing material absorption and radiation -- yield arbitrarily close to perfect LDOS suppression in the limit of vanishing bandwidth. △ Less

Submitted 4 March, 2024; v1 submitted 27 September, 2023; originally announced September 2023.

Charge-State Stability of Color Centers in Wide-Bandgap Semiconductors

Authors: Rodrick Kuate Defo, Alejandro W. Rodriguez, Steven L. Richardson

Abstract: The NV$^-$ color center in diamond has been extensively investigated for quantum sensing, computation, and communication applications. Nonetheless, charge-state decay from the NV$^-$ to its neutral counterpart the NV$^0$ detrimentally affects the robustness of the NV$^-$ center and remains to be fully overcome. In this work, we provide an $ab~initio$ formalism for accurately estimating the rate of… ▽ More The NV$^-$ color center in diamond has been extensively investigated for quantum sensing, computation, and communication applications. Nonetheless, charge-state decay from the NV$^-$ to its neutral counterpart the NV$^0$ detrimentally affects the robustness of the NV$^-$ center and remains to be fully overcome. In this work, we provide an $ab~initio$ formalism for accurately estimating the rate of charge-state decay of color centers in wide-bandgap semiconductors. Our formalism employs density functional theory calculations in the context of thermal equilibrium. We illustrate the method using the transition of NV$^-$ to NV$^0$ in the presence of substitutional N [see Z. Yuan $et~al$., PRR 2, 033263 (2020)]. △ Less

Submitted 20 December, 2023; v1 submitted 29 July, 2023; originally announced July 2023.

Theoretical Investigation of Charge Transfer Between Two Defects in a Wide-Bandgap Semiconductor

Authors: Rodrick Kuate Defo, Alejandro W. Rodriguez, Efthimios Kaxiras, Steven L. Richardson

Abstract: Charge traps in the semiconductor bulk (bulk charge traps) make it difficult to predict the electric field within wide-bandgap semiconductors. The issue is the daunting number of bulk charge-trap candidates which means the treatment of bulk charge traps is generally qualitative or uses generalized models that do not consider the trap's particular electronic structure. The electric field within a w… ▽ More Charge traps in the semiconductor bulk (bulk charge traps) make it difficult to predict the electric field within wide-bandgap semiconductors. The issue is the daunting number of bulk charge-trap candidates which means the treatment of bulk charge traps is generally qualitative or uses generalized models that do not consider the trap's particular electronic structure. The electric field within a wide-bandgap semiconductor is nonetheless a crucial quantity in determining the operation of semiconductor devices and the performance of solid-state single-photon emitters embedded within the semiconductor devices. In this work we accurately compute the average electric field measured at the location of N$V^-$ charged defects for the substitutional N (N$_\text{C}$) concentration of $n_{\text{N}_\text{C}} \approx 1.41\times10^{18}$ cm$^{-3}$ for the commonly used oxygen-terminated diamond (see [D. A. Broadway $et$ $al$., Nature Electronics 1, 502 (2018)]). We achieve this result by evaluating the leading-order contribution to the electric field far away from the surface, which comes from the N$_\text{C}$ defects that induce the ionization of the N$V^-$. Our results use density-functional theory (DFT) and the principle of band bending. Our work has the potential to aid both in the prediction of the functioning of semiconductor devices and in the prediction and correction of the spectral diffusion that often plagues the optical frequencies of solid-state single-photon emitters upon repeated photoexcitation measurements. Our results for the timescales involved in thermally driven charge transfer also have the potential to aid in investigations of charge dynamics. △ Less

Submitted 15 March, 2023; v1 submitted 14 December, 2022; originally announced December 2022.

Calculating the Hyperfine Tensors for Group-IV Impurity-Vacancy Centers in Diamond: A Hybrid Density-Functional Theory Approach

Authors: Rodrick Kuate Defo, Efthimios Kaxiras, Steven L. Richardson

Abstract: The hyperfine interaction is an important probe for understanding the structure and symmetry of defects in a semiconductor. Density-functional theory has shown that it can provide useful first-principles predictions for both the hyperfine tensor and the hyperfine constants that arise from it. Recently there has been great interest in using group-IV impurity-vacancy color centers X$V^-$ (where X =… ▽ More The hyperfine interaction is an important probe for understanding the structure and symmetry of defects in a semiconductor. Density-functional theory has shown that it can provide useful first-principles predictions for both the hyperfine tensor and the hyperfine constants that arise from it. Recently there has been great interest in using group-IV impurity-vacancy color centers X$V^-$ (where X = Si, Ge, Sn, or Pb and $V$ is a carbon vacancy) for important applications in quantum computing and quantum information science. In this paper, we have calculated the hyperfine tensors for these X$V^-$ color centers using the HSE06 screened Hartree-Fock hybrid exchange-correlation functional with the inclusion of core electron spin polarization. We have compared our results to calculations which only use the PBE exchange-correlation functional without the inclusion of core electron spin polarization and we have found our results are in very good agreement with available experimental results. Finally, we have theoretically shown that these X$V^-$ color centers exhibit a Jahn-Teller distortion which explains the observed anisotropic distribution of the hyperfine constants among the neighboring $^{13}$C nuclear spins. △ Less

Submitted 30 May, 2021; originally announced May 2021.

Journal ref: Phys. Rev. B 104, 075158 (2021)

Methods to Accelerate High-Throughput Screening of Atomic Qubit Candidates in van der Waals Materials

Authors: R. Kuate Defo, H. Nguyen, M. J. H. Ku, T. D. Rhone

Abstract: The discovery of atom-like spin emitters associated with defects in two-dimensional (2D) wide-bandgap (WBG) semiconductors presents new opportunities for highly tunable and versatile qubits. So far, the study of such spin emitters has focused on defects in hexagonal boron nitride (hBN). However, hBN necessarily contains a high density of nuclear spins, which are expected to create a strong incoher… ▽ More The discovery of atom-like spin emitters associated with defects in two-dimensional (2D) wide-bandgap (WBG) semiconductors presents new opportunities for highly tunable and versatile qubits. So far, the study of such spin emitters has focused on defects in hexagonal boron nitride (hBN). However, hBN necessarily contains a high density of nuclear spins, which are expected to create a strong incoherent spin-bath that leads to poor coherence properties of spins hosted in the material. Therefore, identification of new qubit candidates in other 2DWBG materials is necessary. Given time demands of $ab~initio$ methods, new approaches for rapid screening and calculation of identifying properties of suitable atom-like qubits are required. In this work, we present two new methods for rapid estimation of the zero-phonon line (ZPL), a key property of atomic qubits in WBG materials. First, this ZPL is calculated by exploiting Janak's theorem. For finite changes in occupation, we provide the leading-order estimate of the correction to the ZPL obtained using Janak's theorem, which is more rapid than the standard method ($Δ$SCF). Next, we also demonstrate an approach to converging excited states that is faster for systems with small strain than the standard approach used in the $Δ$SCF method. We illustrate these methods using the case of the singly negatively charged calcium vacancy in SiS$_2$, which we are the first to propose as a qubit candidate. This work has the potential to assist in accelerating the high-throughput search for quantum defects in materials, with applications in quantum sensing and quantum computing. △ Less

Submitted 11 June, 2021; v1 submitted 2 December, 2020; originally announced December 2020.

How carbon vacancies can affect the properties of group IV color centers in diamond: A study of thermodynamics and kinetics

Authors: Rodrick Kuate Defo, Efthimios Kaxiras, Steven L. Richardson

Abstract: Recently there has been much interest in using Group IV elements from the Periodic Table to fabricate and study X$V$ color centers in diamond where X = Si, Ge, Sn, or Pb and $V$ is a carbon vacancy. These Group IV color centers have a number of interesting spin and optical properties which could potentially make them better candidates than N$V^-$ centers for important applications in quantum compu… ▽ More Recently there has been much interest in using Group IV elements from the Periodic Table to fabricate and study X$V$ color centers in diamond where X = Si, Ge, Sn, or Pb and $V$ is a carbon vacancy. These Group IV color centers have a number of interesting spin and optical properties which could potentially make them better candidates than N$V^-$ centers for important applications in quantum computing and quantum information processing. Unfortunately, the very same ion implantation process that is required to create these X$V$ color centers in diamond necessarily also produces many carbon vacancies ($V_{\rm C}$) which can form complexes with these color centers ($V_{\rm C}-$X$V$) that can dramatically affect the properties of the isolated X$V$ color centers. The main focus of this work is to use density-functional theory (DFT) to study the thermodynamics and kinetics of the formation of these $V_{\rm C}-$X$V$ complexes and to suggest experimental ways to impede this process such as varying the Fermi level of the host diamond material through chemical doping or applying an external electrical bias. We also include a discussion of how the simple presence of many $V_{\rm C}$ can negatively impact the spin coherence times ($T_2$) of Group IV color centers through the presence of acoustic phonons. △ Less

Submitted 24 March, 2020; v1 submitted 26 May, 2019; originally announced May 2019.

Comments: 19 pages, 7 figures

Journal ref: Journal of Applied Physics 126, 195103 (2019)

\textit{Ab-initio} Tight-Binding Hamiltonian for Transition Metal Dichalcogenides

Authors: Shiang Fang, Rodrick Kuate Defo, Sharmila N. Shirodkar, Simon Lieu, Georgios A. Tritsaris, Efthimios Kaxiras

Abstract: We present an accurate \textit{ab-initio} tight-binding hamiltonian for the transition-metal dichalcogenides, MoS$_2$, MoSe$_2$, WS$_2$, WSe$_2$, with a minimal basis (the \textit{d} orbitals for the metal atoms and \textit{p} orbitals for the chalcogen atoms) based on a transformation of the Kohn-Sham density function theory (DFT) hamiltonian to a basis of maximally localized Wannier functions (M… ▽ More We present an accurate \textit{ab-initio} tight-binding hamiltonian for the transition-metal dichalcogenides, MoS$_2$, MoSe$_2$, WS$_2$, WSe$_2$, with a minimal basis (the \textit{d} orbitals for the metal atoms and \textit{p} orbitals for the chalcogen atoms) based on a transformation of the Kohn-Sham density function theory (DFT) hamiltonian to a basis of maximally localized Wannier functions (MLWF). The truncated tight-binding hamiltonian (TBH), with only on-site, first and partial second neighbor interactions, including spin-orbit coupling, provides a simple physical picture and the symmetry of the main band-structure features. Interlayer interactions between adjacent layers are modeled by transferable hopping terms between the chalcogen \textit{p} orbitals. The full-range tight-binding hamiltonian (FTBH) can be reduced to hybrid-orbital k $\cdot$ p effective hamiltonians near the band extrema that captures important low-energy excitations. These \textit{ab-initio} hamiltonians can serve as the starting point for applications to interacting many-body physics including optical transitions and Berry curvature of bands, of which we give some examples. △ Less

Submitted 31 October, 2015; v1 submitted 29 June, 2015; originally announced June 2015.

Comments: 16 pages, 10 figures

Journal ref: Phys. Rev. B 92, 205108 (2015)