We compare the accuracy of conventional semilocal density functional theory (DFT), the DFT+U method, and the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional for structural parameters, redox react ...

Hybrid density functional calculations of redox potentials and formation energies of transition metal compounds

Using hybrid density functional theory based on the Heyd-Scuseria-Ernzerhof (HSE06) functional, we compared polaron migration and phase separation in olivine LiMnPO4 to LiFePO4. The barriers for free hole and electron polaron migration in the Mn olivine system are calculated to be 303 and 196 meV, respectively, significantly higher than the corresponding barriers of 170 and 133 meV, respectively, for the Fe olivine system, in agreement with previous experimental findings. These results suggest that the electronic conductivities of LiMnPO4 and MnPO4 are about 177 and 11 times lower than their respective Fe analogs at room temperature. In the presence of lithium vacancies or ions, the barriers for both hole and electron polaron migration were found to be about 100‐120 meV higher in the Mn olivine. The HSE06 functional, with its more universal treatment of self-interaction error, was found to be essential to the proper localization of a polaron in the Mn olivine but predicted qualitatively incorrect phase separation behavior in the LixFePO4 system.

Comparison of small polaron migration and phase separation in olivine LiMnPO4 and LiFePO4 using hybrid density functional theory

LiFePO4 is a battery cathode material with high safety standards due to its unique electronic structure. We performed systematic experimental and theoretical studies based on soft X-ray emission, absorption, and hard X-ray Raman spectroscopy of LixFePO4 nanoparticles and single crystals. The results clearly show a non-rigid electron-state reconfiguration of both the occupied and unoccupied Fe-3d and O-2p states during the (de)lithiation process. We focus on the energy configurations of the occupied states of LiFePO4 and the unoccupied states of FePO4, which are the critical states where electrons are removed and injected during the charge and discharge process, respectively. In LiFePO4, the soft X-ray emission spectroscopy shows that, due to the Coulomb repulsion effect, the occupied Fe-3d states with the minority spin sit close to the Fermi level. In FePO4, the soft X-ray absorption and hard X-ray Raman spectroscopy show that the unoccupied Fe-3d states again sit close to the Fermi level. These critical 3d electron state configurations are consistent with the calculations based on modified Becke and Johnson potentials GGA+U (MBJGGA+U) framework, which improves the overall lineshape prediction compared with the conventionally used GGA+U method. The combined experimental and theoretical studies show that the non-rigid electron state reshuffling guarantees the stability of oxygen during the redox reaction throughout the charge and discharge process of LiFePO4 electrodes, leading to the intrinsic safe performance of the electrodes.

http://absimage.aps.org/image/MAR16/MWS_MAR16-2015-003297.pdf

Why LiFePO$_{4}$ is a safe battery electrode: Coulomb repulsion induced electron-state reshuffling upon lithiation

Single photon emitters in solid-state crystals have received a lot of attention as building blocks for numerous quantum technology applications. Fluorescent defects in hexagonal boron nitride (hBN) stand out due to their high luminosity and robust operation at room temperature. The fabrication of identical emitters at pre-defined sites is still challenging, which hampers the integration of these defects in optical systems and electro-optical devices. Here, we demonstrate the localized fabrication of hBN emitter arrays by electron beam irradiation using a standard scanning electron microscope with deep sub-micron lateral precision. The emitters are created with a high yield and a reproducible spectrum peaking at 575 nm. Our measurements of optically detected magnetic resonance have not revealed any addressable spin states. Using density functional theory, we attribute the experimentally observed emission lines to carbon-related defects, which are activated by the electron beam. Our scalable approach provides a promising pathway for fabricating room temperature single photon emitters in integrated quantum devices.

https://pubs.aip.org/aip/apm/article-pdf/doi/10.1063/5.0147560/18032524/071108_1_5.0147560.pdf

Localized creation of yellow single photon emitting carbon complexes in hexagonal boron nitride

6H‐SiC(0001)表面上のナノファセットに形成したグラフェンの電子状態について II

Optical quantum technologies promise to revolutionize today’s information processing and sensors. Crucial to many quantum applications are efficient sources of pure single photons. For a quantum emitter to be used in such application, or for different quantum systems to be coupled to each other, the optical emission wavelength of the quantum emitter needs to be tailored. Here, we use density functional theory to calculate and manipulate the transition energy of fluorescent defects in the two-dimensional material hexagonal boron nitride. Our calculations feature the HSE06 functional which allows us to accurately predict the electronic band structures of 267 different defects. Moreover, using strain-tuning we can tailor the optical transition energy of suitable quantum emitters to match precisely that of quantum technology applications. We therefore not only provide a guide to make emitters for a specific application, but also have a promising pathway of tailoring quantum emitters that can couple to other solid-state qubit systems such as color centers in diamond.

/pdf/tailoring-the-emission-wavelength-of-color-centers-in-3bz3fkiy.pdf

Tailoring the Emission Wavelength of Color Centers in Hexagonal Boron Nitride for Quantum Applications

Small hole polarons, charge carriers for electronic conductivity, play crucial roles in the performance of olivine LiMPO4 (M = Fe, Co, Mn, Ni) batteries, yet their formation mechanism and effects have not been completely explored. This work is aimed to apply density functional theory with additional on-site Hubbard corrections (DFT+U) to investigate the formation mechanism and the effects of small hole polarons in olivine LiFe1−xCoxPO4. It is found that small hole polarons always localize first at the Fe sites and then at the Co sites only when the Fe sites are fully occupied. The results show that much less bond-length distortion of CoO6 than that of FeO6 is required to form a small hole polaron; hence, Co doping helps to create small hole polarons and to stabilize the structures, as indicated by lower formation energy. Moreover, Co doping enhances the electronic conductivity of LiFePO4 because of Co donor states and also increases the intercalation voltage due to the difference of 3d electron configurations between the Fe2+/Fe3+ states and the Co2+/Co3+ states.

First principles study of small hole polaron formation in doped olivine LiFe1−xCoxPO4: Effects of Li deficiency

Quantum key distribution (QKD) has matured in recent years from laboratory proof-of-principle demonstrations to commercially available systems. One of the major bottlenecks is the limited communication distance in fiber networks due to the exponential signal damping. To bridge intercontinental distances, low Earth orbit satellites transmitting the quantum signals over the atmosphere can be used. These free-space links, however, can only operate during the night, as the sunlight otherwise saturates the detectors used to measure the quantum states. For applying QKD in a global quantum internet with continuous availability and high data rates, operation during daylight is required. In this work, we model a satellite-to-ground quantum channel for different quantum light sources to identify the optimal wavelength for free-space QKD in ambient conditions. Daylight quantum communication is possible within the Fraunhofer lines or in the near-infrared spectrum, where the intrinsic background from the sun is comparably low. The highest secret key rate is achievable at the Ca II Fraunhofer line. We also propose a true single photon source based on a color center in hexagonal boron nitride coupled to a microresonator that can implement such a scheme. Our results can also be applied in roof-to-roof scenarios and are therefore relevant for near-future quantum networks.

The ideal wavelength for daylight free-space quantum key distribution

Olivine-type LiFePO4 is widely considered as a cathode for lithium-ion batteries owing to its environmental friendliness and low-cost, yet its applicability in the pristine state is limited due to poor electronic and ionic conductivity. To investigate the conductivity enhancement of LiFePO4, first-principles method under the GGA+U framework is implemented to study effects of doping with Ti4+ at Fe2+ sites under the lithium-deficient environment. LiFePO4 crystal and electronic structures as well as conductivity are investigated. Ti doping creates the impurity states at the acceptor level, which are normally degenerate states, but split into multiple states by the crystal field splitting. Doping under the lithium-deficient environment induces small hole polarons localizing at the Fe atoms and creates defect states located in the intermediate band. Both phenomena combine to facilitate charge carrier hopping. The climbing-image nudge elastic band (cNEB) calculation shows that Li hopping can be promoted by doping with high Ti concentration. This co-doping mechanism therefore can enhance both the electronic and ionic conductivities, which can be beneficial benchmark for cathode-material synthesis in the future.

First-Principles Study of Effects of Combined Ti Supervalent Cations and Lithium Ion Vacancies Doping on Crystal and Electronic Structures and Conductivity in LiFePO4

Chanaprom Cholsuk

Papers

Tailoring the Emission Wavelength of Color Centers in Hexagonal Boron Nitride for Quantum Applications

Localized creation of yellow single photon emitting carbon complexes in hexagonal boron nitride

First principles study of small hole polaron formation in doped olivine LiFe1−xCoxPO4: Effects of Li deficiency

The ideal wavelength for daylight free-space quantum key distribution

First-Principles Study of Effects of Combined Ti Supervalent Cations and Lithium Ion Vacancies Doping on Crystal and Electronic Structures and Conductivity in LiFePO4