Theoretical unification of hybrid-DFT and DFT plus U methods for the treatment of localized orbitals

Hybrid functionals serve as a powerful practical tool in different fields of computational physics and quantum chemistry. On the other hand, their applicability for the case of correlated d and f orbitals is still questionable and needs more considerations. In this article we formulate the on-site occupation dependent exchange correlation energy and effective potential of hybrid functionals for localized states and connect them to the on-site correction term of the DFT+ U method. The resultant formula indicates that the screening of the onsite electron repulsion is governed by the ratio of the exact exchange in hybrid functionals. Our derivation provides a theoretical justification for adding a DFT+ U-like on-site potential in hybrid-DFT calculations to resolve issues caused by overscreening of localized states. The resulting scheme, hybrid DFT+ V-w, is tested for chromium impurity in wurtzite AlN and vanadium impurity in 4H-SiC, which are paradigm examples of systems with different degrees of localization between host and impurity orbitals.Funding Agencies|Knut and Alice Wallenberg Foundation "Isotopic Control for Ultimate Materials Properties"; Swedish Research Council (VR) Grants [621-2011-4426, 621-2011-4249]; Swedish Foundation for Strategic Research program SRL Grant [10-0026]; Swedish National Infrastructure for Computing Grants [SNIC 001/12-275, SNIC 2013/1-331]; "Lendulet program" of Hungarian Academy of Sciences; U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-06CH11357]; Linnaeus Environment at Linkoping on Nanoscale Functional Materials (LiLi-NFM) - VR</p

Theoretical unification of hybrid-DFT and DFT plus U methods for the treatment of localized orbitals

Hybrid functionals serve as a powerful practical tool in different fields of computational physics and quantum chemistry. On the other hand, their applicability for the case of correlated d and f orbitals is still questionable and needs more considerations. In this article we formulate the on-site occupation dependent exchange correlation energy and effective potential of hybrid functionals for localized states and connect them to the on-site correction term of the DFT+ U method. The resultant formula indicates that the screening of the onsite electron repulsion is governed by the ratio of the exact exchange in hybrid functionals. Our derivation provides a theoretical justification for adding a DFT+ U-like on-site potential in hybrid-DFT calculations to resolve issues caused by overscreening of localized states. The resulting scheme, hybrid DFT+ V-w, is tested for chromium impurity in wurtzite AlN and vanadium impurity in 4H-SiC, which are paradigm examples of systems with different degrees of localization between host and impurity orbitals.Funding Agencies|Knut and Alice Wallenberg Foundation "Isotopic Control for Ultimate Materials Properties"; Swedish Research Council (VR) Grants [621-2011-4426, 621-2011-4249]; Swedish Foundation for Strategic Research program SRL Grant [10-0026]; Swedish National Infrastructure for Computing Grants [SNIC 001/12-275, SNIC 2013/1-331]; "Lendulet program" of Hungarian Academy of Sciences; U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-06CH11357]; Linnaeus Environment at Linkoping on Nanoscale Functional Materials (LiLi-NFM) - VR</p

Spin and photophysics of carbon-antisite vacancy defect in 4H silicon carbide: A potential quantum bit

Silicon carbide with engineered point defects is considered as very promising material for the next generation devices, with applications ranging from electronics and photonics to quantum computing. In this context, we investigate the spin physics of the carbon antisite-vacancy pair that in its positive charge state enables a single photon source. We find by hybrid density functional theory and many-body perturbation theory that the neutral defect possesses a high spin ground state in 4H silicon carbide and provide spin-resonance signatures for its experimental identification. Our results indicate the possibility for the coherent manipulation of the electron spin by optical excitation of this defect at telecom wavelengths, and suggest the defect as a candidate for an alternative solid state quantum bit.Funding Agencies|MTA Lendulet program of Hungarian Academy of Sciences; Knut and Alice Wallenberg Foundation; Swedish Foundation for Strategic Research program SRL [10-0026]; SNIC [001/12-275, 2013/1-331]; US Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-06CH11357]; Ministry of Education and Science of the Russian Federation [14.Y26.31.0005]; Tomsk State University Academic D. I. Mendeleev Fund Program</p

Твори Марка Вовчка у переспівах і перекладах зарубіжних митців

У німецькому виданні “Історія слов’янських літератур” українській літературі відведено окремий розділ, що давав громадськості Німеччини досить ґрунтовне уявлення про характер і розвиток новітньої української літератури, про її найбільші художні здобутки і, зокрема, творчі досягнення Марка Вовчка.The German edition of The History of Slavic Literatures has a special chapter devoted to the Ukrainian literature. It gives a profound idea of the nature and development of the Ukrainian literature in recent times, of its greatest artistic achievements, particularly, of the oeuvres by Marko Vovchok

Electron paramagnetic resonance and theoretical studies of Nb in 4H- and 6H-SiC

High purity silicon carbide (SiC) materials are of interest from high-power high temperature applications across recent photo-voltaic cells to hosting solid state quantum bits, where the tight control of electrically, optically, and magnetically active point defects is pivotal in these areas. 4H- and 6H-SiC substrates are grown at high temperatures and the incorporation of transition metal impurities is common. In unintentionally Nb-doped 4H- and 6H-SiC substrates grown by high-temperature chemical vapor deposition, an electron paramagnetic resonance (EPR) spectrum with C-1h symmetry and a clear hyperfine (hf) structure consisting of ten equal intensity hf lines was observed. The hf structure can be identified as due to the interaction between the electron spin S - 1/2 and the nuclear spin of Nb-93. Additional hf structures due to the interaction with three Si neighbors were also detected. In 4H-SiC, a considerable spin density of similar to 37.4% was found on three Si neighbors, suggesting the defect to be a complex between Nb and a nearby carbon vacancy (V-C). Calculations of the Nb-93 and Si-29 hf constants of the neutral Nb on Si site, Nb-Si(0), and the Nb-vacancy defect, NbSiVC0, support previous reported results that Nb preferentially forms an asymmetric split-vacancy (ASV) defect. In both 4H- and 6H-SiC, only one Nb-related EPR spectrum has been observed, supporting the prediction from calculations that the hexagonal-hexagonal defect configuration of the ASV complex is more stable than others.Funding Agencies|Swedish Energy Agency||Swedish Research Council VR/Linne Environment LiLI-NFM, FP7|270197|NHDP|TAMOP-4.2.1/B-09/1/KMR-2010-0002|Swedish National Infrastructure for Computing||Knut and Alice Wallenberg Foundation||</p

Isolated Spin Qubits in SiC with a High-Fidelity Infrared Spin-to-Photon Interface

The divacancies in Silicon Carbide are a family of paramagnetic defects that show promise for quantum technologies due to their long-lived electron spin coherence and their optical addressability at near-telecom wavelengths. Nonetheless, a high-fidelity spin-photon interface, which is a crucial prerequisite for such technologies, has not yet been demonstrated. It is demonstrated that such an interface exists in isolated divacancies in epitaxial films of 3C-SiC and 4H-SiC. Our data show that divacancies in 4H-SiC have minimal undesirable spin mixing, and that the optical linewidths in our current sample are already almost the same as those of recent remote entanglement demonstrations in other systems. Moreover, we find that 3C-SiC divacancies have a millisecond Hahn-echo spin coherence time