| 1. |
- Barcza, Gergely, et al.
(författare)
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DMRG on Top of Plane-Wave Kohn-Sham Orbitals: A Case Study of Defected Boron Nitride
- 2021
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Ingår i: Journal of Chemical Theory and Computation. - : AMER CHEMICAL SOC. - 1549-9618 .- 1549-9626. ; 17:2, s. 1143-1154
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we analyze the numerical aspects of the inherent multireference density matrix renormalization group (DMRG) calculations on top of the periodic Kohn-Sham density functional theory using the complete active space approach. The potential of the framework is illustrated by studying hexagonal boron nitride nanoflakes embedding a charged single boron vacancy point defect by revealing a vertical energy spectrum with a prominent multireference character. We investigate the consistency of the DMRG energy spectrum from the perspective of sample size, basis size, and active space selection protocol. Results obtained from standard quantum chemical atom-centered basis calculations and plane-wave based counterparts show excellent agreement. Furthermore, we also discuss the spectrum of the periodic sheet which is in good agreement with extrapolated data of finite clusters. These results pave the way toward applying the DMRG method in extended correlated solid-state systems, such as point defect qubit in wide band gap semiconductors.
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| 2. |
- Benedek, Zsolt, et al.
(författare)
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Symmetric carbon tetramers forming spin qubits in hexagonal boron nitride
- 2023
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Ingår i: npj Computational Materials. - : NATURE PORTFOLIO. - 2057-3960. ; 9:1
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Tidskriftsartikel (refereegranskat)abstract
- Point defect quantum bits in semiconductors have the potential to revolutionize sensing at atomic scales. Currently, vacancy-related defects are at the forefront of high spatial resolution and low-dimensional sensing. On the other hand, it is expected that impurity-related defect structures may give rise to new features that could further advance quantum sensing in low dimensions. Here, we study the symmetric carbon tetramer clusters in hexagonal boron nitride and propose them as spin qubits for sensing. We utilize periodic-DFT and quantum chemistry approaches to reliably and accurately predict the electronic, optical, and spin properties of the studied defect. We show that the nitrogen-centered symmetric carbon tetramer gives rise to spin state-dependent optical signals with strain-sensitive intersystem crossing rates. Furthermore, the weak hyperfine coupling of the defect to their spin environments results in a reduced electron spin resonance linewidth that can enhance sensitivity.
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| 3. |
- Bulancea Lindvall, Oscar, et al.
(författare)
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Dipolar spin relaxation of divacancy qubits in silicon carbide
- 2021
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Ingår i: npj Computational Materials. - : Nature Portfolio. - 2057-3960. ; 7:1
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Tidskriftsartikel (refereegranskat)abstract
- Divacancy spins implement qubits with outstanding characteristics and capabilities in an industrial semiconductor host. On the other hand, there are still numerous open questions about the physics of these important defects, for instance, spin relaxation has not been thoroughly studied yet. Here, we carry out a theoretical study on environmental spin-induced spin relaxation processes of divacancy qubits in the 4H polytype of silicon carbide (4H-SiC). We reveal all the relevant magnetic field values where the longitudinal spin relaxation time T-1 drops resonantly due to the coupling to either nuclear spins or electron spins. We quantitatively analyze the dependence of the T-1 time on the concentration of point defect spins and the applied magnetic field and provide an analytical expression. We demonstrate that dipolar spin relaxation plays a significant role both in as-grown and ion-implanted samples and it often limits the coherence time of divacancy qubits in 4H-SiC.
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| 4. |
- Bulancea Lindvall, Oscar, et al.
(författare)
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Isotope-Purification-Induced Reduction of Spin-Relaxation and Spin-Coherence Times in Semiconductors
- 2023
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Ingår i: Physical Review Applied. - : AMER PHYSICAL SOC. - 2331-7019. ; 19:6
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Tidskriftsartikel (refereegranskat)abstract
- Paramagnetic defects and nuclear spins are often the major sources of decoherence and spin relaxation in solid-state qubits realized by optically addressable point defect spins in semiconductors. It is commonly accepted that a high degree of depletion of nuclear spins can enhance the coherence time by reducing magnetic noise. Here we show that the isotope purification beyond a certain optimal level can become contraproductive when both electron and nuclear spins are present in the vicinity of the qubits, particularly for half-spin systems. Using state-of-the-art numerical tools and considering the silicon-vacancy qubit in various spin environments, we demonstrate that the coupling of the spin-3/2 qubit to a spin bath of spin-1/2 point defects in the lattice can be significantly enhanced by isotope purification. The enhanced coupling shortens the spin-relaxation time that in turn may limit the coherence time of spin qubits. Our results can be generalized to triplet point defect qubits, such as the nitrogen-vacancy center in diamond and the divacancy in silicon carbide.
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| 5. |
- Bulancea Lindvall, Oscar, et al.
(författare)
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Low-Field Microwave-Free Magnetometry Using the Dipolar Spin Relaxation of Quartet Spin States in Silicon Carbide
- 2023
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Ingår i: Physical Review Applied. - : AMER PHYSICAL SOC. - 2331-7019. ; 19:3
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Tidskriftsartikel (refereegranskat)abstract
- Paramagnetic defects and nuclear spins are the major sources of magnetic-field-dependent spin relaxation in point-defect quantum bits. The detection of related optical signals has led to the development of advanced relaxometry applications with high spatial resolution. The nearly degenerate quartet ground state of the silicon-vacancy qubit in silicon carbide (SiC) is of special interest in this respect, as it gives rise to relaxation-rate extrema at vanishing magnetic field values and emits in the first near-infrared transmission window of biological tissues, providing an opportunity for the development of sensing applications for medicine and biology. However, the relaxation dynamics of the silicon-vacancy center in SiC have not yet been fully explored. In this paper, we present results from a comprehensive theoretical investigation of the dipolar spin relaxation of the quartet spin states in various local spin environments. We discuss the underlying physics and quantify the magnetic field and spin-bath-dependent relaxation time T1. Using these findings, we demonstrate that the silicon-vacancy qubit in SiC can implement microwave-free low-magnetic-field quantum sensors of great potential.
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| 6. |
- Davidsson, Joel, 1989-, et al.
(författare)
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ADAQ : Automatic workflows for magneto-optical properties of point defects in semiconductors
- 2021
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Ingår i: Computer Physics Communications. - : Elsevier. - 0010-4655 .- 1879-2944. ; 269
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Tidskriftsartikel (refereegranskat)abstract
- Automatic Defect Analysis and Qualification (ADAQ) is a collection of automatic workflows developed for high-throughput simulations of magneto-optical properties of point defects in semiconductors. These workflows handle the vast number of defects by automating the processes to relax the unit cell of the host material, construct supercells, create point defect clusters, and execute calculations in both the electronic ground and excited states. The main outputs are the magneto-optical properties which include zero-phonon lines, zero-field splitting, and hyperfine coupling parameters. In addition, the formation energies are calculated. We demonstrate the capability of ADAQ by performing a complete characterization of the silicon vacancy in silicon carbide in the polytype 4H (4H-SiC).
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| 7. |
- Davidsson, Joel, 1989-
(författare)
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Color Centers in Semiconductors for Quantum Applications : A High-Throughput Search of Point Defects in SiC
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Point defects in semiconductors have been and will continue to be relevant for applications. Shallow defects realize transistors, which power the modern age of information, and in the not-too-distant future, deep-level defects could provide the foundation for a revolution in quantum information processing. Deep-level defects (in particular color centers) are also of interest for other applications such as a single photon emitter, especially one that emits at 1550 nm, which is the optimal frequency for long-range communication via fiber optics.First-principle calculations can predict the energies and optical properties of point defects. I performed extensive convergence tests for magneto-optical properties, such as zero phonon lines, hyperfine coupling parameters, and zero-field splitting for the four different configurations of the divacancy in 4H-SiC. Comparing the converged results with experimental measurements, a clear identification of the different configurations was made. With this approach, I also identified all configurations for the silicon vacancy in 4H-SiC as well as the divacancy and silicon vacancy in 6H-SiC. The same method was further used to identify two additional configurations belonging to the divacancy present in a 3C stacking fault inclusion in 4H-SiC. I extended the calculated properties to include the transition dipole moment which provides the polarization, intensity, and lifetime of the zero phonon lines. When calculating the transition dipole moment, I show that it is crucial to include the self-consistent change of the electronic orbitals in the excited state due to the geometry relaxation. I tested the method on the divacancy in 4H-SiC, further strengthening the previous identification and providing accurate photoluminescence intensities and lifetimes.Finding stable point defects with the right properties for a given application is a challenging task. Due to the vast number of possible point defects present in bulk semiconductor materials, I designed and implemented a collection of automatic workflows to systematically investigate any point defects. This collection is called ADAQ (Automatic Defect Analysis and Qualification) and automates every step of the theoretical process, from creating defects to predicting their properties. Using ADAQ, I screened about 8000 intrinsic point defect clusters in 4H-SiC. This thesis presents an overview of the formation energy and the most relevant optical properties for these single and double point defects. These results show great promise for finding new color centers suitable for various quantum applications.
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| 8. |
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| 9. |
- Davidsson, Joel, 1989-, et al.
(författare)
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Exhaustive characterization of modified Si vacancies in 4H-SiC
- 2022
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Ingår i: Nanophotonics. - : Walter de Gruyter. - 2192-8606 .- 2192-8614. ; 11:20, s. 4565-4580
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Tidskriftsartikel (refereegranskat)abstract
- The negatively charged silicon vacancy (V-Si(-)) in silicon carbide is a well-studied point defect for quantum applications. At the same time, a closer inspection of ensemble photoluminescence and electron paramagnetic resonance measurements reveals an abundance of related but so far unidentified signals. In this study, we search for defects in 4H-SiC that explain the above magneto-optical signals in a defect database generated by automatic defect analysis and qualification (ADAQ) workflows. This search reveals only one class of atomic structures that exhibit silicon-vacancy-like properties in the data: a carbon anti-site (C-Si) within sub-nanometer distances from the silicon vacancy only slightly alters the latter without affecting the charge or spin state. Such a perturbation is energetically bound. We consider the formation of V-Si(-) + C-Si; up to 2 nm distance and report their zero phonon lines and zero field splitting values. In addition, we perform high-resolution photoluminescence experiments in the silicon vacancy region and find an abundance of lines. Comparing our computational and experimental results, several configurations show great agreement. Our work demonstrates the effectiveness of a database with high-throughput results in the search for defects in quantum applications.
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| 10. |
- Dhungel, Omkar, et al.
(författare)
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Near zero-field microwave-free magnetometry with ensembles of nitrogen-vacancy centers in diamond
- 2024
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 109:22
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Tidskriftsartikel (refereegranskat)abstract
- We study cross -relaxation features near zero magnetic field with ensembles of nitrogen -vacancy (NV) centers in diamond and examine their properties in samples with a range of 0.9-16.0 ppm of NV concentrations. The observed NV -NV cross -relaxation features between differently oriented NV centers in high (>= 0.9 ppm)NV-density samples hold promise for a variety of magnetometry applications where microwave fields (or any bias field) disturb the system under study. We theoretically determine the values of the bias magnetic fields corresponding to cross relaxations between different axes and experimentally validate them. The behavior of zero -field cross -relaxation features as a function of temperature is also investigated.
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