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Ivády, Viktor
Publications (10 of 14) Show all publications
Fashandi, H., Ivády, V., Eklund, P., Lloyd Spetz, A., Katsnelson, M. I. & Abrikosov, I. A. (2015). Dirac points with giant spin-orbit splitting in the electronic structure of two-dimensional transition-metal carbides. Physical Review B. Condensed Matter and Materials Physics, 92(15)
Open this publication in new window or tab >>Dirac points with giant spin-orbit splitting in the electronic structure of two-dimensional transition-metal carbides
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2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 15Article in journal (Refereed) Published
Abstract [en]

We investigated the structural and electrical properties of 2D MXene sheets by means of firstprinciples density functional theory (DFT) calculations. To describe the Kohn-Sham states, plane wave basis set and projector augmented wave method (PAW) were used as implemented in the Vienna ab initio Simulation Package (VASP). We applied PBE parameterization of the generalized gradient approximation of the exchange and correlation energy functional to account for many-body effects of the interacting electron system. Convergent sampling of the Brillouin-zone was achieved by a Γ-centered 15×15×1 grid. In order to model a single sheet of MXene we ensured at least 30 Å vacuum between the periodically repeated sheets. For the structural optimization 1×10−3 eV/Å force criteria was used. The relativistic spin-orbit coupling effects were also included in our simulations regarding band structure and density of states.

Keyword
Cone-point, MAX phase, MXene, Dirac fermion, Spin-orbit coupling
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-113761 (URN)10.1103/PhysRevB.92.155142 (DOI)000363512700002 ()
Available from: 2015-05-01 Created: 2015-01-30 Last updated: 2017-12-05Bibliographically approved
Trinh, X. T., Ivády, V., Kawahara, K., Suda, J., Kimoto, T., Gali, Á., . . . Son, N. T. (2015). Electron paramagnetic resonance studies of carbon interstitial related defects in 4H-SiC.
Open this publication in new window or tab >>Electron paramagnetic resonance studies of carbon interstitial related defects in 4H-SiC
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2015 (English)Manuscript (preprint) (Other academic)
Abstract [en]

In n-type 4H-SiC grown by chemical vapor deposition and irradiated by low-energy (250 keV) electrons, an electron paramagnetic resonance center, labeled EI8a, was observed at room temperature. A short anneal at temperatures in the range of 300-500 °C in darkness changes EI8a to a new center, labeled EI8b, which can be converted back by illumination at room temperature. We show that EI8a and EI8b are the two different configurations of the same defect, labeled EI8, with C1h symmetry and an electron spin S=1/2. The EI8 center is stable up to ~650 °C and annealed out at ~800 °C. Based on the observed hyperfine structures due to the hyperfine interaction between the electron spin and the nuclear spins of four 29Si atoms and three 13C atoms, the EI8 center is suggested to be related to a carbon interstitial cluster.

National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-117880 (URN)
Available from: 2015-05-12 Created: 2015-05-12 Last updated: 2015-05-12Bibliographically approved
Falk, A. L., Klimov, P. V., Ivády, V., Szasz, K., Christle, D. J., Koehl, W. F., . . . Awschalom, D. D. (2015). Optical Polarization of Nuclear Spins in Silicon Carbide. Physical Review Letters, 114(24), 247603
Open this publication in new window or tab >>Optical Polarization of Nuclear Spins in Silicon Carbide
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2015 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 114, no 24, p. 247603-Article in journal (Refereed) Published
Abstract [en]

We demonstrate optically pumped dynamic nuclear polarization of Si-29 nuclear spins that are strongly coupled to paramagnetic color centers in 4H- and 6H-SiC. The 99% +/- 1% degree of polarization that we observe at room temperature corresponds to an effective nuclear temperature of 5 mu K. By combining ab initio theory with the experimental identification of the color centers optically excited states, we quantitatively model how the polarization derives from hyperfine-mediated level anticrossings. These results lay a foundation for SiC-based quantum memories, nuclear gyroscopes, and hyperpolarized probes for magnetic resonance imaging.

Place, publisher, year, edition, pages
American Physical Society, 2015
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-120166 (URN)10.1103/PhysRevLett.114.247603 (DOI)000356407600010 ()
Note

Funding Agencies|Air Force Office of Scientific Research (AFOSR); AFOSR Multidisciplinary Research Program of the University Research Initiative; National Science Foundation; Material Research Science and Engineering Center; Knut and Alice Wallenberg Foundation "Isotopic Control for Ultimate Materials Properties"; Lendulet program of the Hungarian Academy of Sciences; National Supercomputer Center in Sweden

Available from: 2015-07-13 Created: 2015-07-13 Last updated: 2017-12-04
Gällström, A., Magnusson, B., Leone, S., Kordina, O., Son, N. T., Ivády, V., . . . Ivanov, I. G. (2015). Optical properties and Zeeman spectroscopy of niobium in silicon carbide. Physical Review B. Condensed Matter and Materials Physics, 92(7), 1-14, Article ID 075207.
Open this publication in new window or tab >>Optical properties and Zeeman spectroscopy of niobium in silicon carbide
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2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 7, p. 1-14, article id 075207Article in journal (Refereed) Published
Abstract [en]

The optical signature of niobium in the low-temperature photoluminescence spectra of three common polytypes of SiC (4H, 6H, and 15R) is observed and confirms the previously suggested concept that Nb occupies preferably the Si-C divacancy with both Si and C at hexagonal sites. Using this concept we propose a model considering a Nb-bound exciton, the recombination of which is responsible for the observed luminescence. The exciton energy is estimated using first-principles calculation and the result is in very good agreement with the experimentally observed photon energy in 4H SiC at low temperature. The appearance of six Nb-related lines in the spectra of the hexagonal 4H and 6H polytypes at higher temperatures is tentatively explained on the grounds of the proposed model and the concept that the Nb center can exist in both C1h and C3v symmetries. The Zeeman splitting of the photoluminescence lines is also recorded in two different experimental geometries and the results are compared with theory based on phenomenological Hamiltonians. Our results show that Nb occupying the divacancy at the hexagonal site in the studied SiC polytypes behaves like a deep acceptor.

Place, publisher, year, edition, pages
American Physical Society, 2015
National Category
Theoretical Chemistry Other Physics Topics
Identifiers
urn:nbn:se:liu:diva-117972 (URN)10.1103/PhysRevB.92.075207 (DOI)000362204100001 ()
Note

At the time for thesis presentation publication was in status: Manuscript

Funding Agencies|Knut and Alice Wallenberg Foundation; Lendulet program of the Hungarian Academy of Sciences; Hungarian OTKA Project [K101819]; Ministry of Education and Science of the Russian Federation [14.Y26.31.0005]; Tomsk State University Academic D. I. Mendeleev Fund Program [8.1.18.2015]

Available from: 2015-05-19 Created: 2015-05-19 Last updated: 2017-12-04Bibliographically approved
Szasz, K., Ivády, V., Abrikosov, I., Janzén, E., Bockstedte, M. & Gali, A. (2015). Spin and photophysics of carbon-antisite vacancy defect in 4H silicon carbide: A potential quantum bit. Physical Review B. Condensed Matter and Materials Physics, 91(12), 121201
Open this publication in new window or tab >>Spin and photophysics of carbon-antisite vacancy defect in 4H silicon carbide: A potential quantum bit
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2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 12, p. 121201-Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
American Physical Society, 2015
National Category
Physical Sciences Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-116950 (URN)10.1103/PhysRevB.91.121201 (DOI)000351038400004 ()
Note

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

Available from: 2015-04-13 Created: 2015-04-10 Last updated: 2017-12-04
Ivády, V., Szasz, K., Falk, A. L., Klimov, P. V., Christle, D. J., Janzén, E., . . . Gali, A. (2015). Theoretical model of dynamic spin polarization of nuclei coupled to paramagnetic point defects in diamond and silicon carbide. Physical Review B. Condensed Matter and Materials Physics, 92(11), 115206
Open this publication in new window or tab >>Theoretical model of dynamic spin polarization of nuclei coupled to paramagnetic point defects in diamond and silicon carbide
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2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 11, p. 115206-Article in journal (Refereed) Published
Abstract [en]

Dynamic nuclear spin polarization (DNP) mediated by paramagnetic point defects in semiconductors is a key resource for both initializing nuclear quantum memories and producing nuclear hyperpolarization. DNP is therefore an important process in the field of quantum-information processing, sensitivity-enhanced nuclear magnetic resonance, and nuclear-spin-based spintronics. DNP based on optical pumping of point defects has been demonstrated by using the electron spin of nitrogen-vacancy (NV) center in diamond, and more recently, by using divacancy and related defect spins in hexagonal silicon carbide (SiC). Here, we describe a general model for these optical DNP processes that allows the effects of many microscopic processes to be integrated. Applying this theory, we gain a deeper insight into dynamic nuclear spin polarization and the physics of diamond and SiC defects. Our results are in good agreement with experimental observations and provide a detailed and unified understanding. In particular, our findings show that the defect electron spin coherence times and excited state lifetimes are crucial factors in the entire DNP process.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2015
National Category
Physical Sciences Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-121891 (URN)10.1103/PhysRevB.92.115206 (DOI)000361370600003 ()
Note

Funding Agencies|Knut and Alice Wallenberg Foundation "Isotopic Control for Ultimate Materials Properties"; Swedish Research Council (VR) [621-2011-4426, 621-2011-4249]; Swedish Foundation for Strategic Research program SRL [10-0026]; Grant of Ministry of Education and Science of the Russian Federation [14.Y26.31.0005]; Tomsk State University Academic D. I. Mendeleev Fund Program [8.1.18.2015]; Swedish National Infrastructure for Computing Grants [SNIC 2013/1-331]; "Lendulet program" of Hungarian Academy of Sciences

Available from: 2015-10-13 Created: 2015-10-12 Last updated: 2017-12-01
Castelletto, S., Johnson, B. C., Ivády, V., Stavrias, N., Umeda, T., Gali, A. & Ohshima, T. (2014). A silicon carbide room-temperature single-photon source. Nature Materials, 13(2), 151-156
Open this publication in new window or tab >>A silicon carbide room-temperature single-photon source
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2014 (English)In: Nature Materials, ISSN 1476-1122, E-ISSN 1476-4660, Vol. 13, no 2, p. 151-156Article in journal (Refereed) Published
Abstract [en]

Over the past few years, single-photon generation has been realized in numerous systems: single molecules(1), quantum dots(2-4), diamond colour centres5 and others(6). The generation and detection of single photons play a central role in the experimental foundation of quantum mechanics(7) and measurement theory(8). An efficient and high-quality single-photon source is needed to implement quantum key distribution, quantum repeaters and photonic quantum information processing(9). Here we report the identification and formation of ultrabright, room-temperature, photostable single-photon sources in a device-friendly material, silicon carbide (SiC). The source is composed of an intrinsic defect, known as the carbon antisite-vacancy pair, created by carefully optimized electron irradiation and annealing of ultrapure SiC. An extreme brightness (2 x 10(6) counts s(-1)) resulting from polarization rules and a high quantum efficiency is obtained in the bulk without resorting to the use of a cavity or plasmonic structure. This may benefit future integrated quantum photonic devices(9).

Place, publisher, year, edition, pages
Nature Publishing Group, 2014
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-104643 (URN)10.1038/nmat3806 (DOI)000330182700020 ()
Available from: 2014-02-20 Created: 2014-02-20 Last updated: 2017-12-06
Falk, A. L., Klimov, P. V., Buckley, B. B., Ivády, V., Abrikosov, I., Calusine, G., . . . Awschalom, D. D. (2014). Electrically and Mechanically Tunable Electron Spins in Silicon Carbide Color Centers. Physical Review Letters, 112(18), 187601
Open this publication in new window or tab >>Electrically and Mechanically Tunable Electron Spins in Silicon Carbide Color Centers
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2014 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 112, no 18, p. 187601-Article in journal (Refereed) Published
Abstract [en]

The electron spins of semiconductor defects can have complex interactions with their host, particularly in polar materials like SiC where electrical and mechanical variables are intertwined. By combining pulsed spin resonance with ab initio simulations, we show that spin-spin interactions in 4H-SiC neutral divacancies give rise to spin states with a strong Stark effect, sub-10(-6) strain sensitivity, and highly spin-dependent photoluminescence with intensity contrasts of 15%-36%. These results establish SiC color centers as compelling systems for sensing nanoscale electric and strain fields.

Place, publisher, year, edition, pages
American Physical Society, 2014
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-107840 (URN)10.1103/PhysRevLett.112.187601 (DOI)000336099800011 ()
Available from: 2014-06-23 Created: 2014-06-23 Last updated: 2017-12-05
Szasz, K., Ivády, V., Janzén, E. & Gali, A. (2014). First principles investigation of divacancy in SiC polytypes for solid state qubit application. In: SILICON CARBIDE AND RELATED MATERIALS 2013, PTS 1 AND 2: . Paper presented at 15th International Conference on Silicon Carbide and Related Materials, ICSCRM 2013 (pp. 499-502). Stafa-Zurich, Switzerland: Trans Tech Publications, 778-780
Open this publication in new window or tab >>First principles investigation of divacancy in SiC polytypes for solid state qubit application
2014 (English)In: SILICON CARBIDE AND RELATED MATERIALS 2013, PTS 1 AND 2, Stafa-Zurich, Switzerland: Trans Tech Publications , 2014, Vol. 778-780, p. 499-502Conference paper, Published paper (Refereed)
Abstract [en]

We calculated the hyperfine structure and the zero-field splitting parameters of divacancies in 3C, 4H and 6H SiC in the ground state and in the excited state for 4H SiC within the framework of density functional theory. Besides that our calculations provide identification of the defect in different polytypes, we can find some carbon atoms next to the divacancy that of the spin polarizations are similar in the ground and excited states. This coherent nuclear spin polarization phenomenon can be the base to utilize 13C spins as quantum memory.

Place, publisher, year, edition, pages
Stafa-Zurich, Switzerland: Trans Tech Publications, 2014
Series
Materials Science Forum, ISSN 0255-5476 ; Vol 778-780
Keyword
Hyperfine coupling; Optically detected magnetic resonance; Photoluminescence; Solid state quantum bit; Zero-field splitting
National Category
Physical Sciences Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-110538 (URN)10.4028/www.scientific.net/MSF.778-780.499 (DOI)000336634100117 ()2-s2.0-84896083747 (Scopus ID)9783038350101 (ISBN)
Conference
15th International Conference on Silicon Carbide and Related Materials, ICSCRM 2013
Available from: 2014-09-15 Created: 2014-09-12 Last updated: 2014-09-15
Ivády, V., Simon, T., Maze, J. R., Abrikosov, I. & Gali, A. (2014). Pressure and temperature dependence of the zero-field splitting in the ground state of NV centers in diamond: A first-principles study. Physical Review B. Condensed Matter and Materials Physics, 90(23), 235205
Open this publication in new window or tab >>Pressure and temperature dependence of the zero-field splitting in the ground state of NV centers in diamond: A first-principles study
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2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 23, p. 235205-Article in journal (Refereed) Published
Abstract [en]

Nitrogen-vacancy centers in diamond (NV) attract great attention because they serve as a tool in many important applications. The NV center has a polarizable spin S = 1 ground state and its spin state can be addressed by optically detected magnetic resonance (ODMR) techniques. The m(S) = 0 and m(S) = +/- 1 spin levels of the ground state are separated by about 2.88 GHz in the absence of an external magnetic field or any other perturbations. This zero-field splitting (ZFS) can be probed by ODMR. As this splitting changes as a function of pressure and temperature, the NV center might be employed as a sensor operating at the nanoscale. Therefore, it is of high importance to understand the intricate details of the pressure and temperature dependence of this splitting. Here we present an ab initio theory of the ZFS of the NV center as a function of external pressure and temperature including detailed analysis on the contributions of macroscopic and microscopic effects. We found that the pressure dependence is governed by the change in the distance between spins as a consequence of the global compression and the additional local structural relaxation. The local structural relaxation contributes to the change of ZFS with the same magnitude as the global compression. In the case of temperature dependence of ZFS, we investigated the effect of macroscopic thermal expansion as well as the consequent change of the microscopic equilibrium positions. We could conclude that theses effects are responsible for about 15% of the observed decrease of ZFS.

Place, publisher, year, edition, pages
American Physical Society, 2014
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-113573 (URN)10.1103/PhysRevB.90.235205 (DOI)000346860700002 ()
Note

Funding Agencies|NIIF Supercomputer center [1090]; EU [270197, 611143]; Hungarian Academy of Sciences; Knut and Alice Wallenberg Foundation; Swedish National Infrastructure for Computing [SNIC 001/12-275, SNIC 2013/1-331]; Conicyt (Chile) Fondecyt [1141185, PIA ACT1108, PIA ACT1112]; Ministry of Education and Science of the Russian Federation [14.Y26.31.0005]; Tomsk State University

Available from: 2015-01-23 Created: 2015-01-23 Last updated: 2017-12-05
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