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Our group member, Viktor Ivády, has been honored by receiving the Premium Postdoctoral Fellowship from the Hungarian Academy of Sciences. Congratulations!
We use density-functional theory calculations to see if other group-IV elements—namely, germanium, tin, and lead—in a vacancy center exhibit similar optical properties to silicon but with improved spin properties that permit an economical cooling system. We develop a new theory for understanding the interaction of light and magnetic fields with these color centers, and we explore how the dynamical motion of the atoms strongly affects the magneto-optical properties of the color center. By combining our theory with density-functional theory calculations, we find that lead-vacancy color centers should have favorable optical and spin properties for operating at room temperature.
Phys. Rev. X 8, 021063 (2018). DOI:10.1103/PhysRevX.8.021063
Our ab initio simulations determined the magneto-optical properties of the neutral divacancy in silicon carbide that contributed to understanding its qubit operation carried out by Prof. David D. Awschalom group.
Phys. Rev. X 7, 021046 (2017). DOI:10.1103/PhysRevX.7.021046
Our ab initio study identified the nitrogen terminated (111) diamond surface to host shallow nitrogen-vacancy centers for quantum sensing. In cooperation with Alex Retzker we showed, that special quantum simulations can be carried out with the nitrogen nuclear spins in this system
Nano Letters 17 (4), pp 2294-2298. (2017) DOI:10.1021/acs.nanolett.6b05023
We investigated the electron-phonon coupling in small diamond cages called diamondoids. These quantum systems are mostly modelled within the Born-Oppenheimer approximations, where the coupling between the electrons and vibration modes is not fully taken into account. In our study we shown that the severe electron-phonon coupling is a key point to properly describe the overall lineshape of the experimental photoemission spectrum. Our method goes beyond the typical Born-Oppenheimer approximation, where the electrons are pure electron-like quasi-particles. In our model, we deduced a link between the many-body perturbation approach of the electron-phonon coupling and the well-known Jahn-Teller effect.
Nature Communications 7: 11327. (2016). DOI:10.1038/ncomms11327
Working together with Professor Awshalom's group we demonstrate optically pumped dynamic nuclear polarization of 29Si 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 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. The study is featured in Physics and highlighted as Editor's suggestion in PRL.
Physical Review Letters 114 247603 (2015). DOI:10.1103/PhysRevLett.114.247603
Together with colleagues in the United States, we have investigated and proposed a new kind of material for photovoltaics. In strongly correlated insulators, strong Coulomb interactions make highly photoexcited electron-hole pairs decay fast into multiple electron-hole pairs via impact ionization, thus preventing slower energy dissipation to heat by phonons. The prototypical M1-type vanadium-oxide insulator have been used for computational simulations, which may be a first example of a new class of photovoltaic materials.
Phys. Rev. B 90, 165142 (2014). DOI:10.1103/PhysRevB.90.165142
In collaboration with Bremen Center for Computational Materials Science we have identified such a diamond surface that would be ideal to host shallow implanted nitrogen-vacancy centers in diamond for sensing at the nanoscale.
Nano Letters 14 4772-4777 (2014). DOI:10.1021/nl501927y
In the field of methodology development, we have constructed a DFT scheme combining hybrid exchange and DFT+U strategies. This machinery seems to be an efficient tool in DFT calculations involving solid-state defects with highly localized electronic orbitals, as its tests on two typical systems show.
Physical Review B 90 035146 (2014) DOI:10.1103/PhysRevB.90.035146
Our group members at Wigner ADMIL laboratory have succeeded manufacturing carbon antisite–vacancy colour centres in silcon carbide nanoparticles. These colour centres behave as single-photon emitters, and therefore may be used in the future in nanometrology and quantum informatics. The biocompatibiliy of SiC makes these defects ideal candidates for biosensing at the molecular level.
ACS Nano 8 7938-7947 (2014). DOI:10.1021/nn502719y
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 (David Awschalom group at Chicago University) 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.
Physical Review Letters 112 187601 (2014). DOI:10.1103/PhysRevLett.112.187601
In an international cooperation with Japanese and Australian colleagues, we supported the establishment and identification of the carbon antisite–vacancy pair as a new kind of single-photon emitter in silicon carbide with theoretical work and computational simulation. The results have been highlighted in Nature Physics and Nature Photonics.
Nature Materials 13 151-156 (2014) DOI:10.1038/nmat3806
In a cooperation with partners in Russia and Germany, we have taken part in the work of predicting and validating the properties of silicon-vacancy colour centres in nanosized diamond particles. We have contributed with time-dependent DFT calculations, not carried out on such large systems so far. Nanoparticles for validating experiments have been extracted from a meteorite sample, but production of similar artificial nanoparticles seems to be very prospective in e.g. biosensing. This article has been selected for review in Nature News and Views.
Nature Nanotechnology 9, 54–58 (2014) DOI:10.1038/NNANO.2013.255