Choi Research Group

Research

Keywords

: Quantum (Information) Materials, Defect/Doping, Magnetism, Density Functional Theory & Beyond , Mean-field theory

Defects as quantum information platform

The negatively charged nitrogen-vacancy center in diamond is promising for quantum information technology such as quantum sensing and quantum memory due to its long spin lifetimes (~6 ms) at room temperature, coherence time (~a few ms) and high sensitivity. We perform a comprehensive theoretical investigation of the pressure dependence of the negatively charged nitrogen-vacancy center in diamond. We examine how key properties, such as the zero-phonon line, zero-field splitting, charge-state transition level, and photoionization thresholds, evolve under six different types of external pressure. Our results reveal linear pressure dependence of zero-phonon line and zero-field splitting, with distinct trends depending on symmetry-preserving or symmetry-breaking conditions. We also identify pressure regimes where charge-state stability is either enhanced or suppressed.

Hidden magnetism in Cobalt material

The ground-state electron configuration of the octahedral Co3+ ion corresponds to a low-spin (LS) state. However, we theoretically demonstrate that the octahedral Co3+ ion in Co3S4 prefers a high-spin (HS) state, resulting in unusual magnetism. DFT+U calculation and ligand-field theory show that weak crystal-field splitting associated with S2- induces a spin crossover from the LS to HS state of the octahedral Co3+ ion along with a weak Jahn-Teller-like elongation, and as a result, a ferromagnetic (FM) metal phase is energetically stabilized. Nevertheless, this phase is significantly more stable than the antiferromagnetic (AFM) phase experimentally reported at low temperature. Furthermore, phonon calculations and LKDM theory suggest that the FM metal phase is possibly one of the Co3S4 polymorphs, appearing in the certain experimental growth environment, and this phase undergoes a phase transition to the frustrated magnetic state.

Physical Review Research 4, 033171 (2022).

Current Applied Physics 22, 65 (2021).

Two dimensional oxides

Two-dimensional (2D) transition metal oxides have been attracting attention for a variety of future electronic, optoelectronic, magnetic, and energy applications. For a long time, bulk TiO2 has been a technically important material, hence many efforts have been made to obtain its 2D phase for potential electronic and photochemical applications. Several 2D TiO2 phases are computationally predicted, and experimental attempts have been made to realize these phases. We propose a practical way to obtain a truly 2D 1T-TiO2 overcoming experimental difficulty or beyond an earlier computational suggestion.

We also present the combined theoretical study on the structural, electronic, and magnetic properties of 2D 1T-RhO2. DFT+U calculation and Monte Carlo simulation indicate that 1T−RhO2 favors a ferromagnetic (FM) metallic phase with an out-of-plane magnetization and the FM phase is stable below the Curie temperature of 73.9 K based on the Heisenberg Hamiltonian model. We conclude that spin-orbit coupling interaction between Rh 4d stabilizes the out-of-plane magnetization.

Physical Review B 106, 064414 (2022).

Physical Review Materials 4, 094001 (2020).

Hydrogen in VO2

Hydrogen ions provide the most efficient means to induce reversible phase transformation by ionic insertion to modulate physical properties of VO2. With experimentalists, we discover that hydrogen as an extrinsic defect can be externally inserted into a target VO2 channels to alter conductivity and induce phase transition between insulating and metallic phase without disrupting the crystal lattice framework. We also demonstrate that oxygen vacancies help the hydrogen injection into the material by reducing energy barriers for hydrogen diffusion.

Advanced Electronic Materials 7, 2000802 (2021).

Nature Materials 15, 1113 (2016).

See also 경북일보, 동아사이언스, 베리타스알파 (in Korean)

Ti antisite defects & O vacancies in SrTiO3

Many peculiar properties in SrTiO3 have triggered extensive investigations to figure out what causes them. We show that Ti anisite-like defects, whose structures are constructed by one Ti ion off-centered from the Sr site, are energetically favorable and even comparable in formation energy to oxygen vacancies under certain growth condition. We propose that our Ti anisite-like defects are responsible for the some interesting experimental observations.

Although oxygen vacancies or its complexes can be intimately related to a rich variety of phenomena observed in SrTiO3, understanding oxygen vacancies is still limited, although many efforts have been made. We discover that oxygen vacancies induce a local anti-ferrodistortive-like oxygen-octahedron rotation, even in the cubic phase. This feature leads to localized electronic states in the bandgap, giving an excellent explanation to the meastured thermal ionization and optical transition.

Physical Review Letters 103, 185502 (2009).

Advanced Materials 25, 86 (2013).