서호성 교수

Abstract Long coherence times are key to the performance of quantum bits (qubits). Here, we experimentally and theoretically show that the Hahn-echo coherence time of electron spins associated with divacancy defects in 4 H –SiC reaches 1.3 ms, one of the longest Hahn-echo coherence times of an electron spin in a naturally isotopic crystal. Using a first-principles microscopic quantum-bath model, we find that two factors determine the unusually robust coherence. First, in the presence of moderate

Bismuth vanadate is a promising photoanode for solar-to-fuel photocatalytic applications, and it has been extensively studied in recent years. However, the microscopic mechanism underlying the observed changes in electronic conductivity due to oxygen vacancies and nitrogen dopants remains unclear. Here, we combine electronic structure calculations at the hybrid density functional theory (DFT) level with constrained DFT, and we elucidate the role of defects in enhancing the transport properties o

Spin defects in wide-band gap semiconductors are promising systems for the realization of quantum bits, or qubits, in solid-state environments. To date, defect qubits have only been realized in materials with strong covalent bonds. Here, we introduce a strain-driven scheme to rationally design defect spins in functional ionic crystals, which may operate as potential qubits. In particular, using a combination of state-of-the-art ab-initio calculations based on hybrid density functional and many-b

Color centers in two-dimensional hexagonal boron nitride (h-BN) have recently emerged as stable and bright single-photon emitters (SPEs) operating at room temperature. In this study, we combine theory and experiment to show that vacancy-based SPEs selectively form at nanoscale wrinkles in h-BN with its optical dipole preferentially aligned to the wrinkle direction. By using density functional theory calculations, we find that the wrinkle's curvature plays a crucial role in localizing vacancy-bas

Using density functional theory (DFT), scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS), we study the interface structure and electronic properties of the anatase-TiO${}_{2}$/SrTiO${}_{3}$(001) heterostructure epitaxially grown on Si(001) by molecular beam epitaxy (MBE). We show that charge transfer at the TiO${}_{2}$/SrTiO${}_{3}$ interface is induced by the chemical bond formation between Ti and O. Subsequent O lattice polarization is found to be t

We use density functional theory to investigate the influence of surface vacancies on the surface stability of a stoichiometric freestanding LaAlO${}_{3}$ (001) thin film. Defect-free three- and five-unit-cell-thick LaAlO${}_{3}$ (001) thin films show macroscopic electric fields of 0.28 and 0.22 V/\AA{}, respectively. The built-in electric field is sufficiently strong for the five-unit-cell-thick film to undergo a dielectric breakdown in the local density approximation. We show that the electric

Even with renewed interest in Ge as a competitor to Si in field-effect transistors, several key features of the surface electronic structure of Ge(001) have remained controversial. Notably, the origin of strong Fermi-level pinning in Ge has been heavily debated. Using high-resolution angle-resolved photoemission spectroscopy (ARPES) and first-principles hybrid density functional theory calculations, we compare and unambiguously establish the critical differences between the electronic structure

Abstract Negatively charged boron vacancies (V B − ) in hexagonal boron nitride (h-BN) are a rapidly developing qubit platform in two-dimensional materials for solid-state quantum applications. However, their spin coherence time (T 2 ) is very short, limited to a few microseconds owing to the inherently dense nuclear spin bath of the h-BN host. As the coherence time is one of the most fundamental properties of spin qubits, the short T 2 time of V B − could significantly limit its potential as a

Hexagonal boron nitride (h-BN) has been recently found to host a variety of quantum point defects, which are promising candidates as single-photon sources for solid-state quantum nanophotonic applications. Most recently, optically addressable spin qubits in h-BN have been the focus of intensive research due to their unique potential in quantum computation, communication, and sensing. However, the number of high-symmetry, high-spin defects that are desirable for developing spin qubits in h-BN is

We have demonstrated that domain switching in ferroelectric copolymer films can be significantly affected by humidity. With increasing relative humidity (RH), we observed larger domains with highly irregular boundaries as a result of lateral spreading of the tip-induced electric field that originates from water adsorption. Fractal dimension study of irregular domains reveals that the fractal dimension is higher in cases where the RH is higher. The results show that the RH is one of the major swi

Using density functional theory, we investigate the growth mode of Pt (001) on SrTiO3 (001) (STO) and explore the thermodynamic wetting conditions at this interface. The authors calculate the surface energy of Pt (001) to be 2.45 J/m2 and that of TiO2-terminated STO (001) to range from 1.30–2.06 J/m2, depending on the chemical environment. The calculated interface energy is 0.37 J/m2 higher than that of the STO (001) surface across the entire thermodynamically allowed range, suggesting that Pt (

Epitaxial oxide heterostructures are of fundamental interest in a number of problems ranging from oxide electronics to model catalysts. The epitaxial CoO/SrTiO3 (001) heterostructure on Si(001) has been recently studied as a model oxide catalyst for water splitting under visible light irradiation (Ngo et al., J. Appl. Phys. 114, 084901 (2013)). We use density functional theory to investigate the valence band offset at the CoO/SrTiO3 (001) interface. We examine the mechanism of charge transfer an

Half-monolayer Sr on Si(001) is a Zintl template necessary for epitaxial growth of SrTiO3 on Si(001). The authors investigate the reconstruction in the atomic and electronic structure of Si(001) induced by sub-monolayer Sr deposition using in-situ x-ray/ultraviolet photoemission spectroscopy and density functional theory. Sub-monolayer Sr is deposited on Si(001) using molecular beam epitaxy and the structural evolution of the surface is monitored using reflection high-energy electron diffraction

Abstract The negatively charged boron vacancy (V B − ) in h‐BN is a spin‐1 defect functioning as an optically addressable spin qubit in 2D materials. A precise understanding of its spin decoherence is essential to advance it into a robust qubit platform. First‐principles quantum many‐body simulations are employed to investigate V B − decoherence in dense nuclear spin baths of h‐BN under magnetic fields from 0.01 to 3 T, considering isotopic variants h‐ 10 B 14 N, h‐ 11 B 14 N, h‐ 10 B 15 N, and