Perspectives on deterministic control of quantum point defects by scanned probes

Single-photon emitters play an essential role in quantum technologies, including quantum computing and quantum communications. Atomic defects in hexagonal boron nitride ( h-BN) have recently emerged as new room-temperature single-photon emitters in solid-state systems, but the development of scalable and tunable h-BN single-photon emitters requires external methods that can control the emission energy of individual defects. Here, by fabricating van der Waals heterostructures of h-BN and graphene, we demonstrate the electrical control of single-photon emission from atomic defects in h-BN via the Stark effect. By applying an out-of-plane electric field through graphene gates, we observed Stark shifts as large as 5.4 nm per GV/m. The Stark shift generated upon a vertical electric field suggests the existence of out-of-plane dipole moments associated with atomic defect emitters, which is supported by first-principles theoretical calculations. Furthermore, we found field-induced discrete modification and stabilization of emission intensity, which were reversibly controllable with an external electric field.

On the thermoelectric power variation according to temperature of zirconium alloys and its possible application to the estimation of the amount of solubility variations of iron and chromium in zircaloy

Most mineralogical applications of transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) have involved the imaging of extended defects, such as dislocations, stacking faults, Wadsley defects, and antiphase boundaries. Modern electron microscopes, however, are capable of imaging individual, isolated atoms (e.g., Isaacson et al. 1979), raising the question of whether individual or clustered point defects can be resolved within mineral crystals.

Single-photon sources and their optical spin readout are at the core of applications in quantum communication, quantum computation, and quantum sensing. Their integration in photonic structures such as photonic crystals, microdisks, microring resonators, and nanopillars is essential for their deployment in quantum technologies. While there are currently only two material platforms (diamond and silicon carbide) with proven single-photon emission from the visible to infrared, a quantum spin–photon interface, and ancilla qubits, it is expected that other material platforms could emerge with similar characteristics in the near future. These two materials also naturally lead to monolithic integrated photonics as both are good photonic materials. While so far the verification of single-photon sources was based on discovery, assignment and then assessment and control of their quantum properties for applications, a better approach could be to identify applications and then search for the material that could address the requirements of the application in terms of quantum properties of the defects. This approach is quite difficult as it is based mostly on the reliability of modeling and predicting of color center properties in various materials, and their experimental verification is challenging. In this paper, we review some recent advances in an emerging material, low-dimensional (2D, 1D, 0D) hexagonal boron nitride (h-BN), which could lead to establishing such a platform. We highlight the recent achievements of the specific material for the expected applications in quantum technologies, indicating complementary outstanding properties compared to the other 3D bulk materials.

Recent progress on BEEM

We report on a pronounced redistribution of the local electronic density of states at the graphite surface, which is induced by the presence of low energy hydrogen-ion induced point defects. Scanning tunneling microscopy reveals standing waves in the local density of states, which are due to backscattering of electron wave functions at individual point defects. The superstructure thereby formed is directly related to the pointlike structure of the Fermi surface of graphite. For high defect density interference patterns are observed which sensitively change structure on the relative positions of the defects. These patterns could be reproduced by tight binding simulations of various defect distributions.

Grinding processes require a high energy input per unit volume of material removed, which is converted to heat at the grinding zone, resulting in increased force and wear. In the present study, the influence of grinding parameters like work speed and depth of cut on grinding forces and energy was studied. An attempt has been made to study the forces and energy involved while grinding aluminium alloy (A356)/silicon carbide (SiC) composite material with different grinding wheels. Experiments were carried out on a surface grinding machine. Three different types of wheels like SiC, cubic boron nitride (CBN) and diamond wheels were used. The grinding forces increased with increase in depth of cut and work speed. SiC exhibited high grinding force compared to the CBN wheel. In the case of the diamond wheel, it was even less. The specific grinding energy was highest for the diamond wheel followed by CBN and SiC wheels. The specific grinding energy decreased with increase in depth of cut and work speed.

Automated identification and characterization of clustered weld defects, which comprise a plural of closely distributed individual member defects, is challenging. In this paper, time, Cartesian and Polar domain features in phased array ultrasonic sectorial scanning data are studied to segment clustered weld defect echoes by density based and K-means clustering. Through multiple domain segmentation, all individual member defect echoes are successfully identified. Then size and depth estimation is performed on each individual member defect to describe its patterns and to locate its depth. The developed technique is applied to experimental data collected from scanning on a tubular T weld test sample and is shown to discriminate, locate, and measure all the individual defects inside clustered defects. Based on the geometrical features of each individual member defect, they are classified as either point, linear or planar defects.

A fine study of the interfacial part in the silicon carbide fiber (SiCf) reinforced silicon carbide (SiC) composites was conducted by transmission electron microscopy. The boron nitride (BN) and carbon nanotubes (CNTs) were progressively coated on the SiCf by chemical vapor deposition method to form a hierarchical structure. Three composites with different interfaces, SiCf–CNTs/SiC, SiCf@BN/SiC, and SiCf@BN–CNTs/SiC, were fabricated by polymer infiltration and pyrolysis method. The interfaces and microstructures of the three composites were carefully characterized to investigate the improvement mechanism of strength and toughness. The results showed that BN could protect the surface of SiCf from corrosion and oxidation so that improved the possibility of debonding and pullout. CNTs could avoid the propagation of cracks in the composites so that improved the damage resistance of the matrix. The synergistic reinforcement brought by BN and CNTs interfaces made the SiCf@BN–CNTs/SiC composites with a tensile fracture strength as high as 359 MPa, with an improvement of 23% compared to that of SiCf@BN/SiC.

Silicon carbide (SiC) has been considered as one of the alternative power semiconductor materials due to its excellent properties such as wide band gap, high break down field, and high thermal conductivity. Although SiC with 3C (zinc blende) is suitable for high-frequency power devices from its high electron mobility and high electron-saturation-velocity, SiC is well known as having dozens of polytypes including 4H and 6H in addition to 2H (wurtzite) and 3C that consist of different ordered stacking sequences of SiC atomic double layers. The 3C-SiC is believed to be more stable than the hexagonal structure at low temperatures. In contrast, 2H-SiC, which has the simplest stacking sequence, is rarely observed at high temperatures. Moreover, point defects also affect the stable polytype in SiC. Heavily Nitrogen (N)-doped single crystals of 6H-SiC were completely transformed into 3C-SiC by annealing in vacuum at presence of Si vapor at 2080 K and 2180 K (hereafter Case I). The 3C-SiC-like bands was also observed in 4H-SiC epilayers with heavily N-doping after thermal annealing (hereafter Case II). Presuming that vacancy formation is related to temperature, point defects such as vacancy and N in SiC are crucial factor for the structural transformation between hexagonal and cubic structures. In this study, the structural transformation in SiC is simply interpreted by considering point defect formation with the aid of ab initio calculations. Ab initio total energy calculations in a √3×√3×12 unit cell imply that stable stacking for SiC is 6H with Si-vacancy (VSi), 4H with C-vacancy (VC) and N substituting for Si (NSi), and 3C with N substituting for C (NC), whereas 2H-stacking does not appear as a stable stacking in these systems. Furthermore, the formation energy calculations reveal that vacancy formation depends on C chemical potential m C, where VSi and VC formations are favorable at large m C (corresponding to C-rich) and small m C (Si-rich), respectively. On the other hand, NC formation is more favorable than NSi formation over the entire range of m C and independent of conditions because of very small difference in covalent radius between N and C. Comparing these calculated results with the experimental finding in the Case I for transformation from 6H to 3C, thermal treatment with presence of Si vapor at high temperatures suppresses VSi formation favoring 6H-stacking while it enhances VC formation to be occupied by N (NC) in addition to N substitution for C (NC) that transform 6H to 3C. The Case II can be also interpreted by considering NC. Thermal annealing for N-doped 4H-SiC favored by VC easily produces NC with N occupying VC and substituting for C to realize 3C-SiC bands at high N-doped region in 4H-SiC epilayers. These results suggest that NC is crucial for the transformation from hexagonal stacking such as 6H and 4H to cubic stacking 3C. In order to give physical interpretation for the transformation to 3C-SiC with NC, the interaction energies Jn between nth neighbor SiC double layers are estimated in the range of n ≤ 3 using the axial next nearest neighbor Ising (ANNNI) model and calculated total energies for SiC polytypes obtained in this study. The calculated results elucidate that the absolute value of positive J 1 (51.70 meV) with J 3 (9.92 meV) favoring 3C-stacking is much larger than that of negative J 2 (-16.66 meV) inducing hexagonal stacking. It should be noted that the Jn values are one order of magnitude greater than those of perfect SiC such as J 1 (2.33 meV), J 2 (-1.41 meV), and J 3 (0.51 meV). This implies that the layer interaction in SiC becomes strong and long-range due to N-doping. Considering coordinate of (|J 2|/J 1, J 3/J 1) in the ANNNI phase diagram, it is also found that (0.31, 0.19) for SiC with NC is located in the stable region of 3C and is more distant from the multi-phase degeneracy point (0.5, 0) inducing polytype and phase boundary between 3C and 4H than (0.61, 0.21) for perfect SiC. This clarifies that the large J 1 in SiC with NC stabilizes 3C-stacking to dominate the structural transformation to 3C due to N-doping. Furthermore, atomistic interaction between the nearest neighbor layers corresponding to J 1 is also qualitatively discussed in terms of electrostatic interaction between bond charges located at the center of interatomic bonds and that between ionic charges located at the lattice sites on the basis of a simple energy description and charge distribution obtained by ab initio calculations.

Silicon carbide (SiC) is considered as one of the most promising structural and coating materials for advanced nuclear applications, due to its low neutron capture cross section and excellent irradiation resistance. The difference in swelling behavior between monocrystalline and polycrystalline SiC is experimentally investigated by heavy ion irradiation at room temperature (RT). In this work, single crystal hexagonal (6H) SiC and polycrystalline chemically vapor-deposited (CVD) SiC are irradiated by 1.5 MeV Si ions with the fluences of 11014-21016 cm-2 and 11015-21016 cm-2, respectively, at RT. The step height of irradiation swelling is measured by a white light interferometer and the lattice expansion of the damage layer is characterized by using X-ray diffraction (XRD) spectrometry, in addition, the actual irradiation swelling is obtained by dividing the height of swelling step by the depth of damage layer. The XRD profiles show that the lattice expansion in the damage layer increases with the increase of irradiation fluence, and the new diffraction peak relating to the lattice structure of damage layer disappears in a fluence of 21015 cm-2, which means that the damage layer is completely amorphous at this time and the threshold dose of amorphization at RT in single crystal 6H-SiC is less than 0.8 dpa. The direct-impact model is used to fit the swelling step heights of CVD SiC and 6H-SiC irradiated by 1.5 MeV Si, and the swelling results show that the amorphization threshold dose of polycrystalline CVD SiC is larger than that of single crystal 6H-SiC. In the present work, three distinct stages are found in the heavy-ion irradiation swellings between monocrystalline and polycrystalline SiC. i.e., low-fluence region, intermediate-fluence region, and high-fluence region stage. 1) In the low-fluence region, the swellings are similar to each other, since the swelling is mainly contributed to by point defects in this region, and the micron sized grains in polycrystalline CVD SiC are of single crystal structure. 2) In the intermediate-fluence region, the irradiation swelling of the polycrystalline CVD SiC is smaller than that of the single crystal 6H-SiC, since the irradiation-induced amorphousness in polycrystalline CVD SiC is relatively hard to occur due to the existence of grain boundary in this region. 3) The irradiation swellings of 6H-SiC and CVD SiC are almost the same at the high-fluence region stage, since the irradiation swelling is caused by amorphization in this region, and the swelling depends on the difference between densities before and after irradiation. In addition, in the irradiation swelling analysis of SiC materials, XRD swelling measurement method is suitable for irradiation swelling induced by point defects, especially for neutron irradiation experiments.

Quantum technology has grown out of quantum information theory and now provides a valuable tool that researchers from numerous fields can add to their toolbox of research methods. To date, various systems have been exploited to promote the application of quantum information processing. The systems that can be used for quantum technology include superconducting circuits, ultracold atoms, trapped ions, semiconductor quantum dots, and solid-state spins and emitters. In this review, we will discuss the state-of-the-art of material platforms for spin-based quantum technology, with a focus on the progress in solid-state spins and emitters in several leading host materials, including diamond, silicon carbide, boron nitride, silicon, two-dimensional semiconductors, and other materials. We will highlight how first-principles calculations can serve as an exceptionally robust tool for finding novel defect qubits and single-photon emitters in solids, through detailed predictions of electronic, magnetic, and optical properties.

Understanding and engineering atomic defects in hexagonal boron nitride (hBN) provides a powerful platform for realizing solid-state quantum emitters and spin qubits, advancing the field of quantum information science and technologies. However, the full potential of such quantum defects remains locked by the critical lack of a deterministic structure-property relationship at the atomic scale. Here, we demonstrate a strategy to atomically engineer and decipher quantum defects in hBN by integrating scanning tunneling microscopy/spectroscopy (STM/STS) and noncontact atomic force-microscopy with a CO-functionalized tip. We implemented controllable argon ion bombardment to create both boron vacancies (VB) and nitrogen vacancies (VN) in submonolayer hBN grown on Cu(111). Simultaneously, encapsulated Ar species trapped between hBN and Cu(111) locally lift the hBN to form nanobubbles, thereby decoupling atomic vacancies from the metal substrate and enabling direct probing of their electronic states. For the on-bubble VN, STS measurement reveals a prominent in-gap state with a phonon replica. Furthermore, with aid of STM tip-assisted manipulation, we demonstrate that the tuning of nanobubble sizes modulates their strain profile, thereby modulating the energetic positions of electronic states in on-bubble defects, corroborated by density functional calculations. Our studies offer insight into the intrinsic defect structures in hBN and quantum defect engineering via local strain engineering.

Point and small cluster defects in magnesium aluminate spinel have been studiedfrom a first principles viewpoint. Typical point defects that occur during collisioncascade simulations are cation anti-site defects, which have a small formationenergy and are very stable, O and Mg split interstitials and vacancies. IsolatedAl interstitials were found to be energetically unfavourable but could occur aspart of a split Mg–Al pair or as a three atom–three vacancy Al ‘ring’ defect,previously observed in collision cascades using empirical potentials. The structure andenergetics of the defects were investigated using density functional theory (DFT)and the results compared to simulations using empirical fixed charge potentials.Each point defect was studied in a variety of supercell sizes in order to ensureconvergence. It was found that empirical potential simulations significantly overestimateformation energies, but that the type and relative stability of the defects are wellpredicted by the empirical potentials both for point defects and small defect clusters.

Hexagonal boron nitride on metal surfaces as a support and template