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Weng H.K., Nagakubo A., Ogi H., et al. "Phonon propagation in isotopic diamond superlattices", Physical Review B, 104(5), 054112, 2021. https://doi.org/10.1103/physrevb.104.054112., The out-of-plane thermal conductivity and elastic constant of epitaxial [100] C12/C13 superlattice diamonds with layer thicknesses of 1, 30, and 100 nm are evaluated by picosecond ultrasound spectroscopy. The measured elastic constants of the superlattices are equivalent to those of single-layer diamond thin films. This result confirms our success in synthesizing superlattice specimens with few defects at the interfaces. Therefore, the phonon transport behavior is governed by the mass difference, not the interfacial defects. The measured thermal conductivity of the superlattices is lower than that of a pure C12 isotope diamond thin film. We estimated the lattice thermal conductivity using the lattice dynamics calculation, attributing the lowered thermal conductivity to the decrease in the phonon group velocity in superlattices. We further consider the effect of mini-umklapp scattering in the superlattice, which explains the dependence of the thermal conductivity on the layer thickness. We reveal that the mini-umklapp scattering effect becomes significant only for an isotope diamond superlattice because of the high Debye temperature and large relative mass difference.

Wide-field magnetometry can be realized by imaging the optically-detected magnetic resonance of diamond nitrogen vacancy (NV) center ensembles. However, NV ensemble inhomogeneities significantly limit the magnetic-field sensitivity of these measurements. We demonstrate a double-double quantum (DDQ) driving technique to facilitate wide-field magnetic imaging of dynamic magnetic fields at a micron scale. DDQ imaging employs four-tone radio frequency pulses to suppress inhomogeneity-induced variations of the NV resonant response. As a proof-of-principle, we use the DDQ technique to image the dc magnetic field produced by individual magnetic-nanoparticles tethered by single DNA molecules to a diamond sensor surface. This demonstrates the efficacy of the diamond NV ensemble system in high-frame-rate magnetic microscopy, as well as single-molecule biophysics applications.

We investigate phosphorus (P) donors in P-doped diamond epitaxial films by means of electron paramagnetic resonance spectroscopy (EPR). In a diamond thin film on IIa diamond substrate, we could observe an EPR signal of the NIMS1 center, which has been assigned to a neutral P donor with a ${D}_{2d}$ symmetry. In contrast, we could not observe this center in another free-standing diamond film with the same P concentration $([\mathrm{P}]\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3})$. This striking contrast can be reasonably accounted for in a model where the P donor in diamond has a nearly degenerate ground state due to ${t}_{2}$ and $e$, in contrast to P donors in group-IV semiconductors like Si and Ge, where ${a}_{1}$ singlet ground state is present. We observed a strong uniaxial strain only in the former diamond thin film, which was confirmed by Raman microscopy and preferential orientations of NIMS1 EPR centers. This strain splits the nearly degenerate ground state of the P donor, resulting in the observation of the NIMS1 center. We point out that the electronic system of the substitutional P donor in diamond is similar to that of interstitial Li donor in Si.

Dynamical decoupling sequences with multiple pulses can be considered to exhibit filter functions for the time evolution of a qubit superposition state. They contribute to the improvement of coherence time and qubit-phase accumulation due to a time-varying field and can thus achieve high-frequency-resolution spectroscopy. Such behaviors find useful application in highly sensitive detection based on qubits for various external fields such as a magnetic field. Hence, decoupling sequences are indispensable tools for quantum sensing. In this study, we experimentally and theoretically investigated the effects of finite-width pulses in the sequences on AC magnetometry utilizing nitrogen-vacancy centers in an isotopically-controlled diamond. We revealed that the finite pulse widths cause a deviation of the optimum time to acquire the largest phase accumulation due to the sensing field from that expected by filter functions neglecting the pulse widths, even if the widths are considerably shorter than the time period of the sensing field. Moreover, we experimentally demonstrated that the deviation can be corrected by an appropriate time-frequency conversion. Our results provide a guideline for the detection of an AC field with an accurate frequency and linewidth in quantum sensing with multiple-pulse sequences., 6 pages, 5 figures