Spin Dynamics of a Solid-State Qubit in Proximity to a Superconductor.

A broad effort is underway to understand and harness the interaction between superconductors and spin-active color centers with an eye on hybrid quantum devices and novel imaging modalities of superconducting materials. Most work, however, overlooks the interplay between either system and the environment created by the color center host. Here we use a diamond scanning probe to investigate the spin dynamics of a single nitrogen-vacancy (NV) center proximal to a superconducting film. We find that the presence of the superconductor increases the NV spin coherence lifetime, a phenomenon we tentatively rationalize as a change in the electric noise due to a superconductor-induced redistribution of charge carriers near induced redistribution of charge carriers near the NV. We then build on these findings to demonstrate transverse-relaxation-time-weighted imaging of the superconductor film. These results shed light on the dynamics governing the spin coherence of shallow NVs, and promise opportunities for new forms of noise spectroscopy and imaging of superconductors.

Temperature-dependent second-harmonic generation from color centers in diamond.

Under infrared ultrashort pulse laser stimulation, we investigate temperature-dependent second-harmonic generation (SHG) from nitrogen-vacancy (NV)-introduced bulk diamond. The SHG intensity decreases in the temperature range of 20-300°C, due to phase mismatching caused by refractive index modification. We discover that optical phonon scattering outperforms acoustic phonon scattering in NV diamond by fitting the temperature dependence of the SHG intensity using a model based on the bandgap change via the deformation potential interaction. This study presents an efficient and viable way for creating diamond-based nonlinear optical temperature sensing.

Giant nonlinear optical effects induced by nitrogen-vacancy centers in diamond crystals.

We investigate the effect of nitrogen-vacancy (NV) centers in single crystal diamond on nonlinear optical effects using 40 fs femtosecond laser pulses. The near-infrared femtosecond pulses allow us to study purely nonlinear optical effects, such as optical Kerr effect (OKE) and two-photon absorption (TPA), related to unique optical transitions by electronic structures with NV centers. It is found that both nonlinear optical effects are enhanced by the introduction of NV centers in the N + dose levels of 2.0×10 11 and 1.0×10 12 N +/cm 2. In particular, our data demonstrate that the OKE signal is strongly enhanced for the heavily implanted type-IIa diamond. We suggest that the strong enhancement of the OKE is possibly originated from cascading OKE, where the high-density NV centers effectively break the inversion symmetry near the surface region of diamond.

Tip-induced excitation of a single vortex in nano-size superconductors using scanning tunneling microscopy.

A single vortex is excited into nano-size Pb superconducting island structures by a local current injection from a probe tip of a scanning tunneling microscope. For the excitation, a sufficient amount and duration of the pulsed current are required. Injecting the current at peripheral sites is more effective than the center for the vortex excitation. Time-dependent Ginzburg-Landau calculations suggest that a current-induced normal-state area, which can be nucleated by the tunneling current exceeding the critical current and expanded by the Joule heating, reduces the required magnetic field for the vortex penetration and excites a vortex into the islands.

Superconducting Pb island nanostructures studied by scanning tunneling microscopy and spectroscopy.

Superconductivity of nanosized Pb-island structures whose radius is 0.8 to 2.5 times their coherence length was studied under magnetic fields using low-temperature scanning tunneling microscopy and spectroscopy. Spatial profiles of superconductivity were obtained by conductance measurements at zero-bias voltage. Critical magnetic fields for vortex penetration and expulsion and for superconductivity breaking were measured for each island. The critical fields depending on the lateral size of the islands and existence of the minimum lateral size for vortex formation were observed.

Atomically resolved imaging by low-temperature frequency-modulation atomic force microscopy using a quartz length-extension resonator.

Low-temperature ultrahigh vacuum frequency-modulation atomic force microscopy (AFM) was performed using a 1 MHz length-extension type of quartz resonator as a force sensor. Taking advantage of the high stiffness of the resonator, the AFM was operated with an oscillation amplitude smaller than 100 pm, which is favorable for high spatial resolution, without snapping an AFM tip onto a sample surface. Atomically resolved imaging of the adatom structure on the Si(111)-(7x7) surface was successfully obtained.

Adsorbate-induced alloy phase separation: a direct view by high-pressure scanning tunneling microscopy.

The influence of high pressures of carbon monoxide (CO) on the stability of a Au/Ni(111) surface alloy has been studied by high-pressure scanning tunneling microscopy. We show that CO induces a phase separation of the surface alloy at high pressures, and by means of time-lapsed STM movies we find that Ni atoms are removed from the surface layer during the process. Density functional theory calculations reveal the thermodynamic driving force for the phase separation to be the Au-induced compression of the CO overlayer with a resulting CO-CO repulsion. Furthermore, the atomistic mechanism of the process is shown to be kink-site carbonyl formation and evaporation which is found to be enhanced by the presence of Au.