Ensembles of nitrogen-vacancy (NV) centers in diamonds are widely utilized for magnetometry, magnetic-field imaging and magnetic-resonance detection. At zero ambient field, Zeeman sublevels in the NV centers lose first-order sensitivity to magnetic fields as they are mixed due to crystal strain or electric fields. In this work, we realize a zero-field (ZF) magnetometer using polarization-selective microwave excitation in a 13 C-depleted crystal sample. We employ circularly polarized microwaves to address specific transitions in the optically detected magnetic resonance and perform magnetometry with a noise floor of 250 pT/ √ Hz. This technique opens the door to practical applications of NV sensors for ZF magnetic sensing, such as ZF nuclear magnetic resonance, and investigation of magnetic fields in biological systems.

Understanding the mechanisms behind high-Tc Type-II superconductors (SC) is still an open task in condensed ma er physics. One way to gain further insight into the microscopic mechanisms leading to superconductivity is to study the magnetic properties of the SC in detail, for example by studying the properties of vortices and their dynamics. In this work we describe a new method of wide-eld imaging magnetometry using nitrogen-vacancy (NV) centers in diamond to image vortices in an y rium barium copper oxide (YBCO) thin lm. We demonstrate quantitative determination of the magnetic eld strength of the vortex stray eld, the observation of vortex pa erns for di erent cooling elds and direct observation of vortex pinning in our disordered YBCO lm.is method opens prospects for imaging of the magnetic-stray elds of vortices at frequencies from DC to several megahertz within a wide range of temperatures which allows for the study of both high-TC and low-TC SCs. e wide temperature range allowed by NV center magnetometry also makes our approach applicable for the study of phenomena like island superconductivity at elevated temperatures (e.g. in metal nano-clusters[1]).

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