  Electron spin resonance (ESR) spectroscopy has broad applications in physics,
chemistry and biology. As a complementary tool, zero-field ESR (ZF-ESR)
spectroscopy has been proposed for decades and shown its own benefits for
investigating the electron fine and hyperfine interaction. However, the ZF-ESR
method has been rarely used due to the low sensitivity and the requirement of
much larger samples than conventional ESR. In this work, we present a method
for deploying ZF-ESR spectroscopy at the nanoscale by using a highly sensitive
quantum sensor, the nitrogen-vacancy center in diamond. We also measure the
nanoscale ZF-ESR spectrum of a few P1 centers in diamond, and show that the
hyperfine coupling constant can be directly extracted from the spectrum. This
method opens the door to practical applications of ZF-ESR spectroscopy, such as
investigation of the structure and polarity information in spin-modified
organic and biological systems.
