Sensitive J-coupling spectroscopy using high-T_c superconducting quantum interference devices in magnetic fields as low as microteslas

In this work we present a sensitive J-coupling spectroscopy in one shot using high-T_c superconducting quantum interference devices and a flux transformer in microtesla fields. In the proposed NMR detection scheme the precession of proton spin was inductively coupled to the SQUID magnetometer. We enhanced the SNR signal and the spectral resolution significantly by applying a pre-polarization field of 0.045T. In microtesla fields where chemical shifts are absent, we demonstrate proton-phosphate coupling J₃[H,P] = (10.94 0.08)Hz in trimethyl phosphate. The sensitive NMR spectrometer will be of great interest for diagnosis information of molecular structure and biological applications.