High-Performance Low-Field NMR Utilizing a High- $T_{\rm c}$ rf SQUID

High-resolution low-field nuclear magnetic resonance (NMR) signals of selected liquid samples were recorded using a nitrogen-cooled superconducting quantum interference device (SQUID). The NMR measurements were performed at Larmor frequencies (f _L) from 2 Hz to 40 kHz. The natural spectral linewidth of tap water could be measured in magnetic fields below 7 microtesla. To demonstrate the measurement sensitivity and resolution, J-coupling spectra of 2,2,2-trifluoroethanol were recorded at different measurement fields, with signals separated by several hundreds of Hertz. An additional nitrogen-cooled tuned LC-circuit and a signal recovery procedure involving a pi/2 AC pulse were applied in the higher f _L region ( >10 kHz) to enhance the signal-to-noise ratio up to one order of magnitude.