Searching for time-varying nuclear electric dipole moments using precision magnetic resonance*

The Cosmic Axion Spin Precession Experiments (CASPEr) search for ultralight axion-like dark matter. CASPEr-e is sensitive to the time-varying nuclear electric dipole moment, induced by the electric-dipole moment (EDM) coupling g_d. The detection scheme is based on a precision measurement of ²⁰⁷Pb solid-state nuclear magnetic resonance in a polarized ferroelectric crystal. We calibrated the detector and characterized the excitation spectrum and relaxation parameters of the nuclear spin ensemble with pulsed magnetic resonance measurements in a 4.4 T magnetic field. We swept the magnetic field near this value and searched for axion-like dark matter with Compton frequency within a 1 MHz band centered at 39.65 MHz. Our measurements place the upper bound g_d < 9.5 × 10^-4 GeV^-2 (95% confidence level) in this frequency range. This constraint corresponds to an upper bound of 1.0 × 10^-21 e·cm on the amplitude of oscillations of the neutron electric dipole moment, and 4.3 × 10^-6 on the amplitude of oscillations of CP-violating θ-parameter of quantum chromodynamics. Our results demonstrate the feasibility of using solid-state nuclear magnetic resonance to search for axion-like dark matter in the nano-electronvolt mass range.