Magnetic monopole noise.

Abstract

Magnetic monopoles^1-3 are hypothetical elementary particles with quantized magnetic charge. In principle, a magnetic monopole can be detected by the quantized jump in magnetic flux that it generates upon passage through a superconducting quantum interference device (SQUID)⁴. Following the theoretical prediction that emergent magnetic monopoles should exist in several lanthanide pyrochlore magnetic insulators^5,6, including Dy₂Ti₂O₇, the SQUID technique has been proposed for their direct detection⁶. However, this approach has been hindered by the high number density and the generation-recombination fluctuations expected of such thermally generated monopoles. Recently, theoretical advances have enabled the prediction of the spectral density of magnetic-flux noise from monopole generation-recombination fluctuations in these materials^7,8. Here we report the development of a SQUID-based flux noise spectrometer and measurements of the frequency and temperature dependence of magnetic-flux noise generated by Dy₂Ti₂O₇ crystals. We detect almost all of the features of magnetic-flux noise predicted for magnetic monopole plasmas^7,8, including the existence of intense magnetization noise and its characteristic frequency and temperature dependence. Moreover, comparisons of simulated and measured correlation functions of the magnetic-flux noise indicate that the motions of magnetic charges are strongly correlated. Intriguingly, because the generation-recombination time constant for Dy₂Ti₂O₇ is in the millisecond range, magnetic monopole flux noise amplified by SQUID is audible to humans.