Magnetic Monopole Noise

Prof J.C. Seamus Davis, CMP Oxford

Magnetic monopoles are hypothetical elementary particles exhibiting quantized magnetic charge m_0=±(h⁄(μ_0 e)) and quantized magnetic flux Φ_0=±h/e. A classic proposal for detecting such magnetic charges is to measure the quantized jump in magnetic flux Φ threading the loop of a superconducting quantum interference device (SQUID) when a monopole passes through it. Naturally, with the theoretical discovery that a plasma of emergent magnetic charges should exist in several lanthanide-pyrochlore magnetic insulators including Dy2Ti2O7, this SQUID technique was proposed for their direct detection (Nature 451, 42 (2008)). Experimentally, this has proven extremely challenging because of the high number density, and the generation-recombination (GR) fluctuations, of the monopole plasma. Recently, however, theoretical advances at Oxford have allowed the¬ spectral density of magnetic-flux noise S_Φ (ω,T) due to GR fluctuations of ±m_* magnetic charge pairs to be determined. These theories present a sequence of strikingly clear predictions for the magnetic-flux noise signature of emergent magnetic monopoles. In response, we developed a high-sensitivity, SQUID based flux-noise spectrometer, and used this new instrument to measure the frequency and temperature dependence of S_Φ (ω,T) for Dy2Ti2O7 samples. Virtually all the elements of S_Φ (ω,T) predicted for a magnetic monopole plasma, including the existence of intense magnetization noise and its characteristic frequency and temperature dependence, are detected directly. Moreover, comparison of simulated and measured correlation functions C_Φ (t) of the magnetic-flux noise Φ(t) reveals that the motion of magnetic charges is strongly correlated. A final striking observation is that, since the monopole GR time constants τ(T) are in the millisecond range for Dy2Ti2O7, magnetic monopole flux noise amplified by the SQUID is audible to human perception.