Abstract
Magnetic monopoles are hypothetical elementary particles exhibiting quantized
magnetic charge m 0 = ±(h/(μ 0 e)) and quantized magnetic flux Φ 0 = ±h/e.
In principle, such a magnetic charge can be detected by the quantized jump in
magnetic flux Φ it generates upon passage through a superconducting quantum
interference device (SQUID). Naturally, with the theoretical discovery that a
plasma of emergent magnetic charges should exist in several lanthanide-pyrochlore
magnetic insulators, including Dy 2 Ti 2 O 7 , this SQUID technique was proposed
for their direct detection. Experimentally, this has proven challenging because of
the high number density of the monopole plasma. Recently, however, theoretical
advances have allowed the spectral density of magnetic-flux noise S Φ (ω, T ) due
to generation recombination fluctuations of ±m ∗ magnetic charge pairs to be
predicted. Here we report development of a SQUID based flux-noise spectrometer
and consequent measurements of the frequency and temperature dependence of
S Φ (ω, T ) for Dy 2 Ti 2 O 7 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.
Moreover, comparisons of simulated and measured correlation functions C Φ (t) of
the magnetic-flux noise Φ(t) imply that the motion of magnetic charges is strongly
correlated.
ix
Magnetic monopoles are hypothetical elementary particles exhibiting quantized
magnetic charge m 0 = ±(h/(μ 0 e)) and quantized magnetic flux Φ 0 = ±h/e.
In principle, such a magnetic charge can be detected by the quantized jump in
magnetic flux Φ it generates upon passage through a superconducting quantum
interference device (SQUID). Naturally, with the theoretical discovery that a
plasma of emergent magnetic charges should exist in several lanthanide-pyrochlore
magnetic insulators, including Dy 2 Ti 2 O 7 , this SQUID technique was proposed
for their direct detection. Experimentally, this has proven challenging because of
the high number density of the monopole plasma. Recently, however, theoretical
advances have allowed the spectral density of magnetic-flux noise S Φ (ω, T ) due
to generation recombination fluctuations of ±m ∗ magnetic charge pairs to be
predicted. Here we report development of a SQUID based flux-noise spectrometer
and consequent measurements of the frequency and temperature dependence of
S Φ (ω, T ) for Dy 2 Ti 2 O 7 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.
Moreover, comparisons of simulated and measured correlation functions C Φ (t) of
the magnetic-flux noise Φ(t) imply that the motion of magnetic charges is strongly
correlated.
ix