Magnetic Monopole Noise

Magnetic monopoles are hypothetical elementary particles exhibiting quantized magnetic charge $m_0=\pm(h/\mu_0e)$ and quantized magnetic flux $\Phi_0=\pm h/e$. A classic proposal for detecting such magnetic charges is to measure the quantized jump in magnetic flux $\Phi$ 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 Dy$_2$Ti$_2$O$_7$, this SQUID technique was proposed for their direct detection. 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 have allowed the spectral density of magnetic-flux noise $S_{\Phi}(\omega,T)$ due to GR fluctuations of $\pm m_*$ 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. Here we report development of a high-sensitivity, SQUID based flux-noise spectrometer, and consequent measurements of the frequency and temperature dependence of $S_{\Phi}(\omega,T)$ for Dy$_2$Ti$_2$O$_7$ samples. Virtually all the elements of $S_{\Phi}(\omega,T)$ predicted for a magnetic monopole plasma, including the existence of intense magnetization noise and its characteristic frequency and temperature dependence, are detected directly. Moreover, comparisons of simulated and measured correlation functions $C_{\Phi}(t)$ of the magnetic-flux noise $\Phi(t)$ imply that the motion of magnetic charges is strongly correlated because traversal of the same trajectory by two magnetic charges of same sign is forbidden

Common glass-forming spin-liquid state in the pyrochlore magnets Dy2Ti2O7 and Ho2Ti2O7

Despite a well-ordered pyrochlore crystal structure and strong magnetic interactions between the Dy3+ or Ho3+ ions, no long-range magnetic order has been detected in the pyrochlore titanates Ho2Ti2O7 and Dy2Ti2O7. To explore the actual magnetic phase formed by cooling these materials, we measure their magnetization dynamics using toroidal, boundary-free magnetization transport techniques. We find that the dynamical magnetic susceptibility of both compounds has the same distinctive phenomenology, which is indistinguishable in form from that of the dielectric permittivity of dipolar glass-forming liquids. Moreover, Ho2Ti2O7 and Dy2Ti2O7 both exhibit microscopic magnetic relaxation times that increase along the super-Arrhenius trajectories analogous to those observed in glass-forming dipolar liquids. Thus, upon cooling below about 2 K, Dy2Ti2O7 and Ho2Ti2O7 both appear to enter the same magnetic state exhibiting the characteristics of a glass-forming spin liquid

Magnetic Monopole Noise

Magnetic monopoles are hypothetical elementary particles exhibiting quantized magnetic charge m_0 = +/-(h/mu_0 e)) and quantized magnetic flux Phi_0 = +/-h/e. In principle, such a magnetic charge can be detected by the quantized jump in magnetic flux Phi 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 Dy2Ti2O7, 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 _Phi(omega,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 _Phi(omega,T) for Dy2Ti2O7 samples. Virtually all the elements of S _Phi(omega,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_Phi(t) of the magnetic-flux noise Phi(t) imply that the motion of magnetic charges is strongly correlated