Quantum Spin & Monopole Fluids
In collaboration with Prof. Stephen Blundell - Oxford
Magnetic monopoles are hypothetical elementary particles exhibiting quantized magnetic charge m0=Â±hâ„(μ0e) and quantized magnetic flux Φ0=Â±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 fluid of emergent magnetic charges should exist in several lanthanide-pyrochlore magnetic insulators including Dy2Ti2O7, this SQUID technique was proposed for their direct detection (Castelnovo et. al. Nature 451, 42 (2008)). Experimentally, this has proven extremely challenging because of the high number density, and generation-recombination (GR) fluctuations, of the monopole plasma. Recently, however, theoretical advances by Prof. S. Blundell of Oxford University have allowed the spectral density of spin-noise SΦ(ω,T) due to GR fluctuations of Â±m* magnetic charge pairs to be determined.
Fig. 2A The sequence of Dy spin flips in Dy2Ti2O7 that generate two magnetic monopoles of opposite charge propagating through the material. B) When these monopoles are created within the input coil of a DC SQUID and depart to infinity in opposite directions, the flux through the SQUID jumps by Φ =m*μ0.
In 2018 we developed a high-sensitivity, SQUID based spin-noise spectrometer, and measured the frequency and temperature dependence of SΦ(ω,T) for Dy2Ti2O7 samples. Virtually all the elements of SΦ(ω,T) predicted for a magnetic monopole fluid, including the existence of intense magnetization noise and its characteristic frequency and temperature dependence, are detected. This provides the first direct access to the microscopic physics a monopole fluid.
High precision measurement of the spin-noise spectrum is an innovative approach to magnetic quantum fluids. It opens a wide variety of new research avenues including the following projects of immediate interest:
a) Ho2Ti2O7 is a pyrochlore magnetic insulator with many similar characteristics to Dy2Ti2O7 and it is widely believing to also contain a fluid of emergent magnetic monopoles. We plan to use our spin-noise spectroscopy (SNS) technique to search for the flux noise SΦ(ω,T) signature of magnetic monopole fluid in Ho2Ti2O7.
b) Based on our measurements, we estimate that the flux jump of individual magnetic monopoles in Dy2Ti2O7 and Ho2Ti2O7 should be detectable in sub-micron scale samples and at mK temperatures. We plan to develop a millikelvin SNS instrument for this purpose, and to search for individual magnetic monopoles in these and other compounds.
c) Eventually, out SNS approach will be generalized into a visualization technique in the form of a Scanned Spin-Noise Microscope (SSNM). We plan development of this new instrument as part of the suite of new quantum microscopes at our Beecroft Building laboratories.