Magnetic bubble chambers and sub-GeV dark matter direct detection

We propose a new application of single molecule magnet crystals: their use as “magnetic bubble chambers” for the direct detection of sub-GeV dark matter. The spins in these macroscopic crystals effectively act as independent nanoscale magnets. When antialigned with an external magnetic field they form metastable states with a relaxation time that can be very long at sufficiently low temperatures. The Zeeman energy stored in this system can be released through localized heating, caused for example by the scattering or absorption of dark matter, resulting in a spin avalanche (or “magnetic deflagration”) that amplifies the effects of the initial heat deposit, enabling detection. Much like the temperature and pressure in a conventional bubble chamber, the temperature and external magnetic field set the detection threshold for a single molecule magnet crystal. We discuss this detector concept for dark matter detection and propose ways to ameliorate backgrounds. If successfully developed, this detector concept can search for hidden photon dark matter in the meV–eV mass range with sensitivities exceeding current bounds by several orders of magnitude.

Figure 1

DM detector concept based on magnetic deflagration in molecular nanomagnet crystals. A DM event that deposits energy in the form of heat ignites a spin-flip avalanche in the crystal which is detected by the change in magnetic flux through a pick-up loop.

Figure 2

Left: Potential felt by individual molecular magnets in the crystal. Right: Lifting of degenerate ground states in an external magnetic field.

Figure 3

Typical behavior of magnetic relaxation time as a function of temperature for an SMM. The temperature dependence that dominates at high and intermediate temperatures is indicated. The low temperature behavior approaches the limiting quantum mechanical tunneling relaxation time, denoted ${\tau }_{\mathrm{QM}}$.

Figure 4

Molecular structures for [${\mathrm{Mn}}_6{\mathrm{O}}_2(\mathrm{H}\text{−}\mathrm{sao}{)}_6({\mathrm{O}}_2\mathrm{CPh}{)}_2(\mathrm{MeCN}{)}_2({\mathrm{H}}_2\mathrm{O}{)}_2$] (left) and [${\mathrm{Mn}}_6{\mathrm{O}}_2(\mathrm{Et}\text{−}\mathrm{sao}{)}_6({\mathrm{O}}_2\mathrm{CPh}({\mathrm{CH}}_3{)}_2{)}_2(\mathrm{EtOH}{)}_6$] (right). Green, red, blue, and grey spheres represent manganese, oxygen, nitrogen, and carbon, respectively. Hydrogen atoms have been omitted for clarity. In each structure the ${\mathrm{Mn}}_6$ unit has been highlighted. The tunable parts of the molecules have been circled with H-sao and Et-sao in purple circles, ${\mathrm{O}}_2\mathrm{CPh}$ and ${\mathrm{O}}_2\mathrm{CPh}({\mathrm{CH}}_3{)}_2$ in blue circles, and MeCN and EtOH in green circles.

Figure 5

Tuning curves for a SMM with $U=50\mathrm{K}$ ($\simeq 4.3\mathrm{meV}$), ${\tau }_0=5\times {10}^{-12}\mathrm{s}$, and for $\mathrm{\Delta }{E}_{\mathrm{Zee}}=(0,0.001,0.01,0.1)\mathrm{meV}$ (blue solid, green dot-dash, yellow dash, red dot).

Figure 6

Estimated sensitivity to absorption of dark vector DM in ${\mathrm{Mn}}_{12}$-acetate, assuming an aggressive sensitivity of $1\mathrm{event}/\mathrm{kg}\mathrm{year}$ (dashed), and a sensitivity of $1\mathrm{event}/\mathrm{kg}\mathrm{day}$ (dot-dashed). The absorption data from Refs. [62, 65] (described in the appendix) has been smoothed, an interpolation used in the region $m_V\sim 0.2–0.5\mathrm{eV}$, for which no data was available, and we use the approximation $\kappa \simeq {\kappa }_{\mathrm{eff}}$ (see text).

Figure 7

Transmittance data for a Mn12-acetate power, reproduced from Ref. [65].

Figure 8

Absorption spectrum for Mn12-acetate, reproduced from Ref. [62].