Electron spin resonance in silicon MOS structures down to 0.36 K

While transport of 2-dimensional (2D) electrons has been routinely measured down to few mK, performing electron spin resonance (ESR) at low temperatures is challenging. We report measurements of the paramagnetic susceptibility of 2D electrons in a silicon metal-oxide-semiconductor (MOS) structure using ESR at 0.34 T for temperatures down to 0.36 K. When the MOS gate is biased below the threshold voltage, we measure electrons weakly confined below the conduction band edge and find that the susceptibility follows a Curie-like 1/T temperature dependence. The Curie susceptibility of confined electrons suggests that they are independent electrons confined by disorder at the Si-SiO$_2$ interface. At gate voltages above threshold the signal arises from 2D conduction electrons, whose susceptibility is expected to follow a simple Pauli temperature dependence, i.e., constant at low temperatures. Surprisingly, at an electron density of $2.8 \times 10^{11}$ cm$^{-2}$ (Fermi temperature = 20 K), as the temperature is reduced from 4.2 K to 0.36 K, the susceptibility actually drops by a factor of 2. Furthermore, this effect becomes more pronounced at higher 2D electron densities. The drop in susceptibility for 2D conduction electrons at low temperature is unexpected and remains to be explained.