Probing Collective Magnetic Excitations in Magic-Angle Twisted Bilayer Graphene Correlated Insulating States with Electron Spin Resonance Spectroscopy

In MATBG, correlated insulating (CI) states emerge at an integer number of electrons per moiré unit cell. These states are suspected to emerge from successive filling of the fourfold degenerate spin and valley states of the nearly-flat miniband [1][2]. This would imply that different CI states in MATBG have different associated spin/valley flavor polarizations. Indeed evidence of an anomalous hall effect in MATBG, stemming from polarized orbital magnetic moments, has been observed at CI states associated with specific integer filling factors [3][4][5], albeit in only a handful of samples. However, the order in which the spin/valley flavor states are populated remains unknown, as does the nature of how electrons in these states couple to one another and to external magnetic fields. One potential technique for learning the flavor polarization properties of these CI states is to probe their excitations using resistively-detected electron spin resonance (ESR). We implement this by measuring near-DC transport through an encapsulated MATBG device, noting the change in resistivity when we couple in microwave magnetic fields from a microfabricated superconducting coplanar waveguide.<br/><br/>In general, ESR is a form of microwave spectroscopy: Microwaves are shone on the material, and when the energy of the microwaves exactly matches the energy cost to generate an excitation of a magnetic state, the material will absorb the microwaves. In resistively-detected ESR, this resonant microwave absorption is observed via a corresponding change in resistance, through heating of the electron system or some other mechanism. This measurement has been performed on several CVD graphene samples [6][7][8][9] as well as on MATBG with spin-orbit coupling induced by a proximal TMDC layer [10], but has yet to be seen in more canonical MATBG with clear CI states. Given the suspected spin/valley polarization of CI states and the ability of ESR to probe collective magnetic excitations, ESR offers the prospect of unambiguously establishing the flavor polarization of the CI states. Understanding the magnetic ground state of these CI states may also offer clues to the nature of superconducting pairing that occurs upon doping these flavor-polarized states.<br/><br/>This work is primarily supported by the U.S.Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, under contract DE-AC02-76SF00515.<br/><br/>[1] Zondiner, U., et al. <i>Nature</i> <b>582</b>, 203–208 (2020).<br/>[2] Park, J.M., et al. <i>Nature</i> <b>592</b>, 43–48 (2021).<br/>[3]Sharpe, A. L., et al. <i>Science</i> <b>365</b>, 605–608 (2019).<br/>[4] Serlin, M., et al. <i>Science</i> <b>367,</b> 900-903 (2020).<br/>[5] Stepanov, P., et al. <i>Phys. Rev. Lett.</i> <b>127</b>, 197701 (2021).<br/>[6] Sichau, J., et al. <i>Phys. Rev. Lett.</i> <b>122</b>, 046403 (2019).<br/>[7] Mani, R. G., et al. <i>Nature Communications</i> <b>3</b>, 996 (2012)<br/>[8] Singh, U. R., et al. <i>Physical Review B</i> 102.24 (2020): 245134.<br/>[9] Sharma, Chithra H., et al. <i>AIP Advances</i> 12.3 (2022): 035111.<br/>[10] Morissette, Erin, et al. <i>arXiv preprint arXiv:2206.08354</i> (2022).