Enhanced thermopower and large magneto-resistance in compensated electron-hole bands of twisted double bilayer graphene

Finding materials with large thermopower is crucial for the development of novel

electric generators and solid-state cooling devices. Recent advances showed semimetals to

be a viable platform in this regard. Low-magnetic field-induced enhancement of thermopower has

been theoretically predicted in compensated semimetals. Thus, exploring materials with semi-

metallic traits has gained great interest. Here we report strong evidence of semi-

metallic low energy band in twisted double bilayer graphene (TDBLG) with twist angle 1.2^o. We have

carried out a thorough temperature and magnetic field dependent electrical and thermopower

study. An enhancement of two orders of magnitude of thermopower, along with large

magneto-resistance (~2500%), is observed near the charge neutrality point (CNP). Both

thermopower and magnetoresistance saturates within magnetic fields (<1T) much smaller than the

extreme quantum limit. These experimental findings indicate the nearly compensated semimetallic

nature of TDBLG at the CNP, and qualitatively understood by a two-band model. Furthermore,

we observe an unusual sub-linear temperature scaling of resistance at lower temperature (<10K)

near the theoretically predicted region of overlap of the low-energy bands, indicative of a possible

excitonic bound state in TDBLG.