Gap-Opening Transition In Dirac Semimetal Zrte5

We apply Te-125 nuclear magnetic resonance (NMR) spectroscopy to investigate the Dirac semimetal ZrTe5. With the NMR magnetic field parallel to the b axis, we observe significant quantum magnetic effects. These include an abrupt drop at 150 K in spin-lattice relaxation rate. This corresponds to a gapopening transition in the Dirac carriers, likely indicating the onset of excitonic pairing. Below 50 K, we see a more negative shift for the Te-z bridging site, indicating the repopulation of Dirac levels with spin polarized carriers at these temperatures. This is the previously reported 3D quantum Hall regime; however, we see no sign of a charge density wave as has been proposed.