Gap-Opening Transition in Dirac Semimetal ${\mathrm{ZrTe}}_5$

We apply ${}^{125}\mathrm{Te}$ nuclear magnetic resonance (NMR) spectroscopy to investigate the Dirac semimetal ${\mathrm{ZrTe}}_5$. 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 gap-opening transition in the Dirac carriers, likely indicating the onset of excitonic pairing. Below 50 K, we see a more negative shift for the ${\mathrm{Te}}_z$ 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.

Figure 1

(a) Layered structure of ${\mathrm{ZrTe}}_5$. (b) View along $a$ with ${\mathrm{Te}}_d$, ${\mathrm{Te}}_z$, and ${\mathrm{Te}}_a$ labeled, showing the NMR field ($H_0$) along $b$.

Figure 2

(a) ${}^{125}\mathrm{Te}$ line shapes at 290 and 30 K labeled for 3 Te sites. (b) Angular dependence at 290 K. (${\mathrm{Te}}_a$ not mapped at all angles due to low signal strength for this transition). Solid curves: fits to $K\propto A+B{\cos }^2\theta$. (c) $H_0\parallel b$ shift vs $T$.

Figure 3

$1/T_{1}T$ vs $T$ in (a) linear and (b) log scale. (c) Comparison of $H_0\parallel b$ to $H_0\parallel a$ data from Ref. [10]. Solid curves: ${\mu }^2\ln \mu$ fits described in text. See also Supplemental Material [33] for an expanded low-$T$ view.

Figure 4

(a)–(c) Numerical results vs $T$ for continuous Dirac (DC) and quasi-2D Landau level (LL) models in text: (a) Carrier concentrations. (b) Chemical potential. (c) Knight shift. (d) Sketch of Landau levels and low-$T$ spins. (e) Condensation process with CB pocket participation.