Giant Nernst effect and field-enhanced transversal $z_N\mathrm{T}$ in ${\mathrm{ZrTe}}_5$

Thermoelectric materials can recover electrical energy from waste heat and vice versa, which are of great significance in green energy harvesting and solid state refrigerators. The thermoelectric figure of merit ($z\mathrm{T}$) quantifies the energy conversion efficiency, and a large Seebeck or Nernst effect is crucial for the development of thermoelectric devices. Here we present a significantly large Nernst thermopower in topological semimetal ${\mathrm{ZrTe}}_5$, which is attributed to both strong Berry curvature and bipolar transport. The largest in-plane $S_{xy}$ (when B//b) approaches 1900 $\mu \mathrm{V}/\mathrm{K}$ at $T=100$ K and B = 13 T, and the out-of-plane $S_{xz}$ (when B//c) reaches 5000 $\mu \mathrm{V}/\mathrm{K}$. As a critical part of $z_N\mathrm{T}$, the linearly increased in-plane $S_{xy}$ and resistivity ${\rho }_{\mathrm{yy}}$ in regard to B induces an almost linear increasing transversal $z_N\mathrm{T}$ without saturation under high fields. The maximum $z_N\mathrm{T}$ of 0.12 was obtained at B = 13 T and $T=120$ K, which significantly surmounts its longitudinal counterpart under the same condition.

Figure 1

(a) Crystal structure of ZrTe5. (b) Temperature dependence of the Seebeck $S_{xx}$ (black curve) and resistivity ${\rho }_{xx}$ (blue curve) under zero field. (c) $S_{xx}$ and ${\rho }_{xx}$ as a function of field $B$ taken at 2 K; both show clear quantum oscillations. The red dashed line marks the $N=1$ Landau level, above which ${\rho }_{xx}$ decreases sharply, then enters the quantum limit above 1.5 T. (d) Landau fan diagram of the Landau level index $N$ versus 1/$B$ at 3.2 K. Black and red dot indicate the peak and valley positions, respectively.

Figure 2

(a),(b) The in-plane Nernst thermopower $S_{xy}$ as a function of $\mathit{B}$ under different temperatures; the field $\mathit{B}$ is parallel to the $\mathit{b}$ axis. The inset shows the schematic thermoelectric measurements setup of $S_{xy}$. (c),(d) The out-of-plane Nernst $S_{xz}$ as a function of field $\mathit{B}$ under different temperatures, where the field is parallel to the $\mathit{c}$ axis. The inset shows the schematic thermoelectric measurements setup of $S_{xz}$. (e) Fit of the anomalous Nernst signals of $S_{xy}$ from −2 to 2 T under selected temperatures using Eq. (1) (f) The fitted anomalous Nernst coefficient $S^A/T$ as a function of temperature $T$. Black and red lines indicate the in-plane $S_{xy}^A/T$ and out-of-plane $S_{xz}^A/T$, respectively. The dashed line indicates the resistivity peak temperature 90 K.

Figure 3

(a) The magnetoresistivity ${\rho }_{yy}$ at different temperature $T$. (b) The thermal conductivity ${\kappa }_{xx}$ taken from 80 to 270 K. As temperature rises, the ${\kappa }_{xx}$ decreases monotonically. (c) The field dependence of power factor $\mathrm{p}{\mathrm{F}}_{\mathrm{tr}}$. (d) The transversal $z_N\mathrm{T}$ as a function of field $\mathit{B}$. Generally, high field could enhance the $z_N\mathrm{T}$, the greatest enhancement occurs at 120 K. Inset: plot the $z_N\mathrm{T}$ as a function of temperature $T$ under fixed magnetic field ranging from 0 to 13 T.