Anticorrelation between polar lattice instability and superconductivity in the Weyl semimetal candidate ${\mathrm{MoTe}}_2$

The relation between the polar structural instability and superconductivity in a Weyl semimetal candidate ${\mathrm{MoTe}}_2$ has been clarified by finely controlled physical and chemical pressure. The physical pressure as well as the chemical pressure, i.e., the Se substitution for Te, enhances the superconducting transition temperature $T_{\mathrm{c}}$ at around the critical pressure where the polar structure transition disappears. From the heat capacity and thermopower measurements, we ascribe the significant enhancement of $T_{\mathrm{c}}$ at the critical pressure to a subtle modification of the phonon dispersion or the semimetallic band structure upon the polar-to-nonpolar transition. On the other hand, the physical pressure, which strongly reduces the interlayer distance, is more effective on the suppression of the polar structural transition and the enhancement of $T_{\mathrm{c}}$ as compared with the chemical pressure, which emphasizes the importance of the interlayer coupling on the structural and superconducting instability in ${\mathrm{MoTe}}_2$.

Figure 1

(a), (b) The electrical resistivity for the single-crystalline ${\mathrm{MoTe}}_2$ at various pressures. Inset of Fig. 1(b) shows the electrical resistivity at pressures up to 0.46 GPa. The superconductivity at 0 GPa is suppressed by applying the magnetic field of 0.1 T as plotted by the gray line. (c) Phase diagram of the crystal structure and superconductivity. Red and blue circles represent the structural transition temperature $T_{\mathrm{s}}$ and superconducting transition temperature $T_{\mathrm{c}}$, respectively. Inset shows the crystal structure of the ${\mathrm{T}}_{\mathrm{d}}$ (polar) phase and the $1{\text{T}}^{\prime }$ (nonpolar) phase.

Figure 2

(a) The Seebeck coefficient and (b) resistivity for the polycrystalline samples of $\mathrm{Mo}{\left({\mathrm{Te}}_{1-x}{\mathrm{Se}}_x\right)}_2$ below 4 K. (c) Phase diagram of the structure and superconductivity. Structural transition temperature $T_{\mathrm{s}}$ (red circles) and superconducting transition temperature $T_{\mathrm{c}}$ (blue circles) are determined by the Seebeck coefficient and resistivity, respectively.

Figure 3

(a) Temperature dependence of upper critical field $H_{\mathrm{c}2}$ for $x=0.2$. The red curve is the best fit of the equation $H_{\mathrm{c}2}=H_{\mathrm{c}2}^{*}{(1-T/T_{\mathrm{c}})}^{1+\alpha }$ to the experimental data. The inset shows the temperature dependence of the resistivity under magnetic fields up to 7 T. (b) $T^2$ dependence of $C_{\mathrm{p}}/T$ of polycrystalline samples of $x=0$ (black circles) and $x=0.2$ (green circles). The red dotted line shows the fitting of the experimental data with the equation $C_{\mathrm{p}}/T=\gamma +\beta T^2$.

Figure 4

(a) Unit cell volume $V$ divided by number of atoms per unit cell $Z$, (b) $ab$ plane area ($ab$), and (c) interplane distance as a function of $x$ for $1{\text{T}}^{\prime }-\mathrm{Mo}{\left({\mathrm{Te}}_{1-x}{\mathrm{Se}}_x\right)}_2$ and physical pressure $P$ for $1{\text{T}}^{\prime }-{\mathrm{MoTe}}_2$. The lattice parameters as a function of $P$ are taken from Ref. [21].