Electronic Structures of $R{\mathrm{T}\mathrm{e}}_2$ ($R=\mathrm{L}\mathrm{a},\mathrm{C}\mathrm{e}$): A Clue to the Pressure-Induced Superconductivity in ${\mathrm{C}\mathrm{e}\mathrm{T}\mathrm{e}}_{1.82}$

Electronic structures of $R{\mathrm{T}\mathrm{e}}_2$ ($R=\mathrm{L}\mathrm{a},\mathrm{C}\mathrm{e}$) have been investigated by using the local spin density approximation (LSDA) and the $\mathrm{L}\mathrm{S}\mathrm{D}\mathrm{A}+U$ ($U$: on-site Coulomb interaction) band methods. Both ${\mathrm{L}\mathrm{a}\mathrm{T}\mathrm{e}}_2$ and ${\mathrm{C}\mathrm{e}\mathrm{T}\mathrm{e}}_2$ show the very similar Fermi surface nesting features along the [100] direction, which drive the charge-density wave (CDW) instability in the Te(1) sheets. The contribution near $E_F$ from Ce $4f$ states is negligible in agreement with the measured ARPES spectra. In the semimetallic CDW-distorted $R{\mathrm{T}\mathrm{e}}_2$, both Te vacancy and pressure induce the charge transfer from Te(1) $5p$ to $R$ $5d$ states, producing the enhanced density of states at $E_F$. We suggest that these increased self-doped Te(1) $5p$ hole carriers are responsible for the pressure-induced superconductivity in nonstoichiometric ${\mathrm{C}\mathrm{e}\mathrm{T}\mathrm{e}}_{1.82}$.

Figure 1

The LSDA band structure of ${\mathrm{L}\mathrm{a}\mathrm{T}\mathrm{e}}_2$ (top) and the $\mathrm{L}\mathrm{S}\mathrm{D}\mathrm{A}+U$ band structure of ${\mathrm{C}\mathrm{e}\mathrm{T}\mathrm{e}}_2$ (bottom). $\Gamma ,X,Z$, and $M$ represent the (0,0,0), ($1/2,0,0$), ($0,0,1/2$), and ($1/2,1/2,0$) $\mathbf{k}$ points in the simple tetragonal Brillouin zone. The size of the circle represents the contribution of the Te(1) $5p$ states in the wave function.

Figure 2

Fermi surfaces of ${\mathrm{L}\mathrm{a}\mathrm{T}\mathrm{e}}_2$ (top left) and ${\mathrm{C}\mathrm{e}\mathrm{T}\mathrm{e}}_2$ (top right). The arrows correspond to the nesting vectors along the [100] and [110] directions. Bottom: Calculated susceptibilities for ${\mathrm{L}\mathrm{a}\mathrm{T}\mathrm{e}}_2$ and ${\mathrm{C}\mathrm{e}\mathrm{T}\mathrm{e}}_2$. ${\chi }_0(\mathbf{0})$ is normalized to DOS at $E_F$ in each case.

Figure 3

Top: $\mathrm{L}\mathrm{S}\mathrm{D}\mathrm{A}+U$ band structure of ${\mathrm{C}\mathrm{e}\mathrm{T}\mathrm{e}}_2$ with the Ce $4f$ contribution larger than $18\%$. Bottom: The extracted Ce $4f$ ARPES spectra of ${\mathrm{C}\mathrm{e}\mathrm{T}\mathrm{e}}_2$. These spectra were extracted from the Ce $4d\to 4f$ fine-structure resonance (Ref. 16).

Figure 4

The LSDA DOSs of tetragonal ${\mathrm{L}\mathrm{a}\mathrm{T}\mathrm{e}}_2$ (nondistorted) (solid), CDW-distorted ${\mathrm{L}\mathrm{a}\mathrm{T}\mathrm{e}}_2$ with the original (dot-dashed) and the reduced (double dot-dashed) volume, and CDW-distorted ${\mathrm{L}\mathrm{a}\mathrm{T}\mathrm{e}}_{1.75}$ with the original (dashed) and the reduced (dotted) volume, respectively. Left inset: Fermi surface of the CDW-distorted ${\mathrm{L}\mathrm{a}\mathrm{T}\mathrm{e}}_2$ with the original volume. $\Gamma ,Z,C$, and $Y$ represent (0,0,0), ($0,0,1/2$), ($0,1/2,1/2$), and ($0,1/2,0$) $\mathbf{k}$ points in the monoclinic Brillouin zone. Right inset: schematic diagram of the charge transfer from Te(1) $5p$ to $R$ $5d$ states induced by the pressure.