Multiple electronic transitions and superconductivity in ${\text{Pd}}_x{\text{TiSe}}_2$

We report the synthesis and characterization of ${\text{Pd}}_{\text{x}}{\text{TiSe}}_2$ via magnetization, resistivity, specific heat measurements, and low-temperature electron diffraction. At very low levels of intercalation, near ${\text{Pd}}_{0.01}{\text{TiSe}}_2$, the commensurate charge density wave transition in the ${\text{TiSe}}_2$ parent compound near 220 K becomes substantially stronger; a distinct transition to a more resistive state is seen at about 80 K. Originally semimetallic, ${\text{TiSe}}_2$ becomes an insulator at low temperatures for $0.01\le x\le 0.06$ in ${\text{Pd}}_{\text{x}}{\text{TiSe}}_2$. Increasing Pd intercalation beyond this level begins to suppress the insulating behavior until a 2 K superconductor is found in the narrow composition range of $0.11\le x\le 0.12$. Beyond $x=0.12$, a normal metallic state (down to 0.3 K) is induced.

Figure 1

(Color online) (a) Lattice parameters $a$ (right axis) and $c$ (left axis) as a function of $x$ in ${\text{Pd}}_{\text{x}}{\text{TiSe}}_2$, with the crystal structure shown in the inset. (b) Powder x-ray diffraction data and refinement for ${\text{Pd}}_{0.18}{\text{TiSe}}_2$, confirming the 1T polytype of the Pd-intercalated ${\text{TiSe}}_2$ compounds.

Figure 2

(Color online) Temperature-dependent resistivity data (scaled at 300 K) for ${\text{Pd}}_{\text{x}}{\text{TiSe}}_2$ on a semilog plot, with the derivatives $d\rho /dT$ shown in the inset.

Figure 3

(Color online) Low-temperature $\rho \left(T\right)$ data for $0.10\le x\le 0.18$ for ${\text{Pd}}_{\text{x}}{\text{TiSe}}_2$, showing the superconducting transitions for $x=0.11$ and 0.12.

Figure 4

(Color online) ${\mu }_0\text{H}=0.5\text{T}$ temperature-dependent magnetic susceptibility data for ${\text{Pd}}_{\text{x}}{\text{TiSe}}_2$, $0\le x\le 0.06$, indicating a drop in susceptibility associated with the CDW transition. Vertical arrows mark the extreme $T_{\text{CDW}}$ values. Inset: magnetic susceptibility data for $0.08\le x\le 0.14$, where the CDW transition is no longer visible.

Figure 5

$T=11\text{K}$ electron diffraction patterns of the (a) ${\text{TiSe}}_2$ and (b) ${\text{Pd}}_{0.01}{\text{TiSe}}_2$ reciprocal lattices. The crystals are tilted away from the (001) zone axis, to show the superreflections in higher-order Laue zones. The diffraction pattern in (a) shows much weaker superlattice reflections compared to the analogous ones for the doped sample (b).

Figure 6

(Color online) $\rho \left(300\text{K}\right)/\rho \left(4\text{K}\right)$ (left axis) and electronic specific-heat coefficient $\gamma$ (right axis) as a function of $x$ in ${\text{Pd}}_{\text{x}}{\text{TiSe}}_2$. The line is a guide to the eye, emphasizing the increase in both quantities at intermediate compositions, and their nearly linear decrease beyond the superconducting state (for $x>0.12$).

Figure 7

(Color online) Electronic phase diagram for ${\text{Pd}}_{\text{x}}{\text{TiSe}}_2$. Brown and orange points are determined from resistivity and magnetization data respectively. Solid lines are shown for the high-$T$ CDW $\left({\text{CDW}}_{\text{H}}\right)$ and superconducting phase boundaries. The dotted lines for the second electronic transition within the CDW phase $\left({\text{CDW}}_{\text{L}}\right)$ are hypothetical phase boundaries where the transitions are not visible in the data. Inset: the crystal structure of ${\text{Pd}}_{\text{x}}{\text{TiSe}}_2$.