Pressure-Induced Heavy Fermion Superconductivity in the Nonmagnetic Quadrupolar System ${\mathrm{PrTi}}_2{\mathrm{Al}}_{20}$

We report the discovery of a pressure-induced heavy fermion superconductivity in a nonmagnetic orbital ordering state in the cubic compound ${\mathrm{PrTi}}_2{\mathrm{Al}}_{20}$. In particular, we found that the transition temperature and the effective mass associated with the superconductivity are dramatically enhanced as the system approaches the putative quantum critical point of the orbital order. Our experiment indicates that the strong orbital fluctuations may provide a nonmagnetic glue for Cooper pairing.

Figure 1

(a) Magnetic part of the resistivity, ${\rho }_{\mathrm{mag}}$, versus the logarithm of the temperature under various pressures. Here, ${\rho }_{\mathrm{mag}}$ is obtained by subtracting the resistivity of ${\mathrm{LaTi}}_2{\mathrm{Al}}_{20}$ obtained under ambient pressure from that of ${\mathrm{PrTi}}_2{\mathrm{Al}}_{20}$. (b) Temperature dependence of the resistivity and the ac magnetic susceptibility at low temperatures. A large diamagnetic signal due to the SC transition is observed at a lower temperature, which corresponds to nearly 60% superconducting shielding, estimated by comparing to the diamagnetic signal of lead with almost the same size as the sample of ${\mathrm{PrTi}}_2{\mathrm{Al}}_{20}$. (c) Temperature dependence of the ac specific heat divided by temperature for different pressures. The curves are shifted vertically for clarity.

Figure 2

(a) Temperature dependence of the electrical resistivity $\rho (T)$ of ${\mathrm{PrTi}}_2{\mathrm{Al}}_{20}$ at 8.7 GPa under various magnetic fields. (b) Superconducting phase diagram, i.e., the critical magnetic field $B_{c2}$ as a function of temperature, derived from the zero resistivity temperature. (c) $\rho (T)$ versus $T^3$ at various pressures. (d) Pressure dependence of the residual resistivity ${\rho }_0$ and the effective mass $m^{*}$ estimated by using the slope of the critical field curve, assuming a spherical Fermi surface.

Figure 3

Open circles and squares represent the position of $T_{\max }$ determined from the maximum in the temperature dependence of the resistivity and the ferroquadrupole ordering temperature $T_Q$, respectively. The SC transition temperatures $T_{\mathrm{SC}}$ are deduced from the temperature dependence of the resistivity (closed circles), the ac magnetic susceptibility (closed triangles), and the ac specific heat (closed squares), respectively. The inset shows the cubic crystal structure of ${\mathrm{PrTi}}_2{\mathrm{Al}}_{20}$. Cages made by ${\mathrm{PrAl}}_{16}$ and ${\mathrm{TiAl}}_{12}$ are indicated in green (larger cages) and purple (smaller cages), respectively.