We report the discovery of pressure-induced superconductivity in ferromagnetic UIr, which lacks inversion symmetry in the crystal structure. The Curie temperature T C1 = 46 K at ambient pressure decreases with increasing pressure, reaching a value of 11 K at 1.5 GPa. It presumably decreases further up to about P c1 = 1.7 GPa. The ferromagnetic region named 'F1' exists up to P c1 . A second magnetic phase named 'F2' with a low ferromagnetic moment appears in the pressure range from 1.9 to 2.4 GPa. In the 'F2' phase, the magnetic transition temperature T C2 decreases with pressure, from 18 K at 1.9 GPa to approximately zero at P C2 = 2.6-2.7 GPa. In this critical pressure region, superconductivity appears below T sc = 0.14 K.

We have measured the electrical resistivity of a single crystal of YbCo 2 Zn 20 at pressures up to 2.37 GPa and at temperatures from 50 mK to 300 K. Above a critical pressure P c ($ 1 GPa), we have found a resistivity anomaly at T M ($ 0:15 K at 1 GPa) that increases with the pressure. At the ambient pressure, the system shows a nonmagnetic ground state described by the Fermi-liquid model. The T 2 coefficient of the electrical resistivity A strongly increases with the pressure upon approaching P c . However, in the vicinity of P c , the temperature dependence of the resistivity deviates from the Fermiliquid description. These observations suggest that the application of hydrostatic pressure induces a magnetically ordered state for P ! P c and T T M .In intermetallic compounds, including Ce and Yb, the hybridization between the 4f and itinerant conduction-band electrons induces the instability of magnetic moments and charge configurations. In recent years, one of the most interesting topics is the ground state properties of heavy fermion metals located at or close to a magnetic quantum critical point (QCP). 1) The application of external pressure is one of the important tools for controlling the electronic configurations as well as chemical pressure. In the case of heavy fermion Ce compounds exhibiting antiferromagnetic order, such as CeIn 3 and CePd 2 Si 2 , 2) the magnetic order is suppressed by applying pressure. Interestingly, unconventional superconductivity appears in the vicinity of the QCP at which the magnetic ordering temperature is decreased to zero.So far, pressure-induced magnetic transitions have been observed in some Yb-based compounds. [3][4][5][6][7] The key point here is that Yb ions fluctuate between the nonmagnetic Yb 2þ (J ¼ 0) and the magnetic Yb 3þ (J ¼ 7=2) states. Since the ionic volume of the magnetic Yb 3þ state is smaller than that of the nonmagnetic Yb 2þ one, applying pressure stabilizes the magnetic Yb 3þ configuration and induces the appearance of a magnetically ordered state in contrast to the Ce case. However, the critical pressure for these compounds is as high as or higher than 6 GPa, which prevents us from understanding the physics in the vicinity of the magnetic QCP because of difficulties in high-pressure experiments.The series of compounds YbT 2 Zn 20 (T = Fe, Co, Ru, Rh, Or, Ir) belongs to a new heavy fermion system crystallizing in the cubic CeCr 2 Al 20 structure. 8,9) All these compounds show an enhanced Sommerfeld coefficient of the specific heat exceeding 400 mJ/(mol K 2 ). The high-temperature magnetic susceptibility of these compounds follows the Curie-Weiss law with the effective moments close to the value for the free Yb 3þ ion ( eff ¼ 4:54 B ), although there is no indication of magnetic order down to 20 mK. In the YbT 2 Zn 20 family, YbCo 2 Zn 20 exhibits some notable features as follows. The low temperature electrical resistivity and specific heat can be described by the formulas ¼ 0 þ AT 2 and C=T ¼ constant, as expected from the Fermiliquid behavior; the value...

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