We report the discovery of a pressure-induced heavy fermion superconductivity in a nonmagnetic orbital ordering state in the cubic compound PrTi(2)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.

No abstract

No abstract

No abstract

We found a metamagnetic like anomaly at H m ' 5 kOe in a heavy fermion compound YbCo 2 Zn 20 below the characteristic temperature T max ¼ 0:32 K where the ac-susceptibility shows a broad peak, suggesting that an electronic state with a very low Kondo temperature is realized. Interestingly, the metamagnetic like behavior was observed as two peaks at 4.0 and 7.5 kOe at 95 mK in the magnetic field dependence of the electronic specific heat C=T. The extremely large values of the electronic specific heat coefficient ' 8000 mJ/(K 2 Ámol) and A ¼ 160 mÁcm/K 2 in the electrical resistivity Most of the cerium or uranium compounds order antiferromagnetically on the basis of the RKKY interaction. Some cerium or uranium compounds such as CeCu 6 , CeRu 2 Si 2 and UPt 3 , however, exhibit no long-range magnetic ordering.1) The magnetic susceptibility of these compounds increases with decreasing temperature, following the Curie-Weiss law at high temperatures, and shows a maximum at a characteristic temperature T max . Below T max , the susceptibility becomes almost temperature-independent, and the f-electron system is changed into a new electronic state, called the heavy fermion state. The almost localized f-electrons at high temperatures become itinerant at temperatures below T max via the many-body Kondo effect. The crossover from ''localized'' to ''itinerant'' occurs at a characteristic temperature T max , which approximately corresponds to the Kondo temperature T K .The conduction electrons in the heavy fermion state are the interacting electrons or quasiparticles corresponding to the Landau Fermi liquid. An enhanced Pauli susceptibility ðT ! 0Þ ¼ 0 correlates with a large electronic specific heat coefficient C=TðT ! 0Þ ¼ and a large coefficient ffiffiffi A p in the electrical resistivity ¼ 0 þ AT 2 , where 0 is the residual resistivity. Note that the value is roughly estimated as 10 4 =T K [mJ/(K 2 Ámol)] or R ln 2=T K . One of the characteristic properties in the heavy fermion compounds is the metamagnetic behavior or an abrupt nonlinear increase of magnetization at the magnetic field H m at temperatures lower than T max . The metamagnetic behavior appears at H m ¼ 77 kOe in CeRu 2 Si 2 2) and H m ¼ 200 kOe in UPt 3 , 3) for example. Very recently, we found the metamagnetic behavior at H m ' 100 kOe in YbIr 2 Zn 20 with the cubic crystal structure, 4) which is expected to occur even in Yb-based compounds on the basis of the hole-electron analogy between 4f 13 -Yb 3þ and 4f 1 -Ce 3þ electronic configuration. YbIr 2 Zn 20 is a new Yb-based heavy fermion compound with ¼ 540 mJ/(K 2 Ámol), 5) and in fact, the conduction electrons with large cyclotron effective masses of 10 { 30m 0 (m 0 : rest mass of an electron) are detected in the de Haas-van Alphen (dHvA) experiment.4) The corresponding magnetic susceptibility follows the Curie-Weiss law of Yb 3þ at high temperatures, but shows a broad peak at T max ¼ 7:4 K.We continued our experimental study for a similar compound of YbCo 2 Zn 20 . The following characteristic features w...