We studied an effect of pressure on the heavy fermion state of YbCo 2 Zn 20 under magnetic fields. The critical pressure P c , where the electronic state changes from the heavy fermion state to the antiferromagnetic state, is estimated to be P c ≃1.8 GPa. The super-heavy fermion state in the P c region is found to be strongly destroyed in magnetic fields for H ∥<100>. The field-induced quadrupolar phase, which is most likely realized only for H ∥<111>, is found to be robust even in the antiferromagnetic pressure region.
We succeeded in growing single crystals of LaNiC 2 with the non-centrosymmetric orthorhombic structure by the Czochralski method, and measured the electrical resistivity, de Haas–van Alphen effect, and specific heat to clarify the Fermi surface and superconducting properties. This compound has been studied from a viewpoint of the triplet superconducting pairing state. In the present experiment, we observed an ellipsoidal Fermi surface and a multiply-connected-pillar Fermi surface, which are split into two Fermi surfaces, reflecting the antisymmetric spin–orbit interaction based on the non-centrosymmetric crystal structure. The two ellipsoidal Fermi surfaces are split by 230 K, for example. The anisotropy of electrical resistivity and upper critical field H c2 in superconductivity are not large for three principal directions. From the low-temperature specific heat measurement, superconductivity in LaNiC 2 is explained by the framework of the BCS-superconductivity, contrary to the above arguments. The upper critical field H c2 (0), which was obtained from the specific heat under magnetic fields, is about 2 kOe.
We studied the heavy fermion compound YbCo2Zn20 with an electronic specific heat coefficient γ \( \simeq \) 8000 mJ/(K2·mol) by measuring the de Haas–van Alphen (dHvA) oscillation, Hall effect, magnetic susceptibility, and magnetization at ambient pressure, as well as the electrical resistivity in magnetic fields of up to 320 kOe and at pressures of up to 5 GPa. The detected Fermi surfaces are small in volume, reflecting the small Brillouin zone based on the large cubic lattice constant a = 14.005 Å. The cyclotron effective masses, which were determined from the dHvA experiment, are found to be markedly reduced in magnetic fields. In other words, the detected cyclotron masses of 2.2–8.9 m0 (m0: the rest mass of an electron) at Hav = 117 kOe are enhanced to 100–500 m0 at 0 kOe. By applying pressure, the heavy fermion state disappears at Pc \( \simeq \) 1.8 GPa and orders antiferromagnetically for P > Pc. The field-induced antiferroquadrupolar phase, which is observed only for \(H\parallel \langle 111\rangle \) in the magnetic field range from HQ = 60 kOe to \(H'_{\text{Q}} = 210\) kOe, is found to shift to lower magnetic fields and merge with theantiferromagnetic phase at 4.5 GPa.
We measured the de Haas–van Alphen (dHvA) oscillations, electrical resistivity, magnetic susceptibility, magnetization, and specific heat of PrIr 2 Zn 20 , which has recently been suggested as a heavy-fermion superconductor. Several dHvA branches with small frequencies ranging from 0.33×10 6 to 8.00×10 6 Oe were detected in the dHvA experiment, which were well explained by the results of energy band calculations for LaIr 2 Zn 20 using the lattice parameter of PrIr 2 Zn 20 . Moreover, the cyclotron effective mass is small, i.e., less than 1 m 0 ( m 0 : rest mass of an electron). This means that 4 f electrons are fully localized and do not contribute to the Fermi surface. Similar dHvA results were obtained for the reference compound LuIr 2 Zn 20 . Thus, superconductivity in PrIr 2 Zn 20 , which has been observed very recently below 0.05 K, is not due to heavy quasiparticles. In fact, the present superconductivity disappears at a magnetic field of 20 Oe. From the experimental results of magnetic susceptibility, magnetization, and specific heat, we estimated the 4 f crystalline electric field (CEF) scheme of PrIr 2 Zn 20 : Γ 3 (0 K)–Γ 4 (separated from a non-Kramers Γ 3 doublet by 36.0 K)–Γ 1 (86.4 K)–Γ 5 (103 K). On the basis of this CEF scheme, we discuss a broad Schottky peak in the magnetic specific heat at 0.4 K and the metamagnetic-like anomaly of magnetization at H m ≃50 kOe for H ∥<100>.
We measured the de Haas–van Alphen oscillation in divalent compounds YbTIn 5 (T: Co, Rh, Ir) and YbCoGa 5 . The Fermi surfaces, which are characteristic in two kinds of nearly cylindrical Fermi surface, are approximately unchanged in these compounds and are well explained by the results of energy band calculations. The cyclotron mass is found to increase remarkably as the average lattice constant decreases from 5.4 Å in YbIrIn 5 to 5.2 Å in YbCoGa 5 : m c * = 1.1 m 0 in YbIrIn 5 and 5.1 m 0 in YbCoGa 5 for dHvA branch β 1 , revealing a chemical pressure effect. There exists the hybridization between 4 f electrons and conduction electrons in YbCoGa 5 .
We studied the heavy fermion compound YbCo2Zn20 with an electronic specific heat coefficient γ \( \simeq \) 8000 mJ/(K2·mol) by measuring the de Haas–van Alphen (dHvA) oscillation, Hall effect, magnetic susceptibility, and magnetization at ambient pressure, as well as the electrical resistivity in magnetic fields of up to 320 kOe and at pressures of up to 5 GPa. The detected Fermi surfaces are small in volume, reflecting the small Brillouin zone based on the large cubic lattice constant a = 14.005 A. The cyclotron effective masses, which were determined from the dHvA experiment, are found to be markedly reduced in magnetic fields. In other words, the detected cyclotron masses of 2.2–8.9 m0 (m0: the rest mass of an electron) at Hav = 117 kOe are enhanced to 100–500 m0 at 0 kOe. By applying pressure, the heavy fermion state disappears at Pc \( \simeq \) 1.8 GPa and orders antiferromagnetically for P > Pc. The field-induced antiferroquadrupolar phase, which is observed only for \(H\parallel \langle 111\rangl...
