Masahiro Ohya

Heavy Fermion State in YbIr2Zn20

We measured the high-field magnetization and de Haas–van Alphen (dHvA) oscillations for YbIr 2 Zn 20 with a cubic crystal structure, together with the electrical resistivity and magnetic susceptibility. The magnetic susceptibility χ with an effective magnetic moment of Yb 3+ becomes temperature-independent at low temperatures, with a broad peak at T χmax =7.4 K for H ∥ . The corresponding magnetization indicates a metamagnetic transition at H m =120 kOe, consistent with a T χmax vs H m relation in the Ce- and U-based heavy fermion compounds. The large cyclotron masses of 4–27 m 0 are detected in the dHvA experiment, and are found to be reduced at magnetic fields higher than H m =120 kOe. The resistivity follows the Fermi liquid relation ρ=ρ 0 + A T 2 under magnetic field, and the \(\sqrt{A}\) value is also found to have a maximum at H m as a function of magnetic field. From the present experimental results, together with the results of 4 f -itinerant energy band calculations, the 4 f electrons are fou...

Metamagnetic Behavior in Heavy-Fermion Compound YbIr2Zn20

The metamagnetic behavior in the heavy-fermion compound YbIr 2 Zn 20 with a cubic structure was studied by magnetization, de Haas–van Alphen (dHvA) oscillation, magnetoresistance, specific heat, thermal expansion, and magnetostriction measurements in single-crystalline samples. A metamagnetic anomaly at H m ≃100 kOe was found to exist in all field directions in the cubic crystal structure. The cyclotron effective mass m c * , the coefficient A in the Fermi liquid relation of electrical resistivity ρ=ρ 0 + A T 2 , and the electronic specific heat coefficient C / T indicate a peak at the metamagnetic field H m . The negative thermal expansion and negative volume magnetostriction suggest that the valence of the Yb ion gradually changes from a 4 f -itinerant state to a trivalent state with increasing magnetic field, and an almost trivalent state is realized above the metamagnetic field H m . On the other hand, the topology of the Fermi surface is approximately unchanged below and above H m in the dHvA experiment.

Strong Field Quenching of the Quasiparticle Effective Mass in Heavy Fermion Compound YbCo2Zn20

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 χ max =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 γ≃8000 mJ/(K 2 ·mol) and A =160 µΩ·cm/K 2 in the electrical resistivity ρ=ρ 0 + A T 2 at H =0 kOe are most likely due to the very low Kondo temperature. The \(\sqrt{A}\) value was, however, found to be strongly reduced from \(\sqrt{A}=12.6\) (µΩ·cm/K 2 ) 1/2 at 0 kOe to 0.145 (µΩ·cm/K 2 ) 1/2 at 150 kOe. Therefore, we considered that the corresponding cyclotron effective mass m c * , which was determined from the temperature dependence of the de Haas–van Alphen (dHvA) amplitude, is also reduced ...

Fermi Surface Property and Characteristic Crystalline Electric Field Effect in PrIr2Zn20

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 he...

Low-Temperature Magnetic Orderings and Fermi Surface Properties of LaCd11, CeCd11, and PrCd11 with a Caged Crystal Structure

We succeeded in growing single crystals of cage-structure compounds RCd11 (R: La, Ce, and Pr) and precisely studied their low-temperature magnetic and electronic properties by measuring electrical resistivity, magnetic susceptibility, magnetization, specific heat, and the de Haas–van Alphen (dHvA) effect. We found antiferromagnetic ordering at 0.44 and 0.39 K in CeCd11 and PrCd11, respectively, and clarified the magnetic phase diagrams of the compounds. In addition, low-lying crystalline electric field (CEF) schemes were proposed from the specific heat results of both compounds. From the present study, the antiferromagnetic ordering in PrCd11 is found to be of the exchange-induced type with a singlet ground state. From the dHvA experiment, we detected small dHvA branches ranging from 7 � 10 5 to 2 � 10 7 Oe, which correspond to small Fermi surfaces. This is mainly due to a small Brillouin zone based on a large unit cell. Moreover, the dHvA frequencies and cyclotron masses are approximately the same among RCd11, revealing a localized character of 4f electrons in CeCd11 and PrCd11.

Novel Electronic States of Heavy Fermion Compound YbCo2Zn20

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...

Metamagnetic behavior in a heavy fermion compound YbCo2Zn20

We studied an electronic state at low temperatures by measuring the specific heat and ac-susceptibility under magnetic fields for a heavy fermion compound YbCo2Zn20. The magnetic specific heat in the form of Cmag/T increases steeply and exhibits an extremely large value 8 J/(K2-mol) below 0.2 K, which corresponds to an electronic specific heat coefficient with a very low Kondo temperature of 0.2-1 K. The magnetic entropy increases rapidly at higher temperatures up to 10 K and reaches Rln8 at 50-60 K, which corresponds to a small crystalline-electric-field (CEF) splitting energy. In addition, we found a metamagnetic anomaly at Hm = 5.7 kOe below the characteristic temperature Tχmax =0.32 K, where the temperature dependence of the ac-susceptibility shows a broad peak. Interestingly, the metamagnetic behavior was observed as the double peak at 4.0 and 7.5 kOe in the magnetic field dependence of the specific heat C/T at 95 mK.

Characteristic magnetic phase diagram in heavy-fermion compound YbCo2Zn20

We studied the magnetic field versus temperature phase diagram of the heavy-fermion compound YbCo2Zn20 with the cubic CeCr2Al20-type structure by measuring the specific heat, ac-susceptibility, and electrical resistivity under magnetic fields. In a low magnetic field region, a metamagnetic behavior was observed at Hm = 6 kOe for three principal field directions below Tχmax = 0.32 K, at which the temperature dependence of ac-susceptibility shows a local broad maximum. Two crossover lines were found below and above Hm. At high magnetic fields, the heavy-fermion state was rapidly suppressed with increasing magnetic field, and the field-induced ordered phase appeared in a very limited field direction around H || 111 above the transition field HQ= 60 kOe in the temperature range below TQ = 0.6 K.

Relation between Metamagnetic Transition and Quantum Critical Point in Heavy Fermion Compound YbIr2Zn20

We measured the electrical resistivity in magnetic field H under pressure P for a heavy fermion compound YbIr2Zn20. The quantum critical point is found to be defined as an electronic state where the metamagnetic transition field Hm becomes zero. This is realized at Pc ? 5.2 GPa. The most characteristic feature in the quantum critical pressure region is an anomalously large A value of the electrical resistivity p = p0 + AT2, reaching A = 380???cm/K2 at 5.0 GPa, which is, however, strongly reduced with increasing the magnetic field: A = 1.45???cm/K2 at 80 kOe. From the result of the de Haas-van Alphen experiment at ambient pressure, the corresponding cyclotron effective mass of conduction electrons at 5.0 GPa is 400 ? 500 m0 (m0: rest mass of an electron).