Kunihiko Maezawa

Ce- and Yb-based Kondo semiconductors

Abstract Kondo semiconductors are a class of strongly correlated f-electron materials whose low-energy excitations exhibit a (pseudo)gap at low temperatures. The significant difference between the pseudogap in orthorhombic CeNiSn and CeRhSb and the real gap in cubic Ce 3 Bi 4 Pt 3 and YbB 12 is highlighted by a comparison of transport properties. Low-temparature measurements of the magnetoresistance, Hall coefficient and specific heat of CeNiSn have revealed a field-induced excitation of the coherent low-carrier state. A systematic study of CeNi 1− x T x Sn ( T = Co, Cu and Pt ) has shown that the residual carriers in CeNiSn are immobilized by 1% substitution irrespective of the substitutes. Further substitution with 5% Cu induces a magnetic instability at T = 0.

dHvA Effect Study of Metamagnetic Transition in CeRu 2Si 2 II - The State above the Metamagnetic Transition

We have performed an extensive dHvA effect study of CeRu 2 Si 2 to gain more insight into the nature of the metamagnetic transition. The angular dependence of the dHvA frequencies confirms the transition of the Fermi surfaces from those of the itinerant f electron below the metamagnetic transition field ( H m ) to those of the localized f electron above H m . The field dependence of the frequencies and effective masses are measured for all the frequencies observed with fields parallel to [001]. A waveform analysis suggests that the Fermi surfaces of the up and down spin electrons are similar in size, whereas the effective masses are significantly different. The amplitudes of the dHvA oscillations and their field dependence are anomalous above H m , implying that the dHvA oscillations above H m may possibly come from a state which is not well developed near H m .

Crystal Growth and Characterization of the Kondo Semimetal CeNiSn

Several single crystals of CeNiSn and CeNi 0.95 T 0.05 Sn (T=Co and Cu) were grown and characterized by metallographic examination, electron-probe microanalysis and the measurements of resistivity, magnetoresistance, magnetic susceptibility and specific heat. Below 10 K, the semiconductor-like behavior found for dirty or substituted crystals is suppressed with decreasing impurities. For a crystal purified by the solid-state electrotransport technique, the resistivity along the orthorhombic a axis is metallic with the T 2 dependence, whereas those along the b and c axes increase below 3 K and pass through weak maxima at 0.8 K. These results suggest that the ground state of CeNiSn is not semiconducting but semimetallic with a pseudogap which closes along the a axis.

Metamagnetic Behavior of the Heavy-Fermion Compound CeNi2Ge2.

We have studied the temperature dependence of the electrical resistivity, the magnetic susceptibility and the high-field magnetization of CeNi 2 Ge 2 . Quadratic temperature dependence of the resistivity is observed below 0.7 K. The susceptibility exhibits a broad maximum at 28 K. Magnetization shows metamagnetic behavior at H M ∼42 T as in CeRu 2 Si 2 . These results indicate that CeNi 2 Ge 2 is a new heavy-fermion compound showing the metamagnetic behavior in the nonmagnetic ground state.

Specific heat study of non-fermi liquid behavior in CeNi2Ge2 : Anomalous peak in quasi-particle density-of-states

To investigate the non-Fermi liquid (NFL) behavior in a nonalloyed system CeNi 2 Ge 2 , we have measured the temperature and field dependences of the specific heat C on a CeNi 2 Ge 2 single crystal. The distinctive temperature dependence of C / T (\(\sim\alpha-\beta\sqrt{T}\)) is destroyed in almost the same manner for both field directions of B // c -axis and B // a -axis. The overall behavior of C ( T , B ) and the low-temperature upturn in χ( T ) can be reproduced, assuming an anomalous peak of the quasi-particle-band density-of-states (DOS) at the Fermi energy possessing \(\sqrt{\epsilon}\) energy dependence. Absence of residual entropy around T =0 K in B ∼0 T has been confirmed by the magnetocaloric effect measurements, which are consistent with the present model. The present model can also be applied to the NFL behavior in CeCu 5.9 Au 0.1 using a ln(e)-dependent peak in the DOS. Possible origins of the peak in the DOS are discussed.

The Origin of Magnetic Field Dependence of Specific Heat in Single-Crystalline CeNiSn

The effect of magnetic field on the electronic ground state in CeNiSn has been investigated by means of specific-heat measurement in magnetic fields µ 0 H up to 14 T. A high-quality single crystal allows us to observe a marked field effect on specific heat in the temperature range from 2 to 25 K. The ground state is discussed in terms of a pseudo-gapped density of states in the Kondo resonance band with residual states at the Fermi level inducing the Zeeman splitting.

High Field Magnetoresistance and de Haas-van Alphen Effect in CeNi

We have measured the magnetoresistance and de Haas-van Alphen (dHvA) effect in CeNi in the field up to 150 kOe and at temperature down to 0.4 K. This material is found to be a compensated metal with equal carrier concentration of electrons and holes. The open orbits exist along the b - and c -axes. Four dHvA branches have been found in the field along the b -axis. The dHvA frequencies and the cyclotron masses are 3.60 ×10 7 Oe (10.3 m 0 ), 1.25 ×10 7 Oe (8.91 m 0 ), 6.58 ×10 6 Oe (3.05 m 0 ) and 2.15 ×10 6 Oe (2.58 m 0 ).

Long Range Antiferromagnetic Ordering Observed Below 20 mK in the Heavy Fermion Superconductor UPt3.

Ultralow-temperature neutron scattering experiments have been carried out in order to study the antiferromagnetic ordering in the heavy fermion superconductor UPt 3 . We observed the (0.5 0 1) antiferromagnetic peak below 5 K. We found a finite spin correlation length of about 300 A above 50 mK. The peak intensity increases linearly with decreasing temperature, and is suppressed below the superconducting transition temperature of about 0.56 K. The moment size of 0.04 µ B /U was estimated from the peak intensity. These behaviors are in full agreement with results of previous studies. Surprisingly, the antiferromagnetic peak exhibits considerable narrowing below 50 mK, and a resolution-limited peak is observed below 20 mK. This is direct evidence that the antiferromagnetic long-range ordering is realized in UPt 3 at ultralow temperatures.

Anomaly of Magnetization in the Superconducting Mixed State of UPt3

Magnetization curves of UPt 3 along the c-axis were measured at low temperatures down to 80 mK. Below 350 mK a peak of the irreversible magnetization was found just below the superconducting upper critical field H c 2 . However, the peak disappeared above 390 mK, which is much lower than the superconducting critical temperature of 580 mK, suggesting an unconventional mechanism of the flux pinning near H c 2 . No distinct anomaly was observed in the magnetization curves at the phase transition between B and C superconducting phases.

Magnetic phase diagram and the pressure and field dependence of the Fermi surface in UGe 2

The ac susceptibility and de Haas\char21{}van Alphen (dHvA) effect in