T. Tayama

Magnetic-field induced quantum critical point in YbRh2Si2

We report low-temperature calorimetric, magnetic, and resistivity measurements on the antiferromagnetic (AF) heavy-fermion metal YbRh(2)Si(2) ( T(N)=70 mK) as a function of magnetic field B. While for fields exceeding the critical value B(c0) at which T(N)-->0 the low-temperature resistivity shows an AT2 dependence, a 1/(B-B(c0)) divergence of A(B) upon reducing B to B(c0) suggests singular scattering at the whole Fermi surface and a divergence of the heavy quasiparticle mass. The observations are interpreted in terms of a new type of quantum critical point separating a weakly AF ordered from a weakly polarized heavy Landau-Fermi liquid state.

Antiferro-Quadrupolar Ordering and Multipole Interactions in PrPb3

Antiferro-quadrupolar (AFQ) ordering in the cubic Γ 3 ground state system PrPb 3 has been studied by means of magnetization and specific heat measurements under magnetic fields H . Field variation of the AFQ transition temperature T Q (0.4 K at H =0) has been determined for the three crystallographic directions , and . The obtained T Q ( H ) is strongly anisotropic, with T Q ( H ) the largest (∼0.7 K at 6 T). The AFQ phase diagram and the magnetization behavior observed for H ∥ are well explained by a model that incorporates both antiferromagnetic and antiferroquadrupolar interactions. The interactions between field-induced staggered magnetic moments stabilize the AFQ order. This simple model, however, seriously fails to explain the observed anisotropy: T Q ( H )> T Q ( H )> T Q ( H ). Reconsidering the field-induced multipole moments in the AFQ phase, we propose that inclusion of a weak ferro-octupolar interaction between Γ 5 type octupole moments is importa...

Low-temperature magnetization study of the heavy-electron systems

Abstract The ground-state magnetic properties of Ce0.75La0.25B6 and Sm3Te4 have been studied by means of static magnetization measurements at very low temperatures T ⩾ 50 mK. In Ce0.75La0.25B6, the H − T phase diagram reveals three different ordered phases: in addition to the two phases (phase II and III) known for CeB6, a new antiferromagnetic state appears before the antiferro-quadrupolar order sets in. In the valence fluctuation compound Sm3Te4, a spin-glass ordering is evidenced at Tg ∼ 1.3 K. It is likely that the two valence states (Sm2+, Sm3) are randomly frozen at low temperature to form effectively a randomly diluted magnet.

Tuning heavy fermion systems into quantum criticality by magnetic field

We discuss a series of thermodynamic, magnetic, and electrical transport experiments on the two heavy fermion compounds CeNi2Ge2 and YbRh2Si2 in which magnetic fields, B, are used to tune the systems from a non-Fermi liquid (NFL) into a field-induced FL state. Upon approaching the quantum-critical points from the FL side by reducing B we analyze the heavy quasiparticle (QP) mass and QP-QP scattering cross sections. For CeNi2Ge2 the observed behavior agrees well with the predictions of the spin-density wave (SDW) scenario for three-dimensional (3D) critical spin-fluctuations. By contrast, the observed singularity in YbRh2Si2 cannot be explained by the itinerant SDW theory for neither 3D nor 2D critical spinfluctuations. Furthermore, we investigate the magnetization M(B) at high magnetic fields. For CeNi2Ge2 a metamagnetic transition is observed at 43 T, whereas for YbRh2Si2 a kink-like anomaly occurs at 10 T in M vs B (applied along the easy basal plane) above which the heavy fermion state is completely suppressed.

Magnetic phase diagram of CexLa1−xB6 studied by magnetization measurement

Abstract From DC magnetic susceptibility χ(T) measurement on CexLa1−xB6 (x=0.75, 0.7, 0.65 and 0.4), Ce concentration dependence of the zero-field ordering temperature is established. The new magnetic state (phase-IV) characterized by an isotropic cusp in χ(T) is clearly seen for x=0.75, 0.7 and 0.65. Distinct irreversibility in χ(T) observed for x=0.4 below the cusp temperature indicates that phase-IV changes into a certain glass-like state for x⩽0.5.

Field-history-dependent peak effect in the superconducting mixed state of CeRu2

Abstract Detailed magnetization measurements of CeRu2 single crystals with different purity are performed in order to clarify whether their peak effects are due to the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) transition. The onset field of the peak shifts to lower field side as the sample becomes impure. Regardless of the purity the Ginzburg–Landau parameter κ2 continues to increase on cooling from Tc, indicating little paramagnetic suppression of the superconductivity. These results disagree with the scenario of the FFLO transition.

Low-temperature magnetization of antiferro-quadrupolar ordering system TmTe

Abstract Magnetization of TmTe has been examined at temperatures down to 50 mK, in order to study the antiferroquadrupolar (AFQ) and antiferromagnetic (AF) ordered states. On cooling, magnetic susceptibility χ(T) increases enormously and appears to diverge in zero field at T N =0.22 K , evidencing that a ferromagnetic correlation is induced by the anisotropy due to the underlying AFQ order at T Q ≈1.8 K . In the magnetization process in the AFQ phase for field parallel to the [1 0 0] direction, a small metamagnetic jump is observed at ∼1.9xa0T which might be related to a change in the quadrupolar order parameter.

Peak effect in CeRu2 : Role of crystalline defects

We measured the electrical resistivity in magnetic fields and the Hall voltage to understand the peak effect in a superconductor with the cubic structure CeRu 2 . Two samples with residual resistivity ratios of ρ RT /ρ 0 = 240 and 12 were prepared, and the experimental results in the superconducting mixed state were compared between them. The flux flow begins at a high field and the peak effect is observed in a very narrow window in the T vs. H phase diagram near the upper critical field H c2 for the sample with ρ RT /ρ 0 = 240, while it starts at a lower field and the window where the peak effect is observed becomes wider for the sample with ρ RT /ρ 0 = 12. These characteristic features are mainly related to crystalline defects in the sample.

Low-temperature magnetization of Ce(Ru0.85Rh0.15)2Si2

Magnetization measurements have been performed on a single-crystalline sample of Ce(Ru 0.85 Rh 0.15 ) 2 Si 2 in magnetic fields along the tetragonal c-axis up to 8T in the temperature range between 40 mK and 10 K. Two transitions (Bc = 3.45 T, B M = 5.5 T) are observed in the magnetization process at 40 mK. Further, at low temperatures, hysteresis is observed at B c and this disappears above 2 K. Therefore, the transition at B c is first order at low temperatures, but becomes second order above 2 K.

Magnetization study on the antiferro-quadrupolar ordering in PrPb3

Abstract Quadrupolar ordering in the cubic compound PrPb3 has been studied by DC magnetization measurements at temperatures down to T∼50 mK in fields (H‖[1 0 0]) up to 8xa0T. The quadrupolar ordering temperature TQ(H), below which the magnetic susceptibility is enhanced, is found to increase in H from 0.4xa0K at H=0 to 0.6xa0K at H=5 T . On cooling below TQ(H), sharp metamagnetic transitions develop in the magnetization process at around 7xa0T. The resulting H–T phase diagram can be explained by incorporating magnetic interactions between the field-induced antiferromagnetic moments in the O02 type antiferro-quadrupolar phase.