Kenichi Tenya

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.

Magnetic Phase Diagram of CexLa1-xB6 Studied by Static Magnetization Measurement at Very Low Temperatures.

DC magnetization of single crystals of Ce x La 1- x B 6 , 0.5≤ x ≤1, have been measured at low temperatures ( T ) down to 40 mK in magnetic field H //[100], and the H - T - x phase diagrams were obtained. At zero field, the antiferro-quadrupolar (AFQ) transition temperature T Q rapidly decreases with decreasing x and vanishes abruptly between x =0.75 and 0.7. The Neel temperature T N , which is lower than T Q in CeB 6 , varies more slowly with x and exceeds T Q for \(x \gtrsim 0.8\). A clear peak observed in the susceptibility strongly suggests that a new antiferromagnetic (AF) state having no AFQ moment (phase IV) exists for \(0.8\gtrsim x\gtrsim 0.5\), in addition to the two ordered phases II (AFQ) and III (AF+AFQ). As H increases, a first-order phase IV-III transition is observed with a metamagnetic jump in the magnetization. Successive I (paramagnetic)-IV-III phase transitions are observable in a very narrow region of the H - T - x space.

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...

Neutron Scattering Study on Competition between Hidden Order and Antiferromagnetism in U(Ru1-xRhx)2Si2 (x ≤0.05)

We have performed elastic and inelastic neutron scattering experiments on solid solutions of U(Ru 1- x Rh x ) 2 Si 2 with Ru-rich concentrations: x =0, 0.01, 0.02, 0.025, 0.03, 0.04 and 0.05. Hidden order is suppressed with increasing x , and correspondingly, the onset temperature T m (∼17.5 K at x =0) of weak antiferromagnetic (AF) Bragg reflection decreases. For x =0.04 and 0.05, no magnetic order is detected in the investigated temperature range down to 1.4 K. In the intermediate range, 0.02 ≤ x ≤0.03, we found that the AF Bragg reflection is strongly enhanced. At x =0.02, this enhancement takes place at ∼7.7 K (≡ T M ), which is significantly lower than T m (∼ 13.7 K). T M increases with increasing x , and seems to merge with T m at x =0.03. If the AF state is assumed to be homogeneous, the staggered moment µ o estimated at 1.4 K increases from 0.02(2) µ B /U ( x =0) to 0.24(1) µ B /U ( x =0.02). The behavior is similar to that observed under hydrostatic pressure (µ o increases to ∼0.25 µ B /U at 1.0 ...

Inhomogeneous magnetism in URu2Si2 studied by muon spin relaxation under high pressure

Abstract We have studied the low-temperature phase below 17.5 K in the heavy-fermion superconductor URu 2 Si 2 , using the zero-field μSR technique under hydrostatic pressures P up to 0.85 GPa . The measurements qualitatively confirm the previous results of NMR and neutron scattering that pressure induces unusual evolution of an inhomogeneous antiferromagnetic (AF) phase which spatially competes with some nonmagnetic ‘hidden order’. The AF volume fraction, however, develops abruptly at around a pressure of 0.6 GPa , in contrast to the P -linear variations in the NMR results. Moreover, the onset temperature of the AF-volume evolution is clearly lower than 17.5 K , whereas the nominal Neel temperature seems to be about 20 K . These experimental results suggest that the AF phase exists as a metastable state.

Hidden order and weak antiferromagnetism in URu2Si2

Abstract The origin and nature of the low-temperature phase appearing below 17.5 K (≡T 0 ) in URu2Si2 is still not fully understood. Since the weak antiferromagnetic state (WAF; μord∼0.02–0.04μB) developing below around T0 is not simply reconciled with a large reduction in the magnetic entropy (∼0.2R ln 2) , the existence of some hidden order parameter has been argued, where WAF is considered to be of a secondary nature. The paper reviews recent development of microscopic characterization of WAF based on neutron scattering and 29 Si NMR under high pressure. It is demonstrated that, at least, for pressures ranging from ∼0.3 to ∼0.83 GPa , there are spatially competing two different ordered states in the crystal: a normal-moment (∼0.25μB/U) antiferromagnetic state and some unknown, non-magnetically ordered state. Simple extrapolation to ambient pressure suggests that WAF originates from one percent volume fraction of this antiferromagnetic phase. We also discuss the unusual magnetic properties at ambient pressure, presenting our latest results of zero-field and longitudinal-field μSR experiments.

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.

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.

Single crystal growth, normal and superconducting properties of UPd2Al3

Normal and superconducting properties of single crystalline UPd 2 Al 3 have been studied by static magnetization, AC susceptibility and magnetoresistance measurements. In the normal state, an open orbit is found to exist along the [0001] direction from a saturated magnetoresistance. In the superconducting state, a pronounced peak effect is observed just below the upper critical field H c2 . The peak consists of two structures at low temperatures, becomes one above 0.6 K and continues up to a low field of 0.1 T. These experimental results are compared to those based on the FFLO model. The paramagnetic effect in superconductivity is also found by the static magnetization measurement.

New Magnetic State in Incommensurate Magnetic Phase of Heavy-Fermion Superconductor CeRh0.6Co0.4In5

We performed elastic neutron scattering experiments on solid solution CeRh 1- x Co x In 5 with x =0.4 to clarify the nature of the antiferromagnetic (AF) phase in the vicinity of the quantum critic...