山根 悠

Abstract We performed inelastic neutron scattering experiments on PrMgNi₄ with the cubic MgSnCu₄-type structure. The magnetic excitations were observed at 1.1, 2.5, 5.9, 10.7, and 11.7 meV. Since the energy of the excitations is constant for |Q| < 5 Å⁻¹, they are ascribed to the crystalline electric field (CEF) excitations of the Pr³⁺ ions. We adopted CEF parameters of W = −3.3 K and x = 0.8 for the cubic T_d point group to reproduce the strong excitations at 1.1, 10.7, and 11.7 meV. The calculated CEF level scheme reveals the Γ₃ doublet ground state with the quadrupolar degrees of freedom and the excited states of Γ₄ triplet (1.4 meV), Γ₁ singlet (3.4 meV), and Γ₅ triplet (13.5 meV). This scheme, however, does not explain the observed two excitations at 2.5 and 5.9 meV. These additional excitations may arise from splitting of the doublet by symmetry lowering due to excess Mg atoms occupying the Pr sites. This splitting is probably responsible for the absence of the long-range quadrupole order in PrMgNi₄.

Abstract The thermal expansion of a diluted Ce system La_1-xCe_xCu₆ for (0.6 ≤ x ≤ 1) has been measured between 10 and 150 K to reveal the change from the coherent heavy Fermion state (0.9 ≤ x ≤ 1) to the incoherent Kondo state (0 < x ≤ 0.73). The large Ce concentration x dependence of the linear thermal expansion coefficient along b-axis α_b(T) suggests that the coupling between the 4f¹ electron and the lattice strain is the largest along the b-axis in the three crystallographic axes. The maximum of the magnetic contribution to the volume thermal expansion coefficient β_m(T) at T = 50 K is retained in the x range of 0.6 ≤ x ≤ 1, suggesting the crystalline electric field (CEF) level for x = 1 doesn’t change by the substitution. Furthermore, the upturn in β_m(T) below 25 K, which should be a precursor of the maximum at T = 2.5 K reported for x = 1, is retained when we decrease x from 1 to 0.6. Because the ground state for x = 0.6 is the incoherent Kondo state, the robustness of the maximum at T = 50 K and upturn in the current x value implies that β_m(T) in 10 ≤ T ≤ 150 K is attributed to the CEF and Kondo effects rather than the formation of the heavy Fermion state.

Abstract We performed electrical resistivity measurements of Ce₃TiSb₅ under pressure. From the pressure dependences of antiferromagnetic ordering temperature T_N and lower anomalous temperature T*, temperature–pressure phase diagram of Ce₃TiSb₅ up to 2.3 GPa has been constructed. T* rapidly decreases with increasing pressure and disappears around 1 GPa. T_N increases with increasing pressure, however a hump structure of electrical resistivity below T_N becomes small. Hence, some change for the magnetic ordered state is expected to occur in higher pressure region.

©2020 The Physical Society of Japan Magnetic properties of an Yb-based semiconductor YbCuS2 with a zigzag chain of Yb3+ ions were studied. The Curie–Weiss behavior of the magnetic susceptibility χ(T) indicates antiferromagnetic interaction and an isolated Kramers doublet ground state with effective spin-1=2. Magnetic specific heat Cm=T exhibits a sharp peak at TO = 0.95K, where the magnetic entropy is 20% of Rln2, indicating strong magnetic fluctuations. In magnetic fields, TO is robust for B < 4T, and it shifts to higher temperatures but reverses downward for B ≥ 7T. Below TO, a magnetization plateau manifests itself at around 6T which is followed by transitions at 9.1 and 15.6T. The phase diagram suggests that nontrivial phases arise from magnetic frustration in the Yb zigzag chain.

© 2020 American Physical Society. The cubic Kondo lattice YbCo2Zn20 is one of the heaviest known Fermi liquids. We have measured the low-temperature electrical resistivity ρ(T), magnetic susceptibility χ(T), and heat capacity C(T) in single crystals of Yb(Co1-xNix)2Zn20 (x≤0.126) and Yb(Co1-xFey)2Zn20 (y≤0.07). While pure YbCo2Zn20 displays maxima in ρ(T) and χ(T), ascribed to an enhanced crystal electric field (CEF) degeneracy, the maxima are suppressed by small amounts (≈6 at.%) of substitutions of the Co atoms. This goes along with a suppression of superheavy Fermi liquid behavior as manifested in the divergence of C/T. We ascribe the observations to local distortions at the Yb site due to the chemical disorder in the environment, rather than to a chemical pressure effect. This fragileness to the local distortion indicates that superheavy Fermion behavior in YbCo2Zn20 is closely linked to an enhanced CEF degeneracy.

© 2019 American Physical Society. Acoustic signatures of the single-site quadrupolar Kondo effect in Y0.966Pr0.034Ir2Zn20 are presented. The elastic constant (C11-C12)/2, corresponding to the Γ3(E)-symmetry electric-quadrupolar response, reveals a logarithmic temperature dependence of the quadrupolar susceptibility in the low-magnetic-field region below ∼0.3 K. Furthermore, the Curie-type divergence of the elastic constant down to ∼1 K indicates that the Pr ions in this diluted system have a non-Kramers ground-state doublet. These observations evidence the single-site quadrupolar Kondo effect, as previously suggested based on specific-heat and electrical-resistivity data.

© 2019 American Physical Society. We report the discovery of bulk superconductivity in the ternary intermetallics YNiSi3 and LuNiSi3. High-quality single crystals were grown via the Sn-flux method and studied using magnetization, specific-heat, and resistivity measurements at low temperatures. The critical temperatures obtained from these different techniques are in very good agreement and yield Tc=1.36(3)K and Tc=1.61(2)K for YNiSi3 and LuNiSi3, respectively. Magnetization measurements indicate that both compounds are among the rare cases where type-I superconductivity occurs in a ternary intermetallic, however, the jump in the specific heat at the transition is lower than the value expected from BCS theory (ΔCel/γnTc=1.43) in both materials and is equal to 1.14(9) and 0.71(5) for the Y and Lu compounds, respectively. Resistivity measurements exhibit sharp transitions but with critical fields μ0Hc(0) (≈0.05T for YNiSi3 and ≈0.08T for LuNiSi3) considerably higher than those obtained from the magnetization and specific heat (≈0.01T). First-principles density functional theory calculated electronic structure shows that these compounds have highly anisotropic and complex Fermi surfaces with one electronic and two holelike branches. One hole branch and the electron branch have a large cylindrical topology connecting the first Brillouin-zone boundaries, the former being built up by the hybridization of Y(Lu) d, Ni d, and Si p states, and the latter being built up by Ni d and Si p states. The calculated phononic structures indicate that the coupling of the Y(Lu), Ni d, and Si p electrons in the low-lying optical phonon branches is responsible for the formation of Cooper pairs and the observed superconducting state. Therefore, these compounds can be classified as anisotropic three-dimensional metals with multiband superconducting ground states in the weak-coupling regime.

© 2019 American Physical Society. We report thermal expansion and magnetostriction of the cubic non-Kramers system PrIr2Zn20 with a nonmagnetic Γ3 ground-state doublet. In previous experiments, antiferroquadrupolar order at TQ=0.11 K and a Fermi liquid state at around Bc≈5T for B[001], indicative of possible ferrohastatic order, were discovered. For magnetic fields B[001], the low-temperature longitudinal and transverse thermal expansion and magnetostriction are highly anisotropic. The resulting volume strain is very small, indicating that the Pr valence remains nearly constant as a function of magnetic field. We conclude that the Fermi liquid state around Bc forms through a very little change in c-f hybridization. This result is in sharp contrast to Ce- and Yb-based Kramers Kondo lattices, which show significantly larger volume strains due to the high sensitivity of the Kondo temperature to hydrostatic pressure.

©2019 The Physical Society of Japan The praseodymium-encaged compound PrIr2Zn20 with a non-Kramers doublet ground state undergoes an antiferroquadrupolar transition at TQ = 0.11K. On heating above TQ, the magnetic specific heat Cm=T decreases as –lnT and the electrical resistivity ρ(T) increases with an upward curvature; thereby, formation of the quadrupole Kondo lattice is proposed. In this work, we studied the mechanism of these non-Fermi liquid (NFL) behaviors by measuring C and ρ for alloy systems where the Zn cage was partially substituted by Cd and Ga: PrIr2Zn20–xCdx (x ≤ 0.3) and PrIr2Zn20–yGay (y ≤ 0.25). The cubic lattice parameter increases with increasing x but hardly changes with y. The characteristic temperature T0 of the NFL behaviors in Cm(T), which is defined as the temperature where the magnetic entropy Sm(T) reaches (3=4)Rln2, increases with increasing x and y. This increase in T0 is caused by the symmetry lowering of the Pr site due to the disorder of the cage atoms. On the other hand, another characteristic temperature, TR, where ρ(T) deviates from the T-linear dependence on cooling, decreases with increasing x but increases with y. This trend of TR indicates that the quadrupole Kondo lattice is stabilized owing to the strengthening of the c–f hybridization by the chemical pressure and=or the enhanced density of states of the conduction electrons.

© 2019 The Physical Society of Japan The structural, transport, and magnetic properties of single-crystalline samples of the praseodymium-based cubic compound PrMgNi4 were studied. The single-crystal X-ray structural analysis revealed that 4.5% of the Pr atoms at the 4a site are replaced by Mg atoms. The χ(T) data follow the Curie–Weiss law with an effective moment for the Pr3+ ion. The magnetic specific heat divided by temperature, Cm=T, shows a broad maximum at around 3 K, which is reproduced by a two-level model with a ground-state doublet. On cooling below 1 K, Cm=T approaches a constant value, which is reproduced by a random two-level model. The twofold degeneracy of the ground state is lifted by symmetry lowering due to the substituted Mg atoms for the Pr atoms or strong hybridizations between the 4 f 2 electron states and conduction bands, which hinder the long-range quadrupole order.

©2019 The Physical Society of Japan Electrical resistivity ρ, magnetization M, and specific heat C were measured for 4 f 3 Kramers systems NdT2Zn20 (T = Co and Rh), in which Nd3+ ions form a diamond sublattice. The magnetic susceptibility χ = M=B follows the Curie–Weiss law in the temperature range from 300 to 10 K, from which the paramagnetic Curie temperatures θp are evaluated to be −7.1 and −6.3 K for T = Co and Rh, respectively. A broad maximum in the magnetic specific heat divided by temperature Cm=T at around 13 K is ascribed to the excitations from the crystalline electric field ground state of the magnetic Γ6 doublet to the excited Γ8 quartets. Sharp peaks in Cm(T) at TN = 0.53 and 0.94 K for T = Co and Rh, respectively, coincide with the drops in the resistivity ρ(T), and both temperatures decrease with increasing magnetic field. Moreover, χ(T) for T = Co decreases on cooling at TN, indicating the manifestation of an antiferromagnetic order at TN. ρ(T) shows upward curvature for TN < T < 4 K. Taking account of the significant reduction of the magnetic entropy at TN from the expected value R ln 2=Nd, we attribute the anomalous behavior in ρ(T) to the c–f hybridization of the Γ6 doublet ground state and=or magnetic frustration due to the diamond structure of the Nd sublattice.

© 2018 American Physical Society. The Ce atoms in CeIrSn form a quasikagome lattice in the hexagonal c plane. Our single-crystal x-ray diffraction analysis has confirmed the absence of superstructure in the ZrNiAl-type structure. Motivated by the recent observation of quantum critical behaviors in the isostructural and isoelectronic compound CeRhSn, we have studied low-temperature properties of single-crystalline samples of CeIrSn by measuring the electrical resistivity, magnetic susceptibility χ, magnetization M, and specific heat C down to 0.05 K. Our study does not show evidence of a long-range magnetic ordering down to the lowest temperature. The χ(T) data show an Ising character, χc a, and power-law behaviors at low temperatures: χcT-0.8 and χaT-0.6. When external field B is applied along the a axis, both M(B) and C(B)/T exhibit metamagnetic anomalies at 5.5 T, which resemble those observed in CeRhSn at 3.5 T. We attribute the metamagnetic crossover under in-plane magnetic fields to the alleviation of magnetic frustration inherent in the quasikagome Ising Kondo lattice.

© 2018 Author(s). The specific heat of Y1-xPrxIr2Zn20 with x = 0.044 has been measured down to 0.08 K in magnetic fields of B ≤ 12 T. The 4f contribution to the specific heat divided by temperature C4f/T diverges as -lnT in B = 0 and 2 T applied along the [100] direction. In B = 4 T, however, C4f(T) exhibits a broad maximum, which shifts to higher temperatures with increasing B. This behavior can be explained by the Zeeman effect which splits the ground state doublet through the first excited triplet. The 4f contribution to the entropy, S4f(T), in B ≥ 4 T is well reproduced by the crystalline electric field calculation, whereas the calculation fails to reproduce the S4f(T) data in B ≤ 2 T. We propose that the on-site interaction of the active quadrupole in the ground state doublet with two-channel conduction bands gives rise to the -lnT variation of C4f/T.

© 2018 American Physical Society. Electrical resistivity ρ(T) and specific heat C(T) measurements have been made on the diluted 4f2 system Y(Pr)Ir2Zn20. Both data of ρ and magnetic specific heat Cm per Pr ion are well scaled as a function of T/T0, where T0 is a characteristic temperature of non-Fermi-liquid (NFL) behaviors. Furthermore, the temperature dependences of ρ and Cm/T agree with the NFL behaviors predicted by the two-channel Kondo model for the strong coupling limit. Therefore, we infer that the observed NFL behaviors result from the single-site quadrupole Kondo effect due to the hybridization of the 4f2 states with multichannel conduction electrons.

© 2017 Elsevier B.V. PrIr2Zn20 exhibits an antiferroquadrupolar (AFQ) order at TQ = 0.11 K, above which temperature the electrical resistivity ρ(T) shows an upward curvature and the magnetic specific heat divided by temperature Cm/T follows − lnT dependence. The non-Fermi Liquid (NFL) behaviors have suggested formation of a quadrupole Kondo lattice. In the present work, we have studied the effect of Ga substitution for Zn on the NFL behavior by the measurements of the specific heat C, the magnetic susceptibility χ, and ρ of PrIr2Zn20-xGax (0 ≤ x ≤ 0.25). With increasing x, the characteristic temperature T0 defined as the temperature where the magnetic entropy Sm reaches (3/4)Rln2 is increased by a factor of 3.5. Similarly, another characteristic temperature TR below which ρ(T) starts decreasing with the upward curvature increases with x by a factor of 1.2. The increments of both T0 and TR may be attributed to the possible split of the Γ3 doublet by symmetry lowering of the Pr sites. Otherwise, the quadrupole Kondo lattice would be stabilized by the enhanced c-f hybridization due to the increment of the 4p electronic density and/or the chemical pressure effect.

© 2017 Elsevier B.V. The electrical resistivity ρ and specific heat C of a dilute Pr system Y1-xPrxIr2Zn20 for 0 ≤ x ≤ 0.44 were measured to study the phenomena arising from active quadrupoles of the Pr3+ ion with 4f2 configuration. On cooling, ρ's of all samples monotonically decrease, while the residual resistivity ratio ρ(300 K)/ρ(3 K) drastically decreases with x. In the whole range x ≤ 0.44, the magnetic contribution to the specific heat divided by temperature Cm/T shows a broad maximum at around 10 K, which can be reproduced by a two-level model with a first-excited triplet separated by 30 K from a ground state doublet. This indicates that the crystalline electric field ground state of the Pr ions remains in the Γ3 doublet for the cubic Td point group. On cooling, the Cm/T data for x = 0.085 and 0.44 approach constant values at T<0.3 K as expected from the random two-level model. By contrast, Cm/T for x = 0.044 increases continuously down to 0.08 K, suggesting a non-Fermi liquid state due to the impurity quadrupole Kondo effect.

© 2017 Elsevier B.V. Electrical resistivity ρ magnetization M and specific heat C are reported for polycrystalline samples of the hexagonal system PrRh3. The magnetic susceptibility M/B obeys the Curie-Weiss law with the effective magnetic moment μeff = 3.88 μB/Pr and the paramagnetic Curie temperature θp = +2.9 K, which indicates ferro-type magnetic interaction between the trivalent Pr ions. A cusp in C(T) at 3.0 K coincides with a bend in ρ(T). Applying magnetic fields, the peak broadens and shifts to higher temperatures. The field dependence indicates a ferro-type magnetic order. The magnetic entropy Sm is (1/3)Rln2 at TC = 3.0 K, suggesting that part of the Pr ions take part in the magnetic order. A broad tail of the magnetic specific heat Cm observed above TC may result from short-range correlations and/or fluctuations of the active magnetic dipole and quadrupoles in the ground state doublet.

© 2017 American Physical Society. CeRhSn with a quasikagome lattice of Ce atoms in the hexagonal c plane has been expected to be in close vicinity to a zero-field quantum criticality derived from magnetic frustration. We have studied how the ground state changes with substitution of Pd for Rh in CeRh1-xPdxSn (x≤0.75) by measuring the specific heat C, magnetic susceptibilities χdc and χac, magnetization M, electrical resistivity ρ, and magnetoresistance. For x=0, the field dependence of χac at T=0.03K shows a peak at Ba=3.5T, confirming the spin-flop crossover in the field applied along the hard axis. The temperature dependence of χac shows a broad maximum at 0.1 K whereas C/T continues to increase down to 0.08 K. For x0.1,ρ(T) is dominated by incoherent Kondo scattering and both C/T and χac(T) exhibit peaks, indicating the development of an antiferromagnetic order. The ordering temperature rises to 2.5 K as x is increased to 0.75. Our results indicate that the ground state in the quasikagome Kondo lattice CeRh1-xPdxSn leaves the quantum critical point at x=0 with increasing x as a consequence of suppression of both the magnetic frustration and Kondo effect.

© 2017 The Physical Society of Japan. Electrical resistivity ρ, magnetization M, and specific heat C measurements are reported for single-crystalline samples of the 4 f3 Kramers system NdIr2Zn20. On cooling, ρ(T) monotonically decreases and drops below 0.65 K. The magnetic susceptibility M/B follows the Curie-Weiss law from 300 to 20 K, leading to an effective magnetic moment of μeff = 3.59 μB/Nd and a paramagnetic Curie temperature of θp = -3.6 K. The negative value of θp indicates an antiferrotype interaction, although the enhancement of M(B) in the paramagnetic state implies a ferro-type one. A broad maximum in the magnetic specific heat Cm at around 15K arises from the excitations from the crystalline electric field ground state of a magnetic Γ6 doublet to the excited Γ8 quartet lying at 39K above the ground state. A sharp peak in C(T) at 0.65K coincides with the drop in ρ(T), both of which shift to lower temperatures upon applying a magnetic field. The field dependences indicate that an antiferromagnetic order may occur under the geometrical frustration in the Nd sublattice with a diamond structure.

©2017 The Physical Society of Japan. The electrical resistivity, magnetization, and specific heat of the caged compounds ROs2Zn20 (R = La, Ce, Pr, and Nd) have been measured to study their structural, magnetic, and superconducting properties. These measurements indicate that the compounds undergo structural transitions at Ts = 151, 109, 87, and 62 K, respectively. The decrease in Ts along with the lanthanoid contraction suggests that the high-temperature phase is more stable for a smaller lattice volume. Analysis of the lattice specific heat of LaOs2Zn20 reveals that the Zn atom at the 16c site encapsulated in the R2Zn12 cage vibrates at a low energy of 3 meV. For CeOs2Zn20, the small magnetic susceptibility with a broad maximum indicates the valence-fluctuating state of the Ce ions. In PrOs2Zn20, the crystalline electric field ground state of the 4 f2 state of the Pr3+ ion remains in a non-Kramers doublet at T > Ts, which is lifted by symmetry lowering of the Pr site at T < Ts. Thereby, the quadrupolar degrees of freedom are quenched, avoiding the long-range quadrupole order. PrOs2Zn20 and LaOs2Zn20 exhibit superconducting transitions at 0.06 and 0.07 K, respectively.

© Published under licence by IOP Publishing Ltd. Electrical resistivity ρ, specific heat C, and magnetic susceptibility χ measurements are reported on PrIr2Zn20-xGax for x=0.5 and 1.0. On cooling, ρ(T)'s monotonically decrease to the residual values which are two orders of magnitudes larger than that of PrIr2Zn20. A broad hump in Cm(T)'s at around 10 K can be reproduced by the two-level model with the Γ3 doublet ground state and the Γ4 triplet excited state. The magnetic entropy release shifts to higher temperatures with increasing x. The increase in χ(T) at T < 5 K contrasts with the saturation of χ(T) for x=0. These observations indicate the splitting of the ground state doublet due to the lowering of the local symmetry at the Pr site by the Ga substitution.

<p>In the titled system, the evolution of a long-range antiferromagnetic (AFM) order manifests in sharp peaks in the specific heat at 0.7, 1.0 and 1.5 K for x = 0.4, 0.5 and 0.65, respectively. Our results indicate that the doping of 4d electrons in the non-Fermi liquid system CeRhSn destroys the coherence of the quasi-kagome lattice and weakens the hybridization between the 4f state and conduction bands, leading to the evolution of AFM order</p>