Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.7566/jpsj.91.114703

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.35848/1347-4065/abeb8c

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

DOI： 10.1063/5.0014986

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.jre.2020.12.007

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevb.102.094505

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.102.094505/fulltext

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevb.101.235160

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Publishing type：Research paper (international conference proceedings)   Publisher：Journal of the Physical Society of Japan  

DOI： 10.7566/jpscp.30.011137

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Journal of the Physical Society of Japan  

DOI： 10.7566/jpscp.30.011136

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Publishing type：Research paper (international conference proceedings)   Publisher：Journal of the Physical Society of Japan  

DOI： 10.7566/jpscp.30.011119

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We report the core-level photoemission study of cubic PrBe13, PrIr2Zn20 and PrB6 using linearly polarized hard X-ray. We have successfully observed linear dichroism (LD) in Pr3+ 3d core-level photoemission spectra reflecting the 4f ground-state symmetry for all the compounds. The observed LDs and polarization-dependent spectra are reproduced by ionic-model calculations including the full multiplet theory and the local crystalline electric field (CEF) splitting. We have confirmed that LDs in Pr3+ 3d core-level photoemission spectra are able to determine unambiguously the Pr3+ 4f ground-state symmetry.

DOI： 10.1016/j.elspec.2019.07.006

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevb.100.245124

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

The praseodymium-encaged compound PrIr2Zn20 with a non-Kramers doublet ground state undergoes an anti-ferroquadrupolar transition at T-Q = 0.11 K. On heating above T-Q, the magnetic specific heat C-m/T decreases as -ln T and the electrical resistivity rho(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 rho 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 T-0 of the NFL behaviors in C-m(T), which is defined as the temperature where the magnetic entropy S-m(T) reaches (3/4)R ln 2, increases with increasing x and y. This increase in T-0 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, T-R, where rho(T) deviates from the T-linear dependence on cooling, decreases with increasing x but increases with y. This trend of T-R 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.

DOI： 10.7566/JPSJ.88.054704

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We report thermal expansion and magnetostriction of the cubic non-Kramers system PrIr2Zn20 with a nonmagnetic Gamma(3) ground-state doublet. In previous experiments, antiferroquadrupolar order at T-Q = 0.11 K and a Fermi liquid state at around B-c approximate to 5 T for B parallel to [001], indicative of possible ferrohastatic order, were discovered. For magnetic fields B parallel to [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 B-c 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.

DOI： 10.1103/PhysRevB.99.081117

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Author(s)  

DOI： 10.1063/1.5084614

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DOI： 10.1063/1.5084607

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

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 C-4f/T diverges as -lnT in B = 0 and 2 T applied along the [100] direction. In B = 4 T, however, C-4f(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, S-4f(T), in B >= 4 T is well reproduced by the crystalline electric field calculation, whereas the calculation fails to reproduce the S-4f(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 C-4f/T. (c) 2018 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

DOI： 10.1063/1.5043132

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© 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.

DOI： 10.1103/PhysRevLett.121.077206

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：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.

DOI： 10.1016/j.physb.2017.09.004

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We here report the magnetization, M, and specific heat, C, of Nd3NiGe2, which crystallizes in the orthorhombic Gd3NiSi2-type structure. Nd ions occupy three nonequivalent sites in a unit cell. Upon cooling, magnetization divided by magnetic field, M/B, increased sharply at the Curie temperature, TC, of 87 K and below 40 K. The former result indicates that the increase in M/B observed at TC is due to the long-range ferromagnetic order. The increase below 40 K is derived from a short-range correlation because of the absence of clear anomaly in C(T). At 10 K and 2 K, the values of M undergo metamagnetic transitions. The value of magnetic specific heat divided by temperature shows a shoulder-like anomaly at around 20 K, which is attributed to antiferromagnetic behavior. Furthermore, two peaks in C(T) were observed at 4.5 K and 3.8 K, and these peaks occurred at lower temperatures in the presence of a magnetic field. This behavior is typical of materials with antiferromagnetic order. These observations are attributed to the competition between ferromagnetic and antiferromagnetic interactions, which is a result of the three nonequivalent Nd sites.

DOI： 10.1016/j.physb.2017.10.012

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The electrical resistivity. 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 4f(2) configuration. On cooling, rho's of all samples monotonically decrease, while the residual resistivity ratio rho(300 K)/rho(3 K) drastically decreases with x. In the whole range x (<=) 0.44, the magnetic contribution to the specific heat divided by temperature C-m/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 Gamma(3) doublet for the cubic T-d point group. On cooling, the C-m/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, C-m/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.

DOI： 10.1016/j.physb.2017.07.062

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We investigated the magnetization, M, and specific heat, C, of ThSi2-type PrGe2−δ. A polycrystalline sample of PrGe2−δ was prepared by arc-melting. Magnetization divided by magnetic field, M/B, increased sharply and C showed a clear jump at the Curie temperature, TC, of 14.6 K
these results indicate that PrGe2−δ ordered ferromagnetically. The magnetic entropy at TC reached Rln 3, indicating a quasi-triplet crystalline electric field (CEF) ground state. The maximum value of magnetic entropy change was 11.5 J/K kg with a field change of 7 T, which is comparable to those of other right rare-earth based magnetocaloric materials. This large magnetic entropy change was attributed to the quasi-triplet ground state of the CEF.

DOI： 10.1016/j.physb.2017.12.067

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DOI： 10.1063/1.5042435

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DOI： 10.1016/j.physb.2018.08.032

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We report experimentally observed linear dichroism in angle-resolved core-level photoemission spectra of PrIr2Zn20 and PrB6 with cubic symmetry. The different anisotropic 4f charge distributions between the compounds due to the crystalline-electric-field splitting are responsible for the difference in the linear dichroism, which has been verified by spectral simulations with the full multiplet theory for a single-site Pr3+ ion with cubic symmetry. The observed linear dichroism and polarization-dependent spectra in two different photoelectron directions for PrIr2Zn20 are reproduced by theoretical analysis for the Gamma(3) ground state, whereas those of the Pr 3d and 4d core levels indicate the Gamma(5) ground state for PrB6.

DOI： 10.7566/JPSJ.86.123703

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The magnetization of Gd1-xNdxZn2 (0 &lt; x &lt;= 1) was measured to study the effect of Nd substitution in GdZn2 with a Curie temperature of 85 K and a spin-reorientation transition temperature of 58 K on the magnetocaloric effect. The Nd counterpart NdZn2 shows antiferromagnetic order at 23 K. Samples of Gd1-xNdxZn2 (0 &lt; x &lt;= 1) were prepared by the melt-growth method. In Nd-substituted systems, the anomaly due to spin-reorientation disappeared. For x &gt;= 0.6, field-induced metamagnetic transitions were observed, indicating an antiferromagnetic ground state. This complex magnetism may originate from competition between ferromagnetic and antiferromagnetic interactions. Magnetic entropy change Delta S-m was calculated based on the magnetization measurements. Delta S-m was suppressed by Nd substitution for x values up to 0.6. For x = 1 (NdZn2), the maximum value of Delta S-m was -9 J/K kg, which is almost the same as those of other Nd-based magnetocaloric materials. (C) 2017 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2017.04.064

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Electrical resistivity rho, magnetization M, and specific heat C measurements are reported for single-crystalline samples of the 4f(3) Kramers system NdIr2Zn20. On cooling, rho(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 mu(eff) = 3.59 mu(B)/Nd and a paramagnetic Curie temperature of theta(p) = -3.6 K. The negative value of theta(p) indicates an antiferro-type interaction, although the enhancement of M(B) in the paramagnetic state implies a ferro-type one. A broad maximum in the magnetic specific heat C-m at around 15 K arises from the excitations from the crystalline electric field ground state of a magnetic Gamma(6) doublet to the excited Gamma(8) quartet lying at 39 K above the ground state. A sharp peak in C(T) at 0.65 K coincides with the drop in rho(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.

DOI： 10.7566/JPSJ.86.054708

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

Neutron diffraction measurements conducted under applied magnetic fields evidence the ordered structure of PrIr2Zn20 below approximately 0.1 K. A two-channel (orbital) Kondo effect is expected to act within the non-Kramers doublet ground state of the Pr 4f electrons of this material in cubic symmetry. Antiferromagnetic reflections are clearly induced under the influence of applied magnetic fields, which are characterized by the propagation vector q=(1/2,1/2,1/2). This result indicates antiferromagnetic-type ordering of the Pr f-electron multipoles. The Γ3-type quadrupole, O22, is the dominant order parameter deduced from the observed magnetic structure factor. We also discuss the entanglement of various finite-magnitude multipoles up to third rank, which are relevant to the low-energy crystalline-electric-field split levels in this material.

DOI： 10.1103/PhysRevB.95.155106

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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 T-s = 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 3meV. 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 4f(2) state of the Pr3+ ion remains in a non-Kramers doublet at T &gt; T-s, which is lifted by symmetry lowering of the Pr site at T &lt; T-s. 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.

DOI： 10.7566/JPSJ.86.034707

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Magnetic properties and magnetocaloric effect of the Gd-based compound GdZn2 have been investigated. GdZn2 shows a ferromagnetic transition at T-C = 85K and spin-reorientation transition at T-SR = 58 K. The maximum entropy change and relative cooling power (RCP) are estimated to be 11.5 J/K kg and 690 J/kg, respectively, for a magnetic field change of 7 T. The large value of RCP is suggested that GdZn2 is an attractive candidate for a low-temperature magnetic refrigerant.

DOI： 10.1016/j.jmmm.2016.09.104

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Orbital degrees of freedom in condensed matter could play important roles in forming a variety of exotic electronic states by interacting with conduction electrons. In 4f-electron systems, because of strong intra-atomic spin-orbit coupling, an orbitally degenerate state inherently carries quadrupolar degrees of freedom. The present work has focused on a purely quadrupole-active system PrIr2Zn20 showing superconductivity in the presence of an antiferroquadrupole order at T-Q = 0.11 K. We observed non-Fermi-liquid (NFL) behaviors emerging in the electrical resistivity rho and the 4f contribution to the specific heat, C-4f, in the paramagnetic state at T &gt; T-Q. Moreover, in magnetic fields B &lt;= 6 T, all data sets of rho(T) and C-4f (T) are well scaled with characteristic temperatures T-0's. This observation of the NFL state in the nonmagnetic quadrupole-active system has an origin intrinsically different from that observed in the vicinity of the conventional quantum critical point. It implies possible formation of a quadrupole Kondo lattice resulting from hybridization between the quadrupoles and the conduction electrons with an energy scale of k(B)T(0). At T &lt;= 0.13 K, rho(T) and C-4f (T) exhibit anomalies as B approaches 5 T. This is the manifestation of a field-induced crossover toward a Fermi-liquid ground state in the quadrupole Kondo lattice.

DOI： 10.1103/PhysRevB.94.075134

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To investigate a field-induced level crossing of the ground-state doublet in PrTr2Zn20 (Tr = Rh, Ir), we performed ultrasonic measurements in pulsed magnetic fields applied along the [110] and [001] directions and analyzed the results in the framework of the strain-susceptibility approach. Above 40 T for H parallel to [110], we observed an elastic softening of the transverse modulus (C11 - C-12)/2 corresponding to the ground-state doublet. In both compounds the softening is followed by a minimum at about 47 T at low temperatures. We predict the presence of a new field-induced phase boundary in PrTr2Zn20 at this field with two possible cases. The magnetic field of the minimum cannot be explained by only the quadrupole interaction.

DOI： 10.7566/JPSJ.85.043601

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We have investigated atomic dynamics of caged compound LaRu2Zn20 by the measurements of specific heat C and inelastic x-ray scattering (IXS). The lattice part of the specific heat C-lat divided by T-3, C-lat/T-3, shows a broad peak at around 15 K, which is reproduced by two Einstein modes with characteristic temperatures of theta(E1) = 35 K and theta(E2) = 82 K, respectively. IXS measurements along the [111] and [110] directions reveal weakly dispersive phonon modes at 3 meV (35 K) and 7 meV (80 K), respectively, whose values agree with the values of theta(E)'s. The first-principles calculation has assigned the 3 meV phonon modes as the optical modes of Zn atoms located at the middle of two La atoms, which is likely to lead to the structural instability.

DOI： 10.1103/PhysRevB.93.064105

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Electrical resistivity p, specific heat C, and magnetic susceptibility chi measurements are reported on PrIr2Zn20-xGa for x=0.5 and 1.0. On cooling, rho(T)'s monotonically decrease to the residual values which are two orders of magnitudes larger than that of PrIr2Zn20. A broad hump in C-m(T)'s at around 10 K can be reproduced by the two-level model with the Gamma(3) doublet ground state and the Gamma(4) triplet excited state. The magnetic entropy release shifts to higher temperatures with increasing x. The increase in chi(T) at T &lt; 5 K contrasts with the saturation of chi(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.

DOI： 10.1088/1742-6596/683/1/012011

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The electrical resistivity. and the specific heat C of Pr1-xLaxIr2Zn20 (0 &lt;= x &lt;= 1) were measured to study the effect of La substitution in PrIr2Zn20 on the superconducting transition at T-c = 0.05K and the antiferro-quadrupolar (AFQ) order at T-Q = 0.11 K, which occur in a nonmagnetic Gamma(3) doublet ground state. For x = 0.04, a cusp in C(T) due to the AFQ order was observed at T-Q = 0.07 K, while the cusp disappeared for x &gt;= 0.09, indicating that the AFQ order rapidly collapses with the dilution of the Pr lattice. On the other hand, in the whole range of 0 = x = 1, the superconducting transition to zero resistance was observed. With increasing x, T-c hardly changes up to x = 0.47, above which T-c increases linearly with x. The results indicate a rather weak interplay between T-c and T-Q. We have measured the magnetic field variation of T-c to determine the upper critical field of the superconducting state. The estimated effective mass m* for x = 0 is twice larger than that for x = 1, suggesting that moderately strong hybridization between 4 f and conduction electrons stabilizes the superconducting state in PrIr2Zn20.

DOI： 10.7566/JPSJ.84.063703

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The Laves phase compound CePt2 crystallizing in the cubic C15 (MgCu2 type) structure orders antiferromagnetically at 1.8 K. Single crystalline samples of Ce1-xPbxPt2 (0 &lt; x &lt; 0.5) have been prepared by the self-flux method. It is found that Pb atoms replace preferentially the Ce atoms in the face-center-cubic sublattice, forming a superlattice structure of the cubic C15b (MgSnCu4 type). We have measured the specific heat, magnetic susceptibility, and electrical resistivity. The results reveal that the Ce valence remains trivalent in Ce1-xPbxPt2 (x &lt;= 0.43) but the superlattice formation suppresses the antiferromagnetic ordering temperature below 0.35 K for x = 0.43. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jallcom.2015.02.051

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Low-temperature Seebeck coefficient S/T measurements have been performed on Pr-based 1-2-20 system, PrTr2X20 (Tr = Ti, Ta, V, Ir, X = Al, Zn) with non-Kramers doublet ground states. For PrTr2X20 with X = Al, we find a large S/T, which amounts to those of heavy fermion metals. By contrast, S/T for PrIr2Zn20 is found to be considerably small as the same order of magnitude as those of ordinary metals, despite the commonly enhanced Sommerfeld coefficient gamma throughout the system. A satisfactory of the quasi-universal relation between S/T and gamma as well as the Kadowaki-Woods relation demonstrates that the mass enhancement is realized in PrTr2Al20 due to the hybridization between f-electrons and the conduction electrons. We also find that the small S/T of PrIr2Zn20 is enhanced at low temperatures under the fields on the verge of quadrupole ordered phase, and in the same regime, the electrical resistivity follows the quadratic temperature dependence with a steep slope as a characteristic of Fermi liquid. The results imply an emergence of a nontrivial coherent state with sizable mass enhancement associated with the quadrupole degree of freedom.

DOI： 10.1088/1742-6596/592/1/012025

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Magnetic properties in a caged compound NdOs2Zn20 have been studied by the measurements of magnetization M, magnetic susceptibility M/B, and specific heat C. The measurements indicate that NdOs2Zn20 shows a structural transition at T-s = 62 K and a ferromagnetic transition at T-C = 0.6 K. The structural transition is also observed in the La counter-part LaOs2Zn20 at T-s = 151 K. The results of M(T)/B, M(B), and C(T) of NdOs2Zn20 are reproduced by a crystalline electrical field scheme with a Kramers doublet ground state.

DOI： 10.1016/j.phpro.2015.12.064

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Atomic dynamics of PrT2Zn20 for T=Ru with a structural transition at T-s=138 K and T=Ir without such a transition have been studied by inelastic X-ray scattering (IXS) measurements. The IXS spectra for T=Ru reveal an optical phonon excitation at 3 meV. We assign it to low-energy vibration of the Zn atom at the 16c site by taking account of the first principles calculation [Hasegawa et al. 2012 J. Phys.: Conf. Proc. 391 012016]. For T=Ir, on the other hand, the optical excitation at 3 meV was not observed. The contrasting results indicate that the low-energy optical phonon mode has a role in the structural transition in PrRu2Zn20 and isostructural La counterparts.

DOI： 10.1088/1742-6596/592/1/012024

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DOI： 10.7566/JPSCP.3.011039

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DOI： 10.7566/JPSCP.3.011068

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DOI： 10.7566/JPSCP.3.011091

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DOI： 10.7566/JPSCP.3.011042

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：KOREAN PHYSICAL SOC  

We report a study of La-139(I=7/2) nuclear magnetic resonance (NMR) to investigate the structural phase transition in the two caged compounds LaIr2Zn20 and LaRu2Zn20 from microscopic viewpoints. The former compound shows a second-order structural phase transition around T (s) = 200 K whereas the latter one shows a first-order structural phase transition at T (s) = 150 K. For both compounds, quite narrow La-NMR lines of less that 10 kHz without any electric field gradient (EFG) at the La site were observed at temperatures above T (s) , indicating that the local symmetry at the La site holds a cubic symmetry. On the other hand, an EFG at La site appears at temperatures below T (s) , evidencing symmetry lowering occurs at the La site at temperatures below T (s) . At temperatures around T (s) , both the nuclear spin-lattice relaxation rate (1/T (1)) and the nuclear spin-spin relaxation rate (1/T (2)) show unusual enhancements associated with the slowing down of the EFG fluctuations. However, the temperature dependences of 1/T (1) T and 1/T (2) are different from the typical slowingdown phenomena of the EFG fluctuations due to the classical motional narrowing. The lowering of the local symmetry at the La site is ascribed to a freezing of the rattling motion.

DOI： 10.3938/jkps.63.650

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The magnetic and transport properties of a Pr-encaged compound PrOs2Zn20 have been studied by the electrical resistivity ρ(T), magnetic susceptibility χ(T), and specific heat C(T) measurements down to 2 K. The ρ(T) and the C(T) show distinct anomalies at T s = 87 K, which resemble the anomalies in the isostructural and isoelectronic compound PrRu2Zn20 with the structural phase transition at T s = 138 K. Above 50 K, the χ(T) follows the Curie-Weiss law with the effective magnetic moment of 3.64 μB/f.u., which indicates the trivalent state of the Pr ion. The saturated behavior of χ(T) below 5 K indicates the Van-Vleck paramagnetic state. Hence, the Pr3+ ion falls in a nonmagnetic ground state under the crystalline electric field. © 2013 The Korean Physical Society.

DOI： 10.3938/jkps.62.2143

Scopus

J-GLOBAL

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Inelastic neutron scattering studies of PrT2Zn20 (T = Ir, Rh, and Ru) were carried out in order to reveal f electron states indispensable for the low-temperature properties of PrT2Zn20 associated with multipole degrees of freedom. Sharp excitation peaks, which are located within excitation energies of less than 10 meV, are attributed to crystal field splitting level schemes of 4f(2) in Pr3+ ions. Data of the Ir-based compound is explained by a non-Kramers doublet ground state in the local symmetry T-d for this system. Results of the Rh-based compound are explained by the scheme for the Pr-site local cubic symmetry T in the low-temperature structural phase, which is lower than T-d. In the Ru-based compound, the doublet is split as a result of the loss of local cubic symmetry through structural transformation. These experimental results confirm the non-Kramers doublet ground state that causes the quadrupole ordering or orbital Kondo effect predicted for this series of compounds.

DOI： 10.7566/JPSJ.82.043707

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Superconducting and antiferroquadrupolar (AFQ) transitions in a Pr-based compound PrRh2Zn20 have been found to occur simultaneously at T-c = T-Q = 0.06 K. The superconducting transition manifests itself by zero resistance and large diamagnetic susceptibility. The specific heat exhibits a Schottky anomaly peaking at 14 K, and magnetization curves measured at 2 K show anisotropic behaviors. The analysis of these data indicates that the crystalline electric field ground state of the trivalent Pr ion is the non-Kramers Gamma(3) doublet with the quadrupolar degrees of freedom. A sharp peak in the specific heat at 0.06 K has been attributed not to the superconducting transition but to the AFQ transition because the ordering temperature T-Q decreases in B parallel to [100] but increases in B parallel to [110] and B parallel to [111] with increasing B up to 6 T. This anisotropic behavior of T-Q(B) can be well explained by a two-sublattice mean-field calculation, which corroborates the AFQ ordered state below T-Q. The entropy release at TQ is only 10% of R ln 2 expected for the Gamma(3) doublet, suggesting possible interplay between the quadrupolar degrees of freedom and the superconductivity.

DOI： 10.1103/PhysRevB.86.184426

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We performed electrical resistivity rho, magnetic susceptibility chi, specific heat C and electron diffraction measurements on single-crystalline samples of PrT2Zn20 (T = Ru, Rh and Ir). The three compounds show the Van Vleck paramagnetic behavior, indicating the nonmagnetic crystalline electric field (CEF) ground states. A Schottky-type peak appears at around 14 K, irrespective of the T element, which can be moderately reproduced by a doublet-triplet model. For T = Ru, a structural transition occurs at T-s = 138 K, below which no phase transition appears down to 0.04 K. On the other hand, for T = Ir, antiferroquadrupole (AFQ) ordering arising from the nonmagnetic 03 doublet takes place at T-Q = 0.11 K. For T = Rh, despite a structural transition between 170 and 470 K, the CEF ground state is still the non-Kramers Gamma(3) doublet. However, no phase transition due to the Gamma(3) doublet was observed even down to 0.1 K.

DOI： 10.1088/0953-8984/24/29/294207

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We performed muon spin rotation and relaxation (mu SR) measurements in the caged-structure heavy-fermion system PrIr2Zn20 to elucidate its magnetic and superconducting properties. Temperature-independent mu SR spectra were observed below 1 K, indicating that the phase transition at 0.11 K is of a nonmagnetic origin, most probably pure quadrupole ordering. In the superconducting phase, no sign of unconventional superconductivity, such as superconductivity with broken time-reversal symmetry, was seen below T-c = 0.05 K. We also observed spontaneous muon spin precession in zero field in the paramagnetic phase below 15 K, suggesting that unusual coupling between Pr-141 nuclei and muons is realized in PrIr2Zn20.

DOI： 10.1103/PhysRevB.85.235152

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

La-139 nuclear magnetic resonance (NMR) measurements have been carried out to investigate structural phase transition in LaIr2Zn20 and LaRu2Zn20. The former compound undergoes second-order structural phase transition at T-s = 200 K, whereas the latter undergoes first-order transition at T-s = 150 K. For both compounds, narrow Lorentzian La-NMR spectra with no electric field gradient (EFG) were observed above T-s, indicating that the local symmetry at the La site holds a cubic symmetry. Below T-s, the La NMR spectrum of LaIr2Zn20 splits into more than two lines with broad tails distributed over a frequency range of 0.8 MHz, while that of LaRu2Zn20 broadens with broad tails over a frequency range of 1.6 MHz. The appearance of an EFG at the La site below T-s is clear evidence that the cubic local symmetry at the La site lowers below T-s. Both compounds show unusual increases in the nuclear spin-lattice relaxation rate at approximately T-s, indicating the slowing down of the fluctuation of the EFG associated with the rattling motion of Zn atoms. Below T-s, the lowering of the local symmetry at the La site can be ascribed to the freezing of the rattling motion.

DOI： 10.1143/JPSJ.81.023711

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELSEVIER SCIENCE BV  

We have performed muon spin rotation and relaxation experiments in caged compound PrIr2Zn20 with the non magnetic 3 doublet state at J-PARC MUSE to elucidate the magnetic state. In the paramagnetic phase, we have observed a spontaneous muon spin precession, even in a zero field. This result suggests that a novel coupling between Pr-141 nuclei and muon is formed in PrIr2Zn20.

DOI： 10.1016/j.phpro.2012.04.055

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

We have prepared single crystals of PrRh2Zn20, which is isostructural and isoelectronic to PrIr2Zn20 where coexistence of superconductivity and antiferroquadrupolar order has been recently found. The electrical resistivity rho, magnetic susceptibility chi, isothermal magnetization M, and specific heat C have been measured. Hysteretic behavior appears in rho(T) for 170 K center dot center dot T center dot center dot 470 K, suggesting a structural phase transition. On cooling below 10 K, chi approaches a constant value,indicating a non-magnetic ground state. Anisotropic behavior in M(B)at 1.8 K for B center dot center dot 2 T and abroad peak in C(T) at around 10 K are reproduced by the twocrystal-field levels split by 32 K;a nonmagnetic Gamma(3) doublet ground state and a magnetic Gamma(4) triplet excited state

DOI： 10.1088/1742-6596/391/1/012051

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

The cage compound PrIr2Zn20 with the non-Kramers doublet ground state shows a phase transition at T-Q similar to 0.11 K. To investigate the origin of the phase transition, we have carried out ultrasonic measurements on single-crystalline samples. From the disappearance of electric quadrupole degrees of freedom at T-Q and the negative sign of a quadrupole-quadrupole coupling constant, we clarified that the phase transition is antiferro-quadrupolar ordering at the lowest temperature ever measured. We also found anisotropic magnetic field H dependence and the re-entrant behavior of T-Q(H) from the measurements in H parallel to the [100], [110], and [111] axes. The quadrupole Kondo effect interrupts the long-range quadrupole interaction at very low temperatures and thus lowers T-Q. We first reported the rattling motion of Zn atoms at the 16c site in the RT2Zn20 system.

DOI： 10.1143/JPSJ.80.093601

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

An antiferroquadrupolar ordering at T-Q = 0.11 K has been found in a Pr-based superconductor PrIr2Zn20. The measurements of specific heat and magnetization revealed the non-Kramers Gamma(3) doublet ground state with the quadrupolar degrees of freedom. The specific heat exhibits a sharp peak at T-Q = 0.11 K. The increment of T-Q in magnetic fields and the anisotropic B - T phase diagram are consistent with the antiferroquadrupolar ordered state below T-Q. The entropy release at T-Q is only 20% of Rln(2), suggesting that the quadrupolar fluctuations play a role in the formation of the superconducting pairs below T-c = 0.05 K.

DOI： 10.1103/PhysRevLett.106.177001

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

Elastic properties of the cage compound PrRu(2)Zn(20) have been investigated by means of ultrasonic measurement. The transverse modulus (C(11)-C(12))/2 shows elastic softening below 250 K and exhibits a discontinuous change in the slope at 150 K. An abrupt elastic hardening is observed below T(s)=138 K which is a non-magnetic phase transition temperature. The phase transition at T(s) is accompanied by hysteresis indicating that the transitionis of first-order. The softening above 150 K is well reproduced by Curie-Weiss-like formula for the co-operative Jahn-Teller effect with a negative quadrupole-quadrupole coupling constant. The phase transition at T(s) is not caused by 4f-electrons of Prions. There is a possibility that the elastic hardening below 3 K originates from the rattling motion.

DOI： 10.1088/1742-6596/273/1/012136

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Electrical resistivity rho, specific heat C, magnetization M measurements are reported on four compounds, LaRu2Zn20, PrRu2Zn20, LaIr2Zn20, and PrIr2Zn20, which crystallize in a cubic CeCr2Al20-type structure. LaRu2Zn20, LaIr2Zn20, and PrIr2Zn20 show superconducting transitions at T-C = 0.2, 0.6, and similar to 0.05 K, respectively, whereas PrRu2Zn20 remains a normal state down to 0.04 K. This is the first observation of superconductivity in the family of RT2X20 (T = transition metal; X = Al and Zn). Furthermore, structural phase transitions manifest themselves at T-s = 150, 138, and 200K for LaRu2Zn20, PrRu2Zn20, and LaIr2Zn20, respectively. No magnetic transition is found in PrRu2Zn20 and PrIr2Zn20 down to 1.8 K. On cooling PrIr2Zn20 below 2K, the specific heat divided by temperature, C/T, continuously increases and reaches 5 J/(K-2.mol) at 0.4 K, suggesting that Pr 4f(2) electrons are involved in the heavy-fermion state, as observed in a related compound YbCo2Zn20.

DOI： 10.1143/JPSJ.79.033704

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.71.1.0_2009

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DOI： 10.11316/jpsgaiyo.71.2.0_1814

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DOI： 10.11316/jpsgaiyo.70.1.0_2003

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DOI： 10.11316/jpsgaiyo.70.2.0_1852

CiNii Books

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DOI： 10.11316/jpsgaiyo.70.1.0_2005

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DOI： 10.11316/jpsgaiyo.70.2.0_2028

CiNii Books

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DOI： 10.11316/jpsgaiyo.70.2.0_2030

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<p>4f2配位をとるPr希薄系Y1-xPrxIr2Zn20 (x≤ 0.05)の電気抵抗率ρと磁気比熱Cm/Tは，0.1< T< 0.4 KでそれぞれT^1/2，-lnTに比例する非フェルミ液体(NFL)的挙動を示す。本研究では，不純物四極子近藤効果の可能性について検証するため， NFL的挙動に対する磁場効果を調べた。x= 0.05のCm/Tでは，B= 4 Tで0.3 K付近に肩が現れ，磁場とともに高温へシフトする。これは，磁場により基底二重項が分裂して生じるショットキー異常である。ρは，B= 4 TではT< 0.5 Kで直線的な振る舞いに変化する。これらの結果は，B= 0でのNFL的挙動が基底二重項の四極子自由度に起因することを支持する。</p>

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<p>非クラマースPr希薄系の基底二重項で期待される不純物四極子近藤効果について調べるため，Y_1-x_Pr_x_Ir_2_Zn_20_ (x≤ 0.05)を作製し, 0.04 K< Tでの比熱と電気抵抗を測定した。電気抵抗率ρ(T)はT< 1 Kで降温とともにT^(1/2)^に従って上凸で減少し，磁気比熱Cm(T)/TはT< 2 Kで-lnTで増大する非フェルミ液体的挙動を示す。これらの振る舞いはPrの希薄極限で生じていることから，Pr単サイトのc-f混成効果により生じた四極子近藤効果を示唆する。</p>

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