Determination of spin Hamiltonian in the Ni4_4 magnetic molecule

Magnetic excitations in a Ni$_4$ magnetic molecule were investigated by inelastic neutron scattering and bulk susceptibility ($\chi_\text{bulk}$) techniques. The magnetic excitation spectrum obtained from the inelastic neutron scattering experiments exhibits three modes at energy transfers of $\hbar\omega=0.5$, 1.35, and 1.6 meV. We show that the energy, momentum, and temperature dependences of the inelastic neutron scattering data and $\chi_\text{bulk}$ can be well reproduced by an effective spin Hamiltonian consisted of intra-molecule exchange interactions, a single-ionic anisotropy, biquadratic interactions, and Zeeman term. Under a hydrostatic pressure, the bulk magnetization decreases with increasing pressure, which along with the biquadratic term indicates spin-lattice coupling present in this system.Comment: 6 pages, 6 figures, and 2 table

Observation of Modulated Quadrupolar Structures in PrPb3

Neutron diffraction measurements have been performed on the cubic compound PrPb3 in a [001] magnetic field to examine the quadrupolar ordering. Antiferromagnetic components with q=(1/2+-d 1/2 0), (1/2 1/2+-d 0) (d~1/8) are observed below the transition temperature TQ (0.4 K at H=0) whose amplitudes vary linear with H and vanish at zero field, providing the first evidence for a modulated quadrupolar phase. For H<1 T, a non-square modulated state persists even below 100 mK suggesting quadrupole moments associated with a Gamma3 doublet ground state to be partially quenched by hybridization with conduction electrons.Comment: Physical Review Letters, in press. 4 pages, 4 figure

Antiferro-quadrupole state of orbital-degenerate Kondo lattice model with f^2 configuration

To clarify a key role of $f$ orbitals in the emergence of antiferro-quadrupole structure in PrPb$_{3}$, we investigate the ground-state property of an orbital-degenerate Kondo lattice model by numerical diagonalization techniques. In PrPb$_{3}$, Pr$^{3+}$ has a $4f^{2}$ configuration and the crystalline-electric-field ground state is a non-Kramers doublet $\Gamma_{3}$. In a $j$-$j$ coupling scheme, the $\Gamma_{3}$ state is described by two local singlets, each of which consists of two $f$ electrons with one in $\Gamma_{7}$ and another in $\Gamma_{8}$ orbitals. Since in a cubic structure, $\Gamma_{7}$ has localized nature, while $\Gamma_{8}$ orbitals are rather itinerant, we propose the orbital-degenerate Kondo lattice model for an effective Hamiltonian of PrPb$_{3}$. We show that an antiferro-orbital state is favored by the so-called double-exchange mechanism which is characteristic of multi-orbital systems.Comment: 3 pages, 3 figures, Proceedings of Skutterudite2007 (September 26-30, 2007, Kobe

Detailed Measurements of Characteristic Profiles of Magnetic Diffuse Scattering in ErB2_2C2_2

Detailed neutron diffraction measurements on a single crystalline ErB$_2$C$_2$ were performed. We observed magnetic diffuse scattering which consists of three components just above the transition temperatures, which is also observed in characteristic antiferroquadrupolar ordering compounds HoB$_2$C$_2$ and TbB$_2$C$_2$. The result of this experiments indicates that the antiferroquadrupolar interaction is not dominantly important as a origin of the magnetic diffuse scattering.Comment: 5 pages, 5 figures, submitted to J. Phys. Soc. Jp

Superconductivity in the Ferroquadrupolar State in the Quadrupolar Kondo Lattice PrTi2_2Al20_{20}

The cubic compound PrTi$_2$Al$_{20}$ is a quadrupolar Kondo lattice system that exhibits quadrupolar ordering due to the non-Kramers $\Gamma_3$ ground doublet and has strong hybridization between $4f$ and conduction electrons. Our study using high-purity single crystal reveals that PrTi$_2$Al$_{20}$ exhibits type-II superconductivity at $T_{\rm c} = 200$ mK in the nonmagnetic ferroquadrupolar state. The superconducting critical temperature and field phase diagram suggests moderately enhanced effective mass of $m^*/m_0 \sim 16$

Magnetic Phase Diagrams with Possible Field-induced Antiferroquadrupolar Order in TbB2_2C2_2

Magnetic phase diagrams of a tetragonal antiferromagnet TbB$_2$C$_2$ were clarified by temperature and field dependence of magnetization. It is noticeable that the N{\'e}el temperature in TbB$_2$C$_2$ is anomalously enhanced with magnetic fields, in particular the enhancement reaches 13.5 K for the ${}$ direction at 10 T. The magnetization processes as well as the phase diagrams are well interpreted assuming that there appear field-induced antiferroquadrupolar ordered phases in TbB$_2$C$_2$. The phase diagrams of the AFQ compounds in RB$_2$C$_2$ are systematically understood in terms of the competition with AFQ and AFM interactions.Comment: 4 pages, 4 figures, RevTeX

Itinerant-Electron Magnet of the Pyrochlore Lattice: Indium-Doped YMn2Zn20

We report on a ternary intermetallic compound, "YMn2Zn20", comprising a pyrochlore lattice made of Mn atoms. A series of In-doped single crystals undergo no magnetic long-range order down to 0.4 K, in spite of the fact that the Mn atom carries a local magnetic moment at high temperatures, showing Curie-Weiss magnetism. However, In-rich crystals exhibit spin-glass transitions at approximately 10 K due to a disorder arising from the substitution, while, with decreasing In content, the spin-glass transition temperature is reduced to 1 K. Then, heat capacity divided by temperature approaches a large value of 280 mJ K-2 mol-1, suggesting a significantly large mass enhancement for conduction electrons. This heavy-fermion-like behavior is not induced by the Kondo effect as in ordinary f-electron compounds, but by an alternative mechanism related to the geometrical frustration on the pyrochlore lattice, as in (Y,Sc)Mn2 and LiV2O4, which may allow spin entropy to survive down to low temperatures and to couple with conduction electrons.Comment: 5 pages, 4 figures, J. Phys. Soc. Jpn., in pres

Kondo Effects and Multipolar Order in the cubic PrTr2Al20 (Tr=Ti, V)

Our single crystal study reveals that PrTr2Al20 (Tr = Ti and V) provides the first examples of a cubic {\Gamma}3 nonmagnetic ground doublet system that shows the Kondo effect including a -ln T dependent resistivity. The {\Gamma}3 quadrupolar moments in PrV2Al20 induce anomalous metallic behavior through hybridization with conduction electrons, such as T^{1/2} dependent resistivity and susceptibility below ~ 20 K down to its ordering temperature T_O = 0.6 K. In PrTi2Al20, however, quadrupoles are well-localized and exhibit an order at T_O = 2.0 K. Stronger Kondo coupling in PrV2Al20 than in PrTi2Al20 suppresses quadrupolar ordering, and instead promotes hybridization between the {\Gamma}3 doublet and conduction electrons, leading to most likely the quadrupolar Kondo effect.Comment: 12 pages, 4 figure

Effects of Impurities with Singlet-Triplet Configuration on Multiband Superconductors

Roles of multipole degrees of freedom in multiband superconductors are investigated in a case of impurities whose low-lying states consist of singlet ground and triplet excited states, which is related to the experimental fact that the transition temperature $T_{\rm c}$ is increased by Pr substitution for La in LaOs$_4$Sb$_{12}$. The most important contribution to the $T_{\rm c}$ increase comes from the inelastic interband scattering of electrons coupled to quadrupole or octupole moments of impurities. It is found that a magnetic field modifies an effective pairing interaction and the scattering anisotropy appears in the field-orientation dependence of the upper critical field $H_{{\rm c}2}$ in the vicinity of $T_{\rm c}$, although a uniaxial anisotropic field is required for experimental detection. This would be proof that the Pr internal degrees of freedom are relevant to the stability of superconductivity in (La$_{1-x}$Pr$_x$)Os$_4$Sb$_{12}$.Comment: 10 pages, 5 figures, to appear in J. Phys. Soc. Jp