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Register a new userIsotropic parallel antiferromagnetism in the magnetic-field-induced charge-ordered state of SmRu$_4$P$_{12}$ caused by $p$-$f$ hybridization
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Nature of the field-induced charge ordered phase (phase II) of SmRu$_4$P$_{12}$ has been investigated by resonant x-ray diffraction (RXD) and polarized neutron diffraction (PND), focusing on the relationship between the atomic displacements and the antiferromagnetic (AFM) moments of Sm. From the analysis of the interference between the non-resonant Thomson scattering and the resonant magnetic scattering, combined with the spectral function obtained from x-ray magnetic circular dichroism, it is shown that the AFM moment of Sm prefers to be parallel to the field ($m_{text{AF}} parallel H$), giving rise to large and small moment sites around which the P$_{12}$ and Ru cage contract and expand, respectively. This is associated with the formation of the staggered ordering of the $Gamma_7$-like and $Gamma_8$-like crystal-field states, providing a strong piece of evidence for the charge order. PND was also performed to obtain complementary and unambiguous conclusion. In addition, isotropic and continuous nature of the phase II is demonstrated by the field-direction invariance of the interference spectrum in RXD. Crucial role of the $p$-$f$ hybridization is shown by resonant soft x-ray diffraction at the P $K$-edge ($1sleftrightarrow 3p$), where we detected a resonance due to the spin polarized $3p$ orbitals reflecting the AFM order of Sm.
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Structural properties of SmRu$_4$P$_{12}$ in the anomalous magnetic ordered phase between $T^*sim 14 $ K and $T_{text{N}}=16.5$ K in magnetic fields has been studied by x-ray diffraction. Atomic displacements of Ru and P, reflecting the field-induced charge order of the $p$ electrons, have been deduced by analyzing the intensities of the forbidden Bragg peaks, assuming a cubic space group $Pmbar{3}$. Also, by utilizing high-resolution x-ray diffraction experiment, we observed a splitting of fundamental Bragg peaks, clarifying that the unit cell in the magnetic ordered phase is rhombohedral elongated along the $[1, 1, 1]$ axis. Responses of the rhombohedral domains to the magnetic field, which reflects the direction of the magnetic moment, is studied in detail.
Electronic structures of the filled-skutterudite compounds PrRu$_4$P$_{12}$ and SmRu$_4$P$_{12}$, which undergo a metal-insulator transition (MIT) at $T_{rm MI}$ = 60 K and 16 K, respectively, have been studied by means of optical spectroscopy. Their optical conductivity spectra develop an energy gap of $sim$ 10 meV below $T_{rm MI}$. The observed characteristics of the energy gap are qualitatively different from those of the Kondo semiconductors. In addition, optical phonon peaks in the spectra show anomalies upon the MIT, including broadening and shifts at $T_{rm MI}$ and an appearance of new peaks below $T_{rm MI}$. These results are discussed in terms of density waves or orbital ordering previously predicted for these compounds.
Semiconducting skutterudite CeFe$_4$P$_{12}$ is investigated by synchrotron x-ray photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). Ce 3$d$ core-level PES and 3$d-4f$ XAS, in combination with single impurity Anderson model (SIAM) calculations, confirm features due to $f^0$, $f^1$ and $f^2$ configurations. The Ce 4$f$ density of states (DOS) indicates absence of a Kondo resonance at Fermi level, but can still be explained by SIAM with a small gap in non-$f$ DOS. While Ce 4$f$ partial DOS from band structure calculations are also consistent with the main Ce 4$f$ DOS, the importance of SIAM for core and valence spectra indicates Kondo semiconducting mixed valence for CeFe$_4$P$_{12}$, derived from strong hybridization between non-$f$ conduction and Ce 4$f$ DOS.
We report the de Haas-van Alphen (dHvA) experiment on the filled skutterudite PrFe$_4$P$_{12}$ exhibiting apparent Kondo-like behaviors in the transport and thermal properties. We have found enormously enhanced cyclotron effective mass $m^{rm ast}_{rm c}=81 m_{rm 0}$ in the high field phase (HFP), which indicates that PrFe$_4$P$_{12}$ is the first Pr-compound in which really heavy mass has been unambiguously confirmed. Also in the low field non-magnetic ordered phase (LOP), we observed the dHvA branch with $m^{rm ast}_{rm c}=10 m_{0}$ that is quite heavy taking into account its small Fermi surface volume (0.15% of the Brillouin zone size). The insensitivity of mass in LOP against the magnetic field suggests that the quadrupolar interaction plays a main role both in the mass renormalization and the LOP formation.
We successfully synthesized a verdazyl-based charge-transfer salt $[$$o$-MePy-V-($p$-Br)$_2]$FeCl$_4$, which has an $S_{rm{V}}$=1/2 on the radical $o$-MePy-V-($p$-Br)$_2$ and an $S_{rm{Fe}}$=5/2 on the FeCl$_4$ anion. $Ab$ $initio$ molecular orbital calculations indicate the formation of an $S_{rm{V}}$=1/2 honeycomb lattice composed of three types of exchange interaction with two types of inequivalent site. Further, the $S_{rm{V}}$=1/2 at one site is sandwiched by $S_{rm{Fe}}$=5/2 spins through antiferromagnetic (AF) interactions. The magnetic properties indicate that the dominant AF interactions between the $S_{rm{V}}$ = 1/2 spins form a gapped singlet state, and the remaining $S_{rm{Fe}}$ = 5/2 spins cause an AF order. The magnetization curve exhibits a linear increase up to approximately 7 T, and an unconventional 5/6 magnetization plateau appears between 7 T and 40 T. We discuss the differences between the effective interactions associated with the magnetic properties of the present compound and ($o$-MePy-V)FeCl$_4$. We explain the low-field linear magnetization curve through a mean-field approximation of an $S_{rm{Fe}}$ = 5/2 spin model. At higher field regions, the 5/6 magnetization plateau and subsequent nonlinear increase are reproduced by the $S_{rm{V}}$ = 1/2 AF dimer, in which a particular internal field is applied to one of the spin sites. The ESR resonance signals in the low-temperature and low-field regime are explained by conventional two-sublattice AF resonance modes with easy-axis anisotropy. These results demonstrate that exchange interactions between $S_{rm{V}}$ = 1/2 and $S_{rm{Fe}}$ = 5/2 spins in $[$$o$-MePy-V-($p$-Br)$_2]$FeCl$_4$ realize unconventional magnetic properties with low-field classical behavior and field-induced quantum behavior.
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