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Atomic displacements and lattice distortion in the magnetic-field-induced charge ordered state of SmRu$_{4}$P$_{12}$

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 Added by Takeshi Matsumura
 Publication date 2016
  fields Physics
and research's language is English




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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.



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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.
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.
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