Masafumi Sera

Hiroshima University, Hirosima, Hiroshima, Japan

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Publications (258)466.84 Total impact

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    ABSTRACT: A Kondo semiconductor CeRu$_2$Al$_{10}$ with an orthorhombic crystal structure shows an unusual antiferromagnetic ordering at rather high temperature $T_0$ of 27.3 K, which is lower than the Kondo temperature $T_{\rm K}\sim$ 60 K. In optical conductivity [$\sigma(\omega)$] spectra that directly reflect electronic structure, the $c$-$f$ hybridization gap between the conduction and $4f$ states is observed at around 40 meV along the three principal axes. However, an additional peak at around 20 meV appears only along the $b$ axis. With increasing $x$ to 0.05 in Ce(Ru$_{1-x}$Rh$_x$)$_2$Al$_{10}$, the $T_0$ decreases slightly from 27.3 K to 24 K, but the direction of the magnetic moment changes from the $c$ axis to the $a$ axis. Thereby, the $c$-$f$ hybridization gap in the $\sigma(\omega)$ spectra is strongly suppressed, but the intensity of the 20-meV peak remains as strong as for $x=0$. These results suggest that the change of the magnetic moment direction originates from the decreasing of the $c$-$f$ hybridization intensity. The magnetic ordering temperature $T_0$ is not directly related to the $c$-$f$ hybridization but is related to the charge excitation at 20 meV observed along the $b$ axis.
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    ABSTRACT: We have studied the Pr- and La-doping effects on the magnetic anisotropy in the antiferro-magnetic (AFM) phase of ${\mathrm{CeRu}}_{2}{\mathrm{Al}}_{10}$. The crystalline electric field (CEF) splitting in ${\mathrm{PrRu}}_{2}{\mathrm{Al}}_{10}$ was found to be as large as $\ensuremath{\sim}800$ K with a singlet ground state. In ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Ru}}_{2}{\mathrm{Al}}_{10}$, the CEF level scheme of the Pr ion is not changed with $x$. The AFM moment (${m}_{\mathrm{AF}}$) is rotated from $c$ to $b$ axis in both systems at ${x}_{c}^{\mathrm{sr}}\ensuremath{\sim}0.03$ and $\ensuremath{\sim}0.07$ for Ln=Pr and La, respectively. As the ionic radius of La and Pr is larger and smaller than that of Ce, respectively, these results indicate that the chemical pressure effect is not associated with the rotation of ${m}_{\mathrm{AF}}$, but is caused by the suppression of the $c\ensuremath{-}f$ hybridization originating from the decrease of $4f$ electrons of Ce ions by Ce-site substitution. Since a small amount of Pr or La doping changes easily the magnetization easy axis of all the moments on Ce sites, the origin of the magnetic anisotropy is not the local single ion effect but the bandlike effect through the anisotropic $c\ensuremath{-}f$ hybridization. The magnetic phase diagrams of ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Ln}}_{x}{\mathrm{Ru}}_{2}{\mathrm{Al}}_{10}$ indicate that above ${x}_{c}^{\mathrm{sr}}$, the AFM order with ${m}_{\mathrm{AF}}\ensuremath{\parallel}b$ continues to exist up to ${x}_{c}$, which is $\ensuremath{\sim}0.4$ and $\ensuremath{\sim}0.6$ in Ln=Pr and Ln=La, respectively. This indicates that even in the sample with an AFM transition temperature (${T}_{0}$) near ${x}_{c}$, the anisotropic $c\ensuremath{-}f$ hybridization dominates the AFM order. A large positive transverse magnetoresistance is seen below ${T}_{0}$, but a very small one above ${T}_{0}$. Together with the results of Hall resistivity and the observation of Shubnikov\char21{}de Haas oscillation, we propose that there exist large Fermi surfaces above ${T}_{0}$ and small ones below ${T}_{0}$. A gap is opened by the AFM order on almost the area of the large Fermi surface, and small Fermi surfaces are constructed below ${T}_{0}$, although we do not know the mechanism, which might be specific to the AFM order in Kondo semiconductors. The largest suppression of the magnetic scattering below ${T}_{0}$ is observed for the current $I\ensuremath{\parallel}a$ and the smallest one for $I\ensuremath{\parallel}b$. This anisotropy may be associated with the anisotropic $c\ensuremath{-}f$ hybridization, which may contribute to the anisotropic magnetic scattering of the conduction electron below ${T}_{0}$.
    Physical Review B 06/2015; 91(23). DOI:10.1103/PhysRevB.91.235124 · 3.66 Impact Factor
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    ABSTRACT: We have investigated the anomalous ordered phase of CeTe under high pressure, which has been suggested to be an antiferroquadrupole ordered phase. An anisotropic magnetic phase diagram has been obtained from magnetization and specific heat measurements for the three main field directions along [100], [110], and [111]. We discuss the magnetic phase diagram using a two-sublattice mean-field calculation including antiferromagnetic and antiferroquadrupolar interactions. The anomalous ordered phase can be interpreted as an antiferromagnetic ordered phase, which is strongly affected by the antiferroquadrupolar interaction through the off-diagonal matrix element between the Gamma(7) crystal-field ground state and the Gamma(8) excited state.
    Journal of the Physical Society of Japan 04/2015; 84(4):044708. DOI:10.7566/JPSJ.84.044708 · 1.48 Impact Factor
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    ABSTRACT: Numerical investigations on muon sites in Ce-based Kondo semiconductors, Ce(Ru,Rh)2Al10 were carried out by using the Density Functional Theory. From the view point of simple electrostatic potential calculations, we found all the previously reported muon sites, suggested by di↵erent groups (Kambe S et al. 2010 J. Phys. Soc. Jpn. 79 053708 and Khalyavin D D et al., 2010 Phys. Rev. B 82 100405(R)), can be possibly chosen as muon stopping sites. We also investigated the changes in the potential of the Rh-doped case. We discovered that the electronic potential around the nearest Ru atom to the substituted Rh atom is a↵ected and the potential becomes asymmetric around the nearest Ru ion. Although big changes in hyperfine fields at muon sites have been reported (Guo H et al. 2013 Phys. Rev. B 88 115206), the muon positions estimated from the potential calculations do not change much.
    13th International Conference on Muon Spin Rotation, Relaxation and Resonance, Grindelwald, Switzerland; 12/2014
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    ABSTRACT: We have performed Ru-NQR measurements on CeRu2Al10 exhibiting novel phase transition at an abnormally high temperature T-0 = 27.3K and on NdRu2Al10 with a magnetic transition temperature T-m = 2.4K as a reference RKKY system. The splitting of the NQR line due to internal fields below T-0 shows a mean-field-like monotonic increase, indicating no change in the magnetic structure below T-0. The internal field strength is one order larger than those at Al sites in CeRu2Al10, and is comparable to that at the Ru site in NdRu2Al10 despite the sevenfold smaller magnitude of the 4 f moment, being indicative of an enhanced conduction electron polarization at the Ru site. One of the causes of the high T-0 might be the enhanced exchange coupling through the Ce-Ru-Ce path mediated by the enhanced conduction electron polarization. Being similar to that in the Al site, the nuclear spin-lattice relaxation rate 1/T-1 shows a gap-like decrease below T-0 without enhancement owing to a critical slowing down at T-0, in contrast to the mean field 2nd-order transition.
    Journal of the Physical Society of Japan 10/2014; 83(10):103705. DOI:10.7566/JPSJ.83.103705 · 1.48 Impact Factor
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    ABSTRACT: The effect of electron doping by the substitution of Rh for Ru on unconventional magnetic order in CeRu2Al10 was investigated via neutron powder diffraction. In Ce(Ru1-xRhx)(2)Al-10 with x = 0.05, 0.12, and 0.2, reorientation of the ordered moment from the c-axis as in pure CeRu2Al10 to the a-axis takes place in all samples, while the ordering vector q = (0, 1, 0) remains unchanged within this concentration range. The moment reorientation is accompanied by an increase in its size by a factor of similar to 2.4, from mu = 0.43 mu(B) at x = 0 to mu = 1.06, 1.04, and 1.02 mu(B) for x = 0.05, 0.12, and 0.2, respectively. The continuous decrease in the Neel temperature T-0(T-N), despite an abrupt increase in mu, underlines the strong anisotropy in the exchange interaction in CeRu2Al10 and the fact that this anisotropy is easily suppressed by electron doping.
    Journal of the Physical Society of Japan 10/2014; 83(10):104707. DOI:10.7566/JPSJ.83.104707 · 1.48 Impact Factor
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    ABSTRACT: We investigated the Ce- and Ru-site substitution effects on the spin gap in CeRu2Al10 by measuring the magnetic and thermal properties of Ce(1-y)Ln(y)Ru(2)Al(10) (Ln = La, Pr, Y) and Ce(Ru1-xTx)(2)Al-10 (T = Re, Rh) system, respectively. The magnetic susceptibility shows that the itinerant and localized nature is enhanced by Re and Rh doping, respectively. In the latter, the orientation of the magnetic moment in the antiferromagnetic ordered phase suddenly changes from the c to a axis at x = 0.03. The orientation of the magnetic moment to the a axis is consistent with the large anisotropy of the magnetic susceptibility in the paramagnetic region. The specific heat shows that the exponential temperature dependence, e(-Delta/kBT), and the electronic specific coefficient gamma is not changed in the Ce-site substituted systems at least up to y = 0.1 but in the Ru-site substituted systems, the e(-Delta/kBT) dependence disappears, and the gamma value increases rapidly with x. These indicate that although the spin gap is robust against the Ce-site substitution by Ln ion at least up to y = 0.1, the spin gap is rapidly collapsed by the Ru-site substitution and in place, the conduction electron with heavy effective mass appears at the Fermi level. The spin gap which is formed under the subtle balance between the localized and itinerant nature is easily collapsed by a small amount of Re or Rh doping.
    Physical Review B 10/2014; 90(16). DOI:10.1103/PhysRevB.90.165124 · 3.66 Impact Factor
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    ABSTRACT: The compound NdFe$_2$Al$_{10}$ ($Cmcm$, space group #63) has been studied by both powder and single-crystal neutron diffraction. Below T$_N$ = 3.9 K, the Nd$^{3+}$ magnetic moments order in a commensurate magnetic structure, whose unit cell consists of four orthorhombic unit cells stacked along the $b$ direction. It can be described either as double-$k$ [$\mathbf{k}_1$ = (0, 1/4, 0), $\mathbf{k}_3$ = (0, 3/4, 0)] on the original base-centered orthorhombic lattice or, equivalently, as single-$k$ (wave vector $\mathbf{k}_1$ alone) on the primitive orthorhombic lattice obtained by considering corners and centers of (0 0 1) faces as inequivalent. The intensity refinements point to a structure consisting of (0 1 0) ferromagnetic planes stacked along the $b$ direction, in which the moments are collinear and oriented along the $a$ axis (easy direction according to bulk magnetization measurements). The alternating sequence providing the best refinement turns out to be that which yields the lowest exchange energy if one assumes antiferromagnetic near-neighbor exchange interactions with $J_1 \gg J_2, J_3$. Information is also presented regarding the temperature and magnetic field dependence of the magnetic structure.
    Physical Review B 08/2014; 90(22). DOI:10.1103/PhysRevB.90.224425 · 3.66 Impact Factor
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    ABSTRACT: We examined the magnetic and transport properties of the Kondo semiconductor CeFe2Al10. A large increase in the electrical resistivity along the a- and b-axes (rho(a), rho(b)) is observed below 20 K, but only a small increase in rho(c). A clear shoulder is seen at T similar to 20 K in the thermoelectric power along the b-axis. The upturn of rho(a) and rho(b) below 20 K is quite easily suppressed by pressure as well as by a magnetic field along the a-axis. From these results, we conclude that the c-f hybridization gap below 20 K is characterized by a charge gap along the a- and b-axes, and a spin gap along the a-axis, while a small charge gap effect is observed along the c-axis.
    Journal of the Physical Society of Japan 08/2014; 83(8):084708. DOI:10.7566/JPSJ.83.084708 · 1.48 Impact Factor
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    ABSTRACT: We carried out a high-resolution x-ray diffraction experiment on ${\mathrm{Ce}}_{0.7}{\mathrm{La}}_{0.3}{\mathrm{B}}_{6}$ that exhibits staggered order of $4f$ octupole moments below ${T}_{O}=1.4$ K. Theoretically, ferroquadrupole moments that accompany the antiferro-octupole order and a resultant rhombohedral deformation of the cubic lattice were predicted. In contrast, experimentally, no direct evidence of the rhombohedral lattice has been obtained. We observe the splitting of Bragg peaks below ${T}_{O}$ and find that the unit cell is a rhombohedron being elongated along the [111] axis. The response of rhombohedral domains to magnetic fields also well agrees with theoretical calculations. A particular outcome of this experiment is that the magnitude of the induced quadrupole moments is precisely evaluated from the obtained shear strain. Using this result, the magnitude of the hidden octupole moments is also discussed.
    Physical Review B 07/2014; 90(4). DOI:10.1103/PhysRevB.90.041108 · 3.66 Impact Factor
  • Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
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    ABSTRACT: Kondo semiconductors CeT2Al10 (T = Ru and Os) with the orthorhombic YbFe2Al10-type structure have attracted attentions due to the extremely high Néel temperatures. We have investigated the magnetic properties of NdFe2Al10 with the same structure as CeT2Al10 to obtain information of magnetic interactions in these compounds. From the results of magnetization measurements, it was confirmed that the long range antiferromagnetic order occurs at TN = 3.77(1) K in NdFe2Al10. We have measured 57Fe Mössbauer spectra of NdFe2Al10 in the temperature range from 2.8 to 300 K. All observed Mössbauer spectra can be fitted using one Fe site. The temperature dependence of the refined magnetic hyperfine field below TN cannot be explained by using the Brillouin function with J = 9/2. Furthermore, the Mössbauer spectra in the temperature from TN to 4.2 K indicate magnetic hyperfine fields at the Fe site in the paramagnetic state. These results reveal that there are magnetic fluctuations in the paramagnetic state of NdFe2Al10.
    Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
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    ABSTRACT: The Γ5u-type antiferro-octupole (AFO) ordered state of Ce0.7La0.3B6, emerging below 1.5 K, has been studied by resonant x-ray diffraction in magnetic fields. We studied in detail what kind of multipole moments are induced by the field in the AFO phase. In a mean-field model for the Γ5u-AFO order within the Γ8 quartet crystal-field ground state, the Γ3g-type antiferroquadrupole (AFQ) is expected to be induced most strongly. However, contrary to this expectation, the main induced moment was the Γ5g-type AFQ, which is the order parameter of the field induced AFQ phase above 1 T.
    Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
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    ABSTRACT: The magnetoresistance of CeT2Al10 (T = Ru and Os) and the Hall resistivity of CeOs2Al10 have been measured using non destructive pulsed magnets. The longitudinal magnetoresistance of CeRu2Al10 shows a negative magnetoresistance for the magnetic field (H) along the a- and c-axes but shows less field dependence for H||b. These behaviors are probably attributed to the suppression of the conduction electron scattering by the localized moments. On the other hand, the transverse magnetoresistance exhibits a complex H dependence, which cannot be explained simply by a cyclotron motion of the conduction electrons. The longitudinal magnetoresistance of CeOs2Al10 for H||a rapidly decreases with increasing field below ∼10 K, indicating that the semiconducting behavior below ∼15 K at zero field is rapidly suppressed by the magnetic field up to ∼15 T. The Hall resistivity of CeOs2Al10 exhibits a non linear behavior below 4.2 K, which reflects the fact that CeOs2Al10 is a two-carrier system.
    Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
  • Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
  • Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
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    ABSTRACT: In order to investigate the anisotropy of the pressure-induced antiferroquadrupole ordered phase of CeTe, we carried out magnetization measurements under high pressure for three field directions of [001], [110], and [111]. The results show that the transition temperature is the highest for H||[110] and the lowest for H||[111]. This anisotropy is roughly explained by a mean field calculation which assumes Oxy type antiferroquadrupole order. In addition, we carried out resistivity measurements under high pressure to study the Kondo effect. It is shown that the Kondo effect is enhanced by pressure.
    Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
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    ABSTRACT: The CeT2Al10 family of orthorhombic compounds exhibits a very peculiar evolution from a Kondo-insulator (T: Fe) to an unconventional long-range magnetic order (T: Ru, Os). Inelastic neutron scattering experiments performed on single-crystal CeFe2Al10 reveal that this material develops a spin-gap in its magnetic spectral response below ~ 50 K, with a magnetic excitation dispersing from $E = 10.2 \pm 0.5$ meV at the Y zone-boundary point [q = (0,1,0)] to $\approx 12$ meV at the top of the branch. The excitation shows a pronounced polarization of the magnetic fluctuations along a, the easy anisotropy axis. Its behavior is contrasted with that of the (magnonlike) modes previously reported for CeRu2Al10, which have transverse character and exist only in the antiferromagnetic state. The present observation is ascribed to a "magnetic exciton" mechanism invoked to explain a similar magnetic response previously discovered in YbB12.
    Physical Review B 01/2014; 89(16). DOI:10.1103/PhysRevB.89.161103 · 3.66 Impact Factor
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    ABSTRACT: The multipole ordered phase in Ce0.7La0.3B6, emerging below 1.5 K and named phase IV, has been studied by resonant x-ray diffraction in magnetic fields. By utilizing diamond x-ray phase plates to rotate the incident linear polarization and a conventional crystal analyzer system, full linear polarization analysis has been performed to identify the order parameters. The analysis shows that the Γ5g(Oyz, Ozx, Oxy) quadrupoles are more induced by the field than the Γ3g (O20 and O22) quadrupoles on the Γ5u (Tx+y +zβ) antiferro-octupole order in phase IV. The problem is that this result is contradictory to a mean-field calculation, which inevitably gives the Γ3g quadrupole as the main induced moment. This result indicates that the Γ5g quadrupole order is close in energy. We consider that a large fluctuation of the Γ5g quadrupole is hidden behind the primary ordering of the Γ5u octupole and that the multipolar fluctuation significantly affects the ordering phenomenon.
    Physical Review B 12/2013; 89(1). DOI:10.1103/PhysRevB.89.014422 · 3.66 Impact Factor
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    ABSTRACT: We investigated the magnetic properties of the antiferromagnetic (AFM) compound GdT2Al10 (T=Ru and Fe). Although GdRu2Al10 could be understood well as a simple AFM compound, the exchange interaction in GdFe2Al10 is found to be greatly varied with temperature. The magnitude of the AFM exchange interaction is reduced with decreasing temperature. We ascribed its origin to the exchange enhancement of the Fe ion with a decrease of temperature as was observed in YFe2Al10. The ordered moment is found to be along the [011] direction in the bc plane in both compounds from the anisotropic magnetic susceptibility. The origin of the magnetic anisotropy below TN could not be understood by the magnetic dipole interaction, which might come from the AFM order on the zigzag chain.
    Physical Review B 09/2013; 88(10). DOI:10.1103/PhysRevB.88.100404 · 3.66 Impact Factor

Publication Stats

2k Citations
466.84 Total Impact Points

Institutions

  • 1999–2015
    • Hiroshima University
      • • Department of Quantum Matter
      • • Graduate School of Advanced Sciences of Matter
      Hirosima, Hiroshima, Japan
  • 2007–2010
    • Kochi University
      • • Graduate School of Integrated Arts and Sciences
      • • Faculty of Science
      Kôti, Kōchi, Japan
  • 2005
    • Okayama University
      • Department of Physics
      Okayama, Okayama, Japan
  • 1998–2002
    • Aoyama Gakuin University
      Edo, Tōkyō, Japan
  • 1999–2001
    • Kagoshima University
      • Department of Physics
      Kagosima, Kagoshima, Japan
  • 2000
    • Saitama University
      • Department of Physics
      Saitama, Saitama, Japan
  • 1983–1999
    • Tohoku University
      • • Institute for Materials Research
      • • Department of Physics
      Sendai, Kagoshima-ken, Japan
  • 1996
    • Nippon Telegraph and Telephone
      Edo, Tōkyō, Japan
  • 1987–1993
    • Nagoya University
      • Department of Material Science
      Nagoya, Aichi, Japan
    • Université de Toulon
      La Garde-près-Toulon, Provence-Alpes-Côte d'Azur, France
  • 1989
    • Hokkaido University
      • Division of Physics
      Sapporo, Hokkaidō, Japan