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S. J. Moon, M. W. Kim,
K. W. Kim,
Y. S. Lee,
J.-Y. Kim,
J.-H. Park,
B. J. Kim,
S.-J. Oh,
S. Nakatsuji,
Y. Maeno,
I. Nagai,
S. I. Ikeda,
G. Cao,
T. W. Noh
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ABSTRACT: We investigated the electronic structures of the two-dimensional layered perovskite Sr2MO4 (M=4d Ru, 4d Rh, and 5d Ir) using optical spectroscopy and polarization-dependent O 1s x-ray absorption spectroscopy. While the ground states of the series of compounds are rather different, their optical conductivity spectra σ(ω) exhibit similar interband transitions, indicative of the common electronic structures of the 4d and 5d layered oxides. The energy splittings between the two eg orbitals, i.e., d3z2−r2 and dx2−y2, are about 2 eV, which is much larger than those in the pseudocubic and 3d layered perovskite oxides. The electronic properties of the Sr2MO4 compounds are discussed in terms of the crystal structure and the extended character of the 4d and 5d orbitals.
Phys. Rev. B. 09/2006; 74(11).
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S J Moon, M W Kim,
K W Kim,
Y S Lee,
J Y Kim,
J H Park,
B J Kim,
S J Oh,
S. Nakatsuji,
Y Maeno,
I. Nagai,
S I Ikeda,
G Cao,
T. W. Noh
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ABSTRACT: We investigated the electronic structures of the two-dimensional layered perovskite Sr$_{2}$\textit{M}O$_{4}$ (\textit{M}=4\textit{d} Ru, 4\textit{d} Rh, and 5\textit{d} Ir) using optical spectroscopy and polarization-dependent O 1\textit{s} x-ray absorption spectroscopy. While the ground states of the series of compounds are rather different, their optical conductivity spectra $\sigma(\omega)$ exhibit similar interband transitions, indicative of the common electronic structures of the 4\textit{d} and 5\textit{d} layered oxides. The energy splittings between the two $e_{g}$ orbitals, $i.e.$, $d_{3z^{2}-r^{2}}$ and $d_{x^{2}-y^{2}}$, are about 2 eV, which is much larger than those in the pseudocubic and 3\textit{d} layered perovskite oxides. The electronic properties of the Sr$_{2}$\textit{M}O$_{4}$ compounds are discussed in terms of the crystal structure and the extended character of the 4\textit{d} and 5\textit{d} orbitals.
07/2006;
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ABSTRACT: We investigated the ab-plane absorption spectra of RMnO3 (R=La, Pr, Nd, Gd, and Tb) thin films. As the ionic radius of the R ion decreases, we observed a drastic suppression of the 2 eV peak, i.e., the intersite optical transition between spin- and orbital-aligned states across the Mott gap. We found that, in addition to orbital rotation, orbital mixing in the orbital-ordered state should play an important role in the suppression of 2 eV peak. We also found that the spectral weight of 2 eV peak is proportional to the A-type antiferromagnetic ordering temperature, which suggests that the magnetic interaction should be sensitively coupled to the orbital degree of freedom.
Physical Review Letters 07/2006; 96(24):247205. · 7.37 Impact Factor
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ABSTRACT: We investigated optical excitations of transition-metal (TM) oxides with metal oxygen octahedra taking account of the orbital multiplicity effects. We predicted excitation energies of intersite d–d transitions and p–d transitions of TM oxides. We compared the evaluated excitation energies with reported experimental data, and found that they are in good agreement with each other. Moreover, we could demonstrate possible answers for a few long-standing problems of the low-frequency spectral features in some early 3d TM oxides: (i) the broad and multi-peak structures of the d–d transitions, (ii) the low values (around 2 eV) of the d–d transition energies for some t2g1 and t2g2 systems, and (iii) the lack of the d–d transition below 4.0 eV region for LaCrO3, one of the t2g3 systems. These indicate that our approach considering the orbital multiplicity effects could provide good explanations of intriguing features in the optical spectra of some early TM oxides. In addition, we showed that optical spectroscopy can be useful as a powerful tool to investigate spin and/or orbital correlations in the TM ions. Finally, we discussed the implications of the orbital multiplicity in the Zannen–Sawatzky–Allen scheme, which has been used successfully to classify correlated electron systems.
New Journal of Physics 06/2005; 7(1):147. · 4.18 Impact Factor
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ABSTRACT: We investigated the $ab$-plane absorption spectra of $R$MnO$%_{3}$ ($R$ = La, Pr, Nd, Gd, and Tb) thin films. As the $R$-ion size decreases, we observed a drastic suppression of the 2 eV peak, \textit{i.e.} the inter-site optical transition between spin- and orbital-aligned states across the Mott gap. We found that both lattice distortion and the corresponding orbital mixing of the ordered orbital state should play an important role in the 2 eV peak suppression. We also found that the 2 eV spectral weight is proportional to the $A$-type antiferromagnetic ordering temperature, which suggests that the magnetic interaction might be sensitively coupled to the orbital mixing.
05/2005;
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ABSTRACT: We investigated the temperature-dependent optical absorption spectra of LaMnO3 and NdMnO3 epitaxial thin films. We found that both spectra have strong absorption features around 2 eV, and that the 2 eV peak of LaMnO3 has a weak temperature dependence. As the La ions are replaced with Nd ions, the spectral weight of the peak becomes suppressed. From careful comparisons with the orbitally degenerate Hubbard model, we could explain most of the spectral features of the 2 eV peak observed in the absorption spectra. We also found a fine structure of three sub-peaks in the 2 eV peak, whose origin was considered based on currently available ideas.
New Journal of Physics 11/2004; 6(1):156. · 4.18 Impact Factor
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ABSTRACT: Optical spectroscopy was used to determine nearest-neighbor spin correlations in pyrochlore molybdates R2Mo2O7 (R=Y, Sm, and Nd), which exhibit ferromagnetic metal to spin-glass insulator transition as the R ion size decreases. Using an analysis based on the orbitally degenerate Hubbard model, we could estimate important physical parameters, such as the effective on-site Coulomb energy U(eff) and the Hund rule exchange coupling J(H). We demonstrated experimentally and theoretically that the effective superexchange interaction between the Mo ions depends on J(H)/U(eff), which determines the phase boundary of the magnetic ground states.
Physical Review Letters 02/2004; 92(2):027202. · 7.37 Impact Factor
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ABSTRACT: We investigated the optical conductivity spectra σ(ω) of double perovskites Sr2(Fe1-zMnz)MoO6, which show filling controlled metal–insulator transition. Based on systematic analyses of optical conductivity and O 1s x-ray absorption spectroscopy, the electronic structures of both Sr2FeMoO6 and Sr2MnMoO6 near the Fermi level are presented, which turn out to agree with the recent LSDA+U calculation results [H. Wu, Phys. Rev. B 64, 125126 (2001)]. With the Mo carrier doping (with z decreasing), the in-gap spectral weight is formed, below the gap of Sr2MnMoO6, and finally developed into a Drude peak in Sr2FeMoO6. Due to the possible site disorder, finite energy peaks rather than Drude-like peaks were observed for most of the doping ranges 0.2<~z<~0.8. With the ferrimagnetic ordering, we observed redistribution of spectral weight over a wide energy region from 0 to 3 eV. The high energy spectral weight was transferred to the low energy region, similar to the manganites. We discussed possible scenarios relevant to the Fe–Mo hybridization.
Phys. Rev. B. 09/2002; 66(10).
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ABSTRACT: We systematically investigated optical properties of Nd1-xSrxMnO3 single crystals ( x = 0.40, 0.50, 0.55, and 0.65). They are similar in their spin-orbital (SO) disordered states at room temperature. At low temperature, the crystals enter into various SO ordered states, i.e., F-, CE-, A-, and C-type orderings, and their mid-infrared absorptions become quite different. The remarkable variation can be explained by polaron dynamics which depend on the ordering patterns. This SO pattern dependent polaron model can also explain the pseudo CE-type ordering case, demonstrating that this scheme can explain the carrier dynamics in complex SO configurations.
Physical Review Letters 08/2002; 89(1):016403. · 7.37 Impact Factor
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ABSTRACT: We investigated the magnetic field (H)-dependent optical conductivity spectra of MgB2 thin film in the far-infrared region. The H-dependences can be explained by the increase of normal metallic regions embedded in the superconducting background. The area fraction of the normal metallic region increases rather quickly at low field, but slowly at high field. It follows neither H- nor H^{1/2}-dependences. The results suggest the complicated nature of the superconducting gap in MgB2. Comment: 8 pages, 4 figures, submitted to PRB (Corrected typos : changed form)
04/2002;
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ABSTRACT: We investigated optical properties of Nd(1-x)Sr(x)MnO3 (x= 0.40, 0.50, 0.55, and 0.65) single crystals. In the spin/orbital disordered state, their conductivity spectra look quite similar, and the strength of the mid-infrared absorption peak is proportional to x(1-x) consistent with the polaron picture. As temperature lowers, the Nd(1-x)Sr(x)MnO3 samples enter into various spin/orbital ordered states, whose optical responses are quite different. These optical responses can be explained by the spin/orbital ordering pattern-dependent polaron hopping. Comment: 3 figures (gzipped)
03/2002;
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ABSTRACT: We investigated temperature (T)- and magnetic field-dependent optical conductivity spectra (\s\w) of a La_5/8-yPr_yCa_3/8MnO_3 (y~0.35) single crystal, showing intriguing phase coexistence at low T. At T_C < T < T_CO, a dominant charge-ordered phase produces a large optical gap energy of ~0.4 eV. At T < T_C, at least two absorption bands newly emerge below 0.4 eV. Analyses of (\s\w) indicate that the new bands should be attributed to a ferromagnetic metallic and a charge-disordered phase that coexist with the charge-ordered phase. This optical study clearly shows that La_5/8-yPrCa_3/8MnO_3 (y~0.35) is composed of multiphases that might have different lattice strains. Comment: A single file with 9 figures embedded, to appear in Phys. Rev. B
03/2002;
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ABSTRACT: We reported far-infrared transmission measurements on a c-axis oriented superconducting MgB$_{2}$ thin film in the frequency range of 30 $\sim$ 250 cm$^{-1}$. We found that these measurements were sensitive to values of scattering rate $1/\tau$ and superconducting gap $2\Delta$. By fitting the experimental transmission spectra at 40 K and below, we obtained $1/\tau =$ (700 $\sim$ 1000) cm$^{-1}$ and $2\Delta (0)\cong$ 42 cm$^{-1}$. These two quantities suggested that MgB$_{2}$ belong to the dirty limit. Comment: submitted at May 4
05/2001;
