arXiv subjects

T. W. Noh

Publications and source records attributed to T. W. Noh.

90 records · Page 5Linked to original sources

arXiv2002

Non-Fermi liquid behavior and scaling of low frequency suppression in optical conductivity spectra of CaRuO$_3$

Optical conductivity spectra $σ_1(ω)$ of paramagnetic CaRuO$_3$ are investigated at various temperatures. At T=10 K, it shows a non-Fermi liquid behavior of $σ_1(ω)\sim 1/ω^{\frac 12}$, similar to the case of a ferromagnet SrRuO$_3$. As the temperature ($T$) is increased, on the other hand, $σ_1(ω)$ in the low frequency region is progressively suppressed, deviating from the $1/ω^{\frac 12%}$-dependence. Interestingly, the suppression of $σ_1(ω)$ is found to scale with $ω/T$ at all temperatures. The origin of the $% ω/T$ scaling behavior coupled with the non-Fermi liquid behavior is discussed.

cond-mat.str-el↗

arXiv2002

Complicated nature of the gap in MgB2: magnetic field-dependent optical studies

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.

cond-mat.str-el↗

arXiv2002

Spin/Orbital Pattern-Dependent Polaron Absorption in Nd(1-x)Sr(x)MnO3

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.

cond-mat.str-el↗

arXiv2002

Charge Ordering Fluctuation and Optical Pseudogap in La$_{1-x}$Ca$_{x}$MnO$_{3}$

Optical spectroscopy was used to investigate the optical gap (2$Δ$) due to charge ordering (CO) and related pseudogap developments with x and temperature (T) in La$_{1-x}$Ca$_{x}$MnO$_{3}$ (0.48 <= x <= 0.67). Surprisingly, we found 2$Δ$/k_{B}T_{CO} is as large as 30 for x ~0.5, and decreases rapidly with increasing x. Simultaneously, the optical pseudogap, possibly starting from T^* far above T_{CO} becomes drastically enhanced near x=0.5, producing non-BCS T-dependence of 2$Δ$ with the large magnitude far above T_{CO}, and systematic increase of T^* for x~0.5. These results unequivocally indicate systematically-enhanced CO correlation when x approaches 0.5 even though T_{CO} decreases.

cond-mat.str-el↗

arXiv2002

Optical Evidence of Multiphase Coexistence in Single Crystalline (La,Pr,Ca)MnO3

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.

cond-mat.str-el↗

arXiv2001

Pseudogap formation of four-layer BaRuO$_3$ and its electrodynamic response changes

We investiaged the optical properties of four-layer BaRuO$_{3}$, which shows a fermi-liquid-like behavior at low temperature. Its optical conductivity spectra clearly displayed the formation of a pseudogap and the development of a coherent peak with decreasing temperature. Temperature-dependences of the density $n$ and the scattering rate $1/τ$ of the coherent component were also derived. As the temperature decreases, both $n$ and $1/τ$ decrease for four-layer BaRuO$_{3}$. These electrodynamic responses were compared with those of nine-layer BaRuO$_{3}$, which also shows a pseudogap formation but has an insulator-like state at low temperature. It was found that the relative rates of change of both $n$ and $1/τ$ determine either metallic or insulator-like responses in the ruthenates. The optical properties of the four-layer ruthenate were also compared with those of other pseudogap systems, such as high $T_{c}$ cuprates and heavy electron systems.

cond-mat.str-el↗

arXiv2001

Optical investigation on the electronic structures of Y_{2}Ru_{2}O_{7}, CaRuO_{3}, SrRuO_{3}, and Bi_{2}Ru_{2}O_{7}

We investigated the electronic structures of the bandwidth-controlled ruthenates, Y$_{2}$Ru$_{2}$O$_{7}$, CaRuO$_{3}$, SrRuO$_{3}$, and Bi$_{2}$Ru$% _{2}$O$_{7}$, by optical conductivity analysis in a wide energy region of 5 meV $\sim $ 12 eV. We could assign optical transitions from the systematic changes of the spectra and by comparison with the O 1$s$ x-ray absorption data. We estimated some physical parameters, such as the on-site Coulomb repulsion energy and the crystal-field splitting energy. These parameters show that the 4$d$ orbitals should be more extended than 3$d$ ones. These results are also discussed in terms of the Mott-Hubbard model.

cond-mat.str-el↗

arXiv2001

Far-infrared transmission studies of c-axis oriented superconducting MgB2 thin film

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 $1/τ$ and superconducting gap $2Δ$. By fitting the experimental transmission spectra at 40 K and below, we obtained $1/τ=$ (700 $\sim$ 1000) cm$^{-1}$ and $2Δ(0)\cong$ 42 cm$^{-1}$. These two quantities suggested that MgB$_{2}$ belong to the dirty limit.

cond-mat.supr-con↗

arXiv2000

Optical Studies of a Layered Manganite La_{1.2}Sr_{1.8}Mn_2O_7 : Polaron Correlation Effect

Optical conductivity spectra of a cleaved ab-plane of a La_{1.2}Sr_{1.8}Mn_2O_7 single crystal exhibit a small polaron absorption band in the mid-infrared region at overall temperatures. With decreasing temperature (T) to Curie temperature (T_C), the center frequency of the small polaron band moves to a higher frequency, resulting in a gap-like feature, and that it collapses to a lower frequency below T_C. Interestingly, with decreasing T, the stretching phonon mode hardens above T_C and softens below T_C. These concurring changes of lattice and electronic structure indicate that short range polaron correlation exist above T_C but disappear with a magnetic ordering.

cond-mat.str-el↗

arXiv2000

Magnetic Field Dependent Optical Studies of a Layered Antiferromagnet Pr$%_{1/2}$Sr$_{1/2}$MnO$_3$

Magnetic field ($H$)-dependent optical conductivity spectra $σ(ω) $ and dielectric constant spectra $ε(ω)$ of a layered antiferromagnet ($A$-type) Pr$_{1/2}$Sr$_{1/2}$MnO$_3$ were presented. At 0.0 T in 4.2 K, the $σ(ω)$ showed a small Drude-like peak in the far-infrared region and a broad absorption peak in the mid-infrared region. With increasing $H$, large spectral weights were transferred from high to low energy regions with increase of the Drude-like peak. We found that a polaron picture could explain most of the $H$-dependent spectral changes. By comparing with the $H$-dependent spectral weight changes and $ε(ω)$ of Nd$_{1/2}$Sr$_{1/2}$MnO$_3$, which has the $CE$-type spin ordering, we showed that a dimensional crossover from a 2-dimensional to a 3-dimensional metal occurs in Pr$_{1/2}$Sr$_{1/2}$MnO$_3$.

cond-mat.str-el↗

arXiv1999

Melting of Charge/Orbital Ordered States in Nd$_{1/2}$Sr$_{1/2}$MnO$_3$: Temperature and Magnetic Field Dependent Optical Studies

We investigated the temperature ($T=$ 15 $\sim $ 290 K) and the magnetic field ($H=$ 0 $\sim $ 17 T) dependent optical conductivity spectra of a charge/orbital ordered manganite, Nd$_{1/2}$Sr$_{1/2}$MnO$_3$. With variation of $T$ and $H$, large spectral weight changes were observed up to 4.0 eV. These spectral weight changes could be explained using the polaron picture. Interestingly, our results suggested that some local ordered state might remain above the charge ordering temperature, and that the charge/orbital melted state at a high magnetic field (i.e. at $H=$ 17 T and $% T=$ 4.2 K) should be a three dimensional ferromagnetic metal. We also investigated the first order phase transition from the charge/orbital ordered state to ferromagnetic metallic state using the $T$- and $H$% -dependent dielectric constants $ε_1$. In the charge/orbital ordered insulating state, $ε_1$ was positive and $dε_1/dω\approx 0$. With increasing $T$ and $H$, $ε_1$ was increased up to the insulator-metal phase boundaries. And then, $ε_1$ abruptly changed into negative and $dε_1/dω>0$, which was consistent with typical responses of a metal. Through the analysis of $% ε_1$ using an effective medium approximation, we found that the melting of charge/orbital ordered states should occur through the percolation of ferromagnetic metal domains.

cond-mat.str-el↗

arXiv1999

Dimensional Crossover driven by Magnetic Ordering in Optical Conductivity of Pr_{1/2}Sr_{1/2}MnO_3

We investigated optical properties of Pr_{0.5}Sr_{0.5}MnO_3, which has the A-type antiferromagnetic ordering at a low temperature. We found that T- dependence of spectral weight transfer shows a clear correlation with the magnetic phase transition. In comparison with the optical conductivity results of Nd_{0.5}Sr_{0.5}MnO_3, which has the CE-type antiferromagnetic charge ordering, we showed that optical properties of Pr_{0.5}Sr_{0.5}MnO_3 near the Neel temperature could be explained by a crossover from 3D to 2D metals. Details of spectral weight changes are consistent with the polaron picture.

cond-mat.str-el↗

arXiv1999

Electrodynamics of Nd0.7Sr0.3MnO3 Single Crystal investigated by Optical Conductivity Analyses

We investigated temperature dependent optical conductivity spectra of Nd0.7Sr0.3MnO3 single crystal. We found that polishing and surface scattering effects on the Nd0.7Sr0.3MnO3 crystal surfaces could significantly distort the optical responses, especially in the mid-infrared region. However, oxygen annealing and gold normalization processes seemed to remedy the distortions. We found that the spectral weight of Nd0.7Sr0.3MnO3 in the metallic state might be composed of a Drude carrier term and a strong incoherent mid-infrared absorptions. The temperature dependence of the spectral weight suggests that the electron-lattice coupling should be important in optical properties of colossal magnetoresistance manganites.

cond-mat.str-el↗

arXiv1999

Optical Investigations of Charge Gap in Orbital Ordered La1/2Sr3/2MnO4

Temperature and polarization dependent electronic structure of La1/2Sr3/2MnO4 were investigated by optical conductivity analyses. With decreasing temperature, for E//ab, a broad mid-infrared (MIR) peak of La1/2Sr3/2MnO4 becomes narrower and moves to the higher frequency, while that of Nd1/2Sr3/2MnO4 nearly temperature independent. We showed that the MIR peak in La1/2Sr3/2MnO4 originates from orbital ordering associated with CE-type magnetic ordering and that the Jahn-Teller distortion has a significant influence on the width and the position of the MIR peak.

cond-mat.str-el↗

arXiv1998

Optical Investigations of La_{7/8}Sr_{1/8}MnO_3

We investigated temperature dependent optical conductivity spectra of La_{7/8}Sr_{1/8}MnO_3. Its phonon bending mode starts to be splitted near a structural phase transition temperature. With further cooling, strength of a mid-infrared small polaron peak, which is located at 0.4 eV, becomes increased. These temperature dependent changes in the phonon spectra and the polaron absorption peaks were explained by a phase separation and a percolation-type transition.

cond-mat.str-el↗

arXiv1998

Spectral evolution in (Ca,Sr)RuO_3 near the Mott-Hubbard transition

We investigated the optical properties of (Ca,Sr)RuO_3 films on the borderline of a metal-insulator (M-I) transition. Our results show all of the predicted characteristics for a metallic Mott-Hubbard system, including (i) a mass enhancement in dc-limit, (ii) an U/2 excitation, and (iii) an U excitation. Also, a self-consistency is found within the Gutzwiller-Brinkman-Rice picture for the Mott transition. Our finding displays that electron correlation should be important even in 4d materials.

cond-mat.str-el↗

arXiv1998

A Scaling Behavior of Spectral Weight Changes in Perovskite Manganites La_{0.7-y}Pr_{y}Ca_{0.3}MnO_3

Optical conductivity spectra of La_{0.7-y}Pr_{y}Ca_{0.3}MnO_3 were systematically investigated. For metallic samples, the spectral weight below 0.5 eV, whose magnitude can be represented by the effective carrier number N_{eff}(0.5 eV), increases as temperature becomes lower. Regardless of the Pr doping, all the measured values of N_{eff}(0.5 eV)/T_C fall into one scaling curve. This scaling behavior could be explained by the theoretical model by Roeder et al. [Phys. Rev. Lett. 76, 1356 (1996)], which includes spin double exchange and Jahn-Teller lattice coupling to holes. With the Pr doping, far-infrared conductivities were found to be suppressed, probably due to the Anderson localization.

cond-mat.str-el↗

arXiv1998

Polaron Absorption in a Perovskite Manganite La0.7Ca0.3MnO3

Temperature dependent optical conductivity spectra of a La0.7Ca0.3MnO3 (LCMO) sample were measured. In the metallic regime at very low temperatures, they clearly showed two types of absorption features, i.e., a sharp Drude peak and a broad mid-infrared absorption band, which could be explained as coherent and incoherent bands of a large lattice polaron. This elementary excitation in LCMO was found to be in a strong coupling regime and to have interactions with the spin degree of freedom.

cond-mat.str-el↗