Suppressed Coherence due to Orbital Correlations in the Ferromagnetically Ordered Metallic Phase of Mn Compounds

Small Drude weight $D$ together with small specific heat coefficient $\gamma$ observed in the ferromagnetic phase of R$_{1-x}$A$_x$MnO$_3$ (R=La, Pr, Nd, Sm; A=Ca, Sr, Ba) are analyzed in terms of a proximity effect of the Mott insulator. The scaling theory for the metal-insulator transition with the critical enhancement of orbital correlations toward the staggered ordering of two $e_g$ orbitals such as $3x^2-r^2$ and $3y^2-r^2$ symmetries may lead to the critical exponents of $D \propto \delta^{u}$ and $\gamma \propto \delta^v$ with $u=3/2$ and $v=0$. The result agrees with the experimental indications.Comment: 4 pages LaTeX using jpsj.sty. To appear in J. Phys. Soc. Jpn. 67(1998)No.

Dynamic Kerr Effect and Spectral Weight Transfer in the Manganites

We perform pump-probe Kerr spectroscopy in the colossally magnetoresistive manganite Pr0.67Ca0.33MnO3. Kerr effects uncover surface magnetic dynamics undetected by established methods based on reflectivity and optical spectral weight transfer. Our findings indicate the connection between spin and charge dynamics in the manganites may be weaker than previously thought. Additionally, important differences between this system and conventional ferromagnetic metals manifest as long-lived, magneto-optical coupling transients, which may be generic to all manganites.Comment: 12 text pages, 4 figure

Optical conductivity in doped manganites with planar x2^2-y2^2 orbital order

We investigate a planar model for the ferromagnetic (FM) phase of manganites, which develops orbital order of $e_g$ electrons with x$^2$-y$^2$-symmetry at low temperature. The dynamic structure factor of orbital excitations and the optical conductivity $\sigma(\omega)$ are studied with help of a finite-temperature diagonalization method. Our calculations provide a theoretical prediction for $\sigma(\omega)$ for the 2D FM state and are of possible relevance for the recently found A-type phase of manganites at high doping which consists of FM layers coupled antiferromagnetically. In the x$^2$-y$^2$ ordered regime $\sigma(\omega)$ shows both a Drude peak and a gapped incoherent absorption due to a gap in the orbital excitations.Comment: 5 pages, 5 figures, to appear in Phys. Rev. Let

Ultrafast Photoinduced Formation of Metallic State in a Perovskite-type Manganite with Short Range Charge and Orbital Order

Femtosecond reflection spectroscopy was performed on a perovskite-type manganite, Gd0.55Sr0.45MnO3, with the short-range charge and orbital order (CO/OO). Immediately after the photoirradiation, a large increase of the reflectivity was detected in the mid-infrared region. The optical conductivity spectrum under photoirradiation obtained from the Kramers-Kronig analyses of the reflectivity changes demonstrates a formation of a metallic state. This suggests that ferromagnetic spin arrangements occur within the time resolution (ca. 200 fs) through the double exchange interaction, resulting in an ultrafast CO/OO to FM switching.Comment: 4 figure

Dominant role of charge ordering on high harmonic generation in Pr_{0.6}Ca_{0.4}MnO_{3}

High-harmonic generation (HHG) is a typical high-order nonlinear optical phenomenon and can be used to probe electronic structures of solids. Here, we investigate the temperature dependence of HHG from Pr_{0.6}Ca_{0.4}MnO_{3} in the range of 7 K to 294 K including the charge ordering (CO) transition and magnetic transition temperatures. The high-harmonic intensity remains almost constant in the high-temperature charge-disordered phase. However, as the temperature is lowered, it starts to gradually increase near the CO transition temperature where an optical gap related to the CO phase appears. The anomalous gap energy dependence resembles the one recently reported in a Mott insulator. We attribute the HHG suppression at high temperatures to the destructive interference among high-harmonic emissions from thermally activated multiple CO configurations. Our results suggest that HHG is a promising tool for probing the fluctuation of local order in strongly correlated systems.Comment: 16 pages, 8 figure

Colossal magnetooptical conductivity in doped manganites

We show that the current carrier density collapse in doped manganites, which results from bipolaron formation in the paramagnetic phase, leads to a colossal change of the optical conductivity in an external magnetic field at temperatures close to the ferromagnetic transition. As with the colossal magnetoresistance (CMR) itself, the corresponding magnetooptical effect is explained by the dissociation of localized bipolarons into mobile polarons owing to the exchange interaction with the localized Mn spins in the ferromagnetic phase. The effect is positive at low frequencies and negative in the high-frequency region. The present results agree with available experimental observations.Comment: 4 pages, REVTeX 3.0, two eps-figures included in the tex

Comparison and Contrast of Genes and Biological Pathways Responding to Marek’s Disease Virus Infection Using Allele-Specific Expression and Differential Expression in Broiler and Layer Chickens.

Background Marek’s disease (MD) is a commercially important neoplastic disease of chickens caused by the Marek’s disease virus (MDV), a naturally occurring oncogenic alphaherpesvirus. Enhancing MD genetic resistance is desirable to augment current vaccines and other MD control measures. High throughput sequencing was used to profile splenic transcriptomes from individual F1 progeny infected with MDV at 4 days of age from both outbred broilers (meat-type) and inbred layer (egg-type) chicken lines that differed in MD genetic resistance. The resulting information was used to identify SNPs, genes, and biological pathways exhibiting allele-specific expression (ASE) in response to MDV infection in each type of chicken. In addition, we compared and contrasted the results of pathway analyses (ASE and differential expression (DE)) between chicken types to help inform on the biological response to MDV infection. Results With 7 individuals per line and treatment group providing high power, we identified 6,132 single nucleotide polymorphisms (SNPs) in 4,768 genes and 4,528 SNPs in 3,718 genes in broilers and layers, respectively, that exhibited ASE in response to MDV infection. Furthermore, 548 and 434 genes in broilers and layers, respectively, were found to show DE following MDV infection. Comparing the datasets, only 72 SNPs and 850 genes for ASE and 20 genes for DE were common between the two bird types. Although the chicken types used in this study were genetically different, at the pathway level, both TLR receptor and JAK/STAT signaling pathways were enriched as well as exhibiting a high proportion of ASE genes, especially at the beginning of both above mentioned regulatory pathways. Conclusions RNA sequencing with adequate biological replicates is a powerful approach to identify high confidence SNPs, genes, and pathways that are associated with transcriptional response to MDV infection. In addition, the SNPs exhibiting ASE in response to MDV infection provide a strong foundation for determining the extent to which variation in expression influences MD incidence plus yield genetic markers for genomic selection. However, given the paucity of overlap among ASE SNP sets (broilers vs. layers), it is likely that separate screens need to be incorporated for each population. Finally, comparison of gene lists obtained between these two diverse chicken types indicate the TLR and JAK/STAT signaling are conserved when responding to MDV infection and may be altered by selection of genes exhibiting ASE found at the start of each pathway

Charge and orbital order in half-doped manganites

An explanation is given for the charge order, orbital order and insulating state observed in half-doped manganese oxides, such as Nd$_{1/2}$Sr$_{1/2}$MnO$_{3}$. The competition between the kinetic energy of the electrons and the magnetic exchange energy drives the formation of effectively one-dimensional ferromagnetic zig-zag chains. Due to a topological phase factor in the hopping, the chains are intrinsically insulating and orbital-ordered. Most surprisingly, the strong Coulomb interaction between electrons on the same Mn-ion leads to the experimentally observed charge ordering. For doping less than 1/2 the system is unstable towards phase separation into a ferromagnetic metallic and charge-ordered insulating phase.Comment: To appear in Phys. Rev. Lett., 4 pages, 4 figure