Sources of Variation in Holding Returns for Fed Funds Futures Contracts

This paper relates predictable gains from positions in fed funds futures contracts to violations of the expectations hypothesis of the term structure of interest rates. Although evidence for predictable gains from positions in short-horizon contracts is mixed, we find that gains in longer horizon contracts can be well described using Markov-switching models, with predictability associated with particular episodes in which economic activity was weak and variability in the returns to these contracts was quite high.

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

Thinning practices in rehabilitated mangroves: Opportunity to synergize climate change mitigation and adaptation

Mangrove trees act important roles in the coastal ecosystems, protecting community against high-tide and\ud storms, controlling land erosion and providing fish breeding ground. In the last few decades, the massive area has\ud devastated due to commercial shrimp and fish ponds development. To rehabilitate the coastal ecosystems, some\ud mangrove has been planted with spacing distances of 1x1 m with minimal forest management. Those dense-spaced\ud stands enhanced light competitions and inhibit growth. These poor quality and immature stands that reach an early\ud climax in 10-15 years were observed in two adjacent sites near Nam Dinh and Thanh Hoa in northern Vietnam, where\ud Kandelia candel were planted. To cultivate the resurgent stands and increase their growth, thinning mangrove is\ud essential. Stand densities of the mangrove trees with and without the thinning practice were 17,800 and 5,200 trees ha-1,\ud respectively. Their potential of the maximum above-ground biomass were 303 and 239 Mg ha-1, respectively. However,\ud quality of the single tree was largely different whether or not thinning practice is conducted, as the thinned one of 46 kg\ud tree-1 was about three times higher than the non-thinned of 17 kg tree-1. The thinning practice enhances stand biomass\ud growth with improved growth condition in the forest, which advances carbon sequestration for the climate change\ud mitigation. The cultivated trees also ensure the climate change adaptation of coastal protection, fishery products and\ud bio-diversity. Synergizing mitigation and adaptation strategies with the mangrove thinning would enhance the benefits\ud for coastal communities most vulnerable to climate change

Orbital dynamics: The origin of the anomalous optical spectra in ferromagnetic manganites

We discuss the role of orbital degeneracy in the transport properties of perovskite manganites, focusing in particular on the optical conductivity in the metallic ferromagnetic phase at low temperatures. Orbital degeneracy and strong correlations are described by an orbital t-J model which we treat in a slave-boson approach. Employing the memory-function formalism we calculate the optical conductivity, which is found to exhibit a broad incoherent component extending up to bare bandwidth accompanied by a strong suppression of the Drude weight. Further, we calculate the constant of T-linear specific heat. Our results are in overall agreement with experiment and suggest low-energy orbital fluctuations as the origin of the strongly correlated nature of the metallic phase of manganites.Comment: To appear in: Phys. Rev. B 58 (Rapid Communications), 1 November 199

Oxygen isotope effect and phase separation in the optical conductivity of (La0.5_{0.5}Pr0.5_{0.5})0.7_{0.7}Ca0.3_{0.3}MnO3_3 thin films

The optical conductivities of films of (La$_{0.5}$Pr$_{0.5}$)$_{0.7}$Ca$_{0.3}$MnO$_3$ with different oxygen isotopes ($^{16}$O and $^{18}$O) have been determined in the spectral range from 0.3 to 4.3 eV using a combination of transmission in the mid-infrared and ellipsometry from the near-infrared to ultra-violet regions. We have found that the isotope exchange strongly affects the optical response in the ferromagnetic phase in a broad frequency range, in contrast to the almost isotope-independent optical conductivity above $T_C$. The substitution by $^{18}$O strongly suppresses the Drude response and a mid-infrared peak while enhancing the conductivity peak at 1.5 eV. A qualitative explanation can be given in terms of the phase separation present in these materials. Moreover, the optical response is similar to the one extracted from measurements in polished samples and other thin films, which signals to the importance of internal strain.Comment: 11 pages, 11 figures, to appear in PR

Cooperative Jahn-Teller Effect and Electron-Phonon Coupling in La1−xAxMnO3La_{1-x}A_xMnO_3

A classical model for the lattice distortions of \lax is derived and, in a mean field approximation, solved. The model is based on previous work by Kanamori and involves localized Mn d-electrons (which induce tetragonal distortions of the oxygen octahedra surrounding the Mn) and localized holes (which induce breathing distortions). Parameters are determined by fitting to the room temperature structure of $LaMnO_3$. The energy gained by formation of a local lattice distortion is found to be large, most likely $\approx 0.6$ eV per site, implying a strong electorn-phonon coupling and supporting polaronic models of transport in the doped materials. The structural transition is shown to be of the order-disorder type; the rapid x-dependence of the transition temperature is argued to occur because added holes produce a "random" field which misaligns the nearby sites.Comment: 24 pages. No figures. One Table. Late

Static and Dynamical Properties of the Ferromagnetic Kondo Model with Direct Antiferromagnetic Coupling Between the localized t2gt_{2g} Electrons

The phase diagram of the Kondo lattice Hamiltonian with ferromagnetic Hund's coupling in the limit where the spin of the localized $t_{2g}$ electrons is classical is analyzed in one dimension as a function of temperature, electronic density, and a direct antiferromagnetic coupling $J'$ between the localized spins. Studying static and dynamical properties, a behavior that qualitatively resembles experimental results for manganites occurs for $J'$ smaller than 0.11 in units of the $e_g$ hopping amplitude. In particular a coexistence of ferromagnetic and antiferromagnetic excitations is observed at low-hole density in agreement with neutron scattering experiments on $\rm{La_{2-2x}Sr_{1+2x}Mn_2O_7}$ with$x=0.4$. This effect is caused by the recently reported tendency to phase separation between hole-rich ferromagnetic and hole-undoped antiferromagnetic domains in electronic models for manganites. As $J'$ increases metal-insulator transitions are detected by monitoring the optical conductivity and the density of states. The magnetic correlations reveal the existence of spiral phases without long-range order but with fairly large correlation lengths. Indications of charge ordering effects appear in the analysis of charge correlations.Comment: 14 pages with 25 eps figures embeded in the tex

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.Comment: 4 pages and separate 4 figure

Metallic ferromagnetism without exchange splitting

In the band theory of ferromagnetism there is a relative shift in the position of majority and minority spin bands due to the self-consistent field due to opposite spin electrons. In the simplest realization, the Stoner model, the majority and minority spin bands are rigidly shifted with respect to each other. Here we consider models at the opposite extreme, where there is no overall shift of the energy bands. Instead, upon spin polarization one of the bands broadens relative to the other. Ferromagnetism is driven by the resulting gain in kinetic energy. A signature of this class of mechanisms is that a transfer of spectral weight in optical absorption from high to low frequencies occurs upon spin polarization. We show that such models arise from generalized tight binding models that include off-diagonal matrix elements of the Coulomb interaction. For certain parameter ranges it is also found that reentrant ferromagnetism occurs. We examine properties of these models at zero and finite temperatures, and discuss their possible relevance to real materials

Optical spectral weights and the ferromagnetic transition temperature of CMR manganites: relevance of double-exchange to real materials

We present a thorough and quantitative comparison of double-exchange models to experimental data on the colossal magnetoresistance manganese perovskites. Our results settle a controversy by showing that physics beyond double-exchange is important even in La$_{0.7}$Sr$_{0.3}$MnO$_3$, which has been regarded as a conventional double-exchange system. We show that the crucial quantity for comparisons of different calculations to each other and to data is the conduction band kinetic energy $K$, which is insensitive to the details of the band structure and can be experimentally determined from optical conductivity measurements. The seemingly complicated dependence of $T_c$ on the Hund's coupling $J$ and carrier concentration $n$ is shown to reflect the variation of $K$ with $J$, $n$ and temperature. We present results for the optical conductivity which allow interpretation of experiments and show that a feature previously interpreted in terms of the Hund's coupling was misidentified. We also correct minor errors in the phase diagram presented in previous work.Comment: 13 pages, 7 eps figure