Circadian Organization in Hemimetabolous Insects

The circadian system of hemimetabolous insects is reviewed in respect to the locus of the circadian clock and multioscillatory organization. Because of relatively easy access to the nervous system, the neuronal organization of the clock system in hemimetabolous insects has been studied, yielding identification of the compound eye as the major photoreceptor for entrainment and the optic lobe for the circadian clock locus. The clock site within the optic lobe is inconsistent among reported species; in cockroaches the lobula was previously thought to be a most likely clock locus but accessory medulla is recently stressed to be a clock center, while more distal part of the optic lobe including the lamina and the outer medulla area for the cricket. Identification of the clock cells needs further critical studies. Although each optic lobe clock seems functionally identical, in respect to photic entrainment and generation of the rhythm, the bilaterally paired clocks form a functional unit. They interact to produce a stable time structure within individual insects by exchanging photic and temporal information through neural pathways, in which serotonin and pigment-dispersing factor (PDF) are involved as chemical messengers. The mutual interaction also plays an important role in seasonal adaptation of the rhythm

Temperature-dependent photoemission spectral weight transfer and chemical potential shift in Pr1−x_{1-x}Cax_xMnO3_3 : Implications for charge density modulation

We have studied the temperature dependence of the photoemission spectra of Pr$_{1-x}$Ca$_x$MnO$_3$ (PCMO) with $x=0.25$, 0.3 and 0.5. For $x=0.3$ and 0.5, we observed a gap in the low-temperature CE-type charge-ordered (CO) phase and a pseudogap with a finite intensity at the Fermi level ($E_F$) in the high-temperature paramagnetic insulating (PI) phase. Within the CO phase, the spectral intensity near $E_F$ gradually increased with temperature. These observations are consistent with the results of Monte Carlo simulations on a model including charge ordering and ferromagnetic fluctuations [H. Aliaga {\it et al.} Phys. Rev. B {\bf 68}, 104405 (2003)]. For $x=0.25$, on the other hand, little temperature dependence was observed within the low-temperature ferromagnetic insulating (FI) phase and the intensity at $E_F$ remained low in the high-temperature PI phase. We attribute the difference in the temperature dependence near $E_F$ between the CO and FI phases to the different correlation lengths of orbital order between both phases. Furthermore, we observed a chemical potential shift with temperature due to the opening of the gap in the FI and CO phases. The doping dependent chemical potential shift was recovered at low temperatures, corresponding to the disappearance of the doping dependent change of the modulation wave vector. Spectral weight transfer with hole concentration was clearly observed at high temperatures but was suppressed at low temperatures. We attribute this observation to the fixed periodicity with hole doping in PCMO at low temperatures.Comment: 5pages, 7figure

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

Chemical potential shift induced by double-exchange and polaronic effects in Nd_{1-x}Sr_xMnO_3

We have studied the chemical potential shift as a function of temperature in Nd$_{1-x}$Sr$_x$MnO$_3$ (NSMO) by measurements of core-level photoemission spectra. For ferromagnetic samples ($x=0.4$ and 0.45), we observed an unusually large upward chemical potential shift with decreasing temperature in the low-temperature region of the ferromagnetic metallic (FM) phase. This can be explained by the double-exchange (DE) mechanism if the $e_g$ band is split by dynamical/local Jahn-Teller effect. The shift was suppressed near the Curie temperature ($T_C$), which we attribute to the crossover from the DE to lattice-polaron regimes.Comment: 5 pages, 6 figure

Soft spin waves in the low temperature thermodynamics of Pr_{0.7}Ca_{0.3}MnO_{3}

We present a detailed magnetothermal study of Pr(0.7)Ca(0.3)MnO(3), a perovskite manganite in which an insulator-metal transition can be driven by magnetic field, but also by pressure, visible light, x-rays, or high currents. We find that the field-induced transition is associated with an enormous release of energy which accounts for its strong irreversibility. In the ferromagnetic metallic state, specific heat and magnetization measurements indicate a much smaller spin wave stiffness than that seen in any other manganite, which we attribute to spin waves among the ferromagnetically ordered Pr moments. The coupling between the Pr and Mn spins may also provide a basis for understanding the low temperature phase diagram of this most unusual manganite.Comment: 10 pages, LATEX, 5 PDF figures, corrected typo

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

Growth, transport, and magnetic properties of Pr0.67Ca0.33MnO3 thin films

We have grown Pr0.67Ca0.33MnO3 thin films on LaAlO3 using pulsed laser deposition. Below 50 K, a field induced insulator-metal transition results in changes in resistivity of at least 6 orders of magnitude. The field induced conducting state is metastable at low temperature. The temperature dependence of the resistivity exhibits considerable hysteresis in a field of 40 kOe but becomes reversible in a field of 80 kOe

Effect of Co doping on the in-plane anisotropy in the optical spectrum of underdoped Ba(Fe1-xCox)2As2

We investigated the anisotropy in the in-plane optical spectra of detwinned Ba(Fe1-xCox)2As2. The optical conductivity spectrum of BaFe2As2 shows appreciable anisotropy in the magnetostructural ordered phase, whereas the dc resistivity is almost isotropic at low temperatures. Upon Co doping, the resistivity becomes highly anisotropic, while the finite-energy intrinsic anisotropy is suppressed. It is found that anisotropy in resistivity arises from anisotropic impurity scattering from doped Co atoms, extrinsic in origin. Intensity of a specific optical phonon mode is also found to show striking anisotropy in the ordered phase. The anisotropy induced by Co impurity and that observed in the optical phonon mode are hallmarks of the highly polarizable electronic state in the ordered phase.Comment: 5 pages, 4 figure