Doping Dependence of Polaron Hopping Energies in La(1-x)Ca(x)MnO(3) (0<= x<= 0.15)

Measurements of the low-frequency (f<= 100 kHz) permittivity at T<= 160 K and dc resistivity (T<= 430 K) are reported for La(1-x)Ca(x)MnO(3) (0<= x<= 0.15). Static dielectric constants are determined from the low-T limiting behavior of the permittivity. The estimated polarizability for bound holes ~ 10^{-22} cm^{-3} implies a radius comparable to the interatomic spacing, consistent with the small polaron picture established from prior transport studies near room temperature and above on nearby compositions. Relaxation peaks in the dielectric loss associated with charge-carrier hopping yield activation energies in good agreement with low-T hopping energies determined from variable-range hopping fits of the dc resistivity. The doping dependence of these energies suggests that the orthorhombic, canted antiferromagnetic ground state tends toward an insulator-metal transition that is not realized due to the formation of the ferromagnetic insulating state near Mn(4+) concentration ~ 0.13.Comment: PRB in press, 5 pages, 6 figure

Low Temperature Magnetic Properties of the Double Exchange Model

We study the {\it ferromagnetic} (FM) Kondo lattice model in the strong coupling limit (double exchange (DE) model). The DE mechanism proposed by Zener to explain ferromagnetism has unexpected properties when there is more than one itinerant electron. We find that, in general, the many-body ground state of the DE model is {\it not} globally FM ordered (except for special filled-shell cases). Also, the low energy excitations of this model are distinct from spin wave excitations in usual Heisenberg ferromagnets, which will result in unusual dynamic magnetic properties.Comment: 5 pages, RevTeX, 5 Postscript figures include

Low-temperature electrical transport and double exchange in La(Pb,Ca)MnO

The resistivity in the ferromagnetic state of flux-grown La_{2/3}(Pb,Ca)_{1/3}MnO_3 single crystals, measured in magnetic fields up to 7 T, reveals a strong quadratic temperature dependence at and above 50 K. At lower temperatures, this contribution drops precipitously leaving the resistivity essentially temperature independent below 20 K. The Seebeck coefficient also reflects a change of regime at the same temperature. We attribute this behavior to a cut-off of single magnon scattering processes at long wavelengths due to the polarized bands of a double-exchange ferromagnet.Comment: 10 pages, TeX, 4 figures. Revised version. Submitte

What’s so bad about scientism?

In their attempt to defend philosophy from accusations of uselessness made by prominent scientists, such as Stephen Hawking, some philosophers respond with the charge of ‘scientism.’ This charge makes endorsing a scientistic stance, a mistake by definition. For this reason, it begs the question against these critics of philosophy, or anyone who is inclined to endorse a scientistic stance, and turns the scientism debate into a verbal dispute. In this paper, I propose a different definition of scientism, and thus a new way of looking at the scientism debate. Those philosophers who seek to defend philosophy against accusations of uselessness would do philosophy a much better service, I submit, if they were to engage with the definition of scientism put forth in this paper, rather than simply make it analytic that scientism is a mistake

Actin-binding rho activating protein (Abra) is essential for fluid shear stress-induced arteriogenesis

OBJECTIVE: Arteriogenesis, the development of a collateral circulation, is important for tissue survival but remains functionally defective because of early normalization of fluid shear stress (FSS). Using a surgical model of chronically elevated FSS we showed that rabbits exhibited normal blood flow reserve after femoral artery ligature (FAL). Inhibition of the Rho pathway by Fasudil completely blocked the beneficial effect of FSS. In a genome-wide gene profiling we identified actin-binding Rho activating protein (Abra), which was highly upregulated in growing collaterals. METHODS AND RESULTS: qRT-PCR and Western blot confirmed highly increased FSS-dependent expression of Abra in growing collaterals. NO blockage by L-NAME abolished FSS-generated Abra expression as well as the whole arteriogenic process. Cell culture studies demonstrated an Abra-triggered proliferation of smooth muscle cells through a mechanism that requires Rho signaling. Local intracollateral adenoviral overexpression of Abra improved collateral conductance by 60% in rabbits compared to the natural response after FAL. In contrast, targeted deletion of Abra in CL57BL/6 mice led to impaired arteriogenesis. CONCLUSIONS: FSS-induced Abra expression during arteriogenesis is triggered by NO and leads to stimulation of collateral growth by smooth muscle cell proliferation

High Temperature Thermopower in La_{2/3}Ca_{1/3}MnO_3 Films: Evidence for Polaronic Transport

Thermoelectric power, electrical resistivity and magnetization experiments, performed in the paramagnetic phase of La_{2/3}Ca_{1/3}MnO_3, provide evidence for polaron-dominated conduction in CMR materials. At high temperatures, a large, nearly field-independent difference between the activation energies for resistivity (rho) and thermopower (S), a characteristic of Holstein Polarons, is observed, and ln(rho) ceases to scale with the magnetization. On approaching T_c, both energies become field-dependent, indicating that the polarons are magnetically polarized. Below T_c, the thermopower follows a law S(H) prop. 1/rho (H) as in non saturated ferromagnetic metals.Comment: 10 pages, 5 .gif figures. Phys. Rev B (in press

Specific heat and heat conductivity of the BaTiO3 polycrystalline films with the thickness in the range 20 - 1100 nm

Thermal properties - specific heat and heat conductivity coefficient - of polycrystalline BaTiO3 films on massive substrates were studied as a function of the temperature and the film thickness by ac-hot probe method. The anomalies of specific heat with decreasing of the film thickness from 1100 to 20 nm revealed the reducing of critical temperature (Tc) and excess entropy of the ferroelectric phase transition, which becomes diffused. The critical thickness of the film at which Tc = 0 estimated as 2.5 nm.Comment: 12 pages, 7 figures, 2 tables, 450kb; submitted to J.Phys.:Cond.Mat

Extrinsic Magnetotransport Phenomena in Ferromagnetic Oxides

This review is focused on extrinsic magnetotransport effects in ferromagnetic oxides. It consists of two parts; the second part is devoted to an overview of experimental data and theoretical models for extrinsic magnetotransport phenomena. Here a critical discussion of domain-wall scattering is given. Results on surfacial and interfacial magnetism in oxides are presented. Spin-polarized tunnelling in ferromagnetic junctions is reviewed and grain-boundary magnetoresistance is interpreted within a model of spin-polarized tunnelling through natural oxide barriers. The situation in ferromagnetic oxides is compared with data and models for conventional ferromagnets. The first part of the review summarizes basic material properties, especially data on the spin-polarization and evidence for half-metallicity. Furthermore, intrinsic conduction mechanisms are discussed. An outlook on the further development of oxide spin-electronics concludes this review.Comment: 133 pages, 47 figures, submitted to Rep. Prog. Phy