212 research outputs found
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
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
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
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
Country-Level Aid Coordination at the United Nations: Taking the Resident Coordinator System Forward
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