6,835 research outputs found
Large and Small Polaron Excitations in La2/3(Sr/Ca)1/3MnO3 Films
We present detailed optical measurements of the mid-infrared (MIR)
excitations in thin films of La2/3Sr1/3MnO3 (LSMO) and La2/3Ca1/3MnO3 (LCMO)
across the magnetic transition. The shape of the excitation at about 0.2 eV in
both samples is analyzed in terms of polaron models. We propose to identify the
MIR resonance in LSMO as the excitation of large polarons and that in LCMO as a
small polaron excitation. A scaling behavior for the low-energy side of the
polaronic MIR resonance in LSMO is established
Colossal Effects in Transition Metal Oxides Caused by Intrinsic Inhomogeneities
The influence of quenched disorder on the competition between ordered states
separated by a first-order transition is investigated. A phase diagram with
features resembling quantum-critical behavior is observed, even using classical
models. The low-temperature paramagnetic regime consists of coexisting ordered
clusters, with randomly oriented order parameters. Extended to manganites, this
state is argued to have a colossal magnetoresistance effect. A scale T* for
cluster formation is discussed. This is the analog of the Griffiths
temperature, but for the case of two competing orders, producing a strong
susceptibility to external fields. Cuprates may have similar features,
compatible with the large proximity effect of the very underdoped regime.Comment: 4 pages, 4 figure
Spin-phonon coupling in antiferromagnetic chromium spinels
The temperature dependence of eigenfrequencies and intensities of the IR
active modes has been investigated for the antiferromagnetic chromium spinel
compounds CdCr2O4, ZnCr2O4, ZnCr2S4, ZnCr2Se4, and HgCr2S4 by IR spectroscopy
for temperatures from 5 K to 300 K. At the transition into the magnetically
ordered phases, and driven by spin-phonon coupling, most compounds reveal
significant splittings of the phonon modes. This is true for geometrically
frustrated CdCr2O4, and ZnCr2O4, for bond frustrated ZnCr2S4 and for ZnCr2Se4,
which also is bond frustrated, but dominated by ferromagnetic exchange. The
pattern of splitting is different for the different compounds and crucially
depends on the nature of frustration and of the resulting spin order. HgCr2S4,
which is almost ferromagnetic, exhibits no splitting of the eigenfrequencies,
but shows significant shifts due to ferromagnetic spin fluctuations.Comment: 15 pages, 6 figure
Amorphous thin film growth: theory compared with experiment
Experimental results on amorphous ZrAlCu thin film growth and the dynamics of
the surface morphology as predicted from a minimal nonlinear stochastic
deposition equation are analysed and compared. Key points of this study are (i)
an estimation procedure for coefficients entering into the growth equation and
(ii) a detailed analysis and interpretation of the time evolution of the
correlation length and the surface roughness. The results corroborate the
usefulness of the deposition equation as a tool for studying amorphous growth
processes.Comment: 7 pages including 5 figure
Onium ions. 18. Static protonated and exchanging diprotonated ambivalent heteroorganic systems. hydroxylamines, acetone oxime, and dimethyl sulfoxide
Untangling the Conceptual Isssues Raised in Reydon and Scholz’s Critique of Organizational Ecology and Darwinian Populations
Reydon and Scholz raise doubts about the Darwinian status of organizational ecology by arguing that Darwinian principles are not applicable to organizational populations. Although their critique of organizational ecology’s typological essentialism is correct, they go on to reject the Darwinian status of organizational populations. This paper claims that the distinction between replicators and interactors, raised in modern philosophy of biology but not discussed by Reydon and Scholz, points the way forward for organizational ecologists. It is possible to conceptualise evolving Darwinian populations providing the inheritance mechanism is appropriately specified. By this approach, adaptation and selection are no longer dichotomised, and the evolutionary significance of knowledge transmission is highlightedPeer reviewe
Phonon anomalies and charge dynamics in Fe_{1-x}Cu_{x}Cr_{2}S_{4} single crystals
A detailed investigation of phonon excitations and charge carrier dynamics in
single crystals of Fe_{1-x}Cu_{x}Cr_{2}S_{4} (x = 0, 0.2, 0.4, 0.5) has been
performed by using infrared spectroscopy. In FeCr_{2}S_{4} the phonon
eigenmodes are strongly affected by the onset of magnetic order. Despite
enhanced screening effects, a continuous evolution of the phonon excitations
can be observed in the doped compounds with x = 0.2 (metallic) and x = 0.4, 0.5
(bad metals), but the effect of magnetic ordering on the phonons is strongly
reduced compared to x = 0. The Drude-like charge-carrier contribution to the
optical conductivity in the doped samples indicates that the colossal
magneto-resistance effect results from the suppression of spin-disorder
scattering.Comment: 8 pages, 6 figure
The Bak-Sneppen Model on Scale-Free Networks
We investigate by numerical simulations and analytical calculations the
Bak-Sneppen model for biological evolution in scale-free networks. By using
large scale numerical simulations, we study the avalanche size distribution and
the activity time behavior at nodes with different connectivities. We argue the
absence of a critical barrier and its associated critical behavior for infinite
size systems. These findings are supported by a single site mean-field analytic
treatment of the model.Comment: 5 pages and 3 eps figures. Final version appeared in Europhys. Let
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