1,055 research outputs found
Time-, Frequency-, and Wavevector-Resolved X-Ray Diffraction from Single Molecules
Using a quantum electrodynamic framework, we calculate the off-resonant
scattering of a broad-band X-ray pulse from a sample initially prepared in an
arbitrary superposition of electronic states. The signal consists of
single-particle (incoherent) and two-particle (coherent) contributions that
carry different particle form factors that involve different material
transitions. Single-molecule experiments involving incoherent scattering are
more influenced by inelastic processes compared to bulk measurements. The
conditions under which the technique directly measures charge densities (and
can be considered as diffraction) as opposed to correlation functions of the
charge-density are specified. The results are illustrated with time- and
wavevector-resolved signals from a single amino acid molecule (cysteine)
following an impulsive excitation by a stimulated X-ray Raman process resonant
with the sulfur K-edge. Our theory and simulations can guide future
experimental studies on the structures of nano-particles and proteins
Clustering of matter in waves and currents
The growth rate of small-scale density inhomogeneities (the entropy
production rate) is given by the sum of the Lyapunov exponents in a random
flow. We derive an analytic formula for the rate in a flow of weakly
interacting waves and show that in most cases it is zero up to the fourth order
in the wave amplitude. We then derive an analytic formula for the rate in a
flow of potential waves and solenoidal currents. Estimates of the rate and the
fractal dimension of the density distribution show that the interplay between
waves and currents is a realistic mechanism for providing patchiness of
pollutant distribution on the ocean surface.Comment: 4 pages, 1 figur
Power-law tail distributions and nonergodicity
We establish an explicit correspondence between ergodicity breaking in a
system described by power-law tail distributions and the divergence of the
moments of these distributions.Comment: 4 pages, 1 figure, corrected typo
Representations of hom-Lie algebras
In this paper, we study representations of hom-Lie algebras. In particular,
the adjoint representation and the trivial representation of hom-Lie algebras
are studied in detail. Derivations, deformations, central extensions and
derivation extensions of hom-Lie algebras are also studied as an application.Comment: 16 pages, multiplicative and regular hom-Lie algebras are used,
Algebra and Representation Theory, 15 (6) (2012), 1081-109
THEORY OF INTEGRAL INDIVIDUALITY BY V. S. MERLIN: HISTORY AND NOWADAYS
The study is devoted to overview and analysis of V. S. Merlinβ theory of integral individuality.The aim of the study is to reveal a systemβs background of the theory of integral individuality; to designate its current issues and to put new tasks of its further advancement. Methodology and research methods. Problematic and comparative analyses are used. A systematization of the main assumptions of the theory by V. S. Merlin shows that it is based on a general systemic approach and current ideas about integration. Results. It is demonstrated that the system-based approach provides a multi-focus perspective to view the integral individuality. Mostly, the following system ideas are embedded in V. S. Merlinβs theory. They are the concepts of structural levels, teleology, and polymorphism. With respect to the theory of integral individuality, a human is shown as a big system. It consists of a hierarchical set not included in each other, but relatively autonomous operative multilevel subsystems. They link one to another in a polymorphic multi-valued (many-tomany) way. The main features of integral individuality are seen as the hierarchical arrangement and levels, integration and differentiation, teleology and causality, flexibility of polymorphic links (between levels) and rigidity of causal links (within levels). In spite of its maturity, this theory can be put in a further progress. This perspective has been elaborated based on three key ideas β multi-quality, commonality, and isomerism. Scientific novelty. The routes of the phenomenon of integral individuality are uncovered. Its main properties are described: a systemic version of integrity, hierarchy, and polymorphism. Some topical problems are highlighted within the theory of integral individuality. Next tasks can be set to further develop the theory of integral individuality. They focus on shift from the systemic viewpoint to a multi-systemic outline, to combine integrity and commonality, to provide an isomerism coming from the polymorphic framework.Practical significance. The materials and thesis stressed in this article can be useful for researchers studying holistic conceptions of human. The theory of integral individuality can guide investigations designated to test Merlinβs assumptions under various conditions.Π‘ΡΠ°ΡΡΡ ΠΏΠΎΡΠ²ΡΡΠ΅Π½Π° Π°Π½Π°Π»ΠΈΠ·Ρ ΡΠ΅ΠΎΡΠΈΠΈ ΠΈΠ½ΡΠ΅Π³ΡΠ°Π»ΡΠ½ΠΎΠΉ ΠΈΠ½Π΄ΠΈΠ²ΠΈΠ΄ΡΠ°Π»ΡΠ½ΠΎΡΡΠΈ, ΡΠ°Π·ΡΠ°Π±ΠΎΡΠ°Π½Π½ΠΎΠΉ Π²ΠΈΠ΄Π½ΡΠΌ Π΄Π΅ΡΡΠ΅Π»Π΅ΠΌ ΠΎΡΠ΅ΡΠ΅ΡΡΠ²Π΅Π½Π½ΠΎΠΉ ΠΏΡΠΈΡ
ΠΎΠ»ΠΎΠ³ΠΈΡΠ΅ΡΠΊΠΎΠΉ Π½Π°ΡΠΊΠΈ, ΠΎΡΠ½ΠΎΠ²Π°ΡΠ΅Π»Π΅ΠΌ ΠΠ΅ΡΠΌΡΠΊΠΎΠΉ ΠΏΡΠΈΡ
ΠΎΠ»ΠΎΠ³ΠΈΡΠ΅ΡΠΊΠΎΠΉ ΡΠΊΠΎΠ»Ρ Π. Π‘. ΠΠ΅ΡΠ»ΠΈΠ½ΠΎΠΌ. Π¦Π΅Π»Ρ ΠΏΡΠ±Π»ΠΈΠΊΠ°ΡΠΈΠΈ β Π²ΡΡΠ²ΠΈΡΡ ΡΡΠ½Π΄Π°ΠΌΠ΅Π½ΡΠ°Π»ΡΠ½ΡΠ΅ ΡΠΈΡΡΠ΅ΠΌΠ½ΡΠ΅ ΠΈΠ΄Π΅ΠΈ Π² ΡΠ΅ΠΎΡΠΈΠΈ ΠΈΠ½ΡΠ΅Π³ΡΠ°Π»ΡΠ½ΠΎΠΉ ΠΈΠ½Π΄ΠΈΠ²ΠΈΠ΄ΡΠ°Π»ΡΠ½ΠΎΡΡΠΈ, ΠΎΠ±ΠΎΠ·Π½Π°ΡΠΈΡΡ Π΅Π΅ Π°ΠΊΡΡΠ°Π»ΡΠ½ΡΠ΅ Π½Π°ΠΏΡΠ°Π²Π»Π΅Π½ΠΈΡ ΠΈ ΠΏΠΎΡΡΠ°Π²ΠΈΡΡ ΠΎΡΠ΅ΡΠ΅Π΄Π½ΡΠ΅ Π·Π°Π΄Π°ΡΠΈ Π΅Π΅ Π΄Π°Π»ΡΠ½Π΅ΠΉΡΠ΅Π³ΠΎ ΡΠ°Π·Π²ΠΈΡΠΈΡ. ΠΠ΅ΡΠΎΠ΄ΠΎΠ»ΠΎΠ³ΠΈΡ ΠΈ ΠΌΠ΅ΡΠΎΠ΄Ρ ΠΈΡΡΠ»Π΅Π΄ΠΎΠ²Π°Π½ΠΈΡ. ΠΡΠΏΠΎΠ»ΡΠ·ΠΎΠ²Π°Π»ΠΈΡΡ ΠΏΡΠΎΠ±Π»Π΅ΠΌΠ½ΡΠΉ Π°Π½Π°Π»ΠΈΠ·, ΡΠΈΡΡΠ΅ΠΌΠ°ΡΠΈΠ·Π°ΡΠΈΡ ΠΎΡΠ½ΠΎΠ²Π½ΡΡ
ΠΏΠΎΠ»ΠΎΠΆΠ΅Π½ΠΈΠΉ ΡΠ°Π·Π±ΠΈΡΠ°Π΅ΠΌΠΎΠΉ ΡΠ΅ΠΎΡΠΈΠΈ. ΠΠ΅ΡΠΎΠ΄ΠΎΠ»ΠΎΠ³ΠΈΡΠ΅ΡΠΊΠΎΠΉ Π±Π°Π·ΠΎΠΉ ΠΈΠ·ΡΡΠ΅Π½ΠΈΡ ΠΎΠ±ΡΡΠΆΠ΄Π°Π΅ΠΌΡΡ
ΠΏΡΠΎΠ±Π»Π΅ΠΌ ΠΏΠΎΡΠ»ΡΠΆΠΈΠ» ΡΠΈΡΡΠ΅ΠΌΠ½ΡΠΉ ΠΏΠΎΠ΄Ρ
ΠΎΠ΄ ΠΈ ΡΠΎΠ²ΡΠ΅ΠΌΠ΅Π½Π½ΡΠ΅ ΠΏΡΠ΅Π΄ΡΡΠ°Π²Π»Π΅Π½ΠΈΡ ΠΎΠ± ΠΈΠ½ΡΠ΅Π³ΡΠ°ΡΠΈΠΈ. Π Π΅Π·ΡΠ»ΡΡΠ°ΡΡ. ΠΠΎΠΊΠ°Π·Π°Π½ΠΎ, ΡΡΠΎ ΡΠΈΡΡΠ΅ΠΌΠ½ΡΠΉ ΠΏΠΎΠ΄Ρ
ΠΎΠ΄ ΠΎΡΠΊΡΡΠ²Π°Π΅Ρ ΠΌΠ½ΠΎΠ³ΠΎΠ°ΡΠΏΠ΅ΠΊΡΠ½ΠΎΠ΅ Π²ΠΈΠ΄Π΅Π½ΠΈΠ΅ ΡΠ²Π»Π΅Π½ΠΈΡ ΠΈ ΠΏΠΎΠ·Π²ΠΎΠ»ΡΠ΅Ρ ΡΠ°ΡΡΠΌΠ°ΡΡΠΈΠ²Π°ΡΡ Π΅Π³ΠΎ Π² Π½Π΅ΡΠΊΠΎΠ»ΡΠΊΠΈΡ
ΡΠΈΡΡΠ΅ΠΌΠ°Ρ
ΠΊΠΎΠΎΡΠ΄ΠΈΠ½Π°Ρ. Π£ΡΠ΅Π½ΠΈΠ΅ ΠΎΠ± ΠΈΠ½ΡΠ΅Π³ΡΠ°Π»ΡΠ½ΠΎΠΉ ΠΈΠ½Π΄ΠΈΠ²ΠΈΠ΄ΡΠ°Π»ΡΠ½ΠΎΡΡΠΈ ΠΏΡΠ΅Π΄ΡΡΠ°Π²Π»ΡΠ΅Ρ ΡΠΎΠ±ΠΎΠΉ Π²Π°ΡΠΈΠ°Π½Ρ ΡΠ΅Π»ΠΎΡΡΠ½ΠΎΠ³ΠΎ ΠΏΠΎΠ΄Ρ
ΠΎΠ΄Π° ΠΊ ΡΠ΅Π»ΠΎΠ²Π΅ΠΊΡ Ρ ΠΏΠΎΠ·ΠΈΡΠΈΠΉ ΠΏΡΠΈΠ½ΡΠΈΠΏΠΎΠ² ΠΎΠ±ΡΠ΅ΠΉ ΡΠ΅ΠΎΡΠΈΠΈ ΡΠΈΡΡΠ΅ΠΌ. Π ΡΠ΅ΠΎΡΠΈΠΈ ΠΠ΅ΡΠ»ΠΈΠ½Π° ΡΠ΅Π°Π»ΠΈΠ·ΠΎΠ²Π°Π½Ρ, ΠΏΡΠ΅ΠΆΠ΄Π΅ Π²ΡΠ΅Π³ΠΎ, ΡΠ»Π΅Π΄ΡΡΡΠΈΠ΅ ΡΡΠ½Π΄Π°ΠΌΠ΅Π½ΡΠ°Π»ΡΠ½ΡΠ΅ ΠΈΠ΄Π΅ΠΈ: ΠΎ ΡΡΡΡΠΊΡΡΡΠ½ΡΡ
ΡΡΠΎΠ²Π½ΡΡ
, ΡΠ΅Π»Π΅ΠΎΠ»ΠΎΠ³ΠΈΡΠ΅ΡΠΊΠΎΠΉ Π΄Π΅ΡΠ΅ΡΠΌΠΈΠ½Π°ΡΠΈΠΈ ΠΈ ΠΏΠΎΠ»ΠΈΠΌΠΎΡΡΠΈΠ·ΠΌΠ΅. Π’Π΅ΠΎΡΠΈΡ ΠΈΠ½ΡΠ΅Π³ΡΠ°Π»ΡΠ½ΠΎΠΉ ΠΈΠ½Π΄ΠΈΠ²ΠΈΠ΄ΡΠ°Π»ΡΠ½ΠΎΡΡΠΈ ΡΡΠ°ΠΊΡΡΠ΅Ρ ΡΠ΅Π»ΠΎΠ²Π΅ΠΊΠ° ΠΊΠ°ΠΊ Π±ΠΎΠ»ΡΡΡΡ ΡΠΈΡΡΠ΅ΠΌΡ, ΠΊΠΎΡΠΎΡΠ°Ρ ΡΠΊΠ»Π°Π΄ΡΠ²Π°Π΅ΡΡΡ ΠΈΠ· ΠΈΠ΅ΡΠ°ΡΡ
ΠΈΡΠ΅ΡΠΊΠΎΠΉ ΡΠΎΠ²ΠΎΠΊΡΠΏΠ½ΠΎΡΡΠΈ Π½Π΅ Π²Ρ
ΠΎΠ΄ΡΡΠΈΡ
Π΄ΡΡΠ³ Π² Π΄ΡΡΠ³Π°, ΠΎΡΠ½ΠΎΡΠΈΡΠ΅Π»ΡΠ½ΠΎ Π°Π²ΡΠΎΠ½ΠΎΠΌΠ½ΠΎ ΡΠΎΡΡΡΠ΅ΡΡΠ²ΡΡΡΠΈΡ
ΡΠ°Π·Π½ΠΎΡΡΠΎΠ²Π½Π΅Π²ΡΡ
ΠΏΠΎΠ΄ΡΠΈΡΡΠ΅ΠΌ, ΠΌΠ½ΠΎΠ³ΠΎ-ΠΌΠ½ΠΎΠ³ΠΎΠ·Π½Π°ΡΠ½ΠΎ (ΠΏΠΎΠ»ΠΈΠΌΠΎΡΡΠ½ΠΎ) ΡΠ²ΡΠ·Π°Π½Π½ΡΡ
ΠΌΠ΅ΠΆΠ΄Ρ ΡΠΎΠ±ΠΎΠΉ. ΠΠ΅ΡΠ°ΡΡ
ΠΈΡΠ΅ΡΠΊΠΈΠΉ ΡΠΏΠΎΡΠΎΠ± ΠΎΡΠ³Π°Π½ΠΈΠ·Π°ΡΠΈΠΈ ΠΈ ΡΡΠΎΠ²Π½ΠΈ, Π΅Π΄ΠΈΠ½ΡΡΠ²ΠΎ ΠΈΠ½ΡΠ΅Π³ΡΠ°ΡΠΈΠΈ ΠΈ Π΄ΠΈΡΡΠ΅ΡΠ΅Π½ΡΠΈΠ°ΡΠΈΠΈ, ΡΠ΅Π»Π΅ΠΎΠ»ΠΎΠ³ΠΈΡΠ΅ΡΠΊΠΈΠΉ ΠΈ ΠΊΠ°ΡΠ·Π°Π»ΡΠ½ΡΠΉ ΡΠΈΠΏΡ Π΄Π΅ΡΠ΅ΡΠΌΠΈΠ½Π°ΡΠΈΠΈ, Π³ΠΈΠ±ΠΊΠΎΡΡΡ ΠΌΠ½ΠΎΠ³ΠΎ-ΠΌΠ½ΠΎΠ³ΠΎΠ·Π½Π°ΡΠ½ΡΡ
(ΠΏΠΎΠ»ΠΈΠΌΠΎΡΡΠ½ΡΡ
) ΠΈ ΠΆΠ΅ΡΡΠΊΠΎΡΡΡ ΠΎΠ΄Π½ΠΎΠ·Π½Π°ΡΠ½ΡΡ
ΡΠ²ΡΠ·Π΅ΠΉ β ΡΠ°ΠΊΠΎΠ²Ρ Π³Π»Π°Π²Π½ΡΠ΅ ΠΎΡΠΎΠ±Π΅Π½Π½ΠΎΡΡΠΈ Π²Π·Π³Π»ΡΠ΄Π° Π½Π° ΡΠ΅Π»ΠΎΠ²Π΅ΠΊΠ° Π² ΡΡΠΎΠΉ ΡΠ΅ΠΎΡΠΈΠΈ. Π£ΡΠ΅Π½ΠΈΠ΅ ΠΠ΅ΡΠ»ΠΈΠ½Π° ΠΈΠΌΠ΅Π΅Ρ Π·Π½Π°ΡΠΈΡΠ΅Π»ΡΠ½ΡΠΉ ΠΏΠΎΡΠ΅Π½ΡΠΈΠ°Π» Π΄Π»Ρ Π΄Π°Π»ΡΠ½Π΅ΠΉΡΠ΅Π³ΠΎ ΡΠ°Π·Π²ΠΈΡΠΈΡ. ΠΡΡ Π·Π°Π΄Π°ΡΡ ΠΌΠΎΠΆΠ½ΠΎ ΡΠ΅ΡΠ°ΡΡ, ΡΠΎΡΡΠ΅Π΄ΠΎΡΠΎΡΠΈΠ²ΡΠΈΡΡ Π½Π° ΡΡΠ΅Ρ
Π°ΠΊΡΡΠ°Π»ΡΠ½ΡΡ
ΠΏΡΠΎΠ±Π»Π΅ΠΌΠ°Ρ
β ΠΌΠ½ΠΎΠ³ΠΎΠΊΠ°ΡΠ΅ΡΡΠ²Π΅Π½Π½ΠΎΡΡΠΈ, ΠΎΠ±ΡΠ½ΠΎΡΡΠΈ ΠΈ ΠΈΠ·ΠΎΠΌΠ΅ΡΠΈΠΈ.ΠΠ°ΡΡΠ½Π°Ρ Π½ΠΎΠ²ΠΈΠ·Π½Π°. ΠΡΠΊΡΡΡΠ° ΡΡΡΠ½ΠΎΡΡΡ ΡΠ΅Π½ΠΎΠΌΠ΅Π½Π° ΠΈΠ½ΡΠ΅Π³ΡΠ°Π»ΡΠ½ΠΎΠΉ ΠΈΠ½Π΄ΠΈΠ²ΠΈΠ΄ΡΠ°Π»ΡΠ½ΠΎΡΡΠΈ. ΠΠΏΠΈΡΠ°Π½Ρ Π΅Π³ΠΎ Π³Π»Π°Π²Π½ΡΠ΅ Π°ΡΡΠΈΠ±ΡΡΡ β ΡΠΈΡΡΠ΅ΠΌΠ½ΡΠΉ Π²Π°ΡΠΈΠ°Π½Ρ ΡΠ΅Π»ΠΎΡΡΠ½ΠΎΡΡΠΈ, ΠΈΠ΅ΡΠ°ΡΡ
ΠΈΡ ΠΈ ΠΏΠΎΠ»ΠΈΠΌΠΎΡΡΠΈΠ·ΠΌ. ΠΠ±ΠΎΠ·Π½Π°ΡΠ΅Π½Ρ Π°ΠΊΡΡΠ°Π»ΡΠ½ΡΠ΅ ΠΏΡΠΎΠ±Π»Π΅ΠΌΡ ΠΈ Π½Π°ΠΌΠ΅ΡΠ΅Π½Ρ ΠΎΡΠ΅ΡΠ΅Π΄Π½ΡΠ΅ Π·Π°Π΄Π°ΡΠΈ Π΄Π°Π»ΡΠ½Π΅ΠΉΡΠ΅Π³ΠΎ ΡΠ°Π·Π²ΠΈΡΠΈΡ ΡΠ΅ΠΎΡΠΈΠΈ ΠΈΠ½ΡΠ΅Π³ΡΠ°Π»ΡΠ½ΠΎΠΉ ΠΈΠ½Π΄ΠΈΠ²ΠΈΠ΄ΡΠ°Π»ΡΠ½ΠΎΡΡΠΈ ΠΏΠΎ Π»ΠΈΠ½ΠΈΡΠΌ ΠΏΠ΅ΡΠ΅Ρ
ΠΎΠ΄Π° ΠΎΡ ΡΠΈΡΡΠ΅ΠΌΠ½ΠΎΠ³ΠΎ ΠΊ ΠΏΠΎΠ»ΠΈΡΠΈΡΡΠ΅ΠΌΠ½ΠΎΠΌΡ Π΅Π΅ ΡΠ°Π·Π²ΠΈΡΠΈΡ, ΠΎΠ±ΡΠ½ΠΎΡΡΠΈ ΠΈ ΠΈΠ·ΠΎΠΌΠ΅ΡΠΈΠΈ. ΠΡΠ°ΠΊΡΠΈΡΠ΅ΡΠΊΠ°Ρ Π·Π½Π°ΡΠΈΠΌΠΎΡΡΡ. ΠΠ°ΡΠ΅ΡΠΈΠ°Π»Ρ ΡΡΠ°ΡΡΠΈ ΠΌΠΎΠ³ΡΡ Π±ΡΡΡ ΠΏΠΎΠ»Π΅Π·Π½Ρ ΠΈΡΡΠ»Π΅Π΄ΠΎΠ²Π°ΡΠ΅Π»ΡΠΌ, Π·Π°Π½ΠΈΠΌΠ°ΡΡΠΈΠΌΡΡ ΠΈΠ·ΡΡΠ΅Π½ΠΈΠ΅ΠΌ ΡΠ΅Π»ΠΎΡΡΠ½ΡΡ
ΠΏΡΠ΅Π΄ΡΡΠ°Π²Π»Π΅Π½ΠΈΠΉ ΠΎ ΡΠ΅Π»ΠΎΠ²Π΅ΠΊΠ΅, ΠΏΡΠΎΠ²ΠΎΠ΄ΡΡΠΈΠΌ ΡΠΌΠΏΠΈΡΠΈΡΠ΅ΡΠΊΠΈΠ΅ ΠΈΡΡΠ»Π΅Π΄ΠΎΠ²Π°Π½ΠΈΡ Π² ΡΡΡΠ»Π΅ ΡΠ΅ΠΎΡΠΈΠΈ ΠΈΠ½ΡΠ΅Π³ΡΠ°Π»ΡΠ½ΠΎΠΉ ΠΈΠ½Π΄ΠΈΠ²ΠΈΠ΄ΡΠ°Π»ΡΠ½ΠΎΡΡΠΈ Π. Π‘. ΠΠ΅ΡΠ»ΠΈΠ½Π°
Rational Approximate Symmetries of KdV Equation
We construct one-parameter deformation of the Dorfman Hamiltonian operator
for the Riemann hierarchy using the quasi-Miura transformation from topological
field theory. In this way, one can get the approximately rational symmetries of
KdV equation and then investigate its bi-Hamiltonian structure.Comment: 14 pages, no figure
Applications of Temperley-Lieb algebras to Lorentz lattice gases
Motived by the study of motion in a random environment we introduce and
investigate a variant of the Temperley-Lieb algebra. This algebra is very rich,
providing us three classes of solutions of the Yang-Baxter equation. This
allows us to establish a theoretical framework to study the diffusive behaviour
of a Lorentz Lattice gas. Exact results for the geometrical scaling behaviour
of closed paths are also presented.Comment: 10 pages, latex file, one figure(by request
Minimal Stochastic Model for Fermi's Acceleration
We introduce a simple stochastic system able to generate anomalous diffusion
both for position and velocity. The model represents a viable description of
the Fermi's acceleration mechanism and it is amenable to analytical treatment
through a linear Boltzmann equation. The asymptotic probability distribution
functions (PDF) for velocity and position are explicitly derived. The diffusion
process is highly non-Gaussian and the time growth of moments is characterized
by only two exponents and . The diffusion process is anomalous
(non Gaussian) but with a defined scaling properties i.e. and similarly for velocity.Comment: RevTeX4, 4 pages, 2 eps-figures (minor revision
Crystallization of the ordered vortex phase in high temperature superconductors
The Landau-Khalatnikov time-dependent equation is applied to describe the
crystallization process of the ordered vortex lattice in high temperature
superconductors after a sudden application of a magnetic field. Dynamic
coexistence of a stable ordered phase and an unstable disordered phase, with a
sharp interface between them, is demonstrated. The transformation to the
equilibrium ordered state proceeds by movement of this interface from the
sample center toward its edge. The theoretical analysis dictates specific
conditions for the creation of a propagating interface, and provides the time
scale for this process.Comment: 8 pages and 3 figures; to be published in Phys. Rev. B (Rapid
Communications section
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