54 research outputs found
Spin rotation for ballistic electron transmission induced by spin-orbit interaction
We study spin dependent electron transmission through one- and
two-dimensional curved waveguides and quantum dots with account of spin-orbit
interaction. We prove that for a transmission through arbitrary structure there
is no spin polarization provided that electron transmits in isolated energy
subband and only two leads are attached to the structure. In particular there
is no spin polarization in the one-dimensional wire for which spin dependent
solution is found analytically. The solution demonstrates spin evolution as
dependent on a length of wire. Numerical solution for transmission of electrons
through the two-dimensional curved waveguides coincides with the solution for
the one-dimensional wire if the energy of electron is within the first energy
subband. In the vicinity of edges of the energy subbands there are sharp
anomalies of spin flipping.Comment: 9 oages, 7 figure
Spin Dynamics and Spin Transport
Spin-orbit (SO) interaction critically influences electron spin dynamics and
spin transport in bulk semiconductors and semiconductor microstructures. This
interaction couples electron spin to dc and ac electric fields. Spin coupling
to ac electric fields allows efficient spin manipulating by the electric
component of electromagnetic field through the electric dipole spin resonance
(EDSR) mechanism. Usually, it is much more efficient than the magnetic
manipulation due to a larger coupling constant and the easier access to spins
at a nanometer scale. The dependence of the EDSR intensity on the magnetic
field direction allows measuring the relative strengths of the competing SO
coupling mechanisms in quantum wells. Spin coupling to an in-plane electric
field is much stronger than to a perpendicular field. Because electron bands in
microstructures are spin split by SO interaction, electron spin is not
conserved and spin transport in them is controlled by a number of competing
parameters, hence, it is rather nontrivial. The relation between spin
transport, spin currents, and spin populations is critically discussed.
Importance of transients and sharp gradients for generating spin magnetization
by electric fields and for ballistic spin transport is clarified.Comment: Invited talk at the 3rd Intern. Conf. on Physics and Applications of
Spin-Related Phenomena in Semiconductors, Santa Barbara (CA), July 21 - 23.
To be published in the Journal of Superconductivity. 7 pages, 2 figure
Spintronics: Fundamentals and applications
Spintronics, or spin electronics, involves the study of active control and
manipulation of spin degrees of freedom in solid-state systems. This article
reviews the current status of this subject, including both recent advances and
well-established results. The primary focus is on the basic physical principles
underlying the generation of carrier spin polarization, spin dynamics, and
spin-polarized transport in semiconductors and metals. Spin transport differs
from charge transport in that spin is a nonconserved quantity in solids due to
spin-orbit and hyperfine coupling. The authors discuss in detail spin
decoherence mechanisms in metals and semiconductors. Various theories of spin
injection and spin-polarized transport are applied to hybrid structures
relevant to spin-based devices and fundamental studies of materials properties.
Experimental work is reviewed with the emphasis on projected applications, in
which external electric and magnetic fields and illumination by light will be
used to control spin and charge dynamics to create new functionalities not
feasible or ineffective with conventional electronics.Comment: invited review, 36 figures, 900+ references; minor stylistic changes
from the published versio
Semiconductor Spintronics
Spintronics refers commonly to phenomena in which the spin of electrons in a
solid state environment plays the determining role. In a more narrow sense
spintronics is an emerging research field of electronics: spintronics devices
are based on a spin control of electronics, or on an electrical and optical
control of spin or magnetism. This review presents selected themes of
semiconductor spintronics, introducing important concepts in spin transport,
spin injection, Silsbee-Johnson spin-charge coupling, and spindependent
tunneling, as well as spin relaxation and spin dynamics. The most fundamental
spin-dependent nteraction in nonmagnetic semiconductors is spin-orbit coupling.
Depending on the crystal symmetries of the material, as well as on the
structural properties of semiconductor based heterostructures, the spin-orbit
coupling takes on different functional forms, giving a nice playground of
effective spin-orbit Hamiltonians. The effective Hamiltonians for the most
relevant classes of materials and heterostructures are derived here from
realistic electronic band structure descriptions. Most semiconductor device
systems are still theoretical concepts, waiting for experimental
demonstrations. A review of selected proposed, and a few demonstrated devices
is presented, with detailed description of two important classes: magnetic
resonant tunnel structures and bipolar magnetic diodes and transistors. In most
cases the presentation is of tutorial style, introducing the essential
theoretical formalism at an accessible level, with case-study-like
illustrations of actual experimental results, as well as with brief reviews of
relevant recent achievements in the field.Comment: tutorial review; 342 pages, 132 figure
meso-Me2Si(1-indenyl)2ZrCl2/ methylalumoxane catalyzed polymerization of the ethylene to ethyl-branched polyethylene
The meso ansa zirconocene with dimethylsilyl bridge, activated by methylalumoxane, catalyses the ethylene polymerization, producing ethyl-branched polyethylene. With respect to the polymers obtained with the previously investigated meso zirconocenes, we have found higher branching amount and lower molecular weight. The rapid \u3b2-H transfer from the growing chain to the coordinated monomer could account for both these features of the polymer. The investigation on the structural parameters of the complex, through X-ray diffraction analysis, and on the electrophilicity of the cationic center, through NMR experiments, suggests, as a possible rationalization of this behavior, the obstruction in the inward site. \ua9 2004 Elsevier B.V. All rights reserved
Connection of Stereoselectivity, Regioselectivity, and Molecular Weight Capability in rac-R′2Si(2-Me-4-R-indenyl)2ZrCl2 Type Catalysts
A set of 19 silicon-bridged C2-symmetric zirconocenes rac-R′2Si(2-Me-4-R-indenyl)2ZrCl2 of varying steric demand in position 4 were synthesized and screened in
propene homopolymerization in a high-throughput experimental setup. The size and accuracy of the experimental data set allow to identify surprisingly good correlations among stereoselectivity, regioselectivity, and molecular weight capability
(R2 ≈ 0.8−0.9) over a broad range. We rationalize this trend by assuming that steric tuning in the 4-position affects both preferred insertion and stereoerror formation similarly but leaves other barriers largely unaffected. A quantitative structure−activity relationship based on one single computational descriptor, Δ%VBur
using the difference in the percent of buried volume between the “blocked” and “open” quadrants of the catalyst precursoris established. Provided that a large sphere of 5.0 Å is used, stereoselectivity can be predicted with unprecedented accuracy, i.e., a mean average deviation (MAD) of 0.18 kcal/mol (ΔΔG‡
enantio), 0.0007 (σ, probability that the preferred propene enantioface is selected at an active site of given chirality), or 0.3% (mmmm pentads). On the basis of this
empirical model, we predicted that the catalyst with R = o-tolyl is an ideal candidate for high stereoselectivity/high MW capability. Ad hoc synthesis and testing of the precursor confirmed the expectations: the catalyst shows the highest stereoselectivity reported so far (σ = 0.9999) for metallocenes at 60 °C, while maintaining a high MW capability (Mw > 1 MDa) and relatively high regioselectivity
Role of Solvent Coordination on the Structure and Dynamics of ansa-Zirconocenium Ion Pairs in Aromatic Hydrocarbons
The solution structure and dynamics of three prototypical bis-indenyl ansa-zirconocenium methyl cations (Me2Si(2-methyl-4-phenyl-6-tert-butylindenyl)2ZrMe+, [1Me]+ Me2Si(2,4-dimethylindenyl)2ZrMe+, [2Me]+ Me2Si(indenyl)2ZrMe+, [3Me]+) paired with the weakly coordinating B(C6F5)4- anion have been investigated by NMR spectroscopy in aromatic hydrocarbons with different polarities (toluene, ϵr = 2.38; chlorobenzene, ϵr = 5.69; 1,2-difluorobenzene (ODFB), ϵr = 13.38). These highly electrophilic cations are stabilized by solvent coordination, as evidenced by the unequivocal identification of [1Me·C7H8]+B(C6F5)4-, a rare example of a toluene-stabilized ion pair that has been fully characterized in solution. Combining spectroscopic and DFT methods allowed us to evaluate how solvent coordination modulates the dynamic processes typical of this class of catalysts. An exchange between the two faces of coordinated toluene (face inversion, FI) occurs without solvent decomplexation (ΔH⧧FI = 14.6 kcal mol-1 ΔS⧧FI = 3 cal mol-1 K-1 ΔG⧧FI(298) = 13.7 kcal mol-1) and is about 20 times faster than the exchange between coordinated and free solvent (solvent decomplexation, SD, ΔH⧧SD = 17.9 kcal mol-1 ΔS⧧SD = 10 cal mol-1 K-1 ΔG⧧SD(298) = 14.9 kcal mol-1) and ion pair symmetrization (IPS, ΔH⧧IPS = 18.6 kcal mol-1 ΔS⧧IPS = 12 cal mol-1 K-1 ΔG⧧IPS(298) = 14.9 kcal mol-1). For the ion pairs [1-3Me·solvent]+B(C6F5)4-, IPS rate constants (kIPS) do not correlate with the solvent polarity (kIPS(ODFB) > kIPS(toluene) > kIPS(chlorobenzene)). For the more coordinating toluene and chlorobenzene solvents, ΔG⧧IPS nicely tracks with the amount of positive charge at the metal, increasing as the positive charge increases (qZrMe2,CM5; 1Me2 > 3Me2 > 2Me2). In contrast, in the less coordinating ODFB, differences in the IPS barriers for [1-3Me·ODFB]+B(C6F5)4- appear to correlate better with steric parameters, as measured by the percent buried volume on the open quadrants (%VBur,open) for the corresponding neutral precursors (%VBur,open = 34.8, 35.5, 34.6% for 1Me2, 2Me2, and 3Me2, respectively)
Synthesis and olefin polymerization performance of new ansa-zirconocene with OSiO-bridged bis(2-indenyl) ligand
New ansa-zirconocene with bis(inden-2-yloxy)dimethylsilane ligand was synthesized and characterized by NMR spectroscopy and X-ray crystallography. The zirconocene was found to be highly active catalyst of ethylene polymerization and ethylene/hex-1-ene copolymerization upon methylaluminoxane activation
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