Spin relaxation mechanism of hopping transport in a 2D asymmetric quantum dot array

Spin relaxation is studied in the hopping conduction mode in 2D arrays of quantum dots (QDs) with structural asymmetry. It is shown that the absence of the “up-down” symmetry in a QD leads to the emergence of a new spin relaxation mechanism in tunneling in a 2D QD array. The difference in spin relaxation mechanisms for symmetric and asymmetric QDs is demonstrated on the basis of theoretical analysis of an elementary event (jump between two tunnel-coupled dots). It is shown that spin flip during tunneling between QDs is the main spin relaxation mechanism in the transport in dense arrays of QDs in Ge placed in weak (1–10 T) magnetic fields.     

Application of XAFS spectroscopy to studying the microstructure and electronic structure of quantum dots

Variations in the microscopic structural parameters of semiconductor nanoclusters (interatomic distances, coordination numbers, and types of neighboring atoms) under variation of the preparation conditions of Ge/Si, GaN/AlN, and InAs/AlAs heterostructures are determined by the EXAFS-and XANES spectroscopy methods. The effect of preparation conditions on interphase diffusion and structure parameters in semiconductor nanoclusters is revealed. The effect of the temperature of synthesis at different stages, the substrate orientation and pretreatment, the composition of the molecular beam (in particular, the presence of Ge⁺ cations) on the local composition of nanostructures is studied. Relations between the size and shape of nanoparticles and their local spatial characteristics (interatomic distances, stoichiometric composition, and phase boundary characteristics) are examined.     

Microscopic Parameters of Heterostructures with Ge Nanoclusters and Thin Films in a Si Matrix

Microstructural parameters (interatomic distances, coordination numbers, and their anisotropy) of heterostructures with Ge nanoclusters and thin films were determined by EXAFS spectroscopy. The variation of these parameters is correlated to the morphology of the germanium quantum points on Si(001), and adequate structural models are suggested. It is found that the pseudomorphic four-monolayer germanium films buried in a Si matrix contain up to 50% Si atoms. The pyramidal germanium islands (lateral size 15 nm, height 1.5 nm) formed during further Stranski–Krastanov heteroepitaxial growth have a thinner Ge–Si intermediate boundary layer near two monolayers and Ge–Ge interatomic distances 0.04 shorter than those in pure germanium in agreement with the results of bond length calculations in the framework of the valence force field (VFF) model.     

Photoluminescence enhancement in double Ge/Si quantum dot structures

The luminescence properties of double Ge/Si quantum dot structures are studied at liquid helium temperature depending on the Si spacer thickness d in QD molecules. A seven-fold increase in the integrated photoluminescence intensity is obtained for the structures with optimal thickness d = 2 nm. This enhancement is explained by increasing the overlap integral of electron and hole wavefunctions. Two main factors promote this increasing. The first one is that the electrons are localized at the QD base edges and their wavefunctions are the linear combinations of the states of in-plane Δ valleys, which are perpendicular in k-space to the growth direction [001]. This results in the increasing probability of electron penetration into Ge barriers. The second factor is the arrangement of Ge nanoclusters in closely spaced QD groups. The strong tunnel coupling of QDs within these groups increases the probability of hole finding at the QD base edge, that also promotes the increase in the radiative recombination probability.     

Composite Films Based on Carbon Quantum Dots in a Matrix of PEDOT:PSS Conductive Polymer

Physics of the Solid State - Carbon quantum dots (CQDs), as well as their composites with the PEDOT:PSS conductive polymer, are hydrothermally synthesized from glucose, and their electrical...     

A note on Fröberg's conjecture for forms of equal degrees

In this note we study ideals generated by generic forms in polynomial rings over any algebraicly closed field of characteristic zero. We prove for many cases that the $(d+k)$-th graded component of an ideal generated by generic forms of degree d has the expected dimension (given by dimension count). And as a consequence of our result, we obtain that ideals generated by several generic forms of degrees d usually have the expected Hilbert series. The precise form of this expected Hilbert series, in general, is known as Fröberg's conjecture.     

Electron spins in an array of tunnel-coupled quantum dots

Mechanisms of electron spin relaxation in semiconductor arrays with tunnel-coupled quantum dots are reviewed. The contribution for anisotropic exchange interaction is shown for asymmetrical quantum dots having no inversion axis relative to their plane. The configuration of vertically coupled double Ge/Si quantum dots is found where anisotropic exchange coupling does not contribute to spin decoherence. It could be a basic configuration of spin-based quantum computation schemes.     

Projective push-forwards in the Witt theory of algebraic varieties

We define push-forwards along projective morphisms in the Witt theory of smooth quasi-projective varieties over a field. We prove that they have standard properties such as functoriality, compatibility with pull-backs and projection formulas.