Spin excitations in quantum hall systems and their effect on nuclear spin relaxation and photoluminescence

Two problems with respect to spin excitations in quantum Hall systems are studied by means of exact numerical diagonalization. The first one is related to the formation of reversed-spin quasielectrons (QER) in a two-dimensional electron gas (2DEG). The single—particle properties of QER’S as well as the pseudopotentials of their interaction with one another and with Laughlin quasielectrons (QE’s) and quasiholes (QH’s) are calculated. Based on the short-range character of the QER—QER and QER—QE repulsion, the partially unpolarized incompressible states at the filling factors 1/ = 14—1 and 1/ = T55 are postulated within Haldane’s hierarchy scheme. To describe photoluminescence, the family of bound h(QER)n states of a valence hole h and n QER’S are predicted in analogy to the found earlier fractionally charged excitons hQEn. The binding energy and optical selection rules for both families are compared. The hQER is found radiative in contrast to the dark hQE, and the h(QER)2 is found nonradiative in contrast to the bright hQEg. The second problem involves the numerical study of the relaxation rates of nuclear spins coupled through the hyperfine interaction to a 2DEG at magnetic fields corresponding to both fractional and integral Landau level fillings 1/. The spectral functions T_1(E) describing the response of the 2DEG to the reversal of an embedded localized spin are calculated. In a (locally) incompressible 1/ = 1 or 31,; state, the finite Coulomb energy of short spin waves, together with the small nuclear Zeeman energy, prevent nuclear spin relaxation even in the limit of vanishing electron Zeeman energy (EZ). However, we find that the nuclear spins can couple to the internal excitations of mobile finite-size skyrmions that appear in the 2DEG at sufficiently low EZ and at V slightly different from 1 or %. The experimentally observed dependence of nuclear spin relaxation rate on E2 and 1/ is explained in terms of the occurrence of skyrmions and anti-skyrmions of various topological charge

Effects of Interfacial Bonding on Friction and Wear at Silica/Silica Interfaces

Static friction between amorphous silica surfaces with a varying number of interfacial siloxane (Si–O–Si) bridges was studied using molecular dynamic simulations. Static friction was found to increase linearly with the applied normal pressure, which can be explained in the framework of Prandlt–Tomlinson’s model. Friction force was found to increase with concentration of siloxane bridges, but with a decreasing gradient, with the latter being due to interactions between neighboring siloxane bridges. In addition, we identified atomic-level wear mechanisms of silica. These mechanisms include both transfer of individual atoms accompanied by breaking interfacial siloxane bridges and transfer of atomic cluster initialized by rupturing of surface Si–O bonds. Our simulations showed that small clusters are continually formed and dissolved at the sliding interface, which plays an important role in wear at silica/silica interface.National Science Foundation (U.S.) (Grant EAR-0910779)United States. Army Research Office (Grant W911NF-12-1-0548

Size Dependence of Nanoscale Wear of Silicon Carbide

Nanoscale, single-asperity wear of single-crystal silicon carbide (sc-SiC) and nanocrystalline silicon carbide (nc-SiC) is investigated using single-crystal diamond nanoindenter tips and nanocrystalline diamond atomic force microscopy (AFM) tips under dry conditions, and the wear behavior is compared to that of single-crystal silicon with both thin and thick native oxide layers. We discovered a transition in the relative wear resistance of the SiC samples compared to that of Si as a function of contact size. With larger nanoindenter tips (tip radius around 370 nm), the wear resistances of both sc-SiC and nc-SiC are higher than that of Si. This result is expected from the Archard's equation because SiC is harder than Si. However, with the smaller AFM tips (tip radius around 20 nm), the wear resistances of sc-SiC and nc-SiC are lower than that of Si, despite the fact that the contact pressures are comparable to those applied with the nanoindenter tips, and the plastic zones are well-developed in both sets of wear experiments. We attribute the decrease in the relative wear resistance of SiC compared to that of Si to a transition from a wear regime dominated by the materials' resistance to plastic deformation (i.e., hardness) to a regime dominated by the materials' resistance to interfacial shear. This conclusion is supported by our AFM studies of wearless friction, which reveal that the interfacial shear strength of SiC is higher than that of Si. The contributions of surface roughness and surface chemistry to differences in interfacial shear strength are also discussed