Friction and energy dissipation mechanisms in adsorbed molecules and molecularly thin films

This review provides an overview of recent advances that have been achieved in understanding the basic physics of friction and energy dissipation in molecularly thin adsorbed films and the associated impact on friction at microscopic and macroscopic length scales. Topics covered include a historical overview of the fundamental understanding of macroscopic friction, theoretical treatments of phononic and electronic energy dissipation mechanisms in thin films, and current experimental methods capable of probing such phenomena. Measurements performed on adsorbates sliding in unconfined geometries with the quartz crystal microbalance technique receive particular attention. The final sections review the experimental literature of how measurements of sliding friction in thin films reveal energy dissipation mechanisms and how the results can be linked to film-spreading behavior, lubrication, film phase transitions, superconductivity-dependent friction, and microelectromechanical systems applications. Materials systems reported on include adsorbed films comprised of helium, neon, argon, krypton, xenon, water, oxygen, nitrogen, carbon monoxide, ethane, ethanol, trifluoroethanol, methanol, cyclohexane, ethylene, pentanol, toluene, tricresylphosphate, t-butylphenyl phosphate, benzene, and iodobenzene. Substrates reported on include silver, gold, aluminum, copper, nickel, lead, silicon, graphite, graphene, fullerenes, C60, diamond, carbon, diamond-like carbon, and YBa₂Cu₃O₇, and self-assembled monolayers consisting of tethered polymeric molecules.     

Theory of multi-exciton states in semiconductor nanocrystals

In this article, the fundamental physics of multi-exciton states in semiconductor nano-crystals is reviewed focusing on the mesoscopic enhancement of the excitonic radiative decay rate and the excitonic optical nonlinearity and the mechanism of their saturation with increase of the nanocrystal size. In the case of the radiative decay rate the thermal excitation of excited exciton states having small oscillator strength within the homogeneous linewidth of the exciton ground state is essential in determining the saturation behavior. The weakly correlated exciton pair states are found to cause a cancellation effect in the third-order nonlinear optical susceptibility at the exciton resonance, providing the first consistent understanding of the experimentally observed saturation of the mesoscopic enhancement of the excitonic optical nonlinearity. The presence of the weakly correlated exciton pair states is confirmed convincingly from the good correspondence between theory and experiments on the induced absorption spectra from the exciton state in CuCl nanocrystals. Furthermore, ultrafast relaxation processes of biexcitons are discussed in conjunction with the observed very fast rise of the biexciton gain in nanocrystals. In prospect of future progress in research, the theoretical formulation to calculate the triexciton states as one of the multi-exciton states beyond the biexciton is presented for the first time including the electron-hole exchange interaction.     

Charge transport through ferromagnet/two-dimensional electron gas/d-wave superconductor junctions with Rashba spin-orbit coupling

Along the lines of Blonder, Tinkham and Klapwijk, we investigate the charge transport through ferromagnet/two-dimensional electronic gas/d-wave superconductor (F/2DEG/S) junctions in the presence of Rashba spin-orbit (SO) coupling and focus our attention on the interplay between spin polarization and spin precession. At zero spin polarization, the spin-mixing scattering resulted from Rashba SO coupling decreases the zero-bias conductance peak. Under spin polarization, spin precession introduces novel Andreev reflection, which competes with the effect of spin-mixing scattering. If the F layer is a half metal, the later effect is overwhelmed by that of novel Andreev reflection. As a result, the zero-bias conductance dip caused by spin polarization is enhanced, and at strong Rashba SO coupling, a split zero-bias peak is found in the gap. In an intermediate region where the two effects are comparable with each other, the zero-bias conductance shows a reentrant behavior as a function of Rashba SO coupling.     

Coherent quantum transport in two-dimensional electron gas/superconductor double junctions with Rashba spin-orbit coupling

Based on the extended Blonder-Tinkham-Klapwijk (BTK) approach, we have investigated the coherent quantum transport in two-dimensional electron gas/superconductor (2DEG/SC) double tunneling junctions in the presence of the Rashba spin-orbit coupling (RSOC). It is found that all the reflection coefficients in BTK theory as well as conductance spectra oscillate with the external voltage and energy. The oscillation feature of conductance can be tuned largely by the RSOC for low insulating barriers, while for high insulating barriers it is almost independent of the RSOC. These phenomena are essentially different from those found in ferromagnet/superconductor double tunneling junctions.     

RF-sputtered CrB2 diffusion barrier for Ni/Au Ohmic contacts on p-CuCrO2

Ohmic contacts to p-type CuCrO₂ using Ni/Au/CrB₂/Ti/Au contact metallurgy are reported. The samples were annealed in the 200–700 °C range for 60 s in flowing oxygen ambient. A minimum specific contact resistance of 2 × 10⁻⁵ Ω cm² was obtained after annealing at 400 °C. Further increase in the annealing temperature (>400 °C) resulted in the degradation of contact resistance. Auger Electron Spectroscopy (AES) depth profiling showed that out-diffusion of Ti to the surface of the contact stacks was evident by 400 °C, followed by Cr at higher temperature. The CrB₂ diffusion barrier decreases the specific contact resistance by almost two orders of magnitude relative to Ni/Au alone.     

Ir-based diffusion barriers for Ohmic contacts to p-GaN

Ir-based electrical contacts to p-type GaN have been fabricated and characterized. Both GaN//Ni/Au/Ir/Au and GaN//Ni/Ir/Au contact structures were deposited, however, only the former produced Ohmic current-voltage characteristics. At an anneal temperature of 500 °C, the Ni/Au/Ir/Au contact had a specific contact resistance of ∼2 × 10⁻⁴ Ω cm², comparable or superior to conventional Ni/Au contacts that are less thermally stable. Anneal temperatures above 500 °C caused the Ir-based contact to fail. Auger electron spectroscopy was used to obtain depth profiles of both types of contacts at a variety of temperatures in order to provide insight into the mechanism of Ohmic formation as well as potential reasons for failure. A comparison to other metallization schemes on p-GaN is also given.     

在二部无标度网上的两性疾病传播

利用易感-感染-易感(SIS)传播模型研究人类性接触网上的病毒传播.当仅仅考虑异性性接触时,该网络是一个二部的无标度网.对这个网络上的SIS传播模型,通过率方程的方法分析了男性感染率和女性感染率与传染阈值之间的关系,发现女性感染者与男性感染者之比由网络的拓扑和男女感染率之比所确定.这一结果表明性接触网的拓扑对性传染病传播的重要性.最后给出了支持理论结果的数值模拟. 关键词： 性传染病 两性性接触网 无标度网络 二部图     

Investigation of unstable motion characteristics of vacuum arc cathode spots between transverse magnetic field contacts

With the improvement of the current level of power grids, the requirements of the opening level of the vacuum switches are also increasing. Vacuum arc cathode spots provide steam and electrons and, to a certain extent, determine the opening capacity of the vacuum switch. In this paper, a vacuum arc cathode spot research platform based on the de-mountable vacuum chamber is constructed. The characteristics of the vacuum arc cathode spots under the transverse magnetic field (TMF) contacts are assessed by a high-speed charge coupled device. The experimental results show that the cathode spot diffusion process can be divided into three processes through cathode spot distribution, arc voltage and current: initial diffusion stage of cathode spots, unstable motion stage of cathode spots, and extinguishing stage. The motion mode of cathode spots during unstable motion stage can be divided into cathode spots group stagnation (CSGS) to multi-cathode jet (MCJ) switch mode, cathode spots group motion (CSGM) to MCJ switch mode, CSGM mode, and MCJ mode. The effects of peak current and contact diameter on unstable motion mode were analysed.     

An Exceptionally Close,Non‐Bonded Hydrogen–Hydrogen Contact with Strong Through‐Space Spin–Spin Coupling

Condensation of 1,8,13‐tris(mercaptomethyl)triptycene and tris(bromomethyl)methane yields an in,in‐cyclophane with two inwardly directed methine groups. Based on X‐ray analysis and DFT and MP2 calculations, the hydrogen–hydrogen non‐bonded contact distance is estimated to be 1.50–1.53 Å. Furthermore, the two in‐hydrogen atoms show obvious spin–spin coupling with J=2.0 Hz.     

Quantum corrections to the conductance of a square quantum dot with soft confinement

We study the conductance of a square quantum dot, modeling the potential with a self-consistent Thomas-Fermi approximation. The resulting potential is characterized by level statistics indicative of mixed chaotic and regular electron dynamics within the dot in spite of the regular geometry of the gates defining the dot. We calculate numerically, for the case of a quantum dot with soft confinement, the weak localization (WL) correction. We demonstrate that this confining potential may generate either Lorentzian or linear lineshapes depending on the number of modes in the leads. Finally, we present experimental WL data for a lithographically square dot and compare the results with numerical calculations. We analyze the experimental results and numerical simulations in terms of semiclassical and random matrix theory (RMT) predictions and discuss their limitations as far as real experimental structures are concerned. Our results indicate that direct application of the above predictions to distinguish between chaotic and regular dynamics in a particular cavity can not always lead to reliable conclusions as the shape and magnitude of the WL correction can be strongly sensitive to the geometry-specific, non-universal features of the system. Received 13 May 1998