总结NIF实验失败基础上，我们提出了间接-直接驱动组合的混合驱动（HD）点火模型，吸取了ID和DD优点，同时克服了它们的缺点。HD分两个阶段，第一阶段用较低辐射温度ID内爆预压缩靶球，同时，辐射烧蚀靶球烧蚀层表面膨胀产生了 一个大尺度冕区等离子体。第二阶段发生在ID后期，具有平顶脉冲的DD激光在ID冕区等离子体中临界面上被吸收，形成一个超声电子热波从临界面向较远的辐射烧蚀阵面传播。当慢化到声波时形成高压压缩波，后者在稳定的DD激光支持下像一个“雪耙”，不斷地把在压缩波阵面前低的冕区等离子体密度堆积为高密度。另一方面，靶球烧蚀层不斷被烧蚀，这样，在“雪耙”和辐射烧蚀阵面之间形成一个高密度区，从而产生了压力比传统ID烧蚀压高多倍的驱动内爆的HD压力，实现了 “雪耙''模型增压过程。
We assess the sensitivity of the LHC, its high energy upgrade, and a prospective 100 TeV hadronic collider to the Dirac Yukawa coupling of the heavy neutrinos in left-right symmetric models (LRSMs). We focus specifically on the trilepton final state in regions of parameter space yielding prompt decays of the right-handed gauge bosons (W R ) and neutrinos (N R ). In the minimal LRSM, the Dirac Yukawa couplings are completely fixed in terms of the mass matrices for the heavy and light neutrinos. In this case, the trilepton signal provides a direct probe of the Dirac mass term for a fixed W R and N R mass. We find that while it is possible to discover the W R at the LHC, probing the Dirac Yukawa couplings will require a 100 TeV pp collider. We also show that the observation of the trilepton signal at the LHC would indicate the presence of a non-minimal LRSM scenario.
Cherenkov radiation (CR) is the electromagnetic wave generated by moving charges passing through a dielectric medium with velocity larger than a certain threshold. To generate CR in natural medium, the electron energy should be as large as hundreds to thousands of keV. We demonstrate in hyperbolic metamaterial that there is no velocity threshold for electron to generate CR. Based on this threshold-less CR, the first on-chip integrated free electron light source was realized. This work opens up the possibility of exploring high performance on-chip integrated free-electron optoelectronic devices.
The combination of Scanning Tunneling Microscopy (STM) and optical excitation merges two successful experimental techniques in solid-state physics. The combination of optical pump-probe techniques with Scanning Tunneling Microscopy (STM) enables us to get atomic resolution of an STM with time resolution on the ns time scale, i.e. well beyond the bandwidth of the current amplifier. This approach provides the prospect to resolve surface dynamics on the atomic scale. More specifically, optical excitation and Scanning Tunneling Microscopy (STM) is discussed to study the carrier dynamics at the GaAs(110) surface. By illuminating the tunnel contact between a tip and an n-doped GaAs crystal, we generate electron-hole pairs, which will be separated in the tip-induced space charge region (SCR). A detailed spectroscopic analysis shows that photo-excited charge carriers, trapped in a local region beneath the STM tip, contribute to the tunneling current. By adjusting the current in a controlled manner we are able to actively access different screening conditions of the electric potential at the surface. Studying the time evolution of the photo-induced tunnel current gives access to the charge dynamics. We discuss different processes determining the relaxation characteristic of the excited system. By using the lateral resolution of the STM, the influence of single dopants on the relaxation dynamics of the system is investigated. We discuss the impact of these defects in terms of their depth dependent binding energy of the donors.
报告人：Kai Yi，University of Iowa
The discoveries of numerous new bound quark states since 2003 have revitalized interest in the spectroscopy of exotic bound quark and antiquark systems. These structures do not fit easily into the standard quark model. Proposals like four quark (plus one antiquark) states and other hybrid states have been suggested as explanations. Since 2009, several new bound states have been reported by experiments at colliders. The developing story on our understanding of these structures from collider experiments like CDF, LHCb, CMS, D0, BaBar and BES will be discussed, including the motivation for these new exotic hadron states and potential new physics.
地点：Room W663, Physics Building, Peking University
The transport properties of epitaxial graphene have been subject of intense theoretical and experimental investigations since its invention. Besides electron-electron and electron-phonon scattering, the charge transport is determined by structural defects such as impurities, substrate steps or monolayer/bilayer junctions. The latter are leading to a spatially varying potential landscape as well as an inhomogeneous current density. Scanning Tunneling Microscopy combined with a potentiometric extension, called Scanning Tunneling Potentiometry (STP), has opened a way to study these transport properties down to the nanometer scale. Using an STP setup based on a home-built low-temperature STM operating down to 6 K and applicable magnetic field of up to 6T, we have investigated the sheet resistance of graphene focusing on charge transport across different localized defects on a sub-nanometer scale. We find that the voltage drop at a monolayer-bilayer boundary in graphene clearly extends spatially up to a few nanometers into the bilayer and hence is not located strictly at the structural defect. We explain this behavior by the weak coupling between the two bilayer sheets. From magneto-transport STP measurements mapping the local electrochemical potential as a function of the applied magnetic field, we have extracted the local charge carrier concentration by the emerging Hall field. Additionally, we show that the defect resistance at local defects such as steps, wrinkles and ML/BL-junctions remains constant for all magnetic fields applied here. To determine local resistances quantitatively, the local driving field as well as the local current density are needed. While STP is measuring the local chemical potential with high precision, the local current density is a priory unknown. In all STP studies up to now, the local current density is replaced by an averaged value, e.g. given by the total current and the geometry of the sample. Graphene grown on 6H-silicon carbide (0001) prepared by polymer assisted sublimation growth (PASG) are characterized by a high degree of a spatial homogeneity. This allows analyzing transport properties quantitatively on the nanometer scale. We demonstrate this new possibility by determining the sheet resistance as a function of the stacking sequence of 6H-SiC. At 8 Kelvin, highly resolved STP measurements show a significant variation of up to 240% demonstrating the strong influenced of the underlying substrate on a local scale.