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With the rapid development of laser plasma accelerator, multiple functional beamlines have been designed to transport such pulsed particle beam with large energy spread and divergence angle. Accordingly, the demand for optimal design and online feedback of laser beamlines has also been put forward. We have developed a graphics processing unit–accelerated multiparticle beam transport simulation algorithm based on the open source project “HPSim” to meet the above need. The algorithm implements a series of functions such as the simulation of solenoids, the simulation of exponentially decreasing energy spectrum beams, and cross-programming and cross-language calls. In terms of computational efficiency, the algorithm is approximately 20 times more efficient than multiparticle simulation program tracewin, meeting the simulation time-consumption requirement of a hundred milliseconds for virtual accelerator software. The algorithm’s monoenergetic and exponentially decreasing energy spectrum beam simulation capabilities are verified in the beamline design for CLAPA-I and CLAPA-II at Peking University. The algorithm can be applied not only to virtual accelerator for real-time beam transport simulations but also to fast and efficient beamline magnets optimization design.

The recently discovered intrinsic magnetic topological insulator MnBi_{2}Te_{4} has been met with unusual success in hosting emergent phenomena such as the quantum anomalous Hall effect and the axion insulator states. However, the surface-bulk correspondence of the Mn-Bi-Te family, composed by the superlatticelike MnBi_{2}Te_{4}/(Bi_{2}Te_{3})_{n} (n=0,1,2,3…) layered structure, remains intriguing but elusive. Here, by using scanning tunneling microscopy and angle-resolved photoemission spectroscopy techniques, we unambiguously assign the two distinct surface states of MnBi_{4}Te_{7} (n=1) to the quintuple-layer (QL) Bi_{2}Te_{3} termination and the septuple-layer (SL) MnBi_{2}Te_{4} termination, respectively. A comparison of the experimental observations with theoretical calculations reveals diverging topological behaviors, especially the hybridization effect between the QL and SL, on the two terminations. We identify a gap on the QL termination, originating from the hybridization between the topological surface states of the QL and the bands of the SL beneath, and a gapless Dirac-cone band structure on the SL termination with time-reversal symmetry. The quasiparticle interference patterns further confirm the topological nature of the surface states for both terminations, continuing far above the Fermi energy. The QL termination carries a spin-helical Dirac state with hexagonal warping, while at the SL termination, a strongly canted helical state from the surface lies between a pair of Rashba-like splitting bands from its neighboring layer. Our work elucidates an unprecedented hybridization effect between the building blocks of the topological surface states and also reveals the termination-dependent time-reversal symmetry breaking in a magnetic topological insulator.

A combination of electron cooling and rf system is an effective method to compress the beam bunch length in storage rings. A simulation code based on multiparticle tracking was developed to calculate the bunched ion beam cooling process, in which the electron cooling, intrabeam scattering (IBS), ion beam space-charge field, transverse and synchrotron motion are considered. Meanwhile, bunched ion beam cooling experiments have been carried out in the main cooling storage ring (CSRm) of the Heavy Ion Research Facility in Lanzhou, to investigate the minimum bunch length obtained by the cooling method, and study the dependence of the minimum bunch length on beam and machine parameters. The experiments show comparable results to those from simulation. Based on these simulations and experiments, we established an analytical model to describe the limitation of the bunch length of the cooled ion beam. It is observed that the IBS effect is dominant for low intensity beams, and the space-charge effect is much more important for high intensity beams. Moreover, the particles will not be bunched for much higher intensity beam. The experimental results in CSRm show a good agreement with the analytical model in the IBS dominated regime. The simulation work offers us comparable results to those from the analytical model both in IBS dominated and space-charge dominated regimes.

The novel pseudogap state has been systematically studied on flat-top In, Al, and Pb islands/films grown on Si(111) surfaces with scanning tunneling microscopy. Our observations and analysis suggest that the interplay between electron-phonon interaction and quantum confinement, the dynamic Coulomb blockade, and the orthodox Coulomb blockade are, respectively, the dominant mechanism of the formation of such pseudogap states in the flat-top metal islands at different island size scales. Our results can help to settle the dispute on the origins of the pseudogap state in such systems. [ABSTRACT FROM AUTHOR]

We have mapped the potential energy landscape for a Ag atom around Ag clusters on Si(111), via positioning a probing atom in the vicinity of clusters and monitoring the subsequent thermal motions with scanning tunneling microscopy. Both the probing atom's modulated diffusion and its combination with the clusters are quantitatively measured in a wide temperature range, showing strong dependence on the cluster size as well as specific sites. The atom-cluster interactions, as large as over 200 meV, are determined and found to alternate between attraction and repulsion at different separations. The demonstrated ability of studying atomic scale dynamics of single adsorbates in complex environments can provide opportunities to understand various phenomena in nanostructure growth and nanocatalysis. [ABSTRACT FROM AUTHOR]

We show with systematic photoemission spectroscopy and scanning tunneling spectroscopy data that a spin helical surface state appears on the (110) surface of noncentrosymmetric, three-dimensional HgTe. The topological surface state in HgTe exhibits sharp, linear dispersion without κz variation, as well as clear, left-right imbalanced spin polarization and circular dichroism. Chemical gating by alkali metal deposition on the surface causes the unexpected opening and/or increase of a surface insulating gap without changing its topological property. Such an unusual behavior we uncover in three-dimensional HgTe sheds light on a convenient control of the Fermi surface and quantum transport in a topological insulator. [ABSTRACT FROM AUTHOR]

Combining scanning tunneling microscopy and first-principles calculations, we have shown that single Co atoms adsorbed on a Si(111)-(7 × 7) surface have eight different configurations that possess different spin magnetic moments. Despite the large adsorption energy, we have demonstrated that both the position and the spin state of single adsorbed Co atoms can be well controlled through the vertical atomic manipulation and the "tip-touch" operation that converts Co atoms among various adsorption configurations. Our approach to construct atomic-scale magnetic structures on a semiconductor surface can provide a new pathway to realize spin-based devices, including a scalable solid-state quantum computer. [ABSTRACT FROM AUTHOR]

Using scanning tunneling microscopy supplemented with first-principles calculations, we examine all the thermally activated atomistic processes of Agn (n = 1–26) constructed atom-by-atom on a Si(111)-(7×7) substrate, and we exploit such cluster dynamical information to further determine the energetic stability (or magicity) of the clusters. By generalizing the traditional concept of cluster magicity solely based on cluster association/dissociation to also include various complex collective cluster motions, we identify the existence of two classes of magic clusters. The most stable class, Ag10 and Ag25, is defined by geometrical shell closure; the less stable class of Agn (n = 3, 5, 13, 16, 19) is associated with lower kinetic barriers against internal restructuring of, or atom detachment from, their respective clusters of neighboring sizes. Our detailed analysis also reveals that the substrate effect, rather than the number of bonds within the clusters, dominates the cluster stabilities. The conceptual advances gained in the present study are broadly applicable to many related cluster systems in contact with external media, and they are expected to be instrumental in tuning the dynamical behaviors of clusters in surface catalysis, nanoplasmonics, and other technological areas. [ABSTRACT FROM AUTHOR]

Adsorption of one-third monolayer of Sn on an atomically clean Si(111) substrate produces a two-dimensional triangular adatom lattice with one unpaired electron per site. This dilute adatom reconstruction is an antiferromagnetic Mott insulator; however, the system can be modulation doped and metallized using heavily doped p-type Si(111) substrates. Here, we show that the hole-doped dilute adatom layer on a degenerately doped p-type Si(111) wafer is superconducting with a critical temperature of 4.7±0.3 K. While a phonon-mediated coupling scenario would be consistent with the observed Tc, Mott correlations in the Sn-derived dangling-bond surface state could suppress the s-wave pairing channel. The latter suggests that the superconductivity in this triangular adatom lattice may be unconventional. [ABSTRACT FROM AUTHOR]