Progress in Surface Science最新文献

Light-control of structural dynamics at surfaces promises switching of chemical and physical functionality at rates limited only by the velocity of directed atomic motion. Following optical stimulus by femtosecond light pulses (1 fs = 10^-15 s), transient electronic and lattice excitations can drive phase transitions in solids. Coherent control schemes facilitate a selective transfer of optical energy to specific electronic or vibrational degrees of freedom, as exemplified by the steering of molecular reactions via optical pulse sequences in femtochemistry. However, a transfer of this concept from molecules to solids requires coupling of few decisive phonons to optical transitions in the electronic band structure, and a weak coupling to other lattice modes to maximize coherence times. In this respect, atomic indium wires on the (111) surface of silicon represent a highly attractive model system, with a Peierls-like phase transition between insulating (8×2) and metallic (4×1) structures, governed by shear and rotation phonons. This review provides a survey on our advances in the time-resolved probing and coherent vibrational control of the In/Si(111) surface. In particular, we discuss how coherent atomic motion can be harnessed to affect the efficiency and threshold of the phase transition. Starting from a description of the (8×2) and (4×1) equilibrium structures and key vibrational modes, we study the structural dynamics following single-pulse optical excitation of the (8×2) phase. Our results highlight the ballistic order-parameter motion in the nonequilibrium transition as well as the impact of microscopic heterogeneity on the excitation and subsequent relaxation of the metastable photo-induced (4×1) phase. Furthermore, we discuss our results on the combination of ultrafast low-energy electron diffraction (ULEED) with optical pulse sequences to investigate the coherent control over the transition, mode-selective excitation and the location of the transition state.

The study of processes concerning adsorption, diffusion and reaction of atoms and molecules on surfaces is one of the core areas of surface science research. Resolving these dynamic processes with atomic resolution in real space and at real time is of great significance for the understanding of catalytic reaction mechanism and the development of new materials. A scanning tunneling microscope with fast imaging function, a so-called “high-speed scanning tunneling microscope” combining both high temporal and high spatial resolution, is an ideal instrument to characterize processes within this area. This review aims to highlight some recent developments of high-speed scanning tunneling microscope technique and its application to study the structural dynamics on surfaces. Firstly, factors that limit the time resolution of scanning tunneling microscope are analyzed from the aspects of both hardware and software. Secondly, strategies and instrument designs enabling imaging rate up to 100 frames per second are introduced. Then, recent breakthroughs on resolving surface structural dynamics, such as atom diffusion, on-surface synthesis of low-dimensional materials and chemical reaction, by high-speed scanning tunneling microscope are highlighted. Finally, the challenges and opportunities of high-speed scanning tunneling microscope technique are outlined and a perspective is provided.

Many fundamental processes in nature occur on ultrashort time scales within picoseconds to attoseconds, and on intrinsic length scales from nanometers to picometers. The structure of crystalline solids is dictated by long range order and the periodic arrangement of atoms, but the elementary excitations that define its interaction with the environment may vary locally at the atomic scale. Multiple domains and phases can coexist on length scales down to a few nanometer, and impurities and defects can influence the collective many-body response of solids at the single-atom level. Ultrafast pump–probe techniques provide valuable information about fundamental many-body interactions in solids and at surfaces, but spatially average over macroscopic spot sizes such that the influence of local order or disorder at angstrom scales is not directly accessible. Therefore, real-space observation of ultrafast dynamics with atomic spatial resolution is highly desirable, and motivates the development of time-resolved ultrafast scanning tunneling microscopy (USTM) since the early 1990’s. Tremendous progress has been made in this field in the past decade, and a number of breakthrough achievements have significantly advanced our possibilities to add ultrafast time resolution to the angstrom spatial resolution of STM. This article reviews new technical approaches and developments in the field of USTM. A particular focus will be the classification of light-matter interaction in tunnel junctions, based on the criteria for adiabatic tunneling from Keldysh's theory of strong-field ionization and a tunneling time as defined by Büttiker and Landauer, and on Tucker's definition of quantum detection in a tunnel junction mixer. Moreover, various mechanisms to generate an ultrafast tunneling current in USTM are discussed and are to some extent related to those from other techniques such as optical spectroscopy or photoemission spectroscopy. The resulting new possibilities for imaging the ultrafast dynamics of electronic and vibrational excitations at surfaces with USTM will be highlighted. Finally, the article outlines possible future directions of USTM for studying ultrafast processes and light-induced phenomena at surfaces and in quantum materials.

Plasmons are self-sustained collective excitations of electron liquid, which have received increasing attention since its proposal by David Pines at 1960s. For the great potential in applications, the researches on plasmons make great advances all the way from semiconductors, metals, semimetals, to monolayer graphene. With the fast development of the field of topological materials, the research of plasmons has been extended into topological insulators, generating many exciting discoveries related to the topologically protected surface states. Topological semimetals, exhibiting various fantastic properties different from topological insulators, have become another research focus in condensed matter. Recently the plasmons in topological semimetals, providing a new perspective to further understand and utilize the topological states, have been attracting more and more attention. In this article, we review the recent theoretical and experimental investigations on the plasmons of topological semimetals, including the Dirac, Weyl and nodal line semimetals. In theoretical aspects, main different behaviors between the plasmons of topological semimetals and traditional metals are reviewed, such as the quantum nature, unusual dependence on temperature and charge carrier density, and the properties related to the chiral anomaly and Fermi arcs. The experimental studies are less reported, and the review is mainly focused on the measurements of optical conductivity and electron energy loss spectra in several typical real materials. Finally, the prospects of the future of the plasmons in topological semimetals in theories and experiments are outlooked.

With the development of laser and magneto-optical technology and the discovery of a broad range of magnetic quantum materials exhibiting exotic properties and new physics, ultrafast magnetization dynamics has become increasingly appealing to advanced magnetic information technology. Furthermore, manipulating magnetization via light provides insights into interactions among multiple degrees of freedom in condensed matters and has revealed a wide range of nonequilibrium phenomena. In this minireview, we first present the theoretical considerations of ultrafast magnetization dynamics from both classical and ab initio points of view. We then discuss several aspects of state-of-the-art experimental studies on light-induced magnetization dynamics in various materials, including ultrafast demagnetization and magnetization reversal, as well as coherent-phonon-driven magnetization precession and phase transitions. In particular, we highlight the role of light-induced phonons from some recent work in the latter two aspects, providing a completely new perspective as well as an alternative approach for optical control of magnetization dynamics. As a powerful means of dynamical control and thanks to the progress and advances of experimental techniques, all-optical quantum manipulation of emergent materials is becoming one of the most far-reaching frontier research areas of ultrafast sciences.

Recent high resolution multiphoton photoemission studies of low index Ag surfaces have revealed spectral features whose energetics was controlled by multiple quanta of plasmon energy rather than the photon energies appearing in the standard Einstein’s one-electron energy scaling in photoeffect. To elucidate these peculiar features we introduce and elaborate the mechanism of bulk- and surface plasmon-induced electron emission from metal surfaces, conveniently termed plasmoemission. Our point of departure is the cloud of hot plasmons generated in the primary interactions of external electromagnetic (EM) field(s) with the system. Such hot plasmon distributions acquire the form of a coherent state plasmonic bath which may serve as a source of energy and momentum required for electron emission from the system. These plasmoemission channels are complementary to the standard photoemission channels driven directly by the primary EM fields. Adopting this paradigm we analyze the plasmonically induced electron yield by using perturbative and nonperturbative approaches in the length and velocity gauge representations of the electron–plasmon interaction. Pursuing the perturbative approach to one- and two bulk plasmon-induced electron emission from Ag(110) surface we have investigated the effects of underlying band structure on the electron yield and proposed as how to discern them in the measured spectra. This also enables putting the perturbative descriptions of plasmoemission into the general context of pump–probe spectroscopy. The more demanding nonperturbative approach has been implemented by invoking the Volkov ansatz type of electron wavefunction in the velocity gauge and applied to surface plasmon-induced electron emission from quasi-two-dimensional surface bands on Ag(111). In this formulation the electrons emanate from the surface Floquet bands generated from the parent surface state band by the action of prepumped plasmonic coherent state field. A quantitative assessment of the multiplasmoemission yield is presented in terms of the plasmonic coherent state parameters controlled by the external pumping fields. The opposite limit of plasmonically induced electron tunneling regime is recovered in the quasistatic strong field limit. The pump–probe concept can be established also in the nonperturbative picture albeit in a more complex form.

Light at optical frequencies interacting with a metal surface can excite interband quantum transitions, or intraband currents at frequencies approaching the PHz range. Momentum conservation enables the interband excitation to occur in first order as a dipole transition, while intraband excitations involve second-order momentum scattering processes. The free electron response to optical fields can also be collective, causing the optical field to be screened by the multipole plasmon response. We describe the exitation of single crystal silver surfaces in the region where the dielectric response transits from negative to positive passing through the epsilon near zero (ENZ) condition. There, electrons can no longer screen the optical field, so that it penetrates as a collective charge density wave of the free electron plasma, in other words, as a bulk transverse or longitudinal plasmon field. We examine two-photon photoemission (2PP) signals from Ag(1 1 1), (1 0 0) and (1 1 0) surfaces through the ENZ region under conditions where intraband, and interband single particle, and bulk plasmon collective responses dominate. We are specifically interested in the bulk plasmon decay into plasmonic photoemission. Plasmonic decay into excitation of electrons from the Fermi level, which we observe as a nonlinear 2PP process, has been established for the free electron and noble metals, but its significance to transduction of optical-to-electronic energy has not penetrated the plasmonics community. 2PP spectra show evidence for intraband hot electron generation, interband surface and bulk band excitation, and nonlinear bulk plasmon driven plasmonic single particle excitation. Because the intraband and plasmonic decay into hot electron distributions have been extensively considered in the literature, without reference to explicit experimental measurements, we discuss such processes in light of the directional anisotropy of the electronic structure of single crystalline silver. We note that projected band gaps in silver exclude large regions of the unoccupied state density from hot electron generation, such that it predominantly occurs in the (1 1 0) direction. Moreover, the excited hot electron distributions do not follow expectations from the joint density of the occupied and unoccupied states of a free electron metal, as assumed in majority of research on hot electron processes. We describe the strongly anisotropic hot electron distributions recorded by 2PP spectroscopy of Ag surfaces, and the plasmonic photoemission process that occurs on all surfaces irrespective of the momentum-dependent single particle band structure of silver. Plasmonic photoemission, or its linear analog that excites hot electrons at energies below th