Progress in Surface Science最新文献

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.

Electronic states in quantum materials can be engineered by light irradiation, which is greatly advanced by ab-initio computational predictions in realistic light-matter coupled systems. Here we review the most recent progresses from first principles computation in the light-driven Floquet steady states and transient dynamical states with topological electronic bands in real crystals. We first introduce the first-principles modeling approach, dubbed time-dependent Wannier scheme, for simulating real quantum materials under light irradiation. Then, we present a few examples of theoretically-predicted Floquet-Bloch electronic bands engineered by time-periodic light fields, which include the three types of Floquet-Dirac fermions in graphene and black phosphorus, the Floquet-Chern flat bands with an unprecedented high flatness ratio of band width over band gap in a Kagome material, and the Floquet conversion between bright and dark valley excitons in monolayer transition-metal dichalcogenides. Next, we show the ultrafast dynamical evolution of Weyl nodal points in orthorhombic WTe₂ driven by a time-aperiodic short light pulse, and discuss the connection between the Floquet and transient states engineered by light. After that, we introduce three prominent experiments, inspired by theoretical predictions, on the light-induced topological Floquet electronic bands in quantum crystalline materials. Finally, we make a brief summary and perspective on the engineering of topological electronic states through light-matter interactions.

Borophene, a two-dimensional (2D) planar boron sheet, has attracted dramatic attention for its unique physical properties that are theoretically predicted to be different from those of bulk boron, such as polymorphism, superconductivity, Dirac fermions, mechanical flexibility and anisotropic metallicity. Nevertheless, it has long been difficult to obtain borophene experimentally due to its susceptibility to oxidation and the strong covalent bonds in bulk forms. With the development of growth technology in ultra-high vacuum (UHV), borophene has been successfully synthesized by molecular beam epitaxy (MBE) supported by substrates in recent years. Due to the intrinsic polymorphism of borophene, the choice of substrates in the synthesis of borophene is pivotal to the atomic structure of borophene. The different interactions and commensuration of borophene on various substrates can induce various allotropes of borophene with distinct atomic structures, which suggests a potential approach to explore and manipulate the structure of borophene and benefits the realization of novel physical and chemical properties in borophene due to the structure–property correspondence. In this review, we summarize the recent research progress in the synthesis of monolayer (ML) borophene on various substrates, including Ag(1 1 1), Ag(1 1 0), Ag(1 0 0), Cu(1 1 1), Cu(1 0 0), Au(1 1 1), Al(1 1 1) and Ir(1 1 1), in which the polymorphism of borophene is present. Moreover, we introduce the realization of bilayer (BL) borophene on Ag(1 1 1), Cu(1 1 1) and Ru(0 0 0 1) surfaces, which possess richer electronic properties, including better thermal stability and oxidation resistance. Then, the stabilization mechanism of polymorphic borophene on their substrates is discussed. In addition, experimental investigations on the unique physical properties of borophene are also introduced, including metallicity, topology, superconductivity, optical and mechanical properties. Finally, we present an outlook on the challenges and prospects for the synthesis and potential applications of borophene.

Quantum coherent physics and chemistry concern the creation and manipulation of an excited-state manifold that contains the superposition and entanglement of multiple quantum levels. Electromagnetic waves such as light and microwave can be used to generate and probe different quantum coherent phenomena. The recent advances in scanning tunneling microscopy (STM) techniques including ultrafast laser coupled STM and electron spin resonance STM combine electromagnetic excitation with tunneling electron detection, bringing the investigation of quantum coherence down to the atomic and molecular level. Here, we survey the latest STM studies of different quantum coherent phenomena covering molecular vibration, electron transfer, surface plasmon resonance, phonon, spin oscillation, and electronic transition, and discuss the state and promise of characterizing and manipulating quantum coherence at the atomic or molecular scale.

An excitonic insulator phase is expected to arise from the spontaneous formation of electron–hole pairs (excitons) in semiconductors where the exciton binding energy exceeds the size of the electronic band gap. At low temperature, these ground state excitons stabilize a new phase by condensing at lower energy than the electrons at the valence band top, thereby widening the electronic band gap. The envisioned opportunity to explore many-boson phenomena in an excitonic insulator system is triggering a very active debate on how ground state excitons can be experimentally evidenced. Here, we employ a nonequilibrium approach to spectrally disentangle the photoinduced dynamics of an exciton condensate from the entwined signature of the valence band electrons. By means of time- and angle-resolved photoemission spectroscopy of the occupied and unoccupied electronic states, we follow the complementary dynamics of conduction and valence band electrons in the photoexcited low-temperature phase of Ta₂NiSe₅, the hitherto most promising single-crystal candidate to undergo a semiconductor-to-excitonic-insulator phase transition. The photoexcited conduction electrons are found to relax within less than 1 ps. Their relaxation time is inversely proportional to their excess energy, a dependence that we attribute to the reduced screening of Coulomb interaction and the low dimensionality of Ta₂NiSe₅. Long after ($>$ 10 ps) the conduction band has emptied, the photoemission intensity below the Fermi energy has not fully recovered the equilibrium value. Notably, this seeming carrier imbalance cannot be rationalized simply by the relaxation of photoexcited electrons and holes across the semiconductor band gap. Rather, a rate equation model involving different photoemission crosssections of the valence electrons and the condensed excitons is able to reproduce the delayed recovery of the photoemission intensity below the Fermi energy. The model shows that electron quantum tunnelling between the exciton condensate and the valence band top is enabled by an extremely small activation energy of $4\times {10}^{-6}$ eV and explains the retarded recovery of the exciton condensate. Our findings not only determine the energy gain of ground state exciton formation with exceptional energy resolution, but also demonstrate the use of time-resolved photoemission to unveil the re-formation dynamics of an exciton condensate with femtosecond time resolution.

The ultrafast dynamics of electrons and collective modes in systems out of equilibrium is crucially governed by the energy transfer from electronic degrees of freedom, where the energy of the pump source is usually absorbed, to lattice degrees of freedom. In conventional metals such process leads to an overall heating of the lattice, usually described by an effective lattice temperature $T_{\mathrm{ph}}$, until final equilibrium with all the degrees of freedom is reached. In specific materials, however, few lattice modes provide a preferential channel for the energy transfer, leading to a non-thermal distribution of vibrations and to the onset of hot phonons, i.e., lattice modes with a much higher population than the other modes. Hot phonons are usually encountered in semiconductors or semimetal compounds, like graphene, where the preferential channel towards hot modes is dictated by the reduced electronic phase space. Following a different path, the possibility of obtaining hot-phonon physics also in metals has been also recently prompted in literature, as a result of a strong anisotropy of the electron–phonon (el-ph) coupling. In the present paper, taking MgB₂ as a representative example, we review the physical conditions that allow a hot-phonon scenario in metals with anisotropic el-ph coupling, and we discuss the observable fingerprints of hot phonons. Novel perspectives towards the prediction and experimental observation of hot phonons in other metallic compounds are also discussed.

High-reflective multilayer laser coatings are widely used in advanced optical systems from high power laser facilities to high precision metrology systems. However, the real interface quality and defects will significantly affect absorption/scattering losses and laser induced damage thresholds of multilayer coatings. With the recent advances in the control of coating design and deposition processes, these coating properties can be significantly improved when properly engineered the interface and defects. This paper reviews the recent progress in the physics of laser damage, optical losses and environmental stability involved in multilayer reflective coatings for high power nanosecond near-infrared lasers. We first provide an overview of the layer growth mechanisms, ways to control the microstructures and reduce layer roughness, as well as the nature of defects which are critical to the optical loss and laser induced damage. Then an overview of interface engineering based on the design of coating structure and the regulation of deposition materials reveals their ability to improve the laser induced damage threshold, reduce the backscattering, and realize the desirable properties of environmental stability and exceptional multifunctionality. Moreover, we describe the recent progress in the laser damage and scattering mechanism of nodule defects and give the approaches to suppress the defect-induced damage and scattering of the multilayer laser coatings. Finally, the present challenges and limitations of high-performance multilayer laser coatings are highlighted, along with the comments on likely trends in future.

In this review, we summarize the recent progress in the understanding of the spin-polarized electronic states in two-dimensional (2D) atomic layer materials (ALMs) formed on solid surfaces. The spin-polarized electronic states caused by the combination of spin-orbit coupling (SOC) with broken spatial inversion symmetry along the surface normal direction is one of the most exotic phenomena that appears on ALMs formed on solid surfaces as well as clean solid surfaces. The so-called Rashba-Bychkov (RB) effect that arises from the potential gradient induced by broken inversion symmetry was believed to be the main origin of these spin-polarized electronic states. However, the spin texture of most ALMs are different from that caused by the ideal RB effect. Due to the high impact of the spin-polarized electronic states of 2D materials in not only spin-related fundamental science but also in applications since they are the key concepts to realize future semiconductor spintronics devices, much efforts have been made to elucidate the origin of these peculiar spin textures. So far, the deviations in spin texture from the ideal one have been attributed to be induced by perturbation, such as entanglement of spin and orbital momenta. In this review, we first illustrate how the symmetry of the ALM’s atomic structure can affect the spin texture, and then introduce that various spin textures, ranging from the RB-type and symmetry-induced type to spin textures that cannot be explained based on the origins proposed so far, can be simply induced by the orbital angular momentum. This review aims to provide an overview on the insights gained on the spin-polarized electronic states of ALMs and to point out opportunities for exploring exotic physical properties when combining spin and other physics, e.g. superconductivity, and to realize future spintronics-based quantum devices.