Search

Growth of Xene, referred to as the analog of graphene made of atoms other than carbon, with higher lattice quality and more intrinsic electronic structures, is an important challenge to the development of two-dimensional (2D) materials. We report an approach using Pb-atom deposition to striped-phase (SP) germanene grown on Ag(111) to trigger it to a quasifreestanding phase (QP). Using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM), we demonstrate that the 2D monatomic Pb layer, instead of being on top, grows side-by-side with QP germanene, not only driving the SP-to-QP transition but further enhancing the lattice order of QP as revealed by the dramatic evolution of LEED-spot shape and uniform STM-imaged array of moir\'e patterns. Originally oriented at 30\ifmmode^\circ\else\textdegree\fi{} to Ag(111), the QP germanene transits first to a narrow distribution of rotations then to two sharp domains at \ifmmode\pm\else\textpm\fi{}3\ifmmode^\circ\else\textdegree\fi{} from 30\ifmmode^\circ\else\textdegree\fi{}, while the monoatomic Pb domains twist from \ifmmode\pm\else\textpm\fi{}4.3\ifmmode^\circ\else\textdegree\fi{} to \ifmmode\pm\else\textpm\fi{}9.5\ifmmode^\circ\else\textdegree\fi{} with respect to Ag(111). The analysis of higher-order coincidence for both layers on Ag(111) is in complete agreement with LEED results and reveals a picture of two insoluble monatomic layers driving each other to a more optimal configuration. Moreover, the corresponding density of states measured by scanning tunneling spectroscopy for quality-enhanced QP germanene exhibits a clear V-shape distribution at the energy \ensuremath{\sim}0.4 eV above Fermi level, indicating the Fermi-Dirac cone, which agrees with our density functional theory calculation.

Ultrathin Pb and Ge films deposited on Ag(111) surfaces have been investigated and compared. We found that at 1/3 ML, both films formed surface alloys, ${\mathrm{Ag}}_{2}\mathrm{Pb}$ and ${\mathrm{Ag}}_{2}\mathrm{Ge}$, with $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}R{30}^{\ensuremath{\circ}}$ and $\frac{19}{20}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\frac{19}{20}\sqrt{3}R{30}^{\ensuremath{\circ}}$ structures on Ag(111) but the surface electronic structures exhibit a most evident difference at the Ag(111) surface zone boundary ${\overline{M}}_{\mathrm{Ag}(111)}$, where the single band and the splitting ones were observed, respectively. Up to 1 ML, ${\mathrm{Ag}}_{2}\mathrm{Ge}$ subsequently develops into germanene with a striped phase and then a quasifreestanding phase, as previously reported [Lin et al., Phys. Rev. Mater. 2, 024003 (2018)], while ${\mathrm{Ag}}_{2}\mathrm{Pb}$ evolves to a dense Pb(111) phase that also reveals splitting bands at ${\overline{M}}_{\mathrm{Ag}(111)}$. We discover that the larger (smaller) atomic size of a Pb (Ge) atom with respect to an Ag atom causes the commensurate (incommensurate) interfaces and further demonstrate that the splitting bands of ${\mathrm{Ag}}_{2}\mathrm{Ge}$ surface alloy and 1-ML Pb film originated from the commonly incommensurate interface with Ag(111), which mediates umklapp scattering by inducing the mirror image of the pristine ${\mathrm{Ag}}_{2}\mathrm{Ge}$ and Pb(111) bands relative to ${\overline{M}}_{\mathrm{Ag}(111)}.$

Here we report the evolution of bulk band structure and surface states in rare earth mono-bismuthides with partially filled f shell. Utilizing synchrotron-based photoemission spectroscopy, we determined the three-dimensional bulk band structure and identified the bulk band inversions near the X points, which, according to the topological theory, could give rise to nontrivial band topology with odd number of gapless topological surface states. Near the surface Gamma bar point, no clear evidence for predicted gapless topological surface state is observed due to its strong hybridization with the bulk bands. Near the M bar point, the two surface states, due to projections from two inequivalent bulk band inversions, interact and give rise to two peculiar sets of gapped surface states. The bulk band inversions and corresponding surface states can be tuned substantially by varying rare earth elements, in good agreement with density-functional theory calculations assuming local f electrons. Our study therefore establishes rare earth mono-bismuthides as an interesting class of materials possessing tunable electronic properties and magnetism, providing a promising platform to search for novel properties in potentially correlated topological materials., Comment: 7 figures

Two-dimensional (2D) honeycomb lattices beyond graphene promise new physical properties such as quantum spin Hall effect. While there have been claims of growth of such lattices (silicene, germanene, stanene), their existence needs further support and their preparation and characterization remain a difficult challenge. Our findings suggest that two distinct phases associated with germanene, the analog of graphene made of germanium (Ge) instead of carbon, can be grown on Ag(111) as observed by scanning tunneling microscopy, low-energy electron diffraction, and angle-resolved photoemission spectroscopy. One such germanene exhibits an atom-resolved alternatively buckled full honeycomb lattice, which is tensile strained and partially commensurate with the substrate to form a striped phase (SP). The other, a quasifreestanding phase (QP), is also consistent with a honeycomb lattice with a lattice constant incommensurate with the substrate but very close to the theoretical value for freestanding germanene. The SP, with a lower atomic density, can be driven into the QP and coexist with the QP by additional Ge deposition. Band mapping and first-principles calculations with proposed SP and QP models reveal an interface state exists only in the SP but the characteristic \ensuremath{\sigma} band of freestanding germanene emerges only in the QP---this leads to an important conclusion that adlayer-substrate commensurability plays a key role to affect the electronic structure of germanene. The evolution of the dual germanene phases manifests the competitive formation of Ge-Ge covalent and Ge-Ag interfacial bonds.

We report on helicity-dependent terahertz emissions that originate from the helicity-dependent photocurrents in topological insulator $\mathrm{S}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{3}$ thin films due to ultrafast optical excitation. The polarity of the emitted terahertz radiation is controlled by both the incident angle and the helicity of optical pulses. Using an unprecedented decomposition-recombination procedure in the time domain, the signals of the Dirac fermions are fully separated from bulk contributions. These results provide insights into the optical coupling of topological surface states and open up opportunities for applying helicity-dependent terahertz emission spectroscopy in spintronics.

Films of $\mathrm{Pb}(111)$ with thickness ranging from 1 to 16 monolayers (ML) were grown on $\mathrm{Ge}(111)$ and examined by angle-resolved photoemission spectroscopy. The measured thickness dependences of the work function and surface energy from the same set of samples were both well described by damped sinusoidal oscillatory functions with the same period, but the oscillation phases differ by $1/4$ period between the two quantities, as judged by the node positions. This difference agrees with the prediction of a generic quantum-well model. The varied node positions of damped oscillations of the work function and surface energy were examined in terms of the details of Pb subband dispersions, indicating the different roles of the density of states at the Fermi level to both thin-film properties. The surface dipoles of Pb films were found to have no relevant effects on the phase shift of thickness-dependent work functions except for the amplitudes of oscillations. The total boundary phase shifts of the quantum well states, extracted from the energy positions of the quantum-well states for both $\mathrm{Pb}/\mathrm{Ge}(111)$ and freestanding Pb films, are compared and related to the thickness-dependent work function and surface energy.

To investigate the surface diffusion of chemisorbed H on Ag/Au(111) thin films with surface steps, we characterized the distribution of H atoms on the surface by measuring the Shockley states of these surfaces with angle-resolved photoelectron spectroscopy. Our results show that H atoms tend to diffuse and to remain on the terrace at the lower side of the surface steps during annealing. This diffusive behavior is explained qualitatively through the existence of an Ehrlich-Schwoebel potential barrier associated with the surface steps.

The dependence on thickness of electronic and lattice structures of Pb films deposited on Ge(100) substrate was investigated with angle-resolved photoemission spectroscopy and low-energy electron diffraction. Pb films grew in the (111) surface direction with two (1 \ifmmode\times\else\texttimes\fi{} 1) domains rotated 90 \ifmmode^\circ\else\textdegree\fi{} relative to each other despite a square lattice of Ge(100); correspondingly, the heavy-hole band edge of Ge does not interact with quantum-well--state bands of Pb films. Symmetry arguments in terms of orbitals of Pb quantum-well states and Ge band edges correlate the consonant diminution of lattice and electronic coupling between the film and the substrate.

The wetting of thin Ag films on Cu(111) is characterized in detail with angle-resolved photoelectron spectroscopy. After constructing disordered Ag films on Cu(111), we monitored the evolution of their layer-resolved Shockley states during wetting. Our results provide the first observation of a transitional state, which implies a coexistence of clean Cu(111), 1 ML of Ag on Cu(111), and 2 ML of Ag on Cu(111). The evolution of these Shockley states indicates a two-process model of wetting.

Based on measurements of angle resolved photoemission, we report that in the Pb/Ge(111)- \sqrt{3}x\sqrt{3} R30^\circ structure, in addition to three bands resembling Ge heavy hole (HH), light hole (LH), and split off (SO) bulk band edges, a fourth dispersive band resembling the non split off (NSO) band is found near the surface zone center. While three Ge bulk-like bands get distorted due to strong coupling between Pb and Ge, the NSO-like band gets weaker and disappears for larger thickness of Pb, which, when combined with ab initio calculations, indicates its localized nature within space charge layer. Our results are clearly important for designing electronics involved with metal-semiconductor contacts., 21 pages, 4 figures, Phys. Rev. B 81, 245406 (2010)

We report a mechanism for rapid deterioration of metallic surfaces during photoemission. The surface of interest is Cu(111), but a Cu(111) surface partially covered with Ag was chosen for testing. The surface of Ag/Cu(111) is characterized with its Shockley states, of which the binding energy and line width are very sensitive to surface order and composition; the evolution of the surface can be characterized by monitoring the change of its Shockley states. We observed a rapid degradation of the Shockley state of Cu(111) during photoemission at temperatures less than 220 K. A different evolution of the Shockley states during annealing and cooling was also discovered. All these observations favor a mechanism based on photonstimulated chemisorption of hydrogen on the surface. We suggest that this mechanism of surface deterioration affects all investigations of a surface involving intense radiation. Our experiments were performed on Beamline 21B1. All measurements were performed with photons of energy 21 eV; the beam flux and position are presumed constant. The base pressure of the system was better than 6×10 torr. The sample was prepared on depositing 0.5 monolayer (ML) of Ag on Cu(111) and subsequently annealing to 450 K. After annealing, clean Cu(111) and Cu(111) covered by Ag at 1 ML, denoted “1-ML Ag/Cu(111)”, coexist on the sample surface. Figure 1 illustrates how the Shockley states evolve with prolonged exposure to radiation at various temperatures. After exposures at 220 K and 150 K, the sample was moved ~1.5 mm vertically; a further spectrum was recorded and is plotted at the bottom of the stacked plot. Each spectrum in Fig. 1 comprises two peaks, labeled SCu and S1 pertaining to the Shockley states of Cu(111) and 1-ML Ag/Cu(111), respectively. Dissimilarity in the evolution of spectra with temperature is evident: the spectra remain essentially unchanged at 300 K, whereas SCu degrades noticeably at 220 and 150 K. Although showing a slight broadening, the bottom spectra in Figs. 1(b) and 1(c) resemble the top spectra measured at the beginning of the experiments, which is a strong indication that rapid degradation of SCu exists only in the area under irradiation. Figure 2 is a stacked plot of photoemission spectra measured at various temperatures. The measurement was completed with an annealing and a cooling cycle. As observed in the figure, the width of SCu is noticeably wider during annealing. The dependence of the line widths of the Shockley states on temperature is determined from fitting the spectra in Fig. 2. The results from the fitting are plotted in Fig. 3. The line widths of the Shockley states of Cu(111) (SCu) and 1-ML Ag/Cu(111) (S1) are shown as triangles and diamonds, respectively. The line widths measured during annealing and cooling are labeled with filled and open symbols, respectively. The results show noticeably the difference in the temperature-dependent variation of the line widths between SCu and S1. Furthermore, the widths of both SCu and S1 vary differently between annealing and cooling. The widths of the Shockley states underwent a large increase and then decreased to the values for a clean ordered surface; our measurement can be considered to constitute temperature-programmed desorption (TPD). The result indicates that the desorption of the contaminants from Cu(111) and Ag/Cu(111) is complete at ~350 K and ~180 K, respectively; these numbers match satisfactorily the temperatures for chemisorbed hydrogen atoms to desorb from Cu(111) and Ag(111). The results suggest that chemisorbed hydrogen atoms are likely a principal contaminant involved in the rapid deterioration of metallic surfaces observed in our experiments. We believe that all photoemission measurements on surfaces are subject to the mechanism of surface deterioration that we here report. The surface deterioration according to the proposed mechanism is likely to become a major obstacle to the success of future experiments on surfaces utilizing micro-focused beams.

We observed atomic-layer-resolved surface states and quantum-well states using angle-resolved photoelectron spectroscopy on atomically flat Ag/Au(111) thin films grown with a two-step method. The layer dependence of binding energies of the bound states was determined unequivocally for a Ag coverage up to 17 monolayers. We analyzed both the surface states and the quantum-well states using a one-dimensional pseudopotential approach. Our results show a fruitful combination of growth techniques and theoretical analysis for thin-film systems.

Subband dispersions of quantum-well states in Ag films on Ge(111) have been determined by angle-resolved photoemission. The effective masses of the subbands at the zone center increase substantially for decreasing film thicknesses. This peculiar behavior is attributed to a kinetic constraint for standing-wave formation governed by a momentum-dependent phase-shift function. No evidence is found for in-plane electron localization within the confined geometry.