[en] Angle-resolved photoemission has been utilized to study the surface electronic structure of 1/3 monolayer of Sn on Ge(111) in both the room-temperature (3x3 )R30^o phase and the low-temperature (3x3 ) charge-density-wave phase. The results reveal a gap opening around the (3x3 ) Brillouin zone boundary, suggesting a Peierls-like transition despite the well-documented lack of Fermi nesting. A highly sensitive electronic response to doping by intrinsic surface defects is the cause for this unusual behavior, and a detailed calculation illustrates the origin of the (3x3 ) symmetry

[en] The initial stages of high temperature CaF₂ growth by molecular beam epitaxy on Si(111) substrates with a 3 deg. miscut were characterized using atomic force microscopy and low energy electron diffraction. At a growth temperature of 750 deg. C, electron diffraction measurements showed that the surface retained the (3x1) surface reconstruction up to a deposition thickness of at least 1.2 nm. The overall topography of the surface was defined by atomically flat terraces decorated with a large number of clusters. These clusters were confined to step edges and were typically 10-20 nm tall. The clusters appear to nucleate at the top of step edges and then grow in size until they extend across the step onto the neighboring terrace below. These results indicate that in this growth regime, the CaF₂ molecules diffuse across terraces to aggregate into relatively large nanostructures after the formation of a thin wetting layer. The unusually rounded features and large heights seen in these clusters appear to arise from the topography of substrate terraces.

[en] We present the results of extensive Monte Carlo simulations of intercalated manganese–titanium (Mn–Ti) layered TiS₂ crystals. The computational model involves mixtures of Mn and Ti in various percentages placed on a triangular lattice with fixed lattice sites and up to five layers. The range of concentrations of intercalated Mn studied was 5%  ⩽  X _Mn  ⩽  33% and for Ti, 0%  ⩽  X _Ti  ⩽  15%, where X _A denotes the percentage of the total number of lattice sites occupied by species A. The species are allowed to interact spatially through a screened Coulomb potential and magnetically with external and RKKY field terms. Structurally, the pure Mn systems present as disordered at very low densities and evolve through a 2  ×  2 structure (perfect at X _Mn  =  25%) up to a $\sqrt{3}$  ×  $\sqrt{3}$ lattice (perfect at X _Mn  =  33%), with variations of the two ‘perfect’ lattice structures depending on density. Changes in density for pure Mn systems as well as those intercalated with both Mn and Ti dramatically affects the system’s structural and magnetic properties, and the magnetic behavior of various morphological features present in the system are discussed. The RKKY interaction is adjusted based on the intercalant compositions and is very sensitive to structural variations in the intercalant layers. The composition ranges studied here encompass and exceed those that are experimentally accessible, which helps place experimentally relevant densities in perspective. (paper)

[en] Two infrared (IR)-active vibrational modes, observed at 93 and 113 cm⁻¹ in Raman scattering, are evidence of an inversion symmetry breakdown in thin (∼10 nm) nanoplates of topological insulator Bi₂Te₃ as-grown on SiO₂. Both Raman and IR modes are preserved after typical device fabrication processes. In nanoplates transferred to another SiO₂ substrate via contact printing, however, the IR modes are absent, and the Raman spectra are similar to those from bulk samples. The differences between as-grown and transferred nanoplates may result from nanoplate–substrate interactions. (paper)

[en] By examining thermally excited states above the Fermi level in angle resolved photoemission spectroscopy, we show that the disconnected Fermi arcs observed in the pseudogap state of the cuprate phase diagram are actually components of fully enclosed hole pockets. The spectral weight of these pockets is vanishingly small at the Luttinger surface, coincident with the antiferromagnetic zone-boundary formed by the underlying spin lattice, creating the illusion of the Fermi arcs. The area of the pockets as measured in this study is consistent with the doping level, and hence the carrier density, of the samples measured. Furthermore, the shape and area of the pockets is well reproduced by phenomenological models of the pseudogap phase that reflect its proximity to a Mott insulator