Stripe and Short Range Order in the Charge Density Wave of 1T-Cu_{x}TiSe_{2}.

We study the impact of Cu intercalation on the charge density wave (CDW) in 1T-Cu_{x}TiSe_{2} by scanning tunneling microscopy and spectroscopy. Cu atoms, identified through density functional theory modeling, are found to intercalate randomly on the octahedral site in the van der Waals gap and to dope delocalized electrons near the Fermi level. While the CDW modulation period does not depend on Cu content, we observe the formation of charge stripe domains at low Cu content (x<0.02) and a breaking up of the commensurate order into 2×2 domains at higher Cu content. The latter shrink with increasing Cu concentration and tend to be phase shifted. These findings invalidate a proposed excitonic pairing as the primary CDW formation mechanism in this material.

Doping nature of native defects in 1T-TiSe2.

The transition-metal dichalcogenide 1T-TiSe2 is a quasi-two-dimensional layered material with a charge density wave (CDW) transition temperature of T(CDW) ≈ 200 K. Self-doping effects for crystals grown at different temperatures introduce structural defects, modify the temperature-dependent resistivity, and strongly perturbate the CDW phase. Here, we study the structural and doping nature of such native defects combining scanning tunneling microscopy or spectroscopy and ab initio calculations. The dominant native single atom dopants we identify in our single crystals are intercalated Ti atoms, Se vacancies, and Se substitutions by residual iodine and oxygen.

Infrared- and Raman-spectroscopy measurements of a transition in the crystal structure and a closing of the energy gap of BiTeI under pressure.

BiTeI is a giant Rashba spin splitting system, in which a noncentrosymmetric topological phase has recently been suggested to appear under high pressure. We investigated the optical properties of this compound, reflectivity and transmission, under pressures up to 15 GPa. The gap feature in the optical conductivity vanishes above p∼9 GPa and does not reappear up to at least 15 GPa. The plasma edge, associated with intrinsically doped charge carriers, is smeared out through a phase transition at 9 GPa. Using high-pressure Raman spectroscopy, we follow the vibrational modes of BiTeI, providing additional clear evidence that the transition at 9 GPa involves a change of crystal structure. This change of crystal structure possibly inhibits the high-pressure topological phase from occurring.

Layer-by-layer entangled spin-orbital texture of the topological surface state in Bi2Se3.

We study Bi(2)Se(3) by polarization-dependent angle-resolved photoemission spectroscopy and density-functional theory slab calculations. We find that the surface state Dirac fermions are characterized by a layer-dependent entangled spin-orbital texture, which becomes apparent through quantum interference effects. This explains the discrepancy between the spin polarization obtained in spin and angle-resolved photoemission spectroscopy-ranging from 20% to 85%-and the 100% value assumed in phenomenological models. It also suggests a way to probe the intrinsic spin texture of topological insulators, and to continuously manipulate the spin polarization of photoelectrons and photocurrents all the way from 0 to ±100% by an appropriate choice of photon energy, linear polarization, and angle of incidence.

Hydrogen peroxide diffusion dynamics in dental tissues.

The aim of this study was to investigate the diffusion dynamics of 25% hydrogen peroxide (H2O2) through enamel-dentin layers and to correlate it with dentin's structural alterations. Micro-Raman Spectroscopy (MRS) and Fourier Transform Infrared Photoacoustic Spectroscopy (FTIR-PAS) were used to measure the spectra of specimens before and during the bleaching procedure. H2O2 was applied to the outer surface of human enamel specimens for 60 minutes. MRS measurements were performed on the inner surface of enamel or on the subsurface dentin. In addition, H2O2 diffusion dynamics from outer enamel to dentin, passing through the dentin-enamel junction (DEJ) was obtained with Raman transverse scans. FTIR-PAS spectra were collected on the outer dentin. MRS findings revealed that H2O2 (O-O stretching µ-Raman band) crossed enamel, had a more marked concentration at DEJ, and accumulated in dentin. FTIR-PAS analysis showed that H2O2 modified dentin's organic compounds, observed by the decrease in amides I, II, and III absorption band intensities. In conclusion, H2O2 penetration was demonstrated to be not merely a physical passage through enamel interprismatic spaces into the dentinal tubules. H2O2 diffusion dynamics presented a concentration gradient determined by the chemical affinity of the H2O2 with each specific dental tissue.

Rashba spin-splitting control at the surface of the topological insulator Bi2Se3.

The electronic structure of Bi(2)Se(3) is studied by angle-resolved photoemission and density functional theory. We show that the instability of the surface electronic properties, observed even in ultrahigh-vacuum conditions, can be overcome via in situ potassium deposition. In addition to accurately setting the carrier concentration, new Rashba-like spin-polarized states are induced, with a tunable, reversible, and highly stable spin splitting. Ab initio slab calculations reveal that these Rashba states are derived from 5-quintuple-layer quantum-well states. While the K-induced potential gradient enhances the spin splitting, this may be present on pristine surfaces due to the symmetry breaking of the vacuum-solid interface.