Absence of Partial Amorphization in GeSbTe Chalcogenide Superlattices

Phase-change materials (PCMs) are widely used for optical data storage due to their fast and reversible transitions between a crystalline and an amorphous phase that exhibit reflectivity contrast. In the last decade, PCMs have been found to be promising candidates for the development of nonvolatile electronic memories, as well. In this context, superlattices of thin layers of GeTe and Sb2Te3 show an unprecedented performance gain in terms of switching speed and power consumption with respect to bulk GeSbTe compounds. Models of crystalline–crystalline transitions, proposed to explain the improved properties, however, are challenged by recent experiments in which GeTe–Sb2Te3 superlattices are observed to reconfigure toward a van der Waals heterostructure of rhombohedral GeSbTe and Sb2Te3. Herein, ab initio molecular dynamics simulations are used to explore an alternative switching mechanism that comprises amorphous–crystalline transitions of ultrathin GeSbTe layers between crystalline Sb2Te3. Despite some positive results obtained by tailoring the quenching protocol, overall the extensive simulations do not yield clear evidence for this mechanism. Therefore, they suggest that the switching process probably involves a transition between two crystalline states

Wave function mapping in graphene quantum dots with soft confinement

Using low-temperature scanning tunneling spectroscopy, we map the local density of states (LDOS) of graphene quantum dots supported on Ir(111). Due to a band gap in the projected Ir band structure around the graphene K point, the electronic properties of the QDs are dominantly graphene-like. Indeed, we compare the results favorably with tight binding calculations on the honeycomb lattice based on parameters derived from density functional theory. We find that the interaction with the substrate near the edge of the island gradually opens a gap in the Dirac cone, which implies soft-wall confinement. Interestingly, this confinement results in highly symmetric wave functions. Further influences of the substrate are given by the known moir{\'e} potential and a 10% penetration of an Ir surface resonanceComment: 7 pages, 11 figures, DFT calculations directly showing the origin of soft confinment, correct identification of the state penetrating from Ir(111) into graphen

Cadmium influences the 5-fluorouracil cytotoxic effects on breast cancer cells

The aim of the research was to evaluate a heavy metal, cadmium (Cd), which was used to produce alterations in human breast cancer cell line MCF-7. Moreover, we analyzed both immunohistochemical and ultrastructural alterations induced by the antineoplastic drug, 5-fluorouracil (5-FU), after exposure to different concentrations of cd. Also, we compared the effects of these compounds on actin and tubulin cytoskeleton proteins. Under ultramicroscopic observation, control cells looked polymorphous with filopodia. In cells already treated with small concentrations of Cd, after brief times of incubation, we observed an intense metabolic activity with larger, clearer, and elongated mitochondria characterized by thin and numerous dilated cristae. 5-FU-treated cells showed cytotoxicity signs with presence of pore-like alterations in the cell membrane and evident degeneration of cytoplasm and cell nuclei. The addition of 5-FU (1.5 µM) to the cells treated with Cd (5 µM–20 µM) did not induce significant ultrastructural changes in comparison with cells treated only with Cd. In Cd+5FU-treated cells mitochondria with globular aspect and regular cristae indicated the active metabolic state. In cells treated only with Cd we observed alterations in actin distribution, while tubulin branched out throughout the cytoplasm. With the association of Cd+5FU, we observed less morphological alterations in both tubulin and actin cytoskeleton proteins. Although the mechanism remains unknown at present, our findings suggest that Cd prevents the cytotoxic effect of 5-FU on breast cancer cells. These preliminary results could have an important clinical application in patients with breast cancer

Ab initio molecular dynamics and materials design for embedded phase-change memory

The Ge2Sb2Te5 alloy has served as the core material in phase-change memories with high switching speed and persistent storage capability at room temperature. However widely used, this composition is not suitable for embedded memories—for example, for automotive applications, which require very high working temperatures above 300 °C. Ge–Sb–Te alloys with higher Ge content, most prominently Ge2Sb1Te2 (‘212’), have been studied as suitable alternatives, but their atomic structures and structure–property relationships have remained widely unexplored. Here, we report comprehensive first-principles simulations that give insight into those emerging materials, located on the compositional tie-line between Ge2Sb1Te2 and elemental Ge, allowing for a direct comparison with the established Ge2Sb2Te5 material. Electronic-structure computations and smooth overlap of atomic positions (SOAP) similarity analyses explain the role of excess Ge content in the amorphous phases. Together with energetic analyses, a compositional threshold is identified for the viability of a homogeneous amorphous phase (‘zero bit’), which is required for memory applications. Based on the acquired knowledge at the atomic scale, we provide a materials design strategy for high-performance embedded phase-change memories with balanced speed and stability, as well as potentially good cycling capability

Materials Screening for Disorder-Controlled Chalcogenide Crystals for Phase-Change Memory Applications

Tailoring the degree of disorder in chalcogenide phase-change materials (PCMs) plays an essential role in nonvolatile memory devices and neuro-inspired computing. Upon rapid crystallization from the amorphous phase, the flagship Ge–Sb–Te PCMs form metastable rocksalt-like structures with an unconventionally high concentration of vacancies, which results in disordered crystals exhibiting Anderson-insulating transport behavior. Here, ab initio simulations and transport experiments are combined to extend these concepts to the parent compound of Ge–Sb–Te alloys, viz., binary Sb2Te3, in the metastable rocksalt-type modification. Then a systematic computational screening over a wide range of homologous, binary and ternary chalcogenides, elucidating the critical factors that affect the stability of the rocksalt structure is carried out. The findings vastly expand the family of disorder-controlled main-group chalcogenides toward many more compositions with a tunable bandgap size for demanding phase-change applications, as well as a varying strength of spin–orbit interaction for the exploration of potential topological Anderson insulators

Metal - Insulator transition driven by vacancy ordering in GeSbTe phase change materials

Phase Change Materials (PCMs) are unique compounds employed in non-volatile random access memory thanks to the rapid and reversible transformation between the amorphous and crystalline state that display large differences in electrical and optical properties. In addition to the amorphous-to-crystalline transition, experimental results on polycrystalline GeSbTe alloys (GST) films evidenced a Metal-Insulator Transition (MIT) attributed to disorder in the crystalline phase. Here we report on a fundamental advance in the fabrication of GST with out-of-plane stacking of ordered vacancy layers by means of three distinct methods: Molecular Beam Epitaxy, thermal annealing and application of femtosecond laser pulses. We assess the degree of vacancy ordering and explicitly correlate it with the MIT. We further tune the ordering in a controlled fashion attaining a large range of resistivity. Employing ordered GST might allow the realization of cells with larger programming windows

IGLV3-21∗01 is an inherited risk factor for CLL through the acquisition of a single-point mutation enabling autonomous BCR signaling

© 2020 National Academy of Sciences. All rights reserved. The prognosis of chronic lymphocytic leukemia (CLL) depends on different markers, including cytogenetic aberrations, oncogenic mutations, and mutational status of the immunoglobulin (Ig) heavy-chain variable (IGHV) gene. The number of IGHV mutations distinguishes mutated (M) CLL with a markedly superior prognosis from unmutated (UM) CLL cases. In addition, B cell antigen receptor (BCR) stereotypes as defined by IGHV usage and complementaritydetermining regions (CDRs) classify ∼30% of CLL cases into prognostically important subsets. Subset 2 expresses a BCR with the combination of IGHV3-21-derived heavy chains (HCs) with IGLV3- 21-derived light chains (LCs), and is associated with an unfavorable prognosis. Importantly, the subset 2 LC carries a single-point mutation, termed R110, at the junction between the variable and constant LC regions. By analyzing 4 independent clinical cohorts through BCR sequencing and by immunophenotyping with antibodies specifically recognizing wild-type IGLV3-21 and R110-mutated IGLV3-21 (IGLV3-21R110), we show that IGLV3-21R110-expressing CLL represents a distinct subset with poor prognosis independent of IGHV mutations. Compared with other alleles, only IGLV3-21∗01 facilitates effective homotypic BCR-BCR interaction that results in autonomous, oncogenic BCR signaling after acquiring R110 as a single-point mutation. Presumably, this mutation acts as a standalone driver that transforms IGLV3-21∗01-expressing B cells to develop CLL. Thus, we propose to expand the conventional definition of CLL subset 2 to subset 2L by including all IGLV3-21R110-expressing CLL cases regardless of IGHV mutational status. Moreover, the generation ofmonoclonal antibodies recognizing IGLV3-21 or mutated IGLV3-21R110 facilitates the recognition of B cells carrying this mutation in CLL patients or healthy donors

Metal - Insulator transition driven by vacancy ordering in GeSbTe phase change materials

Phase Change Materials (PCMs) are unique compounds employed in non-volatile random access memory thanks to the rapid and reversible transformation between the amorphous and crystalline state that display large differences in electrical and optical properties. In addition to the amorphousto-crystalline transition, experimental results on polycrystalline GeSbTe alloys (GST) films evidenced a Metal-Insulator Transition (MIT) attributed to disorder in the crystalline phase. Here we report on a fundamental advance in the fabrication of GST with out-of-plane stacking of ordered vacancy layers by means of three distinct methods: Molecular Beam Epitaxy, thermal annealing and application of femtosecond laser pulses. We assess the degree of vacancy ordering and explicitly correlate it with the MIT. We further tune the ordering in a controlled fashion attaining a large range of resistivity. Employing ordered GST might allow the realization of cells with larger programming windows

The local adsorption geometry of benzenethiolate on Cu(1 0 0)

The local adsorption geometry of benzenethiolate in the ordered c(2 × 6) phase on Cu(1 0 0) has been investigated by a combination of S K-edge near-edge X-ray absorption fine structure (NEXAFS), normal incidence X-ray standing waves (NIXSW) and S 1s scanned-energy mode photoelectron diffraction (PhD). NEXAFS and PhD show that the molecular plane is tilted from the surface normal by 20 ± 15°, while NIXSW clearly identifies the S head-group as occupying the four-fold coordinated hollow sites. PhD shows the S atoms lies 1.34 ± 0.04 Å above the outermost Cu atomic layer, leading to a Cu–S bondlength of 2.25 ± 0.02 Å. The combination of the PhD and NIXSW results shows the Cu surface layer has an outward relaxation of 0.15 ± 0.06 Å. Possible origins for this large adsorbate-induced relaxation are discussed

Magnetolocalization in disordered quantum wires

The magnetic field dependent localization in a disordered quantum wire is considered nonperturbatively. An increase of an averaged localization length with the magnetic field is found, saturating at twice its value without magnetic field. The crossover behavior is shown to be governed both in the weak and strong localization regime by the magnetic diffusion length L_B. This function is derived analytically in closed form as a function of the ratio of the mean free path l, the wire thickness W, and the magnetic length l_B for a two-dimensional wire with specular boundary conditions, as well as for a parabolic wire. The applicability of the analytical formulas to resistance measurements in the strong localization regime is discussed. A comparison with recent experimental results on magnetolocalization is included.Comment: 22 pages, RevTe