Type-II Ising Pairing in Few-Layer Stanene

Spin-orbit coupling has proven indispensable in realizing topological materials and more recently Ising pairing in two-dimensional superconductors. This pairing mechanism relies on inversion symmetry breaking and sustains anomalously large in-plane polarizing magnetic fields whose upper limit is expected to diverge at low temperatures, although experimental demonstration of this has remained elusive due to the required fields. In this work, the recently discovered superconductor few-layer stanene, i.e. epitaxially strained $\alpha$-Sn, is shown to exhibit a new type of Ising pairing between carriers residing in bands with different orbital indices near the $\Gamma$-point. The bands are split as a result of spin-orbit locking without the participation of inversion symmetry breaking. The in-plane upper critical field is strongly enhanced at ultra-low temperature and reveals the sought for upturn

Electron interaction-driven insulating ground state in Bi2Se3 topological insulators in the two dimensional limit

We report a transport study of ultrathin Bi2Se3 topological insulators with thickness from one quintuple layer to six quintuple layers grown by molecular beam epitaxy. At low temperatures, the film resistance increases logarithmically with decreasing temperature, revealing an insulating ground state. The sharp increase of resistance with magnetic field, however, indicates the existence of weak antilocalization, which should reduce the resistance as temperature decreases. We show that these apparently contradictory behaviors can be understood by considering the electron interaction effect, which plays a crucial role in determining the electronic ground state of topological insulators in the two dimensional limit.Comment: 4 figure

Dynamics of spin-2 Bose condensate driven by external magnetic fields

Dynamic response of the F=2 spinor Bose-Einstein condensate (BEC) under the influence of external magnetic fields is studied. A general formula is given for the oscillation period to describe population transfer from the initial polar state to other spin states. We show that when the frequency and the reduced amplitude of the longitudinal magnetic field are related in a specific manner, the population of the initial spin-0 state will be dynamically localized during time evolution. The effects of external noise and nonlinear spin exchange interaction on the dynamics of the spinor BEC are studied. We show that while the external noise may eventually destroy the Rabi oscillations and dynamic spin localization, these coherent phenomena are robust against the nonlinear atomic interaction.Comment: 16 pages, 7 figures. accepted by Phys. Rev.

Doublecortin-Expressing Cells Persist in the Associative Cerebral Cortex and Amygdala in Aged Nonhuman Primates

A novel population of cells that express typical immature neuronal markers including doublecortin (DCX+) has been recently identified throughout the adult cerebral cortex of relatively large mammals (guinea pig, rabbit, cat, monkey and human). These cells are more common in the associative relative to primary cortical areas and appear to develop into interneurons including type II nitrinergic neurons. Here we further describe these cells in the cerebral cortex and amygdala, in comparison with DCX+ cells in the hippocampal dentate gyrus, in three age groups of rhesus monkeys: young adult (12.3 ± 0.2 years, n = 3), mid-age (21.2 ± 1.9 years, n = 3) and aged (31.3 ± 1.8 years, n = 4). DCX+ cells with a heterogeneous morphology persisted in layers II/III primarily over the associative cortex and amygdala in all groups (including in two old animals with cerebral amyloid pathology), showing a parallel decline in cell density with age across regions. In contrast to the cortex and amygdala, DCX+ cells in the subgranular zone diminished in the mid-age and aged groups. DCX+ cortical cells might arrange as long tangential migratory chains in the mid-age and aged animals, with apparently distorted cell clusters seen in the aged group. Cortical DCX+ cells colocalized commonly with polysialylated neural cell adhesion molecule and partially with neuron-specific nuclear protein and γ-aminobutyric acid, suggesting a potential differentiation of these cells into interneuron phenotype. These data suggest a life-long role for immature interneuron-like cells in the associative cerebral cortex and amygdala in nonhuman primates

Evidence for electron-electron interaction in topological insulator thin films

We consider in our work high quality single crystal thin films of Bi2Se3, grown by molecular beam epitaxy, both with and without Pb doping. Our ARPES data demonstrate topological surface states with a Fermi level lying inside the bulk band gap in the Pb doped filims. Transport data show weak localization behavior, as expected for a 2D system, but a detailed analysis within the standard theoretical framework of diffusive transport shows that the temperature and magnetic field dependences of resistance cannot be reconciled in a theory that neglects inter-electron interactions. We demonstrate that an excellent account of quantum corrections to conductivity is achieved when both disorder and interaction are taken into account. These results clearly demonstrate that it is crucial to include electron electron interaction for a comprehensive understanding of diffusive transport in topological insulators.Comment: Submitted to Phys. Rev.

Optical Monitoring of BL Lacertae Object OJ 287: a 40-Day Period?

We present the results of our optical monitoring of the BL Lacertae object OJ 287 during the first half of 2005. The source did not show large-amplitude variations during this period and was in a relatively quiescent state. A possible period of 40 days was derived from its light curves in three BATC wavebands. A bluer-when-brighter chromatism was discovered, which is different from the extremely stable color during the outburst in 1994--96. The different color behaviors imply different variation mechanisms in the two states. We then re-visited the optical data on OJ 287 from the OJ-94 project and found as well a probable period of 40 days in its optical variability during the late-1994 outburst. The results suggest that two components contribute to the variability of OJ 287 during its outburst state. The first component is the normal {\sl blazar} variation. This component has an amplitude similar to that of the quiescent state and also may share a similar periodicity. The second component can be taken as a `low-frequency modulation' to the first component. It may be induced by the interaction of the assumed binary black holes in the center of this object. The 40-day period may be related to the helical structure of the magnetic field at the base of the jet, or to the orbital motion close to the central primary black hole.Comment: 31 pages, 8 figures, accepted by A

Type-II Ising pairing in few-layer stanene

Spin-orbit coupling has proven indispensable in the realization of topological materials and, more recently, Ising pairing in two-dimensional superconductors. This pairing mechanism relies on inversion symmetry–breaking and sustains anomalously large in-plane polarizing magnetic fields whose upper limit is predicted to diverge at low temperatures. Here, we show that the recently discovered superconductor few-layer stanene, epitaxially strained gray tin (α-Sn), exhibits a distinct type of Ising pairing between carriers residing in bands with different orbital indices near the Γ-point. The bands are split as a result of spin-orbit locking without the participation of inversion symmetry–breaking. The in-plane upper critical field is strongly enhanced at ultralow temperature and reveals the predicted upturn

Phase Separation and Magnetic Order in K-doped Iron Selenide Superconductor

Alkali-doped iron selenide is the latest member of high Tc superconductor family, and its peculiar characters have immediately attracted extensive attention. We prepared high-quality potassium-doped iron selenide (KxFe2-ySe2) thin films by molecular beam epitaxy and unambiguously demonstrated the existence of phase separation, which is currently under debate, in this material using scanning tunneling microscopy and spectroscopy. The stoichiometric superconducting phase KFe2Se2 contains no iron vacancies, while the insulating phase has a \surd5\times\surd5 vacancy order. The iron vacancies are shown always destructive to superconductivity in KFe2Se2. Our study on the subgap bound states induced by the iron vacancies further reveals a magnetically-related bipartite order in the superconducting phase. These findings not only solve the existing controversies in the atomic and electronic structures in KxFe2-ySe2, but also provide valuable information on understanding the superconductivity and its interplay with magnetism in iron-based superconductors