New Low-Mass Stars and Brown Dwarfs with Disks in Lupus

Using the Infrared Array Camera and the Multiband Imaging Photometer aboard the {\it Spitzer Space Telescope}, we have obtained images of the Lupus 3 star-forming cloud at 3.6, 4.5, 5.8, 8.0, and 24 \micron. We present photometry in these bands for the 41 previously known members that are within our images. In addition, we have identified 19 possible new members of the cloud based on red 3.6-8.0 \micron colors that are indicative of circumstellar disks. We have performed optical spectroscopy on 6 of these candidates, all of which are confirmed as young low-mass members of Lupus 3. The spectral types of these new members range from M4.75 to M8, corresponding to masses of 0.2-0.03 $M_\odot$ for ages of $\sim1$ Myr according to theoretical evolutionary models. We also present optical spectroscopy of a candidate disk-bearing object in the vicinity of the Lupus 1 cloud, 2M 1541-3345, which Jayawardhana & Ivanov recently classified as a young brown dwarf ($M\sim0.03$ $M_\odot$) with a spectral type of M8. In contrast to their results, we measure an earlier spectral type of M5.75$\pm$0.25 for this object, indicating that it is probably a low-mass star ($M\sim0.1$ $M_\odot$). In fact, according to its gravity-sensitive absorption lines and its luminosity, 2M 1541-3345 is older than members of the Lupus clouds ($\tau\sim1$ Myr) and instead is probably a more evolved pre-main-sequence star that is not directly related to the current generation of star formation in Lupus.Comment: 18 pages, 3 tables, 6 figure

Interstellar C60+

Buckminsterfullerene (C60) was recently detected through its infrared emission bands in the interstellar medium (ISM), including in the proximity of massive stars, where physical conditions could favor the formation of the cationic form, C60+. In addition, C60+ was proposed as the carrier of two diffuse interstellar bands in the near-IR, although a firm identification still awaits for gas-phase spectroscopic data. We examined in details the Spitzer IRS spectra of the NGC 7023 reflection nebula, at a position close (7.5") to the illuminating B star HD 200775, and found four previously unreported bands at 6.4, 7.1, 8.2 and 10.5 \mu m in addition to the classical bands attributed to Polycylic Aromatic Hydrocarbons (PAHs) and neutral C60. These 4 bands are observed only in this region of the nebula, while C60 emission is still present slightly further away from the star, and PAH emission even further away. Based on this observation, on theoretical calculations we perform, and on laboratory studies, we attribute these bands to C60+. The detection of C60+ confirms the idea that large carbon molecules exist in the gas-phase in these environments. In addition, the relative variation of the C60, and C60+, band intensities constitutes a potentially powerful probe of the physical conditions in highly UV-irradiated regions.Comment: Accepted for publication in A&A, v2: minor corrections of typos and language and additional reference include

The interior angular momentum of core hydrogen burning stars from gravity-mode oscillations

A major uncertainty in the theory of stellar evolution is the angular momentum distribution inside stars and its change during stellar life. We compose a sample of 67 stars in the core-hydrogen burning phase with a $\log\,g$ value from high-resolution spectroscopy, as well as an asteroseismic estimate of the near-core rotation rate derived from gravity-mode oscillations detected in space photometry. This assembly includes 8 B-type stars and 59 AF-type stars, covering a mass range from 1.4 to 5\,M$_\odot$, i.e., it concerns intermediate-mass stars born with a well-developed convective core. The sample covers projected surface rotation velocities $v\sin\,i \in[9,242]\,$km\,s$^{-1}$ and core rotation rates up to $26\mu$Hz, which corresponds to 50\% of the critical rotation frequency. We find deviations from rigid rotation to be moderate in the single stars of this sample. We place the near-core rotation rates in an evolutionary context and find that the core rotation must drop drastically before or during the short phase between the end of the core-hydrogen burning and the onset of core-helium burning. We compute the spin parameter, which is the ratio of twice the rotation rate to the mode frequency (also known as the inverse Rossby number), for 1682 gravity modes and find the majority (95\%) to occur in the sub-inertial regime. The ten stars with Rossby modes have spin parameters between 14 and 30, while the gravito-inertial modes cover the range from 1 to 15.Comment: Manuscript of 5 pages, including 2 figures, accepted for publication in The Astrophysical Journal Letter

Periodic spatial variation of the electron-phonon interaction in epitaxial graphene on Ru(0001

We have performed low temperature scanning tunnelling spectroscopy (STS) measurements on graphene epitaxially grown on Ru(0001). An inelastic feature, related to the excitation of a vibrational breathing mode of the graphene lattice, was found at 360 meV. The change in the differential electrical conductance produced by this inelastic feature, which is associated with the electron-phonon interaction strength, varies spatially from one position to other of the graphene supercell. This inhomogeneity in the electronic properties of graphene on Ru(0001) results from local variations of the carbon-ruthenium interaction due to the lattice mismatch between the graphene and the Ru(0001) lattices.Comment: 6 Pages, 3 figure

Parity-breaking phases of spin-orbit-coupled metals with gyrotropic, ferroelectric and multipolar orders

We study Fermi liquid instabilities in spin-orbit-coupled metals with inversion symmetry. By introducing a canonical basis for the doubly degenerate Bloch bands in momentum space, we derive the general form of interaction functions. A variety of time-reversal-invariant, parity-breaking phases is found, whose Fermi surface is spontaneously deformed and spin-split. In terms of symmetry, these phases possess gyrotropic, ferroelectric and multipolar orders. The ferroelectric and multipolar phases are accompanied by structural distortions, from which the electronic orders can be identified. The gyrotropic phase exhibits a unique nonlinear optical property. Based on recent experiments, we identify several interesting quantum materials including pyrochlore oxides, which show evidence of these parity breaking orders.Comment: 5 page

At the cutting edge : structural analysis and chemical modification of the edges of mechanically cleaved graphene nanoribbons : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Nanoscience at Massey University, Manawatū, New Zealand

The first decade of the new carbon nanomaterial graphene has been a time of great discovery and excitement as the exceptional properties of this material were uncovered and its promise for numerous applications realised. The unique properties of graphene, including its exceptional electronic structure, are now well-established, and investigations into how these properties can be manipulated and exploited are rapidly taking off. This research contributes to the emerging field by exploring the structure and chemistry of the edges of mechanically cleaved graphene nanoribbons; groundwork for the future development of edge-modified nanoribbons that could be used to form selfassembled graphene nanoribbon composite structures with potential for devices in solar energy conversion. For this purpose, a Raman microscope was built that enabled for various aspects of the structure of graphene nanoribbons to be probed, in particular the geometry and smoothness of the edges, which have important implications for the specific reactivity of the edge carbon atoms. Chemical approaches for the specific functionalisation of the edges of the nanoribbons were developed, involving reactions tailored to the reactive groups present at the edges, and these were found to be highly successful and selective