arXiv subjects

C. Lane

Publications and source records attributed to C. Lane.

23 records · Page 2Linked to original sources

arXiv2014

Electronic structure and excitations in oxygen deficient CeO$_{2-δ}$ from DFT calculations

The electronic structures of supercells of CeO$_{2-δ}$ have been calculated within the Density Functional Theory (DFT). The equilibrium properties such as lattice constants, bulk moduli and magnetic moments are well reproduced by the generalized gradient approximation (GGA). Electronic excitations are simulated by robust total energy calculations for constrained states with atomic core- or valence-holes. Pristine ceria CeO$_2$ is found to be a non-magnetic insulator with magnetism setting in as soon as oxygens are removed from the structure. In the ground state of defective ceria, the Ce-$f$ majority band resides near the Fermi level, but appears at about 2 eV below the Fermi level in photoemission spectroscopy experiments due to final state effects. We also tested our computational method by calculating threshold energies in Ce-M$_5$ and O-K x-ray absorption spectroscopy and comparing theoretical predictions with the corresponding measurements. Our result that $f$ electrons reside near the Fermi level in the ground state of oxygen deficient ceria is crucial for understanding catalytic properties of CeO$_2$ and related materials.

cond-mat.mtrl-sci↗

arXiv2008

Confirmation of the Electron Cyclotron Maser Instability as the Dominant Source of Radio Emission from Very Low Mass Stars and Brown Dwarfs

We report on radio observations of the M8.5 dwarf LSR J1835+3259 and the L3.5 dwarf 2MASS J00361617+1821104, which provide the strongest evidence to date that the electron cyclotron maser instability is the dominant mechanism producing radio emission in the magnetospheres of ultracool dwarfs. As has previously been reported for the M9 dwarf TVLM 513-46546, periodic pulses of 100% circularly polarized, coherent radio emission are detected from both dwarfs with periods of 2.84 +/- 0.01 and 3.08 +/- 0.05 hours respectively for LSR J1835+3259 and 2MASS J00361617+1821104. Importantly, periodic unpolarized radio emission is also detected from 2MASS J00361617+1821104, and brightness temperature limitations rule out gyrosynchrotron radiation as a source of this radio emission. The unpolarized emission from this and other ultracool dwarfs is also attributed to electron cyclotron maser emission, which has become depolarized on traversing the ultracool dwarf magnetosphere, possibly due to propagations effects such as scattering. Based on available v sin i data in the literature and rotation periods derived from the periodic radio data for the three confirmed sources of electron cyclotron maser emission, TVLM 513-46546, LSR J1835+3259 and 2MASS J00361617+1821104, we determine that the rotation axes of all three dwarfs are close to perpendicular to our line of sight. This suggests a possible geometrical selection effect due to the inherent directivity of electron cyclotron maser emission, that may account for the previously reported relationship between radio activity and v sin i observed for ultracool dwarfs. We also determine the radius of the dwarf LSR J1835+3259 to be > 0.117 +/- 0.012 R_Sol. (abridged)

astro-ph↗

arXiv2007

Rotational Modulation of M/L Dwarfs due to Magnetic Spots

We find periodic I-band variability in two ultracool dwarfs, TVLM 513-46546 and 2MASS J00361617+1821104, on either side of the M/L dwarf boundary. Both of these targets are short-period radio transients, with the detected I-band periods matching those found at radio wavelengths (P=1.96 hr for TVLM 513-46546, and P=3 hr for 2MASS J00361617+1821104). We attribute the detected I-band periodicities to the periods of rotation of the dwarfs, supported by radius estimates and measured $v$ sin $i$ values for the objects. Based on the detected period of rotation of TVLM 513-46546 (M9) in the I-band, along with confirmation of strong magnetic fields from recent radio observations, we argue for magnetically induced spots as the cause of this periodic variability. The I-band rotational modulation of L3.5 dwarf 2MASS J00361617+1821104 appeared to vary in amplitude with time. We conclude that the most likely cause of the I-band variability for this object is magnetic spots, possibly coupled with time-evolving features such as dust clouds.

astro-ph↗

arXiv2007

Periodic Bursts of Coherent Radio Emission from an Ultracool Dwarf

We report the detection of periodic (p = 1.96 hours) bursts of extremely bright, 100% circularly polarized, coherent radio emission from the M9 dwarf TVLM 513-46546. Simultaneous photometric monitoring observations have established this periodicity to be the rotation period of the dwarf. These bursts, which were not present in previous observations of this target, confirm that ultracool dwarfs can generate persistent levels of broadband, coherent radio emission, associated with the presence of kG magnetic fields in a large-scale, stable configuration. Compact sources located at the magnetic polar regions produce highly beamed emission generated by the electron cyclotron maser instability, the same mechanism known to generate planetary coherent radio emission in our solar system. The narrow beams of radiation pass our line of sight as the dwarf rotates, producing the associated periodic bursts. The resulting radio light curves are analogous to the periodic light curves associated with pulsar radio emission highlighting TVLM 513-46546 as the prototype of a new class of transient radio source.

astro-ph↗

arXiv2004

Proposal for U.S. participation in Double-CHOOZ: A New theta-13 Experiment at the Chooz Reactor

It has recently been widely recognized that a reactor anti-neutrino disappearance experiment with two or more detectors is one of the most cost-effective ways to extend our reach in sensitivity for the neutrino mixing angle theta-13 without ambiguities from CP violation and matter effects. The physics capabilities of a new reactor experiment together with superbeams and neutrino factories have also been studied but these latter are considered by many to be more ambitious projects due to their higher costs, and hence to be farther in the future. We propose to contribute to an international collaboration to modify the existing neutrino physics facility at the Chooz-B Nuclear Power Station in France. The experiment, known as Double-CHOOZ, is expected to reach a sensitivity of sine squared of twice the mixing angle > 0.03 over a three year run, 2008-2011. This would cover roughly 85% of the remaining allowed region. The costs and time to first results for this critical parameter can be minimized since our project takes advantage of an existing infrastructure.

hep-ex↗