766 research outputs found
Seeing many-body effects in single- and few-layer graphene: Observation of two-dimensional saddle-point excitons
Significant excitonic effects were observed in graphene by measuring its
optical conductivity in a broad spectral range including the two-dimensional
{\pi}-band saddle-point singularities in the electronic structure. The strong
electron-hole interactions manifest themselves in an asymmetric resonance
peaked at 4.62 eV, which is red-shifted by nearly 600 meV from the value
predicted by ab-initio GW calculations for the band-to-band transitions. The
observed excitonic resonance is explained within a phenomenological model as a
Fano interference of a strongly coupled excitonic state and a band continuum.
Our experiment also showed a weak dependence of the excitonic resonance in
few-layer graphene on layer thickness. This result reflects the effective
cancellation of the increasingly screened repulsive electron-electron (e-e) and
attractive electron-hole (e-h) interactions.Comment: 9 pages, 3 figures, In PR
Time-resolved Raman Spectroscopy of Optical Phonons in Graphite: Phonon Anharmonic Coupling and Anomalous Stiffening
Time-resolved Raman spectroscopy has been applied to probe the anharmonic
coupling and electron-phonon interaction of optical phonons in graphite. From
the decay of the transient anti-Stokes scattering of the G-mode following
ultrafast excitation, we measured a lifetime of 2.2+/-0.1ps for zone-center
optical phonons. We also observed a transient stiffening of G-mode phonons, an
effect attributed to the reduction of the electron-phonon coupling for high
electronic temperatures
Bidding in a Possibly Common-Value Auction
We analyze a second-price auction with two bidders in which only one of the bidders
is informed as to whether the object is valued commonly. We show that any equilibrium
strategy of the bidder who is uninformed must be part of an equilibrium when both bidders
instead know that the auction is not common value, regardless of the way in which the
values are different. We derive su¢ cient conditions for equilibrium existence
Collective Spin Modes in Superconducting Double Layers
We investigate a double layer system with tight-binding hopping, intra-layer
and inter-layer interactions, as well as a Josephson like coupling. We find
that an antiferromagnetic spin polarization induces additional spin-triplet
pairing (with ) to the singlet order parameter. This causes an undamped
collective mode in the superconducting state below the particle-hole threshold,
which is interpreted as a Goldstone excitation.Comment: 7 pages, latex, 2 postscript figure
Detection of H_2 Pure Rotational Line Emission from the GG Tauri Binary System
We present the first detection of the low-lying pure rotational emission lines of H_2 from circumstellar disks around T Tauri stars, using the Short Wavelength Spectrometer on the Infrared Space Observatory. These lines provide a direct measure of the total amount of warm molecular gas in disks. The J = 2 → 0 S(0) line at 28.218 μm and the J = 3 → 1 S(1) line at 17.035 μm have been observed toward the double binary system GG Tau. Together with limits on the J = 5 → 3 S(3) and J = 7 → 5 S(5) lines, the data suggest the presence of gas at T_(kin) ≈ 110 ± 10 K with a mass of (3.6 ± 2.0) × 10^(-3) M_☉ (±3 σ). This amounts to ~3% of the total gas + dust mass of the circumbinary disk as imaged by millimeter interferometry, but it is larger than the estimated mass of the circumstellar disk(s). Possible origins for the warm gas seen in H_2 are discussed in terms of photon and wind-shock heating mechanisms of the circumbinary material, and comparisons with model calculations are made
Interferometric Observations of Formaldehyde in the Protoplanetary Disk around LkCa15
Emission from the line of HCO has been detected and
marginally resolved toward LkCa15 by the Nobeyama Millimeter Array. The column
density of HCO is higher than that observed in DM Tau and than predicted by
theoretical models of disk chemistry; also the line-intensity profile is less
centrally peaked than that for CO. A similar behavior is observed in other
organic gaseous molecules in the LkCa 15 disk.Comment: 5 pages, 4 figures. accepted to PASJ (Publication of Astronomical
Society of Japan
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