Invariant expansion for the trigonal band structure of graphene

We present a symmetry analysis of the trigonal band structure in graphene, elucidating the transformational properties of the underlying basis functions and the crucial role of time-reversal invariance. Group theory is used to derive an invariant expansion of the Hamiltonian for electron states near the K points of the graphene Brillouin zone. Besides yielding the characteristic k-linear dispersion and higher-order corrections to it, this approach enables the systematic incorporation of all terms arising from external electric and magnetic fields, strain, and spin-orbit coupling up to any desired order. Several new contributions are found, in addition to reproducing results obtained previously within tight-binding calculations. Physical ramifications of these new terms are discussed.Comment: 10 pages, 1 figure; expanded version with more details and additional result

Direct Measurement of Neutron-Star Recoil in the Oxygen-Rich Supernova Remnant Puppis A

A sequence of three Chandra X-ray Observatory High Resolution Camera images taken over a span of five years reveals arc-second-scale displacement of RX J0822-4300, the stellar remnant (presumably a neutron star) near the center of the Puppis A supernova remnant. We measure its proper motion to be 0.165+/-0.025 arcsec/yr toward the west-southwest. At a distance of 2 kpc, this corresponds to a transverse space velocity of ~1600 km/s. The space velocity is consistent with the explosion center inferred from proper motions of the oxygen-rich optical filaments, and confirms the idea that Puppis A resulted from an asymmetric explosion accompanied by a kick that imparted roughly 3*10^49 ergs of kinetic energy (some 3 percent of the kinetic energy for a typical supernova) to the stellar remnant. We discuss constraints on core-collapse supernova models that have been proposed to explain neutron star kick velocities

Giant anisotropy of Zeeman splitting of quantum confined acceptors in Si/Ge

Shallow acceptor levels in Si/Ge/Si quantum well heterostructures are characterized by resonant tunneling spectroscopy in the presence of high magnetic fields. In a perpendicular magnetic field we observe a linear Zeeman splitting of the acceptor levels. In an in-plane field, on the other hand, the Zeeman splitting is strongly suppressed. This anisotropic Zeeman splitting is shown to be a consequence of the huge light hole-heavy hole splitting caused by a large biaxial strain and a strong quantum confinement in the Ge quantum well.Comment: 5 figures, 4 page

Compact strain-sensitive flexible photonic crystals for sensors

A promising fabrication route to produce absorbing flexible photonic crystals is presented, which exploits self-assembly during the shear processing of multi-shelled polymer spheres. When absorbing material is incorporated in the interstitial space surrounding high-refractive-index spheres, a dramatic enhancement in the transmission edge on the short-wavelength side of the band gap is observed. This effect originates from the shifting optical field spatial distribution as the incident wavelength is tuned around the band gap, and results in a contrast up to 100 times better than similar but nonabsorbing photonic crystals. An order-of-magnitude improvement in strain sensitivity is shown, suggesting the use of these thin films in photonic sensors

Coherent optical transfer of Feshbach molecules to a lower vibrational state

Using the technique of stimulated Raman adiabatic passage (STIRAP) we have coherently transferred ultracold 87Rb2 Feshbach molecules into a more deeply bound vibrational quantum level. Our measurements indicate a high transfer efficiency of up to 87%. As the molecules are held in an optical lattice with not more than a single molecule per lattice site, inelastic collisions between the molecules are suppressed and we observe long molecular lifetimes of about 1 s. Using STIRAP we have created quantum superpositions of the two molecular states and tested their coherence interferometrically. These results represent an important step towards Bose-Einstein condensation (BEC) of molecules in the vibrational ground state.Comment: 4 pages, 5 figure

Tuning the scattering length with an optically induced Feshbach resonance

We demonstrate optical tuning of the scattering length in a Bose-Einstein condensate as predicted by Fedichev {\em et al.} [Phys. Rev. Lett. {\bf 77}, 2913 (1996)]. In our experiment atoms in a $^{87}$Rb condensate are exposed to laser light which is tuned close to the transition frequency to an excited molecular state. By controlling the power and detuning of the laser beam we can change the atomic scattering length over a wide range. In view of laser-driven atomic losses we use Bragg spectroscopy as a fast method to measure the scattering length of the atoms.Comment: submitted to PRL, 5 pages, 5 figure

Anisotropic Fermi Contour of (001) GaAs Electrons in Parallel Magnetic Fields

We demonstrate a severe Fermi contour anisotropy induced by the application of a parallel magnetic field to high-mobility electrons confined to a 30-nm-wide (001) GaAs quantum well. We study commensurability oscillations, namely geometrical resonances of the electron orbits with a unidirectional, surface-strain-induced, periodic potential modulation, to directly probe the size of the Fermi contours along and perpendicular to the parallel field. Their areas are obtained from the Shubnikov-de Haas oscillations. Our experimental data agree semi-quantitatively with the results of parameter-free calculations of the Fermi contours but there are significant discrepancies.Comment: 5 pages, 5 figure

Extrinsic Entwined with Intrinsic Spin Hall Effect in Disordered Mesoscopic Bars

We show that pure spin Hall current, flowing out of a four-terminal phase-coherent two-dimensional electron gas (2DEG) within inversion asymmetric semiconductor heterostructure, contains contributions from both the extrinsic mechanisms (spin-orbit dependent scattering off impurities) and the intrinsic ones (due to the Rashba coupling). While the extrinsic contribution vanishes in the weakly and strongly disordered limits, and the intrinsic one dominates in the quasiballistic limit, in the crossover transport regime the spin Hall conductance, exhibiting sample-to-sample large fluctuations and sign change, is not simply reducible to either of the two mechanisms, which can be relevant for interpretation of experiments on dirty 2DEGs [V. Sih et al., Nature Phys. 1, 31 (2005)].Comment: 5 pages, 3 color EPS figure