The effective fine-structure constant of freestanding graphene measured in graphite

Electrons in graphene behave like Dirac fermions, permitting phenomena from high-energy physics to be studied in a solid-state setting. A key question is whether or not these fermions are critically influenced by Coulomb correlations. We performed inelastic x-ray scattering experiments on crystals of graphite and applied reconstruction algorithms to image the dynamical screening of charge in a freestanding graphene sheet. We found that the polarizability of the Dirac fermions is amplified by excitonic effects, improving screening of interactions between quasiparticles. The strength of interactions is characterized by a scale-dependent, effective fine-structure constant, α(g)* (k,ω), the value of which approaches 0.14 ± 0.092 ~ 1/7 at low energy and large distances. This value is substantially smaller than the nominal α(g) = 2.2, suggesting that, on the whole, graphene is more weakly interacting than previously believed.     

Magnetic Field and Plasma Observations at Mars: Initial Results of the Mars Global Surveyor Mission

The magnetometer and electron reflectometer investigation (MAG/ER) on the Mars Global Surveyor spacecraft has obtained magnetic field and plasma observations throughout the near-Mars environment, from beyond the influence of Mars to just above the surface (at an altitude of approximately 100 kilometers). The solar wind interaction with Mars is in many ways similar to that at Venus and at an active comet, that is, primarily an ionospheric-atmospheric interaction. No significant planetary magnetic field of global scale has been detected to date (<2 x 10(21) Gauss-cubic centimeter), but here the discovery of multiple magnetic anomalies of small spatial scale in the crust of Mars is reported.     

MAGNETIC RESONANCE IMAGING—A GENERAL OVERVIEW OF PRINCIPLES AND EXAMPLES IN VETERINARY NEURODIAGNOSIS

Magnetic resonance imaging (MRI), a noninvasive technique that provides accurate, detailed, anatomic images, has had a major impact in the diagnosis of human disease. This technique is based upon the inherent magnetic properties of certain nuclei. Induction of the nuclei into a low energy state is achieved by placing them in a static magnetic field. The nuclei may then be excited into a high energy state by application of a radio frequency pulse. When the second field is stopped, the nuclei return to ground state and emit the absorbed energy in the form of a radio signal. This signal is received by a coil that generally surrounds the specimen and converted to an anatomic image through a process of computer-assisted reconstruction. Contrast is altered by applying the second pulse in different sequences (saturation recovery, inversion recovery, and spin echo) and using enhancing agents such as gadolinium. In this paper, we present an overview of the general principles of MRI and some clinical examples in dogs and cats with central nervous system disease.