Spin Excitations of a Kondo-Screened Atom Coupled to a Second Magnetic Atom

Screening the electron spin of a magnetic atom via spin coupling to conduction electrons results in a strong resonant peak in the density of states at the Fermi energy, the Kondo resonance. We show that magnetic coupling of a Kondo atom to another unscreened magnetic atom can split the Kondo resonance into two peaks. Inelastic spin excitation spectroscopy with scanning tunneling microscopy is used to probe the Kondo effect of a Co atom, supported on a thin insulating layer on a Cu substrate, that is weakly coupled to a nearby Fe atom to form an inhomogeneous dimer. The Kondo peak is split by interaction with the non-Kondo atom, but can be reconstituted with a magnetic field of suitable magnitude and direction. Quantitative modeling shows that this magnetic field results in a spin-level degeneracy in the dimer, which enables the Kondo effect to occur

EXCITATION CONDITIONS IN THE MULTI-COMPONENT SUBMILLIMETER GALAXY SMM J00266+1708

We present multiline CO observations of the complex submillimeter galaxy SMMJ00266+1708. Using the Zpectrometer on the Green Bank Telescope, we provide the first precise spectroscopic measurement of its redshift (z = 2.742). Based on followup CO(1–0), CO(3–2), and CO(5–4) mapping, SMMJ00266+1708 appears to have two distinct components separated by ∼ 500 kms−1 that are nearly coincident along our line of sight. The two components show hints of different kinematics, with the blue-shifted component dispersion-dominated and the red-shifted component showing a clear velocity gradient. CO line ratios differ slightly between the two components, indicating that the physical conditions in their molecular gas may not be alike. We tentatively infer that SMMJ00266+1708 is an on going merger with a mass ratio of (7.8±4.0)/ sin2(i), with its overall size and surface brightness closely resembling that of other merging systems. We perform large velocity gradient modeling of the CO emission from both components and find that each component’s properties are consistent with a single phase of molecular gas (i.e., a single temperatures and density); additional multi-phase modelling of the red-shifted component, although motivated by a CO(1–0) size larger than the CO(3–2) size, is inconclusive. SMMJ00266+1708 provides evidence of early stage mergers within the submillimeter galaxy population. Continuum observations of J00266 at the ∼ 1′′ resolution of our observations could not have distinguished between the two components due to their separation (0.′′73 ± 0.′′06), illustrating that the additional velocity information provided by spectral line studies is important for addressing the prevalence of unresolved galaxy pairs in low-resolution submillimeter surveys

Excitation Conditions in the Multi-component Submillimeter Galaxy SMM J00266+1708

We present multiline CO observations of the complex submillimeter galaxy SMM J00266+1708. Using the Zpectrometer on the Green Bank Telescope, we provide the first precise spectroscopic measurement of its redshift (z=2.742). Based on followup CO(1-0), CO(3-2), and CO(5-4) mapping, SMM J00266+1708 appears to have two distinct components separated by ~500 km/s that are nearly coincident along our line of sight. The two components show hints of different kinematics, with the blue-shifted component dispersion-dominated and the red-shifted component showing a clear velocity gradient. CO line ratios differ slightly between the two components, indicating that the physical conditions in their molecular gas may not be alike. We tentatively infer that SMM J00266+1708 is an ongoing merger with a mass ratio of (7.8+/-4.0)/sin^2(i), with its overall size and surface brightness closely resembling that of other merging systems. We perform large velocity gradient modeling of the CO emission from both components and find that each component's properties are consistent with a single phase of molecular gas (i.e., a single temperatures and density); additional multi-phase modeling of the red-shifted component, although motivated by a CO(1-0) size larger than the CO(3-2) size, is inconclusive. SMM J00266+1708 provides evidence of early stage mergers within the submillimeter galaxy population. Continuum observations of J00266 at the ~1" resolution of our observations could not have distinguished between the two components due to their separation (0.73" +/- 0.06"), illustrating that the additional velocity information provided by spectral line studies is important for addressing the prevalence of unresolved galaxy pairs in low-resolution submillimeter surveys

Characterisation of the Mouse Vasoactive Intestinal Peptide Receptor Type 2 Gene, Vipr2, and Identification of a Polymorphic LINE-1-like Sequence That Confers Altered Promoter Activity

The VPAC(2) receptor is a seven transmembrane spanning G protein-coupled receptor for two neuropeptides, vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP). It has a distinct tissue-specific, developmental and inducible expression that underlies an important neuroendocrine role. Here, we report the characterisation of the gene that encodes the mouse VPAC(2) receptor (Vipr2), localisation of the transcriptional start site and functional analysis of the promoter region. The Vipr2 gene contains 12 introns within its protein-coding region and spans 68.6 kb. Comparison of the 5′ untranslated region sequences for cloned 5′-RACE products amplified from different tissues showed they all were contained within the same exon, with the longest extending 111 bp upstream of the ATG start site. Functional analysis of the 3.2-kb 5′-flanking region using sequentially deleted sequences cloned into a luciferase gene reporter vector revealed that this region is active as a promoter in mouse AtT20 D16:16 and rat GH4C1 cell lines. The core promoter is located within a 180-bp GC-rich region proximal to the ATG start codon and contains potential binding sites for Sp1 and AP2, but no TATA-box. Further upstream, in two out of three mice strains examined, we have discovered a 496-bp polymorphic DNA sequence that bears a significant identity to mouse LINE-1 DNA. Comparison of the promoter activity between luciferase reporter gene constructs derived from the BALB/c (which contains this sequence) and C57BL/6J (which lacks this sequence) Vipr2 promoter regions has shown three-fold difference in luciferase gene activity when expressed in mouse AtT20 D16:16 and αT3-1 cells, but not when expressed in the rat GH4C1 cells or in COS 7 cells. Our results suggest that the mouse Vipr2 gene may be differentially active in different mouse strains, depending on the presence of this LINE-1-like sequence in the promoter region

The role of magnetic anisotropy in the Kondo effect

In the Kondo effect, a localized magnetic moment is screened by forming a correlated electron system with the surrounding conduction electrons of a non-magnetic host. Spin S=1/2 Kondo systems have been investigated extensively in theory and experiments, but magnetic atoms often have a larger spin. Larger spins are subject to the influence of magnetocrystalline anisotropy, which describes the dependence of the magnetic moment's energy on the orientation of the spin relative to its surrounding atomic environment. Here we demonstrate the decisive role of magnetic anisotropy in the physics of Kondo screening. A scanning tunnelling microscope is used to simultaneously determine the magnitude of the spin, the magnetic anisotropy and the Kondo properties of individual magnetic atoms on a surface. We find that a Kondo resonance emerges for large-spin atoms only when the magnetic anisotropy creates degenerate ground-state levels that are connected by the spin flip of a screening electron. The magnetic anisotropy also determines how the Kondo resonance evolves in a magnetic field: the resonance peak splits at rates that are strongly direction dependent. These rates are well described by the energies of the underlying unscreened spin states.Comment: 14 pages, 4 figures, published in Nature Physic

The RR Lyrae Distance Scale

We review seven methods of measuring the absolute magnitude M_V of RR Lyrae stars in light of the Hipparcos mission and other recent developments. We focus on identifying possible systematic errors and rank the methods by relative immunity to such errors. For the three most robust methods, statistical parallax, trigonometric parallax, and cluster kinematics, we find M_V (at [Fe/H] = -1.6) of 0.77 +/- 0.13, 0.71 +/- 0.15, 0.67 +/- 0.10. These methods cluster consistently around 0.71 +/- 0.07. We find that Baade-Wesselink and theoretical models both yield a broad range of possible values (0.45-0.70 and 0.45-0.65) due to systematic uncertainties in the temperature scale and input physics. Main-sequence fitting gives a much brighter M_V = 0.45 +/- 0.04 but this may be due to a difference in the metallicity scales of the cluster giants and the calibrating subdwarfs. White-dwarf cooling-sequence fitting gives 0.67 +/- 0.13 and is potentially very robust, but at present is too new to be fully tested for systematics. If the three most robust methods are combined with Walker's mean measurement for 6 LMC clusters, V_{0,LMC} = 18.98 +/- 0.03 at [Fe/H] = -1.9, then mu_{LMC} = 18.33 +/- 0.08.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles', A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press. 21 pages including 1 table; uses Kluwer's crckapb.sty LaTeX style file, enclose