Aharonov-Bohm Scattering, Contact Interactions and Scale Invariance

We perform a perturbative analysis of the Aharonov-Bohm problem to one loop in a field-theoretic formulation, and show that contact interactions are necessary for renormalizability. In general, the classical scale invariance of this problem is broken quantum mechanically. There exists however a critical point for which this anomaly disappears.Comment: 9 pages, TEX, CTP#218

On the Bergman-Milton bounds for the homogenization of dielectric composite materials

The Bergman-Milton bounds provide limits on the effective permittivity of a composite material comprising two isotropic dielectric materials. These provide tight bounds for composites arising from many conventional materials. We reconsider the Bergman-Milton bounds in light of the recent emergence of metamaterials, in which unconventional parameter ranges for relative permittivities are encountered. Specifically, it is demonstrated that: (a) for nondissipative materials the bounds may be unlimited if the constituent materials have relative permittivities of opposite signs; (b) for weakly dissipative materials characterized by relative permittivities with real parts of opposite signs, the bounds may be exceedingly large

Effect of 2-Substitution on the Rearrangement of 1-Cyclopropylvinyl Cations

2-Substitution in 1-cyclopropylvinyl cations produces a steric effect on cation generation and solvent trapping, but an electronic charge-stabilizing effect on cyclopropyl-to-cyclobutyl rearrangement

High field magnetotransport in composite conductors: the effective medium approximation revisited

The self consistent effective medium approximation (SEMA) is used to study three-dimensional random conducting composites under the influence of a strong magnetic field {\bf B}, in the case where all constituents exhibit isotropic response. Asymptotic analysis is used to obtain almost closed form results for the strong field magnetoresistance and Hall resistance in various types of two- and three-constituent isotropic mixtures for the entire range of compositions. Numerical solutions of the SEMA equations are also obtained, in some cases, and compared with those results. In two-constituent free-electron-metal/perfect-insulator mixtures, the magnetoresistance is asymptotically proportional to $|{\bf B}|$ at {\em all concentrations above the percolation threshold}. In three-constituent metal/insulator/superconductor mixtures a line of critical points is found, where the strong field magnetoresistance switches abruptly from saturating to non-saturating dependence on $|{\bf B}|$, at a certain value of the insulator-to-superconductor concentration ratio. This transition appears to be related to the phenomenon of anisotropic percolation.Comment: 16 pages, 3 figure

Identification of Binuclear Acyl Complexes as Intermediates in the CO-induced Conversion of [(η^5-C_5H_5)Co(CO)(Me)]_2 into Acetone, and [(η^5-C_5H_5)Co(CO)(Et)]_2 into Pentan-3-one

Studies of the carbonylation of [(η^5-C_5H_5)Co(CO)(R)]_2(R = Me, Et) at temperatures below ambient have revealed that these complexes may be converted into ketones via binuclear diacyl complexes [(η^5-C_5H_5)Co(CO)(COR)]_2; the postulated mechanistic pathway circumvents the earlier-identified intermediate (η^5-C_5H_5)Co(CO)R_2, and involves alkyl transfer from a Co^(II) acyl complex as a critical step

Brane Transfer Operations and T-Duality of Non-BPS States

Using the relation between D-brane charges and K-theory, we study non-BPS D-branes and their behavior under T-duality. We point out that in general compactifications, D-brane charges are classified by relative K-theory groups. T-duality is found to act as a symmetry between the relative K-theory groups in Type II and Type I/IA theories. We also study Type \tilde\IA theory (which contains an O8^- plane and an O8^+ plane), using K-theory and T-duality to identify its stable D-branes. Comparison with string theory constructions reveals two interesting effects. One of them involves the transfer of branes between O-planes, while in the other, a D-brane charge which seems conserved near one O-plane in fact decays due to the presence of another type of O-plane.Comment: 28 pages harvmac, 4 figures; Expanded argument for relative K-theory in subsection 2.2, and added explicit K-theory groups of a point in eq.(A.3) of the appendi

3-Oxabicyclo[3,2,0]hepta-1,4-diene

3-Oxabicyclo[3,2,0]hepta-1,4-diene (3) has been synthesized by partial hydrogenation of 3-oxabicyclo-[3,2,0]hepta-1,4,6-triene (2)

Bulk metals with helical surface states

In the flurry of experiments looking for topological insulator materials, it has been recently discovered that some bulk metals very close to topological insulator electronic states, support the same topological surface states that are the defining characteristic of the topological insulator. First observed in spin-polarized ARPES in Sb (D. Hsieh et al. Science 323, 919 (2009)), the helical surface states in the metallic systems appear to be robust to at least mild disorder. We present here a theoretical investigation of the nature of these "helical metals" - bulk metals with helical surface states. We explore how the surface and bulk states can mix, in both clean and disordered systems. Using the Fano model, we discover that in a clean system, the helical surface states are \emph{not} simply absorbed by hybridization with a non-topological parasitic metallic band. Instead, they are pushed away from overlapping in momentum and energy with the bulk states, leaving behind a finite-lifetime surface resonance in the bulk energy band. Furthermore, the hybridization may lead in some cases to multiplied surface state bands, in all cases retaining the helical characteristic. Weak disorder leads to very similar effects - surface states are pushed away from the energy bandwidth of the bulk, leaving behind a finite-lifetime surface resonance in place of the original surface states