1,931 research outputs found
Bostonia: v. 63, no. 3
Founded in 1900, Bostonia magazine is Boston University's main alumni publication, which covers alumni and student life, as well as university activities, events, and programs
Occam's Higgs: A Phenomenological Solution to the Electroweak Hierarchy Problem
We propose a phenomenological solution to the Electroweak hierarchy problem.
It predicts no new particles beyond those in the Standard Model. The Higgs is
arbitrarily massive and slow-roll inflation can be implemented naturally. Loop
corrections will be negligible even for large cutoffs.Comment: 7 pp., 2 figs., LaTeX. Slight rewordin
Bostonia: v. 64, no. 1
Founded in 1900, Bostonia magazine is Boston University's main alumni publication, which covers alumni and student life, as well as university activities, events, and programs
Geometrical (2+1)-gravity and the Chern-Simons formulation: Grafting, Dehn twists, Wilson loop observables and the cosmological constant
We relate the geometrical and the Chern-Simons description of
(2+1)-dimensional gravity for spacetimes of topology , where
is an oriented two-surface of genus , for Lorentzian signature and general
cosmological constant and the Euclidean case with negative cosmological
constant. We show how the variables parametrising the phase space in the
Chern-Simons formalism are obtained from the geometrical description and how
the geometrical construction of (2+1)-spacetimes via grafting along closed,
simple geodesics gives rise to transformations on the phase space. We
demonstrate that these transformations are generated via the Poisson bracket by
one of the two canonical Wilson loop observables associated to the geodesic,
while the other acts as the Hamiltonian for infinitesimal Dehn twists. For
spacetimes with Lorentzian signature, we discuss the role of the cosmological
constant as a deformation parameter in the geometrical and the Chern-Simons
formulation of the theory. In particular, we show that the Lie algebras of the
Chern-Simons gauge groups can be identified with the (2+1)-dimensional Lorentz
algebra over a commutative ring, characterised by a formal parameter
whose square is minus the cosmological constant. In this
framework, the Wilson loop observables that generate grafting and Dehn twists
are obtained as the real and the -component of a Wilson loop
observable with values in the ring, and the grafting transformations can be
viewed as infinitesimal Dehn twists with the parameter .Comment: 50 pages, 6 eps figure
Unconventional London penetration depth in Ba(Fe0.93Co0.07)2As2 single crystals
The London penetration depth, , has been measured in several
single crystals of Ba(FeCo)As. Thermodynamic,
electromagnetic, and structural characterization measurements confirm that
these crystals are of excellent quality. The observed low temperature variation
of follows a power-law, with , indicating the existence of normal quasiparticles down to at least
. This is in contrast to recent penetration depth measurements on
single crystals of NdFeAsOF and SmFeAsOF, which
indicate an anisotropic but nodeless gap. We propose that a more
three-dimensional character in the electronic structure of
Ba(FeCo)As may lead to an anisotropic wave gap
with point nodes that would explain the observed .Comment: 4 page
An Optical-Lattice-Based Quantum Simulator For Relativistic Field Theories and Topological Insulators
We present a proposal for a versatile cold-atom-based quantum simulator of
relativistic fermionic theories and topological insulators in arbitrary
dimensions. The setup consists of a spin-independent optical lattice that traps
a collection of hyperfine states of the same alkaline atom, to which the
different degrees of freedom of the field theory to be simulated are then
mapped. We show that the combination of bi-chromatic optical lattices with
Raman transitions can allow the engineering of a spin-dependent tunneling of
the atoms between neighboring lattice sites. These assisted-hopping processes
can be employed for the quantum simulation of various interesting models,
ranging from non-interacting relativistic fermionic theories to topological
insulators. We present a toolbox for the realization of different types of
relativistic lattice fermions, which can then be exploited to synthesize the
majority of phases in the periodic table of topological insulators.Comment: 24 pages, 6 figure
Topological phase transitions in the non-Abelian honeycomb lattice
Ultracold Fermi gases trapped in honeycomb optical lattices provide an
intriguing scenario, where relativistic quantum electrodynamics can be tested.
Here, we generalize this system to non-Abelian quantum electrodynamics, where
massless Dirac fermions interact with effective non-Abelian gauge fields. We
show how in this setup a variety of topological phase transitions occur, which
arise due to massless fermion pair production events, as well as pair
annihilation events of two kinds: spontaneous and strongly-interacting induced.
Moreover, such phase transitions can be controlled and characterized in optical
lattice experiments.Comment: RevTex4 file, color figure
Non-Universal Behavior of Finite Quantum Hall Systems as a Result of Weak Macroscopic Inhomogeneities
We show that, at low temperatures, macroscopic inhomogeneities of the
electron density in the interior of a finite sample cause a reduction in the
measured conductivity peak heights compared to the
universal values previously predicted for infinite homogeneous samples. This
effect is expected to occur for the conductivity peaks measured in standard
experimental geometries such as the Hall bar and the Corbino disc. At the
lowest temperatures, the decrease in is found to
saturate at values proportional to the difference between the adjacent plateaus
in , with a prefactor which depends on the particular realization
of disorder in the sample. We argue that this provides a possible explanation
of the ``non-universal scaling'' of observed in a
number of experiments. We also predict an enhancement of the ``non-local''
resistance due to the macroscopic inhomogeneities. We argue that, in the Hall
bar with a sharp edge, the enhanced ``non-local'' resistance and the size
corrections to the ``local'' resistance are directly related. Using
this relation, we suggest a method by which the finite-size corrections may be
eliminated from and in this case.Comment: REVTEX 3.0 file (38 pages) + 5 postscript figures in uuencoded
format. Revised version includes an additional figure showing unpublished
experimental dat
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