136 research outputs found
Jahn-Teller Solitons, Structural Phase Transitions and Phase Separation
It is demonstrated that under common conditions a molecular solid subject to
Jahn-Teller interactions supports stable Q-ball-like non-topological solitons.
Such solitons represent a localized lump of excess electric charge in periodic
motion accompanied by a time-dependent shape distortion of a set of adjacent
molecules. The motion of the distortion can correspond to a true rotation or to
a pseudo-rotation about the symmetric shape configuration. These solitons are
stable for Jahn-Teller coupling strengths below a critical value; however, as
the Jahn-Teller coupling approaches this critical value, the size of the
soliton diverges signaling an incipient structural phase transition. The
soliton phase mimics features commonly attributed to phase separation in
complex solids.Comment: 8 pages, 3 figure
Dissipative Effects on Quantum Sticking
Using variational mean-field theory, many-body dissipative effects on the
threshold law for quantum sticking and reflection of neutral and charged
particles are examined. For the case of an ohmic bosonic bath, we study the
effects of the infrared divergence on the probability of sticking and obtain a
non-perturbative expression for the sticking rate. We find that for weak
dissipative coupling , the low energy threshold laws for quantum
sticking are modified by an infrared singularity in the bath. The sticking
probability for a neutral particle with incident energy behaves
asymptotically as ; for a charged
particle, we obtain . Thus, "quantum
mirrors" --surfaces that become perfectly reflective to particles with incident
energies asymptotically approaching zero-- can also exist for charged
particles.Comment: 10 pages, 0 fig
Theory of Spontaneous Polarization of Endohedral Fullerenes
A pseudo-Jahn-Teller model describing central atom distortions is proposed
for endohedral fullerenes of the form A@C where A is either a rare gas
or a metal atom. A critical (dimensionless) coupling is found, below
which the symmetric configuration is stable and above which inversion symmetry
is broken. Vibronic parameters are given for selected endohedral fullerenes.Comment: 4 pages, REVTEX, 1 Postscript figure. [Phys. Rev. Lett. (in press)
Looking for Design in Materials Design
Despite great advances in computation, materials design is still science
fiction. The construction of structure-property relations on the quantum scale
will turn computational empiricism into true design.Comment: 3 pages, 1 figur
Symmetry protected Z2-quantization and quaternionic Berry connection with Kramers degeneracy
As for a generic parameter dependent hamiltonian with the time reversal (TR)
invariance, a non Abelian Berry connection with the Kramers (KR) degeneracy are
introduced by using a quaternionic Berry connection. This quaternionic
structure naturally extends to the many body system with the KR degeneracy. Its
topological structure is explicitly discussed in comparison with the one
without the KR degeneracy. Natural dimensions to have non trivial topological
structures are discussed by presenting explicit gauge fixing. Minimum models to
have accidental degeneracies are given with/without the KR degeneracy, which
describe the monopoles of Dirac and Yang. We have shown that the Yang monopole
is literally a quaternionic Dirac monopole. The generic Berry phases
with/without the KR degeneracy are introduced by the complex/quaternionic Berry
connections. As for the symmetry protected quantization of these
general Berry phases, a sufficient condition of the -quantization
is given as the inversion/reflection equivalence. Topological charges of the
SO(3) and SO(5) nonlinear -models are discussed in their relation to
the Chern numbers of the and models as well.Comment: Submitted for New J. Physics, Special issue on Topological
Insulators. 18 pages, 2 figure
Orthogonality Catastrophe in Quantum Sticking
The probability that a particle will stick to a surface is fundamental to a
variety of processes in surface science, including catalysis, epitaxial growth,
and corrosion. At ultralow energies, how particles scatter or stick to a
surface affects the performance of atomic clocks, matter-wave interferometers,
atom chips and other quantum information processing devices. In this energy
regime, the sticking probability is influenced by a distinctly quantum
mechanical effect: quantum reflection, a result of matter wave coherence,
suppresses the probability of finding the particle near the surface and reduces
the sticking probability. We find that another quantum effect can occur,
further shaping the sticking probability: the orthogonality catastrophe, a
result of the change in the quantum ground state of the surface in the presence
of a particle, can dramatically alter the probability for quantum sticking and
create a superreflective surface at low energies.Comment: 10 pages, 2 figure
Theory and Application of Dissociative Electron Capture in Molecular Identification
The coupling of an electron monochromator (EM) to a mass spectrometer (MS)
has created a new analytical technique, EM-MS, for the investigation of
electrophilic compounds. This method provides a powerful tool for molecular
identification of compounds contained in complex matrices, such as
environmental samples. EM-MS expands the application and selectivity of
traditional MS through the inclusion of a new dimension in the space of
molecular characteristics--the electron resonance energy spectrum. However,
before this tool can realize its full potential, it will be necessary to create
a library of resonance energy scans from standards of the molecules for which
EM-MS offers a practical means of detection. Here, an approach supplementing
direct measurement with chemical inference and quantum scattering theory is
presented to demonstrate the feasibility of directly calculating resonance
energy spectra. This approach makes use of the symmetry of the
transition-matrix element of the captured electron to discriminate between the
spectra of isomers. As a way of validating this approach, the resonance values
for twenty-five nitrated aromatic compounds were measured along with their
relative abundance. Subsequently, the spectra for the isomers of nitrotoluene
were shown to be consistent with the symmetry-based model. The initial success
of this treatment suggests that it might be possible to predict negative ion
resonances and thus create a library of EM-MS standards.Comment: 18 pages, 7 figure
Synergistic Effects of Traffic-Related Air Pollution and Exposure to Violence on Urban Asthma Etiology
Background: Disproportionate life stress and consequent physiologic alteration (i.e., immune dysregulation) has been proposed as a major pathway linking socioeconomic position, environmental exposures, and health disparities. Asthma, for example, disproportionately affects lower-income urban communities, where air pollution and social stressors may be elevated. Objectives: We aimed to examine the role of exposure to violence (ETV), as a chronic stressor, in altering susceptibility to traffic-related air pollution in asthma etiology. Methods: We developed geographic information systems (GIS)–based models to retrospectively estimate residential exposures to traffic-related pollution for 413 children in a community-based pregnancy cohort, recruited in East Boston, Massachusetts, between 1987 and 1993, using monthly nitrogen dioxide measurements for 13 sites over 18 years. We merged pollution estimates with questionnaire data on lifetime ETV and examined the effects of both on childhood asthma etiology. Results: Correcting for potential confounders, we found an elevated risk of asthma with a 1-SD (4.3 ppb) increase in NO2 exposure solely among children with above-median ETV [odds ratio (OR) = 1.63; 95% confidence interval (CI), 1.14–2.33)]. Among children always living in the same community, with lesser exposure measurement error, this association was magnified (OR = 2.40; 95% CI, 1.48–3.88). Of multiple exposure periods, year-of-diagnosis NO was most predictive of asthma outcomes. Conclusions: We found an association between traffic-related air pollution and asthma solely among urban children exposed to violence. Future studies should consider socially patterned susceptibility, common spatial distributions of social and physical environmental factors, and potential synergies among these. Prospective assessment of physical and social exposures may help determine causal pathways and critical exposure periods
International trade and domestic competition: Evidence from Belgium
We investigate the effect of domestic market competition on firm-level export intensity. We employ a comprehensive dataset of Belgian firms from 2005–2008, when the fall in the number of firms engaged in trade was accompanied by a growing amount of transactions. The resulting increase in the domestic concentration of Belgian firms has sparked numerous debates, since the direction of causality between domestic market structure and export performance is unclear. We apply the fractional logit estimator and control for both self-selection and simultaneity bias. We find that a positive linkage exists between the level of competition and export intensity
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