824 research outputs found
PT-Symmetric Quantum Mechanics
This paper proposes to broaden the canonical formulation of quantum
mechanics. Ordinarily, one imposes the condition on the
Hamiltonian, where represents the mathematical operation of complex
conjugation and matrix transposition. This conventional Hermiticity condition
is sufficient to ensure that the Hamiltonian has a real spectrum. However,
replacing this mathematical condition by the weaker and more physical
requirement , where represents combined parity reflection
and time reversal , one obtains new classes of complex Hamiltonians
whose spectra are still real and positive. This generalization of Hermiticity
is investigated using a complex deformation of the
harmonic oscillator Hamiltonian, where is a real parameter. The
system exhibits two phases: When , the energy spectrum of is
real and positive as a consequence of symmetry. However, when
, the spectrum contains an infinite number of complex
eigenvalues and a finite number of real, positive eigenvalues because symmetry is spontaneously broken. The phase transition that occurs at
manifests itself in both the quantum-mechanical system and the
underlying classical system. Similar qualitative features are exhibited by
complex deformations of other standard real Hamiltonians
with integer and ; each of these
complex Hamiltonians exhibits a phase transition at . These -symmetric theories may be viewed as analytic continuations of conventional
theories from real to complex phase space.Comment: 20 pages RevTex, 23 ps-figure
Universality in Random Walk Models with Birth and Death
Models of random walks are considered in which walkers are born at one
location and die at all other locations with uniform death rate. Steady-state
distributions of random walkers exhibit dimensionally dependent critical
behavior as a function of the birth rate. Exact analytical results for a
hyperspherical lattice yield a second-order phase transition with a nontrivial
critical exponent for all positive dimensions . Numerical studies
of hypercubic and fractal lattices indicate that these exact results are
universal. Implications for the adsorption transition of polymers at curved
interfaces are discussed.Comment: 11 pages, revtex, 2 postscript figure
Broadband Records of Earthquakes in Deep Gold Mines and a Comparison with Results from SAFOD, California
For one week during September 2007, we deployed a temporary network of field recorders and accelerometers at four sites within two deep, seismically active mines. The ground-motion data, recorded at 200 samples/sec, are well suited to determining source and ground-motion parameters for the mining-induced earthquakes within and adjacent to our network. Four earthquakes with magnitudes close to 2 were recorded with high signal/noise at all four sites. Analysis of seismic moments and peak velocities, in conjunction with the results of laboratory stick-slip friction experiments, were used to estimate source processes that are key to understanding source physics and to assessing underground seismic hazard. The maximum displacements on the rupture surfaces can be estimated from the parameter Rv, where v is the peak ground velocity at a given recording site, and R is the hypocentral distance. For each earthquake, the maximum slip and seismic moment can be combined with results from laboratory friction experiments to estimate the maximum slip rate within the rupture zone. Analysis of the four M 2 earthquakes recorded during our deployment and one of special interest recorded by the in-mine seismic network in 2004 revealed maximum slips ranging from 4 to 27 mm and maximum slip rates from 1.1 to 6:3 m=sec. Applying the same analyses to an M 2.1 earthquake within a cluster of repeating earthquakes near the San Andreas Fault Observatory at Depth site, California, yielded similar results for maximum slip and slip rate, 14 mm and 4:0 m=sec
Effect of bevelled silo outlet in the flow rate during discharge
We investigate the effect of a bevelled (or slanted) outlet on the discharge rate of mono-sized spheres from a quasi-two-dimensional silo, using the discrete element method. In contrast to hopper discharges, where the bevelling is across the entire base of the container, we study a bevelled opening that is significantly smaller than the silo width and in which the slanting is limited to half a sphere diameter at the boundary of the outlet. We show that the bevelling increases the flow rate comparably to the inclination in hopper walls. Using Beverloo's model, we relate this increase in rate to what we define as the ‘effective opening’ of the silo and analyse the velocity profiles associated with the discharges. We show that different openings, having effectively the same discharge rates, give rise to distinctly different internal dynamics in the silo. These results have the potential to aid industrial processes by fine-tuning and improving control of silo discharges, with a minimal impact on silo design, thus significantly reducing production and handling costs.Fil: Gago, Paula Alejandra. Consejo Nacional de Investigaciones CientÃficas y Técnicas; Argentina. Imperial College London; Reino UnidoFil: Madrid, Marcos Andres. Universidad Tecnológica Nacional; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas; ArgentinaFil: Boettcher, Stefan. University of Emory; Estados UnidosFil: Blumenfeld, Raphael. Imperial College London; Reino UnidoFil: King, Peter. Imperial College London; Reino Unid
Bound States of Non-Hermitian Quantum Field Theories
The spectrum of the Hermitian Hamiltonian
(), which describes the quantum anharmonic oscillator, is real and
positive. The non-Hermitian quantum-mechanical Hamiltonian , where the coupling constant is real and positive, is
-symmetric. As a consequence, the spectrum of is known to be
real and positive as well. Here, it is shown that there is a significant
difference between these two theories: When is sufficiently small, the
latter Hamiltonian exhibits a two-particle bound state while the former does
not. The bound state persists in the corresponding non-Hermitian -symmetric quantum field theory for all dimensions
but is not present in the conventional Hermitian field theory.Comment: 14 pages, 3figure
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