11,538 research outputs found
Spin of the ground state and the flux phase problem on the ring
As a continuation of our previous work, we derive the optimal flux phase
which minimizes the ground state energy in the one-dimensional many particle
systems, when the number of particles is odd in the absence of on-site
interaction and external potential. Moreover, we study the relationship between
the flux on the ring and the spin of the ground state through which we derive
some information on the sum of the lowest eigenvalues of one-particle
Hamiltonians
Influence of a Z+(1540) resonance on K+N scattering
The impact of a (I=0, J^P=1/2^+) Z^+(1540) resonance with a width of 5 MeV or
more on the K+N (I=0) elastic cross section and on the P01 phase shift is
examined within the KN meson-exchange model of the Juelich group. It is shown
that the rather strong enhancement of the cross section caused by the presence
of a Z^+ with the above properties is not compatible with the existing
empirical information on KN scattering. Only a much narrower Z^+ state could be
reconciled with the existing data -- or, alternatively, the Z^+ state must lie
at an energy much closer to the KN threshold.Comment: 9 pages, RevTeX, 3 eps figure
Protostar Formation in Magnetic Molecular Clouds beyond Ion Detachment: I. Formulation of the Problem and Method of Solution
We formulate the problem of the formation of magnetically supercritical cores
in magnetically subcritical parent molecular clouds, and the subsequent
collapse of the cores to high densities, past the detachment of ions from
magnetic field lines and into the opaque regime. We employ the six-fluid MHD
equations, accounting for the effects of grains (negative, positive and
neutral) including their inelastic collisions with other species. We do not
assume that the magnetic flux is frozen in any of the charged species. We
derive a generalized Ohm's law that explicitly distinguishes between flux
advection (and the associated process of ambipolar diffusion) and Ohmic
dissipation, in order to assess the contribution of each mechanism to the
increase of the mass-to-flux ratio of the central parts of a collapsing core
and possibly to the resolution of the magnetic flux problem of star formation.
We show how our formulation is related to and can be transformed into the
traditional, directional formulation of the generalized Ohm's law, and we
derive formulae for the perpendicular, parallel and Hall conductivities
entering the latter, which include, for the first time, the effect of inelastic
collisions between grains. In addition, we present a general (valid in any
geometry) solution for the velocities of charged species as functions of the
velocity of the neutrals and of the effective flux velocity (which can in turn
be calculated from the dynamics of the system and Faraday's law). The last two
sets of formulae can be adapted for use in any general non-ideal MHD code to
study phenomena beyond star formation in magnetic clouds. The results,
including a detailed parameter study, are presented in two accompanying papers.Comment: 17 pages, emulateapj; accepted for publication in the Astrophysical
Journa
Search for Anomalous Couplings in Top Decay at Hadron Colliders
We present a quantitative study on sensitivities to the top-decay anomalous
couplings, taking into account realistic experimental conditions expected at
Tevatron and LHC. A double angular distribution of W and charged lepton in the
top decay is analyzed, using ttbar events in the lepton+jets channel. In order
to improve sensitivities to the anomalous couplings, we apply two techniques:
(1) We use a likelihood fitting method for full kinematical reconstruction of
each top event. (2) We develop a new effective spin reconstruction method for
leptonically-decayed top quarks; this method does not require spin information
of the antitop side. For simplicity, we neglect couplings of right-handed
bottom quark as well as CP violating couplings. The 95% C.L. estimated bound on
a ratio of anomalous couplings reads -0.81 < f_2/f_1 < -0.70,
-0.12<f_2/f_1<0.14 using 1000 reconstructed top events at Tevatron, while
-0.74<f_2/f_1<-0.72, -0.01<f_2/f_1<0.01 is expected with 100k reconstructed top
events at LHC, where only statistical errors are taken into account. A two-fold
ambiguity in the allowed range remains when the number of events exceeds a few
hundred.Comment: 21 pages, 15 figure
The flux phase problem on the ring
We give a simple proof to derive the optimal flux which minimizes the ground
state energy in one dimensional Hubbard model, provided the number of particles
is even.Comment: 8 pages, to appear in J. Phys. A: Math. Ge
A Spherical Model for "Starless" Cores of Magnetic Molecular Clouds and Dynamical Effects of Dust Grains
In the standard picture of isolated star formation, dense ``starless'' cores
are formed out of magnetic molecular clouds due to ambipolar diffusion. Under
the simplest spherical geometry, I demonstrate that ``starless'' cores formed
this way naturally exhibit a large scale inward motion, whose size and speed
are comparable to those detected recently by Taffala et al. and Williams et al.
in ``starless'' core L1544. My model clouds have a relatively low mass (of
order 10 ) and low field strength (of order 10 G) to begin with.
They evolve into a density profile with a central plateau surrounded by a
power-law envelope, as found previously. The density in the envelope decreases
with radius more steeply than those found by Mouschovias and collaborators for
the more strongly magnetized, disk-like clouds.
At high enough densities, dust grains become dynamically important by greatly
enhancing the coupling between magnetic field and the neutral cloud matter. The
trapping of magnetic flux associated with the enhanced coupling leads, in the
spherical geometry, to a rapid assemblage of mass by the central protostar,
which exacerbates the so-called ``luminosity problem'' in star formation.Comment: 27 pages, 4 figures, accepted by Ap
Acceleration of dust particles by low-frequency Alfv\'en waves
We investigate the efficiency of acceleration of charged dust particles by
low-frequency Alfv\'en waves in nonlinear approximation. We show that the
longitudinal acceleration of dust particles is proportional to the square of
the soliton amplitude , while the transversal acceleration is of
. In the conditions of the interstellar medium the resulting velocity
of dust particles can reach 0.3 to 1 km s.Comment: 8 pages, no figs, accepted in Phys. Lett.
Vacuum fluctuations and the thermodynamics of chiral models
We consider the thermodynamics of chiral models in the mean-field
approximation and discuss the relevance of the (frequently omitted) fermion
vacuum loop. Within the chiral quark-meson model and its Polyakov loop extended
version, we show that the fermion vacuum fluctuations can change the order of
the phase transition in the chiral limit and strongly influence physical
observables. We compute the temperature-dependent effective potential and
baryon number susceptibilities in these models, with and without the vacuum
term, and explore the cutoff and the pion mass dependence of the
susceptibilities. Finally, in the renormalized model the divergent vacuum
contribution is removed using the dimensional regularization.Comment: 9 pages, 5 figure
Solitonic approach to the dimerization problem in correlated one-dimensional systems
Using exact diagonalizations we consider self-consistently the lattice
distortions in odd Peierls-Hubbard and spin-Peierls periodic rings in the
adiabatic harmonic approximation. From the tails of the inherent spin soliton
the dimerization d_\infty of regular even rings is found by extrapolations to
infinite ring lengths. Considering a wide region of electron-electron onsite
interaction values U>0 compared with the band width 4t_0 at intermediately
strong electron-phonon interaction g, known relationships obtained by other
methods are reproduced and/or refined within one unified approach: such as the
maximum of d_\infty at U \simeq 3 t_0 for g \simeq 0.5 and its shift to zero
for g \to g_c \approx 0.7. The hyperbolic tangent shape of the spin soliton is
retained for any U and g <~ 0.6. In the spin-Peierls limit the d_\infty are
found to be in agreement with results of DMRG computations.Comment: 4 pages, 4 figures, Physical Review B, Rapid Communications, v. 56
(1997) accepte
Single-Crystal Organic Charge-Transfer Interfaces probed using Schottky-Gated Heterostructures
Organic semiconductors based on small conjugated molecules generally behave
as insulators when undoped, but the hetero-interfaces of two such materials can
show electrical conductivity as large as in a metal. Although charge transfer
is commonly invoked to explain the phenomenon, the details of the process and
the nature of the interfacial charge carriers remain largely unexplored. Here
we use Schottky-gated heterostructures to probe the conducting layer at the
interface between rubrene and PDIF-CN2 single crystals. Gate-modulated
conductivity measurements demonstrate that interfacial transport is due to
electrons, whose mobility exhibits band-like behavior from room temperature to
~ 150 K, and remains as high as ~ 1 cm2V-1s-1 at 30 K for the best devices. The
electron density decreases linearly with decreasing temperature, an observation
that can be explained quantitatively based on the heterostructure band diagram.
These results elucidate the electronic structure of rubrene-PDIF-CN2 interfaces
and show the potential of Schottky-gated organic heterostructures for the
investigation of transport in molecular semiconductors.Comment: 37 pages, 9 Figures (including supplementary information
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