3,733 research outputs found
Combinatorial formulation of Ising model revisited
In 1952, Kac and Ward developed a combinatorial formulation for the two
dimensional Ising model which is another method of obtaining Onsager's formula
for the free energy per site in the thermodynamic limit of the model. Feynman
gave an important contribution to this formulation conjecturing a crucial
mathematical relation which completed Kac and Ward ideas. In this paper, the
method of Kac, Ward and Feynman for the free field Ising model in two
dimensions is reviewed in a selfcontained way.Comment: 27 pages, 17 figure
The Electron Temperature Gradient in the Galactic Disk
We derive the electron temperature gradient in the Galactic disk using a
sample of HII regions that spans Galactocentric distances 0--17 kpc. The
electron temperature was calculated using high precision radio recombination
line and continuum observations for more than 100 HII regions. Nebular
Galactocentric distances were calculated in a consistent manner using the
radial velocities measured by our radio recombination line survey. The large
number of nebulae widely distributed over the Galactic disk together with the
uniformity of our data provide a secure estimate of the present electron
temperature gradient in the Milky Way. Because metals are the main coolants in
the photoionized gas, the electron temperature along the Galactic disk should
be directly related to the distribution of heavy elements in the Milky Way. Our
best estimate of the electron temperature gradient is derived from a sample of
76 sources for which we have the highest quality data. The present gradient in
electron temperature has a minimum at the Galactic Center and rises at a rate
of 287 +/- 46 K/kpc. There are no significant variations in the value of the
gradient as a function of Galactocentric radius or azimuth. The scatter we find
in the HII region electron temperatures at a given Galactocentric radius is not
due to observational error, but rather to intrinsic fluctuations in these
temperatures which are almost certainly due to fluctuations in the nebular
heavy element abundances. Comparing the HII region gradient with the much
steeper gradient found for planetary nebulae suggests that the electron
temperature gradient evolves with time, becoming flatter as a consequence of
the chemical evolution of the Milky Way's disk.Comment: 43 pages, 9 figures (accepted for publication in the ApJ
Comparação entre os métodos de dilatação cervical para a inseminação artificial em ovelhas Santa Inês.
Dynamical matrix for arbitrary quadratic fermionic bath Hamiltonians and non-Markovian dynamics of one and two qubits in an Ising model environment
We obtain the analytical expression for the Kraus decomposition of the
quantum map of an environment modeled by an arbitrary quadratic fermionic
Hamiltonian acting on one or two qubits, and derive simple functions to check
the non-positivity of the intermediate map. These functions correspond to two
different sufficient criteria for non-Markovianity. In the particular case of
an environment represented by the Ising Hamiltonian, we discuss the two sources
of non-Markovianity in the model, one due to the finite size of the lattice,
and another due to the kind of interactions.Comment: 11 pages, 10 figure
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