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d Communicattons sectton ts intended fo the accelerated publication of important ne h in th editort'al their submittal letter why the workj ustiftes this special handling A. Rapid Communication sltould be no longer than 3jri printed pages and must be accompanied by an abstract Pa.ge proofs are sent to authors

A double stranded DNA molecule when pulled with a force acting on one end of the molecule can become either partially or completely unzipped depending on the magnitude of the force F. For a random DNA sequence, the number M of unzipped base pairs goes as M approximately (F - Fc)(-2) and diverges at the critical force Fc with an exponent chi = 2. We find that when excluded volume effect is taken into account for the unzipped part of the DNA, the exponent chi = 2 is not changed but the critical force Fc is changed. The force versus temperature phase diagram depends on only two parameters in the model, the persistence length and the denaturation temperature. Furthermore a scaling form of the phase diagram can be found. This scaling form is parameter independent and depends only on the spatial dimension. It applies to all DNA molecules and should provide a useful framework for comparison with experiments.

Using the Monte Carlo method, we have studied the pulsed laser deposition process at the submonolayer regime. In our simulations, dissociation of an atom from a cluster is incorporated. Our results indicate that the pulsed laser deposition resembles molecular-beam epitaxy at very low intensity, and that it is characteristically different from molecular-beam epitaxy at higher intensity. We have also obtained the island size distributions. The scaling function for the island size distribution for pulsed laser deposition is different from that of molecular-beam epitaxy.