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The critical phenomena of a single defect

We consider the critical system with a point defect and study the variation of thermodynamic quantities, which are the differences between those with and without the defect. Within renormalization group theory, we show generally that the critical exponent of the internal energy variation is the specific heat exponent of a pure system, and the critical exponent of the heat capacity variation is that for the temperature derivative of specific heat of a pure system. This conclusion is valid for the isotropic systems with a short-range interaction. As an example we solve the two dimensional Ising model with a point defect numerically. The variations of the free energy, internal energy and specific heat are calculated with bond propagation algorithm. At the critical point, the internal energy variation diverges with the lattice size logarithmically and the heat capacity variation diverges with size linearly. Near the critical point, the internal energy variation behaves as $\ln |t|$ and the heat capacity variation behaves as $|t|^{-1}$, where $t$ is the reduced temperature.