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arXiv 0906.0868cond-mat.quant-gas
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相互作用玻色子的凝聚密度:泛函重整化群方法

Condensate density of interacting bosons: a functional renormalization group approach

Christopher Eichler, Nils Hasselmann, Peter Kopietz

摘要:

利用泛函重整化群计算了三维相互作用玻色子的温度依赖凝聚密度$ρ^0 (T)$。从数值解的适当截断FRG流方程的不可约顶点,我们得到$ρ^0 (T)$为任意温度。我们仔细地外推我们的数值结果的临界点,并确定序参量指数$β\approx 0.32$,在合理的协议与XY-普适性类相关的预期值$ 0.345$。我们还计算了冷凝物的密度在两个维度在零温度下使用截断的FRG流方程的基础上的衍生物的扩展,包括立方和四次项的有效潜力的权力的密度的扩展。与广泛使用的有效势的二次近似相比,与三次项和四次项相关的耦合常数使凝聚体密度的结果提高了几个百分点。然而,三次和四次耦合常数流到相当大的值,这使一些疑问的FRG计算的基础上的有效势的低阶多项式近似。

英文摘要:

We calculate the temperature dependent condensate density $ρ^0 (T)$ of interacting bosons in three dimensions using the functional renormalization group (FRG). From the numerical solution of suitably truncated FRG flow equations for the irreducible vertices we obtain $ρ^0 (T)$ for arbitrary temperatures. We carefully extrapolate our numerical results to the critical point and determine the order parameter exponent $β\approx 0.32$, in reasonable agreement with the expected value $ 0.345$ associated with the XY-universality class. We also calculate the condensate density in two dimensions at zero temperature using a truncation of the FRG flow equations based on the derivative expansion including cubic and quartic terms in the expansion of the effective potential in powers of the density. As compared with the widely used quadratic approximation for the effective potential, the coupling constants associated with the cubic and quartic terms increase the result for the condensate density by a few percent. However, the cubic and quartic coupling constants flow to rather large values, which sheds some doubt on FRG calculations based on a low order polynomial approximation for the effective potential.

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